J-integral Fracture Toughness of High-Mn Steels at Room and Cryogenic Temperatures
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IGH-MANGANESE (Mn) steel has received considerable interest as a replacement for expensive stainless steels and nickel steels at cryogenic temperatures.[1–3] The lower cost and greater yield strength of high-Mn steel compared to those of stainless steel have urged researchers to find various applications for this material, particularly at cryogenic temperatures.[4,5] The production cost of high-Mn steels is expected to be approximately 30 pct lower than that of STS304.[6] A number of studies have been published on the deformation behaviors of high-strength, high-ductility, high-Mn steels and associated twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effects.[7–9] It has been well established that the stacking fault energy (SFE) of high-Mn steels affects the TWIP and TRIP effects, with the TWIP effect being dominant for steels with SFE values between 20 and 40
JUNHYEOK PARK, KWANHO LEE, HYOKYUNG SUNG, and SANGSHIK KIM are with the Department of Materials Engineering and Convergence Technology, ReCAPT, Gyeongsang National University, Jinju 52828, Republic of Korea. Contact e-mail: [email protected] YONG JIN KIM is with the The 4th R&D Institute, Agency for Defense Development, Daejeon 34186, Republic of Korea. SUNG KYU KIM is with the Steel Products Research Group, Technical Research Laboratories, POSCO, Gwangyang 57807, Republic of Korea. Manuscript submitted July 21, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
mJ m2 and the TRIP effect being more active for steels with SFE values below 20 mJ m2.[5,10] A combination of high tensile strength and high ductility is possible in high-Mn steels as newly formed mechanical twins act as barriers to dislocation movement while necking is restrained by the high work-hardening rate during TWIP-affected tensile deformation.[9,11] However, martensitic transformation tends to occur with decreasing temperature as the SFE value decreases below approximately 20 mJ m2.[4,12] Along with the excellent tensile properties of high-Mn steels at cryogenic temperatures, Charpy impact values of over 40 J have been reported at 196 °C.[13] Such high impact values of high-Mn steels are comparable to those of 304L stainless steel and 9 pct Ni steel, which are widely used as structural materials at cryogenic temperature. Currently, the fracture resistance of high-Mn steels has mostly been evaluated using Charpy impact tests because of experimental convenience. Only a few studies that are based on the concept of fracture mechanics are available on the fracture behavior of high-Mn steels.[14–16] The failure mode of high-Mn steel is normally ductile even at cryogenic temperatures because of the large plastic deformability resulting from the TRIP and/or TWIP effects.[17–19] The fracture resistance of ductile metals measured using V-notch impact tests may be misleading and differ from that measured using fracture mechanics tests using precracked specimen.[20] It is thus necessary to understand the fracture behavior of high-Mn steels at cryogenic temperatures under quasi-sta
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