The characteristics of heat treatment are high hardness, wear resistance, compressive strength, high strength and toughness, certain heat resistance and fatigue resistance, and enough tempering resistance.
(1) High strength and toughness can be obtained by lower limit temperature quenching and tempering.
(2) Long time tempering or multiple tempering is adopted to eliminate stress, improve toughness and avoid random size.
(3) For brittle failure and insufficient toughness, isothermal quenching process is adopted,
(4) Surface strengthening shall be applied.
(5) In the process of use, low temperature stress relief tempering is carried out.
Analysis of heating defects and control during heat treatment:
（1） . overheating of heat treatment
1. General overheating: overheating refers to the coarsening of austenite grains caused by too high heating temperature transition or too long holding time under high temperature. The coarse austenite grain will reduce the strength and toughness of the steel, increase the brittleness transition temperature, and increase the deformation cracking tendency during quenching. The cause of overheating is out of control of furnace temperature instrument or mixture (often without understanding the process). After annealing, normalizing or multiple high temperature tempering, the overheated structure can be re austenitized to refine the grains under normal conditions.
2. Fracture heredity: steel with overheated structure can refine austenite grain after reheating and quenching, but sometimes it still has coarse granular fracture. There are many theoretical controversies on the generation of fracture heredity. It is generally believed that MNS and other impurities were once dissolved into austenite and enriched in the crystal interface due to the high temperature transition of heating, and these inclusions would precipitate along the crystal interface when cooling, and fracture along the coarse austenite grain boundary when impacted.
3. Heredity of coarse structure: when the steel with coarse martensite, bainite and widmanstatten structure is re austenitized, the austenite grain is still coarse when it is heated slowly to the normal quenching temperature or even lower, which is called heredity of structure. In order to eliminate the heredity of coarse structure, intermediate annealing or multiple high temperature tempering can be used.
（2） . over burning
If the temperature transition is too high, not only the austenite grains will be coarse, but also the local oxidation or melting of the grain boundary will lead to the weakening of the grain boundary, which is called overburning. After overburning, the function of steel deteriorates severely and cracks are formed during quenching. The burnt tissue cannot be recovered and can only be scrapped. Therefore, in the work to avoid the occurrence of overburning.
（3） , decarburization and oxidation
When the steel is heated, the surface carbon reacts with the oxygen, hydrogen, carbon dioxide and water vapor in the medium (or atmosphere), reducing the carbon concentration of the surface, which is called decarburization. After quenching, the surface hardness, fatigue strength and wear resistance of decarburized steel are reduced, and the residual tensile stress on the surface is easy to form the surface network crack. When heated, the iron and alloy on the steel surface react with the elements and the oxygen, carbon dioxide, water vapor in the medium (or atmosphere), which is called oxidation. The dimensional accuracy and surface brightness of high temperature (generally above 570 ℃) workpiece are deteriorated after oxidation, and the steel with poor hardenability of oxide film is easy to appear the soft spot of quenching. In order to prevent oxidation and reduce decarburization, the following measures are taken: coating on the surface of the workpiece, sealing and heating with stainless steel foil packaging, heating with salt bath furnace, heating with protective atmosphere (such as purified inert gas, controlling the carbon potential in the furnace), and flame combustion furnace (reducing the furnace gas).
（4） Hydrogen embrittlement
Hydrogen embrittlement is the phenomenon that the plasticity and toughness of high strength steel decrease when heated in hydrogen rich atmosphere. Hydrogen embrittlement can also be eliminated by dehydrogenation treatment (such as tempering, aging, etc.), which can be avoided by heating in vacuum, low hydrogen atmosphere or inert atmosphere. Like the continuous heat treatment furnace, which can be treated in time after quenching, it can also be treated in the process of tempering, and according to the current use and statistics, the products treated in the continuous controlled atmosphere heat treatment furnace generally will not appear hydrogen embrittlement.
Of course, everything has its two sides. In practice, some people use this phenomenon to serve people (such as alloy crushing and destruction).
On 2018-08-08 11:45:46
