Maraging steel is a class of ultra-high-strength steels that achieve exceptional mechanical properties through a unique precipitation-hardening mechanism. Among various grades, 18Ni maraging steel is the most widely used commercial grade, with a typical composition of:
Fe-18Ni-8Co-5Mo-0.1Ti-0.1Al (mass%)
After solution treatment, maraging steel exhibits an austenitic structure. During subsequent cooling, the austenite transforms into a low-carbon martensitic structure. Aging treatment then promotes the precipitation of intermetallic compounds within the martensitic matrix, resulting in significant precipitation strengthening, known as maraging.
Common maraging steel grades include 18Ni, 20Ni, and 25Ni series, among which 18Ni maraging steels have the widest industrial applications due to their excellent balance of strength, toughness, and manufacturability.
Typical chemical compositions of representative 18Ni maraging steels are shown below:
The basic composition of maraging steel can be generally represented as Fe-X-Y:
X elements are martensite-forming elements, mainly austenite stabilizers such as Ni and Co, which enable the formation of a martensitic matrix after cooling.
Y elements are precipitation-strengthening elements, such as Mo and Ti, which form fine intermetallic compounds and significantly increase the strength of the steel.
The excellent strength of maraging steel is primarily attributed to the formation of a large amount of finely dispersed intermetallic precipitates. Increasing the content of precipitation-strengthening elements can further enhance the strengthening effect.
Compared with conventional steels, maraging steels offer outstanding combinations of:
Ultra-high strength
High fracture toughness
Excellent ductility
Superior machinability
Good weldability
Low heat-treatment distortion
Because of these advantages, maraging steels are widely used in advanced industries including:
Aerospace engineering
Space exploration
Marine engineering
Nuclear energy
Aircraft manufacturing
Automotive applications
Precision tooling and pressure vessels
Typical applications include:
Aircraft engine drive shafts
Rocket engine casings
Precision plastic injection molds
CVT (Continuously Variable Transmission) steel belts for automobiles
High-performance sporting equipment such as golf club heads
The physical properties of 18Ni maraging steel vary depending on alloy composition, heat treatment condition, and manufacturing process. The values listed in technical specifications represent typical characteristics and may differ slightly from actual products.
Key physical properties include:
High elastic modulus
Low thermal expansion coefficient
Excellent dimensional stability during heat treatment
These characteristics make 18Ni maraging steel particularly suitable for precision components requiring high strength and dimensional accuracy.
The mechanical properties of 18Ni maraging steels are commonly classified according to their 0.2% yield strength levels, such as:
18Ni Maraging 250
18Ni Maraging 300
18Ni Maraging 350
(The grade number represents the approximate yield strength level in ksi.)
Higher-strength grades contain increased amounts of precipitation-strengthening elements.
The precipitation strengthening effect is closely related to the Mo equivalent, calculated as:
Mo equivalent = Mo (%) + Co (%) / 3 + 3Ti (%)
Research has shown that approximately 1% increase in Mo equivalent can increase steel strength by about 100 MPa. In commercial 18Ni maraging steels, strength levels are mainly controlled through adjustment of Ti content.
YAG285 maraging steel is a high-strength grade developed by reducing expensive alloying elements such as Mo and Co while increasing Ti content. It provides excellent strength with improved cost efficiency.
The outstanding characteristics of maraging steel include:
Tensile strength up to approximately 2000 MPa
High fracture toughness
High specific strength
Excellent ductility
Superior cold working performance
Excellent weldability and machinability
Unlike conventional carbon steels, maraging steel contains almost no carbon. The matrix is primarily composed of Fe-Ni-Co martensite. After solution treatment, the steel contains a high density of dislocations, but because of its extremely low carbon content, the hardness remains around 300 HV, allowing excellent machinability and weldability.
During aging treatment, fine intermetallic compounds precipitate, providing significant strengthening while maintaining good toughness and dimensional stability.
The heat treatment process of maraging steel consists of two main stages:
Solution Treatment
Aging Treatment
These processes generate the martensitic matrix and precipitation-strengthened structure, respectively.
