Abstract: Effect of different off/on-line quenching processes on microstructure and mechanical properties of the NM400 steel microalloyed with Nb, B and a small amount of Cr was researched. Through optical microscope, SEM and TEM observation, the evolution of martensite sub-structure by off/on-line quenching process was analyzed. The results show that the size of martensite in on-line quenching process is smaller and the proportion of large angle grain boundaries is larger, the crystalline defects such as deformation dislocation of rolled austenite are retained, which provides the location for the precipitation of carbides during tempering, makes the distribution of precipitates more dispersed, and has martensite-lower bainite duplex structure improving the hardness, strength and toughness of steel.

Key words: wear resistant steel, on-line quenching, hardness, martensite-lower bainite duplex structure