The subject of grinding cracks has arisen, and the question has been asked, “What is the cause of grinding cracks?” The question was asked relating to heat-treated gear teeth. Although the cracks are very often seen on ground gear teeth, they are not always peculiar to or related to gear teeth.

The major cause of grinding cracks and grid burns on gear teeth is generally caused (but not in all cases) as a result of:

• Not keeping the grind wheel dressed. That is not keeping the wheel face clean of metal fine buildup, resulting in trying to grind metal with metal.
• Taking too large of a grinding cut and generating surface frictional heat on the tooth flank surface.
• Not using enough coolant to flush away the ground metal fines and to keep the gear tooth cool and free of frictional heat buildup at the ground surface.
• Using a grinding wheel that is too hard for the grinding application. In other words, incorrect grinding wheel choice.

If frictional heat is allowed to build up (particularly on a carburized gear-tooth surface) and the surface is not cooled in an appropriate manner, the temperature of the carburized surface (which is a martensitic structure) will rise sufficiently to initiate tempering of the martensitic face. This will cause what is known as “grind burn.”

A very simple test to observe grind burn is to take a cotton-wool swab dipped in NITAL (3-5% nitric acid in alcohol) and wipe it across the ground surface. The surface (if localized, overheating has occurred) will turn a light shade of brown, indicating overheat and “grind burn.”

The hardness of the brown area will be lower in hardness than the non-brown area, thus creating a hardness differential on the immediate surface. There will also be a precipitation of carbides at the grain boundaries, which will cause a volumetric change (contraction) and a reduction in the surface residual compression. Therefore, there will be a very strong likelihood that surface cracks would be initiated.

Another grinding phenomenon can be if too much friction is generated at the interface between the gear tooth and the grinding wheel. Surface overheating can occur to the extent that the contact point will turn to red heat (austenitizing the immediate surface area). If coolant is present, the overheated surface will be transformed to untempered martensite, thus giving rise to surface cracking of the fresh unstable martensite.

Part 2 will continue the discussion on grind burn and grind cracking of carburized ground gear teeth.