MACHINABILITY TESTING OF MOLD STEELS

i

INDUSTRIAL RESEARCH+DEVELOPMENT INSTITUTE

649 PROSPECT BLVD., P.O. BOX 518 518 MIDLAND, ONTARIO, CANADA L4R 4L3 4L3

TEL: (705) 526-2163 FAX: (705) 526-2701

June 30, 1999

MACHINABILITY TESTING

OF MOLD STEELS

SF-2000 @ 321Bhn SF-2000 @ 350Bhn

versus DIN 1.2738 @ 311Bhn

Presented to: Hoang LeHuy Sorel Forge

By

Victor SONGMENE Sasi Ratnasabapathy

Ref. # PRO 310 229

Machinability Testing of New Mold Steels

June 30, 1999 ii

TABLE OF CONTENTS

Table of content ........................................................................................................................................ ii

List of Figures and Tables........................................................................................................................ iii

Summary ………………………………………………………..…………………………………………1

1. Introduction...........................................................................................................................2

2. Objective ................................................................................................................................2

3. Background of machinability of mold steel .........................................................................2

4. Testing Procedure .................................................................................................................4

4.1 Materials ........................................................................................................................................4

4.2 Equipment .....................................................................................................................................4

4.3 Tool life testing..............................................................................................................................5

4.4 Cutting force testing .....................................................................................................................5

4.5 Surface finish testing ....................................................................................................................6

5. Results & Discussions...........................................................................................................7

5.1 Tool Life.........................................................................................................................................7

5.2 Cutting forces tests .......................................................................................................................7

5.3 Surface finish tests ........................................................................................................................7

5.4 Global machinability rating.........................................................................................................8

5.5 Wear pattern and Tool Life .........................................................................................................9

5.6 Taylor Model ...............................................................................................................................11

6. Concluding Remarks ..........................................................................................................12

7. References ...........................................................................................................................12

Appendixes start pages 13-16

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List of figures

Figure 1: Partial Machinability indexes ...........................................................................................1

Figure 2 : Global Machinability rating .……………………......………….…..……..………….1 Figure 3: Wear progression of inserts of two series of test ...……………………..……………..10 Figure 4: Taylor Model on Cutting Speed-Tool Life relationship ..…………………………….11

LIST OF TABLES

Table 1: Effect of alloying elements on machinability

…………………………………………….3

Table 2: Chemical composition of steels, conditions & hardness ……………………...…………5 Table 3: Machinability rating data ……………………………………………………………….8

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Summary.

Milling and drilling tests were performed to compare new SF-2000 mold steel manufactured by Sorel Forge to that of standard DIN 1.2738. Partial machinability ratings of the three tested steels are computed based on cutting speed of 30 minutes tool life, cutting force and surface finish of machined parts (Fig.1). In this graph the partial machinability represents the ratio of performance index (cutting speed for 30 minutes, force, and surface finish) of the selected material to that of the standard.

100

74

97100

107

117

100

136

88

0

25

50

75

100

125

150

SF-2000-321Bhn SF-2000-350 Bhn DIN 1.2738- 311Bhn

Materials

Par

tial

mac

hin

abili

ty in

dex

es (

%)

Speed Force Ra

FIG. 1: Partial Machinability indexes

From Global machinability rating (Fig. 2), it can be noted that:

100

8278

0

25

50

75

100

125

SF-2000-321Bhn SF-2000-350 Bhn DIN 1.2738- 311Bhn

Materials

Mac

hin

abili

ty r

atin

g (

%)

FIG. 2: Global Machinability Rating

The SF-2000 even at high hard condition is easier to machine than the DIN 1.2738. The standard SF-2000 can be machined 20% quicker than the DIN 1.2738. This was confirmed with validation tests.

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1. INTRODUCTION

Increasingly, manufacturing companies are looking for short lead times and greater machining efficiency. Along with the introduction of new work piece material and cutting tool, it has become necessary to standardize the tool, to characterize work piece machinability and to search for appropriate machining parameters ranges for each work piece/tool/operation. Each operation/work piece/tool system has unique requirement to be tested to achieve maximum productivity, and low unit cost. Traditional machinability comparisons look at either tool life or power alone. A new approach developed in this project includes surface finish addressing many machinists’ concerns, especially for mold makers. A generalized procedure is designed to obtain global machinability rating for a given material. The procedure includes three tests to generate individual indices for tool life, cutting force and surface finish.

