Vacuum heat treatment of high speed steel cutting tools

An investigation of under hardening of small diameter HSS drills following vacuum heat treatment is presented. Poor hardening response was found to correlate with a loss of carbon in the HSS blanks. It is shown that full hardness could be attained by removing up to 200 μm off diameter prior to vacuum heat treatment. Metallographic examination revealed that the probable cause of carbon loss was the presence of oxide scale which set up a diffusion gradient leading to rapid grain boundary diffusion of carbon. Accelerated drill testing revealed that the under hard drill had a low tool life but with lower variance. The fully hardened drills exhibited higher variance largely due to the occurrence of early life failures. Metallographic examination suggests that the latter is due to the presence of large defects in the form of fractured primary carbides formed in the drawing process.