Vibratory Stress Relieving Finishing Processes

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    FINISHING PROCESSES

    Vibrations reduce stress

    levelsReprinted from European Surface Treatment Winter 1992/93 by VSR(Africa)cc August2001

    Roger Claxton, The Vibratory

    Stress Relieving Co.

    Various vibratory methods have beenemployed, in the last 60 years, tomodify the stress patterns of

    engineering components beneficially.Portable equipment developed in the

    UK, has established Vibratory StressRelieving (VSR) as a viable

    alternative to Thermal StressRelieving (TSR) for the reduction and

    redistribution of such stress, ensuringthe stability of the component during

    both machining and service. In thecase of surface coated components

    where TSR is precluded, VSR isinvaluable. These stresses are

    responsible for many common problems, from instability during

    machining and inability to achievetight tolerances, to change in size orshape between the manufactures andcustomers inspections, either in

    transit or during installation. VSRcan, in the main, eliminate such

    occurrences, even in finish-machinedcomponents.

    Components ranging from less than1kg to over 100t are treated, in

    minutes rather than hours. Castings,welded fabrications, bars rolls and

    shafts are all treated, and in a widevariety of metals. On accurate

    components, the process is generallyapplied after rough machining. VSR

    is even applied to some finishedmachined components. This

    combined with VSRs option oftreating mixed metal fabrications,gives the potential user many

    advantages

    VSR versus TSRAs an alternative to TSR, VSR hasmany advantages and some

    disadvantages, and both must betaken into account.

    Time and cost savings with VSR areconsiderable. Floor to floor treatment

    time is often as little as 15 minutes.Cost always compares favourably

    with that of TSR, and often offers

    substantial savings. The process doesnot significantly distort or discolour

    components as the furnace would.Environmental factors are also

    significant; VSR uses only a smallamount of electricity per component,

    there are no flue gases, no cleaningand of course, no transport involved.

    Noise was once a problem with somecomponents but as internationalresearch has refined the process,

    noise has been dramatically reducedto acceptable dB levels.

    Unlike TSR, the process doesnot confer any metallurgical

    benefits. Depending on

    requirements, this canoccasionally preclude VSR.

    Conversely, the fact that VSRdoes not alter metallurgy can

    allow greater freedom of design;the process is suitable for mixed

    metal fabrications where there issurface hard facing, and even

    rubber coating.It is often said that TSR isrequired initially to improve the

    machinability of steel. This onlyapplies where thick, flame cutitems have edges to be

    machined, and even then good practise dictates such items

    should have been normalised prior to fabricating to ensure

    good weldability. The cost of

    1

    Shaft EG with strain gauges being VSRd by RAE

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    TSR of these small items and

    VSR of the finished componentsis less than the cost of thermal

    treatment of the finished item.

    Occasionally steel plate containsabnormal impurities, such aschrome inclusions, which make

    machining difficult. As thiscondition would be unknown in

    advance of machining, TSR atapproximately 625C would not

    improve machinability.Obviously a high temperature

    TSR is one solution. With VSR,an initial, low-cost stress relief

    charge is the only expense. TSRwould involve treatment, andtransport, twice. In addition, two

    thermal treatments can have a

    detrimental effect on acomponent.

    As well as being quicker, cheaper and

    cleaner, VSR has another majoradvantage; if greater accuracy than

    can be achieved with TSR is required,VSR can be reapplied near to or, with

    consultation, after final machining.Effectively treating machining

    stresses and improving accuracy. Aleading military instrument builder

    also applies VSR to semi-finishedassemblies to give better joint

    stiffness and stability, re-torquing thejoint after VSR. Finally, it should be

    noted that VSR is unsuitable forstabilising severely cold-workedmaterials, or those with exceptionally

    high yields, and it is only suitable forhardened components where specialmeasures are taken.

    Considerations and ProcedureVSR is now widely accepted andendorsed in many company standards.

