Abstract
The changes of hardness and magnetic properties of mild steel (0.18% C) and commercial pure nickel due to quenching were investigated. When mild steel specimens are quenched hardening (mechanical and magnetic) occurs by enormous thermal stress by which slips take place at the outer layer of specimens, and at the same time, softening occurs. So when mild steel is quenched at a sufficiently low temperature, such as 100∼250°, hardening and softening is just balanced, while remarkable hardening is observed in a specimen which has been quenched at a temperture higher than about 400°. This is not the case with nickel specimens, in which hardness-change mainly depends on the thermal stress alone. The magnetization curve decreases its slope by quenching, but when a certain external tension σc was applied at a temperature below the recrystallization temperature all show the same magnetization curve; because residual compressive stress in a quenched specimen diminishes by external tension and therefore if the quenched specimen is stretched, it will act as if a stretched annealed specimen will do. When the quenching is done at a temperature higher than 450°, such coincidence of magnetization curves are not observed, and perhaps it is because that the higher the heating temperature the more the complicated distribution of residual stress becomes.
