MARAGING STEEL

Anshu Nirmal

INTRODUCTION MARAGING STEELS comprise a special class of high-

strength steels that differ from conventional steels in that they are hardened by a metallurgical reaction that does not involve carbon

These steels are strengthened by the precipitation of intermetallic compounds at temperatures of about 480 °C (900 °F)

Yield strength of maraging steel ranges from 1030 to 2420 MPa but it can be as high as 3450 MPa

Carbon content in these alloys is kept very low in order to prevent the formation of TiC.

The low carbon martensite formed after annealing has hardness value very low(30-35 HRC). intricate shapes can be machined in soft conditions and then it can be hardened as there is only a slight change in dimensions during age hardening

The precipitate particles are of a lattice size comparable to that of the martensite matrix and cause little distortion of the matrix. This characteristic, together with the absence of carbon, allows the steel to be age hardened to very high strength levels while minimizing changes in the shape of the part being hardened.

Composition of Maraging steels MDN 250:- Ni- 17-19% Co- 7-8.5% Mo- 4.6-5.2 Ti - 0.3-0.5% C - 0.03% Si- 0.10% Mn- 0.10% S/p- 0.01% B - 0.03%

Metastable diagram plotting the austenite-to-martensitetransformation upon cooling and the martensite-to-austenite reversion upon heating.

Equilibrium diagram showing that at higher nickel contents the equilibrium phases at low temperatures are austenite and ferrite

There is no transformation of phases until the temperature Ms, the temp. at which the martensite starts transforming to the austenite, is reached

Alloying elements alter the Ms temperature significantly, but do not alter the characteristic that transformation is independent of cooling rate.

In addition to nickel, the other alloy elements present in maraging steels generally lower the martensite transformation range, with the exception of cobalt, which raises it. One of the roles of cobalt in maraging steels is to raise the Ms temperature so that greater amounts of other alloying elements (that is, titanium and molybdenum, which lower the Ms temperature) can be added while still allowing complete transformation to martensite before the steel cools to room temperature.

Ms temperature ranges from 200 to 300 °C. Most of the maraging steel grades are martensitic at

room temperature. The martensite is normally a low-carbon, body-

centered cubic (bcc) lath martensite containing a high dislocation density but no twinning. This martensite is relatively soft ( 30 HRC), ductile, and machinable.

The age hardening of maraging steels is produced by heat treating for 3 to 9 h at temperatures of the order of 455 to 510 °C (850 to 950 °F).

Two type of possible martensite structures can be formed, one containing 5 to 10% Ni which is formed when cooling rate is faster and when % of nickel is increased then the cooling rate necessary to form martensite is lowered and fully martensitic structure is obtained. This type of martensite is called lath martensite and it contain upto 25% Ni. Ni in access to 25% will lead to the formation twinned martensite.

The lath martensitic structure of maraging steels consists of several martensitic packets within each prior austenitic grain and numerous blocks within each packet.

The packets and blocks are planar, lie along one direction, and are parallel to each other.

Packets are the predominant structure of lath martensite followed by the block structures that appear as discrete areas within each packet.

The typical packet/block structure of 18 Ni Marage 250

The substructure of each individual block of lathmartensite consists of a series of fine parallel laths as shown by TEM

Hardness of MD250 maraging steel for different temperature as a function of aging time

The above graph clearly shows that the hardness first increases and then decreases with the increase in the time of aging because of the fact that its structure has a tendency to revert to equilibrium phases-mainly ferrite and pearlite on prolonged aging but enough hardness is achieved up to 1035 MPa before this reaction starts.

This softening occurs due to 2 reasons a) over aging i.e. coarsening of precipitate particles. b) austenitic reversion These two processes are interlinked i.e. the dissolution of

metastable nickel-rich precipitate particle in favor of equilibrium iron rich precipitate locally enriches the matrix in nickel, which favors austenite formation.

Role of Mo in maraging steel Age hardening in maraging steels results primarily

from the precipitation of intermetallic compounds. Precipitation takes place preferentially on dislocations and within the lath martensite to produce a fine uniform distribution of coherent particles.

The major hardener is molybdenum, which upon aging initially forms Ni3Mo, with an orthorhombic Cu3Ti-type structure.

The metastable Ni3Mo phase forms initially because of its better lattice fit with the bcc martensitic matrix.

Growth of the Ni3Mo is restricted by coherency strains, and as such, further aging results in the in situ transformation of Ni3Mo to the equilibrium Fe2Mo phase, which has a hexagonal C14-type structure

Molybdenum also plays the necessary supplemental role of minimizing localized grain-boundary precipitation by lowering the diffusion coefficients of a number of elements in solid solution. Precipitation of these grain-boundary phases severely impairs the toughness of most molybdenum-free ferrous alloys.

It has been observed that the discrete particles of austenite are formed on the grain and the grain boundaries of Mo free 18Ni(300),whose precipitation adversely affect the toughness and ductility at localized scale

Ti in maraging steels Titanium, which is generally present in maraging

steels, promotes additional age hardening through the precipitation of Ni3Ti, which has a DO24 ordered hexagonal structure

Titanium are susceptible to the formation of TiC films at austenite grain boundaries.

The solutioning of carbides at temperatures greater than 1150 °C (2100 °F), followed by holding at temperatures of the order of 900 to 1095 °C (1650 to 2000 °F), results in the reprecipitation of titanium carbide in the form of films on austenite grain boundaries.

