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

摘要

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℃ and tempering at 200-600℃, 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.% M_(23)(B, C)_6, M_3(B_(0.7)C_(0.3)) and M_2(B, C) type carboborides in the matrix. Microhardness values of M_(23)(B, C)_6, M_3(B_(0.7)C_(0.3)) and M_2(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℃, the eutectic M_(23)(B, C)_6 and M_3(B_(0.7)C_(0.3)) carboborides dissolve into the matrix, and many granular M_(23)(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℃, and then, with increasing temperature, decreases considerably. Impact toughness of the B alloyed high speed steel increases slightly until tempering temperature reaches 525℃, and then increases considerably. B alloyed high speed steel has higher hardness and has an excellent wear resistance after tempering under 550℃ that can be attributed to the effect of boron.