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In-Situ Observation of Acicular Ferrite Transformation in High-Strength Low-Alloy Steel Using Confocal Laser Scanning Microscopy

Lee Sang-In, Shin Seung-Hyeok, Park Hyeonwoo, Kim Hansoo, Lee Joonho, Hwang Byoungchul

Archives of Metallurgy and Materials

2022

Vol. 67, iss. 4

1497--1501

In-situ observation of the transformation behavior of acicular ferrite in high-strength low-alloy steel using confocal laser scanning microscopy was discussed in terms of nucleation and growth. It is found that acicular ferrite nucleated at dislocations and slip bands in deformed austenite grains introduced by hot deformation in the non-recrystallization austenite region, and then proceeded to grow into an austenite grain boundary. According to an ex-situ EBSD analysis, acicular ferrite had an irregular shape morphology, finer grains with sub-grain boundaries, and higher strain values than those of polygonal ferrite. The fraction of acicular ferrite was affected by the deformation condition and increased with increasing the amount of hot deformation in the non-recrystallization austenite region.

In-situ Observation of High-Temperature Fracture Behaviour of 347 Stainless Steel Subjected to Simulated Welding Process

Ko Seok-Woo, Park Hyeonwoo, Yoo Il, Kim Hansoo, Lee Joonho, Hwang Byoungchul

Archives of Metallurgy and Materials

2021

Vol. 66, iss. 4

1019--1022

In-situ study on the high-temperature fracture behaviour of 347 stainless steel was carried out by using a confocal laser scanning microscope (CLSM). The welding microstructures of the 347 stainless steel were simulated by subjecting the steel specimen to solution and aging treatments. Undissolved NbC carbides were present within grains after solution treatment, and M23C6 carbides were preferentially formed at grain boundaries after subsequent aging treatment. The M23C6 carbides formed at grain boundaries worked as stress concentration sites and thus generated larger cracks during high-temperature tensile testing. In addition, grain boundary embrittlement was found to be a dominant mechanism for the high-temperature fracture of the 347 stainless steel because vacancy diffusion in the Cr-depleted zones enhances intergranular fracture due to the precipitation of M23C6 carbides at grain boundaries.