Improvement of fracture toughness and tensile ductility of massive rings made of 18 % nickel maraging steel

Abstract

Massive rings of 18% nickel maraging steel produced using the ring rolling process do not consistently attain high fracture toughness (KIc) and tensile ductility values. Due to large section sizes involved, it becomes difficult to suppress precipitation of carbonitride particles and prevent thermal embrittlement. The conventional heat treatment of solutioning at 820 °C followed by aging at 480 °C is not effective in undoing this embrittlement. Different multi-stage solution treatments were designed and tried out to improve the mechanical properties. A two-stage (950 °C + 820 °C)/three-stage (950 °C + 950 °C + 820 °C) solution treatment, depending on the section size, proved to be effective. This treatment leads to recrystallisation of the as-hot-worked microstructure and a fine grain size; it is believed that a delinking of grain boundaries and carbonitrides occurs simultaneously. The observed improvement of fracture toughness and tensile ductility can thus be explained.