In order to improve metal plasticity, reduce deformation resistance, facilitate metal deformation, and obtain good microstructure after forging, the metal is heated before forging. In the process of metal heating, metal materials will undergo a series of changes with the increase of temperature.
In terms of organizational structure and physics. In the aspect of microstructure, not only the microstructure of the metal will change after heating, but also the grain size will become larger, resulting in overheating and overheating; In physics, some physical parameters of metals are related to temperature, such as thermal conductivity. The expansion coefficient changes with the increase of temperature.
In terms of mechanical and chemical properties. In terms of mechanical properties, metal heating will not only improve plasticity and reduce deformation resistance, but also produce internal stress. Excessive internal stress will lead to metal cracking; In terms of chemical properties, many metal surfaces will be oxidized with surrounding media. Decarburization, hydrogen absorption and other chemical reactions to produce oxide scale and decarburization layer will reduce the metal surface quality.
Oxidation When the steel is heated to a high temperature, the surface iron reacts with the oxidation gas () in the furnace to form oxide scale on the steel surface. During the heating process, the heating temperature and heating time have influence on the oxidized skin. When the temperature is below 600 ℃, the rate of skin oxidation is very slow. When the temperature is more than 900 ℃, the more oxidized skin with random temperature rise. The longer the heating time, the more oxidized the skin.
Decarburization. When the steel is heated, the surface carbon reacts with the oxidation gas H in the furnace to reduce the surface carbon content. Heating temperature and heating time will also affect decarburization. When the heating temperature is 700-1000 ℃, the oxide scale on the surface will hinder the diffusion of carbon. At this time, the decarbonization speed is relatively slow. With the increase of temperature, the oxidation and decarbonization speed is faster. When the oxidized skin loses its protective effect, the decarburization speed is faster. The longer the heating time, the deeper the decarburization depth, but the heating time and decarburization depth can not form a positive relationship. When decarburization reaches a certain depth, decarburization will gradually slow down.
overheated. When the heating time is too long and the heating temperature is too high, the grains grow into coarse grains, which is called overheating. Overheating can be eliminated by proper heat treatment and thermal deformation. Common overheat-prone elements such as C.Mn. S.P, and overheat-resistant elements such as Al.Ti.W.V.NB.
Excessive combustion. If the metal heating temperature is too high, the melting point between crystals will melt, or the oxidation between crystals caused by the infiltration of oxidizing gas into the crystal interface is called excessive combustion. Excessive metal materials are generally discarded. Ni.Mo will increase excessive combustion, and Al.Cr.W will resist excessive combustion.
There are also differences in heat conducting materials when heating metals, and the heat generated during heating should be added. Therefore, when forging forgings, heating conditions such as heating temperature and speed should also be considered.