Heating defects and control during heat treatment
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: If the heating temperature is too high or the holding time is too long at high temperature, the austenite grain coarsening is 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 tissue, after reheating quenching, although the austenite grain refinement, but sometimes there are still large grainy fracture. There are many theoretical controversies that cause fracture inheritance. It is generally believed that impurities such as MNS are dissolved into austenite and enriched at 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. Like the current continuous heat treatment furnace after quenching and timely tempering treatment, the oxygen treatment can be considered in the tempering process. According to the current use and statistics, the products processed in the continuous controlled atmosphere heat treatment furnace generally do not appear. Hydrogen embrittlement.
Of course, everything has its two sides. In practice, some people use this phenomenon to serve people (such as crushing of alloys, etc.).
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