Mechanical properties and corrosion resistance of steel X210CrW12 after semi-solid processing and heat treatment

Abstract

Semi-Solid Metal Processing (SSM) is one of the modern methods of steel forming. SSM of hot rolled X210CrW12 tool steel was conducted at 1250°C, in order to obtain about 30% of liquid phase. The microstructure consisted of globular austenitic grains surrounded by eutectic mixture (ferrite, austenite and M7C3 carbides). The average hardness of as-SSM samples was 470HV5, while the coefficient of abrasion resistance reached 7.9*10−6mm3/Nm. The compression strength of thixo-casts of X210CrW12 steel reached 4850MPa at the plastic strain of 34%, although the yield stress was only 810MPa. The X210CrW12 alloy just after thixo-casting revealed disadvantageously small wear resistance due to a large amount of metastable austenitic structure. A heat treatment consisting of 2h tempering at 555°C and additional low temperature tempering at 250°C led to the hardness increase up to 650HV5 as well as to the improvement of abrasion resistance to 1.8*10−6mm3/Nm. The compression strength and plastic strain decreased to 3140MPa and 8.9%, respectively, while the yield stress increased up to 2050MPa. Such very good mechanical properties were due to a complex microstructure consisting of globular grains containing the tempered martensite, fine perlite and M7C3 carbides surrounded by eutectic mixture. Tempering at 630°C for 2h led to the transformation of primary austenitic grains into perlite and carbides. It caused a decrease in hardness down to 495HV5 and the compression strength to 2300MPa, and the deterioration of abrasion resistance to 7.4*10−6mm3/Nm. The corrosion measurements revealed that the X210CrW12 steel directly after SSM was susceptible to the pitting corrosion in 0.1M sodium chloride solution. The corrosion resistance of thixo-casts after tempering at 555°C/2h slightly decreased compared with the as-thixoformed samples.