What is a grinding burn? During grinding, due to the instantaneous high temperature in the grinding area (generally 400-1500 ℃), the surface layer of the workpiece tissue changes locally, and some parts of the surface appear discoloration (namely the color of the oxide film – yellow, brown, purple, green and other colors), this phenomenon is called grinding burn.
Grinding burns can be distinguished according to appearance, properties of surface microstructure changes, and depth. Depending on the appearance of the burn, there are total burns (where the entire surface of the part is burned), porphyritic burns (where there are scattered burns on the surface), and linear burns (where there are streaks on the entire surface of the part), which are the most common burns in production.
According to the properties of surface microstructure, there are tempering burns: when the surface layer temperature of the workpiece does not exceed the phase transition temperature Ac3 but exceeds the transition temperature of martensite, the martensite will change into the tempered trostenite or Soxhlet with lower hardness. Quench burn – when the surface layer temperature of the workpiece exceeds the phase transition temperature Ac3, the martensite is converted into austenite. And then there is sufficient cooling fluid, the surface layer will be quickly cooled to form a secondary quench Martensite, hardness higher than the tempered Martensite. But very thin (only a few microns), and the lower layer is the tempered Soxhlet and trostenite with lower hardness. Annealing burn – When the surface layer temperature of the workpiece exceeds phase transition temperature Ac3, the martensite is converted into austenite. And at this time, if there is no coolant, the surface hardness drops sharply, causing the surface of the workpiece to be annealed. This can easily happen in dry grinding.
Burns can also be classified as shallow (up to 0.005mm), medium (up to 0.001mm), and deep (up to 0.01mm), depending on the depth of the surface microstructure.