Effect of Cooling Rate on Carbide Banding in High-Chromium Bearing Steel After Spheroidization
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DUCTION
BANDED microstructures are produced in hypoeutectoid steels through the microsegregation of alloying elements caused by dendritic growth during solidification. Dendritic solidification increases the concentration of alloying elements in the interdendritic regions (IRs). This phenomenon results in the low concentrations of alloying elements in the dendritic regions (DRs). This concentration difference indicates dendritic microsegregation.[1,2] Pokorny and Pokorny[3] showed that the alloying elements of Mn, Ni, and Cr cause the formation of pre-eutectoid ferrite in the DRs, whereas those of P, Si, and Mo cause the formation of pre-eutectoid ferrite in the IRs. The formation of bands is first explained with the microsegregation of P during solidification as reported by Stead.[4,5] Given that P raises the A3 temperature during cooling, the pre-eutectoid ferrite starts in P-rich zones. In addition, P-poor zones with high C concentration occur in the vicinity of P-rich zones. Thus, the alternating band structure with P-rich
HONG-YI WU and JUIi-CHAO are with the Department of Materials Science and Engineering, National Cheng Kung University, No. 1, University Road, Tai-nan 70101, Taiwan. e-mail: [email protected] YEN-HAO FRANK SU and GUAN-RU LIN are with the Steelmaking Process Development Section, China Steel Corporation, Kaohsiung, No. 1, Zhonggang Road, Kao-hsiung 81233, Taiwan. Manuscript submitted February 3, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
and P-poor zones is due to the P segregation at a slow cooling rate. Furthermore, Kirkaldy et al.,[6] Jatczak et al.,[7] and Bastien[8] proposed two mechanisms, namely, presegregation and trans-segregation. Presegregation indicates the microsegregation from the dendritic solidification to the start of the ferrite precipitation (i.e., the Ar3 temperature) during cooling. Trans-segregation refers to the segregation from austenite to ferrite and pearlite during solid-state transformation. Studies of banded microstructures in the past decade are briefly summarized in the following. Walker et al.[9] analyzed 100CrMnMoSi8-4-6 bearing steel and found that increasing the reduction ratio breaks up the dendritic microstructure but does not affect the peak segregation. Hays and Stemmler[10] observed that banding in maraging steel C350 is almost nil under the solution annealing condition, and bandwidth reveals a linear coarsening trend as a function of time. Caballero et al.[11] found that microstructural banding in cold-rolled dual-phase steels can be eliminated by increasing the cooling rate during hot rolling, lowering annealing temperatures, and prolonging soaking time. Deldar et al.[12] revealed that air-cooling results in the suppression of microstructural banding and in the formation of a fine microstructure for 1.4 wt pct Cu-bearing steel. Maalekian et al.[13] showed the simulation results that increasing the cooling rate and austenite grain size leads to banding disappearance as the transformation temperature shifts to low temperatures such that ferrite forms i
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