UNDERSTANDING THE EFFECTS OF CRYOGENIC TREATMENT ON M2 TOOL PROPERTIES Cryogenically treated ver the years, the possibility tify any property improvements and of enhancing the properties correlate them with microstructural hardened steel of hardened by changes. theoretically should treating them at tempera- Otures below room temperature has Experimental Procedure improve properties. been of interest. Two types of low To understand property and re- This work is aimed at temperature thermal treatments are sponse changes, M2 tool steel subzero, or cold treatment, and cryo- (molybdenum high speed tool steel understanding the genic treatment. Subzero treatment having nominal composition of 0.85- underlying mechanisms is carried out at -145°C (-230°F), while 1.00% C, 4.0% Cr, 2.0% V, 6.0% W, 5% cryogenic treatment is carried out at Mo) was subjected to five different for the improvements. -195°C (-320°F), the boiling point of heat treatment sequences including: liquid nitrogen. Interest continues 1. Austenitize and quench Rajendra Kelkar based on anecdotal evidence of the 2. Austenitize, quench, and temper SSI Technologies benefits of “cryo” treatment not only 3. Austenitize, quench, temper, Janesville, Wis. for steels but also in nonferrous and cryo treat metals like copper, aluminum and ti- 4. Austenitize, quench, and cryo Philip Nash* tanium. The literature contains many treat Thermal Processing Technology claims about the advantages of cryo- 5. Austenitize, quench, cryo, and Center, IIT genics[1-5]. However, there is little in- temper Chicago, Ill. formation available in the scientific was carried out by literature regarding understanding austenitizing in a vacuum furnace at Yuntian Zhu* the mechanism behind property im- 1200°C (2190°F) followed by gas Los Alamos National Laboratory provements[6-8] beyond the obvious using 20 bar nitrogen. Los Alamos, N. Mex. conversion of austenite to marten- was performed in a box site, which only applies to steels. The type furnace at 565°C (1050°F) *Member of ASM International objectives of this study were to quan- for two hours followed by fur- nace cooling. Cryogenic treatment was performed in a cryoprocessor at -184°C (-300°F) followed by 40 hours of holding. Heating and cooling were carried out at the con- trolled rate of 21°C/min (40°F/min). Conventional processing of M2 tool steel includes hardening fol- lowing by triple tempering. When cryogenic treatment is used, only a single temper is used, which seems sufficient to convert martensite to (a) (b) tempered martensite. Sequence 3 rep- resents the current industrial prac- tice for cryogenic treatment. Figure 1 shows the various test samples used for compression and hardness testing, x-ray diffraction, internal fric- tion, and dilatometry. Additional testing included metallography, re- sistivity measurements, and photon induced positron annihilation (PIPA). (c) (d) Fig. 1 — Test specimens: internal friction “dog-bone” type (a), wear resistance and x-ray Test Results diffraction (b), tensile (c), and dilatometry and compression (d). A plot of hardness as a function of PROGRESS • AUGUST 2007 57 66 Further tempering may produce very small spherical carbide precipitation, 65.5 which is important with respect to 65 wear resistance, but may not provide 64.7 64.5 64.4 an improvement in hardness. 64 Compression testing was used to determine the yield strength (Fig. 3). 63.5 63.5 63.3 Yield strength values are clustered Hardness, HRC 63 62.9 for tempered and untempered sam- 62.5 ples, respectively. The highest value for the quenched and cryotreated 62 0 1 2 3 4 5 sample can be attributed to the Processing Sequence No. greater dislocation density resulting Fig. 2 — Hardness values for different processing sequences. from cryogenic treatment, which con- verts almost all retained austenite to 500 the different thermal processing se- martensite. 485 480 476 quences is shown in Fig. 2. Quenched Internal friction tests were used to and tempered samples have higher 460 quantify dissolved carbon and ni- 444 hardness (by 0.7 HRC) compared trogen in solution and to detect the 440 434 430 with that for the quenched only presence of mobile dislocations. A 420 sample, which could be due to sec- plot of internal friction versus fre- 400 Yield Strength, ksi Yield 1 2 3 4 5 ondary hardening from tempering at quency is shown in Fig. 4. The com- Processing sequence No. 565°C. Cryo treatment of the tem- plete absence of the carbon peak can Fig. 3 — Yield strength for different pro- pered sample results in a boost of 1 be attributed to the quench aging, cessing sequences. HRC, possibly due to conversion of which allows carbon to form clusters around the dislocations or possibly 4.0E-03 form small size carbides. Higher level 3.5E-03 of background for sequences 1 and 4 indicates higher density of disloca- )

