Overheating and overburning are two common defects in the processing of metal materials, and they have a significant impact on the performance and service life of the material.
I. Definition of overheating and overcooking
1. Overheating
Overheating refers to the metal material in the heating process, due to the temperature is too high or the holding time is too long, resulting in abnormal grain growth phenomenon. Overheating of the metal organisation becomes coarse, the grain boundaries weakened, mechanical properties such as strength, toughness and other significant decline. Overheating usually occurs in the process of heat treatment, welding or casting, is a defect that needs to be strictly controlled in the processing of metal materials.
2. overcooking
Overcooking is a metal material in the heating process, because the temperature is too high and the holding time is too long, resulting in melting or oxidation of the grain boundaries, the formation of brittle phases or holes and other defects. Overcooking of the metal organisation is seriously damaged, the mechanical properties of a sharp decline, or even completely lose the use of value. Overcooking is an extremely serious heating defects, once it occurs, it is often difficult to completely restore the original properties of the material through subsequent processing.
Second, the causes of overheating and overcooking
1. Causes of overheating
- Heating temperature is too high: in the heating process of metal materials, if the temperature exceeds the normal heating range of the material, the grain will begin to grow abnormally.
- Holding time is too long: at high temperatures for a long time insulation, the grain has enough energy and time to grow, resulting in overheating.
- Inaccurate control of heating equipment: the temperature control of the heating equipment is not accurate, or the temperature sensor failure, resulting in the actual heating temperature is higher than the set temperature.
2. Causes of overheating
- Heating temperature is too high and the holding time is too long: when the heating temperature is much higher than the melting point or critical temperature of the material, and the holding time is too long, the grain boundary will melt or oxidise.
- Influence of heating medium: In some heating medium, such as the atmosphere containing oxidising gases, metal materials are more prone to overcooking.
- Material itself factors: certain metal materials are more sensitive to overheating and overfiring, such as materials with high carbon content or alloying elements.
III. Effects of overheating and overcooking
1. The impact on mechanical properties
- Overheating: overheating of metal materials, coarse grains, grain boundaries weakened, resulting in strength, toughness and other mechanical properties decline. At the same time, overheating may also make the material plasticity deterioration, processing performance is reduced.
- Overcooking: overcooked metal material organisation is seriously damaged, the formation of brittle phase or holes and other defects, resulting in a sharp decline in mechanical properties. Overcooked materials often can not withstand normal processing and use of loads, and may even occur brittle fracture.
2. Effects on processing performance
- Overheating: overheated metal materials due to coarse grain, processing is prone to cracks, deformation and other defects, processing difficulties.
- Overcooking: overcooked metal materials have very poor machining performance and cannot be effectively cut, forged and other processing operations.
3. Impact on service life
- Overheating: overheated metal materials due to the decline in mechanical properties, the service life is shortened. In high temperature, high pressure and other harsh environments, overheated materials are more likely to fail.
- Overcooking: overcooked metal materials almost completely lost the use of value, can not withstand the normal use of load, service life is extremely short.
Four, overheating and overcooking correction method
1. Correction method of overheating
- Normalising treatment: For overheated metal materials, normalising treatment can be used to refine the grain. Normalising treatment is to heat the material to a critical temperature above, hold for a period of time after air cooling. Through the normalising treatment, it can eliminate the overheated organisation and restore the normal grain structure and mechanical properties of the material.
- Annealing treatment: Annealing treatment is also an effective method of correcting overheating. Annealing treatment is to heat the material to a certain temperature, hold for a period of time after cooling with the furnace. Annealing treatment can eliminate internal stresses, reduce hardness and improve machinability. Annealing can be a simple method of correcting overheating for certain materials that do not require high performance.
- Reheating and quenching: For metal materials requiring high hardness and strength, such as tool steel and bearing steel, reheating and quenching can be used to correct overheating. Reheating quenching involves heating the material to the quenching temperature, holding it for a period of time and then cooling it rapidly. By reheating and quenching, fine martensitic organisation can be obtained and the hardness and strength of the material can be improved.
2. Correction of overheating
- Overcooking generally can not be completely corrected: overcooking is a very serious defect in the heating process of metal materials, once occurred, it is often difficult to completely restore the original properties of the material through subsequent treatment. For overcooked materials, usually can only be taken to scrap or for other purposes and other measures.
- Preventive measures: Since overheating cannot be completely corrected, it is crucial to prevent overheating from occurring. In the processing of metal materials, the heating temperature and holding time should be strictly controlled to avoid too high a temperature or holding time is too long. At the same time, the maintenance and repair of the heating equipment should be strengthened to ensure the accuracy and stability of the temperature control. In addition, methods such as protective atmosphere heating or vacuum heating can be used to reduce the occurrence of overfiring.
