Optimising heat treatment requirements for improved toughness of V containing 3%NiCrMo steel

Abstract

One major field of application of medium carbon low alloy steels is military hardware, especially the production of mortars. In manufacturing, it is usually necessary to adhere to specific standards; however, since economic and technological requirements depend on using scrap extensively, it can be difficult to keep minor additions within the specification. Heat treatment will therefore play an important role in obtaining the required mechanical properties. The combined effect of vanadium level and heat treatment cycles on strength and toughness of type 4337V (3%NiCrMo) 'gun steel' has been investigated. Vanadium is added mainly as a grain refiner, but it is also a strong carbide former, and therefore likely to reduce toughness. Conventional heat treatment (normalising at 900°C followed by quenching from 850°C), a modified heat treatment (double normalising at 900 and 800°C, successively, before the conventional treatment), and thermomechanical treatment (quenching in oil directly from the forging temperature at about 800–850°C), were applied. All specimens were tempered at 600°C for 3 h. For all heat treatments, it was concluded that for maximum toughness, a marginal content of 0·08–0·14%V should not be exceeded. Using heat treatment cycles other than the conventional one made it possible to accommodate V at the upper end of the range. Thermomechanical processing increased strength but only increased toughness slightly, whereas double normalising improved both strength and toughness compared with conventional heat treatment.

Keywords:

• HEAT TREATMENT
• CRNIMO STEELS
• VANADIUM
• THERMOMECHANICAL TREATMENT