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Mold Heat Treatment Deformation: Basics & Preventive Measures

1. The influence of mold material

Material selection of mold

In consideration of material selection and heat treatment, T10A steel was selected by a machinery factory to manufacture complicated dies with a large differences in section size and small deformation after quenching, and the hardness requirement is 56-60HRC.

After heat treatment, the hardness of the die meets the technical requirements, but the deformation of the die is large and can not be used, resulting in the scrapping of the die.

After heat treatment, the hardness and deformation of the die meet the requirements.

Therefore, in order to manufacture precise and complex dies with small deformation, micro deformation steel, such as air quenching steel, should be selected as far as possible.

The influence of mold material

Generally speaking, Cr12MoV steel is a micro deformation steel, and should not have large deformation.

The metallographic analysis of the die with severe deformation shows that there are a lot of eutectic carbides in the die steel, which are distributed in strips and blocks.

(1) Causes of die ellipse (deformation)

This is due to the existence of non-uniform carbides distributed in a certain direction in the die steel, and the expansion coefficient of carbides is about 30% smaller than that of the matrix structure of the steel.

When heated, it prevents the expansion of the inner hole of the die, and when cooled, it prevents the shrinkage of the inner hole of the die, resulting in uneven deformation of the inner hole of the die and ellipse of the round hole of the die.

(2) Preventive measures

① In the manufacture of precision and complex mold, we should try to choose the mold steel with small carbide segregation, not cheap, and choose the steel with poor material produced by small steel plants.

② The die steel with serious carbide segregation should be forged reasonably to break the carbide crystal block, reduce the grade of uneven distribution of carbide and eliminate the anisotropy of properties.

③ The forged die steel should be quenched and tempered to obtain the sorbite structure with uniform distribution of carbides, fine and dispersion, so as to reduce the deformation of precision complex die after heat treatment.

④ For the die with larger size or unable to be forged, the solution double refining treatment can be used to refine and distribute carbides evenly, and round the edges and corners, so as to reduce the heat treatment deformation of the die.

2. Influence of mold structure design

Reasonable design

Mold is mainly designed according to the use requirements, and its structure sometimes can not be completely reasonable and symmetrical.

This requires designers to take some effective measures to pay attention to the manufacturability, the rationality of structure and the symmetry of geometric shape when designing the mold without affecting the performance of the mold.

(1) Try to avoid sharp corners and sections with different thickness

Sections, thin edges and sharp corners with great thickness difference should be avoided.

Smooth transition should be made at the junction of die thickness.

In this way, the temperature difference and thermal stress of the die section can be effectively reduced. At the same time, the different time of microstructure transformation on the section can be reduced and the microstructure stress can be reduced. The transition fillet and transition cone are used.

(2) Increase process hole appropriately

For some molds that can not guarantee the uniform and symmetrical cross section, it is necessary to change the non through hole into through hole or add some process holes appropriately on the premise of not affecting the service performance.

The die with narrow cavity will deform after quenching.

If two process holes can be added in the design, the temperature difference of the cross section during quenching will be reduced, the thermal stress will be reduced, and the deformation will be improved obviously.

Increasing process holes or changing non-uniform holes into through holes can reduce the cracking sensitivity due to uneven thickness.

(3) Close and symmetrical structure should be adopted as far as possible

When the shape of the die is open or asymmetric, the stress distribution is uneven after quenching, and it is easy to deform.

Therefore, for the general deformable groove die, the rib should be retained before quenching, and then cut off after quenching.

The groove workpiece is deformed at R after quenching, and can be effectively prevented from quenching deformation after stiffened.

(4) Combined structure is adopted

For the large die with complex shape and size > 400mm and the punch with small thickness and large length, it is better to adopt the combined structure to simplify the complex and reduce the large to small

Changing the inner surface of the die to the outer surface is not only convenient for hot and cold processing, but also can effectively reduce the deformation and cracking.

When designing a combined structure, it should be decomposed according to the following principles without affecting the fitting accuracy:

(1) Adjust the thickness to make the cross-section of the die with great difference basically uniform after decomposition.