Co in maraging steels Cobalt does not play any direct role in the age

hardening of the maraging steel as it does not form any precipitates with the elements present in the maraging steels i.e. iron, molybdenum, nickel or titanium

The main contribution of cobalt is to lower the solubility of molybdenum in the martensitic matrix and thus increase the amount of Ni3Mo precipitate formed during age hardening. Some hardening also results from a short-range ordering reaction in the matrix that involves cobalt.

Steels with high titanium contents have greater hot strength than conventional steels and require higher hot-working loads or higher working temperatures. Working above about 1260 °C (2300 °F) should be avoided. To maximize their mechanical properties, maraging steels should be hot worked at the lowest temperatures that equipment power limitations permit.

The precipitation of TiC films should be avoided as it occurs when billets are hot worked at very high temp and is cooled slowly through a temp. range of 750-1095 0C, therefore the long dwell time should be avoided in this temp region after hot working so that Ti and C remain in the solution.

It is permissible to heat maraging steels to temperatures in excess of 1175 °C (2150 °F) and rapidly cool or hot work through the 750 to 1090 °C (1380 to 2000 °F) range.

After a material has been heated above 1175 °C (2150 °F) and successfully cooled to room temperature, it may be heated into the 750 to 1090 °C (1380 to 2000 °F) range without problems because any carbides and possibly carbonitrides will have precipitated as discrete, stable particles during previous cooling to below 750 °C (1380 °F), and thus cannot reprecipitate as a film on reheating.

Austenitic reversion Austenite reversion in maraging steels is caused by

nickel enrichment of the bcc matrix. Ni3Mo precipitates, which are most responsible for the

age hardening are metastable Formation of Ni3Mo is favored initially over the

formation of Fe2Mo because of its better fit with the bcc martensitic matrix

As the orthorhombic Ni3Mo precipitates grow, the coherency strains between the precipitate and the matrix also increase and it reaches to a stage where the stability of Ni3Mo is upset.

At this point Ni3Mo starts dissolving and is replaced by Fe2Mo

Nitriding

Hardness levels equivalent to 65 to 70 HRC can be achieved at depths up to 0.15 mm (0.006 in.) after nitriding for 24 to 48 h at 455 °C (850 °F).

Nitriding at this temperature allows simultaneous age hardening to occur.

Such treatment must be very carefully controlled to avoid excessive overaging.

Fatigue strength and the wear resistance of maraging steels are improved by nitriding.

Forging temperature for maraging steels 0C 0F 1100-1250 2010-2280

Heat treatments and typical mechanical properties of standard 18Ni maraging steels

Treatment A; solution treat 1 h at 820 °C (1500 °F), then age 3 h at 480 °C (900 °F).

Treatment B: solution treat 1 h at 820 °C (1500 °F), then age 12 h at 480 °C (900 °F).

Treatment C: anneal 1 h at 1150 °C (2100 °F), age 1 h at 595 °C (1100 °F), solution treat 1 h at 820 °C (1500 °F) and age 3 h at 480 °C (900 °F)

The standard aging treatments listed above produce contraction in length of 0.04% in 18Ni(200), 0.06% in 18Ni(250), and 0.08% in both 18Ni(300) and 18Ni(350). These very small dimensional changes during aging allow many maraging steel components to be finish machined in the annealed condition, then hardened. When precise dimensions must be held, an allowance for contraction can be made.

Microstructure containing coarse precipitate particles and finely distributed austenite particles should have good resistance to both fracture and stress-corrosion cracking.

Surface treatments Grit blasting is the most efficient technique for

removing oxide films formed by heat treatment. Maraging steels can be chemically cleaned by pickling

in sulfuric acid or by duplex pickling in hydrochloric acid and then in nitric acid plus hydrofluoric acid.

As with conventional steels, care must be taken to avoid over-pickling.

The sodium hydride cleaning of maraging steels should be avoided to minimize problems with crack formation.

Grease and oils can be removed by cleaning in trichloroethane-type solutions.

Maraging steel can be nickel plated in chloride bath These steel are less susceptible to hydrogen

embrittlement during plating but it is not immune and baking is recommended after plating

Baking should be done at temperatures of about 150 to 205 °C (300 to 400 °F) for periods of 3 to 10 h, depending on size and baking temperature.

Surface hardening can be achieved by nitriding maraging steels in dissociated ammonia. Hardness levels equivalent to 65 to 70 HRC can be achieved at depths of up to 0.15 mm (0.006 in.) after nitriding for 24 to 48 h at 455 °C (850 °F).

Plane-strain fracture toughness of maraging steels compared with fracture toughness of several ultrahigh strength steels as a function of tensile strength.

Effect of temperature on properties of maraging steel a) stress

2)Ductility

3) Hardness

Some physical properties of maraging steel

Martensitic microstructure of 18Ni(250) maraging steel in the solution-annealed condition(305 HV)

Martensitic microstructure of 18Ni(250) maraging steel in the solution-annealed and aged condition (523 HV).

Martensitic microstructure of low-residual 18Ni(250) maraging steel in the solution-annealedcondition (319 HV)

Martensitic microstructure of low-residual 18Ni(250) maraging steel in the solution-annealed and aged condition (565 HV).

Martensitic microstructure of solution-annealed and aged 18Ni(300) maraging steel (596 HV).

Martensitic microstructure of solution-annealed and aged cobalt-free Vascomax T-250 maragingsteel (535 HV).

At high temperatures the flow stress is influenced by strain rate and can be approximated by following formula

Where is strain rate and C and m are empirical constants