2. OBJECTIVE

To generate global and partial machinability indices that designates the degree of difficulty (or ease) with which materials can be machined. The machinability rating will enable the manufacturing engineer quickly to evaluate the manufacturing time and cost of the tested material for any type of jobs.

The influence of the different machinability criteria is determined and recorded. The test assesses the global machinability rating of a work piece material compared to a reference material. The parameters used for machinability assessment are the tool life, the cutting forces and the surface finish.

3. BACKGROUND OF MACHINABILITY OF MOLD STEEL

Machinability is a general term used to rate the ease (or difficulty) of machining. It is a work piece property depending on thermo-mechanical, structure, and compatibility with tool material. Caren [1] was noticing that P-20 mold steel is widely used for making plastic injection molds in North America. It offers a good combination of hardness, machinability, and toughness, but its

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properties are not always consistent. Frequently, the hardness is not uniform throughout a block, which leads to machining problems. The alloying elements in steels have a profound effect on its properties and machinability. The following are the effects of various elements in steels in regards to machinability [2]:

• Carbon is the principal strengthening element in steel. It can have a great effect on numerous metallurgical properties. Its effect on machinability depends on the presence of other alloying elements.

• Manganese increases strength and toughness and improves the machinability. Higher levels have a negative effect on weldability

• Sulfur improves strength when combined to manganese, lowers impact strength and ductility; impairs surface quality. The chips produced are small and break up easily. The shape, orientation, distribution and concentration of manganese sulfides inclusions (second-phase particles) formed significantly influence the machinability. Considered an impurity, except when intentionally added to improve machinability.

• Lead and Phosphor are considered as impurities, except when intentionally added to improve the machinability

• Silicon is added to steel to tie up free oxygen. It decreases the machinability while strength, hardness and corrosion resistance

• Nickel improves strength and toughness when combined with other alloying elements. • Molybdenum has a strong effect on hardenability (similar to manganese). • Copper adversely affects hot working characteristics and surface quality and reduces little

the ductility. • The presence of aluminum and silicon in steels is always harmful on machinability

because they combine with oxygen and form aluminum oxides and silicates. These compounds are hard and abrasive but they makes the material more brittle.

The Table 1 summarizes the effect of different alloying elements on machinability.

Table 1: Effect of alloying elements on machinability [3] Positive Pb S P C 0.3-0.6% Negative Mn Ni Si Al Cu Cr V Mo C>0.6% C<0.3%

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4. TESTING PROCEDURE

The procedure used in this project for machinability testing is the one established in the project Tool life & Machinability Testing [4] and the results are computed using the IRDI’s TL&M software. The test consists of milling parts and the surface finish generated. In addition cutting forces are recorded during drilling operations for comparison purposes.

4.1 Materials

The chemical composition and properties of the mold steels materials submitted for testing are summarized in Table 2.

Table 2: Chemical composition of steels, conditions & hardness

Chemical composition Materials Part

number

Hardness

(Bhn) C Mn S Si Ni Cr Mo V Cu Al

SF-2000

“Standard

”

10807-3 321 .33 .68 .008 .65 .15 1.61 .35 .011 .11 .033

SF-2000

“Hi-Hard”

10547-1 350 .34 .82 .005 .40 .26 1.89 .48 .011 .12 .017

DIN 1.2738 11258-1 311 .39 1.37 .007 .29 1.05 1.89 .18 .008 .07 .011

4.2 Equipment

• Machine-Tool: FADAL VMC 6030 CNC milling machine, 22 hp, 10 000 rpm • Profilometer: Mitutoyo SURFPAK • Table dynamometer: KISTLER 3 components dynamometer 9255B

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4.3 Tool life testing

Side milling tests were used to set the partial machinability based on tool life. The cutting speed tested ranges from 61 m/min to 122 m/min. All the tests were run at the following cutting parameters:

Tool diameter: 38.1 mm

Inserts: TiN-coated carbides

Feed per tooth: 0.1016mm

Radial depth of cut: 12.7mm

Axial depth of cut: 2.54mm

Coolant: Blasocut Universal

(63% mineral oil; 4% water,

Additives: chlorinated paraffin;

Viscosity @40º = 39mm²/sec)

Once the tool life at different speed recorded, the Taylor exponent “n” and the constant “C” are

computed to obtain the tool-life cutting speed relationships described in Equation 1.

