Evaluation of the effect of cryogenic treatment of HSS drills at different holding time in drilling AISI 316-SS

Cryogenic treatment has been acknowledged as an effective means of extending tool life of many cutting tool materials, thus improving productivity significantly. However, the real mechanisms which guarantee better tool performance have not been very clearly understood. This work aims to study the effect the effect of different holding time during cryogenic treatment on M2 high speed steel (HSS) drills. The performance of these cryo-treated tools were compared on the basis of thrust force, surface roughness, tool wear and chip formation by drilling on AISI 316 SS. The thrust force was least at 600 rpm and the rapid deep cryogenic treated (DCT) drills had much less thrust force (45%-55% lower) compared to untreated drills. From the various parameters involved it was observed that the cryo-treated tool bit at holding time of 24 h and at a cutting speed of 600 rpm and feed rate of 0.06 mm rev(-1) showed better performance. The relative wear resistance of the cryo-treated drill at 24 h was at least two times better than the untreated drill while the surface roughness of the holes drilled with this drill was half that of those drilled with untreated drills. This investigation was further extended by gradual shallow cryogenic treatment (GSCT) and gradual deep cryogenic treatment (GDCT) of drills at the optimum holding time of 24 h. Both GSCT and GDCT treated drills showed 35% and 15% less thrust force respectively when compared to DCT. The better performance can be attributed to the lack of internal residual stresses. Microstructure SEM images and Rockwell hardness testing was done to support the findings.