The influence of heating temperature on coercive force and hardness changes in carbon hypoeutectoid steels

Abstract

Hypoeutectoid steel with 0,25% carbon without preliminary heat treatment was investigated. The article describes an experimental study of the hardness and coercive force of this steel during heating and subsequent cooling in calm air. The magnetization depth of the device used and the penetration depth of the indenter when measuring hardness are significantly greater than the thickness of the total oxide films, therefore the measurement result is a complex value depending on the properties of the base metal and oxides. The influence on the studied parameters is proved not only of the structure of the base metal, but also of the properties of oxide films that appear on the steel surface in an oxygen-containing medium during heating. As a result, hardness and coercive force do not correlate with each other at all temperature intervals of heating. It is shown that the visual assessment of temperature by temper colors is subjective, and when the temperature threshold exceeds 500ºС (for the research steel in given modes), the visually determined dependence between the temperature and the color of the sample surface after temperature exposure disappears.

Keywords

• magnetic properties
• coercive force
• temper colors
• cooling in calm air
• temperature interval