Do you know the reason of heat treatment deformation?

Do you know the reason of heat treatment deformation? The control method is here

In the actual production, the heat treatment deformation adds a lot of difficulties to the subsequent processes, especially the mechanical processing, which affects the production efficiency, and leads to the scrapping due to the excessive deformation, which increases the cost. Deformation is a difficult problem to be solved in heat treatment. It is impossible not to deform at all. Generally, deformation is controlled within a certain range.

I. causes of heat treatment deformation

During the heating and cooling process of heat treatment, the steel may deform or even crack due to the existence of quenching stress. Quenching stress is divided into thermal stress and tissue stress. Because of the action of thermal stress and tissue stress, different residual stresses may be generated after heat treatment, which may cause deformation. Deformation occurs when the stress is greater than the yield strength of the material. Therefore, quenching deformation is also related to the yield strength of the steel. The greater the plastic deformation resistance of the material, the smaller the deformation degree.

1. The thermal stress

During heating and cooling, thermal stress occurs because of the difference in temperature between the interior and interior of the parts, which causes the thermal expansion and contraction. When the part is cooled by high temperature, the surface dissipates heat quickly and the temperature is lower than the core. Therefore, the surface has a larger tendency of volume contraction than the core. However, the surface is subjected to tensile stress due to the obstruction of the core, while the core is subjected to compressive stress. When the temperature difference between inside and outside increases, the stress also increases.

2. Tissue stress

Tissue stress is caused by the difference in the specific volume of austenite and its transformation products, and the difference in the time of tissue transformation between the surface and the core of the part or between parts of the part. Since the austenite volume is the smallest, the volume must increase when quenching. During quenching, martensite transformation begins on the surface and the volume increases, while the core remains the same austenite volume. As the volume of the obstructed surface of the core increases, compressive stress is generated on the surface and tensile stress is generated at the core [1].

Methods to reduce and control deformation during heat treatment

1. Reasonable material selection and higher hardness requirements

For the parts with complex shape, large cross section size difference and small deformation requirements, the material with better hardenability should be selected, so as to use a more gentle quenching cooling medium quenching. For the thin plate precision parts, the two - way rolling plate should be used to make the fiber direction of the parts symmetrical. For the hardness requirements of parts, under the premise of meeting the requirements for use, try to choose the lower limit of hardness.

2. Design the parts correctly

The appearance of the parts should be as simple as possible, uniform, symmetrical structure, so as not to increase the tendency of deformation and cracking due to uneven cooling. Try to avoid sudden changes in section size, reduce grooves and thin edges, and avoid sharp edges. Avoid deep holes. Long - shaped parts avoid cross - section trapezoid.

3. Reasonably arrange production routes and coordinate the relationship between cold and hot processing and heat treatment

For the parts with complex shape and high precision requirements, should be processed in advance between rough and finishing, such as stress relief, annealing, etc. 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

Under the condition of meeting the requirements of heat treatment process, reduce the quenching heating temperature and shorten the holding time as far as possible, so that the high temperature strength loss of the parts is less. The plastic resistance is enhanced, and the parts' comprehensive ability to resist stress deformation and quenching deformation is enhanced, thus reducing deformation.

(2) select a more gentle quenching cooling medium

Cooling is the key process of quenching, it is related to the quality of quenching, is also the most prone to problems in the quenching process, after quenching of metal cooling process on deformation is also a very important reason. The quenching deformation of hot oil is smaller than that of cold oil, which is generally controlled at about 90℃. Under the premise of ensuring the hardness, try to use the oil medium, oil medium cooling rate is slow, while water medium cooling rate is relatively fast, and the water temperature change has a greater impact on the cooling characteristics of water medium.

The use of fractional quenching can significantly reduce the thermal stress and microstructure stress of the cattle during metal quenching. This quenching method, because the temperature of each part of the part tends to be even before martensite transformation, and the martensite transformation is completed under the condition of slow cooling, not only reduces the quenching thermal stress, but also significantly reduces the tissue stress, thus effectively reducing or preventing the quenching deformation of the part.

Isothermal quenching can significantly reduce the deformation of parts, isothermal quenching and grading the difference between quenching bainite organization, due to the lower bainite organization strength, high hardness, and toughness, good heat capacity is smaller than the heat capacity of the martensite, organization transformation components inside and outside temperature, quenching organization stress is small. Isothermal quenching and fractional quenching can only be used for smaller parts.

(3) master the operation method of heat treatment correctly

In order to reduce and control the deformation of heat treatment, the operation method of heat treatment must be mastered correctly. When the cross section is cooled evenly, such as vertical quenching of slender shaft (tap, shaft), moving up and down, or vertical gradually quenching and static. When the cross section is unevenly cooled, it should be quickly quenched horizontally or obliquely. If the thickness is uneven, the thick part should be quenched first. The thin sheet shall be entered laterally, and the blind hole and concave surface shall be quenched upward for parts with holes and concave surface to facilitate bubble discharge. In short, to make the parts everywhere cooling rate uniform, some need to protect the parts before quenching protection.

Three, has been deformed parts of the rescue method

Although the measures to reduce the deformation are taken, the deformation is still inevitable, so it is necessary to remedy it. If the hardness of the parts is less than 40HRC after quenching and tempering, it can be corrected by cold straightening press. If the metal in the quenching cooling process, has not cooled to martensite began to change below the temperature point, the plastic parts at this point is good, pressure straightening effect is good, straightening force should pay attention to the slow.

Four, conclusion

Heat treatment can improve the mechanical properties of the parts, improve the strength and hardness of the parts, to meet the needs of various properties, but the resulting deformation is inevitable. When choosing specific methods to prevent deformation in heat treatment, specific methods should be formulated according to specific conditions. We should pay attention to the present stage of heat treatment technology, continue to learn advanced technology, and develop independent innovation, continue to improve the quality and qualification rate of heat treatment parts, make contributions to the development of heat treatment.

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