    However, in order to use VSR andconform to the requirements of

    BS5750, the process should bespecified by the design authority or a

    written consent sought. (The authorscompany has 25 years experience of

    VSR and offers a free advice servicerecognised by the The Welding

    Institute in the UK.) The differences between VSR and TSR must be

    considered when choosing theappropriate process: remember, VSR

    does not change the metallurgicalcondition of components and this can be good or bad, depending on the

    circumstances. VSR does not degradematerial properties.Where necessary, VSR can be

    reapplied prior to final machining.This accounts for its success in

    achieving improved accuracy and

    stability compared to TSR. HoweverVSR is generally applied once, eitherbefore or after rough machining.

    Components are placed on rubbersupports and the exciter is attached.

    The operator then scans the frequencyrange of the equipment (0-220Hz),

    noting instrumental feedback on the

    components resonant responses.These are then dwelt at, on the sub-resonant base point and mid point on

    the flank of each response. The peakof each response is then traversed

    before progressing to the next. A testcertificate is produced for each

    component either graphically or bydata logging.

    Those with no experience of

    VSR often ask how complexfabrications or castings can beadequately stabilised. Each

    resonant response of thecomponent has an associatedloading pattern (mode).

    Different modes affect

    different areas of acomponent. The higher theorder of the resonant

    frequency, the more complexthe loading pattern, exciting

    interspersed, individualelements of the structure. Thesuperimposition of the cyclic

    strains of VSR on the materialstrains, the full assessing and

    maintaining between fixedstrain limits of the materials

    cyclic properties, and the

    consequent lowering andredistribution of stresses,render the component stable.

    The potential VSR usershould remember that some

    VSR systems, particularlyolder ones, have a much more

    limited frequency range thanthe most up-to date models,

    and would be unsuitable forsome components, not least

    some of the examples givenlater in this article.

    EXAMPLES

    Precision pump bodiesVSR is invaluable in thetreatment of materials whoseproperties would be impaired

    by a full thermal stress

    2

    Hewson & Turrel fan on dynamic balancing machine after VSR

    Repaired pump

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    relieving treatment.Westinghouse have found thiswith their precision-machined

    pump bodies, manufacturedfrom precipitation-hardened

    aluminium alloy castings(LM25TF). Over a number of

    years, the pump bodies,

    measuring approximately 375x 300 x 150mm, had beensubjected to highly skilled

    machining and extensiveinspection, but were

    frequently rejected followingdelivery to the customer,

    because of distortion up to0.15mm across the machined

    surface. This wasunacceptable for the intended

    use.The delicate metallurgical balance of quenching and

    ageing required to achieve the

    desired mechanical propertiesdoes not allow any further

    TSR treatment.A finished machined and

    inspected casting wassubjected to VSR, and a

    distortion during treatment of between 0.10 and 0.15mm

    was observed over themachined surfaces. Following

    a series of trials, a satisfactorysolution was found; VSR was

    applied to each casting justprior to the finish machining

    operation. At this stage, therewas still 0.5mm machining

    allowance on all faces. Sincethe introduction of this cycleof operations, the rejection

    rate for the pump bodies has been reduced to zero withover 500 treated, and no

    subsequent problems withdimensional stability have

    been encountered. Nowseveral associated castings are

    also treated, with the effectthat the pump life has been

    considerably extended, andquieter, smoother running

    conditions are achieved,

    Machining tools /

    supreme accuracySome eight years ago, DeanSmith & Grace became

    disillusioned with thermalstress relieving when cast iron

    saddles, consistently intolerance on final inspection

    in the UK, were20% out onarrival in the USA,necessitating rework. VSR

    solved the problem and today,rely on it solely to stabilisesaddles, beds, etc. Before that

    time QA records showed that,

    using thermal stress relief98% of beds were reworkedin-houseafter finalmachining

    due to movement duringhandling. Subsequently, only

    one of 533 beds was reworked 0.2%.

    Dean Smith & Graces

    subcontract machine shop alsofinishes mild steel fabricatedbeds up to 10m long and 1m x

    900mm section, basically in12mm plate, but with sideway

    sections up to 100 x 300mmand weight up to 12 tons. A

    10m bed has a welding timeof approximately 50 hours.

