Effects of niobium carbide additions upon the production and properties of sintered high-speed steel

Abstract

1. A study was made of the comminution to powder, in a rotary ball mill, of R18 high-speed steel swarf containing niobium carbide additions and of the subsequent processing of the resultant composite powders to dense blanks. 2. Niobium carbide was found to intensify the swarf comminution process: In powder milled for 48 h, the presence of 5 vol.% NbC increased the yield of the −0.25-mm fraction from 46 to 81% and that of the finest (−0.04-mm) fraction from 35 to 45.5%. At the same time, the mean particle size decreased from 0.31 to 0.18 mm. 3. A study was made of the pressing and sintering of the high-speed steel powder with niobium carbide additions. Strong compacts with a relative density of 70–73% were produced by pressing the powder, with a rubber addition, under a pressure of 8–10 tons/cm2. 4. Sintering, in the absence of a liquid phase, for 60 min at 1250°C in very dry hydrogen was found to yield blanks with a relative density of more than 95%. The blanks were characterized by a fine-grained structure and a mean carbide grain size of 1.5–2μ. 5. The addition of niobium carbide affected the size of the austenitic grain, reducing it from 18μ at 1 vol.% NbC to 7μ at 5 vol.% NbC. 6. The introduction of niobium carbide raised the hardness of the sintered steel, from 59 HRC (without NbC) to 65 HRC at a NbC content of 5 vol.%.