Modeling Microstructure Evolution in a Martensitic Stainless Steel Subjected to Hot Working Using a Physically Based Model

Abstract

The microstructure evolution of a martensitic Stainless steel subjected to hot compression is simulated with a physically based model. The model is based on coupled sets of evolution equations for dislocations, vacancies, recrystallization, and grain growth. The advantage of this model is that with only a few experiments, the material-dependent parameters of the model can be calibrated and used for a new alloy in any deformation condition. The experimental data of this work are obtained from a series of hot compression, and subsequent stress relaxation tests performed in a Gleeble thermo-mechanical simulator. These tests are carried out at various temperatures ranging from 900 to 1200°C, strains up to 0.7, and strain rates of 0.01, 1, and 10 s À1. The grain growth, flow stress, and stress relaxations are simulated by finding reasonable values for model parameters. The flow stress data obtained at the strain rate of 10 s À1 were used to calibrate the model parameters and the predictions of the model for the lower strain rates were quite satisfactory. An assumption in the model is that the structure of second phase particles does not change during the short time of deformation. The results show a satisfactory agreement between the experimental data and simulated flow stress, as well as less than 5 pct difference for grain growth simulations and predicting the dominant softening mechanisms during stress relaxation according to the strain rates and temperatures under deformation.

Study of the mean size and fraction of the second-phase particles in a 13% chromium steel at high temperature

“…The model used to analyse our experimental results were described in detail elsewhere [11,13] and for example, it can predict the flow stress based on the dislocation density evolution during plastic deformation. The dislocation densitydependent flow stress in the model is stated as follows:…”

“…Generally, in modelling it is common to assume that the particles are uniformly distributed spheres of radius r; the same concept was adopted in this work as well. All these calculations for mean size and volume fraction are based on the assumption that, due to a relatively short deformation time, the fraction and size of carbides do not change considerably before and after the hot compression tests, from which the values of σ 0 and the f/r ratio were determined [13].…”

“…Both σ 0 , i.e. the strengthening effect from the carbides, and f/r ratio were obtained for the test material in another work [13], and the results are summarised in Table 2.…”

“…At this stage, only particle size increases further by coarsening and the fraction remains constant during an isothermal heat treatment. These variations in fraction and size will affect the particle spacing, which is an important parameter in calculating the Orowan strengthening mechanism [9] and calculating the flow stress [10][11][12][13]. Therefore, the knowledge about the size and fraction of precipitations would be crucial to adjust the final microstructure of the product as well as the process parameters especially those related to the flow stress modelling.…”

“…In this study, the existing second-phase particles of a 13% chromium steel at the temperatures from 850 to 1200°C are investigated and the results are compared with those obtained from the model used to simulate the flow stress for this alloy [13]. This comparison is made assuming that any change in the size and fraction of the carbides is negligible due to the short time of deformation, i.e.…”