Defects caused by improper heating during forging can be divided into oxidation, decarburization, carbon increase, sulfur penetration, copper penetration and other defects.; Defects caused by abnormal changes in internal organizational structure, such as excessive internal stress (such as temperature stress and organizational stress) such as overheating, overheating, and non thermal penetration.
Decarburization: Decarburization refers to the oxidation of carbon on the surface of metal at high temperature, which makes the carbon content on the surface significantly lower than that inside. The depth of decarburization layer is related to the composition of steel, the composition of furnace gas, temperature and holding time at this temperature. It is easy to decarburize when heated in oxidizing atmosphere, high carbon steel and steel with high silicon content. Decarburization reduces the strength and fatigue performance of the parts and weakens the wear resistance.
Carbon increase: forgings heated by oil furnace often add carbon on the surface or part of the surface. Sometimes the thickness of carbon increase layer is 1.5~1.6mm, the content of carbon increase layer is about 1% (mass fraction), and the local carbon content even exceeds 2% (mass fraction), resulting in ledeburite structure. This is mainly because when the blank position is close to the nozzle of the oil furnace or when two nozzles cross the fuel injection area when the oil furnace is heated, due to the poor mixing of oil and air and incomplete combustion, the blank surface will form a reducing carbon infiltration atmosphere, resulting in the effect of increased surface carbon. The increase of carbon worsens the machinability of forgings.
Heating crack: when heating high alloy steel and superalloy steel billets with large section size and poor thermal conductivity, if the heating speed is too fast at the low temperature stage, the billets will produce large thermal stress due to large internal and external temperature difference. In addition, due to low temperature, the plasticity of the billet is poor. If the thermal stress value exceeds the strength limit of the blank, a heating crack radiating from the center to the periphery will be generated, thus cracking the entire section.
Overheating: overheating refers to the excessive heating temperature of the metal blank, or the excessive residence time within the specified forging and heat treatment temperature range, or the coarse grains caused by excessive temperature rise due to thermal effects. Generally, after normal heat treatment (normalizing and quenching), the structure of overheated structural steel can be improved and its properties can be restored. This overheating is often referred to as unstable overheating; The severe overheating of alloy structural steel is normalized generally (including high-temperature normalizing).
Overheating: It means that the heating temperature of the metal billet is too high or the time spent in the high-temperature heating zone is too long, and the oxygen and other oxidation gases in the furnace penetrate into the gap between the metal particles, and form eutectic of fusible oxides with the oxidation of iron, sulfur and carbon, which destroys the relationship between the particles and greatly reduces the plasticity of the material. The seriously overheated metal will crack slightly when removing the roughness, and the overheated part will have horizontal cracking. There is no strict temperature limit between overheating and overheating. Generally speaking, overheating is judged by the oxidation and melting of particles. Forgings are often irretrievable after burning and have to be scrapped.