Alloy steels are called such steels., which get their improved properties due to:
– one or more special alloying elements;
– higher content, than in conventional carbon steels such elements as magnesium and silicon.
Alloy steels contain manganese, silicon and copper in higher concentrations, than is allowed for conventional carbon steels (1,65 % manganese; 0,60 % for silicon and 0,60 % on copper).
Alloying elements increase the mechanical and technological properties of steels. Usually alloy steels divided into three groups according to the total content of alloying elements (apart from carbon):
– low alloy steels - less 5 %;
– medium alloy steels - from 5 to 10 %;
– high-alloy steels - more 10 %.
Low alloy steels
Low alloy steels form a group of steels, which exhibit higher mechanical properties compared to conventional carbon steels. This is the result of the addition of alloying elements such as nickel., chromium and molybdenum. For many low alloy steels, the main function of alloying elements is to increase the hardenability of the steel., to then optimize the strength and toughness properties by means of heat treatment. In some cases, However, alloying elements are used to, to increase the resistance of steel to any specific influences.
Low alloy steels , in its turn, share:
- by chemical composition based on the main alloying elements: nickel, chromonic, molybdenum, chromium-molybdenum and similar steels;
- for heat treatment: hardened and tempered (martensitic), normalized and released, annealed and so on;
- by weldability.
Steels can have a huge variety of chemical compositions and, Besides, the same steels can receive different heat treatments. Therefore, there are certain "overlaps" in that classification of low alloy steels, which is presented above.
For this reason, low-alloy steels are often divided into four large groups., such as:
- low-alloy martensitic (improved) become;
- medium carbon high strength steels;
- ball bearing steels;
- heat-resistant chromium-molybdenum steels.
Low-alloy martensitic steels
Low alloy martensitic steels are characterized by relatively high strength with a minimum yield strength 690 MPa and good toughness and ductility, corrosion resistance and weldability. They are also called low alloy tempering steels., meaning improvement by heat treatment. Slabs are made from these steels., sheets, rods, profiles and forged products. They are widely used for the manufacture of pressure vessels, earthmoving and mining equipment, as well as critical elements of large steel structures.
Medium carbon high strength steels
Medium carbon high strength steels are structural and have very high strength. The minimum yield strength of steels of this class reaches 1380 MPa.
GUEST 4543-71 splits these alloys into five groups - in ascending degree of alloying. As the degree of alloying increases, the size of the section of the product increases., where through hardenability can be achieved. The strongest steels from the fifth group are alloyed 1,2-1,5 % chrome; 3,0-3,4 % nickel; 0,35-0,45 % molybdenum and 0,1-0,2 % vanadium.
An example of such a steel is 30XM chromium-molybdenum steel from the third group according to GOST 4543-71 (analogue of the famous steel 4130, from which bicycle frames are made abroad). The minimum yield strength of steel 30XM is 735 MPa, minimum tensile strength - 930 MPa, and the minimum impact strength KCU is 78 J / cm2.
Ball bearing steels
Ball bearing steels must have high hardness. Therefore, they usually have a carbon content of about 1 %. For good hardenability when quenched in oil, these steels have 0,4 dl 1,65 % chrome. Low alloy bearing steel is sometimes used (0,10-0,20 % carbon). In this case, high surface hardness is achieved by carburizing.
Chrome-molybdenum heat-resistant steels
Chrome-molybdenum heat-resistant steels contain 0,5-9 % chrome, 0,5-1,0 % molybdenum and usually less 0,20 % carbon. They are subjected to various heat treatments: normalization with leave, quenching with tempering or annealing. These steels are used in oil and gas equipment, chemical industry, equipment for conventional and nuclear power plants for the manufacture of pipes, heat exchangers and pressure vessels.
Steel Heat Treatment: Metallurgy and Technologies, ed. G. E. Totten, 2006
Gulyaev A. P. Metallurgy, 1986.