What Is Maraging Steel?
Ultra-high-strength steel based on carbon-free (or micro-carbon) martensite, which can produce precipitation hardening of intermetallic compounds when aging. Unlike traditional high-strength steel, it does not use carbon but is strengthened by the precipitation of intermetallic compounds. This makes it have some unique properties: high strength and toughness, low hardening index, good formability, simple heat treatment process, almost no deformation during aging, and good welding performance. As a result, maraging steel has been widely used in sectors requiring such characteristics.
- Martensitic aging steel with industrial application value was first developed by the International Nickel Corporation (INCO) in the early 1960s. From 1961 to 1962, the company B0.F0.Decker et al. Added different contents of cobalt, molybdenum, and titanium to iron-nickel martensitic alloys, and obtained 18Ni (200) with yield strengths of 1400, 1700, and 1900 MPa through age hardening. , 18Ni (250) and 18Ni (300) steel, and 18Ni (200) and 18Ni (250) were first applied to the rocket engine casing. The emergence of this type of steel immediately attracted great attention from metallurgical workers in various countries. The middle and late 1960s were the golden age of research and development of maraging steel. During this period, the International Nickel Corporation and the Vanadium Alloy Steel Corporation (VasCo) developed 18Ni (350) with a yield strength of 2400 MPa. Researchers have also done a lot of work on the processing technology, various properties and strengthening and toughening mechanisms of maraging steel, and also explored the so-called 400 and 500 martensitic aging steels with yield strengths up to 2800 and 3500 MPa. . However, due to the low toughness of these two grades, and the production process is too complicated, it has not been practically used. During this period, maraging steel also has a certain market in the field of tooling. At the same time, countries such as the former Soviet Union and the Federal Republic of Germany also began research on maraging steel. In the 1970s, due to the development of uranium enriched centrifuges, Japan conducted a systematic and in-depth study of maraging steels. Since the 1980s, due to the continuous rise in cobalt prices, great progress has been made in the development of cobalt-free martensitic aging steels, such as T-250 in the United States (18Ni- 3Mo-10.4Ti-0.1A1) and Japan's 14Ni- 3Cr- 3Mo10.5Ti alloy, South Korea s w250 (18Ni40.5w10.4Ti0.1A1) and the former Soviet Union s H161 ~ 6M6 (16Ni6V6Mo) have all come out. These steels not only reduce production costs by 20% to 30%, but also the properties of cobalt-containing martensitic aging steels that are very close to the corresponding strength levels.
- Maraging steel is one of the ultra high strength steels. The outstanding advantage of this steel is that the heat treatment process is simple and convenient. After solution treatment, mechanical processing is performed before aging. The heat treatment has small deformation, and the processability and welding performance are very good. The heat treatment process and properties of maraging steel are shown in the table below. In recent years, martensitic aging steel has been widely used to make molds abroad; however, domestically, martensitic aging steel contains noble metal elements such as Ni, Co, etc., and its content is high and expensive, which is still difficult to be widely used. This type of steel is mainly used for precision forging dies and plastic molds.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
- China has been developing maraging steel since the mid 1960s. At the beginning, imitation of 18Ni (250) and 18Ni (300) were the main ones. By the mid-1970s, they began to study higher-grade steels and cobalt-free or nickel-nickel-cobalt maraging steels. They also developed ultra-high-purity, high-strength and toughness martensitic steels for high-speed rotating bodies cM-1 steel), developed high elastic martensitic aging steel (TM210, etc.) and low-nickel cobalt-free martensitic aging steel (12Ni-3Mn3Mo-TiAlV).
- Martensitic aging steel is not strengthened by carbon supersaturated solid solution or carbide precipitation, but is strengthened by the precipitation of intermetallic compounds during aging of certain alloying elements. Therefore, carbon in steel is the same as sulfur and phosphorus. , Is a harmful impurity element. The lower the carbon content is, the better, generally it should not exceed 0.03% (for important applications, it should be less than 0.01%). The main alloying elements in steel are nickel, cobalt, molybdenum, titanium,
- Chromium and manganese can be used to partially replace nickel and cobalt in maraging steels, and tungsten-molybdenum or vanadium-cobalt have been used in the recent development of cobalt-free steels. Silicon is an impurity element, and its content should not exceed 0.1%. Aluminum is generally added as a deoxidizer during steelmaking, and its residual amount is in the range of 0.05% to 0.2%. In addition, boron, zirconium, calcium, magnesium, and rare earth elements can be used for trace element treatment to improve certain properties of steel.
- Typical martensitic aging steels commonly used are classified by strength level
- 18Ni (200), 18Ni (250), 18Ni (300), and 18Ni (350) are widely used steels.
- In recent years, some countries have developed many variants of martensitic aging steel, especially many cobalt-free martensitic aging steels with good properties.
