Strategy for the design of thermomechanical processes for AISI type 304L stainless steel using dynamic materials model (DMM) stability criteria and model for the evolution of microstructure

Abstract

The dynamic materials model (DMM) [1] is a proven technique for studying constitutive behavior of materials. In this model, the efficiency of power dissipation through microstructural changes, given by η = 2m/(m + 1), where m is the strain rate sensitivity, is plotted as a function of temperature and strain rate to obtain a DMM processing map. The different domains exhibited by the map are correlated with specific microstructural processes occurring during hot working. Prasad [1] has shown that flow instability will occur during hot deformation if ξ (e) = {∂ ln[m/(m + 1)]/ ∂ ln e} + m < 0, where e, is the strain rate. The variation of the instability parameter, ξ (e) with temperature and strain rate, constitutes an instability map, which may be superimposed on the processing map for delineating the regimes of flow instability. The optimal domains predicted by the DMM processing maps are quite wide. In practice, in a wide domain it is very difficult to control the microstructure of the product. Hence, some refining procedure needs to be established for the precise control of the microstructure during working. Malas and Seetharaman [2] proposed stability criteria based on DMM. According to them the optimal processing windows for safe working are: 0 < m ≤ 1, ṁ < 0, s ≥ 1, and ṡ < 0, where ṁ = ∂m/∂ ln e, s = ∂ log σ/∂(1/T ), ṡ = ∂s/∂ log e and T = temperature in K . The apparent activation energy Q = sRT/m, where R = gas constant. In this methodology the reasonable “safe” processing range corresponds to the processing condition where a desirable and fairly constant value of Q is operative. These criteria can be used to refine the safe processing window to achieve better microstructural control during processing. In order to control the development of microstructure during hot working, a new strategy for systematically calculating near-optimal control parameters for hot deformation process has been proposed [3]. This approach involves developing state-space models from available material behavior and hot deformation process models. The control system design consists of two basic stages, and analysis and optimization are critical in both stages. In the first stage, the kinetics of certain dynamic microstructural behavior and intrinsic hot workability of the material are used, along with an approximately chosen optimality criterion, to calculate the optimum strain (e(t)), strain-rate (e(t)), and temperature (T (t)) trajectories for processing. A suitable process simulation model is then used in the second stage to calculate process control parameters, such as ram velocity, die profiles, and billet temperature, which approximately achieve the strain, strain-rate, and temperature trajectories calculated in the first stage. This process design approach treats the deforming material as a dynamic system and involves developing statespace models from available material behavior and hot deformation process models. The design approach requires three basic components for defining and setting up the optimization problem: (1) a dynamic system model, (2) physical constraints, and (3) an optimality criterion. The system models of interest are material behavior and deformation process models. Constraints include the hot workability of the workpiece and the limitations of the forming equipment. Optimality criteria could be related to achieving a particular final microstructure (grain size), regulating temperature, and/or maximising deformation speeds. Fig. 1 describes the steps involved in the proposed new approach [3]. The microstructure development optimization determines optimal trajectories of strain, strain rate, and temperature. From these optimal trajectories, the process optimization stage determines optimal process control parameters, namely the die shape, the ram velocity profile and billet temperature. Goals of the first stage are to achieve enhanced workability and prescribed microstructural parameters. In the second stage, a primary goal is to achieve the thermo-mechanical conditions obtained from stage one for predetermined regions of the deforming workpiece. In the first stage, models of material behavior that describe the kinetics of primary metallurgical mechanisms such as dynamic recovery, dynamic recrystallization, and grain growth during hot working are required for analysis and optimization of material system dynamics. The objective is to define the acceptable ranges of temperature and strain rate over which the material exhibits a “safe” processing window. The complete details of this approach are available elsewhere [3]. The aim of the present investigation is to evaluate the constitutive flow behavior of austenitic stainless steel