Abstract: Annealing---quenching---tempering one. Type of annealing Complete annealing and isothermal annealing Complete annealing, also known as recrystallization annealing, is generally referred to as annealing. This annealing is mainly used for casting, forging and hot-rolled profiles of various carbon steels and alloy steels with sub-eutectoid compositions, and sometimes for welding. structure. It is often used as a final heat treatment for some non-heavy workpieces or as a pre-heat treatment for certain workpieces. 2. Spheroidizing annealing spheroidizing annealing is mainly used for hypereutectoid carbon steel and alloying...
Annealing---quenching---tempering
One. Type of annealing
1. Full annealing and isothermal annealing
Complete annealing, also known as recrystallization annealing, is generally referred to as annealing. This annealing is mainly used for casting, forging and hot-rolled profiles of various carbon steels and alloy steels of hypoeutectoid composition, and sometimes for welded structures. It is often used as a final heat treatment for some non-heavy workpieces or as a pre-heat treatment for certain workpieces.
2. Spheroidizing annealing
Spheroidizing annealing is mainly used for hypereutectoid carbon steel and alloy tool steel (such as steel cutting tools, measuring tools, and steels used in molds). Its main purpose is to reduce hardness, improve machinability, and prepare for later quenching.
3. Stress relief annealing
Stress relief annealing, also known as low temperature annealing (or high temperature tempering), is mainly used to eliminate residual stresses in castings, forgings, welded parts, hot rolled parts, cold drawn parts, and the like. If these stresses are not eliminated, it will cause deformation or cracking of the steel after a certain period of time or during subsequent cutting operations.
two. When quenching, the most common cooling media are brine, water and oil. Brine-quenched workpieces are easy to obtain high hardness and smooth surface, and it is not easy to produce hard spots that are hard to quench, but it is easy to deform the workpiece seriously and even crack. The use of oil as a quenching medium is only suitable for the quenching of some alloy steels or small-sized carbon steel workpieces with relatively high stability of supercooled austenite.
three. Purpose of steel tempering
1. Reduce brittleness, eliminate or reduce internal stress, there is great internal stress and brittleness after quenching steel. If it is not tempered in time, it will cause deformation or even cracking of steel.
2. Obtaining the mechanical properties required for the workpiece, the workpiece has high hardness and high brittleness after quenching. In order to meet the different performance requirements of various workpieces, the hardness can be adjusted by appropriate tempering to reduce the brittleness and obtain the required toughness. Plasticity.
3. Stabilize workpiece size
4. For some alloy steels that are difficult to soften by annealing, high temperature tempering is often used after quenching (or normalizing) to properly aggregate the carbides in the steel and reduce the hardness to facilitate cutting.
Furnace type selection
The furnace type should be determined according to different process requirements and the type of workpiece.
1. For those that cannot be batch-formed, the workpieces are not equal in size, and there are many types, which require versatility and versatility in the process. Box furnaces can be selected.
2. When heating long shafts and long screws, pipes and other workpieces, deep well type electric furnaces can be used.
3. For small batches of carburized parts, a well gas carburizing furnace can be used.
4. For the production of large quantities of automobiles, tractor gears and other parts, continuous carburizing production line or box type multi-purpose furnace can be selected.
5. For the mass production of stamping sheet blanks, it is best to use a rolling furnace and a roller hearth furnace.
6. For batches of shaped parts, push rod or belt type resistance furnaces (push furnace or cast strip furnace) are available for production.
7. For small mechanical parts such as screws and nuts, a vibrating bottom furnace or a mesh belt furnace can be used.
8. Steel ball and roller heat treatment can be selected from the inner spiral rotary tube furnace.
9. Non-ferrous metal ingots can be used in push-type furnaces in mass production, and air-circulating furnaces can be used for small parts and materials of non-ferrous metals.
Heating defects and control
First, overheating
We know that heating overheating during heat treatment is most likely to cause coarsening of austenite grains and degrade the mechanical properties of the parts.
1. General overheating: The heating temperature is too high or the holding time is too long at high temperature, causing the austenite grain coarsening to be called overheating. The coarse austenite grains lead to a decrease in the toughness of the steel, an increase in the brittle transition temperature, and an increase in the tendency of deformation cracking during quenching. The cause of overheating is that the furnace temperature meter is out of control or mixed (often not knowing the process). The superheated structure can be re-austenized under normal conditions to refine the grain after annealing, normalizing or multiple high temperature tempering.
2. Fracture inheritance: Steel with overheated structure, after reheating and quenching, although the austenite grains can be refined, sometimes large coarse fractals still appear. There are many theoretical controversies for the generation of fracture inheritance. It is generally believed that impurities such as MnS are dissolved into austenite and enriched in the crystal interface due to excessive heating temperature, and these inclusions will precipitate along the crystal interface during cooling. It is easy to break along the coarse austenite grain boundary when subjected to impact.
3. Inheritance of coarse tissue: When the steel with coarse martensite, bainite and weiss body is re-austenated, it is heated at a slow speed to the conventional quenching temperature, and even lower, its austenite crystal The granules are still coarse, a phenomenon known as tissue heredity. To eliminate the heritability of the large tissue, intermediate annealing or multiple high temperature tempering treatments can be used.
Second, overburning
If the heating temperature is too high, not only the austenite grains are coarsened, but also the localized oxidation or melting of the grain boundaries causes the grain boundaries to weaken, which is called over-burning. After the steel is over-fired, the performance is seriously deteriorated, and cracks are formed during quenching. The burnt tissue cannot be recovered and can only be scrapped. Therefore, it is necessary to avoid over-burning during work.
Third, decarburization and oxidation
When the steel is heated, the carbon in the surface layer reacts with oxygen, hydrogen, carbon dioxide and water vapor in the medium (or atmosphere), and the carbon concentration in the surface layer is reduced, which is called decarburization. The surface hardness, fatigue strength and resistance of the decarburized steel after quenching The abrasiveness is lowered, and the residual tensile stress is formed on the surface to form a surface network crack.
When heated, the phenomenon that iron and alloys of the steel surface layer react with elements, and oxygen, carbon dioxide, water vapor, etc. in the medium (or atmosphere) to form an oxide film is called oxidation. At high temperatures (generally 570 degrees or more), the dimensional accuracy and surface brightness of the workpiece are deteriorated after oxidation, and the steel having poor hardenability of the oxide film is prone to quenching soft spots.
In order to prevent oxidation and reduce decarburization, there are: surface coating of workpiece, sealed and sealed with stainless steel foil, heated by salt bath furnace, heated by protective atmosphere (such as purified inert gas, controlled carbon potential in furnace), flame burning furnace (making the furnace gas reductive)
Fourth, hydrogen embrittlement
The phenomenon that the high strength steel is reduced in plasticity and toughness when heated in a hydrogen-rich atmosphere is called hydrogen embrittlement. Hydrogen embrittlement can be eliminated by hydrogen removal treatment (such as tempering, aging, etc.), and hydrogen embrittlement can be avoided by heating in a vacuum, low hydrogen atmosphere or inert atmosphere.
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