The Research Of Plastic Instability On Austenitic Stainless Steel During Hot Deformation

When the nonuniform strain which occurs in hot deformation of Austenitic stainless steel is concentrated in some region, it can cause strain localization and lead to local geometry instability on the surface of workpiece, and even cracking. In order to study the phenomenon, this paper, the Cr17Mn6Ni4Cu2N, Cr15Mn9Cu2Ni1N and304austenitic stainless steel are taken as the main research materials. The tensile instability characteristics of austenitic stainless steel and the evolution mechanism of surface strain localization have been investigated by the high temperature tenile test and high temperature tensile unloading test and the finite element analysis method which material model is defined by user. The influence of deformation condition on geometry instability and strain localization evolution rule have been investigationg by the hot deformation tests which deformation temperature, strain rate and stress state were changed. Have cognized the process and mechanism of geometry instability in high temperature tensile, obtained the geometry instability trajectory austenitic stainless steel in various stress states, put forward the model of strain localization evolution in high temperature, determined the relation between material parameters which have relationship with deformation conditions and geometry instability strain and given the processing conditions to reduce strain localization in high temperature deformation.In the condition of high temperature deformation, it make the load instability and geometric instability do not occur at the same time in uniaxial tensile that the strain rate sensitivity of Austenitic stainless steel increase. As well, because of the reason strain rate sensitivity increase, it restrain the deformation of geometry instability region that the deformation resistance increase with the strain rate in potential geometry instability region which is formed in the specimen after the load instability and make it transfer to other positions. This maintains the sample homogeneous deformation and delay the geometric instability occurence.The occurrence of geometry instability relates with the maximum length Lmax and the maximum depth dmax of strain localization region. In950℃, the Lmax and dmax respectively is9.8mm and0.56mm when the geometry instability occur and in1150℃, the Lmax and dmax respectively is11.1mm and0.82mm when the geometry instability occur. Under the influence of elongation and deformation temperature, the number, the maximum length and the maximum depth of strain localization zone of austenitic stainless steel in high temperature tensile. With the increase of elongation, the number is first increases then decreases and the Lmax and dmax is increases. With the decrease of deformation temperature, the number is decreases and the Lmax is increases and dmax is increase.There are "active" and "sleep" two kinds of strain localization region in austenitic stainless steel surface in deformation tensile. In fact, the evolution of strain localization is the process that the "sleep" strain localization region is divided and merged by "active" strain localization region. With the temperature rise, the process is more obvious. The model that is built by the mechanism that two "active" strain localization regions merger "sleep" strain localization regions at the same time by competition showed it is determined by deformation resistance which one "active" strain localization region is able to merge the "sleep" strain localization around the region in the effect of strain rate sensitivity and the deformation resistance is greater and its ability stronger to merger. The deformation resistance depends on the size of the cross-sectional area of strain localization region and the amount of cross-sectional area to reduce.In950-1200℃temperature range, geometry instability strain of austenitic stainless steel is increase with temperature rise and in0.01to10s-1strain rate range, geometry instability strain of austenitic stainless steel is increase with strain rate reduce. Through the analysis of the relationship between material parameters and instability strain, found that strain hardening parameter index n control the load instability strain and strain rate sensitivity coefficient m control the strain from load instability to the geometric instability. This strain from load instability to the geometric instability is the main part of geometric instability strain, and is increase with the increase of m value. The relationship between the geometry instability strain and and material parameters isε1=an+bnm, and among themα,,=-0.153+1.268n, bn=4.167+5.389n. The relationship between、m valuesand load instability strain and the strain from load instability to geometric instability respectively isεn=en-1, ε1-εn=an+bnm-en-1.In0.1-2.5s-1strain rate range, load instability and geometrical instability is separation in the process of geometry instability, and geometric instability and the load instability occurs at the same time in10s-1.The evolution mechanism of strain localization has also changed with the strain rate change. At low strain rate, there are a few "active" strain localization regions in specimen, and they merger "sleep" localized region by competition, however, at high strain rate, there are only one "active" strain localization region in specimen to merger "sleep" localized region. To analyse the material’s parameter in various strain rate, found there are strain strengthening and strain rate strengthening at low strain rate and there are only strain strengthening at higher strain rate in austenitic stainless steel.The trajectorys of geometry instability in various triaxial stess degree7and Lode parameter μ6show that the geometry instability strain is first increases then decreases with η and μ6value increase and the maximum geometry instability strain is not in where η value equals to0.333and μ6value equals to-1. The increase of nonuniform of stress and deformation in specimen is the main reason that geometry instability strain decrease with with77and μ6value increase. With η and μ6value increase, the form of geometry instability change by load instability and geometrical instability separation to geometric instability and the load instability occurs at the same time. With77and μ6value increase, the gap that was taken as strain localization region change from "active" strain localization regions to the "active" strain localization region that is fixed inside the gap or direct geometric instability region.In this paper, the geometry instability process and evolvement mechanism of strain localization of austenitic stainless steel has benn carried on the comprehensive research and analysis in high temperature. The work provides experimen and theoretical foundation to recognize the plastic instability of matrial in high temperature, and expect the results of the work can help to improve the manufacturing technique in thermal deformation.