In actual production, the heat treatment deformation adds a lot of difficulties to the subsequent processes, especially the machining, which affects the production efficiency, and the scrap is too large, resulting in scrapping and increasing the cost. Deformation is a problem that is difficult to solve by heat treatment. It is impossible to completely deform without deformation. Generally, the deformation amount is controlled within a certain range.
First, the cause of heat treatment deformation
Steel may be deformed or even cracked during the heating and cooling process of heat treatment because of the existence of quenching stress. Quenching stress is divided into thermal stress and tissue stress. Due to thermal stress and tissue stress, the residual stress is generated in the parts after heat treatment, which may cause deformation. When the stress is greater than the yield strength of the material, deformation occurs. Therefore, the quenching deformation is also related to the yield strength of the steel. The greater the plastic deformation resistance of the material, the smaller the degree of deformation.
Thermal stress
During heating and cooling, thermal stress is generated due to the inconsistency of thermal expansion and contraction due to the temperature difference in the parts table. When the parts are cooled by high temperature, the surface dissipates quickly and the temperature is lower than the core. Therefore, the surface has a larger volume contraction tendency than the core, but the surface is subjected to tensile stress by the core, and the core is subjected to compressive stress. The temperature difference in the table increases the stress.
2. Tissue stress
The tissue stress is due to the different specific volume of austenite and its transformation products, and the surface transition time of the part and the heart or parts of the part are different. Since the austenite specific volume is the smallest, an increase in volume is inevitable when quenching and cooling. When quenching, the surface begins to transform with martensite, the volume increases, and the austenite volume remains unchanged. Since the core obstructs the surface volume, the surface generates compressive stress and the core generates tensile stress.
Second, methods for reducing and controlling heat treatment deformation
1. Reasonable material selection and hardness improvement
For parts with complex shapes and large differences in cross-sectional dimensions and requiring less deformation, materials with better hardenability should be selected for quenching with a milder quenching cooling medium. For thin-plate precision parts, bi-directional rolled sheets should be used to make the fiber direction of the parts symmetrical. For the hardness requirements of the parts, under the premise of meeting the requirements of use, try to choose the lower limit hardness.
2. Correct design of parts
The shape of the part should be as simple, uniform and structurally symmetrical as possible to avoid the uneven tendency of deformation and cracking due to uneven cooling. Try to avoid sudden changes in section size, reduce grooves and thin edges, and do not have sharp edges and corners. Avoid deeper holes. Long parts avoid cross-sectional trapezoids.
3. Reasonably arrange production routes and coordinate the relationship between hot and cold processing and heat treatment
For parts with complex shapes and high precision requirements, pre-treatment between roughing and finishing should be carried out, such as stress relief and annealing. Prepare the blank for heat treatment to make the tissue more uniform.
4. Improve heat treatment process and operation
(1) Control of heat treatment temperature
In the case of meeting the heat treatment process requirements, the quenching heating temperature is minimized and the holding time is shortened, so that the high temperature strength loss of the parts is less. The plastic resistance is enhanced, and the comprehensive resistance of the parts to stress deformation and anti-quenching deformation is enhanced, thereby reducing deformation.
(2) Select a milder quenching cooling medium
Cooling is the key process of quenching. It is related to the quality of quenching, and it is also the most prone to problems in quenching process. The effect of cooling process on deformation after metal quenching is also a very important reason. Hot oil quenching is less deformed than quenching of cold oil, and is generally controlled at about 90 °C. Under the premise of ensuring hardness, oily medium is used as much as possible. The cooling rate of oily medium is slower, while the cooling rate of aqueous medium is relatively faster, and the change of water temperature has a great influence on the cooling characteristics of aqueous medium.
The use of graded quenching can significantly reduce the thermal stress and microstructure stress of the cattle during quenching, which is an effective way to reduce some complex parts. This quenching method is effective because the temperature of each part of the part has become uniform before the martensite transformation and the martensite transformation is completed under slow cooling conditions, which not only reduces the quenching thermal stress but also significantly reduces the structural stress, thus effectively Reduce or pre-prevent parts from quenching deformation.
Austempering can also significantly reduce the deformation of the part. The difference between austempering and fractional quenching is that the former obtains the lower bainite structure. Because the lower bainite structure has higher strength and hardness, and the toughness is good, the specific heat capacity is higher than the specific heat capacity of martensite. Small, the temperature inside and outside the part is the same when the structure is changed, so the quenching structure stress is also small. Austempering and staged quenching can only be applied to smaller parts.
(3) Correctly master the heat treatment operation method
In order to reduce and control the heat treatment deformation, it is necessary to correctly grasp the heat treatment operation method. When the section is evenly cooled, if the elongated shaft (screw, shaft) is vertically quenched, moving up and down, there is also vertical gradual quenching and standing still. When the section is unevenly cooled, the horizontal rapid quenching or oblique quenching, such as uneven thickness parts, should be thickened first. For sheets with lateral entry, for holes with and for concave parts, the blind holes and concave surfaces should be quenched upwards to facilitate bubble discharge. In short, to make the parts have a uniform cooling rate, some parts that need protection are protected before quenching.
Third, the salvage method of the deformed parts
Although the deformation reduction measures are taken, the deformation is still inevitable, so it is necessary to carry out remediation. The commonly used methods are cold straightening and thermal straightening. If the hardness is less than 40HRC after quenching and tempering, it can be directly corrected by cold straightening. If the metal is not cooled to the martensite transformation temperature point during the quenching and cooling process, the plasticity of the part is good, and the pressurization alignment effect is good. The force should be slow when straightening.
Fourth, the use of precision heat treatment equipment
V. Conclusion
Heat treatment can improve the mechanical properties of the parts, improve the strength and hardness of the parts, and meet various performance needs, but the deformation effects are inevitable. When selecting specific methods to prevent heat treatment deformation, specific methods should be formulated according to specific conditions. Many methods are derived from practice, and trial and error can be used to explore the law. We must pay attention to the current heat treatment technology, continue to learn advanced technology, and at the same time develop independent innovation, continuously improve the quality and qualified rate of heat-treated parts, and contribute to the development of heat treatment.
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