Processing Map and Hot Deformation Characteristics of 21Cr-11Ni-N-RE Lean Austenitic Heat-Resistant Steel

Abstract

Hot deformation behavior of 21Cr‐11Ni‐N‐RE lean austenitic heat‐resistant steel is investigated within the temperature of 1173–1473 K and the strain rate of 0.01–10 s−1. Hot deformation equation and prediction modeling of peak stress and strain are obtained. Based on the dynamic material model (DMM) theories, the processing maps are developed. The results show that at lower strain (0.3), the efficiency of power dissipation (η) increases with increasing temperature and decreasing strain rate; however at the strain of 1.0, η exhibits an obvious decrease at 1373–1473 K and 0.01 s−1, in which significant coarsening of grains is found. At large strain (1.0), the optimum hot working domain of test steel should be at 1423–1473 K and strain rate of 1–10 s−1, in which more uniform and finer microstructure is formed due to the complete dynamic recrystallization (DRX). The average grain diameter of recrystallized grains shows a power law relationship with Z. The larger the Z‐value is, the finer the grains are. Besides, the unstable hot working regions are delineated in the processing maps using Prasad instability criterion. As predicted, twinning bands of flow localization and localized kinking observed at 1173–1223 K and 1–10 s−1 should be responsible for the flow instability.

Hot Deformation Behavior and Microstructural Evolution of High Nitrogen Martensitic Stainless Steel 30Cr15Mo1N

“…As the deformation temperature is increased to 1000 °C, the microstructure of specimen (Figure c) consists of few precipitates, elongated grains, and limited fine grains. This phenomenon demonstrates that DRX is just beginning under such deformation condition . When the deformation temperature is further increased to 1100 °C, precipitates disappear and typical necklace‐type microstructure (Figure e) can be observed, revealing that grain boundary bulging through strain‐induced grain boundary migration is the dominant nucleation mechanism of DRX, and higher temperature promotes the DRX process.…”

“…It can be seen that the peak and steady‐state stresses increase with decreasing temperature and increasing strain rate, and the flow stress decreases dramatically when deformation temperature is increased from 1050 to 1100 °C (Figure a). The reason is that low strain rate can provide enough time for energy accumulation, and the high deformation temperature can accelerate dissolution of precipitates, dislocation movement, and grain boundary migration for nucleation and growth of DRX grains, thus lowering the stress level . At the initial stage of deformation, the rapid increase in flow stress is a result of work hardening caused by the increment of dislocation density .…”

“…Based on previous studies, the power law Equation and exponential law Equation are suitable for low stress and high stress level, respectively. The hyperbolic‐sine type Arrhenius Equation is suitable for entire stress levels.…”

“…Consequently, it is useful to investigate the hot deformation behavior of the steels for the design and optimization of hot forming processes. Arrhenius‐type constitutive model, which takes into account the effects of strain, strain rate, and temperature on flow stress is a powerful tool to describe high temperature deformation behavior of alloys . Moreover, the constitutive model can be used in computer code to model materials response under specified loading conditions.…”

Constitutive Equation and Hot Processing Map of a Nitrogen-Bearing Martensitic Stainless Steel

“…True stress increased with increasing strain rate ( Figure 4a) under a given deformation temperature, while it decreased with increasing deformation temperature at a given strain rate ( Figure 4b). This is because the higher temperature increases the vacancy diffusion rate, the mobility of dislocation and grain boundary which are beneficial for the nucleation and growth of DRX grains, and the lower strain rate can provide a longer time for energy accumulation thus reducing the true stress [25]. According to Figure 4, the shape of the true stress curves can be roughly divided into two groups: (1) at the lower strain rates (less than 1 s −1 ), true stress increased to a peak at small strain, after which it continuously decreased, which indicated the occurrence of DRX [26]; (2) at the strain rates no less than 1 s −1 , true stress increased rapidly at the initial deformation stage and then gradually reached a steady state without any notable peak stress, which was a result of the dynamic recovery (DRV) mechanism.…”