Optimisation of the fracture toughness of a novel ultra-high strength maraging steel

This thesis details work that was carried out to optimise the fracture toughness and thermal stability of a new maraging steel called F1E. This steel was designed to precipitate Laves phase to improve the creep properties, and it was initially found that this was detrimental to toughness properties, and that further precipitation occurred during extended time at desired operating temperatures, embrittling the alloy. Initial work focussed on development of a heat treatment to stabilise the Laves phase, using a slow cool to the service temperature to fully precipitate the equilibrium volume fraction of Laves phase at this temperature without nucleating fresh particles, as it was believed that it was fresh particles forming during service which led to the loss of ductility after thermal exposure. This heat treatment process successfully stabilised the properties. Modifications were then made to the composition in an attempt to improve the ductility and toughness without losing strength or creep performance. Two changes were made – an increase in the nickel content, as increasing the concentration of nickel in the matrix of maraging steels has been shown to improve toughness; and a decrease in molybdenum and tungsten content to reduce the Laves phase volume fraction by 25%. These changes, along with further optimisations of the heat treatment used and refinement of the prior austenite grain size, were successful in improving the fracture toughness of F1E (or RR9922 as the modified composition is known) by a factor of 2, from 23 MPa m\(^0\)\(^.\)\(^5\) to 46.9 MPa m\(^0\)\(^.\)\(^5\)