Heat treatment of steel. Types of heat treatment of metals
(MOT) is a very important final operation in the manufacture of parts and tools. It gives them the necessary mechanical properties and ensures normal operation.
Masters long before our time used a variety of methods of hardening: immersed heated metal strip in wine, oil, in plain or salted water. Such a method is also mentioned: the blacksmith heated the damask dagger, and then mounted his horse and rushed quickly, cooling the product in the air.
In the first half of the XIX century. types of heat treatment were imperfect: hard and brittle cast iron was placed in a vessel with ice, peppered with layers of sugar. After that, the tank was heated for 20 hours, and the iron turned into soft and ductile iron.
Old methods are complemented by new, improved, based on scientific studies of heat treatment.For example, the Belgian experts have developed a technology for quenching tool blanks in a vacuum.
Heat treatment of metals is a set of strictly sequential operations of heating, holding and subsequent cooling of billets or finished products in certain modes to change their structure and provide them with the necessary mechanical, physical, chemical and other properties. The basis of heat treatment are the transformations in the internal structure of materials during heating and subsequent cooling.
Types of heat treatment
The determining factors that influence the results of maintenance are the speed and temperature of heating, as well as the time of exposure to a heated state and the rate of cooling. Depending on temperature indicators and cooling rate of products, the following stages of heat treatment are distinguished:
- further normalization;
- quenching and tempering of steel.
To reduce stiffness and increase the viscosity of steel, to achieve chemical and structural uniformity, relieve internal stresses, annealing is carried out. The process consists of heating steel products above critical points (with the exception ofrecrystallization annealing) and, accordingly, holding at a temperature of heating followed by slow cooling (mainly with the furnace). Depending on the purpose, distinguish the following modes of heat treatment of steel:
- diffuse annealing;
- complete and incomplete;
- granular perlite;
Also called homogenization. Used for large steel castings in order to reduce chemical heterogeneity (segregation). At the first stage, the processed material is heated to temperatures of 1050-1150 ° C. After heating, incubated for about 10-15 hours and then slowly cooled. The characteristics of the steels are improved.
The technology is used to form the fine-grained structure of steel products made by hot stamping, forging, casting. After the procedure of complete annealing, the steels become plastic, soft, without internal stresses. The internal (crystalline) structure becomes homogeneous, fine-grained, consists of ferrite and perlite. By complete annealing, the steel is prepared for machining and subsequent hardening.So predominantly hypoeutectoid steels are treated.
Heat treatment of steel is carried out according to the following technical process: products (billets) are heated to temperatures exceeding 30-50 ° C the so-called critical upper point (in materials science designated as Ac3), then slowly cooled. Cooling to a temperature of 500-550 ° C occurs at the following speed:
- for carbon steels - 150-200 ° C per hour;
- for alloyed - 50-75 ° C per hour.
This technology of heat treatment of steel is applied to hypoeutectoid and hypoeutectoid metals to reduce stiffness, relieve internal stresses and obtain a homogeneous structure. The procedure is subjected to forgings and forgings, processed at temperatures that do not cause significant grain growth.
Technical process: the steel is heated at a temperature above the lower critical point (in the graphs it is designated Ac1) in the temperature range of 740-750 ° C, it is kept for a certain time at this temperature, then it is cooled slowly.
Used for products made of alloyed steels when heated to 20-30 ° C above Ac3, exposure and rapid cooling to a temperature of 630-700 ° C.Billets (products) are aged until austenite is decomposed, then cooled at a positive temperature. After isothermal annealing, steel has similar properties to metals subjected to complete annealing. Heat treatment of metals according to this technical process has an important advantage - reduction of processing time.
Annealing for granular perlite
It is widely used before machining tool eutectoid and proeutectoid alloyed and carbon steels. The material is heated to 25-30 ° C above QD and maintained for a specified time. To a temperature of 600 ° C, the billets are cooled very slowly (30 ° C per hour) together with the furnace, and then cooled naturally. As a result, the carbides acquire a granular (rounded) shape, and the hardness decreases, which favors the process of metal cutting.
The second name is low annealing. The process contributes to the removal of internal stresses and work hardening in products manufactured by cold rolling, cold forming, drawing and sizing (sheets, rods, tubes, wire). In this case, the material is heated to the recrystallization temperature at 50-100 ° C below the Ac1 point (630-680 ° C), kept, then cooled naturally (in air).After recrystallization annealing, a homogeneous structure is formed with a small hardness.
The process involves the heating of metals above Ac3 values by 30-50 ° C, keeping in the temperature corridor and subsequent cooling in air. Heat treatment of steel by the method of normalization is ideal for forming a fine-grained structure, increasing strength and toughness, as well as reducing stiffness before cutting and leveling the structure before subsequent heat treatment.
The structure of normalized steel becomes ferritic-pearlitic (low carbon steel) and sorbitol-like in the presence of structurally free ferrite (medium carbon and low alloyed steels). The hardness of perlite depends on whether it has a thin or coarse structure. During normalization, when cooling is faster, perlite has a thinner structure than during annealing, and higher hardness. Therefore, normalized steel is harder than annealed (150-300 HB). Normalization of hot-rolled steels as opposed to annealing increases the resistance of products to brittle fracture and provides high performance during machining.
Used to smooth out the internal stresses of the crystal lattice and reduce the rigidity of metals, as well as to increase the toughness of hardened products. Allocate:
- low vacation.
High tempering is carried out at a temperature of 500-650 ° C with a smooth cooling. In this case, the steel acquires the structure of sorbitol, which ensures the elimination of internal stresses. Structural, carbon and alloyed steels from which shafts, gears and others are made are subjected to this type of tempering. Characteristics of steels have greater strength, ductility and toughness with their sufficient hardness.
The average tempering is carried out at a temperature of 350-450 ° C, is kept for a certain time and cooled. With this tempering, martensite turns into troostite, the hardness of steel decreases to about 400 HB, and the viscosity increases significantly. Apply (after quenching) tempering to the treatment of springs, springs, dies and other products operating at moderate shock loads.
Low tempering is carried out in the temperature range of 150-250 ° C, maintained and cooled. In this case, the structure of the released martensite is formed.Therefore, the internal stresses in the product decrease, the viscosity somewhat increases, and the glowing brittleness disappears, and the hardness remains almost unchanged. Used for cutting and measuring instruments, which must be hard and not brittle, have high wear resistance, including for cemented products.
Heat treatment of steel is an integral stage in the production of most metal products. Thanks to a wide range of technical processes, it is possible to obtain materials with the required characteristics.