    The fabrication is solely VSRtreated. Operation procedure

    is to fabricate, apply VSR,inspect, rough machineremoving up to 35mm to

    produce slideway profile, ship

    to Dean Smith & Grace, applyVSR and finish machine to

    five microns in 6m bygrinding. No machinability

    problems are encountered atany stage, despite extensive

    machining of flame cut edgesup to 100mm thick. For QA

    purposes, a graphic record isproduced. This is also used by

    the end customer as valuabledesign feedback data with

    regard to the vibrationcharacteristics of the beds.

    Following comparison trialsin 1990 between thermal

    stress relieving and VSR,Bradford Universityrecommended VSR for

    stabilising the famous ButlerElgamill machine beds. Theseare of similar size and require

    similar accuracy, but are castiron and have slideways on

    two adjacent slides.

    Repaired pumps and fansHewson & Turrel have found

    VSR indispensable for twovastly different product

    groups: fans/impellers and pump housing repairs.

    Aluminium, stainless steel,cast steel and chrome

    molybdenum steels, steel pump housings passing

    corrosive or abrasive mediawear badly in service, and are

    built up by MMA and MIGwelding then machined backto correct dimensions. Prior to

    the introduction of VSR,service leaks occurred.Housings treated after rough

    machining, at frequenciesbetween

    80Hz and 170Hz, were foundto change shape up to 0.8mm

    TIR. On Finish machining andin service, components have

    been stable with no leaks.Similar advantages are

    reported from Turkeysleading metallurgy laboratory,

    Zumre Metallurgi, but heremarine valve stems and bodies

    350mm diameter with 4140steel base metal are first

    preheated to 250-350C, thendeposited with Stellite gr6while the part is vibrating at

    an undisclosed highfrequency. This technique hasresulted in an approximate

    20% reduction in pinholes andflaws generally, improved

    dilution and given an easiersurface to machine as the bead

    profile is reduced byapproximately 50%. (If the

    damage is more than 6mmdeep, the crater is first built up

    with SS GR316L by TIG andthen Stellite deposit overlaid.)

    New and reconditioned,Stellite deposited plastic

    moulding machine screws andbarrels are similarly treated at

    many companies worldwide,both to stabilise and to makestraightening easier.

    The other main use of VSR atHewson & Turrel is tostabilise fans and impellors

    ranging in size from 800mm

    diameter x 100mm to 2mdiameter x 900mm infabricated mild steel and

    stainless steel. Sometimesthese are repaired

    3

    Another type of repaired pump HSG

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    components, and sometimesreplacements. After fabricating, but prior to

    dynamic balancing, thecomponents are subjected to

    VSR. Since introducing thistreatment, no fans or

    impellors have gone out of

    balance in service, even underhot conditions hitherto a

    troublesome area. Installationsare now much quieter and lastlonger between overhauls.

    Novenco Aerex, the UKslargest fan and impellormanufacturer, have had their

    own VSR unit for many yearsand endorse the benefits stated

    above. Their Canadian plantalso uses VSR. In both cases

    the system paid for itself in 4 5 months. Rubber coated,

    steel fan blades have beentreated to overcome

    instability.

    Moulds and diesThe periodic application of

    VSR to moulds and dies toreduce the build up of in

    service surface stresses hasreceived much publicity

    recently especially in theUSA. This application isneither new nor demanding,

    and most VSR can be used.For over 20 yearscomponents, typically 1m x

    200-300mm have been roughmachined and VSRd by

    many leading companies. Sub

    resonant vibrations aretraditionally used for solidcomponents. More flexible

    components are treated by thenormal resonant method.

    Coopers Payen Ltd found thatcomplex surface stress

    patterns resulting from

    induced stress relievingduring grinding, made itimpossible to obtain the

    required P20 die and mouldcomponents (typically 300 x

    100 x 250 x 8mm). Treatingthe components on a vibrating

    table has proved veryeffective. Components are

    required flat and parallel towithin a few microns, even at

    high temperatures

    Minesweeping multiplanes

    flatness of 5mm to 500mmMultiplanes for minesweeping weresubject to severe vibration and stressduring operation, and suffered from

    cracking which leads to prematurefailure. The cracking was of acorrosion fatigue type. The

    components were fabricated from13mm thick 316 and 347 stainless

    steel, and varied from approximately1.15m to 1.65m square by about

    250mm deep. The root runs were TIGcapped with five runs of manual

    metal arc, using 19/12 Oerlikon fillermaterial.

