20 KHZ CRACK GROWTH RATE TESTING IN ADVANCED HIGH STRENGTH TOOL STEELS
Mohamed SadekGrand Ballroom A
High nitrogen chromium alloyed PM tool steels display, among other attractive properties, high strength and improved corrosion resistance. Also, powder metallurgy is a manufacturing process that allows the fabrication of near net shape complex geometries and high-quality components in an economical way. Nitrogen acts in an effective way replacing the carbon by the formation of hard carbonitride phases and permits higher amounts of chromium in solid solution. The low carbon and high nitrogen contents, where most of the carbon is replaced by nitrogen, suppresses the formation of metal carbides is in favor of the formation of metal nitrides (MX) and carbonitrides (M2X) allowing a higher nominal amount of chromium in solid solution. As a result of the PM rout well dispersed and fine distribution of nitrides and carbonitrides is created in a martensitic matrix. The microstructure obtained controls the mechanical properties of the steel grade, and a balance of high strength and high toughness is required. In the present study the influence of the nitride and carbonitride distributions on the fatigue properties, and in particular the stress intensity threshold and the early crack growth, was investigated using a 20 kHz ultrasound test equipment.