Influence of Steel Grade and Degree of Hardening on Quenching Stresses

Abstract

The quenching stresses of 18mmφ commercial steel cylinders were measured by the Sachs’ method utilizing electric spark machining. The influences of carbon content of steel and of the bulk of unhardened core upon these stresses were investigated. The results area summarized as follows: (1) The stress distribution of water-quenched, through-hardened steels were of thermal stress-transformational stress superposition type. The center compression declined continuously with decrease of the carbon content and the surface compression exhibited its maximum at medium carbon content. The tension peak in the intermediate zone, was located near the surface in high-carbon steel and shifted deep inward with decrease of the carbon content, which constitutes one of the reasons why a low-carbon steel is less susceptible to quenching crack than a high-carbon steel. (2) The stress distribution of oil-quenched, through-hardened high-carbon steel was of transformational stress type. But, when the steel has a small unhardened core, the longitudinal stress in this area was converted into an exceedingly great tension and that in the hardened area into a small compression; as the core increased in the bulk, the tension in the core again dropped abruptly, while the compression in the hardened area increased. Owing to these stress distribution, a through-hardened steel is more liable to suffer a quenching crack (longitudinal) than a core-hardened, but an inner-crack (transverse) would occur even in the latter, if the core is small. Meanwhile, between these two dangerous cases, there exists a remarkable hardened state, in which the quenching stresses are small as a whole and no cracking is caused.