In Situ Observation of Bainite Transformation and Simultaneous Carbon Enrichment in Austenite in Low-Alloyed TRIP Steel
- PDF / 3,847,687 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 32 Downloads / 212 Views
HEAT treatment is one of the most fundamental processes in the manufacturing of metallic materials.[1–3] In particular, in steel industries, the researchers are focusing on austempering, which involves isothermal heating at approximately 673 K after cooling from a higher temperature. This process develops complex microstructures, including bainite and retained austenite, leading to the production of transformation-induced plasticity (TRIP) steels.[4] Hence, several in situ diffraction studies have focused on the behavior of austenite and the development of the bainite structure during austempering.[5–8] In general, a diffraction experiment
YUSUKE ONUKI and AKINORI HOSHIKAWA are with the Frontier Research Center for Applied Atomic Sciences, Ibaraki University, Tokai 319-1106, Japan. Contact e-mail: [email protected] TAKASHI HIRANO and SHIGEO SATO are with the Graduate School of Science and Engineering, Ibaraki University, Hitachi 316-8511, Japan. TOSHIRO TOMIDA is with the Planning Division, Ibaraki Prefectural Government, Tokai 319-1106, Japan. Manuscript submitted December 19, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
can provide the phase fractions and lattice parameter. The latter for austenite is particularly important, because it is the function of carbon concentration. The carbon enrichment in austenite is one of the most important controlling mechanisms of bainite transformation.[1] In addition, it affects the deformation behavior of the product as well as the fraction of austenite.[3] From these studies, the understanding of bainite transformation has progressed significantly during the past three decades. However, the knowledge obtained is not necessarily applicable to the industrial processes. The recent trend of industrial research is to focus on low-alloyed TRIP steels. The basic composition of low-alloyed TRIP steel is Fe-(1-2)Si-(1-2)Mn-(0.1-0.2)C in wt pct.[9] The bainite transformation of this type of steel is relatively fast. Therefore, austempering typically takes less than 1000 seconds. This situation is somewhat different from the lower bainite transformation of high-carbon (~ 0.6 wt pct) steels, reported by Rakha et al.,[7] Stone et al.,[8] and Nishijima et al.[6] These studies reported inhomogeneous carbon enrichment in austenite during austempering, which was observed as the asymmetrical irregular shapes of diffraction peaks for austenite. It should be confirmed whether the relatively fast reaction
in the low-carbon TRIP steel show similar behavior. Guo et al. found that during austempering, the untransformed austenite consisted of two different parts containing high (~ 1.2 wt pct) and low (0.2 to 0.4 wt pct) amounts of carbon.[5] They used Fe-0.91Si-1.37Mn1.54Cr-0.1Cu-0.14Mo-0.06Ni-0.22C. The higher value of the carbon concentration, 1.2 wt pct, was almost on the T0 line, where the Gibbs free energies for ferrite and austenite are the same. This appears somewhat contradict the idea suggested by Bhadeshia that the bainite transformation should stop at the T0¢ concentration, wh
Data Loading...
