Definitions of Maraging Steel
Maraging steel is ultra-high-strength steel based on carbon or micro-carbon martensite, and precipitation hardening intermetallic compounds are produced during the aging process.
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.
Maraging steel properties
Solid solution strengthening. The first is the solid solution strengthening of martensite by carbon. The over-saturated interstitial carbon causes square distortion in the A-phase crystal lattice, forming a strong stress field. The stress field has a strong exchange effect with the dislocation, which hinders the movement of the dislocation and improves the hardness and strength of the martensite.
Phase change enhancement. The second is phase change strengthening. During the transformation of martensite, substructures with a high density of lattice defects are created in the crystal lattice, such as high-density dislocations in lath martensite, twins in lamellar martensite, etc. These defects are prevented from being the wrong Movement makes martensite strengthen. This is the so-called phase-change strengthening. Experiments have proved that the yield strength of carbon-free martensite is about 284Mpa, which is very close to the yield strength of deformation-strengthened ferrite, while the yield strength of ferrite in the annealed state is only 98~137Mpa, which indicates that phase transformation strengthening Increase the yield strength by 147~186MPa
Aging strengthening. Time strengthening is also an important strengthening factor. After the formation of martensite, since the point Ms of general steel is mostly above room temperature, self-tempering will occur during the quenching process and when staying at room temperature, or under the action of external force. That is, the atoms of carbon atoms and alloying elements diffuse and segregate to dislocations and other crystal defects, or carbides are dispersed and precipitated, pinning dislocations, making it difficult for dislocations to move, resulting in aging strengthening of martensite.
The influence of original austenite grain size and lath martensite bundle size on martensite strength. The original austenite grain size and the size of the lath martensite bundle also have a certain effect on the strength of martensite. The finer the original austenite grains and the smaller the martensite lath, the higher the martensite strength. This is due to the martensite strengthening caused by the phase interface hindering the movement of dislocations.
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