Microstructural evolution during scuffing of hardened 4340 steel-implication for scuffing mechanism

Microstructural changes occurring during scuffing failure in hardened 4340 steel were studied with a block-on-ring test rig using a step-loading test protocol. Tests were interrupted before, during, and after scuffing. Both surface and sub-surface changes in the original tempered microstructure were characterized by scanning electron microscopy. Results of our study showed that scuffing occurred by severe and sudden plastic deformation of near-surface material in a sub-second time frame. Based on this observation and other previously observed microstructural changes, i.e. formation of a large fraction of retained austenite during scuffing, a scuffing mechanism based on adiabatic shear instability is proposed. The proposed scuffing mechanism can effectively explain most of the phenomenological observations associated with scuffing failure.