- The type of steel that has high ductility, toughness and ultra-high strength is maraging steel. The material science for developing this steel is high-purity, high-nickel, ultra-low-carbon steel. In this way, the martensite structure can be obtained by quenching even at a small cooling rate, and it has excellent impact resistance and fracture toughness. The three main maraging steels are:
- Nil8 maraging steel
- There are three types of this steel, with yield strengths of 1350 MPa, 1650 MPa, and 1950 MPa, respectively. This type of steel has a very low impurity content and requires one or two vacuum smelters. And it contains 0.003% B, 0.002% Zr and 0.005% Ca to remove impurities and help improve thermoplastic processability.
- The heat treatment process includes solution treatment at 850 ° C to 870 ° C, air cooling or water quenching, and aging at 480 ° C for 3 hours. In addition to Co, the alloying elements added all lower the Ms point, but the Mf point can be kept above room temperature, so that it can be completely transformed into martensite after quenching after solid solution. Aging precipitation hardening phase is mainly small flakes
- The main production processes are smelting, hot working, cold working, welding, heat treatment and surface treatment.
Smelting of maraging steel
- Generally, a double vacuum smelting process of vacuum induction furnace melting and vacuum consumable furnace remelting is used. For steel grades below 1500 MPa, non-vacuum smelting or non-vacuum smelting plus electroslag remelting can be used. However, for high-strength grades and important steel types, a double vacuum smelting process must be used. During vacuum self-consumption remelting, the current and the temperature of the molten pool should be strictly controlled to avoid severe dendritic segregation of the ingot.
Hot working of maraging steel
- Maraging steel has good thermoplasticity at high temperature, and its hot workability is about the same as 1Crl8Ni9Ti. For steel grades with high titanium and molybdenum content, microsegregation of these elements is prone to occur when the ingot is solidified, and an anisotropic band-like microstructure is formed after hot working. An effective measure to reduce or eliminate microsegregation is to choose a suitable steel ingot size and perform sufficient high temperature homogenization treatment during hot working. In order to prevent the high temperature slow cooling brittleness caused by the precipitation of Ti (C, N) and other compounds along the austenite grain boundary, the workpiece should be cooled or stayed in the temperature range of 1100 750C as much as possible after hot working. In order to obtain fine grains and better mechanical properties, the final forging should be completed at a lower temperature (950 ~ 850C) with a large amount of deformation (greater than 25%).
Cold working of maraging steel
- Cold workability is very good in the solid solution state. Drawing, cold rolling, bending, and deep drawing are easy to perform. The work hardening index of steel is 0.02 to 0.03, which is an order of magnitude lower than that of ordinary steel. Therefore, it is possible to perform cold working with a deformation of more than 90% without softening annealing during processing.
Martensitic aging steel welding
- Good weldability is one of the advantages of maraging steel. Almost all welding processes are applicable. The composition of the welding wire is basically the same as that of the steel being welded. There is no need to preheat before welding, no cracks will not occur after welding, and the joint coefficient can exceed 90% after direct aging.
Heat treatment of maraging steel
- Simple heat treatment process is another important advantage of maraging steel. After the steel has been hot-worked, it should be solution-treated before cold-worked and aged. The purpose is to dissolve the precipitate remaining after hot processing; to dissolve the matrix with sufficient strengthening elements; and to obtain a uniform martensitic structure with a high dislocation density. The solution temperature is usually 820 840 , the solution time is 1h per 25 ram thickness, the solution is air-cooled after solution, and the cooling rate has little effect on the structure and performance. The high strength of maraging steel is obtained by aging treatment. The aging temperature is generally 480 ° C. For high-grade steels, 510 ° C can be used. The aging time is 3 to 6 hours. The aging is air-cooled. After aging, a large number of scattered and ultramicroscopic intermetallic compound particles are precipitated on the martensite matrix, which makes the strength of the material doubled and the loss of toughness is small.
- The properties of maraging steel can also be improved by austenite deformation, martensite deformation, or a combination of both. The austenite deformation treatment reduces the austenite grain size to less than 10um, thereby obtaining a martensitic aging steel with a certain ductility and a strength greater than 3500 MPa. Martensitic deformation treatment after solution treatment and before aging usually increases the strength by 200 MPa because more dislocations are generated. Martensite deformation before solution, can refine austenite grains and increase the strength after aging of steel.
Surface treatment of maraging steel
- Without surface treatment, the wear resistance and fatigue strength of maraging steel are not better than ordinary high-strength steel. Therefore, the surface treatment (gas nitriding, ion nitriding, ion implantation, etc.) must be performed on parts for this purpose. Ion nitridation can increase the contact fatigue life of 18Ni (250) steel rolling bearings by more than double.
- The application of maraging steel has been used in high-speed simplification of rocket engine housings, missile housings, uranium isotope centrifugal separators, helicopter landing gear, high pressure vessels, shafts, gears, bearings, high pressure sensors, fasteners, Springs, as well as industrial dies such as aluminum alloy extrusion dies and casting dies, precision dies, and cold dies have been widely used.