(2) It is easy to decompose in the place where stress concentration occurs, so as to disperse the stress and prevent cracking.

(3) Matching with the process hole to make the structure symmetrical.

(4) It is convenient for cold and hot processing and assembly.

(5) The most important thing is to ensure usability.

If the integral structure of large die is adopted, not only the heat treatment is difficult, but also the shrinkage of the cavity after quenching is inconsistent, even the concave convex edge and plane distortion will be caused, and it is difficult to remedy in the future processing.

Therefore, the combined structure can be used.

After heat treatment, it can be assembled, ground and matched again, which not only simplifies the heat treatment, but also solves the problem of deformation.

3. Influence of die manufacturing process and residual stress

In factories, it is often found that some molds with complex shape and high precision have large deformation after heat treatment.

After careful investigation, it was found that no preheat treatment was carried out during machining and final heat treatment.

1. Causes of deformation

The superposition of the residual stress in the machining process and the stress after quenching increases the deformation of the die after heat treatment.

2. Preventive measures

(1) After rough machining and before semi finish machining, stress relief annealing should be carried out once, i.e. (630-680) ℃ ×( 3-4) H furnace cooling to 500 ℃ or 400 ℃ ×( 2-3) H.

(2) Reduce the quenching temperature, reduce the residual stress after quenching.

(3) The oil is quenched at 170 ℃ for air cooling (step quenching).

(4) The residual stress can be reduced by isothermal quenching.

The above measures can reduce the residual stress and deformation of the die after quenching.

4. Influence of heat treatment on heating process

1. Influence of heating rate

The deformation of die after heat treatment is generally considered to be caused by cooling, which is incorrect.

Mold, especially complex mold, the correct processing technology often has a greater impact on mold deformation.

By comparing the heating process of some molds, it is obvious that the heating speed is faster, which often leads to larger deformation.

(1) The cause of deformation any metal expands when heated

When the steel is heated, the non-uniform temperature of each part in the same mold (that is, the non-uniform heating) will inevitably cause the non-uniform expansion of each part in the mold, thus forming the internal stress of uneven heating.

Below the transformation point of steel, thermal stress is mainly produced by uneven heating

When the temperature is higher than the transformation temperature, the uneven heating will result in the uneven transformation of the microstructure, which will cause the structural stress.

Therefore, the faster the heating speed is, the greater the temperature difference between the surface and the core of the die is, the greater the stress is, and the greater the deformation of the die after heat treatment is.

(2) Preventive measures

The complex mold should be heated slowly below the phase transition point.

Generally speaking, the deformation of mold in vacuum heat treatment is much smaller than that in salt bath furnace.

For low alloy steel die, one preheating (550-620 ℃) can be used; For high alloy die, the second preheating (550-620 ℃ and 800-850 ℃) should be used.

2. Influence of heating temperature

In order to ensure the high hardness of the die, some manufacturers think it is necessary to increase the quenching temperature.

However, the production practice shows that this method is not appropriate. For complex dies, the normal heating temperature is also used for heating and quenching, and the heat treatment deformation after heating at the allowable upper limit temperature is much larger than that at the allowable lower limit temperature.

(1) Causes of deformation

As we all know, the higher the quenching temperature is, the larger the grain size of the steel is. Because the larger the grain size can increase the hardenability, the greater the stress produced during quenching and cooling.

Moreover, because most of the complex dies are made of medium and high alloy steel, if the quenching temperature is high, the residual austenite in the structure will increase due to the low MS point, which will increase the deformation of the die after heat treatment.

(2) Preventive measures

In the case of ensuring the technical conditions of the mold, the heating temperature should be selected reasonably, and the lower limit quenching heating temperature should be selected as far as possible, so as to reduce the stress during cooling and reduce the complex heat treatment deformation.

5. Effect of retained austenite

The deformation and cracking of heat treatment are closely related to the steel and its quality, so it should be based on the performance requirements of the mold.

