This treatment mainly refers to the rapid cooling of the steel material from the high-temperature Ostend iron zone to obtain a specific low-temperature structure, such as Matian loose iron, toughened iron, etc. Generally, water, oil or low-temperature salt baths are used for this cooling, but sometimes fans can also be used to accelerate air cooling to achieve the same purpose. From the iron-carbon phase equilibrium diagram, we know that the general carbon steel is normalized or annealed, and its low-temperature structure is the coexistence structure of ferrous iron and Fe C carbide.
However, this kind of phase change is an atomic diffusion control mechanism, and its formation has a great relationship with the cooling time. If the cooling time is very short and the supersaturated carbon atoms do not have enough time to diffuse to produce carbides, there are two kinds of metastable state <metasable state> organization may be produced. For example, if it is rapidly cooled to a low temperature state, it will be a deformed body core structure that crosses the saturated carbon atoms, also known as the Asada scattered iron structure.
Because these two structures have very special mechanical properties (the former has high strength and toughness, the latter has the highest strength and hardness), they have very high utility value. Because the structure of Asada loose iron or toughened iron is closely related to the supersaturated carbon content, and the detection is related to the precipitation rate of carbides, it is necessary to use different cooling rates for steels with different carbon content.
The second type of organization. Also for alloy steel, different alloying elements and content will affect the required cooling rate. Generally speaking, adding alloying elements, such as chromium, manganese, molybdenum, nickel, etc., will delay the formation of wave iron, so we can use slower cooling rate or high hardness of Asada scattered iron structure. In other words, this type of steel is easier to harden and has inherently better hardening ability.
The best quenching condition is to cool down quickly at high temperature to avoid the formation of porphyrite, that is, to avoid the nose end of the nose-shaped TTT curve (temperature-time-phase change curve), and then slow at medium temperature Cool down slowly. In this way, the hardening effect can be obtained on the one hand, and the ineffective results caused by thermal stress can be reduced.
For pure water or brine, the former task can be achieved, but the latter cannot be achieved, so it is not suitable for finished products with complex shapes. For oil quenching, although the cooling rate at high temperature is slower than that of water or salt water, the cooling rate at its medium temperature is also slower, so the heat dissipation of the finished product is more uniform and there are fewer thermal stress problems.