Effect of heat treatment on structure and properties of B alloyed high speed steel for rolling mill rolls

Solidification structure of two kinds of B alloyed high speed steel for rolling mill rolls, which carbon contents are 0.50 % and 1.00 %, respectively, and the changes in the microstructure and mechanical properties after quenching from 1050◦C and tempering at 200–600◦C, respectively, are studied. The results of metallographic, scanning electron microscopy analysis, X-ray diffraction analysis, hardness and impact toughness measurements of as-cast and heat-treated B alloyed high speed steel are discussed. As-cast matrix of B alloyed high speed steel consists of martensite and troostite. There are 19–26 vol.% M23(B, C)6, M3(B0.7C0.3) and M2(B, C) type carboborides in the matrix. Microhardness values of M23(B, C)6, M3(B0.7C0.3) and M2(B, C) are 1940–2030 HV, 1380–1460 HV and 1460–1530 HV, respectively. Macrohardness of as-cast B alloyed high speed steel reaches 58–60 HRC. After quenching from 1050◦C, the eutectic M23(B, C)6 and M3(B0.7C0.3) carboborides dissolve into the matrix, and many granular M23(B, C)6 precipitate from the matrix, and the whole matrix transforms into martensite. Microhardness of matrix and macrohardness of quenched B alloyed high speed steel have a slight increase comparing with as-cast sample. Hardness of the B alloyed high speed steel remains constant while tempering temperature is below 550◦C, and then, with increasing temperature, decreases considerably. Impact toughness of the B alloyed high speed steel increases slightly until tempering temperature reaches 525◦C, and then increases considerably. B alloyed high speed steel has higher hardness and has an excellent wear resistance after tempering under 550◦C that can be attributed to the effect of boron. K e y w o r d s: high speed steel for rolling mill roll, B alloyed steel, carboboride, quenching, tempering, hardness, impact toughness