The Effects of Austenitizing Temperature, Carbon Content and Subzero Treatment on the Mechanical Properties of Hardened Steels

Abstract

Static torsional properties of hardened steels were determined in relation to austenitizing temperature, austenitic grain size, carbon content and subzero treatment in order to provide a guide for heat treating of steels used in a low temperature tempered state, such as some tool steels, case hardening steels and urtra high strength steels.
Four plain carbon steels and three low alloy steels were used in the experiment.
(1) The torsional properties of hardened steels, such as torsion strength, plastic torsion angle and absorbed energy required for breaking, are affected markedly by the austenitizing temperature. The utmost degree of torsional strength, plasticity and toughness is obtained when they are austenitized just above the critical temperature, and it extremely decreases as the temperature increases.
However, the plasticity and the toughness and sometimes the strength start to increase again from a certain austenitizing temperature and the maximum values are obtained in hardened steels when they are austenitized at about 900°C.
When the temperature is over the degree, which coinsides, with the overheating temperature accompanying abrupt austenitic grain coarsening, these properties decrease.
(2) The effect of carbon content on these properties is also prominent. The greatest degree of strength, plasticity and toughness is obtained in hardened 0.2 to 0.3%C steel, and it decreases extremely as the carbon content increases. But when the content is over about 0.6%C, the properties again increase, showing a maximum at about 1.0%C, decreasing as the carbon content increases.
(3) These properties are generally decreased by subzero treatment, and each maximum state of properties, especially that of plasticity, obtained by austenitizing at about 900°C or by a carbon content of about 1.0%, almost disappear. It suggests that the ductile retained austenite would be available in some degrees improving the plasticity, toughness and also strength of hardened steels.