UNIVERSITY OF HERTFORDSHIRE

VIBRATORY STRESS RELIEVING

AN

ALTERNATIVE TO THERMAL

Dissertation submitted to the University of Hertfordshire in partial fulfilment of the requirements for the degree BSc (Honours)

Supervisor: Wolf Richrath

Name: Stuart Michael Lloyd February 1998SUMMARY

Stress in steel structures and components are probably the most common reason for

premature failure of the component. Induced stresses are inevitable in every form of the

steel process be it casting, welding or machining, and has to be relieved either by thermal or

mechanical stress relieving.

Thermal Stress Relieving involves the uniform heating of the component or part thereof to a

suitable temperature below the transformation range, holding at this temperature for a

predetermined period and then uniformly cooling to avoid the re-induction of stress. Stress

Relief heat treating can reduce distortion and high stresses from welding that can effect

service performance. The process of thermal stress relief is complex and has many

limitations and drawbacks, as an alternative, mechanical stress relief may be used.

Vibratory Stress Relieving (VSR) is probably the best alternative to thermal with achieved

stress reductions of up to 80% (RA Claxton,1988).

The origin of VSR dates back to the early 1930’s with its use in the navy and airforce. The

process, now well established as an alternative method to thermal has a distinct advantage,

in that it can reduce in-built stresses and stabilise components at any stage of the

manufacturing process without the component ever leaving the workshop.

The process of VSR is to excite the component into one or more resonant or sub-resonant

states. (Resonance is when the frequency of the workpiece is equal to that of the excitor

output). This in turn causes the component to undergo elastic distortion and re-distribute the

overall stress equally throughout the component.

Over the last decade, VSR has developed from a little known art into an indispensable basic

process which is now well-tried and established as an alternative to thermal treatment for

stabilising castings, fabrications and machined components.

Although the technique has not yet been fully exploited, fifteen years of commercial

experience and evaluation have determined its valuable areas of application.

(i)

It is also important to emphasise that Vibratory Stress Relieving is not claimed to be a

substitute for all stress relieving treatments, there is some common ground where some

processes are predominant than others.

The main factor that has hindered acceptance of the newer technique was the basic

reluctance to accept that a low cost vibratory treatment could replace thermal treatment

involving high-energy costs and consumption.

Thermal and vibratory treatments share a capability in three areas, namely dimension

control, dimension stabilisation, and overall stress relief. Total stress relief is difficult to

obtain using any commercial process, but only vibratory stress relieving can stabilise / stress

relieve components at any stage of the work in process with no metallurgical change, no

scaling, no discolouring and at a low cost. Conversely only thermal treatment of steel can

change the materials metallurgical property.

Reports and tests contained in the study evaluates the stress reduction in steels with the use

of two common processes namely, thermal and vibratory. Test reports by individuals as

well as institutions clearly show the possibility of VSR as an alternative to thermal on

castings, weldments and machined components. (RW Nichols, 1977).

This study will clearly indicate with various comparison tests to thermal that VSR has

reduced stress levels by up to 80% and in many cases achieved better results than thermal.

It should, however, be mentioned that VSR is certainly not a replacement for thermal but

certainly an alternative.

(ii)

ACKNOWLEDGEMENTS

This dissertation has benefited greatly from the co-operation and effort given by the

Vibratory Stress Relieving company of South Africa and the United Kingdom with notes

and practical literature on Vibratory Stress Relieving.

TUV Rheinland, Quality Services Division South Africa supplied literature and reports on

Thermal Stress Relieving.

Thanks and appreciation to Highveld Steel for the opportunity to conduct tests to verify

supporting materials.

S.M. Lloyd

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TABLE OF CONTENTSPAGE No.

CHAPTER 1.................................................................................................................1

1. PROBLEM DEFINITIONS AND OBJECTIVES........................................1

1.1 Problem Definition.....................................................................................1

1.2 Objectives...................................................................................................1

1.3 Project Results............................................................................................1

1.4 Structure.....................................................................................................2

1.5 Scope..........................................................................................................4

1.6 Methodology..............................................................................................4

1.7 Highveld Steel and Vanadium Corporation Limited....................................6

1.8 Highveld Steel and Vanadium, Product Range............................................8

CHAPTER 2...............................................................................................................14

2. LITERATURE STUDY............................................................................14

2.1 Introduction..............................................................................................14

2.2 Advantages and Limitations of the Vibratory and Thermal Process............17

2.3 Vibratory Stress Relieving, Why and Effective Alternative to Heat

Treatment ?...............................................................................................19

2.4 Vibratory Stress Relieving, An Alternative to Thermal..............................23

2.5 Vibratory Lowering of Residual Stresses in Weldments............................26

2.6 Vibratory Stress Relieving – Its Advantages and Limitations to the

Thermal Process.......................................................................................31

2.7 Conclusion...............................................................................................37

CHAPTER 3...............................................................................................................38

3. ANALYSIS..............................................................................................38

3.1 Introduction..............................................................................................38

3.2 Stress........................................................................................................38

3.3 Stress Relief of Welded Components – A Practical Example.....................39

3.4 Measurement of Vibration – Induced Stress Relief in the Heavy

Fabrication Industry..................................................................................47

3.5 Research Findings....................................................................................49

3.6 Need For Measurement of Residual Stress Relief in Structures..................49

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PAGE No.

3.7 Measurement of Residual Stress Relief on Engineering Structures.............50

3.8 Thermal Stress Relief of Austenitic Stainless Steel....................................59

3.9 Results Obtained by Various Treatments...................................................61

3.10 Heat Treatment of Castings and Special Steels..........................................62

3.11 Heat Treating of Tool Steels.....................................................................65

3.12 Conclusion...............................................................................................67

CHAPTER 4...............................................................................................................68

4. STRESS RELIEVING DIFFERENT OPTIONS.......................................68

4.1 Introduction..............................................................................................68

4.2 Common Areas of Stress and their Effects................................................68

4.3 Methods of Stress Relief...........................................................................68

4.4 Thermal Stress Relief................................................................................70

4.5 Conclusion...............................................................................................71

CHAPTER 5...............................................................................................................72

5. COST EVALUATION AND JUSTIFICATION OF STRESS RELIEF.....72

5.1 Introduction..............................................................................................72

5.2 Case Study 1 – Opencast Mine Dragline Bucket Repairs...........................72

5.3 Case Study 2 – Opencast Mine Coal Hauler Axle......................................74

5.4 Case Study 3 – Coal Loading Terminal Structure......................................75

5.5 Case Study 4 – Segment Roll Reclamation...............................................76

5.6 Recommendations....................................................................................79

CHAPTER 6...............................................................................................................81

6. IMPLEMENTATION AND REVIEW......................................................81

6.1 Introduction..............................................................................................81

6.2 Repair and Treatment Procedure...............................................................81

6.3 Roller Reclamation with the use of VSR ..................................................84

APPENDICES............................................................................................................86

BIBLIOGRAPHY......................................................................................................95

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LIST OF FIGURES

PAGE No.

1.1 Administrative Offices...............................................................................7

1.2 Iron Plant One and Two.............................................................................7

1.3 Corporate and Inter-Trade Structure...........................................................9

1.4 Hot Rolled Rail........................................................................................12

1.5 Highveld Steel Rolled Beams...................................................................13

1.6 Highveld Rolling Plate.............................................................................13

2.1 Bending Modes........................................................................................15

2.2 Residual Stress Profiles Induced by Cold Rolling.....................................21

2.3 Effect of Cyclic Loading upon Surface Residual Stress in Mill

Steel EN3b...............................................................................................22

2.4 Time Temperature Graph.........................................................................25

2.5 Yield Strength to Temperature Relationship.............................................25

2.6 Three Dimensional Stress Comparison.....................................................28

2.7 Measured Values of VSR Treated Beams.................................................29

2.8 Measured Values of Notch Toughness......................................................30

2.9 Fatigue Tests............................................................................................31

3.1 Primary Reducer Gearbox Casing.............................................................40

3.2 Casing During VSR.................................................................................41

3.3 Pattern “B” Casing, Dimensional Check before and after VSR.................44

3.4 Pattern “B” Casing, Surface Flatness before and after VSR.......................44

3.5 Pattern “B” Casing, Side One VSR..........................................................45

3.6 Pattern “B” Casing, Side Two VSR..........................................................45

3.7 Pattern “A” Casing, Side One Thermal.....................................................45

3.8 Pattern “A” Casing, Side Two Thermal....................................................45

3.9 Bedplate...................................................................................................51

3.10 Location of Strain Gauges on Bedplate.....................................................52

3.11 Residual Stress in Bedplate......................................................................52

3.12 Graphical Acceleration on Location One..................................................53

3.13 Graphical Acceleration on Location Two..................................................53

3.14 Bowl Assembly with Composite Metal Construction................................55

3.15 Location of Vibrator on Bowl Assembly..................................................56

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PAGE No.

3.16 Typical Acceleration Spectrum during Vibratory Treatment of

Bowl Assembly........................................................................................56

3.17 Dimensions of bowl Assembly after final Machining................................57

3.18 Stress Relief Obtained in 347 Stainless Steel............................................60

3.19 Effects of Stress Relieving on Tensile Strength and Hardness of

Grey Irons................................................................................................63

3.20 Effects of Stress Relieving Temperature on Hardness of Grey Irons..........64

3.20 Effects of Stress Relieving Temperature on Residual Stress on

Grey Irons................................................................................................64

3.21 Effects of Stress Relieving Temperature on Residual Stress on

Grey Irons................................................................................................64

3.22 Examples of the Causes of Residual Stresses............................................66

4.1 Graphical Representation of a Frequency..................................................69

5.1 Pre-heat Temperature Calculation Table...................................................77

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LIST OF TABLES

PAGE No.

2.1 Composition and Properties of EN3b.......................................................20

3.1 Definition of Tests...................................................................................40

3.2 Vibration Stress Relief Conditions...........................................................42

3.3 Check of Dimensions before and after Stress Relief..................................43

3.4 Test Results.............................................................................................54

3.5 Dial Gauge Measurements........................................................................58

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LIST OF APPENDICES

PAGE No.

1. HEIC Industries Report............................................................................86

2. Rover Group Report.................................................................................87

3. Langley Alloys Report.............................................................................88

4. Stantech Engineering Report....................................................................89

5. John Thompson Report............................................................................90

6. 70 Ton Dragline Bucket...........................................................................91

7. 70 Ton Dragline Bucket...........................................................................91

8. Opencast Mine, Coal Hauler Axle............................................................92

9. Coal Loading Terminal Stacker................................................................92

10. Aluminium and Copper Strengthening Beam............................................93

11. EN30b Marine Drive Shafts, VSR Treated Before Final Machining..........93

12. 25 Ton Ball Mill Steel VSR Treated Before Rubber Lining......................94

13. 8 Ton Dragline Sheave Wheel, VSR Treated after Extensive Repairs........94

(ix) 1

CHAPTER 1

PROBLEM DEFINITIONS AND OBJECTIVES

1.1 PROBLEM DEFINITION

Induced stress in steels invariably means early failure due to fatigue cracking or warpage.

Thermal stress relieving is probably the most commonly practised method of stress relief, it

is, however, expensive due to high transport and power costs and not always a success.

Along with Thermal Stress Relief comes the side effects of component discoloration, scaling

and metallurgical change which may not be desirable for the final product.

Thermal stress relief to complex components comprising of varied material specifications is

not possible due to metallurgical change and large components in many instances simply

cannot be stress relieved due to size.

1.2 OBJECTIVES

The objective will be to ascertain whether Vibratory Stress Relieving can be used as an

alternative to Thermal in normal situations and whether or not it will be suitable for use

where no stress relief is possible at all. VSR cannot be used to induce metallurgical changes

to the process only to stress relieve. Reports contained will establish the success of VSR for

stress relief and weld conditioning as well as the recommendations and reports to its success

(S. Hornsey, 1998).

1.3 PROJECT RESULTS

The objectives of stress relief have now been established, it will now be necessary to

compare the objectives required to those of achieved and to establish a sound process of

stress relief.

In respect to results achieved, the literature study and analysis, Chapter 2 and 3 clearly

shows the more compatible process for the required application.

2

1.3.1 Advantages and Disadvantages

1.3.1.1 Vibratory Stress Relieving

ADVANTAGES DISADVANTAGES

Low cost process.

Portable.

Short treatment time.

No material damage, such as scaling, or

No metallurgical change possible.

discoloration.

Graphical printout of stress re-distribution.

Can be applied at any point of the

manufacturing process.

1.3.1.2 Thermal Stress Relief

ADVANTAGES DISADVANTAGES

Metallurgical change possible. Extremely long treatment times.

Scaling and discoloration to the component.

High energy cost.

Furnace size restricts component treatment.

1.4 STRUCTURE

1.4.1 Chapter 1

This chapter concentrates on establishing the causes and effects of induced stress together

with methods and processes used to reduce these stresses. The Company Profile will give

an overall view of Highveld Steel and Vanadium with respect to products and certain

processes.

3

1.4.2 Chapter 2

The literature study will contain factual reports by leading stress relieving specialists from

various parts of the world. Tests and results conducted on various components to establish

the effects and success of VSR as an alternative to thermal treatment.

1.4.3 Chapter 3

The analysis is a more concise report on the success of the VSR process, backing up the

information contained in the literature study. The analysis will contain actual problems and

various processes available to create a stable component.

1.4.4 Chapter 4

The discussion of the various methods to stress relieve, stabilise and condition components

to prolong the life of the component. Comparisons shall be made of both the VSR process

and the thermal process.

1.4.5 Chapter 5

This chapter will concentrate on cost benefit analysis of the VSR and thermal process with

its recommendations. Various studies conducted by VSR Africa as well as the author to

ascertain the benefits of both processes and recommendations thereof.

