Cause of Heat Checking 3

[evilscott]

Have a question regarding the physics of heat checking. What I've seen in the literature is that it is due to alternating heating and cooling. However, I haven't seen anything that indicates whether it is influenced (and by how much) by rate of change of temperature rather than absolute temperature only. Also, is it an issue regarding thermal differential between the surface layer of the material vs a core temperature ?

 

[swall]

In steel and cast iron friction surfaces, the frictional heating heats the material into the austenitic range (roughly 1400F and above).Then it cools back to to ambient.This cooling back to ambient will cause the austenite to transform to martensite, with a volume expansion.At some point,after numerous cycles, the surface material will be constrained due the hardness/strength of the martensitic layer and the formation of martensite will lead to minute cracks, as the material can not undergo the normal volume expansion.These cracks, once they grow to a size large enough to see are "heat checks".

 

[metengr]

To add to swall's comments, heat checking is also known as thermal fatigue crack propagation. Tensile stresses generated by exposure to temperature transients (with a delta T as low as 300 deg F) can generate low cycle, thermal fatigue damage, as well. The appearance is similar to a checker board pattern of surface cracks. What is known is that thermal fatigue crack propagation can become self-limiting in depth in certain situations; once the tensile stress field from exposure to thermal cycles diminishes, crack growth effectively stops (unless other global stresses take over to further drive crack growth).

We have various components in the Electric Power Generation industry that are exposed to this type of damage, where crack propagation basically stops at a certain depth, with no need for immediate repair.