V x T n = C (1)

From the Taylor model, the cutting speed for 30 min. tool life is predicted and used for

comparison.

4.4 Cutting force testing

Drilling operations were chose to evaluate the penetration force on each of the tested material. The test consisted of making holes with uncoated High Speed Steel (HSS) drill and recording the thrust forces. The following drilling parameters were used:

Drill diameter: 9.92mm Spindle speed: 225 rpm Feed rate: 57.15 mm/min Hole depth: 12.7mm Coolant: Blasocut Universal (63% mineral oil; 4% water, Additives: chlorinated paraffin, Viscosity @ 40º = 39mm²/sec)

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4.5 Surface finish testing

The surface finish tests consisted of a face-milling operation at constant cutting parameters on each of the material tested. Then the surface texture is recorded and analyzed using a profilometer. The cutting parameters used for racing the parts are the followings:

Cutting tool diameter: 38.1 mm Inserts: uncoated carbides Cutting speed: 140.5 m/min Feed per tooth: 0.1016 mm Radial depth of cut: 38.1 mm Axial depth of cut: 0.508 mm Coolant: Blasocut Universal (63 % mineral oil; 4% water) There are many parameters defining the surface texture but the more used parameter is the arithmetical mean deviation Ra. We retained Ra for Machinability rating calculations but others parameters were also recorded and can be found in Appendix C. They are: Ra: arithmetical mean deviation of the

profile

Rz: Ten point height of irregularities Rt: Total height of the profile Dq: Root-mean square of the profile Sk: skewness of the profile Rk: Core roughness depth

Rpk: Reduced peak height Rvk: Reduced valley depth Mrl: Material ratio 1 (Upper limit of bearing length ratio) Mr2: Material ratio 2 (Lower limit of bearing length ratio)

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5. RESULTS & DISCUSSIONS

5.1 Tool Life

The tool life was set using the standard [4,5] value of flank wear (VB = 0.3mm). Two failure modes were recorded: catastrophic failure due to depth of cut notch wear and flank wear. The first wear mechanism (notch or flank wear) that reached the maximum limit admissible ended the life of the tool. The Table 3 summarized the tool life obtained at different cutting speeds, the Taylor’s model exponents and the machinability indexes. A first set of tests with uncoated carbide showed that the standard SF-2000 (321 Bhn) is easier to machine than the Hi-Hard SF-2000 (350 Bhn). However, the tool life was so short for the 350 Bhn material that we changed for TiN-coated carbide inserts for others tests. The SF-2000 (321 BHN) was then choose as standard reference material for comparing the other molds steels while using TiN-coated carbides tools. The standard SF-2000 material has the highest speed (141.4 m/min) for 30 minutes tool life, followed by the Hi-Hard SF-2000 (104.6 m/min). The Din 1.2738 has the lowest cutting speed (91.8 m/min) for the same tool life (Table 3).

5.2 Cutting forces tests An example of drilling tests is showed in Appendix 1 where the thrust force is recorded in function of the time. From this data, we determine the mean cutting forces in drilling operations. The Hi-Hard SF-2000 material and the DIN 1.2738 mold steels required respectively 3423 and 3737 Newtons of forces during drilling tests while the standard SF-2000 required only 3205 Newtons (Appendix 2). The same tendency were observed for drilling torque. Less energy (power) is required to cut the standard steel. The chip thickness ratio (feed /deformed chip thickness) confirmed that the softer material deforms better than the harder.

5.3 Surface finish tests

After milling with the same parameters, the same arithmetic roughness 60 microns was recorded on standard SF-2000 (321 BHN) and on DIN 1.2738 materials while the High Hard SF-2000 was 30 microns higher (Table 3).