1.4.6 Chapter 6

The implementation and review of the tests conducted, how they will be installed at

Highveld and the cost implications.

4

1.5 SCOPE

This report will contain many references to comparison reports carried out from small

aluminium components used on Boeing aeroplanes to dragline buckets used on the South

African open cast mines. Reference will be made to as many published reports conducted

by individuals as well as institutions (VSR Africa; VSR UK).

Results of these reports and recommendations can only be given where no conflict of

interest exists and where no confidentiality will be lost to processes or products. The

investigation of the VSR process has been targeted upon the problem of providing a rational

explanation of the effects of VSR.

1.6 METHODOLOGY

The use of text materials supplied by VSR Africa and VSR UK will be used to determine

the success of VSR as an alternative to thermal on tests carried out on identical components

and some only VSR treated. The decision of VSR treatment shall be further strengthened

with cost analysis reports and company references (VSR Africa) by those using VSR. In-

house tests carried out within Highveld to determine the success of VSR to reduce stresses

and increase yield where possible.

5

HIGHVELD STEEL AND VANADIUM

CORPORATION LIMITED

COMPANY PROFILE

(Courtesy Highveld Steel)

6

1.7 HIGHVELD STEEL AND VANADIUM CORPORATION LIMITED

1.7.1 Company Review

In November 1964 Anglo American Corporation decided to build an iron and steel works

near Witbank, Transvaal, at a cost of R127 million. Four years later, Highveld Steel and

Vanadium had an iron plant, a steel plant and a universal structural mill in operation.

The decision to proceed with a project of this magnitude was based on a two year pilot plant

in correlation with other respected studies. Pilot plant studies revealed that titaniferous

magnetite ore from the bushveld igneous complex could be successfully processed to

produce liquid pig iron and vanadium bearing slag - materials which could be used to

produce finished steel and vanadium pentoxide.

In 1977 the structural mill was complimented with the addition of the plate mill division. In

1981 iron plant one with six furnaces and ten pre-reduction kilns was at capacity production.

In 1982 the flat products division was complimented further with the addition of a hot

reversing strip mill producing coils and sheet. In 1985 Highveld extended its iron

production with the addition of iron plant two (Figure 1.1 and 1.2).

1.7.2 De-Centralised Divisions and Partnerships

In 1962 Anglo American purchased Transvaal Metals a vanadium producing concern

situated outside Witbank. In 1975 Highveld Steel in conjunction with Samancor group and

Ferrometals, Ferroveld was formed for the production of electrode paste used at the iron

plant divisions and in 1976 Highveld acquired a 65% share in Transalloys.

In 1978 with the acquisition of Rand Carbide, Highveld was able to move further into the

ferrosilicone range of carbonaceous products, the division also produces steel drums for its

own use. In 1985 Rheem South Africa became a division of Highveld with plants in

Durban, Johannesburg, Cape Town and Vanderbijlpark for the production of metal

containers and closures.

7

(Courtesy Highveld Steel)

FIGURE 1.1 – The administrative offices with the Steel Plant in the background.

(Courtesy Highveld Steel)

FIGURE 1.2 – Iron Plant One and Two.

8

In October 1991 the Rheem division was granted board approval to establish an aluminium

can production facility to serve the South African beverage industry, this facility was

commissioned in August 1993. In 1991 the corporation, in equal partnership with Samancor

Limited formed the Columbus venture, probably the largest venture ever undertaken by the

Corporation to date. The expansion of Columbus is at present in progress and plans are

being made to quadruple production and make Columbus the largest and only primary

stainless steel producer in Africa.

In 1998 the board approved expenditure of R180 million for the construction of a third

roasting kiln and plant expansion at Vanchem to increase vanadium production and to

enable Highveld to remain the largest producer of vanadium in the world.

1.7.3 Corporate and Inter-Trade Structure

The corporate and inter-trade structure is shown in Figure 1.3

1.8 HIGHVELD STEEL AND VANADIUM, PRODUCT RANGE

1.8.1 Vanadium

1.8.1.1 Vanadium Spinnel (Slag)

This is a by-product of the steel making process, the slag is removed during the purification

process of the shaking ladles. This is then dry milled and sold locally, internationally or

used for the production of vanadium products at Vanchem.

1.8.1.2 Ferro Vanadium

This is produced at Highveld by means of an electric smelter furnace with a monthly

capacity of 300 tons. Ferro-vanadium is sold locally and internationally to foundries and

specialised steel manufactures as a final product.

9

FIGURE 1.3

CORPORATE AND INTER-TRADE STRUCTURE

Control

Inter-company Trade

Sales Products

10

1.8.1.3 Vanadium Pentoxide

ANGLO AMERICAN

INDUSTRIALCORPORATION

LIMITED

CORPORATEBODIES

INSTITUTIONSINDIVIDUALS

SAMANCOR INDUSTRIALDEVELOPMENTCORPORATION

HIGHVELD STEEL AND VANADIUM CORPORATION LIMITED

RHEEM DIVISION

RAND CARBIDEDIVISION

VANCHEMDIVISION

AND WAPADS-

KLOOFMAPOCHS

MINE

STEEL-WORKS

DIVISIONTRANS-ALLOYS

DIVISION

SPITSKOPQUARRY

FERRO-VELD

COLUMBUSJOINT

VENTURE

Crown closures

DrumsPailsCans(Aluminium &Steel)

Ferro-silicon

Electrically calcinedanthracite

ElectrodepasteChar

Vanadium pentoxide Vanadium trioxide (Hivox)Vanadium chemicals

Ferro-vanadium

Cast billetsRolled steel products

Vanadium slag Titania slagBOF slag

Silico-manganese

Medium carbon Ferro-manganese

Silica Quartz by-products

Electrically calcined anthraciteElectrode

Rolled stainless steel products

52% 48%

33.3%

33.3%

33.3%

50%50%

Silico-manganeseMed. CarbonFerro manganese

Iron oreIron ore

SteelSheet Drums

ElectrodePaste

Ferro-silicon

ElectrodePaste

Vanadium pentoxide is used for the manufacture of ferro-vanadium and is sold locally or

internationally to producers who wish to manufacture their own ferro-vanadium. The bulk

is for in-house use at the ferro-vanadium plant.

1.8.1.4 Vanadium Trioxide

This process involves the smelting of polyvanadate in a controlled atmosphere, the final

product being a black sooty powder is used for the manufacture of ferro-vanadium. This is

more financial benefit due to the higher concentrate of vanadium compared to that of

vanadium pentoxide. It is, however, a very messy material to drum or work with.

1.8.1.5 Vanadium Chemicals

These products are produced at the re-located plant recently purchased for the production of

various vanadium chemicals. Vanadium chemicals are mainly in liquid form and are widely

used for the manufacture of fertilisers and other products not related to the steel industry.

1.8.2 Steel Products

1.8.2.1 Billets, Blooms and Slabs

Highveld has five continuous casting machines, one billet machine, three bloom machines

and a slab caster. The mould size on the billet machine can range form 98 mm square to

140mm. The billets for re-rolling are produced on the billet machine and are made available

for sale from time to time but the bulk is for in-house use at the mills.

1.8.2.2 Hot Rolled Steel Rails

Highveld produces rails in six standard (mass/metre) sizes to meet customer requirements.

Rails used in mines and sidings are produced to Highveld’s commercial rail specification

(Figure 1.4).

11

1.8.2.3 Universal Beams, Bearing Piles, Universal Columns, Joists and Channels

Three bloom sizes are used to roll the finished products at the structural mill. This is a

semi-automated combination mill for rolling universal columns, parallel flanged beams,

standard joists, channels, angles, rounds and squares. In addition to this the mill is equipped

to roll rails of up to 36m in length (Figure 1.5).

1.8.2.4 Carbon Manganese Rounds

From its beginning Highveld has been a producer of a large range, large diameter round bar.

In many ways Highveld has pioneered the production of this product from continuously cast

steel. The market segment is mainly localised but still an important element in Highvelds

marketing programme (Figure 1.5 background).

1.8.2.5 Flat Products

In these divisions Highveld produces five main product ranges, namely: strip, coiled plate,

light, medium and heavy plate (Figure 1.6)

12

FIGURE 1.4 - HOT ROLLED RAIL AT THE STRUCTURAL MILL

(Courtesy Highveld Steel)

13

FIGURE 1.5 - HIGHVELD STEEL ROLLED BEAMS AT THE STRUCTURAL

MILL

(Courtesy Highveld Steel)

FIGURE 1.6 - HIGHVELD ROLLING PLATE

(Courtesy Highveld Steel)

4

CHAPTER 2

LITERATURE STUDY

2.1 INTRODUCTION

The following research relates to the use of VSR as an alternative to thermal treatment.

2.1.1 Vibratory Stress Relieving

This technique has evolved under the description of Vibratory Stress relief or VSR. It

consists in essence of using a vibrating device to induce vibration at the natural resonant

frequency of the component. The frequency is varied until resonance (component frequency

is equal to that of the excitors output.) is obtained and the amplitude has become steady.

The frequency of the machine is then increased until another resonant frequency is found,

this process continues until all resonant frequencies have been treated.

The gradual application of load in a cyclic manner as in VSR would cause the material to

follow its natural monotonic stress strain curve. The consequential stress reduction caused

by VSR would be greater for a given applied strain than that achieved by a simple overload.

In practice, several points of application may be made to excite the maximum number of

nodes of vibration and applied load in a variety of directions, effective relief often comes

after a treatment of 10 000 cycles. During the treatment cycle various bending modes are

forced upon the component to ensure effective stress relief (Figure 2.1)

15

FIGURE 2.1 – BENDING MODES

Principles of Applied Vibration Theory

a. First Torsional Mode.b. Second Torsional Mode (longitudinal direction).c. Third Torsional Mode (longitudinal direction).d. Forth Torsional Mode (longitudinal direction).e. Second Torsional Mode (transverse direction).f. First Longitudinal Bending Mode.g. Second Longitudinal Bending Mode.h. Third Longitudinal Bending Mode.i. Second Bending Mode with Edges only slightly involved.j. Third Bending Mode with Edges Slightly Involved.k. Forth Transverse – Bending Mode.l. Transverse - Bending Vibration with Longitudinal – Bending Superimposed.

16

2.1.2 Furnace Treatment

This may be carried out in a variety of sized furnaces mainly dependent on the component

size and volumes. In all cases four important variables must be controlled: Heating rate,

Temperature of Treatment, Time of Treatment (soak time) and Cooling Rate. If any of the

four variables are incorrect then the stress relief process will fail and may under certain

situations induce a greater stress level than when originally treated. Whatever the specific

requirement, control of temperature at all stages of heat treatment is critical and strict control

is vitally important.

2.1.3 The Objectives of Thermal Stress Relief

The terms normalising, annealing, tempering and stress relieving are often used to mean the

same thing namely stress relieving. It is true that these processes do relieve some stress,

however, each process is unique from the other and each has a specific objective.

2.1.3.1 Annealing

Creates a uniform softness to the component, with its slow cooling down process a reduction

of up to 50 % in the strength of the metal can occur with partial stress relief.

2.1.3.2 Normalising

Creates a uniform grain structure within a metal, the quenching process used, however,

induces stress within a metal and is not recommended for stress relief.

2.1.3.3 Tempering

This process with its mill processing temperatures results in a partial stress relief as well as

improved ductile strength.

17

2.1.4 Conclusion

Before choosing any method of stress relief it is important to establish why the component

necessitates the relief of stress. If a metallurgical process such as normalising or annealing

is required then there is no alternative but to thermally treat the component.

In many situations stress relief by simple overload is sufficient, however, where dimensional

stability and stress reduction is required VSR can offer several advantages over other

processes.

2.2 ADVANTAGES AND LIMITATIONS OF THE VIBRATORY AND

THERMAL PROCESS

2.2.1 Vibratory Stress Relieving

2.2.1.1 Advantages

Completely portable, low cost and low power consumption. Graphic result reports can be

generated when required. The process in itself is very short, often not more than 20 – 30

minutes to achieve 80% stress reduction. There are no metallurgical changes to the

materials, which can be essential on certain materials. Complete material stability can be

achieved even when treated close to final machining operations, causes a balanced

redistribution of internal strain resulting in little or no distortion.

2.2.1.2 Disadvantages

Certain components can be noisy during treatment, although measures can also be taken to

avoid such occurrences. Components have to be damped with the use of rubber pads. This

entails the removal of the component from the rest of the machine (identical for thermal). If

a metallurgical change needs to be achieved then this will not be possible with VSR. Care

and extensive knowledge is needed when treating copper or aluminium to avoid hardening.

18

2.2.2 Thermal Treatment

2.2.2.1 Advantages

Causes a refinement of material grain structure if required. It has the ability to treat a wide

range of materials, as does VSR. Improvement to the mechanical properties of heat affected

zones when correctly applied along with high reductions of residual stresses, 80 – 90% is

achievable when correctly applied to certain materials.

2.2.2.2 Disadvantages

Location and furnace size is more often a problem to manufactures resulting in

transportation costs often exceeding treatment cost. Very high energy costs, discoloration,

distortion and very time consuming on large components. Extreme caution must be

exercised in heat treating tempered steels as yield strength and creep properties may be

seriously affected if temperatures are exceeded.

It is for these reasons that thermal treatment may under no circumstances be applied close to

final machining operations.

2.2.3 Conclusion

The process of VSR may seemingly be new with reports indicating a definite and positive

stress reduction and added stability to treated components with no adverse effects associated

with thermal.

The cost of VSR treatment in itself justifies the need for further investigation, however,

thermal treatment can never be totally replaced by VSR for metallurgical reasons.

