Maraging steel

Maraging steels are steels that possess superior strength and toughness without losing ductility. Aging refers to the extended heat-treatment process. These steels are a special class of very-low-carbon ultra-high-strength steels that derive their strength from precipitation of intermetallic compounds rather than from carbon. The principal alloying metal is 15 to 25 wt% nickel. Secondary alloying metals, which include cobalt, molybdenum and titanium, are added to produce intermetallic precipitates.
The first maraging steel was developed by Clarence Gieger Bieber at Inco in the late 1950s. It produced 20 and 25 wt% Ni steels with small additions of aluminium, titanium, and niobium. The intent was to induce age-hardening with the aforementioned intermetallics in an iron-nickel martensitic matrix, and it was discovered that Co and Mo complement each other very well. Commercial production started in December 1960. A rise in the price of Co in the late 1970s led to cobalt-free maraging steels.
The common, non-stainless grades contain 17–19 wt% Ni, 8–12 wt% Co, 3–5 wt% Mo and 0.2–1.6 wt% Ti. Addition of chromium produces corrosion-resistant stainless grades. This also indirectly increases hardenability as they require less Ni; high-Cr, high-Ni steels are generally austenitic and unable to become martensite when heat treated, while lower-Ni steels can.
Alternative variants of Ni-reduced maraging steels are based on alloys of Fe and Mn plus minor additions of Al, Ni and Ti with compositions between Fe-9wt% Mn to Fe-15wt% Mn qualify used. The manganese has an effect similar to nickel, i.e. it stabilizes the austenite phase. Hence, depending on their manganese content, Fe-Mn maraging steels can be fully martensitic after quenching them from the high temperature austenite phase or they can contain retained austenite. The latter effect enables the design of maraging-transformation-induced-plasticity steels.

Properties

Due to the low carbon content maraging steels have good machinability. Prior to aging, they may also be cold rolled to as much as 90% without cracking. Maraging steels offer good weldability, but must be aged afterward to restore the original properties to the heat affected zone.
When heat-treated the alloy has very little dimensional change, so it is often machined to its final dimensions. Due to the high alloy content maraging steels have a high hardenability. Since ductile Fe-Ni martensites are formed upon cooling, cracks are non-existent or negligible. The steels can be nitrided to increase case hardness and polished to a fine surface finish.
Non-stainless varieties of maraging steel are moderately corrosion-resistant and resist stress corrosion and hydrogen embrittlement. Corrosion-resistance can be increased by cadmium plating or phosphating.

Grades of maraging steel

Maraging steels are usually described by a number, which indicates the approximate nominal tensile strength in thousands of pounds per square inch. The compositions and required properties were defined in US military standard MIL-S-46850D. The higher grades have more cobalt and titanium in the alloy; the compositions below are taken from table 1 of MIL-S-46850D. As of July 1, 2024, that standard was cancelled by the U.S. Military and replaced with a number of SAE AMS specifications, which now govern each grade in a separate specification, as enumerated below.

Element	Grade 200	Grade 250	Grade 300	Grade 350
Iron	balance	balance	balance	balance
Nickel	17.0–19.0	17.0–19.0	18.0–19.0	18.0–19.0
Cobalt	8.0–9.0	7.0–8.5	8.5–9.5	11.5–12.5
Molybdenum	3.0–3.5	4.6–5.2	4.6–5.2	4.6–5.2
Titanium	0.15–0.25	0.3–0.5	0.5–0.8	1.3–1.6
Aluminium	0.05–0.15	0.05–0.15	0.05–0.15	0.05–0.15
Tensile strength, MPa
SAE AMS Specification

This family is known as the 18Ni maraging steels, from its nickel percentage. There is also a family of cobalt-free maraging steels which are cheaper but not quite as strong; one example is Fe-18.9Ni-4.1Mo-1.9Ti. There has been Russian and Japanese research in Fe-Ni-Mn maraging alloys.

