b'\x1b\x1a\x19 \x1f\x18\x1d\x17\x16Ask the Grinding DocRUSTY VERSUS DAMAGED PARTSD ear Doc: Were getting visibleto the naked eye and can be re- oxide layer (type 1) mean that you grinding burn on the ungroundvealed only with testing (nital etch,havegenuinethermaldamage surface. Why there? And how do weBarkhausen, X-ray diffraction, mi- (type 2, 3 or 4)? Maybe. Maybe not. get rid of it? crohardness, etc.). Some peopleYoud have to test it.The Doc replies: First, lets becall types 2, 3 and 4 grinding burn,But wait, you say. Your figure careful with definitions as theres awhich I try to avoid. Type 1 doesntshows high temperatures on the lot of ambiguity and overlap in what people refer to as grinding burn.There are four types of thermal damage in steels.Heres how I define it in The Book of Grinding.There are four types of thermalaffect the strength of the material ground surface and lower tempera-damage in steels: 1) oxidation burn,its simply a thin layer of iron oxide.tures on the unground surface. So which most people call grindingTypes 2, 3 and 4, which I refer to aswhy do I have oxidation burn only burn; 2) over-tempering, also calledgenuine thermal damage, do affecton the unground surface? Things thermal softening; 3) residual tensilethe strength of the steel. became hotter on the ground sur-stresses; and 4) rehardening burn,All steel surfaces develop a thinface than they did on the unground also called white layer. oxide layer. But at higher tempera- surface, but you dont see oxidation Type 1 is simply iron oxidethattures, that layer becomes thicker,burn on the unground surface. That is, rustand visible to the nakedmore visible and more colorful.is because of two reasons: 1) There eye. Types 2, 3 and 4 are invisibleDoes that brown-yellow-blue ironwas coolant on the ground surface J. Badger14SEPTEMBER 2021 GrindingDoc.indd 14 8/17/21 11:40 AM'