19

2.3 VIBRATORY STRESS RELIEVING, WHY AN EFFECTIVE

ALTERNATIVE TO HEAT TREATMENT?

Vibratory Stress Relieving is a general term used in industry to refer to the reduction of

residual stresses by means of cyclic loading treatment, commonly applied a VSR treatment

involves the vibration of the component at resonance (the natural phenomenon found in all

components).

In order to achieve high stress amplitudes using relatively low cost portable equipment. It is

claimed that a high degree of stress relaxation may be achieved in a wide variety of

mechanical structures by running the excitors close to the resonant frequency of the

structure.

The freedom of limited distortion, scale and digression of material properties serves to make

VSR an attractive alternative to thermal. The potential savings in time, equipment and

energy costs are substantial as well as the fact that VSR may be applied at any point of the

manufacturing process, but may be most effective when invoked after the completion of all

probable stress inducing processes.

2.3.1 The Need for a Validated Theory

The investigation has been targeted upon the problem of providing a rational explanation of

the effects of VSR. A standard module postulates that VSR works by a combination of

residual and vibrating stresses, exciting the yield strength of the material. With the

presumption, that the subsequent plastic flow is such, that when the vibrational amplitude is

removed, the previously stressed area can now return to a lower level of residual stress.

(R.A. Walker, 1988)

2.3.2 Experimental Approach

Specimens of simple geometry and known residual stress distribution were subjected to

cyclic loading treatments. The material selected was a standard EN3b mild steel to which

residual stress was induced by cold rolling (Table 2.1). These specimens were then subjected

to VSR through various resonant frequencies.

20

Table 2.1

COMPOSITION AND PROPERTIES OF EN3b

C SI MN NI S P

% 0.25 0.35 1.0 - 0.06 0.06

2.3.3 Residual Stress Measurement

In order to study the effects of mechanical treatments upon residual stress levels within the

sample materials it was first necessary to find a suitable method of materials removal. The

method most suitable was found to be a variation of the layer removal technique, in which

the residual stress within a thin surface skin is inferred from the change in strain occurring in

the remaining material upon the removal of this layer. It was ultimately decided that a

single point fly cutter gave the most convenient and least stress inducing method of layer

removal.

The results and observations made on the EN3b samples, which were subjected to cyclic

loading, reduced the surface residual stress. This will continue with increasing cyclic stress

amplitude. Sufficient measurable stress reduction can be expected to occur after a treatment

of 10 000 cycles.

More detailed insight into the nature of the stress reduction obtained by cyclic loading may

be seen in Figure 2.2. In this instance stress relief is confined to the surface, where original

residual stress levels are highest. As the amplitudes are increased beyond these levels, stress

relief effects penetrate further into the material. Changes do occur at this level thus reducing

the stress gradient close to the surface (Figure 2.2.).

These stress changes that are expected to occur in well-defined regions of the material and

according to this model complete stress relief (80%) should occur within the first few cycles.

21

FIGURE 2.2 - RESIDUAL STRESS PROFILES INDUCED BY COLD ROLLING

2.3.4 X-Ray Diffraction Studies

These results support the conjecture that mechanical stress relief occurs by a process of

plastic deformation upon a microscopic scale. The increase in dislocation density indicated

that irreversible slip was taking place within the material, causing dislocation. In an essence

the grain structure is “moved” to produce better grain materials without the induced stress.

2.3.5 Elastic Property Changes

Throughout the course of resonant loading, the system allowed precise monitoring of the

natural frequencies and response amplitudes. In all cases the resonant frequency was

observed to fall as a result of cyclic loading (Figure 2.3).

22

FIGURE 2.3 - EFFECT OF CYCLIC LOADING UPON SURFACE RESIDUAL

STRESS IN MILD STEEL EN3b

2.3.6 Conclusion

It has been demonstrated that resonant cyclic loading of cold rolled mild steel is capable of

effectively reducing peak residual stresses up to 40%. The level of applied stress required to

produce such effects is relatively low, a typical value of 250mpa represents less than half of

a materials ultimate strength. At this level, the estimated fatigue limit of the material is not

exceeded, so that the treatment may be expected to have a negligible effect upon fatigue life

(ultimate tensile strength is 450mpa and the ultimate yield strength is 250mpa).

Considerable evidence has been accumulated to suggest that the mechanism of stress relief

is one of dislocation motion on a microscopic scale. The standard model of bulk plastic

flow in response to overloading becomes applicable only as the applied stress amplitude

approaches or exceeds the yield point.

It is significant that the material used (EN3b) successfully in this investigation is one that

has been reported in much of the available literature to be unresponsive to mechanical stress

relief. A logical conclusion can be made that even more effective results may be achieved in

conjunction with different materials, such as: castings, welded components, and heat-treated

alloys. (R.A. Walker, 1988).

23

The latter, in particular, offer exciting possibilities, since thermal annealing is often

precluded for such materials by the necessity of avoiding untoward effects upon their

mechanical properties. VSR may now be considered to be an established phenomenon with

measurable results.

2.4 VIBRATORY STRESS RELIEVING, AN ALTERNATIVE TO

THERMAL

2.4.1 Vibratory

Over the last decade, VSR has evolved from a little known art into an indispensable process,

which is now well tried and established as an alternative to thermal treatment for stabilising

castings, fabrications and machined components.

It is also important to emphasise that VSR is not claimed to be a substitute to thermal

although there is some common ground. The VSR process involves inducing metal

structures into one or more resonant and sub-resonant conditions or states using portable

high force exciters. Treatment periods vary according to weight and structure shape with

working frequencies ranging from 0 - 100 Hz. The work piece is supported on rubber pads

and with the use of modern exciters results equal and sometimes better can be achieved over

that of thermal processes. The portability of the equipment enables VSR to tackle the

problem at more convenient times and places, this may be that the work piece may never

leave the workshop. (J.S. Hornsey, 1998).

2.4.2 Thermal

Stress relief heat-treating is used to relieve stresses that remain locked in a structure as a

consequence of a manufacturing sequence of some form. Stress relief heat-treating is the

uniform heating of a structure or portion to a suitable temperature below the transformation

range and holding at this temperature for a fixed deviation, followed by a uniform cooling

process.

24

At this point care is to be taken to uniformly cool the work piece as a non-constant cooling

cycle can result in re-occurring residual stresses. Stress relief heat-treating can reduce

distortion and high stresses from welding that can affect service performance. The presence

of residual stresses can also lead to stress corrosion cracking near welds and in regions of a

component that has been cold strained during processing. Residual stresses in ferritic steel

cause significant reduction in resistance to brittle fracture. (J.B. Giacobbe, 1981)

In materials that are not prone to brittle fracture, such as austenitic stainless steel, residual

stress can be sufficient to provide the stress necessary to promote corrosion cracking even in

environments that appear to be benign.

There are many sources of residual stress: they can occur during processing of the material

from ingot to final product form. Residual stresses can be generated during forming

operations such as shearing, bending and machining.

During fabrication processes such as welding, residual stresses are present whenever a

component is stressed beyond its elastic limit, this results in plastic flow.

Bending a bar in fabrication at a temperature where recovery cannot occur (cold forming)

will result in one surface location containing residual tensile stress, whereas a location 180°

away will contain residual compressive stress on the surface of the material. To alleviate

problems such as cracking of materials and welded areas these stresses induced by rolling,

welding, bending and machining have to be treated either thermally or vibratory to provide

an increased working life of the component.

The cause of residual stresses that has received the most attention in open literature is

welding. The residual stresses associated with the steep thermal gradient of welding can

occur on a macro-scale over relatively long distances (reaction stresses) or can be highly

localised (micro-scale). Welding usually results in localised residual stresses that approach

levels equal to the yield strength of the material at room temperature. A number of factors

influence the relief of residual stresses, including level of stress, permissible time for their

relief, temperature and metallurgical stability.

25

The relief of residual stresses is a time-temperature-related phenomenon (Figure 2.4), this is

usually correlated with the use of the Larson Miller equation whereby:

Thermal effect = T(log t + 20) (10-3 )

Where T = Temperature

t = Hours

FIGURE 2.4 – TIME TEMPERATURE GRAPH

2.4.3 Conclusion

Resistance of a material to the reduction of its residual stresses by thermal treatment can be

estimated with knowledge of the influence of temperature on its yield strength. Figure 2.5

provides a summary of the yield strength to temperature relationship for three generic

classes of steel.

FIGURE 2.5 – YIELD STRENGTH TO TEMPERATURE RELATIONSHIP

26

The room temperature yield strength of these materials provides an excellent estimate of the

level of localised residual stress that can be present in a structure. To relieve these stresses,

it is required that the component be heated, to a temperature where its yield strength

approaches a value that corresponds to an acceptable level of residual stress. Alternatively

these components may be vibratory stress relieved, with a combination of residual and

vibratory stresses exceeding the yield strength of the material. The presumption that plastic

flow is such that when the vibrational amplitude is removed, the previously stressed area can

now return to a low level of residual stress.

Creep resistant materials, such as the chromium bearing low alloy steels and the chromium

rich high alloy steels, normally require higher stress relief heat treating temperatures than

that of conventional low alloy steels. High alloy steels such as austenitic stainless steel

require temperatures as low as 400°C, however, at these temperatures, only modest

decreases in residual stresses are achieved. Significant reduction only occurs at

temperatures as high as 480 - 950°C, heat treating in this range however may result in

sensitising susceptible material, and in turn lead to stress corrosion cracking in service.

Copper alloys may fail by stress corrosion cracking due to the presence of residual stresses.

Thermal stress relief is favoured due to the controllability, cost and dimensional stability.

Vibratory stress relieving of austenitic stainless steel is viewed to be a successful venture

with favourable results, however, vibratory stress of copper is not favourable as surface

hardening occurs resulting in breakage. (J.B. Giacobbe, 1981).

2.5 VIBRATORY LOWERING OF RESIDUAL STRESSES IN

WELDMENTS

2.5.1 Introduction

During the welding process residual stresses, which usually create three-dimensional stresses

are formed within the welded joint. In these welded joints of annealed weldments mainly

with larger material thickness the residual stresses can form unfavourable three-dimensional

tensile stress that can decrease strain properties of the joint as well as lower the dynamic

load capacity of the weldment.

27

These drawbacks are solved in manufacture by proposing stress relief of weldments. In

many cases VSR will indicate poor or defective welds with the effect of the weldment

actually breaking or cracking severely, this form of weld diagnosis will in turn call for better

weld preparation as well as the weld itself.

The majority of weldments are annealed to ensure the stability of dimension, shape and

metallurgical properties during their service life. This is basically the only reason why

weldments are treated, not to treat mechanically or even to lower hydrogen contents within

the weld that causes cracking.

Expensive stress relief annealing of weldments can in the majority of cases be substituted

successfully with VSR, for this, weldments are subjected to vibratory stress relief with an

eccentric vibrator / exciter.

2.5.2 Measured Residual Stresses in Un-vibrated and Vibrated Stress Relieved

Welded Joints

To solve the problem of VSR and its effects on welded joint properties the three-

dimensional residual stresses were measured. The welded specimen a 30 x 650 x 1100mm

was used for the measurement of residual stresses.

One specimen remained un-vibrated and the other was vibrated at four resonance levels.

(M. Jesensky, 1983). The measured three-dimensional stresses in un-vibrated and vibrated

joints can be seen in Figure 2.6.

The values of measured residual stress in the above welded joints prove that after VSR the

three dimensional residual stresses on the surface as well as through the joint are between

40% - 80% lower in comparison to the un-vibrated joint.

28

FIGURE 2.6 – THREE DIMENSIONAL STRESS COMPARISON

Un-Vibrated

A. -50 0 50 100 150 200 300

30 RZ RY RX

Vibrated

B. -50 0 50 100 150 200 300

30 RZ RY RX

A = WELDED

B = WELDED AND VIBRATORY STRESS RELIEVED

RX = ALONG THE WELD

RY = NORMAL TO THE WELD

RZ = ALONG THE WELD THICKNESS

+ = TENSION

- = PRESSURE

2.5.3 Measurement of Strains in Welded Beams After Planning

During the study of the problem, strains in the mentioned beams were measured, Figure 2.8.

Measured strains of welded beams after machining one flange 5mm in depth. Line 1

indicates stress levels prior to vibratory stress relieving and Line 2 after.

One beam was untreated, the other was vibrated at lower residual stresses to establish and

stabilise dimensions. The beam was progressively machined ( 2 + 2 + 1 = 5), the course of

strains designed 1 of a welded beam as 2 corresponds to VSR treated beam.

29

The measured values established in Figure 2.7 prove that after VSR treatment the beams

residual stresses were lowered and the shape stabilised. Subsequently higher strains were

measured in the un-vibrated beam as to the welded, measured and vibrated beam.

FIGURE 2.7 – MEASURED VALUES OF VSR TREATED BEAMS

0.6

0.5

0.4 1

0.3

0.2

0.1 2

0

1500

2.5.4 Strength and Yield Strength of Materials Vibrated and Un-Vibrated

The strength tests of welded joints in different steel with different strength values ranging

between 370 and 700mpa were performed. The measured values of welded joints in bars

have proven that vibration exerts a negligible effect on yield strengths and tensile strengths

of welded materials. A slight increase in yield strength, practically within the scatter of

measured values was proven in the vibratory stress relieved joints.

2.5.5 Notch and Fracture Toughness of Un-Vibrated and Vibratory Stress

Relieved Welded Joints

The samples of measurement of notch and fracture toughness were fabricated from steel

welded joint specimens 30mm in thickness. The course of fracture toughness is dependent

on temperature.

