Metal Heat Treatment Deformation: Causes and Control Strategies

As we all know, after heat treatment, the performance of the workpiece will be more excellent and meet the performance requirements of mechanical parts.

However, in the process of hot processing, metal materials will have some problems, such as deformation.

The appearance deformation of metal materials has a fatal effect on the processing of mechanical parts.

Therefore, this paper focuses on the reasons that affect the deformation in the hot processing of metal materials and the solutions to overcome the difficulties in the processing of metal materials.

In production practice, heat treatment deformation has various forms, including volume and size increase and shrinkage deformation, as well as bending, skew, warpage and other deformation.

In terms of the root causes, it can be divided into stress-plastic deformation caused by internal stress and volume deformation caused by specific volume change.

(1) Internal stress plastic deformation

In the process of heat treatment, the uneven heating and cooling and uneven phase transformation will generate internal stress.

Under the cooperation of certain plastic conditions, internal stress and plastic deformation will occur.

In the process of heating and cooling, the different heating and cooling speeds of the inner and outer layers of the parts cause different temperatures, resulting in different degrees of thermal expansion and cold contraction.

The stress deformation thus generated is called thermal stress plastic deformation.

In the process of heating and cooling, the internal structure of the part changes at different times, and the stress and deformation generated in this way 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 during phase transformation 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 and ferrites, the difference in specific volume change of microstructure before and after quenching, the amount of retained austenite and the hardenability of steel.

Methods of reducing heat treatment deformation

1. Temperature measurement and control

There are many types of heat treatment processes actually used in industry, but their basic processes are all thermal processes, which are composed of heating, insulation and cooling.

The whole 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 the metal heat treatment is carried out in these heating furnaces (such as annealing, quenching, tempering in the basic heat treatment, carburizing, ammoniating, aluminizing, chromizing or dehydrogenation, deoxidization in the chemical heat treatment, etc.).

Therefore, the temperature measurement in the heating furnace becomes 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 not accurate, the heat treatment process specification cannot be correctly implemented, and the product quality will be reduced or even scrapped. Temperature measurement and control is the key of heat treatment process and also the key factor affecting deformation.

2. Temperature-controlled normalizing or isothermal annealing

High normalizing hardness, mixed crystal, and large amount of sorbite or widmanstatten structure will increase the deformation of the inner hole, so the forgings should be treated with temperature-controlled normalizing or isothermal annealing.

Normalizing, annealing and quenching and tempering of metal will have a certain impact on the final deformation of metal, and directly affect the change of metal structure.

The practice has proved that isothermal quenching can effectively make the structure of metal more uniform and reduce deformation.

3. Reasonable cooling method

The influence of the cooling process on the deformation of metal after quenching is also a very important deformation reason.

The deformation of hot oil quenching is smaller than that of cold oil quenching, generally controlled at 100 ℃± 20 ℃.

The cooling capacity of the oil is also critical to deformation.

The stirring mode and speed of quenching affect the deformation.

The faster the cooling speed of metal heat treatment, the more uneven the cooling, the greater the stress generated, and the greater the deformation of the mold.

Precooling can be used as much as possible on the premise of ensuring the hardness requirements of the mold;

Step cooling quenching can significantly reduce the thermal stress and structural stress produced during metal quenching, which is an effective method to reduce the deformation of some complicated workpieces;

For some workpieces which are particularly complex or require high precision, the deformation can be significantly reduced by using isothermal quenching.

4. Reasonable part structure

In the cooling process of metal after heat treatment, the thin part always cools quickly, and the thick part cools slowly.

Under the condition of meeting the actual production needs, the thickness difference of the workpiece shall be reduced as much as possible, and the section of the parts shall be uniform as much as possible to reduce the distortion and cracking tendency of the transition zone due to the stress concentration;

The workpiece shall keep the symmetry of structure, material composition and structure as much as possible to reduce the distortion caused by uneven cooling;

Sharp edges, corners, grooves, etc. shall be avoided as much as possible, and fillet transition shall be provided at the junction of thickness and thickness of the workpiece and at the step;

Try to reduce the asymmetry of holes and groove bars on the workpiece;

For parts with uneven thickness, the method of reserved processing quantity shall be adopted.

5. Reasonable clamping method and fixture

The purpose is to make the workpiece heat and cool evenly, so as to reduce the uneven thermal stress and the uneven organizational stress, so as to reduce the deformation.

The clamping method can be changed.

The disc parts are perpendicular to the oil surface, the shaft parts are installed vertically, the compensation washer, the support washer, the superimposed washer, etc. are used, and the carburizing mandrel can be used for the spline hole parts.

6. Machining

When heat treatment is the last process of workpiece processing, the allowable value of heat treatment distortion shall meet the workpiece size specified in the drawing, and the distortion variable shall be determined according to the processing size of the previous process.

Therefore, according to the distortion law of the workpiece, the size should be pre-corrected before heat treatment to make the distortion of heat treatment just within the acceptable range.

When heat treatment is an intermediate process, the machining allowance before heat treatment shall be regarded as the sum of the machining allowance and heat treatment distortion.

Generally, the machining allowance is easy to determine, but the heat treatment is complicated due to many influencing factors, so enough machining allowance is reserved for the machining, and the rest can be used as the allowable distortion variable of the heat treatment.

After heat treatment, the workpiece is processed again.

According to the deformation law of the workpiece, reverse deformation and shrinkage end pre-expansion holes are used to improve the deformation qualification rate after quenching.

7. Use appropriate media

On the premise of ensuring the same hardness requirements, the oil-based medium shall be used as much as possible.

Experiments and practice show that the cooling speed of oil-based medium is slower, while the cooling speed of water-based medium is relatively faster under the premise of no difference in other conditions.

Moreover, compared with oil-based medium, the change of water temperature has a greater impact on the cooling characteristics of water-based medium.

Under the same heat treatment conditions, the deformation of oil-based medium after quenching is relatively small compared with that of water-based medium.

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