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Causes and Control of Deformation in Metal Vacuum Heat Treatment

As we all know, the performance of the workpiece after vacuum heat treatment will be better and more in line with the performance requirements of mechanical parts. However, in the process of vacuum thermal processing, some problems will occur in metal materials, such as deformation. The appearance deformation of metal materials has a fatal effect on the processing of mechanical parts.
In actual production, vacuum heat treatment deformation manifests in various forms, including volume and size increase and shrinkage deformation, as well as bending, twisting, warping and other deformations. There are two types of stress plastic deformation and volume deformation caused by specific volume changes.

(1) Plastic deformation under internal stress

In the process of vacuum heat treatment, the uneven heating and cooling and the unequal phase transition will generate internal stress, and under the cooperation of certain plastic conditions, internal stress plastic deformation will occur. During the heating and cooling process, the different heating and cooling speeds of the inner and outer layers of the parts cause inconsistent temperatures everywhere, resulting in different degrees of thermal expansion and contraction. The stress deformation produced in this way is called thermal stress plastic deformation. During the heating and cooling process, the internal structure transformation of the part occurs at different times, and the stress deformation generated in this way is called tissue stress deformation plastic deformation.

(2) Volume deformation

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

The Method of Reducing the Distortion of Vacuum Heat Treatment

1. Temperature measurement and control

There are many forms of vacuum heat treatment processes that are actually used in industry, but their basic processes are 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 vacuum heat treatment cycle. In the vacuum heat treatment process, various vacuum furnaces are used, and metal heat treatment is carried out in these vacuum furnaces (such as vacuum annealing, vacuum quenching, vacuum tempering in basic vacuum heat treatment, vacuum carburizing in chemical heat treatment, vacuum ammonification, etc. wait). Therefore, the temperature measurement in the vacuum furnace has become an important process parameter measurement for vacuum heat treatment. In every vacuum heat treatment process specification, temperature is a very important content. If the temperature measurement is inaccurate, the vacuum 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 vacuum heat treatment process, and also the key factor 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 forgings should be treated with temperature-controlled normalizing or isothermal annealing. Metal normalization, vacuum annealing and quenching and tempering before vacuum quenching will all have a certain impact on the final deformation of the metal, directly affecting the change in the metal structure. Practice has proved that the use of austempering 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 cooling process on deformation after metal vacuum quenching is also a very important cause of deformation. The deformation of hot oil quenching is smaller than that of cold oil quenching, and it is generally controlled at 100°C ± 20°C. The cooling capacity of the oil is also critical to deformation. Both the stirring mode and speed of vacuum quenching affect the deformation. The faster the cooling rate of metal vacuum 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 under the premise of ensuring the hardness requirements of the mold; the use of graded cooling and quenching can significantly reduce the thermal stress and structural stress generated during metal quenching, and is an effective way to reduce the deformation of some workpieces with more complex shapes; for some particularly complex or For workpieces with high precision requirements, the deformation can be significantly reduced by austempering.

4. Reasonable parts structure

In the cooling process after metal vacuum heat treatment, the thin part always cools quickly, and the thick part cools slowly. In the case of meeting the actual production needs, the gap in the thickness of the workpiece should be minimized, and the cross-section of the part should be uniform to reduce the distortion and cracking tendency of the transition zone due to stress concentration; the workpiece should try to maintain the symmetry of the structure and material composition and organization to reduce Distortion caused by uneven cooling; sharp edges, grooves, etc. should be avoided as far as possible for the workpiece, and there should be rounded corner transitions at the junction of thickness and steps of the workpiece; try to reduce the asymmetric structure of holes and grooves on the workpiece; uneven thickness Parts adopt the method of reserved machining amount.

5. Reasonable clamping methods and fixtures

The purpose is to make the workpiece heating and cooling evenly, so as to reduce the uneven thermal stress and tissue stress, so as to reduce the deformation. The clamping method can be changed. The disc parts are perpendicular to the oil surface, and the shaft parts are installed vertically. Use compensation washers and support washers , Superimposed washers, etc., spline hole parts can be carburized mandrel, etc.

6. Machining

When vacuum 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 amount of distortion should be determined according to the processing size of the previous process. For this reason, according to the distortion law of the workpiece, the size should be pre-corrected before vacuum heat treatment, so that the heat treatment distortion is just within the acceptable range. When heat treatment is an intermediate process, the machining allowance before vacuum heat treatment should be regarded as the sum of machining allowance and vacuum heat treatment distortion. Generally, the machining allowance is easy to determine, but the vacuum heat treatment is more complicated due to the many influencing factors, so enough machining allowance is reserved for machining, and the rest can be used as the allowable distortion of vacuum heat treatment. Vacuum heat treatment is followed by processing. According to the deformation law of the workpiece, anti-deformation and shrinkage end pre-expansion holes are used to improve the qualified rate of deformation after vacuum quenching.

7. Use a suitable medium

On the premise of ensuring the same hardness requirements, use oily medium as much as possible. 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 watery medium is relatively faster. Moreover, compared with the oily medium, the change of water temperature has a greater influence on the cooling characteristics of the watery medium. Under the same vacuum heat treatment conditions, the deformation of the oily medium is relatively smaller than that of the watery medium after vacuum quenching.