The rate of dynamic recrystallization in 17-4 PH stainless steel
Introduction
Dynamic recrystallization (DRX) is an important phenomenon for controlling microstructure and mechanical properties in hot working of many metallic materials [1], [2], [3], [4], [5], [6]. In some materials such as aluminum, dynamic recovery (DRV) can balance work hardening, and a plateau is achieved. However, in many materials such as austenite phase in steels, the kinetics of DRV is low, and DRX can be initiated at a critical condition of stress accumulation. Due to the great impact of DRX on the high temperature flow stress and its effect on the microstructure and properties of the material after processing, the evaluation of the rate and progress of DRX in terms of deformation conditions is important. The rate of DRX depends on the chemical composition of the material, mode of deformation, and the deformation conditions.
17-4 PH is more common than any other type of precipitation hardening (PH) stainless steels. Its ability to develop very high strength without the catastrophic loss of ductility and its superior corrosion resistance to other steels of similar strength, have made it very attractive to engineers [1], [7]. Industrial hot deformation processing such as forging for this steel is conducted in the temperature range of stability of austenite phase. Due to low stacking fault energy of austenite in the 17-4 PH stainless steel, the major restoration process during hot deformation is dynamic recrystallization (DRX).
The DRX may be considered as a solid state transformation and its kinetics can be modeled by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation as follows:where X is the recrystallized volume fraction. According to the definition of strain rate, the DRX time (t) can be expressed as a function of strain. The recrystallized volume fraction (X) could be directly measured from the metallographic images [8], [9], [10] or EBSD maps [11]. However, the flow softening during DRX can also be related to X. As a result, the DRX volume fraction (X) can be expressed as a function of flow stress, which may be employed in many different ways. For example, in some research works, the softening from the peak to the steady-state stress was considered for calculation of X [12], [13]. The flow curve and strain hardening data were also used to quantify the progress of DRX through the relationship between the flow stress and dislocation density [14], [15], [16]. In another approach, the softening of DRX has been related to the difference between the imaginary flow curve that does not undergo DRX softening (i.e. DRV plus work-hardening flow curve) and the experimental flow curve (i.e. DRX, DRV plus work-hardening one) [17], [18].
In the present work, the rate and the progress of DRX in a 17-4 PH stainless steel was studied during hot compression test.
Section snippets
Experimental materials and procedures
The 17-4 PH stainless steel with chemical composition of 0.03 wt.% C–15.14 wt.% Cr–4.53 wt.% Ni–3.4 wt.% Cu–0.25 wt.% Nb was used in this work. The chemical composition of the experimental alloy also falls within the chemical composition of the 15-5 PH stainless steel. The Rastegaev design [1] was used for hot compression specimens (Fig. 1a) to hold glass powder as a lubricant material at the contact surface of anvils and specimen. A Baehr DIL-805 deformation dilatometer was used for hot compression
Effects of deformation conditions on flow curves
Flow curves obtained at different temperatures and strain rates are shown in Fig. 2. Most of the curves exhibit typical DRX behavior with a single peak stress followed by a gradual fall towards a steady-state stress. However, the peak becomes less obvious when the strain rate increases or the deformation temperature decreases.
The drop in flow stress with deformation temperature may be attributed to the increase in the rate of restoration processes and decrease in the strain hardening rate.
Conclusions
- (1)
The stress–strain curves of 17-4 PH stainless steel exhibited typical DRX behavior with a single peak stress followed by a gradual fall towards a steady-state stress.
- (2)
The rate of DRX increases with deformation temperature. By increasing the deformation temperature, the recrystallization curve shifts to lower strains and recrystallization times.
- (3)
By increasing the strain rate, the recrystallization curve shifts to higher strains. However, when the strain divided by strain rate, results in shorter
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