We know that overheating during heat treatment can easily lead to the coarsening of austenite grains, which will reduce the mechanical properties of the parts.
1. General overheating
The heating temperature is too high or the holding time at high temperature is too long, which causes the austenite grains to coarsen, which is called overheating. 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 the materials are mixed (often caused by people who do not understand the process). The overheated structure can be re-austenized under normal circumstances to refine the grains after annealing, normalizing or multiple high-temperature temperings.
2. Broken inheritance
Although steel with overheated structure can refine the austenite grains after reheating and quenching, coarse granular fractures sometimes still appear. The theory of fracture inheritance is controversial. It is generally believed that impurities such as MnS were dissolved into austenite and enriched at the grain interface because the heating temperature was too high. When cooling, these inclusions will precipitate along the grain interface. It is easy to fracture along the coarse austenite grain boundaries when impacted.
3. Inheritance of coarse tissue
When steel parts with coarse martensite, bainite, and Wignisten structures are re-austenized, they are heated slowly to the conventional quenching temperature, or even lower, and the austenite grains are still coarse. This This phenomenon is called histological heritability. To eliminate the inheritance of coarse tissue, intermediate annealing or multiple high-temperature tempering treatments can be used.
If the heating temperature is too high, it will not only cause the austenite grains to become coarse, but also cause local oxidation or melting of the grain boundaries, resulting in weakening of the grain boundaries, which is called overburning. The properties of steel deteriorate severely after over-burning, and cracks form during quenching. Burned tissue cannot be recovered and can only be scrapped. Therefore, overheating should be avoided at work.
When 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. The surface hardness, fatigue strength and resistance of decarburized steel after quenching The wearability is reduced, and the residual tensile stress formed on the surface is prone to surface network cracks.
When heated, the phenomenon in which the iron and alloys on the surface of steel react with elements and oxygen, carbon dioxide, water vapor, etc. in the medium (or atmosphere) to form an oxide film is called oxidation. After oxidation of workpieces at high temperatures (generally above 570 degrees), the dimensional accuracy and surface brightness deteriorate, and steel parts with poor hardenability with oxide films are prone to quenching soft spots.
Measures to prevent oxidation and reduce decarburization include: surface coating of the workpiece, sealing and heating with stainless steel foil packaging, salt bath furnace heating, protective atmosphere heating (such as purified inert gas, controlling carbon potential in the furnace), flame burning furnace (Making the furnace gas reducing)
The phenomenon of reduced plasticity and toughness of high-strength steel when heated in a hydrogen-rich atmosphere is called hydrogen embrittlement. Workpieces with hydrogen embrittlement can also be eliminated by hydrogen removal treatment (such as tempering, aging, etc.). Hydrogen embrittlement can be avoided by heating in a vacuum, low hydrogen atmosphere or inert atmosphere.
