Properties of Maraging Steels

Abstrakt:

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.

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, since steels constitute an overwhelming majority of all martensite-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 fA-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 γ→Mf 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 f?-phase.

Variations of Properties in Maraging Steels

The dependence of mechanical properties of maraging steels on the temperature of tempering is of the same pattern as that for all precipitation-hardenable alloys, i.e. the strength properties increase to a maximum, after which softening takes place. By analogy with ageing, the stages of hardening and softening tempering may be separated in the process.

The hardening effect is caused by the formation of segregates at dislocations and, what is most important, by the formation of partially coherent precipitates of intermediate phases of the type Ni3Ti or Ni3Mo. The softening is due, in the first place, to replacement of disperse precipitates having greater interparticle spacing and, in the second place, to the reverse γ→Mf martensitic transformation which is accompanied by the dissolution of intermetallics in the austenite.

The ultimate strength of maraging steels increases on tempering roughly by 80% and the yield limit, by 140%, i.e. the relative gain in strength properties is not greater than in typical age-hardening alloys, such as beryllium bronze or aluminum alloy Grade 1915, but the absolute values of ultimate and yield strength on tempering of maraging steels reach record figures among all precipitation hardening alloys. This is mainly due to the fact that maraging steels have a very high strength (Rm = 1100 MPa) in the initial (as-hardened) state.

The high strength of maraging steels on tempering at 480-500°C for 1-3 hours may be explained by the precipitation of very disperse semi coherent particles of the size and interparticle spacing of an order of 103 nm in the strong matrix, these intermetallic precipitates also possessing a high strength. Thus, with the same dispersity of precipitates as that of G. P. zones in precipitation, hardening non-ferrous alloys, maraging steels possess an appreciably higher ultimate strength (Rm = 1800-2000 MPa).

As compared with martensite-hardenable carbon-containing steels, carbonless maraging steels show, for the same strength, a substantially greater resistance to brittle fracture, which is their most remarkable merit. On tempering to the maximum strength, the ductility indices and impact toughness, though diminish somewhat, still remain rather high. The high ductility of the carbonless matrix and the high dispersity of uniformly distributed intermetallic precipitates are responsible for a very high resistance to cracking, which is the most valuable property of modern high-strength structural materials.

The properties of maraging steels clearly indicate that these steels have many potential applications in mechanical components of electro-mechanical data processing machines. Use of these steels in shafts that require good dimensional control following heat treatment should be pursued for two reasons. First, maintaining dimensions should be easier because quenching and tempering are not necessary. Second, wear data indicate that equivalent or better wear resistance is obtained from the maraging steel than from the more commonly used shaft materials.

Impact-fatigue strength of 18% Ni-maraging steels indicates that these steels could be used in repeated impact loading situations. The good fracture toughness, compared to that of quenched and tempered alloy steels at the same strength level, indicates possible use in high-impact low-cycle load applications.

Finally, due to the relatively low temperature of aging, the use of the maraging steels for long, thin parts should be considered. Here, their use as a replacement for some case hardened or nitrided components is indicated that the potential application should be carefully studied.

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