Considering the precision, structure and size of the die, as well as the nature, quantity and processing method of the processing object, the reasonable selection is made.

If there is no deformation and accuracy requirement, carbon tool steel can be used to reduce the cost;

For the easily deformed and cracked parts, the alloy tool steel with higher strength and slower critical quenching cooling rate can be selected;

It can be seen that when the deformation of the die made of carbon steel can not meet the requirements, 9mn2v steel or CrWMn steel should be used instead.

Although the cost of material is a little high, the problems of deformation and cracking are solved

Overall, it’s still cost-effective.

At the same time, we should strengthen the inspection and management of raw materials to prevent the mold heat treatment cracking due to the defects of raw materials.

It is an important way to prevent quenching deformation and cracking to formulate reasonable technical conditions (including hardness requirements).

Local hardening or surface hardening can meet the use requirements, try not to overall quenching.

For the whole quenching die, the local requirements can be relaxed, try not to force consistency.

For the mold with high cost or complex structure, when the heat treatment is difficult to meet the technical requirements, the technical conditions should be changed, and the requirements that have little effect on the service life should be appropriately relaxed, so as to avoid scrapping due to repeated repair.

For the selected steel, the highest hardness can not be taken as the technical conditions specified in the design.

Because the highest hardness is often measured with a small sample with limited size, which is quite different from the hardness that can be achieved by a larger mold with actual size.

Because the pursuit of the highest hardness often needs to improve the quenching cooling rate, so as to increase the quenching deformation and cracking tendency, so using higher hardness as the technical condition, even the small size of the mold will bring some difficulties to the heat treatment operation.

In a word, the designer should formulate reasonable and feasible technical conditions according to the service performance and selected steel grades.

In addition, the hardness range of temper brittleness should be avoided when the hardness requirements for the selected steel grades are put forward.

1. Causes of deformation

Because there is a large amount of retained austenite in alloy steel (such as Cr12MoV steel) after quenching, various structures in the steel have different specific volume, and the specific volume of austenite is the smallest, which is the main reason for the volume reduction of high alloy steel die after quenching and low temperature tempering.

The specific volume of various structures of steel decreases in the following order: martensite – tempered sorbite – pearlite – austenite.

2. Preventive measures

(1) Reduce the quenching temperature properly.

As mentioned above, the higher the quenching temperature is, the larger the retained austenite mass is.

Therefore, choosing the appropriate quenching temperature is an important measure to reduce the mold shrinkage.

Generally, in the case of ensuring the technical requirements of the mold, the comprehensive performance of the mold should be considered, and the quenching heating temperature of the mold should be appropriately reduced.

(2) Some data show that the retained austenite content of Cr12MoV steel tempered at 500 ℃ is half less than that tempered at 200 ℃.

Therefore, under the premise of ensuring the technical requirements of the die, the tempering temperature should be appropriately increased.

The production practice shows that the deformation of Cr12MoV steel die tempered at 500 ℃ is the smallest, but the hardness decreases little (2 ~ 3HRC).

(3) Cold treatment after quenching is the best process to reduce the residual austenite mass, and also the best measure to reduce the deformation of the die and the size change during stable use

Therefore, cryogenic treatment should be used for precision and complex dies.

6. Influence of cooling medium and cooling method

The heat treatment deformation of die is often shown after quenching and cooling.

Although there are various factors above, the influence of cooling process can not be ignored.

1. Causes of deformation

When the die is cooled below MS point, phase transformation occurs in the steel

In addition to the thermal stress due to the cooling inconsistency, there is also the structural stress due to the non isochronous phase transformation.

The faster the cooling speed is, the more uneven the cooling is, the greater the stress is and the greater the deformation is.

2. Preventive measures

(1) On the premise of ensuring the hardness of the die, pre-cooling should be used as far as possible.

For carbon steel and low alloy die steel, it can be precooled until the corner is blackened (720 ~ 760 ℃).

For the steel with stable undercooled austenite in pearlite transformation zone, it can be precooled to about 700 ℃.

