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 ...
Carbides are necessary component parr of the structure in high-speed steels. The wear resistance, heat resistance, and strength of these steels markedly depend on the amount of carbides, the ratio between different types of carbides, and their states. The aim of the present work is to study the phase composition of carbide precipitates obtained from atomized powder and the corresponding tungstenless powder high-speed steel of the type M5F5.Carbides in metallurgically melted high-speed steels have been studied quite completely [1][2][3][4]. Information about carbides in powder high-speed steels is inadequate. In [5][6][7] some results are presented for determining the phase composition in atomized powders and powder high-speed steels.Atomized powder was prepared by gas-jet atomization in nitrogen by the generally accepted technology. It contained 2% C; 5/5% Mo; 7.0% V; 5.3% Cr; 0.55% Si; 0.023% N; 0.02% O; 0.008% S; 0.018% P.It was established that more than 60% of spherical powder particles have a diameter of less than 200 ,am. Powder steel type M5F5* was prepared by hot extrusion of containers fille/d with powder. After pouring and compacting the powder air was pumped from the container to a pressure ofja and the container was welded up. Extrusion was performed tlu-ough a die with a nominal stretch ratio of 9 at 1100-1150°C. The microstructure of extruded bars in longitudinal and transverse sections was the same and was characterized by uniformly distributed fine carbide particles. No porosity was detected in microstructural analysis with magnifications of x 500-1000.Electrochemical separation of carbides from the steel was performed in 10% aqueous HCI solution with addition of 2-3 % citric acid with a current' density of 100-200 A/m 2 and a voltage at the electrodes of -1 V. Austenitic corrosion-resistant steel 12Khl8N10T (<0.12% C; 18% Cr; 10% Ni; 0.7% Ti) was used as the cathode. In order to separate carbides from the atomized powder it was separated into fractions on standard screens. Carbides were separated from each powder fraction in order to explain the effect of droplet cooling conditions during atomization on their phase composition and the state of carbides. For this purpose a thin film of current-conducting polymer adhesive was applied to a glass anode and particles of a specific fraction were glued to it so that about half of each particle projected above the adhesive film. Preparation of carbides from particles prepared in this way was performed with a current density of 1.3 A/m 2. After washing with water and drying the carbide residue was subjected to X-ray analysis.Exposure of carbide residues was performed in chromium or iron Kosadiation in RKD57 cameras. Use of the RKD57 camera was due to the small amount of carbides extracted from powders and the possibility of increasing the exposure to 1.5-2 h with the aim of recording on film the maxinmm possible number of lines including relatively weak lines.Steel phase composition in different heated states and also atomized powder was determined from t...
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