First, the phenomenon of overheating, We know that heat treatment in the process of overheating is most likely to lead to coarse austenite grains, so that the mechanical properties of the parts decreased.
1. General overheating: The heating temperature is too high or at high temperature holding time too long, resulting in the austenite grain coarsening is called overheating. The coarse austenite grains lead to the decrease of the strength and toughness of steels, the increase of brittle transition temperature, and the tendency of deformation and cracking during quenching. The cause of overheating is the furnace temperature meter out of control or mixed (often do not understand the process occurs). Overheating tissue can be annealed, normalizing or several high temperature tempering, under normal conditions of re-austenite to refine the grain.
2. The fracture heredity: has the overheating organization the steel, after reheating quenching, although may make the austenite grain refinement, but sometimes still appears the coarse granular shape fracture. There are many theories of fracture inheritance, it is generally believed that the heating temperature is too high so that the MNS and other impurities dissolved into the austenite and enrichment in the crystal interface, and cooling these inclusions will be precipitated along the crystal interface, under impact easily along the coarse austenite grain boundary fracture.
3. The heredity of coarse tissue: there are coarse martensite, bainite, bainitic microstructure of steel parts re-austenite, at slow heating to conventional quenching temperature, or even lower, the austenite grains are still coarse, this phenomenon is called tissue genetics. To eliminate the hereditary of coarse tissue, it can be treated with intermediate annealing or multiple high-temperature tempering.
Second, the phenomenon of excessive burning, The heating temperature is too high, which not only causes the austenite grains to be coarse, but also the grain boundary is oxidized or melted, which causes the grain boundary to weaken, which is called over burning. Steel after burning performance seriously deteriorated, quenching formation cracking. The burnt tissue cannot be recovered and can only be scrapped. Therefore, in the work to avoid the occurrence of burning.
Thrid, decarbonization and oxidation. When the steel is heated, the surface of carbon and media (or atmosphere) in the oxygen, hydrogen, carbon dioxide and water vapor reaction, reduce the surface carbon concentration known as decarbonization, decarburization steel after quenching surface hardness, fatigue strength and wear resistance, and surface formation of residual tensile stress easily formed surface network crack.
When heating, the iron and alloy in the steel surface and the elements and medium (or atmosphere) in the oxygen, carbon dioxide, water vapor and other reactions to produce oxide film is called oxidation. High temperature (generally more than 570 degrees) after the oxidation of workpiece size and surface brightness deterioration, with the oxidation of the hardenability of the poor steel parts prone to hardening soft spots.
In order to prevent oxidation and reduce decarbonization measures are: workpiece surface coating, with stainless steel foil packaging sealed heating, the use of salt bath furnace heating, the use of protective atmosphere heating (such as the purification of the inert gas, control furnace carbon potential), flame burning furnace (so that the furnace gas is reductive)
Four, hydrogen embrittlement phenomenon
The ductility and toughness reduction of high strength steels when heated in a hydrogen-rich atmosphere is called hydrogen embrittlement. Hydrogen embrittlement can be eliminated by hydrogenation of the workpiece, such as tempering, aging and so on, and the hydrogen embrittlement can be avoided by heating with a vacuum, low hydrogen atmosphere or inert atmosphere.
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