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.
