The defects caused by improper heating can be divided into: (1) the defects caused by the change in the chemical state of the outer layer of the billet due to the influence of the medium, such as oxidation, decarbonization, carburization, sulfurization and copper carburization, etc.Defects caused by abnormal changes in the internal organizational structure, such as overheating, overburning and underheat.(3) due to the uneven distribution of temperature in the blank, the internal stress (such as temperature stress, tissue stress) caused by excessive blank cracking.Several of these common defects are described below, and the rest can be seen as examples.
1. The decarburization
Decarbonization refers to the oxidation of carbon in the surface layer of metal at high temperature, making the carbon content in the surface layer significantly lower than that in the interior.
The depth of the decarbonization layer is related to the composition of the steel, the composition of the furnace gas, the temperature and the holding time at this temperature.Using oxidizing atmosphere heating is easy to decarburize, high carbon steel is easy to decarburize, silicon content of steel is also easy to decarburize.
Decarbonization reduces the strength and fatigue performance of the parts and weakens the resistance to wear.
2. The increase of carbon
Forgings heated by an oil furnace are often carburized on the surface or part of the surface.Sometimes the thickness of the carburized layer is up to 1.5-1.6mm, the carbon content of the carburized layer is up to 1% (mass fraction), and the carbon content at local points is even more than 2% (mass fraction).
This is mainly in the case of oil furnace heating, when the location of the billet near the oil furnace nozzle or just in the area of cross-injection of fuel between the two nozzles, the oil and air do not mix well, so the combustion is incomplete, resulting in the formation of a reducing carburizing atmosphere on the surface of the billet, thus producing the effect of surface carburization.
Carburizing makes the mechanical properties of forging become bad, and it is easy to hit the knife when cutting.
3. The overheating
Overheating refers to the phenomenon that the metal blank is heated at too high a temperature, or stays too long within the specified range of forging and heat treatment temperature, or the grain size is caused by too high a temperature rise due to thermal effect.
Carbon steel (subeutectoid or supereutectoid) often overheats and develops a weissner structure.After the martensite steel overheats, the intra - crystalline texture often appears, and the die steel is usually characterized by the primary carbide angular structure.After titanium alloy overheat, obvious grain boundary and straight slender wei - like structure appeared.The fracture of alloy steel may appear as a stone fracture or a strip fracture after it is overheated.The mechanical properties of the superheated structure, especially the impact toughness, will be reduced due to the coarse grain.
After normal heat treatment (normalizing and quenching), the structure of overheated structural steel can be improved and its performance can be restored.However, the serious overheating of alloy structural steel can not be completely eliminated after normal normalizing (including high temperature normalizing), annealing or quenching, which is often referred to as stable overheating.
4. The burnt
Overburning refers to the metal body heating temperature is too high or in the high temperature heating zone residence time is too long, the furnace oxygen and other oxidizing gas permeate into the metal grain gap, and with iron, sulfur, carbon and other oxidation, the formation of fusible oxide eutectic, the destruction of the grain between the contact, so that the material plasticity sharply reduced.If the metal is badly burned, it will crack with a light blow when the coarse is removed, and transverse cracks will appear at the burned place when the metal is pulled out.
There is no strict temperature boundary between overheating and overheating.In general, the characteristics of oxidation and melting of grains are used to judge overburning.In the case of carbon steel, the grain boundary melts when overburned, and in the case of heavy oxygen chemical die steel (high speed steel, Cr12 section steel, etc.) overburned, the grain boundary appears fish-bone leitritic because of melting.The grain boundary melting triangle zone and the remelting sphere appeared when the aluminum alloy was overburned.After the forging has been burned, it is often impossible to save it, so it has to be scrapped.
5. Heat crack
When heating large ingot with large section size and high alloy steel and superalloy blank with poor thermal conductivity, if the heating speed is too fast in the low temperature stage, the blank will generate great thermal stress due to the large internal and external temperature difference.In addition, due to the low temperature and poor plasticity of the blank, if the value of the thermal stress exceeds the strength limit of the blank, a radiant heating crack will be generated from the center to the four sides, making the whole section crack.
6. The copper brittleness
The copper is brittle on the surface of the forging.At high magnification, yellowish copper (or solid solution of copper) is distributed along grain boundaries.
When the blank is heated, if the residual copper oxide in the furnace remains, the steel is oxidized and reduced to free copper at high temperature. The molten steel atoms expand along the austenitic grain boundary, weakening the connection between grains.In addition, when the copper content of steel is relatively high [BBB 0 2% (mass fraction)], if heated in an oxidizing atmosphere, a copper-rich layer is formed under the skin of iron oxide, which also causes steel brittleness.