Understanding Mold Heat Treatment Deformation: Basics and Preventive Measures

1. The influence of mold material

Material selection of mold

A machinery factory selected T10A steel to make complicated dies with large differences in section size and minimal deformation after quenching, with a hardness requirement of 56-60HRC.

However, after heat treatment, the hardness of the die met the technical requirements, but the deformation was too significant and the die had to be scrapped.

To make precise and complex dies with limited deformation, it is recommended to choose micro deformation steel such as air quenched steel as much as possible.

The influence of mold material

Typically, Cr12MoV steel is considered a micro deformation steel, which should have limited deformation.

The metallographic analysis of the die with excessive deformation showed a high amount of eutectic carbides in the die steel, which were present in stripes and blocks.

(1) Causes of die ellipse (deformation)

The presence of non-uniform carbides distributed in a specific direction in the die steel is the cause of the severe deformation. The expansion coefficient of the carbides is approximately 30% smaller than that of the matrix structure of the steel.

During heating, the carbides prevent the expansion of the inner hole of the die, and during cooling, they prevent the shrinkage of the inner hole of the die, leading to uneven deformation of the inner hole of the die and causing the round hole of the die to become elliptical.

(2) Preventive measures

① In the manufacture of precision and complex molds, we should strive to select mold steel with minimal carbide segregation, even if it’s not the cheapest option. We should avoid steel produced by small steel plants that has poor material quality.

② The die steel with significant carbide segregation should be forged properly to break apart carbide crystal blocks and reduce the uneven distribution of carbides. This will also eliminate any anisotropy in the steel’s properties.

③ The forged die steel should be quenched and tempered to achieve a sorbite structure with uniform distribution of carbides that is fine and dispersed. This will minimize deformation in the precision complex die after heat treatment.

④ For molds with larger sizes or those that cannot be forged, a solution double refining treatment can be used to evenly refine and distribute carbides. This will also round the edges and corners, reducing heat treatment deformation in the die.

2. Influence of mold structure design

Reasonable design

The design of a mold is primarily based on its intended use and as a result, its structure may not always be completely rational or symmetrical. To address this, designers must take effective measures to ensure the manufacturability, rationality of structure, and symmetry of the mold’s geometric shape while still maintaining the mold’s performance. This requires careful consideration during the design process.

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

Designers should avoid sections, thin edges, and sharp corners with large thickness differences in mold design. Instead, smooth transitions should be implemented at the junction of the die’s thickness. This will effectively reduce temperature differences and thermal stress in the die section. Additionally, the difference in time of microstructure transformation and microstructure stress can be reduced by using transition fillets and cones.

(2) Increase process hole appropriately

For molds that cannot guarantee a uniform and symmetrical cross-section, it may be necessary to modify the design by changing non-through holes into through holes or adding additional process holes, provided that this does not impact the mold’s performance.

Molds with narrow cavities can become deformed after quenching. By adding two process holes during the design phase, the temperature difference across the cross-section during quenching can be reduced, resulting in less thermal stress and improved deformations.

Enhancing the number of process holes or converting non-uniform holes into through holes can also reduce the risk of cracking 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 asymmetrical, the stress distribution is uneven after quenching, making it susceptible to deformation. To mitigate this, it is common to retain ribbing on general deformable groove dies before quenching and then cut it off after the process. This helps to prevent deformation at R during quenching and improves the overall stability of the workpiece.

(4) Combined structure is adopted

For large dies with complex shapes and sizes greater than 400mm, as well as punches with small thicknesses and large lengths, it is advisable to adopt a combined structure to simplify the complexity and reduce the size from large to small.

Reorienting the inner surface of the die to the outer surface can make both hot and cold processing easier, and also reduce deformation and cracking.

When designing a combined structure, the following principles should be considered to ensure proper decomposition without affecting the fitting accuracy:

(1) Adjust the thickness to achieve a uniform cross-section after decomposition.

(2) Decompose in areas where stress concentration occurs to disperse stress and prevent cracking.

(3) Match the structure with process holes to make it symmetrical.

(4) Ensure convenience for both cold and hot processing and assembly.

(5) Most importantly, ensure the usability of the structure.

Adopting an integral structure for large dies can make heat treatment difficult, leading to inconsistent shrinkage of the cavity after quenching. This can result in concave-convex edges, plane distortion, and difficulties in rectifying these issues during future processing.

To address these challenges, using a combined structure is a suitable solution. After heat treatment, the structure can be assembled, ground, and matched again. This not only simplifies the heat treatment process, but also effectively resolves deformation issues.