30

The measured values shown in Figure 2.8 and those of other welded joints have proven that

the notch and fracture strength values of VSR joints are higher than those of un-vibrated

joints.

FIGURE 2.8 – MEASURED VALUES OF NOTCH TOUGHNESS

KCV

250

200 2

150 1

100

50

0

-50 0 50 100 150

t(°C)

1 - As welded

2 - Vibratory Stress Relieved

t(°C) – Temperature

KCV – Notch Toughness

2.5.6 The Effect of Vibratory Stress Relieving on Fatigue Strength

To solve the problem of VSR application on fatigue strengths of welded joints and

weldments a test in dissimilar steel was conducted. Figure 2.9 shows the results of fatigue

tests of welded specimens 30mm in thickness with a 30mm welded joint. The measured

values shown in Figure 2.9 clearly show that there is no decrease in the fatigue strength of

the welded joint. Specimen 2 remains constant throughout process A and B. The non-loss

in fatigue strength also represents no metallurgical change to the material.

31

FIGURE 2.9 – FATIGUE TESTS

A.

GA MPA

100 3

80 2

60 1

40

20

0

50 100 150 200 250

GM MPA

B.

GA MPA

100

80 1

60 2

40 3

20

0

50 100 150 200 250

GM MPA

Key:

Measure values of fatigue strength in welded joints.

a. Dependence of stress amplitude on mean stress steel specimens.

b. The same dependence on samples.

1 - as welded

2 - after vibratory stress relief

3 - after stress relief annealing

32

2.5.7 Conclusion

Based on hitherto attained research results and experience in practical use of VSR

weldments, the introduction of VSR into practice can be recommended. VSR can be

employed for the stabilisation of welded joints as well as a means of fitness for purpose

testing.

The VSR process is used for the lowering of residual stresses and the stabilisation of various

sized weldments. It can be used to replace the annealing process in machine frames, and

cast iron castings. According to data collected the shape and size of suitable VSR

weldments were sufficiently stabilised after vibratory stress relieving. (M. Jesensky, 1983).

As a fact VSR does not negatively affect the static and dynamic strength of welded joints

and weldments, fracture and notch toughness of welded joints. Based on the attained data

the implementation of the VSR procedure as a replacement to stress relief annealing for the

stabilisation of size of weldments, castings, forging leads to high savings on production

downtime thus increasing production output.

The saving of thermal energy has to be emphasised first of all because the VSR procedure

does not exceed 3% of energy required for annealing of weldments and in average 5% of

production costs.

2.6 VIBRATORY STRESS RELIEVING – ITS ADVANTAGES AND

LIMITATIONS AS AN ALTERNATIVE TO THE THERMAL

PROCESS

2.6.1 Past Work

It is possible to find technical papers, ostensibly written about vibratory stress relieving,

dating back as far as 1934, but in fact, there have been few genuine research programmes

into the process. In order to bring past work into perspective, the reader may be assisted by

reflecting on the following. (R.A. Claxton, 1974).

33

Many evaluations have been made of the process using many different types of equipment,

specimens, measurement methods and general techniques. Many studies were limited to

simple test bars, which due to restricted budgets, were treated in fatigue test machines. A

recent example of misinterpretation involved a report on the axial cycling of a cold drawn

mild steel tube in which no residual stress relief was observed. On the basis of this report

vibratory stress relief was banned by the company even though vibratory stress relief was

successfully used for two previous years. However, if the research were carried out more

comprehensively it would have been found that vibratory stress relieving is virtually

ineffective on cold worked material. Further it is well established that axial cycling can take

up to 10 000 cycles longer to achieve a given benefit than in bending, the condition

predominantly induced by vibratory stress relieving.

2.6.2 Applications

Discussion of the three main areas in which thermal and vibratory treatments share a

capability allows an appraisal of their relative merits.

2.6.2.1 Dimensional Control

Dimension control involves constraining a structure to near its final form using clamps,

sprags etc. and then setting this form by vibratory means such that, when the constraints

(stress) are removed, the part assumes the desired form. Vibration is sometimes applied

either during or after welding and also before releasing constrained non-welded components

in order to achieve this end.

As the process is often time consuming and subject to a certain amount of investigatory

work, thermal methods of dimensional control should be employed in preference

vibroforming, where a simple part is vibrated under load to remove an initial or machining

distortion also falls under this heading. However, the technique is usually an economic

proposition where high manufacturing costs have already been incurred.

34

2.6.2.2 Stabilisation

Dimensional stabilisation differs in effect from overall stress reduction and total stress relief

in as much as stress levels are not the criterion by which the benefit is judged. The sole

criterion is stability during machining, assembly and service. Attainment of dimensional

stability does not necessarily involve a significant reduction of residual stresses. This is

partly so in the treatment of cast iron where a beneficial modification of the stress relaxation

characteristics may be brought about.

Generally vibratory and thermal stress relief share an equal capability here both with regard

to material groupings and basic manufacturing processes. The main exception is that

vibratory stress relief is applied after rough machining of castings and forgings as the parts

are more flexible, some skin stresses have been removed and machining stresses can be

treated. However, vibratory stress relieving has a clear advantage in as much as it is often

applied to give added stability to components, initially stress relieved by thermal or

vibratory means, at an advanced stage of machining. In common with thermal treatment a

second vibratory treatment may be required for a heavily machined, but very precise,

flexible part if the first treatment is carried out prior to roughing, whether the part is forged,

cast or fabricated. Vibratory stress relieving is applied prior to final grinding, as a standard

practice by leading European machine tool companies.

Results of 0.01m in 2m and 0.002mm in 300mm is commonly found on cast iron columns

and bases of large horizontal boring machines, by applying vibratory stress relieving twice

on soft side-way components and three times on plane hardened side-way components. No

thermal stress relief is used, company trials having shown that such accuracy, combined

with long-term stability is not achievable with furnace treatment.

2.6.2.3 Examples of Stabilisation

Marine stern tube propeller shaft bearings, manufactured in three section from thermally

stress relieved cast steel had a history of instability. After living with the problem for many

years the company tried vibratory stress relieving just prior to finish machining. The

components were excited in a torsional mode using a conventional machine.

35

A dimensional check was carried out before and after vibratory stress revealing that the

bearing cap bore opened 0.05mm at one end and 0.1mm at the other whilst the top half arch

and the bearing badly opened 0.05mm uniformly. Repeat stability tests were carried out and

the unit was found to be stable, vibratory stress relieving is now pre-requisite prior to final

machining. Precision vane pump bodies cast and then final machined were being rejected

by a customer at a rate of 40% because of twist and elongation in the region of 0.125mm in

400mm. Vibratory stress relief applied just prior to finished machining, caused movements

in order 0.075mm and thus the rejection rate has now been reduced to zero. Several hundred

propeller shaft bearings have been treated over a period of time, and it should be noted that

previous tests involving thermal treatment had failed to stabilise these components.

Therefore stability to that achieved by thermal, without discoloration, scaling, delay or high

transport costs. (R.A. Claxton, 1974).

2.6.2.4 Overall Stress Reduction

Defined in terms of a significant reduction in residual stress, overall stress reduction is again

a capability shared by thermal and vibratory stress relief. Carried out correctly thermal

stress relief can produce lower stress levels than vibratory treatment. Around 90% seems to

be the best figure accepted for thermal and 80% for vibratory.

Where very low uniform levels are essential, a metallurgical change is often required. This

applies to components such as “pressure vessels” thermal treatment is essential. However,

in the industries most commonly found a large proportion of the components that would

have gone into a furnace can now be safely vibratory stress relieved, thereby taking

advantage of time and money savings and quality of product. (R.A. Claxton, 1974).

2.6.2.5 Examples of Vibroforming Tests

A. Nineteen finished ground, combination keep strips / packers were found to be distorted

over its length by 0.2mm. The EN9 plates, hardened along one edge, were 12mm thick,

500mm long and 200mm wide and consisted of a large number of slots and holes.

Mechanical and thermal means of straightening were discounted due to the risk of

cracking on the one hand and metallurgical changes on the other. The parts were

flattened using the technique of vibroforming to within 0.05mm and were thus salvaged.

36

B. The process, which is not fully understood and ineffective on some steel, was time

consuming and could only, be justified by virtue of the inconvenience and high cost of

replacement. (R.A. Claxton, 1974).

C. A high strength, annual aluminium roll forging, extensively machined to an approximate

2.5m diameter with overall envelope sectional dimension of approximately 50mm x

350mm was found to open out and twist alarmingly when a 355mm wide segment was

removed to produce the final “C” shape. As thermal treatments had proved inadequate,

vibratory stress relieving was applied prior to slitting, several torsional and bending

obtained. On subsequent slitting very little movement took place and the process was

adopted as standard procedure. (R.A. Claxton, 1974).

D. Conversely, vibratory treatment would have been no match for thermal processing in the

case of a large 1.5 tonne marine gearbox, fabricated from mild steel, which was found to

have distorted out of limits during furnace stress relieving. The consultants overseeing

the work called for the component to be inverted and returned to the furnace where it

was supported in such a manner that the distortion was reversed to bring the parts within

limits again. Distortion is a problem with furnace stress relieving, as few furnace floors

or bodies are flat and many operators stack components to fill the furnace. This results

in the lower components being subjected to pressure and upper ones resting an uneven

surface. (R.A. Claxton, 1974).

2.6.2.6 Observations

As well as the question about its effectiveness, which hopefully has been answered in this

chapter, there are other regularly voiced misgivings that may not have been covered this far

about vibratory stress relief. Reservation about the process will hopefully be answered

during the following test results.

1. A resonant condition does not in practice result in infinite amplitudes or damage since

the supports, the air and the material itself create a clamping effect.

2. VSR equipment is not noisy, some of the components with thin walled panels can,

however, be noisy, this, however, can be prevented as panel modes are not required for

effective stress relief.

37

3. Cracking of weld does occur occasionally as it would if the part were thermally treated

(cracking usually results due to a poor weld). A better analysis of the cause and

prevention can be made using vibratory since in the furnace no observations can be

made during treatment. Cracking during casting treatment can also occur but is

welcomed by foundries as this highlights defects within the cast.

4. Quality assurance personnel find that with vibratory treatment they can visually

supervise the treatment of components. Treatment conditions are unique for each

component and a certificate can also be issued accordingly. As with thermal treatment,

high furnace costs usually involve the furnace being charged resulting in compromise

thermal relieving of some components. (J.S. Hornsey, 1997).

2.7 CONCLUSION

Taking into consideration all discussed it is clear that the current state of the art vibratory

stress relieving apparatus and text materials available on the process constitutes a serious

alternative to thermal stress relieving in many applications on the basis of cost, convenience

and technical grounds. There are many cases within the industrial sector that could profit

from its introduction either by sub-contracting or by purchasing a machine for in-house use.

38

CHAPTER 3

ANALYSIS

3.1 INTRODUCTION

Welded structures inevitably contain residual stresses as a result of the considerable thermal

gradients associated with the localised high temperature heat source required to make the

fused joint. (T.R. Gurney, 1968), maintains that suitable weld preparation can minimise

residual stress but not eliminate it completely.

Residual stresses exists in forgings and castings because of the various cooling rates

associated with different section sizes. The nature and magnitude of these stresses affect

their service life considerably, therefore stress relieving is a common engineering practice.

The existence of these residual stresses in structural components is undoubtedly the cause of

dimensional instability during machining and fabrication. (R.A. Claxton, G.G. Saunders,

1977).

3.2 STRESS

Stress in steels and castings is an inevitable unwanted factor, these residual stresses are

virtually elastic deformations which posses some potential energy accumulated in a body.

(V.A. Vinokurov, 1987).

These stresses on relieving, either by thermal or mechanical means change the shape and in

some instances the size of the component. If stress relieving of the component is not carried

out then the ageing process will in time stress relieve the component and cause premature

failure of the component in operation.

An example of this being the repair of a fan impeller, if stress relief is not carried out then

the fan will certainly become unbalanced in time due to the repair relieving itself by ageing

and in turn create unwanted downtime.

39

The indication of stress is usually induced in steels with the application of intense heat

concentrated in small areas for pre-determined times, welding is probably the most common

process. Castings and large scale fabrications are susceptible to high levels of stress due to

the intense heat usually generated with these processes.

3.3 STRESS RELIEF OF WELDED COMPONENTS – A PRACTICAL

EXAMPLE

3.3.1 Introduction

What is the efficiency of residual stress relieving by mechanical vibration? In this chapter a

study will be carried out on two primary welded reducer gearboxes weighing 1200 Kg each

which were thermally (Box A) and mechanically (Box B) stress relieved. X-ray stress

measurements were made in both cases before and after relieving. Dimensional

measurements were also taken on Box B. During the mechanical treatment the dynamic

behaviour of the box was followed by acceleration measurements in order to estimate the

magnitude of the vibration-induced stresses and to detect the eventual presence of a new

permanent strain. In the case of thermal relieving, the reduction of residual stresses is an

average of 70%. In the case of mechanical relieving, the reduction is between 45 - 100% for

tensile stresses, whereas it is only between 0 - 45% for compressive stresses.

3.3.2 Tests on Reducer Gearbox Casing

3.3.2.1 Definition of the test

The tests were carried out on two primary reducer gearbox casings weighing 1200 Kg each

constructed and made from E-26-4 grade steel (Figure 3.1).

Two types of stress relief were used, thermal on Box A and vibratory on Box B. Stress

relief of welded components usually is achieved using heat treatment, so measurements on

casing A serve as a reference. The efficiency of the stress relief was checked by x-ray stress

measurements, both before and after treatment. During the vibratory stress relief process,

measurements of acceleration were made to determine the dynamic behaviour of the

structure.

