Phase transformations in tool alloys with intermetallic-carbide hardening

Abstract

1. The iron-base tool alloy (0.3–0.4% C, 20% W, 20% Co, 4% Cr), which is hardened by precipitation of intermetallic and carbide phases, consists of Feα phase, M6C carbide, and the intermetallic compound Co7W6 in the annealed condition. 2. The extent of decarburizing can be determined from the relative intensities of the Co7W6 and M6C lines on the x-ray patterns. 3. During heating to quenching temperature the Co7W6 is dissolved first, and then M6C. All the Co7W6 is dissolved at 1150°C, while part of the M6C remains in the form of excess carbide even after quenching from 1350°C. 4. The Ms-Mf points of the alloy are 130–150°C higher than for high-speed steels of the usual composition and the Mf point is above room temperature, due to which the quenched alloy contains practically no residual austenite. 5. The hardness of the alloy as cast, annealed, and quenched is approximately the same (HRC 40-45). 6. The absence of residual austenite after quenching permits single tempering of the alloy. 7. The alloy is hardened in tempering at 560–630°C due to the precipitation of carbide and inter-metallic phases. The hardness is increased to HRC 67 at a red hardness of Kr58 up to 720°C and strength σbend up to 240 kg/mm2, and the cutting and technological properties are high.