As we all know, the performance of the heat-treated workpiece will be better and more in line with the performance requirements of mechanical parts. However, in the process of hot working, metal materials will have some problems, such as deformation. The appearance of metal materials is deformed, which has a fatal impact on the processing of mechanical parts. Therefore, this paper focuses on explaining the reasons for the deformation in the thermal processing of metal materials and the solutions to overcome the difficulties in the processing of metal materials.

In actual production, heat treatment deformation has various manifestations, including volume and size increase and shrinkage deformation, as well as bending, twisting, warping and other deformations. There are two types of volume deformation caused by stress plastic deformation and volumetric deformation.

(1) Internal stress plastic deformation

In the process of heat treatment, the uneven heating and cooling and the unequal time of phase transformation will generate internal stress, and under certain plastic conditions, plastic deformation of internal stress will occur. In the process of heating and cooling, the different heating and cooling rates of the inner and outer layers of the part cause the temperature to be inconsistent everywhere, resulting in different degrees of thermal expansion and contraction, and the resulting stress deformation is called thermal stress plastic deformation. During the heating and cooling process, the internal organization of the part changes at different times, and the resulting stress deformation is called tissue stress deformation plastic deformation.

(2) Volume deformation

Specific volume deformation In the process of heat treatment, the specific volume of various phase structures is different, and the volume and size changes that occur during the phase transition are specific volume deformation. The specific volume deformation is generally only related to the content of carbon and gold elements in austenite, the amount of free phase carbides, ferrite, the difference in specific volume change before and after quenching, the amount of retained austenite, and the hardenability of steel. related.

Methods to reduce heat treatment deformation.

1. Temperature measurement and control

There are many forms of heat treatment processes practically used in industry, but their basic processes are all thermal action processes, which are composed of three stages of heating, heat preservation and cooling. The whole process can be described by several parameters such as heating rate, heating temperature, holding time, cooling rate 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, carburizing, ammoniating, aluminizing, chromizing or dehydrogenation, deoxygenation, etc.). Therefore, the temperature measurement in the heating furnace has become an important process parameter measurement of heat treatment. In each heat treatment process specification, temperature is a very important content. If the temperature measurement is inaccurate, the heat treatment process specification will not be correctly implemented, resulting in a decline in product quality or even scrapping. The measurement and control of temperature is the key to the heat treatment process, and it is also a key factor affecting the deformation.

2. Temperature-controlled normalizing or isothermal annealing

Excessive normalizing hardness, mixed crystals, a large amount of sorbite or Widmanners structure will increase the deformation of the inner hole, so temperature-controlled normalizing or isothermal annealing should be used to treat forgings. Normalizing, annealing, and quenching and tempering before quenching will have a certain impact on the final deformation of the metal, and directly affect the change in the metal structure. Practice has proved that the use of isothermal quenching during normalizing can effectively make the metal structure tend to be uniform, thereby reducing the amount of deformation.

3. Reasonable cooling method

The influence of the cooling process on the deformation after metal quenching is also an important cause of deformation. Hot oil quenching has smaller deformation than cold oil quenching, and is generally controlled at 100 °C ± 20 °C. The cooling capacity of the oil is also critical to deformation. Both 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 and the greater the deformation of the mold. Pre-cooling can be used as much as possible on the premise of ensuring the hardness of the mold; the use of graded cooling and quenching can significantly reduce the thermal stress and structural stress generated during metal quenching, and it is an effective method to reduce the deformation of some workpieces with complex shapes; For workpieces with high precision requirements, the use of isothermal quenching can significantly reduce deformation.

4. Reasonable parts structure

During the cooling process of metal after heat treatment, the thin part always cools quickly and the thick part cools slowly. In the case of meeting the actual production needs, the thickness disparity of the workpiece should be minimized, and the cross-section of the parts should be uniform to reduce the distortion and cracking tendency due to stress concentration in the transition zone; the workpiece should try to maintain the symmetry of the structure and material composition and organization to reduce the Distortion caused by uneven cooling; the workpiece should try to avoid sharp edges and corners, grooves, etc., and rounded transitions at the junction of thickness and steps of the workpiece; minimize the asymmetry of holes and grooves on the workpiece; uneven thickness Parts use the method of reserved machining.

5. Reasonable clamping methods and fixtures

The purpose is to make the workpiece evenly heated and cooled, so as to reduce uneven thermal stress and uneven tissue stress to reduce deformation. The clamping method can be changed. The disc parts are perpendicular to the oil surface, and the shaft parts are installed vertically. , superimposed washers, etc., spline hole parts can be used carburized mandrels, etc.

6. Machining

When heat treatment is the last process of workpiece processing, the allowable value of heat treatment distortion should meet the workpiece size specified on the drawing, and the distortion amount should be determined according to the processing size of the previous process. For this reason, according to the distortion law of the workpiece, pre-correction of the size should be carried out before heat treatment, 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 shall be regarded as the sum of machining allowance and heat treatment distortion. Usually, the machining allowance is easy to determine, and the heat treatment is more complicated due to many influencing factors, so leave enough machining allowance for machining, and the rest can be used as the allowable distortion amount of heat treatment. After heat treatment, after processing, according to the deformation law of the workpiece, the anti-deformation and shrinkage end pre-expansion holes are applied to improve the qualified rate of deformation after quenching.

Seven, the use of suitable media

Under the premise of ensuring the same hardness requirements, try to use oily medium. Experiments and practice have proved that under the premise of no difference in other conditions, the cooling rate of oily medium is slower, while the cooling rate of water-based medium is relatively faster. Moreover, compared with the oily medium, the water temperature change has a greater influence on the cooling characteristics of the aqueous medium. Under the same heat treatment conditions, the deformation of the oily medium after quenching is relatively small compared to the aqueous medium.