Rusty versus damaged parts: General Industry Coverage

Dear Doc: We’re getting visible grinding burn on the unground surface. Why there? And how do we get rid of it?

The Doc replies: First, let’s be careful with definitions as there’s a lot of ambiguity and overlap in what people refer to as grinding burn. Here’s how I define it in “The Book of Grinding.”

There are four types of thermal damage in steels: 1) oxidation burn, which most people call grinding burn; 2) over-tempering, also called thermal softening; 3) residual tensile stresses; and 4) rehardening burn, also called white layer.

Type 1 is simply iron oxide — that is, rust — and visible to the naked eye. Types 2, 3 and 4 are invisible to the naked eye and can be revealed only with testing (nital etch, Barkhausen, X-ray diffraction, microhardness, etc.). Some people call types 2, 3 and 4 grinding burn, which I try to avoid. Type 1 doesn’t affect the strength of the material — it’s simply a thin layer of iron oxide. Types 2, 3 and 4, which I refer to as genuine thermal damage, do affect the strength of the steel.