40

The existence of a resonance frequency was checked and the deformation of the part was

described.

Extensometric measurements were made to define the vibratory process, and to detect any

new permanent deformation after the process was completed. Measurements both before

and after made it possible to keep a check on the changes of the overall dimensions and

surface flatness of the parts. (Measurements to be taken are given in Table 3.1)

FIGURE 3.1 – PRIMARY REDUCER GEARBOX CASING

TABLE 3.1 - DEFINITION OF THE TEST

MEASUREMENT CASING

MARK

METEOROLOGICAL CHECK

EXTENSOMETRIC CHECK &

VIBRATORY ANALYSIS

X-RAY STRESS MEASUREMENT

Before stress relief AB X

XX

During vibratory stress relief B XAfter vibratory stress relief B X XAfter thermal stress relief A XOn year after first vibratory stress relief process

B X

41

3.3.3 Vibratory Stress Relief Operating Parameters

Figure 3.2 shows the part during the vibratory stress relief process using standard VSR

equipment strategically mounted. The apparatus consists of a variable rotation speed;

unbalanced mass motor attached to the part by means of clamps. An accelerometer attached

to the part makes it theoretically possible to detect the primary frequency of the component.

The mechanically welded part is placed on four rubber shock absorbers as to minimise loss

in free movement of the component.

FIGURE 3.2 – CASING DURING VIBRATORY STRESS RELIEF

Vibratory and extensometric analysis shows that the stimulating frequency used in this first

stress relief process is not the same as the structure natural frequency. It corresponds to an

overall movement of the casing on the test suspension.

42

Also the stimulating frequency indicated by the unbalanced mass motor control unit is not

the same as real stimulating frequency measured by CETIM’s accelerometers (50Hz

indicated for 30Hz in real terms). An additional stress relief process at a frequency of 33Hz,

corresponding to the first natural frequency determined by the impact response of the part,

was therefore carried out.

Some research indicates that stress relief only occurs for significant deformation requiring

that the part be made to vibrate at a level equal to its natural resonance level. A more recent

publication indicates that better efficiency can be obtained by using sub-resonant vibration

processes. (E.D. Mordfin, 1988).

Table 3.2 lists the conditions under which vibratory stress relief was performed.

TABLE 3.2 - VIBRATION STRESS RELIEF CONDITIONS

STRESS RELIEF PROCESS

FREQUENCY Hz

ACTUAL FREQUENCY INDICATED BY CETIM, Hz

VIBRATION TIME MINUTES

POSITION OF MASS MOTOR

First 50 30 15 Position 1First 49.5 30 15 Position 2Second - 44 10 Position 1Second Automatic cycle 33.6

51.836.8

555

Position 1

3.3.4 Heat Treatment Parameters

3.3.4.1 Heat Treatment

Temperature rise time, 3 hours.

Held at 620°C for 1 hour.

Cooled down in the furnace.

X-ray stress measurement parameters, x-ray stress measurements are based on material

lattice spacing variations in measurements in several directions.

43

3.3.5 Results

3.3.5.1 Meteorological Check

Checks of dimensions before and after the first and second VSR access shows that variations

of magnitude within the accuracy of the measurements (Table 3.3, Figure 3.3.) No

significant divergence in the surface flatness was recorded (Figure 3.4). Vibratory stress

relief does not cause only significant variation of the overall dimension or the surface

flatness of the component. The efficiency of stress relief cannot, therefore, be checked by

meteorological measurements.

TABLE 3.3 - CHECK OF DIMENSIONS BEFORE AND AFTER STRESS RELIEF.

REFERENCE POINT READING

BEFORE STRESS

RELIEF, MM

READING TAKEN AFTER RELIEF,

MM.

--------------------------------------------------------

FIRST SECOND

Length A 1998.42 1998.36 1998.35

Length B 1640.12 1640.13 1640.10

Centre Distance of Axis:

C 897.64 897.65 897.65

D 697.82 697.84 697.82

E 399.44 399.44 399.46

44

FIGURE 3.3 – PATTERN “B” CASING, DIMENSIONAL CHECK BEFORE AND

AFTER VSR

FIGURE 3.4 – PATTERN “B” CASING, SURFACE FLATNESS BEFORE AND AFTER VSR

3.3.5.2 Measurement of Residual Stresses

The measurement point locations are defined in Figure 3.5, 3.6, 3.7 and 3.8. The reduction

of the residual stresses by heat treatment varied from 70 - 100%. After relieving, the

maximum residual stresses are near 30mpa. These results confirm the efficiency of the heat

treatment cycle. The reduction of the residual stresses, after the first vibration treatment

varied from 45 - 100% for tensile stresses and from 0 - 45% for compressive stresses. After

the first stress relief the maximum residual stresses were -200mpa in compression and

+60mpa in tension. The second vibratory treatment did not cause any further detectable

change to the residual stress.

45

FIGURE 3.5 – PATTERN “B” CASING, SIDE ONE VSR

FIGURE 3.6 – PATTERN “B” CASING, SIDE TWO VSR

FIGURE 3.7 – PATTERN “A” CASING, SIDE ONE THERMAL

FIGURE 3.8 – PATTERN “A” CASING, SIDE TWO THERMAL

46

3.3.5.3 Vibratory and Extensometric Analysis

The first vibratory treatment was carried out at a frequency of 30Hz, which does not

correspond to any natural frequency of the component. The dynamic stress induced by

vibration and measured by three strain gauges placed in the zone of the residual stress

measurement are at the very most, 1.5mpa.

The second vibratory treatment was carried out at the first natural frequency of the structure

(44Hz). A dynamic stress level of 100 - 200 times that obtained in the first stress relief

process was recorded.

3.3.6 Interpretation of Results

The vibratory stress relief process does have an effect. Tensile residual stresses are reduced

easier than compressive residual stresses. The vibration frequency of the structure does not

seem to have an influence on the efficiency of stress relief. It is not necessary to relieve

stress using the component frequency or resonance. The extent of the dynamic stresses

having caused stress relief is of the order of 1.5mpa for the gearbox casing. The partial

stress relief is perhaps due to movements and reorganisation of anomalies at atomic level,

this hypothesis remains to be verified (dislodging of dislocations, movement of interstitial

atoms, internal friction).

3.3.7 Conclusion

VSR can be used for stabilisation of dimensions before machining of welded parts. It is

necessary to carry out a preliminary test to judge how effective the process is for a given

manufacture. If the treatment is sufficient, size variations at the time of machining are

negligible. The process may be used for stress relief of parts for which a stress relieving

heat cannot be used. However, it should be borne in mind that stress relief is only partial. It

is quite obvious that VSR should be ruled out completely as a replacement for heat

treatment when the latter is applied for metallurgical reasons or when heat treatment is

compulsory when following manufacturing rules.

47

3.4 THE MEASUREMENT OF VIBRATION – INDUCED STRESS RELIEF

IN THE HEAVY FABRICATION INDUSTRY.

3.4.1 Bed-Plate Test Summary

A bedplate structure shall be vibrated by means of a commercially available variable

frequency vibrator. Residual stresses were measure near the welded location in the structure

before and after treatment. Stress relief of about 30 – 57% was achieved. During the

vibratory treatment, surface strains were monitored (at resonance the applied surface strain

amplitude was measured at 600 microstrains). Subsequent to machining, the bedplate

showed good dimensional stability.

3.4.2 Stainless Steel Bowl Test Summary

A stainless steel bowl was fabricated with carbon steel cooling jackets. The bowl contained

heavy weldments but could not be thermally stress relieved prior to machining due to the

metallurgical change that would occur to the carbon steel jackets. The bowl was vibratory

stress relieved and the close tolerances required were easily achieved. Both components

described have been installed on site and have maintained dimensional stability.

3.4.3 Manufacturing Process

Manufacturing processes such as casting, welding and machining often cause a build up of

residual stresses in components. These residual stresses must be relieved because they add

to the service stress and may increase the susceptibility of the components to early failure

due to brittle fracture, accelerated corrosion, or stress corrosion cracking. Residual stress

can cause unacceptable distortion in structural components such as machine frames and

bedplates.

Non-uniform cooling after welding or thermal treatment of heavy section castings has

previously resulted in high levels of residual stress, when machined has then resulted in

these castings changing shape as stressed material is removed. Past studies on weathering of

grey cast iron castings has shown that a reduction of only 10% is sufficient to ensure shape

stability.

48

3.4.4 Cyclic Loading or Corrosion

Tensile residual stresses should be reduced to as low a level as possible. In welded

structures a degree of reduction around 80% is thought to be acceptable. Engineering

components are usually thermally stress relieved. A sufficient increase in the temperature of

metallic materials causes their yield strength to decrease to very low levels. At about 600°C

the yield strength of most steels is as low as 10mpa or about 5% of the room temperature

strength.

All internal stresses exceeding this low level are relieved by plastic deformation. Prolonged

holding at stress relieving temperature can cause further reduction in residual stresses by

time dependant creep. In steel 60 – 85% of all stresses are eliminated by holding the

structures at 510 – 570°C for one hour and then slow cooling to room temperature is

necessary to avoid generation of residual stresses due to non-uniform temperature

distribution.

3.4.5 Stress Relieving by Mechanical Means

The objective of stress relief by plastic deformation may be achieved at room temperature by

superimposing an externally applied stress on the residual stress field. The applied stress

must be of a magnitude such that the algebraic sum of the combined stresses exceeds the

room temperature yield strength of the material, causing plastic deformation.

When the externally applied stresses are removed, the level of residual stresses will be

reduced. The mechanism is the basis of vibratory stress relieving or vibratory conditioning.

Vibratory conditioning / stress relieving has been found to be effective not only on castings,

fabrications, weldments and machined components but also special steels.

A. Austenitic Stainless Steel

Austenitic Stainless steels are steels in which the precipitation of chromium carbides at

thermal treatment temperatures reduces their corrosion resistance.

B. Dis-similar Metals

Distortion due to differences in thermal expansion coefficients at elevated temperatures.

49

C. Age Hardened Alloys

Alloys may loose their strength at normal stress relieving temperatures.

3.5 RESEARCH FINDINGS

Many researchers have found similar conclusions in respect to the effect of vibrations on

residual stress in specimens and welded structures. (D.A. Adoyan, 1967).

Some of the important considerations for successful application of vibratory conditioning

are:

1. It is not necessary to completely relieve residual stresses when distortion control is the

prime consideration.

2. It is necessary to visualise the resonance mode shapes of the structure for proper

location of the supports and the vibrator. Higher modes of vibration should be excited

so as to generate a more uniform strain amplitude filed.

3. The externally applied strain amplitude must exceed a threshold value for stress relief

to occur.

3.6 NEED FOR MEASUREMENT OF RESIDUAL STRESS RELIEF IN

STRUCTURES

The research findings demonstrate the usefulness of vibratory treatments in controlling

distortion in fabricated structures. In laboratory test conducted on several types of

components ranging from rotating machinery to pressure vessels containing components

intended to serve as ridged members and supports, rather than a pressure retaining vessel.

The distortion due to fabrication of heavy sections and dissimilar metals were thought to be

easily treated by vibratory methods. (R.D. Ohol, 1988)

50

Vibratory conditioning using proprietary machines is gaining acceptance by designers and

fabricators alike. Although the treatment is primarily intended for shape stabilisation of

structures, the effectiveness of any treatment is often judged by measuring the stress relief

obtained and comparing it with the stress relief obtained by thermal means. (R.A. Claxton,

1983).

3.7 MEASUREMENT OF RESIDUAL STRESS RELIEF ON

ENGINEERING STRUCTURES

3.7.1 Vibratory Treatment of a Bedplate

3.7.1.1 Introduction

A heavy gearbox bedplate for a cement plant-grinding mill was fabricated by welding of

plates rather than “I” beams and channels (Figure 3.9). A weldable structural steel

according to the standards for structural steels was used, with the following properties:

Carbon - 0.20% maximum

Yield strength - 235mpa minimum

Ultimate tensile strength - 410 – 530mpa

Elongation - 23% minimum

The bedplate was expected to distort during machining of its top surface. Though not

mandatory, the bedplate and other fabricated structures has been thermally stress relieved,

vibratory treatment was suggested as a cheaper alternative.

51

FIGURE 3.9 - BEDPLATE

3.7.1.2 Test Details

In order to verify the effectiveness of the technique the following experiments were planned.

1. Residual stress measurements before vibratory treatment.

2. Measurement of dynamic strains during the treatment using a separate strain gauge

connected to the dynamic strain gauge instruments.

3. Monitoring of vibration levels during the increase form rest of the vibrator frequency

to identify the components resonant frequencies.

4. Residual stress measurement after the vibratory treatment.

Figure 3.10 is a schematic of the bedplate structure showing the location of the two residual

stress gauges and the one axial gauge use to conduct the test. The structure was vibrated

using a standard VCM 80 machine and positioned as in Figure 3.11. The orientation of the

excitor at location “1” was intended to excite torsional modes of vibration and motor “2”

was to excite bending modes of the bedplate. The structure was supported on rubber

mountings to prevent damping. During the treatment the vibrator speed at location “1” was

varied continuously form 0 – 200Hz. It was found that the vibrator was sited close to an

antinode location of the bedplate, thus the vibrator was moved closer to a nodal area.

52

FIGURE 3.10 - LOCATION OF STRAIN GAUGES ON BEDPLATE

Figure 3.11 - Residual Stress in Bedplate

An accelerometer mounted on the bedplate allowed the resonant frequencies to be

determined and graphically presented.

53

At location “1” the predominant frequencies were found to be at 90Hz and 112Hz and at

location “2” the predominant frequencies were found to be 112Hz and 164Hz, at each of

these frequencies the vibrators were maintained for 30 sec. (Figure 3.12, Figure 3.13)

FIGURE 3.12 - GRAPHICAL ACCELERATION ON LOCATION 1.

