How to reduce the deformation of metal in heat treatment

Surface treatment network: the heat treatment of metal materials is to heat and keep the solid metal or alloy in a proper way, and sometimes to change the internal structure and structure of the metal alloy by chemical and mechanical actions, so as to obtain the process of improving the material mechanical properties. Heat treatment process is an important means to make all kinds of metal materials obtain excellent function. In many practical applications, reasonable selection of materials and various forming processes can not meet the mechanical, physical and chemical functions required by metal work pieces, and heat treatment process is essential.

However, in addition to the positive role of heat treatment process, it is inevitable to produce more or less deformation in the process of treatment, which is necessary to avoid in machining. The relationship between the two is coexistence and needs to be avoided, and only the corresponding methods can be used to control the deformation as small as possible.

Temperature is the key factor of deformation

There are many types of heat treatment processes applied in industry, but their basic processes are all thermal processes, which are composed of three stages: heating, heat preservation and cooling. The whole process can be described by heating speed, heating temperature, holding time, cooling speed and heat treatment cycle. In the heat treatment process, a variety of heating furnaces are used, and the metal heat treatment is carried out in these furnaces (such as annealing, quenching, tempering, carburizing, ammonification, aluminizing, chromizing or dehydrogenation, deoxidization, etc. in the basic heat treatment). Therefore, the temperature measurement in the heating furnace becomes an important process parameter measurement of heat treatment. In every heat treatment process specification, temperature is very important. If the temperature measurement is not correct, the heat treatment process specification can not be accurately implemented, resulting in product quality decline or even scrap. The measurement and control of temperature is not only the key point of heat treatment process, but also the key factor affecting deformation.

(1) the loss of high temperature strength of the workpiece is relatively reduced and the plastic resistance is enhanced after the process temperature is reduced. In this way, the comprehensive ability of the workpiece to resist stress deformation, quenching deformation and high temperature creep is enhanced, and the deformation will be reduced;

(2) after the process temperature is reduced, the temperature range of workpiece heating and cooling will be reduced, and the temperature inconsistency of each part will also be reduced, and the resulting thermal stress and tissue stress will also be relatively reduced, so the deformation will be reduced;

(3) if the process temperature is reduced and the heat treatment process time is shortened, the high temperature creep time and deformation of the workpiece will be reduced.

II. Other influencing factors of deformation and reduction measures

(I) preparation for heat treatment

High normalized hardness, mixed crystal, large amount of sorbite or widmanstatten structure will increase the deformation of inner hole, so the forging should be treated by temperature controlled normalizing or isothermal annealing. Normalizing, annealing and quenching and tempering of metal will have a certain impact on the deformation of metal, which directly affects the change of metal structure. It has been proved by practice that isothermal quenching can effectively make the metal structure tend to average, and thus reduce its deformation.

(II) use reasonable cooling methods

The influence of cooling process after quenching on deformation is also a very important 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 crucial for deformation. The mixing 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 and the greater the deformation of the die. Pre cooling can be used as much as possible under the condition of ensuring the hardness requirements of the mold; the use of staged 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 workpieces with complex shapes; for some workpieces with particularly complex or high precision requirements, the use of isothermal quenching can significantly reduce the deformation.

(III) Fair structure of parts

After metal heat treatment, in the cooling process, the thin department always cools faster than the thick department. Under the condition of satisfying the actual production needs, the thickness gap of the workpiece shall be minimized, and the cross section of the parts shall be average as far as possible to reduce the distortion and cracking tendency of the transition area due to stress concentration; the symmetry of the structure and material composition and organization shall be kept as far as possible to reduce the distortion caused by uneven cooling; the sharp edges, corners, grooves, etc. of the workpiece shall be avoided as far as possible, and the thickness of the workpiece shall be connected There shall be fillet transition at the soil and steps; the hole and groove reinforcement structure on the workpiece shall be minimized; the method of reserved machining amount shall be adopted for parts with uneven thickness.

(IV) Fair clamping method and fixture

Aim to make the workpiece evenly heated and cooled, so as to reduce the uneven thermal stress and the uneven tissue stress, so as to reduce the deformation. The clamping method can be changed. The plate parts are perpendicular to the oil surface, the shaft parts are vertically installed, the compensation washer, the support washer, the superposition washer, etc. can be used for the spline hole parts, and the carburizing mandrel can be used for the spline hole parts.

(V) machining

When heat treatment is the post process of workpiece processing, the allowable value of heat treatment distortion shall meet the workpiece size defined on the drawing, and the distortion variable shall be determined according to the processing size of the previous process. Therefore, the size of the workpiece should be pre corrected before heat treatment according to the distortion law of the workpiece, so that the distortion of heat treatment 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. In general, machining allowance is easy to determine, and heat treatment because of the more complex factors, so leave enough machining allowance for machining, the rest can be used as heat treatment allowance. After heat treatment, according to the deformation law of the workpiece, reverse deformation and shrinkage end pre expansion are applied to improve the qualified rate of deformation after quenching.

(VI) use appropriate media

Under the condition of ensuring the same hardness requirements, try to use oil-based medium. Experiments and practices have proved that under other conditions without any difference, the cooling speed of oil-based medium is slower, while that of water-based medium is faster. Moreover, compared with the oil-based medium, the change of water temperature has a greater impact on the cooling characteristics of the water-based medium. Under the same premise of heat treatment, the deformation of the oil-based medium after quenching is relatively small.

Heat treatment can improve the mechanical function of workpieces, improve the strength and hardness of workpieces, but the deformation caused by heat treatment is inevitable. We should face up to the improvement of heat treatment technology and equipment at the present stage in China, constantly learn from foreign countries to improve the technology of our predecessors, improve the quality and qualification rate of heat treatment workpiece, and make contributions to the heat treatment industry.

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