Structure and properties of cast, forged, and sintered R8F8K7M6 high-speed steels

Abstract

1. The cast steel quenched in the ingot mold exhibited after tempering exceptionally high secondary hardness (HRC 70-71) and red hardness (HRC 62 after tempering for 4 h at 640°C) but very low impact strength. 2. Annealing and subsequent heat treatment slightly increased the impact strength of the cast steel, but lowered its secondary hardness (HRC 68–70) and red hardness (HRC 55–57). 3. The forged steel was superior in strength and equal in secondary hardness to the cast steel, but was surpassed by it in red hardness (HRC 54–55). 4. The steel produced by the powder metallurgy technique showed the least structural and chemical heterogeneity. After annealing it contained fewer chromium carbide and cementite-type carbide inclusions than did the cast and forged steels. After quenching and tempering the sintered steel was characterized by a fine grain, complete absence of any signs of dendritic structure, and a uniform distribution of extremely small carbide particles. It was less prone to overheating, contained less residual austenite than did the cast and forged steels, and could be more easily and evenly tempered. The sintered steel had high secondary hardness (HRC 66–70), while its red hardness was the same as that of the cast and forged steels (HRC 54–57). The sintered steel was less sensitive to changes in heat treatment conditions than the cast and forged steels. It also had a more homogeneous and stable structure and higher impact strength.