Grinding sensitive parts

Dear Doc: Our supplier in Asia is grinding hardened-steel parts and sending them to us for final processing. Some of them are cracking, so we’re going to visit the supplier. What should we be looking for?

The Doc Replies: Typically, there are two possible causes for cracks. The first is that grinding temperatures are too high, causing residual tensile stresses, rehardening phase transformation or both. This can lead to immediate cracking or cracking when the part is in the field. To confirm, check ground parts for white layer via sectioning and nital-etching, as well as residual tensile stresses via X-ray diffraction. To stop the cracking, lower the grinding temperatures with sharper dressing or use better grinding parameters.

The second possibility is that the workpiece material was “messed up” before it was ground. This is where the solution can get tricky. The material, prior to heat treatment, may contain flaws, such as pores, cracks and inclusions.

Heat treatment can also cause material to become “sensitive” to cracking, even when grinding temperatures are low. Bad heat treatment often occurs when tempering times are too short, temperatures are not low enough between tempers or when quenching occurs too slowly. This results in strange phases in the material, such as bainite.