New processing routes of fabricating high-strength bainitic steels with high hardness and better corrosion resistance

The wear resistance and corrosion resistance of bainitic steels are highly required in railway transportation and construction machinery,and boronization and chromizing are common effective methods to improve surface hardness and corrosion resistance. In the existing research work,the austempering for obtaining bainite matrix and surface modification technologies for strengthening surface are carried out separately. The processing route is complicated and increases the production cost. New processing routes for fabricating medium-carbon high-strength bainite steels with high surface hardness and high corrosion resistance were proposed,which integrated the surface modification technologies and austempering. The new processes routes can not only simplify the processing treatment,but also reduce the production cost and environmental pollution. The integrated process of boriding/chromizing and austempering (boro-austempering/chro-austempering) were utilized to fabricate medium-carbon high-strength bainitic steel with high surface hardness and corrosion resistance. The effects of boro-austempering and chro-austempering processes on the microstructure and properties of a bainitic steel were compared and analyzed by means of microstructural observation,hardness test and corrosion experiment. The results show that,compared with only austempering process,extremely hard chromizing layer and boronization layer with Vickers hardness of over 1 500HV(about 3.3 times that of bainite matrix) were synthesized on the bainitic steel surface,and the corrosion performance in 0.5% NaCl solution was obviously improved by chro-austempering and boro-austempering treatments. After being exposed to 0.5% NaCl solution for 3 h,the boro-austempered steel yielded better corrosion resistance. A new type of high strength bainitic steel with bainite as substrate and produced layers on the surface can be fabricated by two integrated processes. Under the same treating time,compared with the chro-austempering process,the boro-austempered steel showed a faster growth rate of the boronization layer and the higher adhesion strength between the boronization layer and the bainite steel matrix.