On 2018-08-08 14:48:54
Defects caused by improper heating in heat treatment furnace
Decarburization refers to the oxidation of carbon in the surface layer of metal at high temperature, which makes the carbon content in the surface layer significantly lower than that in 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. It is easy to decarbonize high carbon steel and silicon - containing steel by oxidizing atmosphere heating.
Decarburization makes the strength and fatigue performance of the parts decrease, and the wear resistance is weakened.
Carbon addition is often found on the surface or part of the surface of a forging heated by an oil furnace. Sometimes the thickness of the carbonizing layer is up to 1.5 ~ 1.6mm, the carbon content of the carbonizing layer is up to about 1% (mass fraction), and the carbon content of the local point is even more than 2% (mass fraction), resulting in the occurrence of lesterite tissue.
This is mainly in the case of heating in the oil furnace, when the billet position is close to the oil furnace nozzle or in the two nozzles cross injection fuel area, because the oil and air mixing is not good, so the combustion is not complete, resulting in the formation of a reductive carburizing atmosphere on the surface of the billet, resulting in the effect of surface carbon. The increase of carbon makes the machining performance of the forging bad, easy to cut when cutting.
Superheating refers to the phenomenon that the heating temperature of metal billet is too high, or the residence time is too long within the specified forging and heat treatment temperature range, or the grain size is caused by the excessive temperature rise caused by thermal effect.
The weissner structure of carbon steel (hypoeutectoid or eutectoid steel) tends to appear after overheating. After martensitic steel is overheated, there is often an intragranular texture. After the titanium alloy overheated, there appeared obvious beta grain boundary and straight and slender weinberg structure. The fracture of alloy steel after overheat may appear in the shape of stone or strip. Superheated microstructure, due to the coarse grain, will cause a reduction in mechanical properties, especially impact toughness.
General overheating structural steel after the normal heat treatment (normalizing, quenching), the structure can be improved, the performance is also restored, this kind of overheating is often called unstable overheating; However, the serious overheating of alloy structural steel can not be completely eliminated after normal fire (including high temperature normal fire), annealing or quenching treatment.
Heat treatment factory burnt refers to metal billet heating temperature is too high or stay a long time, the high temperature heating area of the furnace oxidation oxygen and other gas penetrated into the space between the metal grain, and with iron, sulfur, carbon, such as oxidation, formed the fusible oxide eutectic, destroyed the relationship between grain, make the material of plastic sharply lower. The metal with serious overburn will crack with a gentle blow when removing coarse, and the transverse crack will appear in the overburned part when pulling long.