Heating defect and control in heat treatment process
It is known that overheating during heat treatment can easily lead to the coarseness of austenite grains and the mechanical properties of parts.
1. General overheating: overheating refers to the coarsening of austenite grains caused by high heating temperature or 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 mixing materials (often do not understand 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 genetic. It is generally believed that MNS and other impurities were once dissolved into austenite and enriched in the crystal interface due to high heating temperature, and these inclusions would precipitate along the crystal interface when cooling, and easily 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 heating temperature 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. The properties of the steel deteriorated seriously after overburning, and cracks 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, iron and alloy on the surface of steel react with elements and oxygen, carbon dioxide, water vapor in the medium (or atmosphere) to form oxide film, which is called oxidation. The size accuracy and surface brightness of high temperature (generally above 570 ℃) workpieces deteriorate after oxidation, and the steel parts with poor hardenability of oxide film are prone to appear soft spots. In order to prevent oxidation and reduce decarburization, the following measures are taken: coating on the surface of workpiece, sealing and heating with stainless steel foil packaging, heating with salt bath furnace, heating with protective atmosphere (such as inert gas after purification, control of carbon potential in furnace), flame combustion furnace (reducing 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. The workpiece with hydrogen embrittlement can also be eliminated by dehydrogenation treatment (such as tempering, aging, etc.), and hydrogen embrittlement 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 with oxygen displacement during tempering. According to the current use and statistics, the products treated in the continuous controlled atmosphere heat treatment furnace generally will not have hydrogen embrittlement.
Of course, everything has its two sides. In practice, some people use this phenomenon to serve people (such as alloy crushing).