Metallurgical Factors Affecting Thermal Shock Cracking Resistance of Work Roll for Cold Strip Mills

Abstract

A frictional thermal shock simulator of drop weight type, by rubbing mild steelbar reproduces slipping accident occurres on cold strip mill work rolls and imposes a thermal shock on a specimen, was made and used to study thermal shock cracking phenomenon on high carbon high chromium hardened steel.
A thermal shock produces two heat affected layers below the roll surface, one is rehardened layer and the other is succeeding tempered layer. The maximum depth of crack (d_max) occurred in a thermal shocked area, as a criterion for the resistance to thermal shock cracking, correlates to the thickness of the tempered layer. The initiation site of thermal shock cracking is the upper position within the tempered layer where just below the boundary between these two layers. There seems to be coincidence of the position between above initiation site and a peak of tensile residual stress generated in the tempered layer.
When a martensitic specimen, with greater degree of supersaturation with carbon results from higher quenching temperature or lower subzero treatment temperature, is imposed a thermal shock on, d_max become larger.
While high temperature tempering decreases d_max. The degree of supersaturation with carbon of martensite is considered to be the governing factor of thermal shock cracking resistance.