Generally the term “hot work steel” includes tool steels which adopt a constant temperature above 200°C (392°F) du ring application. Superimposed on the latter are peak temperatures brought about by the operational cycle. Consequently, the use of hot work steels involves, in addition to the usual stresses which tool steels are subjected to, thermal stresses due to the con tact between tools and hot materials du ring forming. Hot work steels must exhibit good heat checking resistance in order to delay- for as long as possible - the formation of chill cracks appearing on the surface in reticulate shape as a consequence of frequent temperature changes in the surface region. To avoid hot cracks, i. e. tension cracks developing primarily in tools with far cavities at sectional transitions and edges and extending- contrary to chill cracks- far into the tool, hot work steels also have to feature good high temperature toughness. For tools subjected to high impact, pressure, or tensile stresses at elevated temperatures, special attention on must be paid to high strength at the various working temperatures. If the structural state is changed by the influence of heat, the strength at ambient temperature and consequently the strength at working temperature are reduced. This is why good high temperature strength and superior retention of hardness are prerequisites for stability of shape. Excellent high temperature wear resistance is necessary for ensuring satisfactory tool life. Further demands that must be met by hot work steels are low ten den cy to adhere to parts being processed, high resistance to erosion, high temperature corrosion and oxidation, dimensional stability du ring heat treatment, good mach inability, and in some cases also good cold hobbing properties.