Eighteen percent (18%) Ni maraging steels are ultrahigh strength steels possessing outstanding fracture toughness. They are based on Fe-Ni-Co-Mo system and contain about 18 wt% nickel. The heat treatment consists of (i) solution annealing, which involves holding the material in the austenitic phase field to take all alloying elements into solution, followed by cooling to room temperature and (ii) aging to realize a fine and uniformly dispersed (Ni,Fe,Co)3(Ti,Mo) precipitates and Fe-Mo based precipitates. The aging treatment is simple; it is carried out at a relatively low temperature and causes very little dimensional change.Today the heat treatment of maraging steel has matured to such an extent that it can take care of variations in the preceding hot working phase of manufacture and consistently lead to high levels of strength, ductility and toughness in a variety of product shapes and sizes, including large section sizes. Thermal cycling can cause grain refinement in these steels and this phenomenon has been industrially exploited to produce massive maraging steel products with a fine grain size and optimum combination of mechanical properties.Overaging treatment can lead to higher toughness levels with some drop in strength. However, this treatment has not found favor in industry to realize higher toughness levels, due to poor reproducibility of properties. Adopting lower strength variants of 18% Ni maraging steel in peak aged condition has instead been the preferred approach.Cobalt being an expensive element, cobalt-free 18% Ni maraging steels have been developed. Titanium is present at a higher level in these steels, compared to their cobalt-bearing counterparts. Heat treatment for realizing the high strength condition again consists of solution annealing and aging. (Ni,Fe)3(Ti,Mo) is the dominating strengthening phase, with higher Ti level and lower Mo level in the precipitate, compared to cobalt-bearing grades.Both cobalt-bearing and cobalt-free maraging steels show marked degradation in toughness, if they get exposed to temperatures in the range 780-950oC while cooling from high temperatures (e.g. 1200°C). This phenomenon is termed thermal embrittlement and can cause serious problems in processing components of thick sections. © 2010 Nova Science Publishers, Inc. All rights reserved.