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The combination of the chemical composition and a specific heat treatment consisting of austenitising close to solidus temperature and subsequent triple tempering leads to the excellent properties of high speed steels. Usually when high speed steels are heated up to austenitising temperature, intermediate holding stages and specific heating rates are applied. This specific heat treatment affects the microstructure, especially the grain size distribution. In some cases, abnormal grain growth can occur. This study describes the influence of the variation of the parameters, such as intermediate holding temperature and heating rate on the microstructural development. All investigations were carried out on the high speed steel HS 6-5-2-5 using both laboratory tests and vacuum heat treating.

The powder metallurgically processed carbon-free alloy Fe-25Co-15Mo (in m.-%) can be hardened via the precipitation of an intermetallic μ phase (Fe, Co)7Mo6 by solution annealing, quenching and subsequent aging. Solution annealing is carried out in the austenite region which leads to a supersaturation of the matrix with molybdenum and the precipitation of nm sized μ phase particles during subsequent aging. “Primary” μ phase particles that are 1–2 μm in diameter formed during prior hot isostatic pressing are the source of molybdenum for aging. However, during solution annealing, these “primary” μ phase particles also impede grain growth. Therefore, the solution annealing process should be optimized to avoid extensive grain growth while still obtaining sufficient molybdenum contents in the matrix. In order to characterize the solution annealing process as a function of time and temperature, dilatometer experiments were performed to monitor the dissolution kinetics of the μ phase. The remaining μ phase fraction was determined in a complementary way by applying two characterization methods: Quantitative analysis of scanning electron microscope images and X-ray diffraction.

The ternary system Fe -25 at% Co -9 at% Mo shows an age hardening behavior similar to aluminum alloys. After solution annealing followed by rapid quenching, the Fe-Co-matrix is hardened during subsequent aging through precipitation of the intermetallic µ-phase (Fe,Co) 7 Mo 6 . In aged condition the entire Mo content is present in coarse primary and fine µ-phase particles and, therefore, the matrix consists exclusively of 71 at% Fe and 29 at% Co. The binary system Fe-Co shows a transformation from the disordered bcc structure to the ordered B2 structure between 25 and 72 at% Co at a critical ordering temperature ranging from room temperature to 723°C. As a consequence, the remaining overaged matrix in the Fe -25 at% Co -9 at% Mo system should also show such a transition. However, an ordered phase is brittle and, thus, not wanted for many applications. Better mechanical properties in terms of ductility can be achieved with a partially or fully disordered phase. Such a state can be obtained by rapid quenching from temperatures above the critical ordering temperature. In this study such an approach was implemented on the ternary Fe -25 at% Co -9 at% Mo alloy. The effect of different cooling rates on the mechanical properties was investigated by means of hardness testing. The actual ordering transition of the Fe -29 at% Co matrix was determined with differential scanning calorimetry and neutron diffraction.