Maraging Steels: Composition and Characteristics | Metallurgy

In this article we will discuss about:- 1. Meaning of Maraging Steels 2. Composition of Maraging Steels 3. Characteristics.

Meaning of Maraging Steels:

Maraging is age hardening of martensite due to the precipitation of intermetallic compounds. The maraging steels offer a remarkable combination of strength and ductility, with a yield strength as high as 2400 MPa with total elongation of 6%. By virtue of their high cost, maraging steels are used mainly in special applications such as rocket casing and other aerospace applications.

In 1958, Bieber showed that low carbon iron-nickel martensites containing titanium and aluminium could be precipitation hardened to HRC-65. Maraging steels are highly alloyed low carbon iron-nickel martensites which possess an excellent combination of strength and toughness better than most carbon-hardened steels. It is an attempt to replace carbon steels at least in critical applications.

High strength in maraging steels is developed due to the formation of very low carbon, tough and ductile iron-nickel martensite but comparatively soft, about 30 HRc, which is further strengthened by the precipitation of intermetallic compounds during age hardening. These steels have high fracture toughness K_IC = 120 MNm^-3/2. As maraging steels are practically non-carbon (C max. 0.03%), no carbides precipitate here.

Composition of Maraging Steels:

There is practically no carbon in them (less than 0.01-0.03%). A nickel content of around 18% helps to get complete lath type martensite formed by air cooling to room temperature. A larger volume of finely dispersed intermetallic compounds which induce high yield strength in maraging steels require nickel content of around 18%. Presence of more than 23% Nickel results in undesirable twinned martensite.

To ensure high toughness, the contents of residual elements P, S, C and N are kept as low as possible otherwise, segregation or precipitation mainly of Ti (C, N) and Ti₂S at prior austenite grain boundary causes embrittlement. Embrittled material can be restored to ductile state (if large % of elements are present) by a solution treatment at about 980°C followed by rapid cooling.

Molybdenum (≈ 6%) and titanium (≈ max 2%) are necessary additions as these form hardening precipitates Ni₃Mo, Ni₃Ti and even Lave phase, Fe₂Mo. Titanium is a stronger hardener, molybedenum and aluminium are moderate hardeners, and cobalt (does not form the precipitates) is a weak hardener. Higher than 1.2% titanium reduces ductility before and after ageing. Molybdenum forms rod shaped Ni₃Mo (25A° wide and 500 A° long at peak hardness) coherent precipitates, which distort the BCC matrix to increase strength and hardness.

Titanium forms stable Ni₃Ti and distorts the BCC lattice. Molybdenum reduces grain boundary precipitation to avoid severe drop in ductility. The presence of more than 1% Mo promotes undesirable austenite reversion. The yield strength of 18% Ni-8% Co-5% Mo increases from 1375 MPa to 2410 MPa as titanium contents increases from 0.20% to 1.4%.

Cobalt has many roles to play. To obtain complete lath type martensite on cooling to room temperature, M_s temperature should be 200° to 300°C. Additions of all the important elements in iron lower the M_s temperature.

One of the roles of 6-8% cobalt in maraging steel in to raise M_s temperature, so that greater amounts of titanium, molybdenum can be added as hardeners and still allow complete transformation to lath martensite before the steels cools to room temperature.

The absence of cobalt in maraging steels requires reduction of these elements and even nickel is then not present higher than 18%. Cobalt does not form precipitate but increases hardness and strength of maraging steels by forming short-range ordering.

Cobalt has greater affinity for iron, and thus does not allow easily metastable. Ni-rich precipitates to change to equilibrium iron-rich precipitate Fe₂Mo which reduces the hardness. Cobalt also retards reversion to undesirable austenite.

Cobalt reduces solid solubility of molybdenum in BCC matrix and promotes fine dispersion of Ni₃Mo precipitate particles and increases the volume fraction of this precipitate. Aluminium is present up to 0.1% as it also raises M_s temperature.

Table 9.5 shows some maraging steels with their properties:

Characteristics of Maraging Steels:

1. As there is practically no carbon in them, martensite is soft, tough, ductite and readily machinable. Ageing is done after machining operations. No carbides are precipitated.

2. As the CCT diagrams of 18 Ni maraging steels don’t show presence of bainite and pearlite transformations, these steels transform to martensite on air cooling from austenitic range. The problems of distortion thus, are almost non-existent.

3. Of the two types of martensites, alloy composition of maraging steels is so chosen to get complete transformation to lath martensite as the steel cools to room temperature. Lath martensite has high density of dislocations, which are uniformly distributed. These promote improved response to age-hardening, first by providing a large number of preferred nucleation sites for the inter-metallic compound precipitates, to form during ageing; secondly, these provide preferred diffusion paths (during ageing) for substitutional solute atoms. Thus, ageing is done for 3-5 hours at 480°c. Ageing results in fine dispersion of precipitates with no preference for grain boundaries.

4. The Fe-Ni maraging steels exhibit athermal hysteresis in phase transformation, i.e., on heating for ageing, the reversion of martensite to austenite is prevented, or minimised.

5. Retained austenite causes variations in tensile strength, ductility, and toughness and is thus undesirable. Twinned martensite is also not desirable. Thus, M_s temperature of the steels is kept high around 200° to 300°C.

6. Normally, maraging steels ingots are heated to 1250°C with hot rolling finished at about 920°C. The steels are subsequently reheated to 815°C to austenitise them. Lower austenitising temperatures result in lower strength and ductility due to incomplete solution of hardening elements. Higher temperatures result in grain growth to reduce the strength of the steels. Boron 0.001 to 0.003% minimises grain growth and resulting reduction in strength. Normal austenitising time, or called as solution-annealing time is 1 hr.

7. Thermal cycling of these steels between M_f and a temperature above solution-annealing temperature does refine the grain size by recrystallisation, but grain size finer than ASTM 6 or 7 cannot be achieved by this method.

8. Age hardening is normally done at 455 to 510°C for 3-12 hours, and then air cooled to room temperature. Normally 18 Ni steels are aged at 480°C for 3 hrs.

Fig. 9.13 illustrates effect of ageing 18 Ni 250 alloy at different ageing temperatures. Overaging appears to improve significantly the plain-strain fracture toughness (K_IC).

9. During ageing, first short range ordering occurs to form (i) iron and cobalt rich regions, (ii) nickel-rich regions. Ni₃ Mo (rod-shape), Ni₃ Ti compounds are formed in nickel-rich regions. These precipitates form on dislocations and the lath boundaries, i.e., are uniformly and finely formed to increase the hardness. Short range order of cobalt also increases hardness. The size of these precipitates increases to ultimately attain the peak hardness. The coherency strains become large to help in dissolving Ni₃Mo and to precipitate Fe₂Mo particles. This is the overageing. Simultaneously, reversion of austenite also occurs. These factors result in decrease in hardness.

10. Nitriding can be done of maraging steels to induce hardness 65-70 HRC up to a depth of 0.15 mm after nitriding for 24-48 hours at 455°C. This improves wear resistance and fatigue resistance.

11. During age hardening, no incubation period is needed for the precipitation, as here is no free energy barrier to precipitation. This is because there is high degree of super saturation in the alloy, and precipitation occurs due to heterogeneous nucleation on the dislocations. The diffusion paths are available of dislocations for solutes to diffuse easily and quickly to cause precipitation soon.

12. Maraging steels are easily weldable. Conventional welding techniques produce sound welds. However, after welding, solution annealing and ageing may be done.