Carbides in atomized powder and powder tungstenless high-speed steel

Abstract

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...