    Initially, thermal treatment was usedto increased service life. This

    consisted of heating to a temperaturebetween 1,000 and 1,050C, followed

    by rapid air cooling. In addition, theweld toes were ground to a radius of

    about 12mm. Replacement of TSR byVSR resulted in a doubling of servicelife. Shot peening the weld toes to

    induce compressive stresses into thesurface was used after VSR, in anattempt to obtain further increases.

    VSR became standard procedure onthese components after sea trials.

    Rolls and ShaftsFigure 1 shows one of eight unstable

    alloy steel, forged drive shafts, 24mmx 2m in A1, T condition being VSR

    treated and monitored using surfacestrain gauges. The results showed that

    VSR reduced surface stress to safelimits, stabilising the component

    while not reducing fatigue life oraltering material properties. Also,

    VSR and strain measurements clearlyidentified the shafts which had been

    correctly TRSd and those which hadnot.

    Hard power enamelling to soft

    rubber liningKnowing about VSR has saved many

    an engineers skin. Problems

    frequently occur either because stressrelieving is thought unnecessary, orbecause it is omitted by mistake.

    During the machining of pads on 10fabricated RHS beams, 150mm x

    150mm x 6mm x 7m for a mechanicalanalysis machine, unacceptable

    bowing occurred. As the components

    had a powder enamel finish, TSR,was out of the question. All thebeams were subjected to VSR, there

    was no cracking of the coating, andthe components were subsequently

    completely stable.Similar problems were overcome

    with a 1m x 5m x 9m enlargingcamera slideway, this time with a

    spray hammer finish, which wasunstable during final tests.

    Cast steel pipes not called upon tobe stress relieved, were lined with anabrasive and corrosive resistant,

    rubber surface coating. In service the

    flanges distorted; after uncoupling forroutine maintenance work, they

    would not reseal. They weredismantled, VSRd re-skimmed and

    gave no further trouble. Allsubsequent parts have been VSRd

    prior to matching and

    there have been no furtherproblems.

    Extremely precise instrumentsneed to be surface finishedusing the chemical blacking

    process. In case where the

    components are required to bestable and accurate to within afew microns, problems occur.

    This is due to prolonged soak,in a caustic medium at 290C,

    4

    Cooper-Payen dies/moulds

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    which is necessary to producethe FE304 surface coat. VSR,applied just before final

    grinding and blacking, hascompletely overcome

    instability.In the plastics, paper, chemical

    and powder industries, there

    have been many cases wheremirror finishes have beenrequired, often introducing

    high surface stress. Wheneverinstability occurs on a

    component, VSR comes to therescue and is subsequently

    written into the manufacturingspecification for future

    production.

    Surface hardened

    componentsToday there is a greater variety

    of surface hardeningtechniques than ever. VSR cansolve many of the associated problems. With weld

    depositing or surfacetransformation using lasers,induction, carbonisation, Arc

    lamps, etc, surface stressingdue to volumetric changes can

    cause problems with long termstability. As milling introduces

    approximately 10 times morestress than planning, VSR is

    also commonly applied toovercome machining

    instability. VSR is appliedeither before or after rough

    machining as appropriate, andgives stability and flatness

    commensurate

    With the machine producing

    the surface.

    Galvanised beamsFollowing an enquiry from a

    hot dip galvanising company

    some years ago, VSR is nowregularly used by structuralsteel joist manufacturers when

    the end product is to begalvanised. Stresses and stress

    relieving induced duringdipping influence the welding

    and straightening stresses,often causing columns and roof

    beams to drift out of acceptablelimits. The process is most

    effective in beams and mesh baskets, but appears to have

    little effect on sheet metalcomponents. The procedurehas largely eradicated the

    problem of components,particularly asymmetric ones,bowing. All for as little as 20

    per beam!

    Paper Reprinted by

    VSR(Africa)cc

    August 2001

    For more information

    Tel 27 135 6500702

    27 135 6500287

    Fax27 135 6501308

    [email protected]

    Website

    http://www.vsr-

    africa.com

    5

    mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]
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