-1 3.0E-03 tions or dislocation-interstitial inter- Seq. 4 Seq. 1 2.5E-03 action. Internal friction is lower for sequence 2, 3 and 5, which suggests 2.0E-03 Seq. 2 lower mobile dislocation density. Seq. 5 Seq. 3 1.5E-03 Moreover, the background is higher for sequence 4 compared with se- Internal friction (Q 1.0E-03 quence 1, which suggests cryogenic 0.5E-03 treatment results in more dislocations in the sample. These results appear 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 to be logical and consistent with the Frequency, Hz resistivity measurements, which are Fig. 4 — Internal friction versus frequency for different processing sequences. used to characterize changes in inter- stitial contents like carbon, carbide retained austenite to martensite. The precipitates and dislocations (Fig. 5). 0.8 0.7433 0.7527 highest hardness (64.9 HRC) was Higher resistivity in sequences 1 achieved by cryogenic treatment di- and 4 is attributed to clustering of 0.7 rectly after quenching, which is due carbon atoms and the high density 0.5788

mm to transformation of retained of dislocations. Resistivity decreases •

 0.6 0.5524 austenite to martensite. By compar- after tempering due to carbide pre-  ison, the hardness of the quenched, cipitation and reduction or re-

0.5 0.4721 cryotreated, and tempered sample is arrangement in dislocation density. 1.4 HRC lower due to precipitation Two competing processes further de-

Resistivity, Resistivity, of carbides and reduction in tetrag- creases resistivity after cryogenic 0.4 onality of martensite during tem- treatment. Resistivity must increase pering. Cryo treatment after temper due to greater dislocation density. 0.3 produces a higher harness than the Also, carbon atoms could cluster at 1 2 3 4 5 Processing sequence No. quench, cryo, temper sequence as a dislocations and subsequently pre- Fig. 5 — Resistivity values for different result of a double effect of conversion cipitate during tempering. Thus, loss processing sequences (error bars represent of retained austenite to martensite of carbon from the matrix may result the standard deviation of five values. and coarser carbides due to sec- in lower resistivity. The highest resis- ondary hardening from tempering. tivity for sequence 4 suggests higher 58 HEAT TREATING PROGRESS • AUGUST 2007 dislocations and clustering of carbon as-received sample (sequence 0). Se- 0.8 atoms. Further tempering reduces quence 4 has the highest dislocation 0.7369 0.7 mm After 1 week carbon in solution due to precipita- density, the effect of full transforma- • 0.6285  0.6 tion, and there is a reduction in dis- tion of retained austenite to marten-  Room temperature location density, thus lowering resis- site and relative thermal contraction 0.5 0.4985 tivity. An example of the change in of the matrix compared with primary Liquid 0.4 0.4118 resistivity with material condition is carbides during cryogenic treatment. After 2-1/2 weeks shown in Fig. 6. Conversion to martensite results in Resistivity, 0.3 Dilation plots for all treated and a 4% volume expansion; on the other Time as-received samples are shown in Fig. hand, there is thermal contraction Fig. 6 — Effect of different material condi- 7. The starting microstructure was from the cryogenic treatment. The tions on resistivity. martensite or tempered martensite. Samples were heated to a tempera- 0.020 ture of 1000°C (1830°F) with As received 10°C/min. (18°F/min) heating and 0.015 cooling. There is no significant dif- ference for all samples in terms of contraction. During heating, there is 0.010 Seq. 4 Seq. 1 a clear distinction between tempered Seq. 5 Seq. 3 and untempered samples. The latter 0.005 Seq. 2 have a lower thermal expansion

