Mechanical and thermal response of a nickel-base superalloy upon grinding with high removal rates

The formation of a plastically deformed and a heat affected zone during grinding of superalloy IN738LC with a high depth of cut but slow work speed (creep feed grinding) was investigated. In order to obtain a better understanding of material removal mechanisms, the alloy strength and contact temperatures under typical machining conditions were evaluated. Temperature calculations comprised both the stationary equilibrium between global heat input and heat transfer to the coolant, as well as flash-like pulses at small contact spots. As a result, it was found that local melting at contact spots seems to be a rather common mechanism during grinding of superalloys, leading to so-called white layers which can easily be observed on metallographic cross sections. Grinding wheel topography has a significant influence on the maximum contact temperature and the width of the white layer. Sufficient cooling is very essential in limiting the width of the molten zone to few micrometers, which then is not critical in respect to mechanical properties. On the other hand severe damage was observed after creep feed grinding without coolant.