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Niwiding of cutting tools is used to improve their endurance. The small thickness of the nitrided layers does not make it possible to evaluate their heat resistance by the standard method. This has made it necessary to develop a method for independent determination of the heat resistance of nitrided layers on high-speed steels. This makes it possible to optimize the nitriding process, in particular, for improving the heat resistance of the layers. The present work is devoted to assessing the possibility of increasing the heat resistance of high-speed powder steels R6M5K5-MP and R9M4K8-MP by ion nitriding and to developing a method for evaluating the heat resistance of the nitrided layers.In the process of mechanical treatment of parts made of difficultly treatable materials or at high cutting rates the cutting edges of the tool heat to 550-560~ which exceeds the conventional tempering temperatures. As a result of such heating the initial structure of the steels changes, which diminishes the hardness and strength of the tool. The heat resistance of the steels is characterized by the temperature at which a hold for 4 h diminishes the initial hardness to 58 HRC. The softening of a tool steel is a result of decomposition of martensite and coalescence of carbide particles.The heat resistance of high-speed steels melted by the conventional technology is increased by alloying them with elements that either increase the stability of martensite or increase the content of disperse carbide particles. As a rule, an increase in the heat resistance is accompanied by worsening of other properties of the metal. Upon adding 5 -10% Co the heat resistance of the steels increases to 645 -650~ but their strength and, especially, their toughness diminish [I]. An increase in the vanadium content to 2% or more with a simultaneous increase in the carbon content diminishes the strength, worsens the grindability, and complicates hot pressure treatment [ 1 ].Nitrogen alloying in an amount exceeding the usual content (0.02-0.03%) has special value for improving the heat resistance. GOST 19265-73 envisages the production of several grades of high-speed steel with 0.05 -0.10% N. As compared to carbides, nitrides have a higher thermal stability and their particles are less susceptible to coalescence. Dissolution of nitrogen in high-alloy austenite diminishes the thermodynamic activity of carbon, which causes segregation of carbides. As a result, the content of the alloying elements in the austenite decreases and their concentration in martensite diminishes too.The use of powder metallurgy for the production of highspeed steels improves their technological properties. Powder steels retain a satisfactory capacity for pressure heat treatment and grindability at a carbon content that cannot be used in conventionally produced steels because of the unsatisfactory manufacturing ductility of the ingots [2].When the content of carbon in a tool steel is not balanced with that of the alloying elements, growth of the total carbide content manifests itself ...