Influence of microstructure on galling resistance of cold-work tool steels with different chemical compositions when sliding against ultra-high-strength steel sheets under dry condition

The objective of this study was to determine the influence of a microstructure on the galling resistance of three variations of D2-grade steels that are used for the stamping of ultra-high-strength steel (UHSS) sheets. Standard D2 steel and two other types of D2-grade steel, having different microstructures, were used as the pins in a pin-on-flat galling test, and a dual-phase 980 (DP 980) sheet was used as the counterface. The friction coefficient was monitored as a function of the sliding distance, and the critical sliding distance for the onset of severe galling was indicated by the transition to a high and unstable friction coefficient. The best galling resistance was observed for the steel that contained a large amount of relatively small secondary carbides, whereas the worst galling resistance was obtained for standard D2 steel, which contained a large amount of coarse primary carbides. The test data also indicated that the galling resistance of the cold-worked tool steels was influenced by the type, size, and distribution of the carbides rather than simply by the total amount of carbides. Furthermore, it was inferred that the galling resistance of cold-worked tool steels could be improved by larger amounts of secondary carbides, smaller mean free paths between secondary carbides, and smaller mean diameters of both primary and secondary carbides.