FIGURE 3.13 - GRAPHICAL ACCELERATION ON LOCATION 2.

105 120 160 180

100 140 170 190

Frequency of Vibrator

3.7.1.3 Results

Residual stress measurements were carried out before and after vibratory treatment. A

1.59mm diameter slot drill was used to drill a blind hole 1.59mm deep, the drill was driven

at a slow speed so as to minimise heating at the gauge location. The results are shown in

Table 3.4

54

TABLE 3.4 - TEST RESULTS

TIME OF MEASUREMENT

PARAMETER BEFORE TREATMENT AFTER TREATMENT

Strain Gauge:

1 -82 -66

2 -74 -59

3 -10 5

Hole Radius – mm 0.84 0.86

Residual Stresses – mpa

1 88.5 63.2

2 41.7 18.2

Angle Beta (degrees) 19 19.6

The residual stress at “1” was reduced from 88.5 to 63.2mpa showing a 30% decrease, at

point “2” the reduction was from 41.7 to 18.2 showing an alarming decrease of 57%

respectively (Figure 3.11). Although a detailed dynamic analysis of the bedplate was not

conducted, it is evident that the beam on which the stresses are measured mainly experiences

bending along the longitudinal direction, thus aiding the reduction of point “2”. The

reduction in point “1” can only occur if large in plane vibration of the beam flange occurs,

the overall stress relieve appears to be due to redistribution following stress relief in the

significant direction. The bedplate was subsequently machined and has shown good

dimensional stability.

3.7.2 Vibratory Treatment of a Bowl Assembly

The bowl assembly structure featured a jacketed structure (Figure 3.14) with a circular cross

section. The shell and base were fabricated with austenitic stainless steel with carbon steel

flange, lifting lugs and water jacket. The base of the bowl was stiffened with carbon steel

stays. The operating perimeters involved an agitator rotating in the bowl, thus circularity

tolerances were vitally important.

55

The composite construction precluded thermal relieving of stresses prior to machining. The

component was treated on the base as well as the shell, visualisation of mode shapes

indicated that the two vibrators be positioned as in Figure 3.15.

Welding of the carbon steel flange and lifting lugs was achieved with the shell stayed by

means of spiders welded to the internal diameter. The component was vibrated before and

after detaching the spiders. The vibrations were held for 30 seconds at each of the natural

frequencies of the bowl, as determined from the graphical display (Figure 3.16).

FIGURE 3.14 - BOWL ASSEMBLY WITH COMPOSITE METAL

CONSTRUCTION

56

FIGURE 3.15 - LOCATION OF VIBRATOR ON BOWL ASSEMBLY

FIGURE 3.16 - TYPICAL ACCELERATION SPECTRUM DURING VIBRATORY

TREATMENT OF BOWL ASSEMBLY.

57

FIGURE 3.17 - DIMENSIONS OF BOWL ASSEMBLY AFTER FINAL

MACHINING

INTERNAL DIAMETER MEASUREMENTS

POINT MAXIMUM MINIMUM

1 1676.425 1676.400

2 1676.500 1676.450

3 1676.550 1676.515

4 1676.500 1676.475

5 1676.450 1676.425

6 1676.465 1676.440

Required final dimension, diameter = 1676 + (0.40 – 0.65mm)

Required final dimension, length = 1143 + (0.025mm)

Subsequent to the two sets of vibratory treatment, the bowl assembly was final machined,

measurements taken after final machining remained constant with time and satisfied the

requirement that the ovality be within 0.25 mm on a diameter of 1.7 m (Figure 3.17 and

Table 3.5).

58

TABLE 3.5 – DIAL GAUGE MEASUREMENTS

QUALITY OF THE BOWL

LOCATION A B C D E F G

DIAL GAUGE

VARIATION 0.01 0.01 0.03 0.03 0.05 0.01 0.01

Two such bowl assemblies were manufactured and vibratory stress relieved installed and

maintained size and stability.

3.7.3 Conclusion

The VSR that was used on both the bedplate and bowl establishes the effectiveness of the

technique as a suitable stress relieving process. The strain gauge based blind hole drilling

technique, even though thought to be not so effective showed a residual stress reduction of

30% and 57%. Since the strain amplitude during resonance is not uniform over the

structure, varying levels of stress will still remain in the bedplate.

It is likely that the stress relief would have been even greater at the points of stress

concentration (welded areas) and at surfaces where the highest strain amplitudes are imposed

due to vibration of the plate structure in bending modes. The redistribution of residual

stresses that occurs due to vibration appears to be useful when dimensional stability is

important.

Vibratory conditioning was and still is being applied to several types of fabricated

structures, in all cases dimensional stability was maintained and the components continue to

operate satisfactory. It should, however, be noted that strain gauge measurements are

expensive and tedious to perform, dimensional stability should rather be used as proof of

effectiveness of the treatment, as was evident in this case study.

59

3.8 THERMAL STRESS RELIEVING OF AUSTENITIC STAINLESS

STEEL

3.8.1 Introduction

Austenitic stainless steel has good creep resistance, consequently it must be heated to 900°C

to obtain adequate stress relief. In some instances heating to the annealing temperature may

be desirable. Holding at a temperature of less than 870°C results in only partial stress relief.

The most effective stress relieving results are achieved by slow cooling. Quenching or rapid

cooling of austenitic stainless steel as is normal in the annealing of austenitic stainless steel,

will usually re-introduce residual stresses.

3.8.2 Selection of Treatment

Selection of an optimum stress relieving treatment is difficult, because heat treatments that

provides adequate stress relief can impair the corrosion resistance of stainless steel, and heat

treatments that are not harmful to corrosion resistance may not provide adequate stress

relief. To avoid specifying a heat treatment that may prove harmful the ASM (American

Society of Metals) neither requires nor prohibits stress relief of austenitic stainless steels.

3.8.3 Heat Treatment Ranges of Austenitic Stainless Steel

Metallurgical characteristics of austenitic stainless steel that may affect the selection of a

stress relieving treatment ranges are as follows:

3.8.3.1 Heat Treatment, Range 900 – 1500°F

Chromium carbides will precipitate in the grain boundaries of wholly austenitic un-

stabilised grades. After prolonged heating such as is necessary for heavy sections grain

precipitation will occur; for cold worked stainless precipitation may occur as low as 800°F

for certain grades. In this condition the steel is susceptible to intergranular corrosion, by

using stabilised or low carbon grades these intergranular precipitates of chromium carbide

can be avoided.

60

3.8.3.2 Heat Treatment, Range 1000 – 1700°F

The formation of hard and brittle sigma phases may result which in turn can reduce both

corrosion resistance and ductility. During the times necessary for stress relief sigma will not

usually form in fully austenitic steels, cast or welded, however, if the stainless is partly

ferritic then the ferritic will transform to sigma during stress relief.

The composition of most austenitic stainless weldments and castings is internationally

adjusted so that ferritic is always present as a deterrent for cracking. Stress relief at this

temperature should be avoided.

Figure 3.18 shows how the percentage of stress relief increases with an increase in stress

relieving temperature for 347 stainless steel, the data also shows the relative unimportance

of holding time.

FIGURE 3.18 – STRESS RELIEF OBTAINED IN 347 STAINLESS STEEL

61

3.8.4 General Recommendations

In the preparation of the correct stress relieving process consideration must be made to the

specific material used, fabrication involved as well as the design and working conditions of

the component. Stress relieving is not recommended if the working environment is not

known or if it is suspected that stress corrosion will occur. If stress relieving is deemed

necessary due regard should be given to metallurgical factors and their effects on the steel.

3.9 RESULTS OBTAINED BY VARIOUS TREATMENTS

3.9.1 Inadequate Stress Relief

Austenitic stainless steels have in many instances been stress relieved at temperatures

usually used for carbon steels, 1000 - 12000°F. Although in carbon steels 80% of residual

stresses are relieved at these temperatures only 30 – 40% of the residual stress is relieved in

austenitic stainless steel, because this treatment does not provide adequate stress relief,

stainless steels relieved in this range are often susceptible to stress corrosion.

3.9.2 Annealing and Water Quenching

Annealing to 2050°F and following with water quenching will most certainly not stress

relieve but will most certainly re-introduce high residual stresses, sometimes even worse

than before annealing.

3.9.3 Intergranular Corrosion

In a number of instances, partially stress relieved stainless steel parts have failed through

intergranular corrosion.

Example 1.

316 Stainless steel was used in fabrication of a steam unit, the unit was partially relieved at

1200°F, failure due to intergranular corrosion occurred within six months.

62

Example 2.

304 Stainless steel was used to fabricate a heat exchanger unit, the unit was partially stress

relieved at 1200°F for a period of two hours and furnace cooled. Failure by intergranular

attack occurred in seven days.

3.9.4 Conclusion

It is evident that thermal stress relieving of austenitic stainless steels is a very complex

process, the variation in grades creates a problem when attempting to stress relieve a

variance of products within the same furnace, all of which have different process times and

temperatures.

The reported results achieved (Figure 3.18) show a variance in actual stress relief in relation

to temperature, what it does not show are the side effects of such thermal reactions that will

occur to the stainless steels.

The final decision to stress relieve will have to be made by the user, are the benefits

achieved by thermal stress relief out weigh the metallurgical changes or is there maybe

alternate methods of stress relief that has no metallurgical interference.

3.10 HEAT TREATMENT OF CASTINGS AND SPECIAL STEELS

3.10.1 Stress Relieving of Cast Iron

Grey iron in the as-cast condition contains residual stresses (unless the cast is coded

gradually in the mould), because cooling and the subsequent contraction of the casting

proceeds at different rates throughout various sections of the casting. The resultant residual

stresses may reduce strength, cause distortion, and in some extreme cases even result in

failure or cracking. The magnitudes of these stresses depend on the shape and section size

of the casting, on the casting technique employed, on the composition and properties of the

material cast and on whether the casting has been stress relieved.

63

3.10.2 Temperature

The temperature of the stress relieving is usually well below the range of transformation of

perlite to austenite. The effects of stress relieving at 1200°F for 6 hours on the tensile

strength and hardness of Grey cast irons are shown in Figure 3.19, the effects of this process

show that the properties of the three different samples had a considerable effect on the

results. Figure 3.20 shows the effects of stress relieving temperature on the hardness of

unalloyed Grey irons. For the maximum relief in stresses with minimum decomposition of

carbide in unalloyed irons, a temperature range of between 1000 – 1050°F is desirable.

Figure 3.21 indicates that from 75 – 85% of the residual stresses can be removed by holding

for one hour in this range. When almost complete stress relief (over 85%) is required in

unalloyed iron then a minimum temperature of 1100°F is required, however, along with

increased stress reduction comes the partial loss in strength, hardness and the wear resistance

will also be lowered.

FIGURE 3.19 - EFFECTS OF STRESS RELIEVING ON TENSILE STRENGTH

AND HARDNESS OF GREY IRONS

64

FIGURE 3.20 – EFFECT OF STRESS RELIEVING TEMPERATURE ON

HARDNESS OF GREY IRONS

FIGURE 3.21 – EFFECTS OF STRESS RELIEVING TEMPERATURE ON

RESIDUAL STRESS IN GREY IRON

65

3.10.3 Rate of Heating

The rate at which Grey iron castings are heated for stress relief depends on the shape and

size of the part, except for complex shapes where this is no concern. When a batch type

furnace is employed, it is of the utmost importance that the furnace temperature does not

exceed 200°F at the time of loading. After the furnace is loaded, the heating rate may be

fairly high, it is common practice to heat to 1150°F within three hours, hold at temperature

for one hour and then cool to 600°F in about four hours before removing from the furnace

and allowing for air cooling. Total process time can be in excess of ten hours before

machining can commence.

3.10.4 Rate of Cooling

If a casting is allowed to cool rapidly from the stress relieving temperature to room

temperature, new stresses may develop and the object of full stress relieving will not be

achieved, it is for this reason that the upper temperature range is an essential part of stress

relieving. It is generally recommended that castings be furnace cooled to 600°F or lower

before removal, most commercial furnaces cool slowly enough to meet all these

requirements.

3.10.5 Conclusion

The loss in wear resistance, strength and hardness is of a great concern, especially if the

casting may require stress relieving and also must maintain its wear resistance, strength and

hardness properties. The reduction in stress of up to 85% is good enough for almost any

component but the process time of ten hours is excessive, however, if stress relief was not to

be done then premature failure will occur.

3.11 HEAT TREATING OF TOOL STEELS

Tool steels are high quality steels made to close compositional and physical tolerances.

These tools are then used for cutting, forming and shaping. In service most tools are

subjected to extreme high loads that are applied rapidly. They must withstand these loads a

great number of times without breaking and without undergoing excessive wear or

deformation.

66

3.11.1 Stress Relieving

Stress relief removes or reduces residual stress induced in tools by heavy machining or other

cold working and thereby decreases the probability of distortion or cracking during

hardening of the tool. The ground surface may be highly stressed after grinding (grinding

induces compressive and tensile stresses). But not cracked. The high stress may, however,

cause cracks to develop immediately after grinding, before use, or during use. Stress

relieving immediately after grinding (Figure 3.22 (b, c)) at or just below the tempering

temperature in order to maintain hardness can often salvage ground tools with high residual

stresses. (It should be noted that stress relieving after will cause scaling and loss in the high

finish due to discoloration).

FIGURE 3.22 – EXAMPLES OF THE CAUSES OF RESIDUAL STRESSES

3.11.2 Procedure

Stress relieving is most commonly performed in air furnaces or salt baths used for

tempering. Neither the heating nor cooling rate is critical, although cooling should be slow

enough to prevent the re-induction of new stresses. After stress relieving, it may be

necessary to correct certain dimensions before hardening because the relief of stress can

cause some dimensional change.

