What is the effect of heat treatment process on mold deformation?

1. The influence of heating speed

Generally speaking, when quenching and heating, the faster the heating rate, the greater the thermal stress produced in the mold, which is easy to cause the deformation and cracking of the mold, especially for alloy steel and high alloy steel, because of its poor thermal conductivity, it is necessary to pay attention to preheating, and for some high alloy molds with complex shapes, it is also necessary to take several grading preheating.

However, in some cases, rapid heating can sometimes reduce deformation. At this time, only the surface of the mold is heated, and the center also remains “cold”, so the organizational stress and thermal stress are correspondingly reduced, and the core deformation resistance is large, thus reducing the quenching deformation. According to some factory experience, it has certain effect in solving the hole distance deformation.

Aluminum alloy mold

2. Influence of heating temperature

The quenching temperature affects the hardenability of the material, as well as the composition and grain size of austenite.

1) From the perspective of hardenability, the high heating temperature will increase the thermal stress, but at the same time increase the hardenability, so the organizational stress also increases, and gradually dominates. For example, carbon tool steels T8, T10, T12, etc., tend to shrink in the general quenching temperature, but if the quenching temperature is increased to ≥850℃, due to the increase of hardenability, the structure stress gradually dominates, so the inner diameter may show a tendency to expand.

2) From the austenite composition, the quenching temperature increases the carbon content of austenite, and the square degree of martensite after quenching increases (the specific volume increases), so that the volume increases after quenching.

3) From the perspective of the effect on MS point, high quenching temperature will lead to coarse austenite grain, which will increase the tendency of deformation and cracking of parts.

In summary, for all kinds of steel, especially some high carbon medium and high alloy steel, the quenching temperature will obviously affect the quenching deformation of the die, so the correct choice of quenching heating temperature is very important.

Generally speaking, choosing too high a quenching heating temperature is not good for deformation. Under the premise of not affecting the performance of the service, always adopt a lower heating temperature. However, for some steel grades with more residual austenite after quenching (such as Cr12MoV, etc.), the residual austenite volume can also be changed by adjusting the heating temperature to adjust the deformation of the die.

3. Influence of quenching cooling rate

In general, when the cooling rate increases above the MS point, the thermal stress increases significantly, and the deformation caused by the thermal stress tends to increase. When the cooling rate increases below MS point, the deformation caused by microstructure stress tends to increase.

For different kinds of steel, due to the different heights of MS points, so when using the same quenching medium, there are different deformation trends. If the same kind of steel uses a different quenching medium, because its cooling capacity is different, it also has a different deformation tendency.

For example, carbon tool steels have a low MS point, so when water cooling is used, the influence of thermal stress tends to prevail. By cold, it may be that the stress of the organization prevails.

In actual production, the mold is often used in the grading or grading isothermal quenching, which is usually not completely quenched, so the thermal stress is usually the main effect, so that the cavity tends to shrink, but because the thermal stress is not very large at this time, so the total deformation is relatively small. If water-oil quenching or oil quenching is used, the thermal stress will be larger and the cavity shrinkage will increase.

4. Influence of tempering temperature

The effect of tempering temperature on deformation is mainly caused by the microstructure transformation during tempering. If the phenomenon of “secondary quenching” occurs in the tempering process, the residual austenite will be transformed into martensite, because the specific volume of the generated martensite is larger than that of the residual austenite, it will cause the enlargement of the mold cavity. For some high alloy tool steels such as Cr12MoV, when the red hardness is the main requirement and the use of high temperature quenching, repeated tempering, each time the fire, the volume will be enlarged once.

If tempered in other temperature areas, due to the quenched martensite to temper martensite (or tempered sorbsite, tempered trositic, etc.) transformation, specific volume decreases, so the cavity tends to shrink.

In addition, the relaxation of the residual stress in the tempering mold also has an impact on the deformation. After the quenching of the mold, if the surface is in a state of tensile stress, the size will increase after the tempering. On the contrary, if the surface is in a state of compressive stress, it will shrink.

However, the former is the most important of the two effects of tissue transformation and stress relaxation.

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