Effect of surface conditions on generation of stress and strain in quenched steel plates

Abstract

An investigation has been undertaken into the relationships between surface roughness, surface heat transfer coefficients, and thermal stress and strain generated during the quenching of 30 mm thick steel plates in water. Variation in surface roughness influenced the length of the vapour blanket stage and the magnitude of the surface heat transfer coefficient during subsequent nucleate boiling. The latter increased as the surface became smoother, but the former was shorter at intermediate levels of roughness (400 grade) than with specimens that were either smoother (600 grade) or rougher (120 grade). The effect of variations in the duration of the vapour blanket stage had little effect on the residual strain at the end of the quench, although the increase in quenching rate during nucleate boiling produced by a decrease in the surface roughness significantly increased this strain at all points in the plate. In contrast, neither effect produced a significant change to the stress distribution at the end of the quench, which always showed a maximum tensile stress of about 300 MN m⁻² just below the surface and a maximum compressive stress of −200 MN m⁻² at the centre. The results indicate that minor damage to a component surface during the hardening of low-alloy steels will not cause serious modifications to the residual stress distribution, although significant and unexpected changes to the residual strain distribution might be produced by such effects. The removal of the vapour blanket aids the formation of martensite as opposed to decomposition products produced by diffusion processes. The results indicate that such a procedure would not seriously impair the residual stress distribution.

MST/531