Intercritical and subcritical spheroidization of 10B38 cold heading steel

Abstract

For the production of bolts by cold forging, the hardenable steel has to be in a soft and ductile state. The Cold Heading Quality Steel is achievable by a spherodization heat treatment on the hot rolled material. The scope of this study is a boron bearing medium carbon steel which has many promising properties as bolting material. The experimentations covered two different initial conditions before the spheroidization heat treatment: hot rolled (ferrite-pearlite microstructure) and quenched (martensitic microstructure). In combination, two spheroidization procedures were considered too: subcritical and intercritical heat treatments. Intercritical heat treatment process included heating to 750 °C , holding for 2 h and then cooling to 715 °C and then holding for 12 h. In the subcritical heat treatment proess, the materialwas heated to 715 °C , holding for 12 h followed by cooling in the furnace. To assess the cold formability, a special notched tensile test was employed and the results were supported with metallographic and microhardness measurements. For both subcritical and intercritical heat treatment, higher levels of spheroidizatin were achieved with the quenched samples compared with the hot rolled condition. On the other hand the subcritical heat treatment resulted in higher levels of spheroiization. A practical equation is developed which relates the failure strain of notched tensile specimens with pearlite content and the hardness of the 10B38 steel.