Abstract
This study investigated the softening mechanisms of the AISI 410 martensitic stainless steel during torsion simulation under isothermal continuous in the temperature range of 900 to 1150 °C and strain rates of 0.1 to 5.0s-1. In the first part of the curves, before the peak, the results show that the critical (ec) and peak (ep) strains are elevated for higher strain rate and lower temperatures contributing for higher strain hardening rate (h). Moreover, this indicated that dynamic recrystallization (DRX) and dynamic recovery (DRV) are not effective in this region. After the peak, the reductions in stresses are associated to the different DRX/DRV competitions. For lower temperatures and higher strain rates there is a delay in the DRX while the DRV is acting predominantly (with low Avrami exponent (n) and high t0.5). The steady state was reached after large strains showing DRX grains, formation of retained austenite and the presence of chromium carbide (Cr23C6) and ferrite δ at the martensitic grain boundaries. These contribute for impairing the toughness and ductility on the material. The constitutive equations at the peak strain indicated changes in the deformation mechanism, with variable strain rate sensitivity (m), which affected the final microstructure.
Highlights
During hot forming processes, stainless steels are subjected to variations in temperature and strain rate, generating changes in the mechanical and microstructural behavior of the alloy[1,2,3]
The stresses increase until reach a limit. This region presents a high level of work hardening, in conditions of low temperature and high strain rate, where the plastic flow curves become steeper
The focus of this study was the thermomechanical behavior of AISI 410 martensitic stainless steel in isothermal continuous hot torsion tests
Summary
During hot forming processes, stainless steels are subjected to variations in temperature and strain rate, generating changes in the mechanical and microstructural behavior of the alloy[1,2,3]. The main hot working processes are forging and rolling, followed by heat treatment, in which the material is subjected to thermal cycles under variable strains in order to improve their mechanical properties[3] In this thermomechanical cycle, materials initially undergo a hardening process, followed by slow dynamic recovery and predominantly dynamic recrystallization, which are dictated by the stacking-fault energy (SFE) and the applied stain conditions[4]. Stainless steels tend to soften by dynamic recrystallization (DRX), after a certain amount of deformation[1,5,6] This threshold deformation, which is known as the critical strain (εc), corresponds to the minimum deformation necessary for the onset of DRX, and the stress corresponding to this strain is called critical stress (σc)[7,8,9,10].
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