The chemical heat treatment of large forgings is a heat treatment process in which a suitable element is infiltrated into the surface of the steel at a certain temperature in a different active medium while diffusing deep into the steel to obtain the desired structure and properties. The properties of the layer are similar to those of steel with the same chemical composition.
The difference between chemical heat treatment and general heat treatment is that it can change the surface properties of the steel surface by changing the chemical composition and structure of the steel surface, so that the parts made of the same material have two different properties on the surface and the core. The surface quenching is the same chemical composition in the surface, but the structure in the surface is different. Therefore, it can be said that chemical heat treatment is a very effective method when different surfaces and cores of the same part are required to have different compositions and properties.
According to the elements infiltrated, the chemical heat treatment of steel can be divided into the following types:
1) Carburizing treatment using carbon as an infiltration element;
2) osmosis treatment using nitrogen as an infiltration element;
3) Carbonitriding treatment with simultaneous infiltration of carbon and nitrogen
4) Metallization treatment using metal elements - chromium, aluminum, silicon, boron, phosphorus, etc. as infiltrating elements.
Carburizing, nitriding, and carbonitriding are chemical heat treatments based on surface strengthening, which are aimed at improving the surface hardness, wear resistance and fatigue strength of forgings. Nitriding can also improve surface thermo-hardness, corrosion resistance and anti-gluing properties. The chemical heat treatment of large forgings is mainly due to these kinds of chemical heat treatments, and the metallizing heat treatment is rarely applied to large forgings.
Chemical heat treatment has certain conditions in the process, equipment and operation process. For example, the parts must be heated in a closed container. The reaction chamber and the gas must be uniformly circulated in the medium, which makes the chemical heat treatment of large forgings difficult.
In terms of performance requirements, the chemical heat treatment of large forgings is more carbonized, followed by carbonitriding and nitriding. In order to improve the surface hardness and maintain the overall plasticity and toughness of the large forgings, Changyi uses surface quenching to achieve the purpose, because for large forgings, surface quenching is more convenient, flexible and economical than chemical heat treatment.
Since the carburizing chemical heat treatment can improve the wear resistance of the parts, it can also improve the fatigue strength, especially the contact fatigue strength. Therefore, even for large heavy-duty gears, there are some sprocket wheels, large bearings, camshafts, etc., in order to improve their load-bearing capacity and service life, while also taking into account the safety of use, sometimes also to reduce the weight of large forgings and The volume is carburized. —The single-stage transmission gear reducer of the ordinary rolling mill that transmits 10,000 horsepower torque. The hardness of the gear is increased from 260HB to 60HRC of carburizing and quenching, which reduces the weight of the reducer from 91t to 45t.
At present, there are already 40t heavy-duty rolling mill gear shafts and 25t major marine gears for deep carburizing heat treatment. In recent years, there has been a deep carburizing heat treatment of the gear shaft of the 12t re-rolling mill.
Some large forgings must have high wear resistance, erosive and anti-adhesive properties, and high precision requirements, requiring little deformation during heat treatment. This part is suitable for nitriding heat treatment. Such as 1200 cold rolling mill slider, guide plate, and large gears running at high speed in the speed increaser on the steam turbine.
Since the nitriding treatment does not result in a deep infiltrated layer, it is not possible to withstand high contact stresses for large forgings having a large radius of curvature. Therefore, nitriding and carburizing have their own advantages and disadvantages, and they are selected according to the force characteristics of the parts.