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Table 3: Machinability rating data

Parameters Materials

SF-2000 SF-2000 1.2738 321 BHN 350 BHN 311 BHN

Tool Life (min) Cutting Speeds

61 m/min 146 52 85 91.4 m/min 45 21 122 m/min 42 21.75 18

Failure mode Flank wear Notch wear Notch wear

Taylor’s Model of Tool Life Exponents & Constants Exponent “n” 0.528 0.827 0.424 Constant “C” 853 1744 390

Cutting speed for 30 minutes tool life 141.4 m/min 104.6 m/min 91.8 m/min

Other Machinability indexes Thrust forces (N) 3205 3423 3736.6

Torque (N-m) 9.7 10 12.6 Chip thickness ratio .29 .34 .35

Surface finish Ra (µm) 67 91 60

Partial and Global Machinability rating Speed 100% 74% 97%

Thrust forces 100% 107% 117% Surface finish Ra 100% 136% 88%

Global 100% 82% 78%

5.4 Global machinability rating

The global machinability rating given in Figures 1 and 2 takes into account the metal removal rate through the cutting speed, the cutting force and surface finish ratios “RCv”, “RCF”, and “RCRa” respectively.

MRR = 50 * RCV + F

40RC

+ RaRC

10 (2)

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RCv = VoVx

The “constant tool life cutting speed ratio”. (The higher this coefficient the easier to machine the tested material).

RCF = FoFx

The “specific cutting force ratio”. (The higher this coefficient the lower the machinability of the tested material)

RCRa = RaoRax

The “surface roughness ratio”. (The higher this coefficient the lower the machinability of the tested material)

Based on the tool life, cutting forces and finish results, it comes that the SF-2000 even at high hard condition is easier to machine than the DIN 1.2738. The standard SF-2000 can be machined 20% quicker than the DIN 1.2738. The results show:

• As expected the lower hardness of SF-2000 (321Bhn) machines better than the harder SF-

2000 (350 Bhn).

• The machinability of DIN 1.2738 (311 Bhn) is practically equal to that of Hi-hard SF-2000,

although the later is harder (350 Bhn).

• The cutting speed used to machine the standard SF-2000 is equivalent to those used to

machine the DIN 1.2738. On the other hand, the standard SF-2000 (321 Bhn) can be

machined 26% lower than Hi-Hard SF-2000 (350 Bhn).

• The specific cutting forces required to cut the standard SF-2000 samples is lower than that

required for Hi-hard SF-2000 and DIN 1.2738. The Hi- hard SF-2000 required 7 % higher

force while the DIN 1.2738 was 17% higher.

• The finishes, during the milling operations, were slightly different among the materials. The

DIN 1.2738 shows better machinability (12% better), while higher hardness SF-2000 (350

BHN) shows lowest machinability (36% lower.) compared to the standard material SF-2000.

5.5 Wear pattern and Tool Life The milling inserts experienced flank and depth of cut notch wear (Fig. 3). Most of cutting tools experienced progressive flank wear up to 0.2 mm. After this level, notch wear took place and progressed faster than flank wear.

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The depth of cut notch wears lead to catastrophic failure while the flank wears usually progressed up to the wear limit. The notch wear can be seen in the picture of an insert shown in Fig. 3 for Hi-hard SF-2000 (350 Bhn). This wear mechanism is very dependent on the material homogeneity and uniformity of hardness. The failure modes obtained for each steel are summarized in Table 3. The Hi-hard SF-2000 and DIN 1.2738 failed by notch wear, while the standard SF-2000 steel failed by progressive flank wear mode. The tool life was set using the standard value of flank wear (VB = 0.3 mm). The first wear mechanism (notch or flank wear) that reached its maximum limit admissible ended the life of the tool. More information on wear progression at different cutting speeds can be found in Appendix 3. Inserts1 @ 61 m/min

Inserts2 @ 61 m/min

Inserts1 @ 91.4 m/min

Insert2 @ 91.4 m/min

Inserts1 @ 122 m/min

Insert 2 @ 122 m/min

Standard SF-2000 - 321 Bhn

@ 66.3 Minutes @ 41 Minutes @ 15 Minutes

Hi-Hard SF-2000 - 350 Bhn

@ 60 Minutes @ 39 Minutes @ 23 Minutes

Figure 3: Wear progression of inserts of two series of test

The figure 3 depicts the inserts at different times and for speeds of 61, 91.4 and 122 m/min. • The inserts used in High Hard SF-2000 steel wear faster and differently than the inserts used

in standard SF-2000 samples. • Hi-Hard SF-2000 inserts reached notch wear limits at 60, 39,and 23 minutes at speeds of 61,

91.4 and 122 m/min respectively. This gives short tool life.

