Heat treatment deformation cause and control method

In actual production, heat treatment deformation will bring difficulties to subsequent processing, affecting production efficiency, resulting in scrapping due to excessive deformation and increasing costs. 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.

 

Reasons of heat treatment deformation

The steel 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.

 

1.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.

 

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 so as 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 sections with a cross section.

 

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.