The 18% Ni-maraging steels, which belong to the family of iron-base alloys, are strengthened by a process of martensitic transformation, followed by age or precipitation hardening. Precipitation hardenable stainless steels are also in this group.

Maraging steels work well in electro-mechanical components where ultra-high strength is required, along with good dimensional stability during heat treatment. Several desirable properties of maraging steels are:

* Ultra-high strength at room temperature
* Simple heat treatment, which results in minimum distortion
* Superior fracture toughness compared to quenched and tempered steel of similar strength level
* Low carbon content, which precludes decarburization problems
* Section size is an important factor in the hardening process
* Easily fabricated
* Good weldability.

These factors indicate that maraging steels could be used in applications such as shafts, and substitute for long, thin, carburized or nitrided parts, and components subject to impact fatigue, such as print hammers or clutches.

Tempering of maraging steels
Tempering as an operation of heat treatment has been well known from the Middle Ages. It is used with martensite-quenched alloys. The processes of tempering will be considered here for steels only, sinse steels constitute an overwhelming majority of all marensite-hardenable alloys.

Maraging steels are carbonless Fe-Ni alloys additionally alloyed with cobalt, molybdenum, titanium and some other elements. A typical example is an iron alloy with 17-19% Ni, 7-9% Co, 4.5-5% Mo and 0.6-0.9% Ti. Alloys of this type are hardened to martensite and then tempered at 480-500��C. The tempering results in strong precipitation hardening owing to the precipitation of intermetallics from the martensite, which is supersaturated with the alloying elements. By analogy with the precipitation hardening in aluminum, copper and other non-ferrous alloys, this process has been termed ageing, and since the initial structure is martensite, the steels have been called maraging.

The structure of commercial maraging steels at the stage of maximum hardening can contain partially coherent precipitates of intermediate metastable phases Ni3Mo and Ni3Ti. Ni3Ti phase is similar to hexagonal ��-carbide in carbon steels. Of special practical value is the fact that particles of intermediate intermetallics in maraging steels are extremely disperse, which is mainly due to their precipitation at dislocations.

The structure of maraging steels has a high density of dislocations, which appear on martensitic rearrangement of the lattice. In lath (untwined) martensite, the density of dislocations is of an order of 1011-1012 cm-2, i.e. the same as in a strongly strain-hardened metal. In that respect the substructure of maraging steel (as hardened) differs appreciably from that of aluminum, copper and other alloys which can be quenched without polymorphic change.

It is assumed that the precipitation of intermediate phases on tempering of maraging steels is preceded with segregation of atoms of alloying elements at dislocations. The atmospheres formed at dislocations serve as centers for the subsequent concentration stratification of the martensite, which is supersaturated with alloying elements.

In maraging steels the dislocation structure that forms in the course of martensitic transformation, is very stable during the subsequent heating and practically remains unchanged at the optimum temperatures of tempering (480-500��C). Such a high density of dislocations during the whole course of tempering may be due to an appreciable extent, to dislocation pinning by disperse precipitates.

A long holding in tempering at a higher temperature (550��C or more) may coarsen the precipitates and increase the interparticle spacing, with the dislocation density being simultaneously reduced. With a long holding time, semi coherent precipitates of intermediate intermetallics are replaced with coarser incoherent precipitates of stable phases such as Fe2Ni or Fe2Mo.

At increased temperatures of tempering (above 500��C), maraging steels may undergo the reverse ������ martensitic transformation, since the as point is very close to the optimum temperatures of tempering. The formation of austenite is then accompanied with the dissolution of the intermetallics that have precipitated from the ��-phase.