Draft:Grinding burn

Grinding burn is the term used to describe structural and residual stress changes that occur during grinding due to thermal overload.

Basics
During mechanical processing of steels, heat is generated which is introduced into the component. This heat input can lead to irreversible changes in the structure and microstructure as well as to resulting residual stress changes, especially in surface near areas of the component. This effect is called thermal overload.

Similar damage may also caused by other mechanical machining processes, such as hard turning. The heat-induced processes in the material are basically the same here as during grinding. But the "burns" possible in this process differ in their severity from the actual grinding burn due to the used tools. Therefore the structural changes should be named after the machining process that caused them, in the case of hard turning as turning burn. Grinding burn can occur in the form of tempered zones and, with even higher heat input, in the form of re-hardened zones (Fig. 1). It is also possible that grinding burn does not manifest itself through structural changes visible under the optical microscope, but only through changes in residual stresses.

A prerequisite for the development of grinding burn is the property of the material used to form a martensitic microstructure. Grinding burn cannot therefore occur in high-alloy austenitic steels and high-alloy ferritic steels. The carbon content in the steel must be high enough to allow martensite to form during rapid cooling. This is the case from a carbon content above 0.3% by mass.

The structural and residual stress changes caused by grinding burn lead to locally altered component properties and reduce its service life and long-term stability. Figure 2 shows a case of damage to a rolling bearing inner ring caused by grinding burn. The detailed image of the raceway surface shows a crack network (Fig. 3). The metallographic cross-section clearly shows the trigger for the crack formation: severe grinding burn with rehardening zones and tempering zones (Fig. 4).

Detection / testing of grinding burn
Grinding burn can be seen with the naked eye only on very severely damaged workpieces. Then blue to black discolorations form on the component caused by oxidation of the surface, so-called tempering colors. However, this is usually not the case. For this reason, various methods of grinding burn testing have been developed to detect grinding burn in or on the surface of a component. The oldest method is abrasive burn etching, also called nital etching (US standard AGMA 2007-B92, AMS 2649, ISO 14104). Grinding burn etching causes a chemical change in the component surface and discoloration remains. In addition, the process is difficult to automate. For this reason, non-destructive testing methods for the detection of grinding burns, such as the Barkhausen noise method and eddy current testing have become established in recent years.

More information

 * Application of Surface Temper Etching
 * Manufacturing of referernce blocks with artificial defects
 * Application of Barkhausen noise method
 * Application of eddy current testing