Modelling of Dynamic Recrystallisation of 316L Stainless Steel using a Systems Approach

Dynamic recrystallisation (DRX) is an important aspect for industrial applications in hot metal working. Although DRX has been known for more than thirty years, its mechanisms have never been precisely investigated, in part because it was not readily possible to make local texture measurements. In the present work, the material behaviour during DRX is investigated and modelled based on the microstructure of 316L stainless steel. The developed model is based on a constitutive equation Modelling technique which incorporates the strain, strain rate and instantaneous temperature for predicting the flow stress of material being deformed under hot conditions. Introduction In metallurgy, dynamic recrystallisation (DRX) is a softening process that takes place during metal deformation at elevated temperatures. This softening is observed in a large number of metals and alloys and for numerous deformation processes. Careful analyses of the deformation behaviour, including microstructural investigations, have shown that the softening process involves discontinuous formation of new grains within the deformed matrix. During deformation, as the grains develop by nucleation and growth, the average dislocation density drops, leading to significant softening. This type of dynamic recrystallisation is associated with low or medium stacking fault energy metals (copper, silver, nickel, the austenite phase of conventional steels, and austenitic stainless steels). In this paper, a constitutive equation model has been developed for the prediction of the flow stress based on the deformation conditions, such as the strain, strain rate and the instantaneous temperature. The majority of laboratory rolling simulations use constant mean values of temperature and strain rate; i.e. constant Zener-Hollomon parameter (Z). However, in industrial rolling there are variations in strain rate and temperature that can produce deviations from equation of state conditions. Such transients in stress-strain relationships and microstructure can significantly affect subsequent processing and final properties. Industry now recognises that it must understand and incorporate transient behaviour if existing physically based models are to become truly predictive tools. This work aims to investigate the effects of controlled changes in strain rate on the instantaneous flow stress of commercial purity austenitic stainless steel (316L) using plane strain compression (PSC) testing. Dynamic Recrystallisation During deformation, work is carried out on the material which implies the storage of dislocations in the original structure. At the same time as work-hardening increases the dislocation density, the softening process of dynamic recovery takes place, diminishing the dislocation density (the more dislocations there are, the faster the recovery rate) [1]. Dynamic recovery is a thermally activated process and strain rate dependent: when the temperature rises, the time necessary for this process to occur is lower. This is the reason why the stress-strain curve is very dependent on the strain rate at high temperatures. The work hardening may be athermal, but dynamic recovery occurs from the beginning of the curve, and this explains why the first part of the stress-strain curve is temperature dependent. This is schematically shown in Fig. 1, where the microstructural evolution is illustrated together with the corresponding stress-strain curves. Fig 1. Schematic evolution of microstructural changes during deformation. 316L Stainless Steel Alloy Modelling In this study, an austenitic 316L stainless steel was selected for the investigation of the hot deformation behavior of a low stacking fault energy FCC metal. The choice of 316L stainless was motivated by the aim of gaining a better understanding of the occurrence of DRX in austenitic alloys. Furthermore, austenitic stainless steel alloys have relatively low stacking fault energy and have been investigated extensively for more than thirty years [2-4]. The chemical composition of the 316L stainless steel used in this investigation is given in Table 1. The commercial composition material has a stabilized microstructure with a grain size of about 100μm. Cr Ni C Mn Si Mo wt% 16.7 12.2 0.024 1.5 0.29 2.63 Table 1 Composition of the 316L stainless steel used in the present work. Plane strain compression (PSC) specimens with dimensions 50x30x10 mm were machined from the as-received material so that test deformation is parallel to the prior rolling direction. All specimens were coated with a thin film of DAG-2626 lubricant, which was allowed to dry before testing. PSC tests were carried out on a computer controlled servohydraulic machine. The specimens were heated to 1025 C and held for 60 seconds. They were then transferred to the test furnace at 1025 C and then deformed to the desired strains using constant or variable strain rates in the range 0.5, 5.0 and 50 s -1 . The specimens were water quenched immediately after deformation. The PSC tests were utilised to obtain equivalent true stress/equivalent true strain curves for different test conditions as illustrated in Table 2 cases 1-3. Furthermore, previous researchers’ data [5-6] (Table 2) were utilised to develop the model based on calculating the stresses at a strain of 0.1, σ0.1, and steady state σss. These stresses were read directly from the curves and were plotted against  ln (actual value for the strain of interest) for each point of strain and instantaneous temperature of deformation in order to interpolate the strain rates for constant stresses. The instantaneous temperatures during deformation were calculated using a finite difference package based on the dimensions of the samples and the deformation conditions [7]. The temperature rise during deformation is shown in Fig. 2 for the different deformation conditions. case Strain Strain rate (1/s) Temperature ( ̊C) Author 1 1 0.5 1025 2 1 5 1025 Abbod 3 1 5