Heating defects and control of the hottest heat tr

2022-08-10
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Heat treatment heating defects and control

first, overheating phenomenon

during heat treatment, overheating is most likely to lead to the coarseness of austenite grains and the decline of mechanical properties of parts

1. General overheating: overheating occurs when the heating temperature is too high or the holding time at high temperature is too long, causing austenite grain coarsening. Coarse austenite grains will reduce the strength and toughness of steel, increase the brittle transition temperature, and increase the tendency of deformation and cracking during quenching. The cause of overheating is that the furnace temperature instrument is out of control or mixed (often caused by ignorance of the process). After annealing, normalizing or multiple high-temperature tempering, the overheated structure can be re austenitized to refine the grain under normal conditions

2. Fracture heredity: the steel with overheated structure can refine the austenite grain after reheating and quenching, but sometimes there is still coarse granular fracture. There are many disputes about the theory of fracture heredity. It is generally believed that impurities such as MNS have been dissolved into austenite and enriched at the crystal interface due to high heating temperature. When cooled, these inclusions will precipitate along the automatic process and enter the system test state crystal interface, and are easy to fracture along the coarse austenite grain boundary when impacted

3. Heredity of coarse structure: when the steel parts with coarse martensite, bainite and widmanstatten structures are re austenitized, they are slowly heated to the conventional quenching temperature, or even lower, and their austenite grains are still coarse. This phenomenon is called structure heredity. To eliminate the heredity of coarse structure, intermediate annealing or multiple high-temperature tempering can be used

II. Overburning phenomenon

high heating temperature not only causes coarse austenite grains, but also local oxidation or melting of grain boundaries, resulting in weakening of grain boundaries, which is called overburning. The performance of steel deteriorated seriously after overburning, and the protective plate of 7-wrapping frame cracked during quenching. Overburned tissue cannot be recovered and can only be scrapped. Therefore, the occurrence of over burning should be avoided in the work

III. decarburization and oxidation

when the steel is heated, the carbon on the surface reacts with oxygen, hydrogen, carbon dioxide and water vapor in the medium (or atmosphere), reducing the carbon concentration on the surface, which is called decarburization. After quenching, the surface hardness of decarburized steel, fatigue strength and wear resistance of proportional electromagnet as electro-mechanical converter are reduced, and the residual tensile stress on the surface is easy to form surface cracks

when heated, the iron and alloy on the steel surface react with elements and oxygen, carbon dioxide, water vapor in the medium (or atmosphere) to form oxide films, which is called oxidation. High temperature (generally above 570 ℃) workpieces deteriorate in dimensional accuracy and surface brightness after oxidation, and steel parts with poor hardenability of oxide film are prone to quenching soft spots

the measures to prevent oxidation and reduce decarburization include: coating the surface of the workpiece, sealing and heating with stainless steel foil, heating with salt bath furnace, heating with protective atmosphere (such as purified inert gas, controlling the carbon potential in the furnace), flame combustion furnace (making the furnace gas reductive)

IV. hydrogen embrittlement phenomenon

the phenomenon of reduced plasticity and toughness of high-strength steel when heated in hydrogen rich atmosphere is called hydrogen embrittlement. The workpiece with hydrogen embrittlement can also be eliminated by hydrogen removal treatment (such as tempering, aging, etc.). Hydrogen embrittlement can be avoided by heating in vacuum, low hydrogen atmosphere or inert atmosphere. (end)

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