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The defects caused by improper heating can be divided into: ① The defects caused by the change of the histochemical state of the outer layer caused by the medium, such as oxidation, decarbonization, carburization, sulfur, copper, etc.② Defects caused by abnormal changes in the internal tissue structure, such as overheating, overburning and underheat transmission.③ Due to the uneven distribution of temperature in the billet, the internal stress (such as temperature stress, tissue stress) is too large and the billet cracking.
1. Decarbonization
Decarbonization refers to that the carbon on the surface layer of the metal is oxidized at high temperature, so that the carbon content of the surface layer is significantly reduced compared with the internal phenomenon.
The depth of the decarbonization layer is related to the composition of the steel, the composition of the furnace gas, the temperature, and the insulation time at this temperature. Oxoxidation atmosphere heating is easy to decarbonization, high carbon steel is easy to decarbonization, silicon steel is also easy to decarbonization.
Decarbonization reduces the strength and fatigue performance of the parts, and weakens the wear resistance.
2. Carbon increase
Forgings heated by oil furnace often increase on the surface or part of the surface. Sometimes the thickness of the carbon layer reaches 1.5~1.6mm, the carbon content of the carbon layer reaches about 1% (mass fraction), and the local carbon content even exceeds 2% (mass fraction), and lystenite tissue appears.
This is mainly in the case of oil furnace heating, when the billet is close to the oil furnace nozzle or in the area where the two nozzles cross the fuel, because the oil and air are not mixed well, so the combustion is not complete, a reducing carburization atmosphere is formed on the surface of the billet, thus producing the effect of surface carbon.
Carbon increase makes the machining performance of forgings bad, and it is easy to cut when cutting.
3. Overheat
Overheat refers to the heating temperature of metal billet is too high, or in the specified forging and heat treatment temperature range, or the temperature rise caused by the thermal effect.
Carbon steel (sub-or over co-analysis steel) often appears after overheating. After martensitic steel overheat, it often appears in the crystal structure, and the mold steel is often characterized by the overheating tissue. After the titanium alloy overheated, the obvious β phase crystal boundary and straight slender Wei organization appeared. After overheating of alloy steel fracture will appear stone fracture or strip fracture. Superheated tissue, due to the large grain size, will reduce the mechanical properties, especially the impact toughness.
After normal heat treatment (ignition, quenching), general overheating structure steel organization can be improved and the performance is restored. This overheating is often called unstable overheating; and serious overheating of alloy structure steel after general ignition (including high temperature ignition), annealing or quenching treatment, the overheating tissue can not be completely eliminated, this overheating is often called stable overheating.
4. Too much burning
Overburning means that the heating temperature of the metal billet is too high or stays in the high temperature heating area for too long, the oxygen and other oxidation gas in the furnace penetrate into the gap between the metal grains, and oxidation with iron, sulfur, carbon, etc., forming a cocrystal of molten oxide, destroying the connection between the grains, and sharply reducing the plasticity of the material. The metal with serious burn will crack with a gentle blow when removing thick, and transverse cracks will appear at the burn when drawing.
There is no strict temperature boundary between overburning and overheating. Generally, the grain oxidation and melting characteristics to judge the burn. For carbon steel, when the grain boundary melts and the severe oxygen chemical mold steel (high speed steel, Cr 12 type steel, etc.) is overburned, the grain boundary appears fishbone lytenite due to melting. Crystal boundary melting triangle zone and remelting ball appear when aluminum alloy is overburned. Forgings after being burned, often cannot be saved, have to be scrapped.
5. Heat cracks
When heating large ingot with large section size and high alloy steel and high temperature alloy billet with poor thermal conductivity, if the heating speed is too fast in the low temperature stage, the billet will produce great thermal stress due to the large temperature difference between inside and outside. In addition, the billet has poor plasticity due to the low temperature. If the thermal stress value exceeds the strength limit of the billet, the radiant heating crack will be generated from the center to the surrounding area, making the whole section crack.
6. Copper crisp
Copper brittle cracks on the surface of the forging. At high observation, light yellow copper (or solid solution of copper) is distributed along the grain boundary.
When the billet is heated, such as the residual copper oxide in the furnace, the oxide steel is reduced to free copper at high temperature, and the molten steel atoms expand along the austenite grain boundary, which weakens the connection between the grains. In addition, when the copper content in the steel is higher [> 2% (mass fraction)], such as heating in the oxidation atmosphere, the copper-rich layer is formed under the iron oxide skin, which also causes steel brittleness.
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