Precision tools are usually stress relived after machining and before hardening, although it is

often desirable to stress relive after rough machining and before finish machining.

67

3.12 CONCLUSION

In the case studies on reducer gearbox casings and the bedplate a distinct advantage was

given to VSR, especially advantageous was the fact that no metallurgical change was

experienced in either of the cases.

This in it’s own has an immediate advantage over thermal especially if induced stress can

suitably be reduced, 40 – 70% is said to be sufficient in welded components and 10% in

castings. (R.A. Claxton, 1983).

In both cases VSR was most financially viable, quicker, no discoloration occurred and both

components could be treated insitu, eliminating high transport costs. In respect to the

thermal studies on austenitic stainless steel and castings, favourable results on the relief of

induced stress certainly do not favour thermal treatment, firstly the process is far to complex

and the side effects of poor stress relief are far more detrimental than can be caused by

stress. (J.B. Giacobbe, 1981). Grey cast iron was found to be more suitable for thermal

processing, however, loss in hardness and wear resistance was a concerning factor. The

level of stress reduction of 85% is suitable but treatment time was excessive.

It could be concluded that VSR could have successfully reduced induced stress in all four

products with no loss in steel hardness or any other property.

68

CHAPTER 4

STRESS RELIEVING – DIFFERENT OPTIONS

4.1 INTRODUCTION

Residual stresses are virtually elastic deformations that possess some potential energy

accumulated in a body. The main feature of residual stress relieving is as follows: they can

be avoided only through metal plastic deformation. The choice of a residual stress relieving

method greatly depends on the kind of removed residual stresses negative effect on the weld

structure.

There are cases when by decreasing residual stress negative after effects result which could

be avoided with stress relief, premature failure of the component is most common. The

change in the metallurgical property of the component which is a factor of residual stress

relief must take priority when stress reduction is necessary.

4.2 COMMON AREAS OF STRESS AND THEIR EFFECTS

During welding processes residual stresses, which usually create three-dimensional stresses

are formed in welded joints. In welded joints of annealed weldments mainly with larger

material thickness the residual stresses can form an unfavourable three dimensional tensile

stress state which can decrease strain properties of a joint, lower the dynamic load carrying

capacity, causes strains in as machined or brittle failure of weldments. (M. Jesensky, 1983).

Processes involved in the manufacture of structures and components often result in the

formation of residual stresses. These are undesirable for various reasons, namely, they can

contribute to accelerated corrosion and cracking of members, machining of residually

stressed components results in distortion from the desired final dimension and in

combination with service loading, residual stress reduces strength and the fatigue life of

affected parts.

(A. J. Waddel, 1983).

69

4.3 METHODS OF STRESS RELIEF

4.3.1 Vibratory Stress Relieving

Vibratory methods have been used for many years in the USA and Britain with varying

degrees of control to modify the internal stress patterns in castings, forgings and welded

structures. VSR is being applied with increasing success to problems arising from internal

residual stress. The principle areas of use have remained within the field of shape

stabilisation, as opposed to stress relief against brittle fracture or stress corrosion, for which

thermal treatment usually confers important metallurgical benefits.

4.3.1.1 How Does the Process Work?

Unlike thermal treatment, the process of VSR remains practically unchanged for most steels

and structures with change coming in excitor placement and treatment times.

VSR consists of attaching two, sometimes only one variable speed excitor to the component

and varying the frequency of the machine to cover all frequency ranges (0-100HZ). When

the induced frequency of the excitor matches the natural frequency of the component,

resonance occurs (Figure 4.1).

FIGURE 4.1 – GRAPHICAL REPRESENTATION OF A FREQUENCY

A

B

Figure 4.1 represents a graphical representation of a frequency printout with point A

representing resonance and point B representing sub-resonance.

70

4.3.1.2 Why VSR?

Prior to the advent of vibratory techniques, thermal methods were used to reduce internal

stresses. The great majority of stress relief operations are still affected by heat treatment

because of the important metallurgical benefits associated with thermal and necessary to

avoid problems of failure by brittle fracture, stress corrosion and creep. It is for these

reasons that the role of VSR must be viewed in the context of that substantial of the

fabricated component field where these metallurgical benefits are not necessary.

Despite the important metallurgical benefits associated with thermal treatment, it is

generally true that dimensional stability, particularly during subsequent machining

operations, is not always achieved. Vibratory treatment, if properly executed almost

invariable results in shape stability and consequently stress relief.

Where necessary metallurgical changes as well as stabilisation is required then both thermal

and vibratory may be used. In such situations it would be recommended to thermal treat

first to avoid the re-induction of residual stress that can affect dimensional stability.

(G.G. Saunders, 1992).

4.4 THERMAL STRESS RELIEF

Stress relief heat treating is used to relieve stresses that remain locked in a structure as a

consequence of a manufacturing sequence. This definition separates stress relief heat

treating from postweld heat treating in that the goal of postweld heat treating is to provide,

in addition to the relief of residual stresses, some preferred metallurgical structure or

property. Moreover, austenitic and non-ferrous alloys are frequently postweld heat treated

to improve resistance to environmental damage.

Stress relief heat treating can reduce distortion and high stresses from welding that affect

service performance. The presence of residual stresses can lead to stress corrosion cracking

near weld and in regions of a component that has been cold strained during processing.

71

Residual stress in ferretic steel causes significant reduction in resistance to brittle fracture.

In a material that is not prone to brittle fracture, such as austenitic stainless steel, residual

stresses can be sufficient to provide the stress necessary to promote stress corrosion cracking

even where not usually prone.

4.4.1 How Does the Process Work?

Stress relief heat treating is the uniform heating of a structure or a portion thereof to a

suitable temperature below the transformation range, holding at this temperature for a pre-

determined time, followed by uniform cooling, particularly when a component is composed

of variable section sizes. If the cooling rate is not uniform then the re-induction of stress

could occur, resulting in either the same or greater stresses for which the heat treatment was

intended.

4.4.2 Why Thermal Stress Relief?

Thermal stress relief is the oldest and most understood process for stress relief, it is therefore

common practice to thermal stress relief if stress relief is required. It is however, noticeable

that the use of thermal stress relief has its drawbacks such as metallurgic change to the

material (even if not required), discoloration, scaling and extensive treatment times. The

thermal process, however, is the only process that can achieve metallurgical changes to

steels. (J.B. Giacobbe, 1981).

4.5 CONCLUSION

Thermal and VSR share a capability in three areas, namely, dimensional control,

dimensional stability and overall stress reduction. The VSR process is the only process that

can reduce in built stress at any part of the manufacturing process, and thermal treatment is

the only process that can change the metallurgical structure of the component.

It therefore can be said that either process, correctly chosen for the situation will work

satisfactory. The financial benefit, however, is in favour of the VSR process (Chapter 5) as

thermal treatment involves high energy cost, high transportation costs and lengthy process

times.

72

CHAPTER 5

COST EVALUATION AND JUSTIFICATION OF STRESS RELIEVING

5.1 INTRODUCTION

The two stress relieving treatments discussed have their own special characteristics and

advantages, however, for many years thermal treatment has been used on many components

with little effect and at a high cost.

The introduction of VSR in the early 1990’s has offered the customer a choice of processes

with distinct advantages over thermal. VSR was only introduced to South Africa in the

early 1990’s but has had a working history of over 60 years throughout the world.

5.2 CASE STUDY ONE – OPEN CAST MINE, DRAGLINE BUCKET

REPAIRS

5.2.1 Report

Extensive repairs were carried out on the equipment (parts) shown in Appendix 6 and 7,

repairs ranged from the welding and machining of the chain pin locator holes (Appendix 6)

to the replacement of a complete side unit (Appendix 7). With this magnitude of repair to a

critical part of the bucket, induced stress would certainly result in premature failure of the

bucket due to pin seizure.

5.2.2 The Objective of Stress Relief

The most fundamental objective is to re-distribute the induced residual stress within the

welded areas and to stabilise the bucket. The achievement of this would in turn result in

increased life expectancy and in turn reduce downtime. (Tests conducted by VSR Africa

and Ingwe Mines have resulted in a life extension of 40% in the buckets service life).

73

5.2.3 Cost Evaluation

5.2.3.1 Thermal Treatment

Transport – 100 Km on a lowbed vehicle, return trip = R 15 000.00

(This entails the removal of the component to the furnace).

Dragline downtime – Approximately 7 days, @ R900 / hour = R151 200.00

Furnace cost – 10 hours treatment, @ R800 / hour = R 8 000.00

Total cost to thermally treat a dragline bucket to the mine would be= R174 200.00

5.2.3.2 Vibratory Stress Relief

Transport – 100 Km, return trip = R 150.00

(This entails the cost of the transporting of the VSR machine to the

component).

Dragline downtime – 48 hours, @ R900 / hour = R 43 200.00

Vibratory Stress Relieving cost = R 3 000.00

The total cost to VSR treat a dragline bucket to the mine would be = R 46 350.00

5.2.3.3 Total Cost Saving

The mine will save approximately R 127 850.00 for every bucket treated with VSR as an

alternate to thermal. (Market related costing used where costs are unknown).

5.2.4 Conclusion

The increase in service life of up to 40% with an additional cost saving benefit of over

R100000.00 is certainly enough justification to VSR treat dragline bucket repairs. It should

be noted that if no stress relief was used, premature failure would be inevitable.

74

5.3 CASE STUDY TWO – OPEN CAST MINE, COAL HAULER AXLE

5.3.1 Repairs

Repairs to the hauler axle (Appendix 8) varied from locator hole weld to the complete

replacement of the coupler flange. The flange was fabricated from hardened steel different

to that of the axle to utilise the working characteristics of the steel.

5.3.2 Objectives of Stress Relief

The main objective will be to re-distribute high levels of induced stress on the flange and

locator as well as to stabilise the component for increased service life.

5.3.3 Cost Evaluation

5.3.3.1 Thermal

Due to the fact that the component was manufactured using various steels, all with varying

metallurgical properties, thermal treatment would not possible. Thermal treatment could be

used if no alternate was available, but loss in one of the steels characteristics would have to

be borne.

5.3.3.2 Vibratory Stress Relieving

The component was successfully VSR treated at various sub-resonant frequencies for 3000 –

5000 cycles at a cost of R2000.00 The component was installed and has shown good

working results.

5.3.4 Conclusion

Thermal treatment was found to be impractical due to the metallurgical variances in the

component that would occur. In the situation where the component would have been

installed with no treatment, VSR was an alternate with effective results.

75

5.4 CASE STUDY THREE – COAL LOADING TERMINAL STACKER

5.4.1 Repairs

The stacker, (Appendix 9) a complete new fabrication for the use on coal mine recovery

operations was constructed form 100mm x 100mm square tube and welded on site.

5.4.2 Objectives of Stress Relief

The main objective here would not be stress relief itself as the stacker does not have to meet

stringent tolerances. The main function for VSR would be to identify poor welds allowing

for easy repair before installation.

5.4.3 Cost Evaluation

5.4.3.1 Thermal Treatment

Because of the size and awkward shape of the reclaimer, thermal treatment as a finished

product would practically be impossible. Treatment would, however, be possible in sections

and then assembled.

Transport – 100 Km, return trip = R 6 000.00

Furnace Cost – treatment time of component – 60 hours, @ R800 / hour = R 48 000.00

Total cost to thermally treat the stacker would be = R54 000.00

5.4.3.2 Vibratory Stress Relieving

The said stacker was successfully treated in August 1995 and is currently still in operation,

all at a cost of R 3 000.00

76

5.4.3.3 Cost Saving

The stacker was VSR treated insitu with a total cost saving of R 51 000.00 to the mine,

almost the manufacturing cost of the stacker.

5.4.4 Conclusion

If the stacker were to be thermally treated in sections and re-assembled on site, welding

faults could still be present when assembled resulting in premature failure. Treatment with

VSR was not only successful but also presented huge cost savings to the mine.

5.5 CASE STUDY FOUR – SEGMENT ROLL RECLAMATION,

HIGHVELD STEEL

5.5.1 Report

Normally less than 10% of a steel mill roll is used before it has to be scrapped or replaced.

It makes economic sense to reclaim the worn roll by welding and re-machining the barrel

and bearing journals to extend the roll life and reduce costs. It is detrimental to the life of

the new roll that certain pre-weld and weld conditions be adhered to, this in turn will avoid

premature working failure by cracking or bending.

5.5.2 Roller Preparation

Prior to preparation of roll reclamation all surface defects should be removed. Cracks

should be removed to their full depth or these could act as stress raisers and could cause

premature failure of the roll. If no obvious surface cracks appear then dye penetrant could

be used to detect hidden flaws.

5.5.3 Pre-Heating

It is essential that the roll be pre-heated prior to welding in order to avoid the formation of a

lower brittle layer or mechanical strain in the weld deposit.

77

To calculate the pre-heat temperature of the roll it will be necessary to add all the alloy

contents and round off to the nearest integer and where the carbon content crosses the alloy

content a temperature can be read to the left (Figure 5.1).

FIGURE 5.1 – PRE-HEAT TEMPERATURE CALCULATION TABLE

The entire body of the roll should be brought up to the pre-heat temperature. Roll expansion

will take place but no welding should take place until temperature equilibrium is reached. It

is possible to maintain lower levels of pre-heat temperature with the welding factor alone,

however, for larger rolls it may be necessary to use gas burners to maintain pre-heat

temperature. In all cases the inter-pass temperature should not fall below the established

pre-heat temperature.

