Elsevier

Materials & Design

Volume 31, Issue 10, December 2010, Pages 4577-4583
Materials & Design

The rate of dynamic recrystallization in 17-4 PH stainless steel

https://doi.org/10.1016/j.matdes.2010.05.052Get rights and content

Abstract

The hot working behavior of 17-4 PH stainless steel (AISI 630) was studied by hot compression test at temperatures of 950–1150 °C with strain rates of 0.001–10 s−1. The progress of dynamic recrystallization (DRX) was modeled by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) kinetics equation. The flow softening was directly related to the DRX volume fraction and the DRX time was determined by strain rate. For quantification of recrystallization rate, the reciprocal of the time corresponding to the DRX fraction of 0.5% or 50% was used. Analysis of the sigmoid-shaped recrystallization curves revealed that the rate of DRX increases with increasing deformation temperature and strain rate. The Zener-Hollomon parameter (Z) was found to be inappropriate for analysis of DRX kinetics. Therefore, the dynamic recrystallization rate parameter (DRXRP) was introduced for this purpose. The DRXRP may be determined readily from the Avrami analysis and can precisely predict the rate of DRX at hot working conditions.

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:X=1-exp(-ktn)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

References (27)

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