How to temper after surface quenching

As we all know, surface quenching after the need for tempering, and ordinary quenching, induction heating surface quenching after use, in order to reduce quenching stress and reduce brittleness, and sometimes adjust the surface hardness to meet the design requirements of the hardness range. However, the local induction surface quenching or flame surface quenching of large forgings, parts large volume, large weight, without the overall heating of the large tempering furnace, sometimes the large forgings overall heating tempering is also afraid of deformation. Whether it is necessary to send large forgings into the furnace for integral tempering after surface quenching is a concern of heat treatment workers. Especially for large forgings, small local surface quenching range, small residual stress, little impact on the whole.

After surface quenching, no tempering can preserve more compressive stress and is conducive to fatigue resistance. However, for wear-resisting cen parts, low temperature tempering is required. For forgings that require surface quenching and grinding, tempering must be carried out to prevent grinding cracks.

After surface quenching can be carried out local tempering, self-tempering, or heating ceramic block far infrared local tempering.

(1) self-tempering

Self-tempering also known as "self-tempering", is the use of induction heating or flame heating to the heating surface of the heat, quenching is not cold to room temperature, the remaining heat to the quenching layer transfer to the quenching layer temperature rise to tempering. This heat from the depth of the quenching layer and around the layer. Although this heat maintains the quenching layer tempering time is very short, it will play a positive role in reducing residual stress and avoiding quenching cracking. This self-tempering process is simple and economical.

Self-tempering is mainly used to strictly control the coolant temperature, cooling time and injection pressure. The specific parameters should be determined by experiments. The self-tempering temperature is 50-70℃ higher than the tempering temperature in the furnace under the same surface hardness requirements.

(2) induction heating and tempering

Continuous movement of induction quenching shaft or other parts, sometimes using induction heating tempering is more convenient, (but high alloy steel roll, such as parts, is not suitable for induction heating tempering) this tempering method, can be immediately after quenching. Because the tempering temperature is lower than the magnetic transition temperature, the penetration depth of current is smaller. On the other hand, in order to reduce the residual tensile stress in the transition layer of surface quenching parts, the depth of the induction heating layer of tempering should be deeper than that of the quenching layer to meet the purpose of reducing the residual tensile stress. Therefore, the induction heating tempering should adopt very low frequency or very small specific power, extend the heating time, use the heat conduction to make the heat to the depth direction and deepen the heating area. When the surface is heated by fixed method, the heating layer can be thickened by intermittent heating.

Practice shows that the microstructure obtained by induction heating tempering has larger dispersion, higher wear resistance and better impact toughness.

The tempering temperature of induction heating is about 50℃ higher than that of furnace.

Flame surface heating is difficult to be used for tempering, because the flame temperature is difficult to change, and it is difficult to control the heating time, tempering uniformity is also poor, not suitable for use.

(3) far-infrared local heating and tempering

Adopted far infrared ceramic heater on the heating surface area, outside the heater with refractory fiber board again, put between the heater and forging surface thermocouple, can control the heating temperature for local tempering, due to the far infrared ceramic heating piece is assembled, large flexibility, according to the forging surface shape to assemble at any time, and the temperature control is directly, and can be used microcomputer control, the accuracy of the tempering is very high. This method can solve the problem that the forging surface cannot be heated and tempered as a whole after quenching, and it is more economical and convenient.

The above three are tempering methods, you can choose different tempering methods according to different circumstances, so that the surface of the forging after quenching tempering to achieve effective results.

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