The typical solution treatment temperature range is:
800–900°C
During heating, reverse transformation of martensite to austenite begins at approximately 500°C.
For cold-worked materials containing residual strain, increasing the solution treatment temperature can promote recrystallization of austenite, resulting in grain refinement.
After solution treatment, the austenite transforms into martensite during cooling.
Typical transformation temperatures:
Ms (Martensite Start): approximately 200°C
Mf (Martensite Finish): approximately 100°C
Higher alloy content lowers the Mf temperature. If the cooling temperature after solution treatment is insufficient, retained austenite may remain, reducing the final tensile strength.
The typical aging temperature of maraging steel is approximately:
450–500°C
Generally:
Lower aging temperatures produce higher hardness and strength
Longer aging times are required to achieve peak properties
The optimal aging condition should be selected according to required mechanical properties and manufacturing requirements.
During aging, fine intermetallic compounds such as:
Ni₃Mo
Ni₃Ti
precipitate within the martensitic matrix.
These precipitates provide direct strengthening effects.
The role of alloying elements:
Mo and Ti: form strengthening precipitates
Co: improves martensite toughness and reduces Mo solubility, promoting Mo-rich intermetallic precipitation
Therefore, Co indirectly accelerates the precipitation-hardening process.
The primary strengthening mechanism of maraging steel used in automotive CVT applications is precipitation strengthening from:
Ni₃Ti
Ni₃Mo
However, Ti is a highly active element and can react with nitrogen and carbon to form hard non-metallic inclusions such as:
TiN
Ti(C,N)
These inclusions may act as fatigue crack initiation sites, especially under high-cycle fatigue conditions.
Advanced steel manufacturers have developed improved maraging steels by:
Refining non-metallic inclusions through advanced melting technologies
Reducing harmful Ti-based inclusions
Developing Ti-free maraging steels
By optimizing Al and Co contents, fine dispersed precipitates such as:
Ni₃Mo
NiAl
can be formed, significantly improving precipitation strengthening.
The addition of Cr also enables nitriding treatment while maintaining the advantages of conventional maraging steels.
The excellent combination of high strength and ductility is a key feature of maraging steel.
Further improvement in ductility can be achieved through:
Grain refinement
Optimization of cold working and hot working processes
Addition of micro-alloying elements such as boron (B)
Fine grain structures improve both mechanical strength and deformation capability.
For ultra-high-strength maraging steels, toughness is critical.
The fundamental approach to improving toughness is:
Reducing impurity levels
Minimizing non-metallic inclusions
Improving steel cleanliness
Advanced refining technologies are widely used to achieve superior toughness performance.
Maraging steel is recognized as one of the most important ultra-high-strength materials due to its excellent combination of strength and toughness.
However, impurities and non-metallic inclusions have significant effects on fatigue performance and fracture toughness.
To improve steel cleanliness, advanced melting technologies are widely applied, including:
Vacuum Induction Melting (VIM)
Vacuum Arc Remelting (VAR)
These technologies effectively reduce impurity elements and non-metallic inclusions, enabling higher-performance maraging steels.
New generations of maraging steels have been developed based on aerospace applications.
By adding:
Carbon (C)
Carbide-forming elements
additional carbide strengthening mechanisms are introduced alongside traditional intermetallic precipitation strengthening.
This approach further increases the strength capability of maraging steels.
In recent years, maraging steel powders have gained increasing attention in:
Metal additive manufacturing
Selective laser melting (SLM)
3D printed metal components
Due to their outstanding strength, toughness, and heat-treatment response, maraging steels are expected to play an increasingly important role in advanced manufacturing technologies.
Maraging steel is a premium ultra-high-strength alloy offering an exceptional combination of:
Tensile strength up to 2000 MPa
High toughness
Excellent machinability
Good weldability
Superior dimensional stability
With continuous developments in alloy design, advanced melting technology, and additive manufacturing, maraging steel continues to expand its applications in aerospace, automotive, energy, tooling, and high-performance engineering industries.