(2) It is an effective method to reduce the deformation of some complex dies by adopting the step cooling quenching method, which can significantly reduce the thermal stress and microstructure stress during the quenching process.

(3) For some precision and complex dies, the deformation can be reduced significantly by austempering.

7. Improving heat treatment process and reducing heat treatment deformation of die

No matter what method is adopted, the deformation of die after quenching is unavoidable

However, the following methods can be used to control the deformation of the precision complex mold.

(1) Reasonable selection of heating temperature

On the premise of ensuring the hardening, the lower quenching temperature should be chosen as far as possible.

However, for some high carbon alloy steel dies (such as CrWMn, cr12mo steel), the MS point can be reduced and the residual austenite can be increased by increasing the quenching temperature to control the quenching deformation.

In addition, the quenching temperature of high carbon steel die with large thickness can be increased to prevent quenching cracks.

For the die which is easy to deform and crack, stress relief annealing should be carried out before quenching.

(2) Reasonable heating

Uniform heating should be done as far as possible to reduce the thermal stress during heating.

For high alloy steel dies with large cross section, complex shape and high deformation requirements, preheating or limited heating speed is generally required.

(3) Correct selection of cooling mode and cooling medium

Pre cooling quenching, step quenching and step cooling should be selected as far as possible.

Pre cooling quenching has a good effect on reducing deformation of slender or thin die.

For the die with wide thickness difference, it can reduce the deformation to a certain extent.

For the mold with complex shape and great difference in cross section, it is better to adopt step quenching.

If the high speed steel is quenched at 580-620 ℃, the quenching deformation and cracking are basically avoided.

(4) Correctly grasp the quenching operation method

Select the right way to quench the workpiece into the medium to ensure the most uniform cooling of the mold, and enter the cooling medium along the direction of the minimum resistance, and move the slowest cooling side towards the liquid.

When the mold cools below MS, the movement should be stopped.

For example, for the mold with uneven thickness, the thick part should be quenched first;

For the workpiece with large section change, the heat treatment deformation can be reduced by adding process holes, reserving reinforcing ribs and plugging asbestos in the holes;

For the workpiece with concave and convex surface or through hole, the concave surface and hole should be quenched upward to discharge the bubbles in the through hole.

8. Conclusion

The deformation reason of precision complex mold is often complex, but as long as we master its deformation law, analyze its causes, and adopt different methods to prevent mold deformation, it can be reduced and controlled.

Generally speaking, the heat treatment deformation of precision complex mold can be prevented by the following methods.

(1) Reasonable selection of materials. For the precision complex die, the micro deformation die steel with good material (such as air quenched steel) should be selected. For the die steel with serious carbide segregation, the reasonable forging and quenching and tempering heat treatment should be carried out. For the larger die steel and the die steel that cannot be forged, the solid solution double refining heat treatment can be carried out.

2) The mold structure design should be reasonable, the thickness should not be too wide, the shape should be symmetrical. For the large deformation mold, the deformation law should be mastered, and the machining allowance should be reserved. For the large, precise and complex mold, the combined structure can be used.

(3) In order to eliminate the residual stress in the process of machining, heat treatment should be carried out in advance for the precision complex die.

(4) The heating temperature should be selected reasonably and the heating speed should be controlled. Slow heating, preheating and other balanced heating methods can be adopted to reduce the heat treatment deformation of precision and complex dies.

(5) On the premise of ensuring the hardness of the die, pre cooling, step cooling quenching or warm quenching process should be adopted as far as possible.

(6) As far as possible, vacuum heating quenching and cryogenic treatment after quenching should be used for precision and complex dies.

(7) For some precise and complicated dies, pre heat treatment, aging heat treatment and quenching and tempering nitriding heat treatment can be used to control the accuracy of the dies.

In addition, the correct operation of heat treatment process (such as hole plugging, hole binding, mechanical fixation, appropriate heating method, correct selection of cooling direction and movement direction in cooling medium, etc.) and reasonable tempering heat treatment process are also effective measures to reduce the deformation of precision complex mold.

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