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 similar to 800 degrees 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.
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