during heating, which is attributed Expansion, % to the conversion of tetragonal 0.000 martensite to a lower tetragonality 200 400 600 800 1000 1200 martensite due to clustering and/or carbide precipitation. -0.005 Induced strain in martensite and c/a ratio for the martensite were -0.010 measured using x-ray analysis Temperature, °C processed by Rietveld analysis, the Fig. 7 — Dilatometric plots after thermal treatment for different processing sequences with general structure analysis system superimposed as-received M2 steel plot. (GSAS) software from LANL (Fig. 8). two distinct effects produce a greater 1.8 The value of strain depends on the 1.53 1.6 number of dislocations compared 1.45 tetragonality of the martensite and with the as-quenched sample. The 1.4 1.31 the mismatch between precipitates 1.06 negative value of S parameter for se- 1.0 and the matrix. Sequence 3 has the quences 2 and 5 indicates a lower Strain maximum strain of 1.6, which is at- number of dislocations compared 0.6 tributed to secondary carbide precip- with the as-received sample. The 0.2 itation from tempering and lowest S parameter value (sequence 1 2 3 4 5 nanoscale carbide precipitation from 5), could be due to the annihilation (a) Processing sequence No. cryotreatment, in addition to strain of dislocations and recovery from mismatch between primary car- processes. The decrease in the S pa- 1.018 1.0168 bides and matrix from cryotreatment. rameter on tempering (sequence 2) 1.0140 The minimum strain of 1.06 (se- is expected due to a combined effect 1.014 quence 5) is attributed to carbide of precipitation and recovery. Cry- coarsening resulting from cryogenic 1.010 1.0088 1.0089 1.0088 otreatment on the same sample (se- c/a ratio treatment during subsequent tem- quence 3) produces a higher disloca- 1.006 pering. Strains for as-quenched and tion density from transformation of quenched plus cryotreated samples retained austenite to martensitic with 1 2 3 4 5 Processing sequence No. are similar but higher than those a corresponding increase in S param- (b) for the tempered samples. These eter. Tempering after cryotreatment Fig. 8 — Plots of calculated strain (a) and c/a ratio (b) by GSAS analysis. results indicate that the dislocation results in a lower dislocation density density is higher in as-quenched and than tempering before cryotreatment hardness, resistivity, and PIPAresults quenched plus cryotreated samples. (sequence 5). indicate that sequence 5 should have Photon induced positron annihila- higher toughness. Good wear resist- tion is used to identify the defect den- Summary and Conclusions ance requires a combination of hard- sity in materials[9]. Data in Fig. 9 in- There is no large difference in ness and toughness[6], which can be dicate the relative number of terms of hardness and yield strength achieved by cryotreatment followed dislocations present in the material. for processes sequences 2, 3, and 5, by tempering. All samples were normalized to the which simulate actual practices. Also, Cryotreatment produces two ef- HEAT TREATING PROGRESS • AUGUST 2007 59 fects: differential contraction between bination of hardness and toughness, matrix and primary carbides (in the but the effect is small and dif- 0.008 case of tempering before cry- ficult to quantify. otreating) and transformation of re- 0.004 tained austenite to martensite; and a Acknowledgement: Special thanks to C. Rideout, Positron Systems Inc., Boise, 4% volume expansion from austenite 0 Idaho, Fred Otto, Midwest Thermal-Vac, 1 2 3 4 5 transformation to martensite. Both Kenosha, Wis., Rick Diekman for cry- S parameter (normalized) -0.004 effects can induce more strain in the otreatment, and Prof. Sheldon Mostovoy Processing sequence No. material, which, in turn, may pro- for helpful discussions. Fig. 9 — S parameter normalized to as- duce more dislocations. Diffusion of received sample versus processing sequence References number. carbon atoms to dislocations reduces 1. R. Barron and R. Thompson, Effect of the overall energy of the system, and Cryogenic Treatment of Corrosion Resis- cipitations in the Wear Resistance Im- can occur to a limited extent at such tance, Advances in Cryogenic Engi- provements of Fe-12Cr-Mo-V-1.4C Tool a low temperature. However, signif- neering (Materials), Vol. 36, p 1375-1379, Steel by Cryogenic Treatment, ISISJ Intl., Vol. 34, No. 2, p 205-210, 1994. icant segregation of carbon to dislo- 1990. 2. R. Mehta, et. al., Phase Transformation 7. D. Yun, et.al., Deep Cryogenic Treat- cations occurs on warming back to and Residual Stress Analysis in Cryo- ment of High-speed Steel and its Mech- room temperature. Dislocations are treated Single Point Cutting Tool, Indian J. anism, Heat Treatment of Metals, Vol. 25(3), preferred sites for carbon to segre- of Engrg. & Matls. Sci., Vol. 2, p 95-102, 1995. p 55-59, 1998. gate (i.e., it has 0.5 eV binding energy 3. W. Fuming, et. al., Study of Microstruc- 8. J.Y. Huang, et. al., Microstructure of Cryogenic Treated M2 Tool Steel, Matl. compared with 0.4 with the other car- tures and Properties of Several Kinds of Hot Working Die Steel, J. of Shanghai Sci. & Engrg., Vol. A339, p 241-244, 2003. bides). Thus, small clusters of carbon Univ., Vol. 2, p 148-155, 1998. 9.C. Rideout, private communication, can precipitate upon heating. The 4. V. Dyncheako and V. Safronova, Cold Postitron Systems Inc., 2002. precipitates produced in this way are Treatment of Quenched Roll Steel, Soviet homogeneous and very fine. In J. of Heavy Machinery, p 44-48, 1993. For more information: Rajendra Kelkar, service, these precipitates can act as 5. S. Chatterjee, Performance Character- materials engineer, SSI Technologies (Sin- istics of Cryogenically Treated High tered Div.), 3330 Palmer Dr., Janesville, crack arresters and can increase the Speed Steel Drills, Intl. J. of Production, WI 53546; tel: 608-373-2814; fax: 608-755- toughness. Cryogenically treated ma- Vol. 30, p 773-786, 1992. 1004; e-mail: [email protected]; Web site: terial should have an improved com- 6. F. Meng, et. al., Role of Eta-carbide Pre- www.ssisintered.com

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