Research On Austempering Toughening Process Of RM2High-strength Die Steel

High-speed upsetting process was not only highly automated and produced, also the forings were precisely sized and highly utilized, which was considered to be the development trend of rapid prototyping manufacturing in recent years. Hot work dies for high-speed upsetters serve under a mal-condition with high impact loads and entire water cooling, which were required with good combined strong and toughness properties. RM2steel was found to have high hardness, good tempering resistance, wear resistance and thermal stability which was achieved better effects for manufacturing high-speed upsetter dies. However the dies were easy to crack. because of poor toughness. In order to improve the working life, improving the toughness of RM2steel was regarded as the key to manufacture high-speed upsetter dies. Lower bainite characterized with both strength and toughness was heat-treated by austempering process to improve the toughness of RM2steel, which was best fitted by strength and toughness, in order to be used and solve the bottleneck problem of working life for high-speed upsetter.According to austempering process characteristics, SEM, EBSD, X-ray diffraction analysis equipments were used to analyse the effects of microstructure and mechanical properties by different austenitizing temperature, austempering temperature, austempering time and tempering temperature, and the best heat treatment process parameters were selected. The results are shown as belows.1) The best austenitizing temperature was selected as1150℃by grain and mechanical property curves of RM2samples after austenitized by different temperatures;2) Within260～330℃temperature range, the hardness of samples were decreased and the toughness were increased with the increase of austempering temperature. When austempering temperature reached at360℃, the bainite was turned to feathery upper bainite, retained austenite obviously decreased and the toughness decreased; with the increase of austempering time, the content of black needle lower bainite were gradually increased. The lower bainite were rapid increased when austempered at30-60min, then the content stabilized after60min, and the corresponding toughness curve was rapidly increased to gradually smoothed;3) The samples were fractured by cleavage fracture after austempered, and crack propagation were blockaded by lower bainite;4) High temperature tempering brittleness of RM2steel samples were appeared after tempered above400℃, thereby inhibiting tempering brittleness approaches were proposed, the best mechanical performances were tempered at200℃. When the RM2steel samples are austenitized by1150℃and austempered at330℃for60-180min, and tempered at200℃, the hardness reached50HRC which is similar to conventional high-speed upsetters, the toughness improved29percent to268.71J/cm2which is satisfied for the requirements of high-speed upsetter die. Applications of RM2steel on high-speed upsetter die for high impact load work are provided for theoretical basis.