Effect of Additions of B and Al₂O₃ Fine Powders on the Sintering of SKH51 Steel Powder

Abstract

We have already reported that the addition of boron powder, known as the most useful sintering enhancer for ferrous powders, was effective for the sintering of the high speed steel powder (SKH51), while a considerable grain growth, harmful to the mechanical properties of the compacts, occurred during sintering. In order to obtain the sintered compacts with high densities and fine-grained structure, 0.2-0.7 mass% Al₂O₃ fine powders were mixed with SKH51 steel powder together with 0.4 mass% B powder. These mixed powders were compacted and sintered in H₂ atmosphere at 1453K-1493K for 2h. It was shown that the SKH51 steel powder compacts with Al₂O₃ and B powders were appreciably densified during sintering, compared with the compacts containing B powder alone, whereas no significant grain refinements were shown, contrary to our expection. It was considered Al₂O₃ fine powder prevented the formation of isolated large pores which tended to emerge during sintering of SKH51 steel powder compacts containing B alone, and resulted in the high densification. It was also shown that the hardness of the compacts after heat treatment was somewhat improved by the addition of Al₂O₃ fine powder together with B powder.