Modeling the Flow Curve Characteristics of 410 Martensitic Stainless Steel Under Hot Working Condition
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TROLLED hot working is a practical technique for refining the microstructure of many industrial alloys, including stainless steels. Most grades of stainless steels are subjected to high-temperature thermomechanical treatment (HTMT) to modify their microstructure as well as to inhibit the precipitation of undesirable phases. The HTMT process often is applied through industrial hot forging of these steels. For martensitic stainless steels, HTMT is a useful technique to optimize the austenite structure prior to quenching to martensite. It is also worth noting that the finer the austenite grain, the finer the structure of martensite will be. It is well understood that dynamic recrystallization (DRX) is the restoration mechanism associated with the hot deformation of austenite in stainless steels. It often contributes in improving the final mechanical properties through refining the microstructure.[1–7] During hot deformation, DRX occurs once the accumulative stored energy reaches a critical value.[8] The softening caused by DRX results in an abrupt decrease in flow stress after the work-hardening stage, leaving a peak on the flow curve. Although the peak of AMIR MOMENI, Ph.D. Student, and KAMRAN DEHGHANI, Associate Professor, are with the Department of Mining and Metallurgy, AmirKabir University of Technology, PO Box 158754413, Tehran, Iran. Contact e-mail: [email protected] GOLAM REZA EBRAHIMI, Assistant Professor, is with the Metallurgy and Materials Engineering Department, Sabzavar Tarbiat Moallem University, PO Box 397, Sabzevar, Iran. HAMID KESHMIRI, M.D., is with the Esfarayen Steel Complex, PO Box 15896, Esfarayen, Iran. Manuscript submitted September 24, 2009. Article published online July 15, 2010 2898—VOLUME 41A, NOVEMBER 2010
the flow curve commonly is assumed as the onset of restoration process, DRX actually starts at a critical strain ec, which is 0.6 to 0.8 times the peak strain ep.[9] After the peak of flow curve, the rate of work softening increases to a maximum followed by a decrease until a steady state is reached. In the steady-state region, there is a dynamic balance between work hardening and flow softening.[9] Besides peak strain, the strain corresponding to the maximum rate of flow softening e* and the strain at the onset of the steady-state flow es are the important characterizing strains associated with a typical flow curve of DRX. It is well known that the DRX process takes place by the mechanism of nucleation and growth.[10,11] Therefore, its kinetics are often modeled by the Johnson– Mehl–Avrami–Kolmogorov (JMAK) equation. The original form of JMAK equation is as follows: X ¼ 1 expðktn Þ
½1
where X denotes fractional softening, t is time, and k and n are material constants. As the kinetics of DRX is a function of strain, some researchers have tried to establish a relation between the fractional softening X and the characteristic points of a flow curve expressed as follows[9]: e e n c X ¼ 1 exp 0:693 ½2 e Many researchers have used different constitutive equations to study the DRX behavio
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