Effect of heat treatment on microstructure and mechanical properties of Ni- and C-free Co-Cr-Mo alloys for medical applications

The effect of pre-heat treatments, followed by a hot forging process, on the microstructure and strength of Co-Cr-Mo alloys was investigated. Four pre-heat treatments were conducted at 1443, 1473, 1503 and 1533 K for three different time condition: 7.2 ks, 21.6 ks and 54.0 ks and then water cooled to room temperature. Tensile tests and XRD analyses were carried out on both as-cast alloy and heat-treated alloys. The volume fraction of retained γ phase increases with an increase in temperature of heat treatment, suggesting that the γ→ε martensitic transformation is suppressed after experiencing the heat treatment at higher temperatures. Tensile strength slightly decreases with increasing the temperature and the time of heat treatment, whereas the ductility slightly increases. The σ phase completely dissolves into matrix when the alloy is heat-treated at temperature of 1533 K longer than 21.6 ks.