Martensitic precipitation hardened steel grades - Maraging
|Maraging C250 - X2NiCoMo18-8-5 - 1.6359 - AMS 6512|
|Maraging C300 - X2NiCoMo18-9-5 - 1.6358 - AMS 6514|
|Maraging C350 - X2NiCoMoTi18-12-4 - 1.6356 - AMS 6515|
|X3NiCoMoTi18-9-5 - 1.2709|
Maraging steel - basic information
Maraging grades are a separate subgroup of high alloy structural steels which chemical composition consists mainly of Nickel, Cobalt, and Molybdenum. After the chemical composition itself, it can be said that they lie between a group of iron-nickel alloys, tool steels and structural steels for heat treatment Carbon present in the alloy grades for improving, carburizing or nitriding is much higher (usually <0.1%), which is limited to <0.03% for Maraging steel.
The second issue to consider is the solid carbon solution and the carbide phase, which is replaced by the separation of intermetallic phases. Maraging steels are one of the most durable iron-nickel grades with considerable plasticity, which, as one of the few, can work in a wide range of temperatures: -200 to 600 ℃ and their hardness is almost unlimited.
Other features include good machinability, weldability and the ability to work continuously in high and low temperatures (depending on grades). The name Maraging is derived from a combination of two words in the topic of heat treatment and the structure obtained after it - Martensite and Aging - Martensite + aging in English
Heat treatment of Maraging
Maraging heat treatment is a process of supersaturation where steel acquires martensic structure through aging. It depends on the hardening of the product usually in the range 800 - 900 ℃ depending on the grades, with cooling in the air. Tempering is carried out at temperatures of 450-550 ℃. In the hardening process, tensile strength of 1000 - 1200 MPa and hardness in the range 30-40 HRC can be achieved.
Inadequate heat treatment, however, does not guarantee a fully satisfactory mechanical properties. By tempering, Maraging materials achieve a hardness of 40-60 HRC, with a strength of Rm between 1500 and 2200 MPa. The process of hardening or oversaturating Maraging steel results in a martensitic structure.
Chemical composition and characteristics
Chemical composition of steel consists of, among others, nickel, cobalt, titanium, and molybdenum. The remaining components in small quantities are Aluminum, sometimes Chromium, Zirconium, Tungsten, Niobium and Beryllium. Each of the basic alloying additives listed above has a significant influence on the mechanical and physical properties, the material structure and the performance characteristics in the individual environments.
Carbon limitation was introduced to reduce the occurrence of undesired carbides. Nickel, as one of the most valuable alloying additives, lowers the threshold of brittleness, is responsible for hardening, and thus facilitates the transformation of the structure into martensitic. Cobalt significantly affects hardness and durability, especially at elevated temperatures. The absence of a suitable amount of carbon in the composition requires the use of a replacement that will elevate the strength and hardness.
Cobalt further reduces the solubility of Molybdenum and Tungsten in alpha iron. Molybdenum is the third most concentrated element, cooperating closely with Cobalt. It has a positive effect on the ductility of steel, further contributing to the formation of intermetallic phases within the grain.
Maraging steel and it's application
Maraging is undoubtedly a unique class of high-alloy steel, which potential can be justified by its wide application in the armaments, aerospace, chemical, aerospace, shipbuilding, oil and petrochemical, and construction and power industries.
These martensitic steels are used in the manufacture of high-pressure tanks, installations, rotors and steam turbines, rocket engines, hulls and ships, responsible springs, bearings, guns, blanks, helicopters, gas turbine shafts, as well as gears, ballasts, compressor discs, safety valves, rotors for ship generators, some connecting rods, propulsion shafts, vehicle timing components and bearings.
For many readers, it may be interesting to note that maraging products can be classified as either tool or high-speed grades - of course by looking at the application, you can find tools and dies for hot work or special knives, machine parts.