What about the deformation of precision parts after heat treatment
In actual production, heat treatment deformation adds many difficulties to subsequent processes, especially mechanical processing, which affects production efficiency and leads to scrap due to excessive deformation, increasing costs. Deformation is a difficult problem to solve in heat treatment. It is impossible not to deform at all.
Causes of heat treatment deformation
Steel may deform or even crack during heat treatment heating and cooling due to the existence of quenching stress. Quenching stress can be divided into thermal stress and tissue stress. Due to thermal stress and tissue stress, different residual stresses may be generated after heat treatment, which may lead to 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 steel.
1. The thermal stress
When heating and cooling, thermal stress is caused by the inconsistency of thermal expansion and cold shrinkage caused by the temperature difference in the part surface. When the parts are cooled by high temperature, the surface dissipates heat quickly, and the temperature is lower than the heart, so the surface has a larger volume contraction tendency than the heart, but it is blocked by the heart, which makes the surface tension stress, while the heart is under pressure stress. The surface temperature difference increases the stress.
2. Organizational stress
The tissue stress is caused by the different specific volume of austenite and its transformation product, and the different tissue transformation time between the surface of parts and the heart or parts of parts. Because of the minimum austenite specific capacity, the volume of quenching must increase. Martensite transformation begins on the surface when quenching. The volume increases, and the core remains the same as austenite volume. Because the core hinders the surface volume increase, the surface produces compressive stress, and the heart produces tensile stress.
Methods of reducing and controlling heat treatment deformation
1. Reasonable material selection and hardness improvement requirements
For parts with complex shapes, large cross-section sizes and small deformation requirements, materials with better hardenability should be selected so as to use a softer quenching cooling medium for quenching. For thin plate precision parts, bidirectional rolling plate should be used to make the fiber direction of parts symmetrical. For the hardness requirements of parts, under the premise of meeting the requirements for use, the lowest hardness should be selected as far as possible.
2. Design the parts correctly
The appearance of parts should be as simple as possible, uniform and symmetrical in 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 through hole. Long - shaped parts avoid cross - section trapezoid.
3. Reasonably arrange the production route and coordinate the relationship between cold and hot processing and heat treatment
For the parts with complex shape and high precision, they should be preprocessed between rough and precision machining, such as stress elimination and annealing. Prepare the blank for heat treatment to make the organization more uniform.
4. Improve the heat treatment process and operation
(1) control of heat treatment temperature
Under the condition that the heat treatment process is satisfied, the quenching heating temperature is reduced and the heat preservation time is shortened as far as possible, so that the high temperature strength loss of the parts is less. The plastic resistance is enhanced, and the comprehensive ability of the parts to resist stress deformation and quenching deformation is enhanced, thus reducing the deformation.
(2) select the quenching cooling medium that is more moderate
Cooling is the key process of quenching, which is related to the quality of quenching, and also the most problematic part of quenching process. The influence of metal quenching process on deformation is also an important reason. Hot oil quenching oil quenching than cold deformation is small, general control at about 90 ℃. Under the premise of ensuring hardness, oil medium should be adopted as far as possible. The cooling speed of oil medium is slow, while that of water medium is relatively fast.
It is an effective way to reduce the heat stress and tissue stress of some complicated parts when metal quenching is adopted. This quenching method not only reduces the quenching thermal stress, but also significantly reduces the tissue stress, so as to effectively reduce or prevent the quenching deformation of the stop parts.
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 graded quenching can only be applied to smaller parts.
(3) correctly master the heat treatment operation method
In order to reduce and control heat treatment deformation it is necessary to correctly master the heat treatment operation method. When the section is cooled uniformly, such as vertical quenching of slender shafts (taps and shafts), vertical gradual quenching and static immobility are also observed. When the cross section is not cooled uniformly, the thick part should be quenched first. For the sheet should enter laterally, and for the parts with holes and concave surfaces, the blind holes and concave surfaces should be quenched upward to facilitate the discharge of bubbles. In conclusion, some parts need to be protected before quenching in order to keep the cooling speed uniform everywhere.
Iii. Rescue methods of deformed parts
Although the deformation reduction measures are adopted, the deformation is still inevitable, so it is necessary to make up for it. The commonly used methods are cold straightening and hot straightening. If parts whose hardness is lower than 40HRC after quenching and tempering, they can be calibrated by a cold straightening press. If the metal is not cooled below the starting temperature of martensite transformation during quenching and cooling, the parts will have good plasticity, good effect of pressure straightening, and slow increase of force during straightening