3. Influence of die manufacturing process and residual stress

In factories, it is common to find that molds with complex shapes and high precision experience significant deformation after heat treatment. Upon closer inspection, it is often discovered that the cause of this deformation is the lack of preheat treatment during both machining and the final heat treatment process.

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

To reduce the residual stress and deformation of the die after quenching, the following measures can be taken:

(1) Conduct a stress relief annealing process once, at a temperature of (630-680)°C for (3-4) hours with furnace cooling to 500°C or 400°C for (2-3) hours, between rough machining and semi-finish machining.

(2) Lower the quenching temperature to reduce residual stress after quenching.

(3) Quench the die in oil at 170°C and allow for air cooling (step quenching).

(4) Reduce residual stress through isothermal quenching.

By following these steps, the residual stress and deformation of the die after quenching can be minimized.

4. Influence of heat treatment on heating process

1. Influence of heating rate

The commonly held belief that the deformation of a die after heat treatment is caused by cooling is incorrect.

In reality, the proper processing technology for the mold, especially complex molds, has a greater impact on its deformation.

A comparison of the heating processes of some molds shows that faster heating speeds often result in greater deformation.

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

When steel is heated, the non-uniform temperature of each part in the same mold (that is, uneven heating) will result in non-uniform expansion, leading to internal stress caused by uneven heating.

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

When the temperature exceeds the transformation temperature, uneven heating leads to uneven microstructural transformation, which generates structural stress.

As a result, faster heating speeds increase the temperature difference between the surface and core of the die, leading to higher stress levels and greater deformation of the die after heat treatment.

(2) Preventive measures

The complex mold should be heated gradually below the phase transition temperature.

In general, the distortion of the mold during vacuum heat treatment is significantly less compared to that in a salt bath furnace.

For low alloy steel dies, one preheat cycle at a temperature range of 550-620°C is sufficient. For high alloy dies, a two-step preheat cycle at temperatures of 550-620°C and 800-850°C is recommended.

2. Influence of heating temperature

Some manufacturers believe that raising the quenching temperature is crucial in ensuring the high hardness of the die. However, actual production experience shows that this is not a suitable method.

For complex dies, the normal heating temperature is employed for both heating and quenching. The heat treatment deformation that occurs after heating at the maximum allowed temperature is much greater compared to that at the minimum allowed temperature.

(1) Causes of deformation

As is widely known, increasing the quenching temperature leads to an increase in the grain size of the steel. This is because a larger grain size enhances hardenability, resulting in greater stress during quenching and cooling.

Additionally, since most complex dies are made of medium to high alloy steel, a high quenching temperature will result in an increase in residual austenite in the structure due to a low Ms point. This will lead to an increase in the deformation of the die after heat treatment.

(2) Preventive measures

In order to meet the technical requirements of the mold, it is important to select an appropriate heating temperature. To minimize stress during cooling and reduce heat treatment deformation in complex molds, it is advisable to choose the lowest possible quenching temperature.

5. Effect of retained austenite

The degree of deformation and cracking during heat treatment is closely tied to the type of steel and its quality. The selection should be made based on the performance requirements of the mold, taking into account the precision, structure, and size of the die, as well as the nature, quantity, and processing method of the material being processed.

For parts without deformation and accuracy requirements, carbon tool steel can be utilized to reduce costs. For parts that are prone to deformation and cracking, alloy tool steel with higher strength and a slower critical cooling rate during quenching should be selected.

If the deformation of a die made of carbon steel does not meet the requirements, 9Mn2V steel or CrWMn steel should be used instead, even though the cost of the material may be higher. This will resolve the issues of deformation and cracking, resulting in a cost-effective solution in the long run.

It is also important to tighten the inspection and management of raw materials to prevent cracking during heat treatment due to defects in the raw materials.

Formulating reasonable technical specifications (including hardness requirements) is a crucial step in preventing deformation and cracking during quenching. Local hardening or surface hardening can meet the usage requirements, and overall quenching should be avoided whenever possible.

For whole quenching dies, local requirements can be relaxed, and there is no need to enforce uniformity. For molds with high cost or complex structure, if it is difficult to meet technical requirements during heat treatment, it is recommended to adjust the technical specifications and relax requirements that have little impact on the service life, in order to avoid scrapping caused by repeated repairs.

The highest achievable hardness should not be taken as the sole technical specification in the design of the selected steel. This is because the highest hardness is often measured on a small sample with limited size, which may differ significantly from the hardness that can be achieved on a larger mold of actual size.

The pursuit of the highest hardness often requires an increase in the cooling rate during quenching, which can result in increased deformation and cracking. Therefore, specifying higher hardness as the technical condition may pose challenges for heat treatment, even for small molds.

