Investigation Of Grain Boundary Character Distributions In Austenitic Stainless Steel

Optical microscopy (OM) and off-line in-situ observation methods were used at first to study the behaviors of carbides precipitation and dissolution in type 304 austenitic stainless steel. Then, the effects of carbides on the grain boundary character distribution (GBCD), under two types of thermal mechanical processing (TMP) of grain boundary engineering (GBE), were investigated by means of scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Finally, the mechanism of GBCD evolution during TMP was clarified by off-line in-situ EBSD techniques. The main contributions are summarized in following.1) Carbides precipitation is going on till the carbon consumed over when the after solid-solution-treatment type 304 austenitic stainless steel is aged at the temperatures between 650℃～900℃. During ageing, carbides precipitate at first on the grain boundaries of high energy and then on those of lower energy and even inside the grains. When the after-aged samples are heated at 940 ℃, the stability of carbides is decreased and most parts of the carbides are dissolved back into the matrix; when heated at the temperatures higher than 980 ℃, carbides are dissolved completely back into the matrix. In this case, the migration of grain boundaries as well as the evolution of microstructures are active due to the un-pinning of carbides.2) The effects of carbides on GBCD in type 304 austenitic stainless steel are quite different under two types of GBE processing (GBE1 and GBE2). In the case of GBE 1, carbides can promote the activities of conventional recrystallization which is featured with the formation of random high angle grain boundaries (RHABs), and thus negative to the optimization of GBCD. However in the case of GBE2, small amount of carbides can only pin some grain boundaries while the grain boundaries free of carbides can migrate extensively during annealing and it is positive to the optimization of GBCD, implying in the meantime of enhancing the fraction of special grain boundaries the size of Σ3n grain clusters (Σ3n GCs) increased drastically as well as the connectivity of RHABs networks interrupted sufficiently by the special grain boundaries.3) Those grain boundaries free of carbides, which can migrate extensively during the GBE2 processing, are usually characteristic of specific misorientations. Such grain boundaries can yield specific deformation sub-structures and strain gradient in the surroundings when the sample is subjected to small-strain cold rolling, it is good to the so-called strain-induced boundary migration (SIBM) which is accompanied by annealing twinning during the subsequent annealing. This is the main contributor to the formation of high-fraction special grain boundaries during GBE2 processing. Most of the grains bounded by such grain boundaries are oriented to Σ3n relationships, the extensive migration and join-up of such grain boundaries not only contribute to the increment of special grain boundaries but also promote the formation of larger-sized Σ3n GCs as well as the effective interruption of RHABs networks by special grain boundaries.