Improved low temperature mechanical properties of 0·4C–Cr–Mo–Ni steel through modified heat treatments

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Abstract
The low temperature mechanical properties of 0·4C–Cr–Mo–Ni steel can be improved significantly by thefollowing treatments. Modified oil quenching (MOQ): interrupt quenching at 573 K just below the martensitic transformation temperature followed by short time tempering at 673 K (up-quenching) before oil quenching and subsequently 473 K tempering (after conventional 1133 K austenitisation). Modified austempering (MA): the same up-quenching treatment followed by austempering at 673 K and subsequently water cooling. Each modified treatment was compared with its corresponding conventional treatment. The MOQ treatment significantly improved the notched tensile strength of the steel with slightly increased 0·2%PS and UTS, owing to an increase infracture ductility over the temperature range 123–203 K and also improved the Charpy impact energy of the steel over the temperature range 203–373 K. As a result of the MA treatment, the 0·2%PS and UTS and the notched tensile strength were developed remarkably with little change of fracture ductility over the temperature range 123–293 K. This treatment also improved the Charpy impact energy of the steel over the temperature range 203–293 K. The beneficial effect of the modified heat treatments on the mechanical properties is briefly discussed in terms of a modified law of mixtures, fibre loading theory, and fracture profiles. MST/1157