Hot Deformation of 18 % Ni Maraging Steels: A Review

Abstract

The published literature makes reference to the excellent hot workability of 18 % nickel maraging steels. However, production of large cross-sectioned products of 18 % nickel maraging steels by hot deformation can lead to different types of quality problems, such as (i) inadequate recrystallization and grain structure aligned in working direction, resulting in directionality of mechanical properties; (ii) thermal embrittlement that leads to poor values of ductility and fracture toughness; and (iii) sharp increase in flow stress with decreasing hot working temperature, necessitating the application of high loads for deformation at relatively low temperatures. To overcome these problems, primarily two sets of studies have been carried out by researchers. One set of studies was centered around the development of an optimum post–hot work solution treatment. Two-stage solution treatments, as opposed to standard single-stage solution treatment at 820°C, have been developed and are shown to lead to a fine-grained microstructure and an excellent combination of strength and fracture toughness with a high degree of isotropy. It has been documented that such heat treatments can also rejuvenate the quality of the products produced through a nonoptimum hot deformation schedule. The other set of studies was focused on establishing deformation processing maps using hot isothermal uniaxial compression testing. Different grades of 18 % nickel maraging steels have been the subject of investigation. Based on these studies, regimes of strain rate and temperature have been identified for optimum processing. Regimes in which flow localization occurs and hence need to be avoided have been identified. Processing regimes for obtaining fine-grained microstructures have been identified; regimes that lead to coarse grains, which need to be avoided, have been established. Constitutive equations have been developed to predict the deformation behavior as an aid to the design of the hot deformation process.