Defects Caused By Improper Heating Process When Forging Forgings

- Apr 18, 2019-

The defects caused by improper heating when forging forgings can be divided into: defects caused by changes in the chemical state of the outer layer of the billet due to the influence of the medium, such as oxidation, decarburization, carbonization and sulfurizing, copper infiltration, etc.; Defects caused by abnormal changes, such as overheating, over-burning, and unheating; due to uneven distribution of temperature inside the billet, causing cracking of the billet caused by excessive internal stress (such as temperature stress and tissue stress).

Decarburization: Decarburization refers to the phenomenon that the carbon in the lower layer of the metal is oxidized, so that the carbon content of the surface layer is significantly lower than that of the interior. The depth of the decarburization layer is related to the composition of the steel, the composition of the furnace gas, the temperature and the holding time at this temperature. Decarburization is easy to occur by heating in an oxidizing atmosphere, and high carbon steel is easy to decarburize, and steel containing a large amount of silicon is also easy to decarburize. Decarburization reduces the strength and fatigue properties of the part, and the wear resistance is weakened.

Carbon addition: Forgings heated by oil furnaces often have carbonation on the surface or part of the surface. Sometimes the thickness of the carbon-increasing layer is 1.5-1.6 mm, the carbon content of the carbon-increasing layer is about 1% (mass fraction), and the carbon content of the local point is even more than 2% (mass fraction), and the Leysite structure appears. This is mainly caused by the fact that when the oil is heated, when the position of the blank is close to the nozzle of the oil furnace or just in the region where the two nozzles cross-spray the fuel, the oil and air are not mixed well, so the combustion is incomplete, and the result is The surface of the billet forms a reducing carburizing atmosphere, thereby producing a surface carbonizing effect. Carbonization deteriorates the machinability of the forging.

Heating crack: When heating large steel ingots with large cross-sectional dimensions and high-alloy steels and high-temperature alloy blanks with poor thermal conductivity, if the heating rate is too fast in the low temperature stage, the billets will have large thermal stress due to large internal and external temperature differences. In addition, the billet is inferior in plasticity due to low temperature. If the value of the thermal stress exceeds the strength limit of the billet, a heating crack is generated from the center to the periphery, and the entire section is cracked.

Overheating: Overheating refers to the phenomenon that the heating temperature of the metal billet is too high, or the residence time is too long in the specified forging and heat treatment temperature range, or the grain temperature is too high due to the thermal effect. After normal heat treatment (normalized, quenched), the microstructure of the superheated structural steel can be improved, and the performance is also restored. This overheating is often called unstable overheating; and the severe overheating of the alloy structural steel is generally normalized. After (including high temperature normalizing), annealing or quenching, the superheated structure cannot be completely eliminated. This overheating is often referred to as stable overheating.

Over-burning: Over-burning means that the heating temperature of the metal billet is too high or the residence time in the high-temperature heating zone is too long. The oxygen and other oxidizing gases in the furnace penetrate into the gap between the metal crystal grains, and are combined with iron, sulfur, carbon, etc. Oxidation forms a eutectic of fusible oxides, destroying the interfacial bonds and dramatically reducing the plasticity of the material. If the metal is over-burned, it will crack when it is thickened. When it is long, it will have a transverse crack in the over-burning place. There is no strict temperature boundary between overburning and overheating. It is generally characterized by oxidation and melting of crystal grains to judge over-burning. After the forging has been burned, it is often impossible to save and has to be scrapped.

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