Heat treatment process is an important means to obtain excellent properties of various metal materials. In many practical applications, the reasonable selection of materials and various forming processes cannot meet the mechanical, physical and chemical properties required by metal workpieces. In this case, heat treatment processes are indispensable.
However, in addition to the positive effects of the heat treatment process, it will inevitably produce more or less deformation during the treatment process, which must be avoided in mechanical processing. The two coexist and need to be avoided. Relationship, we can only use corresponding methods to control the amount of deformation within as small a range as possible.
1. Temperature measurement and control
There are many forms of heat treatment processes actually applied in industry, but their basic processes are all thermal processes, which are composed of three stages: heating, insulation and cooling. The entire process can be described by several parameters such as heating speed, heating temperature, holding time, cooling speed and heat treatment cycle.
In the heat treatment process, various heating furnaces are used, and metal heat treatment is carried out in these heating furnaces (such as annealing, quenching, tempering in basic heat treatment, carburizing, ammoniation, aluminizing, chromizing or chemical heat treatment). Remove hydrogen, remove oxygen, etc.). Therefore, temperature measurement in the heating furnace has become an important process parameter measurement for heat treatment.
In every heat treatment process specification, temperature is a very important content. If the temperature measurement is inaccurate, the heat treatment process specifications will not be implemented correctly, resulting in reduced product quality or even scrapping. The measurement and control of temperature are the key to the heat treatment process and are also the key factors affecting deformation.
2. Temperature controlled normalizing or isothermal annealing
Excessive normalizing hardness, mixed crystals, a large amount of sorbite or Widmanstatten structure will increase the deformation of the inner hole, so temperature-controlled normalizing or isothermal annealing should be used to process the forgings. Normalizing, annealing and tempering before quenching of metal will have a certain impact on the final deformation of the metal, which directly affects the changes in the metal structure.
Practice has proved that the use of isothermal quenching during normalizing can effectively make the metal structure more uniform, thereby reducing its deformation.
3. Reasonable cooling method
The influence of the cooling process on deformation after metal quenching is also a very important cause of deformation. Hot oil quenching has smaller deformation than cold oil quenching, and is generally controlled at 100℃±20℃. The cooling ability of the oil is also critical to deformation. The stirring method and speed of quenching affect the deformation.
The faster the cooling rate of metal heat treatment, the more uneven the cooling, the greater the stress generated, and the greater the deformation of the mold. Pre-cooling can be used as much as possible while ensuring the hardness requirements of the mold; using graded cooling quenching can significantly reduce the thermal stress and structural stress generated during metal quenching, and is an effective method to reduce the deformation of some workpieces with complex shapes; for some particularly complex or For workpieces with high precision requirements, austempering can significantly reduce deformation.
4. Reasonable parts structure
During the cooling process after metal heat treatment, the thin parts always cool faster and the thick parts cool slower. In order to meet the actual production needs, the thickness difference of the workpiece should be minimized and the cross-section of the parts should be uniform to reduce the tendency of distortion and cracking due to stress concentration in the transition zone;
The workpiece should try to maintain the symmetry of the structure, material composition and organization to reduce distortion caused by uneven cooling; the workpiece should try to avoid sharp edges, grooves, etc., and there should be rounded transitions at the junction of thickness and steps of the workpiece; Minimize the asymmetric structure of holes and grooves on the workpiece; use the method of reserved processing amount for parts with uneven thickness.
5. Reasonable clamping methods and fixtures
The purpose is to make the workpiece heated and cooled evenly to reduce uneven thermal stress and uneven tissue stress to reduce deformation. The clamping method can be changed. Disc parts are perpendicular to the oil surface, and shaft parts are installed vertically. Compensation washers and support washers are used. , superimposed washers, etc., carburized mandrels can be used for spline hole parts, etc.
6. Machining
When heat treatment is the last step in the workpiece processing process, the allowable value of heat treatment distortion should meet the workpiece size specified in the drawing, and the amount of distortion should be determined based on the processing size of the previous process. For this reason, the dimensions should be pre-corrected before heat treatment according to the distortion rules of the workpiece, so that the heat treatment distortion is just within the qualified range.
When heat treatment is an intermediate process, the machining allowance before heat treatment should be regarded as the sum of the machining allowance and heat treatment distortion. Usually, the machining allowance is easy to determine, but heat treatment is more complicated due to many influencing factors, so sufficient machining allowance is left for mechanical processing, and the rest can be used as the allowable distortion amount of heat treatment. After heat treatment and reprocessing, according to the deformation law of the workpiece, anti-deformation and contraction-end pre-expansion holes are used to improve the deformation qualification rate after quenching.
7. Use appropriate media
Under the premise of ensuring the same hardness requirements, try to use oily media. Experiments and practice have proven that, provided there is no difference in other conditions, the cooling rate of oily media is slower, while the cooling rate of water-based media is relatively faster. Moreover, compared with oily media, changes in water temperature have a greater impact on the cooling characteristics of aqueous media. Under the same heat treatment conditions, the deformation of oily media after quenching is relatively smaller than that of aqueous media.
