Maraging steel is a type of ultra-high strength steel, it is based on carbon-free or ultra-low carbon Fe-Ni Martensite as the matrix and making the metal compounds aged to precipitate and harden. The Maraging steel grades 18Ni(200), 18Ni(250) and 18Ni(300) steels with a yield strength of 1400, 1700 and 1900 MPa were obtained by INCO by adding different contents of Co, Mo and Ti into the Fe-Ni Martensitic alloy through aging hardening. The 18Ni(200) and 18Ni(250) steels were first applied to the shell of the rocket engine or some special requirements where need ultra-high strength. The strength of Maraging steel does not come from the carbon but the precipitation of intermetallic compounds. AISI 18Ni steel is the representative grade of Maraging steel. The tensile strength of the steel can be divided into:
| AISI | Grade | UNS | GB |
| 18Ni(200) | Maraging 200 | K92810 | 00Ni18Co8Mo3TiAl |
| 18Ni(250) | Maraging 250 | K92890 | 00Ni18Co8Mo5TiAl |
| 18Ni(300) | Maraging 300 | K93120 | 00Ni18Co9Mo5TiAl |
| 18Ni(350) | Maraging 350 | K93160 | 00Ni18Co12Mo4Ti2Al |
The chemical composition of 18Ni Maraging steel(MIL-S-46850D)
| Grades | C | Si | Mn | P | S | Ni | Co | Mo | Ti | Al |
| Grade 200 | ≤0.03 | ≤0.10 | ≤0.10 | ≤0.01 | ≤0.01 | 17.0~19.0 | 8.0~9.0 | 3.0~3.5 | 0.15~0.25 | 0.05~0.15 |
| Grade 250 | ≤0.03 | ≤0.10 | ≤0.10 | ≤0.01 | ≤0.01 | 17.0~19.0 | 7.0~8.5 | 4.6~5.2 | 0.3~0.5 | 0.05~0.15 |
| Grade 300
&300A |
≤0.03 | ≤0.10 | ≤0.10 | ≤0.01 | ≤0.01 | 18.0~19.0 | 8.5~9.5 | 4.6~5.2 | 0.5~0.8 | 0.05~0.15 |
| Grade 350 | ≤0.03 | ≤0.10 | ≤0.10 | ≤0.01 | ≤0.01 | 18.0~19.0 | 11.5~12.5 | 4.6~5.2 | 1.3~1.6 | 0.05~0.15 |
After solid solution, 18Ni steel forms ultra-low carbon Martensite with a hardness of 28~30HRC. After aging treatment, it gets aging hardening due to the dissolution and precipitation of various intermetallic compounds. The hardness can increase to 50HRC. The excellent characteristic of 18Ni steel is that it still has good toughness and high fracture toughness under the condition of ultra-high-strength and high hardness. At the same time, it’s without cold hardening, small aging heat treatment deformation, good welding performance, metals surface can be nitriding treatment.
18Ni maraging steel is a perfect combination of high strength and hardness, good toughness and plasticity. It has been widely used in aviation, aerospace, precision mold and other industrial fields, such as rocket engine shell, missile shell, uranium isotope centrifuge high-speed rotating cylinder and other high precision load-bearing parts of the material.
