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.
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 ƒÃ-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 ƒ¿¨ƒÁ 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 ƒ¿-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 ƒ¿¨ƒÁ 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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