Heat treatment cycle

The steel is first annealed at approximately for 15–30 minutes for thin sections and for 1 hour per thickness for heavy sections, to ensure formation of a fully austenitized structure. This is followed by air cooling or quenching to room temperature to form a soft, heavily dislocated iron-nickel lath martensite. Subsequent aging of the more common alloys for approximately 3 hours at a temperature of produces a fine dispersion of Ni₃ intermetallic phases along dislocations left by martensitic transformation, where X and Y are solute elements added for such precipitation. Overaging leads to a reduction in stability of the primary, metastable, coherent precipitates, leading to their dissolution and replacement with semi-coherent Laves phases such as Fe₂Ni/Fe₂Mo. Further excessive heat-treatment brings about the decomposition of the martensite and reversion to austenite.
Newer compositions of maraging steels have revealed other intermetallic stoichiometries and crystallographic relationships with the parent martensite, including rhombohedral and massive complex Ni₅₀₅₀.

Processing of maraging steel

The maraging steels are a popular class of structural materials because of their superior mechanical properties among different categories of steel. Their mechanical properties can be tailored for different applications using various processing techniques. Some of the most widely used processing techniques for manufacturing and tuning of mechanical behavior of maraging steels are listed as follows:

Solution treatment: As described in the section of Heat treatment cycle, the maraging steel is heated to a specific temperature range, after which it is quenched rapidly. In this step the alloying elements are dissolved, and a homogeneous microstructure is achieved. Homogeneous microstructure thus achieved improves the overall mechanical behavior of maraging steels such as fracture toughness and fatigue resistance.
Aging of maraging steels: It is an important processing step as this step leads to precipitation of intermetallic compounds such Ni₃Al, Ni₃Mo, Ni₃Ti, etc. The semicoherent precipitates obtained during normal aging and incoherent precipitates obtained after overaging contribute to improvement of mechanical behavior by activating various strengthening mechanisms related to hindering of dislocation motion by precipitates. Strengthening mechanisms such as precipitate hardening where precipitates hinder dislocation motion via Orowan mechanism or dislocation bowing lead to increase in the ultimate tensile strength of maraging steels. Aging is also beneficial for reducing the microstructural heterogeneities which may occur due to non-uniform thermal distribution along the building direction in arc additive manufactured samples.
Laser Powder Bed Fusion : Laser Powder Bed Fusion is an additive manufacturing technique used to create components of intricate geometries using a powder metal which is fused together layer by layer using localized high power-density heat source such as a laser. The materials can be tailored to have specific mechanical properties by optimizing the process parameters associated with LPBF. It has been observed that processing parameters such as laser scanning speed, power and the scanning space can have significant effects on the mechanical properties of 300 maraging steel such as tensile strength, microhardness, and impact toughness. Along with the processing parameters, the type of heat treatment subjected to LPBF steels also play an important role. It is observed that processing parameters which have a higher magnitude reduce the relative density of the sample due to rapid vaporization or creation of voids and pores. It is also observed that the microhardness and strength of the steel decreases after solution treatment due to austenite reversion and disappearance of cellular microstructure. On the other hand, aging treatment after solution treatment increases the microhardness and tensile strength of steel which is attributed to formation of precipitates such as Ni₃Mo, Ni₃Ti, Fe₂Mo. The impact toughness increases after solution treatment but decreases after aging treatment, which can be attributed to the underlying microstructure consisting of tiny precipitates acting as regions of stress concentrators for crack formation. Formation of nanoscale precipitates of intermetallic compounds after aging process lead to marked increase in yield and ultimate tensile strength but substantial reduction in ductility of the material. This change in macroscopic behavior of the material can be linked to the evolution of microstructure from dimple to quasi-cleavage fracture morphology. Aging followed by solution treatment of selective laser melted steels also reduces the amount of retained austenite in the martensitic matrix and lead to change in the grain orientation. Aging can reduce the plastic anisotropy to some extent, but directionality of properties is largely influenced by its fabrication history.
Severe plastic deformation: It leads to increase in dislocation density in the materials which in turn assists in the ease of formation of intermetallic precipitates due to availability of faster diffusion pathways through the dislocation cores. It has been observed that plastic deformation before aging leads to reduced peak aging time and increase in peak hardness. Precipitate morphology in severely plastically deformed steel changes and becomes plate-like when overaged which is attributed to higher dislocation density. This in turn leads to significant reduction in ductility and increase in strength of the material. Along with morphology, the orientation of precipitates also play an important role in micromechanism of deformation as they induce anisotropy to the mechanical properties.