Carbon evolution during vacuum heat treatment of High Speed Steel

•Hardening response after vacuum heat treatment (VHT) investigated as a function of surface oxide present during VHT.•Relationship between surface oxide thickness and carbon content after VHT, shown using infrared absorption mass spectrometry.•CO evolution from the substrate directly observed above ∼800 °C and correlated with loss of sample mass measured by TG.•With the surface oxide fully removed from the sample, full carbon content and hardening response were measured after VHT.

We report on the hardness response of vacuum heat treated (VHT) M35 HSS as a function of the thickness of oxide present during VHT. An almost linear relationship existed between the hardening response post-VHT and the oxide thickness pre-VHT. A similar relationship existed between the thickness of oxide pre-VHT and the carbon content post-VHT. CO evolution during VHT was observed from ∼800 °C. In combination, these measurements show that CO evolution and carbon loss are increased in M35 HSS covered with surface oxide whilst hardening response is reduced. A full hardening response and unchanged carbon content were measured in M35 HSS VHT with surface oxide removed.