Cryogenic treatment for High speed steel cutting tool

Cryogenic Treatment of M1, EN19 and H13 Tool Steels to Improve Wear Resistance P Sekhar Babu, Member P Rajendran, Non-member Dr K N Rao, Fellow Cryogenic treatment is said to improve wear resistance of tool and die steels and implemented at many places for that purpose. Although it has been confirmed that cryogenic treatment improves wear resistance and tool life, the process has not been standardised with inconsistent results varying from researcher to researcher.In this work the authors have studied the improvement in wear resistance of M1, EN19 and H13 tool steels after cryogenic treatment. The materials were tested for improvement in abrasive wear resistance after cryogenic treatment at different temperatures below 0°C. All the samples were first heat treated as per standard norms and re tempered after cryogenic treatment. The samples were treated at 0° C, -20° C, - 40° C, -80° C and -190° C. It was observed that the wear resistance improved for all the samples from 315% to 382% depending on the material. Keywords: Tool and die steels; Wear resistance; Cryogenic treatment

INTRODUCTION NASA engineers were the first to notice the effects of cold temperatures on materials. They noticed that many of the metal parts in the aircraft that had returned from the cold vacuum of space came back stronger than they were before flight. Since then sub-zero treatment (-80° C) has been used for many years, but with inconsistent results. Many of the inconsistencies were reduced by longer soaking periods and with deep cryogenic treatment (-190° C). Tool steels are high quality steels made to close compositional and physical tolerances. These are used to make tools for cutting, forming or shaping a material into a part or component adapted for a specific use. In service most tool steels are subjected to extremely high loads that are applied rapidly. The material must withstand these loads a great number of times without breaking and without undergoing excessive wear or deformation. The performance of a tool in service depends on1,2 (i) (ii) (iii) (iv)

proper tool design, accuracy with which the tool is made, selection of proper tool steel, and application of proper heat treatment.

A tool can perform successfully in service only when all four requirements have been fulfilled. All tool steels must be heat treated to develop specific combinations of wear resistance, resistance to deformation or breaking under high loads, and resistance to softening at elevated temperatures. P Sekhar Babu is with the Mechanical Engineering Department, Vignan Institute of Technology and Science, Deshmukhi 508284, Nalgonda, Andhra Pradesh; P Rajendran is with DLRL, DRDO, Chandrayanagutta, Hyderabad 500 005; while Dr K N Rao is with the Mechanical Engineering Department, College of Engineering, Osmania University, Hyderabad 500 007. This paper was received on October 28, 2004. Written discussion on this paper will be entertained till January 31, 2006.

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For a given tool steel at a given hardness, wear resistance may vary widely depending on the wear mechanism involved and the heat treatment used. Among tool steels with widely differing compositions but identical hardness, wear resistance may vary widely under identical wear conditions. In the heat treatment of tool steels the problem of retained austenite after heat treatment has prevailed since the development of tool steels. The retained austenite is soft and unstable at lower temperatures that it is likely to transform into martensite. Freshly formed martensite is brittle and only tempered martensite is acceptable. The transformation of austenite into martensite yields 4% volume expansion causing distortion, which cannot be ignored3. Therefore, the retained austenite should be transformed to the maximum possible extent before any component or tool is put into service. Treating the material after heat treatment at sub-zero or cryogenic temperatures transforms the retained austenite into martensite. Meng4 proposed that greater wear resistance can be obtained with longer soaking periods (~24h) because of the formation of η−carbides which improves the wear resistance to the maximum possible extent. MATERIALS The materials tested are listed in Table 1 and their chemical composition are given in Table 2. One high speed steel (M1) used for lathe tools, milling cutters, cutter blades, boring tools, twist drills, metal cutting saws etc; one constructional steel (EN19) used for axle shafts, gears, connecting rods, studs, bolts and propeller shaft joint etc and one chromium hot work steel (H13) used for making cutters; were selected to study the improvements in wear resistance. Experiment Samples of 50 mm length were made ( Figure 1) from standard bar stock of 8 mm φ -12 mm φ procured locally. The samples were heat treated separately as per prescribed ASM standards. The samples were divided into six sets. The first set of samples after heat treatment and tempering were kept as reference for measuring the IE(I) Journal-MM

improvements in wear resistance at different temperature treatments. The second set of the samples were cooled down to 0°C slowly at a rate of 0.9 K/min, the third to - 20°C, the fourth to -40°C, the fifth to -80°C and the sixth to -190°C. All the samples were soaked at the respective temperatures for 24 h and slowly brought back to room temperature and retempered.

