Investigation On The Process And Performance Of Bulk Ultrafine Grained Austenitic Stainless Steel By Equal Channel Angular Pressing

Stainless steel is a very important structure material in the field of civil and military industries because of its excellent corrosion resistance and certain mechanical property. With the development of modern chemical engineering, household appliance, medical equipment and military industry, the new style stainless steel with higher mechanical strength and corrosion resistance would be a target for the research and development of stainless steel.Equal-Channel Angular Pressing (ECAP) is one of the most effective grain-refining methods. It is considered one of the most prospective techniques for industrialization application at present and has become a research focus in recent years. Up to now research work has been mainly focused on the pure copper, aluminum alloy, magnesium alloy, titanium alloy, pure iron with low deformation resistance and good forming ability. Meanwhile the research objects have been focused on the process and performance after ECAP. The materials with high deformation resistance and unusual work hardening like stainless steel have been less investigated by ECAP. Therefore the synthesizing process and performance of bulk ultrafine-grained 304 stainless steel by ECAP were investigated by this paper. The major works and conclusions are as follows.The ECAP process for bulk ultrafine-grained 304 stainless steel was designed. At first 304 austenitic stainless steel was pressed at room temperature by route BC of ECAP. The work-hardening and deformation induced martensite during ECAP of 304 stainless steel were reduced by annealing treatment after each ECAP pass. Based on this work, a new style pressing mold with a special heat device was designed to improve the synthesizing process. The 304 stainless steel was processed by ECAP at temperature about 300~500℃and 8 passes samples were obtained.The microstructure evolution of 304 stainless steel processed by ECAP from 0 to 8th pass was investigated by metallurgical microscope. The results demonstrated that the ECAP technology can refine materials indeed. Along with severe plastic deformation, the microstructure with equiaxed grains was not homogeneous anymore, the big equiaxed grains broke into a series of subgrains with fuzzy grain boundaries, and a great deal of slip bands appeared at intra-granular, the grains were elongated along certain direction to form texture. With the increase of pressing pass, the microstructure became more homogeneous and grains were more refined but grain refining rate got lower. By X-ray diffraction analysis, it was found that (111) texture was formed in the extruded specimens and the XRD peaks were broadened because of grain refinement and extrusion strain. The deformation-induced martensite appeared during the process for the increasing of surface energy and deformation stored energy which lead to higher driving force for transformation.Along with the increase of pressing pass, yield stress, tensile strength and yield ratio were significant increased after ECAP especially for the first pass. As compared to the as-received specimen, yield stress had increased from 300MPa to1040MPa with increasing extent of about 247%; tensile strength had increased from 690MPa to 1140MPa with increasing extent of about 65%; the yield ratio had increased from 0.43 to 0.91. The increase trend of these three mechanical properties got low as ECAP pass number increased. At the same time, cross-section shrinkage and elongation rate were significant decreased after ECAP especially for the first pass. As compared to the as-received specimen, cross-section shrinkage had decreased from 92.78% to 28.33% and elongation from 70.33% to 57.43%. The cross-section shrinkage and elongation rate tended to be stable with continuing ECAP.The hardness of stainless steel increased from primitive 187HV to 335HV after the first pass ECAP and the increase extent was about 79%. Hardness was slightly enhanced to about 374HV after the second pass press and kept at about 400HV with continuing ECAP.The hardness (HV) and tensile strength (σb) had a certain relationship for the stainless steel after ECAP, HV/σb≈0.3.In exploring the annealing technique to restore the plasticity while keeping the strength relatively constant, the hardness was found to have a rise at 200~300℃and then declined to its normal value as before annealing. When annealing temperature reached 600℃, the hardness reached 424HV. Afterward, the hardness got obvious decline when the temperature was higher than 600℃. Therefore, the annealing treatment to improve the plasticity without strength decrease for the ECAP specimens should be at 500~550℃for 10 min.The anodic polarization curves showed that the corrosion resistance of ECAP specimens got better than the solution treated specimen. They had higher open potential, lower corrosion current density and higher polarization electrical resistance. The SEM image for samples after anodic polarization demonstrated that ECAP specimens had better corrosion resistance than solution-treated specimen. The solution-treated specimen had obvious corroded pits on the surface while the second, fourth and eighth pass pressed specimens had homogeneously corroded surface with no obvious corrosion pits.