Jong-Kyo Choi

Deformation behavior of duplex austenite and ε-martensite high-Mn steel

Abstract Deformation and work hardening behavior of Fe–17Mn–0.02C steel containing ε-martensite within the austenite matrix have been investigated by means of in situ microstructural observations and x-ray diffraction analysis. During deformation, the steel shows the deformation-induced transformation of austenite → ε-martensite → α′-martensite as well as the direct transformation of austenite → α′-martensite. Based on the calculation of changes in the fraction of each constituent phase, we found that the phase transformation of austenite → ε-martensite is more effective in work hardening than that of ε-martensite → α′-martensite. Moreover, reverse transformation of ε-martensite → austenite has also been observed during deformation. It originates from the formation of stacking faults within the deformed ε-martensite, resulting in the formation of 6H-long periodic ordered structure.

Factors influencing fatigue crack propagation behavior of austenitic steels

In the present study, the fatigue crack propagation (FCP) behaviors of austenitic single phase steels, including STS304, Fe18Mn and Fe22Mn with different grain sizes ranging from 12 μm to 98 μm were investigated. The FCP tests were conducted in air at an R ratio of 0.1 using compact tension specimens and the crack paths and fracture surfaces were documented by using an SEM. The highest ΔKth value of 9.9MPa·m1/2 was observed for the Fe18Mn specimen, followed by 5.2MPa·m1/2 for the Fe22Mn specimen and 4.6MPa·m1/2 for the STS304 specimen, showing a substantial difference in the near-threshold FCP resistance for each microstructure. The crack path and fractographic analyses suggested that the near-threshold FCP behavior of these austenitic steels was largely influenced by the degree of slip planarity, as determined by stacking fault energy and grain size, rather than the tensile properties. In the Paris’ regime, the slip planarity still played an important role while the tensile properties began to affect the FCP. The FCP behavior of austenitic steels with different microstructural features are discussed based on detailed fractographic and micrographic observations.

In-situ neutron diffraction analysis on deformation behavior of duplex high Mn steel containing austenite and ɛ-martensite

The deformation behavior of Fe-17Mn-0.02C steel containing ɛ-martensite within austenite matrix has been investigated via in-situ neutron diffraction study at 298 K and 77 K. Based on the analyses of changes in phase fraction and lattice strain, it has been shown that the steel shows the deformation-induced phase transformation of austenite → ɛ-martensite → α′-martensite and the direct transformation of austenite → α′-martensite at both temperatures. However, the kinetics of such transformations vary with temperature, resulting in a higher and more persistent work hardening at 77 K than at 298 K.

Pearlite morphology in the hypereutectoid steels

Abstract The variation of pearlite morphology with carbon content in hypereutectoid steels was examined analytically. It was shown that the cementite thickness decreases with increasing carbon content if (S/S°) (C°/C). In addition, the significance of these conditions on improving the drawability of the hypereutectoid steels was discussed.

Evaluation of solidification cracking susceptibility of Fe–18Mn–0·6C steel welds

Abstract Solidification cracking susceptibilities of high Mn steel welds were evaluated in the present study. A longitudinal Varestraint technique was utilised to assess the solidification cracking behaviours of the fusion zone. High Mn steel welds were more susceptible to solidification cracking than 304 and 202 austenitic stainless steel welds, however, they were less susceptible than 310S austenitic stainless steel welds. Extensive segregations of Mn and C took place at the dendritic and grain boundaries in the weld metal, and accordingly contributed to the increase of the hot cracking susceptibility of high Mn steel by the enlargement of solidification temperature range. Further, continuous γ-(Fe,Mn)3C eutectic phases formed at 1090°C along the grain boundary primarily resulted in the increase of solidification cracking sensitivity in high Mn steel.