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5.6 Taylor Model The figure 4 displays the tool life cutting speed relationships established based on Taylor model.

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

60 65 70 75 80 85 90 95 100 105 110 115 120 125Cutting Speed (m/min)

Cu

ttin

g t

ime

or

To

ol l

ife

(min

)

DIN 1.2738 311Bhn:VT^0.424=390

SF-2000- 321Bhn: VT^ 0.537=853

SF-2000-350 Bhn: VT^0.827=1040.6 SF-2000 321 Bhn

DIN 1.2738 311Bhn

SF-2000-350 Bhn

Fig.4: Taylor Model on Cutting Speed-Tool Life relationship.

From this graph the following conclusions can be made: • In the studied range of cutting speeds 60-120 m/min, the tool life of standard SF-2000

samples is more than twice that of Hi-Hard SF-2000. • At higher cutting speeds (above 90 m/min), the tool life obtained for Hi-Hard SF-2000 (350

Bhn) is better than DIN 1.2738 (311 Bhn). • DIN 1.2738 (311 Bhn) steels performs better at speed below 90 m/min. . The tool life is dependent on the maximum wear limit allowed as shown in Appendix 4. As the wear limit decreases, the tool life decreases

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6. CONCLUDING REMARKS • Based on the tool life, cutting forces and surface finish results, standard and Hi-Hard SF-2000 are

easier to machine than the DIN 1.2738 (311 Bhn) • There is a difference of 22 % machinability rating between standard SF-2000 and DIN 1.2738. • The machinability of DIN 1.2738 (311 Bhn) is practically equal to that of Hi-hard SF-2000,

although the later is harder (350 Bhn). • The cutting speed used to machine the Hi-Hard SF-2000 (350 Bhn) can be 25 % lower than standard

SF-2000 (321 Bhn) or Din 1.2738 (311 Bhn). • The texture obtained after machining standard SF-2000 (321 Bhn) and DIN 1.2738 (311 Bhn) is

better compare to Hi Hard SF-2000 (350 BHN). • For Hi-Hard SF-2000, we recommend to use cutting speeds between 60 and 90 m/min during end

milling of with coated carbide. At higher cutting speed, the tool life is shorter.

7. REFERENCES

1. Caren, S., “Prehardened mold steels offer machinability and weldability”, reprint from (PM&E - Plastics Machinery & Equipment, October 1993, pp.1-4.

2. Improved Tool Steels for Injection molds, Advanced Materials & Processes, June 1992. 3. Sandvik Coromant, Modern Metal Cutting: A practical Handbook, AB Sandvik Coromant,

Sweden, 199. 4. International Standard ISO 8688-1, Tool life testing in milling - part1: face milling, first edition,

1989. 5. International Standard ISO 8688-2, Tool life testing in milling - part2: End milling, first edition,

1989 6. Songméné V., Stefan, I., Stefan, M., Yan, D., Hirholzer, J, Tool Life & Machinability Testing -Phase

1- Testing Procedure &database, Report of project, IRDI, November 1996. 7. Songméné V., Machinability Testing of Mold steels, Report of project, IRDI, 1996

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Machinability Testing of New Mold steels

June 30, 1999 13

Appendix 1

Cutting Force obtained in drilling

0

500

1000

1500

2000

2500

3000

3500

4000

0 5 10 15 20time (sec.)

Fo

rce

(N)

SF-2000 (350 BHN) mean = 3423 N

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Appendix 2Average Cutting Force

3203

3423

3736.6

2900

3000

3100

3200

3300

3400

3500

3600

3700

3800

SF-2000-321Bhn SF-2000-350Bhn DIN 1.2738-311Bhn

Material

Fo

rce

(N)

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Appendix 3: Tool wear of High Hard SF-2000 (350 Bhn)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 10 20 30 40 50 60 70 80 90

Cutting Time (min)

Fla

nk

wea

r (m

m)

61 m/min

91 m/min

122 m/min

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Appendix 4Tool life as function of Cutting Speed

146

52

85

45

25

42

22 18

0

20

40

60

80

100

120

140

160

SF-2000-321Bhn SF-2000-350Bhn DIN 1.2738-311Bhn

Material

To

ol L

ife (m

in)

61 m/min91.4 m/min122 m/min

Documents

MACHINABILITY TESTING OF MOLD STEELS