Definitions of Maraging Steel
Maraging steel is ultra-high-strength steel with carbon-free (or ultra-low carbon) iron-nickel martensite as the matrix and precipitation hardening of intermetallic compounds after aging.
The steel has good formability before high-strength aging treatment, almost no deformation after aging treatment, and high strength and toughness after aging treatment.
Maraging Steel Heat Treatment
Generally, the heat treatment of maraging steel is solution annealed at 820 ℃, and the holding time is calculated according to its cross-sectional size for 1 h every 25 mm.
The cooling rate after annealing has almost no effect on the microstructure and properties of the steel.
But before the aging treatment, the maraging steel must be cooled to room temperature. It is usually aged at 480 ℃ for 3 to 6 hours.
Production Process of Maraging Steel
Maraging steel is a Fe-Ni alloy containing no carbon and added with cobalt, molybdenum, titanium, and other elements. Typical steel grades such as iron base contain 17%～19% Ni, 7%～9% Co, 4.5%～5% Mo, and 0.6%～0.9% Ti. This type of alloy is quenched into martensite and then tempered at 480-500°C.
During the tempering process, the alloying elements are supersaturated in the martensite, which precipitates from the martensite to form intermetallic precipitates, resulting in a strong precipitation strengthening effect.
According to the analogy of precipitation strengthening of aluminum, copper, and other non-ferrous alloys, this process can be called aging treatment. And because the initial structure is maraging, this type of steel is called maraging steel.
High-strength maraging steel can be obtained through aging treatment. The treatment process is generally: aging temperature is 480℃, high-strength steel can be used at 510℃, the aging time is 3 to 6 hours, and air cooling after aging. After aging treatment, a large number of dispersed ultrafine intermetallic compound particles are deposited on the martensite matrix, thereby doubling the strength of the material with little loss of toughness.
What is maraging steel used for?
1) Maraging steel has high strength and toughness before aging, so it is used to manufacture thin shells of rockets and missiles, aerospace structural steels, pressure vessels, and other high-tech fields such as aviation, aerospace, and atomic energy.
2) Using the stable microstructure performance of maraging steel, even after the temperature is too high and averaging, the softening process is very slow, and it is used as engine parts such as cranks, gears, shafts, and automatic The firing pin of the weapon.
3) Because of its uniform expansion and easy cutting before aging, maraging steel can be used for high-wear units on assembly lines and for mold manufacturing such as forging dies, aluminum alloy extrusion dies, and tool materials.
4) Maraging steel is used in sports equipment. The blade used in fencing is usually made of maraging steel, because the growth rate of cracks in maraging steel is 10 times slower than that in carbon steel, and this reduces the breakage of the blade. People have fewer chances of harm.
5) Maraging steel has been used in medical equipment to make surgical instruments and hypodermic syringes.
Although the price per kilogram of maraging steel is more expensive than ordinary steel, the parts made of maraging steel have a lower cost due to the high strength/mass ratio, and the processing cost is significantly lower.
Maraging Steel Specification
Grade Chemical compositions / % Yield strength/ MPa
Ni Co Mo Ti Al
18Ni(200) 18 8.5 3.3 0.2 0.11 1400
18Ni(250) 18 8.5 5 0.4 0.1 1700
18Ni(300) 18 9 5 0.7 0.1 2000
18Ni(350) 18 12.5 4.2 1.6 0.1 2400
18Ni(cast) 17 10 4.6 0.3 0.1 1650
400Alloy 13 15 10 0.2 0.1 2800
500Alloy 8 18 14 0.2 0.1 3500
Maraging steel properties
High strength, good toughness
Maraging steel is the steel with the highest strength. Although its strength is high, it has higher toughness than other ultra-high-strength steels of the same strength level. It is the steel with the highest strength and toughness among existing materials.
The simple heat treatment process
The simple heat treatment process is another advantage of maraging steel. The traditional process is a solid solution at 850～870 ℃, followed by air cooling or water quenching. The cooling rate has little effect on the structure and performance. Then it is heated to 480 ℃ for aging. For high-strength steel grades, 510 ℃ can be used. The aging time is 3 to 6 hours. After aging, it is air-cooled.
Good welding performance
Excellent welding performance is one of the important advantages of maraging steel. Inert gas shielded welding is the most commonly used welding method. The welding performance of maraging steel is different from the previous ultra-high-strength martensitic steel. It does not require pre-heating and post-heating, and it will not produce welding cracks.
Good cold and hot processing performance
Hot-working is easier, and the workability is roughly the same as that of SU304 steel ingot. For maraging steel, the most commonly used hot working techniques, such as forging and rolling, can be used.
Advantages of maraging steel
- Super high strength at room temperature
- Simple heat treatment to ensure minimal heat treatment distortion
- Compared with hardened steel at the same strength level, it has excellent fatigue toughness
- Low carbon content, thereby eliminating decarburization problems
- Section size is an important factor in the hardening process
- Easy to process
- Good welding performance
- Has high strength and high toughness
- Easy cutting processing, low processing deformation
- Uniform and stable shrinkage during heat treatment
- Easy Nitriding
- Has good corrosion resistance and crack growth ability
- High polished finish
Conclusion
Maraging steel not only has high strength but also has good plasticity, fracture toughness, weldability, hot and cold workability, and stress corrosion resistance. Maraging steel has been widely used in the fields of aerospace, marine engineering, atomic energy industry, structural parts, and tooling.
