The alloy AISI 630 (ASTM A564‐89 17‐4 PH) is a precipitation hardening stainless steel with good mechanical and corrosion properties. Mechanical properties strongly depend on the microstructure. Namely, the formation of the reversed austenite and coarsening of copper rich precipitates cause a substantial drop of hardness. Thus, the evolution of the microstructure during ageing was thoroughly investigated in order to explain the processes that have effect on formation of the reversed autenite. The reaustenitization was analyzed with a dilatometer, while the coarsening of copper rich precipitates was observed by transmission electron microscope. The amount of austenite was measured with x‐ray diffraction and the impact of austenite on the fracture appearance transition temperature was observed. It was found that the amount of the so called reverse austenite does not only depend on the amount of transformed austenite but also on its chemistry, as it dictates its ability to transform into martensite during cooling.
Standard heat treatment of martensitic stainless steel consists of quenching and tempering. However, this results in high strength and hardness, while Charpy impact toughness shows lower values and a large deviation in its values. Therefore, a modified heat treatment of 0.1C-13Cr-3Ni martensitic stainless steel (PK993/1CH13N3) with intercritical annealing between Ac1 and Ac3 was introduced before tempering to study its effect on the microstructure and mechanical properties (yield strength, tensile strength, hardness and Charpy impact toughness). The temperatures of intercritical annealing were 740, 760, 780 and 800 °C. ThermoCalc was used for thermodynamic calculations. Microstructure characterization was performed on an optical and scanning electron microscope, while XRD was used for the determination of retained austenite. Results show that intercritical annealing improves impact toughness and lowers deviation of its values. This can be attributed to the dissolution of the thin carbide film along prior austenite grain boundaries and prevention of its re-occurrence during tempering. On the other hand, lower carbon concentration in martensite that was quenching from the intercritical region resulted in lower strength and hardness. Intercritical annealing refines the martensitic microstructure creating a lamellar morphology.
The potential use of the rare-earth metals in clean steel production was investigated. Reactions between the non-metallic inclusions and the rare-earth metals were investigated in a cold-work tool-steel grade (50CrMoV13-1). A rare-earth metal alloy misch metal (Ce, La, Nd, Pr) was used for the inclusion modification. Aluminium oxide non-metallic inclusions that have a negative effect on the mechanical properties usually occur in the investigated steel. The experimental melts were made in a vacuum induction furnace with different rare-earth metal additions (50, 150, 340, 950 and 2900) ppm. The laboratory cast ingots were hot forged and analysed with light and electron microscopy. It was found that misch metal additions influence the size, composition and type of the non-metallic inclusions in the steel. Most importantly, very small inclusions are formed at lower rare-earth metal additions (up to 340 ppm), but at high additions large non-metallic inclusions are formed (2900 ppm).
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