5.5.4 Buttering Layer

On high carbon rolls it may require that a buttering layer first be deposited to avoid

excessive carbon pick up in the first layer of the required overlay material. If this is not

done then brittle martensite could form. On lower carbon roll a buttering layer should still

be used as higher alloy wear resistant materials can not normally be deposited more than

three or four layers deep without possible crack formation.

5.5.5 Final Layer and Welding Parameters

Highveld currently uses 3,2mm 410s stainless steel wire, using the submerged arc process.

The recommended parameters for this type of weld wire is, 27 – 30 volts, 350 – 500 Amps,

wire stick out 30 – 35mm and a deposition rate of 5 – 7,5 Kg / hr.

78

If these parameters are not set accordingly then the following results may occur:

Low Voltage - Irregular bead shape.

High Voltage - Wide, flat weld with excessive penetration, high flux consumption

and possible loss of alloy content.

Low Amperage - Irregular bead shape, due to arc instability.

High Amperage - Increased penetration resulting in undercut and difficult slag

removal.

5.5.6 Travel Speed

The use of the correct parameters and the correct travel speed will give a smooth, evenly

shaped weld bead which will assist easy slag removal. The most suitable weld should be 13

– 20mm wide and 3 – 4mm high.

5.5.7 Post Weld Treatment

On completion of the welding process it is recommended that the entire roll body be brought

up to at least the original pre-heat temperature. The heat should be applied uniformly and

with the roller revolving. If the surface of the roll cools faster than the core surface, cracks

may develop, it is therefore important to have temperature equilibrium and slow cooling is

essential as a part of post weld treatment. (Welding Alloys of South Africa, August 1996).

5.5.8 The Objective of Stress Relief

Along with the financial benefit of reclaiming steel mill rolls has come the problem of

suitable stress relief to avoid stress cracking and roll warpage. The problems associated

with incorrect stress relief and reclaim welding procedures will be covered in Chapter 6.

The cost saving benefits have been over shadowed with the rejection rate due to insufficient

stress relief resulting in premature failure due to bending of the roll. The surface hardness

has also created problems with the furnace annealing the roll rather than stress relieving

resulting in the surface hardness being under the recommended requirement of 35 – 40

HRC.

79

5.6 RECOMMENDATION

If a metallurgical change to the component is required then thermal treatment is inevitable,

however, stress reduction and weld conditioning can be attained with VSR, often at a

fraction of the cost and shorter process times.

5.6.1 The Benefits Associated with the Use of VSR on Segment Rollers

5.6.1.1 Process Times

The segment rollers can be treated after every stress inducing process, namely welding and

rough machining. The three treatments of VSR at 20 minutes each for 3000 cycles will have

a time saving of 10 hours compared to that of thermal. This relates to three segment rollers

being machined to only one on the thermal process.

5.6.1.2 Financial Benefits

Currently the workshops and the continuous caster stock approximately 200 varied sized

rollers, 200mm, 250mm and 300mm diameters, some as finished goods and others still work

in progress. The inclusion of the VSR process will not only reduce the stress relieving

process from eleven to one but will in the long terms reduce the number of rollers required.

A 30% reduction in rollers will reduce holding space required for finished goods as well as

work in progress.

Cost Analysis

A 30% reduction in rollers estimated at a repair cost of R4730.00 per roller will have a cost

saving of R79 000.00 this figure could only improve with further reductions to the larger

roller diameter stock, as well as more efficient forecasting by both the workshops and the

plant.

80

5.6.1.3 Surface Hardness

The current thermal process can not attain a required minimum hardness of 35 – 40 rockwell

on a regular basis, resulting in the roller being re-welded and machined. The 410S stainless

wire used on the submerged arc process has the property to attain the hardness required

without any thermal process, as previously stated, the process of VSR does not change the

metallurgical state of a component.

5.6.2 Conclusion

Throughout this chapter, reference has been made to various components all with their own

special requirement, even-though these are only a few of the many components requiring

stress relief, VSR has achieved the most favourable results especially with regard to cost.

The steel mill roll reclamation with VSR even-though not fully tested posses to be a

valuable recommendation to reduce storage space and stock levels required to service the

plant, this in itself is just cause to continue with tests and establish forecasted stock levels.

81

CHAPTER 6

IMPLEMENTATION AND REVIEW

6.1 INTRODUCTION

Damaged and worn segment rollers are removed for welding and repair by the workshops

back to OEM standards.

Thermal treatment of the rollers is currently employed but process difficulties in respect to

hardness and stress relief is of concern. Due to these process faults and premature failure it

has now become necessary to use an alternate stress relief process, namely, VSR. The VSR

process will replace all thermal processes and shall be carried out after every stress inducing

process, namely rough machining and welding.

6.2 REPAIR AND TREATMENT PROCEDURE

6.2.1 Receiving and Preparation

The rollers are received by the workshop supervisor for identification and working life

duration record. These rollers are then pre-heated at 680°C for seven hours, the rollers are

then cooled in the oven at 50°C per hour until 300°C and then removed and cooled in still

air.

The rollers are then stacked ready for the next process, rough machining. The barrel and

journals are rough machined by removing 3mm from each in preparation for welding. The

rollers are then pre-heated to 200°C prior to welding and held at this temperature until a

submerged arc welder is available. The welding involves the use of a submerged arc welder

using 410S stainless steel and the journals are welded using 40s stainless steel stick weld.

The final thermal process involves the normalising of the roll at 380°C with gradual cooling

at 50°C per hour until 100°C, the rollers are then air cooled and final machined back to

manufacturers standards.

82

6.2.2 The Objectives of Roller Reclamation and Treatment

The rollers are welded and machined for financial benefit of recovering rollers rather than

replacing.

The thermal treatment is to attempt to stress relieve the roller at various stages in an attempt

to reduce premature failure due to bending and weld corrosion cracking.

6.2.3 Cost Evaluation to Reclaim a 200mm Roller

6.2.3.1 Labour Cost to Undercut Roller

Presuming no stress cracking occurred and the roller is only to be undercut of 3mm then the

following could be presumed:

Machining time = 2 Hrs @ R130.00 P/Hour = R 260.00

Waiting time (roller turn around and crane availability)

= 1 Hr @ R130.00 P/Hour (artisan idle time) = R 130.00

Total undercut cost (per roller) = R 390.00

Machining times can double if crack removal or tool breakage is of concern.

6.2.3.2 Pre-Heating

The pre-heating temperature is 680°C and the normalising is 380°C, to attain these

temperatures from cold start, the oven would use 7819 Mj of electricity at a cost of R250.00.

6.2.3.3 Submerged Arc Cost

The barrel weld is of two runs with a welding time of nine hours per run,

= 18 hours @ R15.00 (unskilled)

= R 270.00 per roll

83

Welding of barrel using 3,2 wire and flux is at a cost of:

Flux = R130.00 per 25Kg, 100Kg per roller

Weld = R2200.00

Total weld cost of = R 2740.00 per roller

The flux cost will, however, reduce as some is reclaimed and re-used. The flux can also be

dry milled and reused over and over resulting in minimal flux cost.

6.2.3.4 Journal Welding

The welding of the roller ends is carried out by a qualified welder. The welding consumable

cost can be calculated to a cost of R30.00 for welding rods. The labour cost is R130.00 for

an average of 4 hours, resulting in a labour cost of R540.00.

6.2.3.5 Final Machining

The machining costs for a 200mm roller is estimated at:

6 Hrs @ R130.00 per hour = R780.00

6.2.3.6 Total Cost

The total cost to refurbish a 200mm roller would be:

Undercut = R 390.00

Pre-heat = R 250.00

Submerged Arc = R2740.00

Journal Welding = R 570.00

Final Machining = R 780.00

Total cost = R 4730.00 per roller

The above figure is based on an average hourly rate of R130.00 for skilled labour and R15.

on un-skilled. The final figure does not include any additional welding due to crack removal

or inadequate weld for sufficient clean up. The omittance of tooling costs at R20.00 per tip

is for ease of calculation.

84

6.2.4 Conclusion

Cost is not a concerning factor, namely the process of repair is of concern. Why are the

rollers pre-heated to 680°C for seven hours on arrival to the workshop, are these rollers not

already stress relieved by the process? The annealing and normalising processes are not

strategic within the process to relieve induced stress they only consume time. The removal

of the roller from the oven at 100°C to still air is not achieved, the rollers are too close to

workshop openings that can have a drastic temperature variance throughout the day or night,

resulting in stress induction. The required surface hardness of 35 – 40 Rockwell is not

always achieved showing a process fault.

6.3 ROLLER RECLAMATION WITH THE USE OF VSR

The procedure of welding and the machining of the rollers as well as the costs associated

with the process will remain unchanged, however, the thermal treatments will be omitted

completely.

6.3.1 Procedure for Inclusion of VSR

The rollers are received by the workshop, identified, marked and rough machined

(machining induces stress). These rollers are then VSR treated for 3000 cycles, treating at

sub-resonant levels for approximately 20 minutes. The roller barrel and journal ends are

then welded by submerged arc and stick welding whilst the roller is cold (heat will be

generated from the welding).

The roller is then treated with VSR for another 3000 cycles treating at sub-resonance. The

VSR roller is then rough machined to 1 – 2mm from size and removed for VSR treatment

(machining induces stress) and then final machined back to manufactures standards.

6.3.2 Cost Evaluation of the VSR Process

3 x VSR treatments @ 3000 cycles = R 500.00 per roller

85

Total Cost

Undercut = R 390.00

VSR treatment = R 500.00

Submerged arc = R2740.00

Journal weld = R 570.00

Final machining = R 780.00

Total Cost = R4980.00 per roller

6.3.3 Test Results

The welding and treating of a roller by the VSR process in various stages of repair have

never been successfully carried out. Tests were carried out on a final machined roller to

determine if movement did occur and these results were negative but not a true

representation of VSR and its effects.

The test and results were not observed by the workshop supervisors and there was no set

standards layed down to ascertain if movement had occurred.

A graphical printout could have been produced to see if the roller did have high stress levels

that could be treated, no feedback was ever received by VSR Africa for further tests.

6.3.4 Conclusion

The process of VSR as a dimensional stabiliser and stress relieving process does work,

however, the test on the segment roller was on both accounts not suitably conducted by both

parties and the results thereof are “nul and void”.

VSR treatment should for optimum stress relief by carried out after every stress inducing

process, such as welding and rough machining. The cost of the VSR roller in respect to the

thermal is higher, however, the treatment time is 10 hours shorter resulting in less rollers

waiting for annealing and normalising and mere waiting for final machining.

86

APPENDICES

1. HEIC INDUSTRIES

HEIC Industries are a heavy construction equipment repair and manufacturing concern

situated in the Transvaal, South Africa. Since VSR’s first inception at HEIC in 1994

substantial VSR treatment has been used on various other components with the same

success as the Axle reference (see Letter of Appendix 1).

APPENDIX 1

87

2. ROVER GROUP

Rover, one of the largest motor vehicle manufactures in the United Kingdom has benefited

form the use of VSR on many of the vehicle jigs for the past 17 years. Within the

boundaries of South Africa, VSR is used extensively in the manufacture of jigs and

bedplates for VW, BMW and Toyota (see Letter of Appendix 2)

APPENDIX 2

88

3. LANGLEY ALLOYS

Langley Alloys, specialist fabricators of corrosion resistant alloys has been benefiting from

the benefits of VSR for over 10 years with success and enormous cost saving results. At the

same time they have benefited from the cleaner components not usually found in thermal

treatment methods. Foundries within South Africa such as Thos Begbies and Scaw Metals

who specialise in corrosion resistant metals have also benefited from the advantages of the

treatment by VSR (see Letter of Appendix 3).

APPENDIX 3

89

4. STANTECH ENGINEERING

Stantech Engineering a division of the foundry, Scaw Metals, has benefited with the use of

VSR in their engineering workshops. VSR was used extensively in the machine shop for

treatment of pins, bushes and fabricated units (see Letter of Appendix 4).

APPENDIX 4

90

5. JOHN THOMPSON

John Thompson Africa, the largest manufacturer of steam and gas boilers in South Africa

has used VSR extensively in the manufacture of boiler components. The increase in life of

boiler welded components has not only improved costs but also the actual production time

has lowered. In 1996 VSR was so successful, John Thompson has purchased a VCM 80

machine to do their own in-house stress relieving (see Letter of Appendix 5).

APPENDIX 5

91

APPENDIX 6 – 70 TON DRAGLINE BUCKET DURING VSR TREATMENT

(Courtesy VSR Africa)

APPENDIX 7 – 70 TON DRAGLINE BUCKET DURING VSR TREATMENT

(Courtesy VSR Africa)

92

APPENDIX 8 – OPENCAST MINE, COAL HAULER AXLE

(Courtesy VSR Africa)

APPENDIX 9 – COAL LOADING TERMINAL STACKER

(Courtesy VSR Africa)

93

6. VIBRATORY STRESS RELIEVING IN OPERATION

This section will show pictures of various components successfully treated with VSR. The

products range from small components ± 20 Kg to components in excess of 50 ton.

APPENDIX 10 – ALUMINIUM AND COPER STRENGTHENING BEAM

(Courtesy VSR UK)

APPENDIX 11 – EN30B MARINE DRIVE SHAFTS, VSR TREATED BEFORE

FINAL MACHINING

(Courtesy VSR UK)

94

APPENDIX 12 – 25 TON BALL MILL SHELL, VSR TREATED BEFORE RUBBER

LINING

(Courtesy VSR Africa)

APPENDIX 13 – 8 TON DRAGLINE SHEAVE WHEEL, VSR TREATED AFTER

EXTENSIVE REPAIRS

(Courtesy VSR Africa)

95

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