Correlation Between the Microstructural Development of Bainitic Ferrite and the Characteristics of Martensite-Austenite
- PDF / 1,161,094 Bytes
- 5 Pages / 593.972 x 792 pts Page_size
- 57 Downloads / 272 Views
nsite-austenite (M-A) constituents are detrimental for the toughness properties in the heat-affected zone (HAZ) or in intercritically coarse-grained HAZ (ICCGHAZ) of high-strength low-alloy steel welds.[1–5] M-A itself acts as a microcrack or is debonded from the matrix, which results in microcracks at the interface. MA is the second phase formed with bainitic ferrite during the microstructural development under the thermal cycle of welding. Consequently, understanding the microstructural development of bainitic ferrite is important and should be related to the characteristics of the formed M-A. Numerous studies elucidated the microstructural characteristics of bainitic ferrite from the viewpoints of morphology and crystallography.[6–9] Furuhara et al.[6,7] showed the effect of the driving force on the crystallographic characteristics of bainitic ferrite in Fe-Ni-C alloys under isothermal transformation conditions. At higher transformation temperatures (i.e., under a weak driving force), laths of two or three variants (V1-V4-V8) with small misorientations often dominate the bainite microstructure. Takayama et al.,[9] who used statistical evidence of the variant pair fraction, reported the same tendency in low-carbon steel. With respect to the
HIDENORI TERASAKI, Associate Professor, and YU-ICHI KOMIZO, Professor, are with the Joining and Welding Research Institute, Osaka University, Osaka 567-0047, Japan. Contact e-mail: [email protected] Manuscript submitted May 10, 2013. Article published online October 1, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
morphology and characteristics of M-A in low-carbon steel under continuous cooling conditions, Lambert et al.[10] showed that small M-A’s are formed at high cooling rates along the simulated cycle of the CGHAZ. In ICCGHAZ, blocky M-A is formed at the prior austenite grain boundary (PAGB) and elongated M-A is formed inside the grains. When the cooling rate is slow, granular bainite and isolated austenite (not mixed M-A) are formed. However, few studies focused on the correlation of M-A formation with the microstructural development of bainitic ferrite are reported in the literature. In a previous study,[11] using high-magnification orientation image maps, we showed the relationship between the misorientation of bainitic ferrite and the characteristics of the M-A constituents. We found that if bainitic ferrite has numerous low-angle boundaries, then a blocky-type retained austenite (referred to as ‘‘isolated austenite’’ by Lambert et al.[10]) is observed and is not stable (it often contains martensite). This type of austenite development is referred as ‘‘type A.’’ However, if bainitic ferrite has few low-angle boundaries, then the retained austenite exhibits a rod-type shape and is stable. This type of austenite development is referred to as ‘‘type B.’’ However, the classification criterion is only the amount of low-angle boundaries and is not enough. In the present work, variant pair analysis of bainitic ferrite in the cases of types A and B is presented by co
Data Loading...