outer diameter 50mm, running at 200 rpm under a load of 20N. Each sample was weighed before and after every abrasion period, using analytical balance with accuracy up to two digits. The analyses are presented in Tables 3-5, the results are plotted in Figures 4-6 and summarised in Figure 7.

The wear test apparatus is shown in schematically in Figure 2, while the weights and other components are shown in Figure 3. Each sample was abraded for 3 min against a course grinding wheel of 2 kg weight

1 kg weight

3 kg weight

Figure 1 Samples after heat treatment

Weight

Pin

Holder Grinding wheel

Improvement in wear resistance, %

Figure 3 Weight, Grinding wheel and holder

Temperature, °C Figure 2 Pin on disc test facility

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Figure 4 Wear resistance improvement in M1 samples

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Improvement in wear resistance, %

Table 2 Composition of materials selected composition, % Material M1 H13 EN19

Mn 0.3 0.3 0.65

Si 0.3 1.0 ---

Cr 4.0 5.0 1.1

V 1.0 1.0 ---

Mo 8.5 1.5 0.3

W 1.5 -----

Table 3 Experimental results: M1 samples

Temperature, °C Figure 5 Wear resistance improvement in H13 samples

Temperature, Initial °C weight HT 16.80 0 20.90 - 20 16.55 - 40 13.80 - 80 17.68 -190 14.72

Final Weight weight loss 14.49 2.31 18.78 2.12 15.05 1.50 12.92 0.88 16.84 0.84 14.18 0.54

Weight loss, % 13.75 10.14 9.06 6.30 4.75 3.60

Improvement in wear resistance 100 135 151 218 289 382

Weight loss, % 9.85 9.60 5.70 5.03 3.51 2.94

Improvement in wear resistance 100 103 172 195 280 335

Improvement in wear resistance, %

Table 4 Experimental results: H13 samples Temperature, Initial °C weight HT 22.42 0 20.00 - 20 28.00 - 40 23.83 - 80 25.60 -190 22.04

Final Weight weight loss 20.21 2.21 18.08 1.92 26.40 1.60 22.63 1.20 24.70 0.90 21.39 0.65

Table 5 Experimental results: EN19 samples analysis

Temperature, °C

Improvement in wear resistance, %

Figure 6 Wear resistance improvement in EN19 samples

Temperature, Initial °C weight HT 32.01 0 26.13 -20 21.67 - 40 30.04 - 80 25.12 -190 31.94

Final Weight weight loss 29.99 2.02 24.73 1.40 20.72 0.95 28.91 1.13 24.30 0.82 31.30 0.64

Weight loss, % 6.31 5.35 4.38 3.76 3.26 2.00

Improvement in wear resistance 100 117 144 167 193 315

CONCLUSION The wear resistance improvement varied from 315% to 382% depending upon the material tested. Therefore, further investigations can be continued with any other material to investigate the improvements in wear resistance by cryogenic treatment. REFERENCES Temperature, °C

Figure 7 Wear resistance analysis Table 1 Materials selected AISI No M1 H13 EN19

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C 0.85 0.35 0.35

Material Molybdenum high speed steel Chromium/molybdenum hot die steel Chromium molybdenum (constructional) steel

1. ASTM, vol 3, pp 421-447. 2. ASM Handbook, vol 4 pp 711-725. 3. D M Lal, S Ranganarayan and A Kalanidhi. ‘Cryogenic Treatment to Augment Wear Resistance of Tool and Die Steels’. Cryogenics International Journal, vol 41, 2001, pp 149-155. 4. Fanger Meng, et al. ‘Role of Eta-carbide Precipitations in the Wear Resistance, Improvement of Fe-12Cr-MoV-1 4C Tool Steels by Cryogenic Treatment. ISIJ International, vol 34, no 20,1994,pp 205-210.

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