In conclusion, the designer should establish reasonable and feasible technical specifications based on the intended usage and selected steel grades. Moreover, the hardness range associated with temper brittleness should be avoided when determining hardness requirements for the selected steel grades.

1. Causes of deformation

Alloy steels, such as Cr12MoV steel, often have a significant amount of retained austenite after quenching. The different structures in the steel have varying specific volumes, with austenite having the smallest specific volume, which is the primary cause of volume reduction in high alloy steel dies after quenching and low-temperature tempering.

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

2. Preventive measures

(1) Properly Reduce the Quenching Temperature

As mentioned earlier, higher quenching temperatures result in a larger retained austenite mass. Therefore, selecting the appropriate quenching temperature is crucial in reducing mold shrinkage. To meet the technical requirements of the mold, the overall performance of the mold should be considered, and the quenching temperature should be appropriately reduced.

(2) Increase the Tempering Temperature

Data shows that the retained austenite content of Cr12MoV steel that is tempered at 500°C is half that of steel tempered at 200°C. Therefore, the tempering temperature should be increased, while still meeting the technical requirements of the die. In practice, the deformation of a Cr12MoV steel die that is tempered at 500°C is minimal, with only a slight decrease in hardness (2-3HRC).

(3) Use Cryogenic Treatment

Cryogenic treatment after quenching is an effective method to reduce the residual austenite mass and minimize deformation and size changes during stable use. Therefore, cryogenic treatment should be used for precision and complex dies.

6. Influence of cooling medium and cooling method

The deformation that occurs during heat treatment of dies is often visible after quenching and cooling. While there are various factors that contribute to this, the impact of the cooling process cannot be overlooked.

1. Causes of deformation

When the die is cooled below the MS point, phase transformation takes place in the steel. This leads to not only thermal stress caused by uneven cooling, but also structural stress due to non-uniform phase transformation. The faster the cooling speed and the more uneven the cooling, the greater the stress and deformation will be.

2. Preventive measures

(1) Use Pre-cooling Whenever Possible

While ensuring the hardness of the die, pre-cooling should be utilized as much as possible. For carbon steel and low alloy die steel, it can be pre-cooled until the corners turn black (720-760°C). For steels with stable undercooled austenite in the pearlite transformation zone, pre-cooling can be done to around 700°C.

(2) Adopt Step Cooling Quenching

The step cooling quenching method is an effective way to reduce deformation in some complex dies by significantly reducing thermal stress and microstructure stress during the quenching process.

(3) Use Austempering

Austempering can significantly reduce deformation in some precision and complex dies.

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

It is impossible to completely eliminate deformation in a die after quenching. However, the following methods can be used to control deformation in precision and complex molds:

(1) Select an Appropriate Heating Temperature

While ensuring hardening, the lowest possible quenching temperature should be selected. However, for high carbon alloy steel dies (such as CrWMn and Cr12Mo steel), increasing the quenching temperature to reduce the MS point and increase residual austenite can be used to control quenching deformation.

Additionally, the quenching temperature of high carbon steel dies with large thickness can be increased to prevent quenching cracks. For dies that are prone to deformation and cracking, stress relief annealing should be performed before quenching.

(2) Optimal Heating

Efforts should be made to achieve uniform heating to reduce thermal stress during heating. For high alloy steel dies with large cross-sections, complex shapes, and high deformation requirements, preheating or limited heating speed is typically necessary.

(3) Appropriate Cooling Mode and Cooling Medium

Whenever possible, pre-cooling quenching, step quenching, and step cooling should be selected. Pre-cooling quenching is effective in reducing deformation in slender or thin dies. It can also reduce deformation to some extent in dies with wide thickness differences.

For molds with complex shapes and significant differences in cross-section, step quenching is recommended. If high speed steel is quenched at 580-620°C, quenching deformation and cracking can be avoided.

(4) Properly Execute Quenching Operations

To ensure the most uniform cooling of the mold, the right method of quenching the workpiece into the medium should be selected. The workpiece should enter the cooling medium in the direction of minimum resistance and the slowest cooling side should be moved towards the liquid. Once the mold has cooled below the MS point, movement should be stopped.

For example, in the case of uneven thickness in the mold, the thicker part should be quenched first. To reduce heat treatment deformation in workpieces with large section changes, process holes, reinforcing ribs, and asbestos plugging in the holes can be added.

For workpieces with concave and convex surfaces or through holes, the concave surface and hole should be quenched upward to release bubbles in the through hole.

8. Conclusion

The cause of deformation in precision and complex molds is often complex, but by understanding its deformation laws, analyzing its causes, and adopting various methods to prevent deformation, it can be reduced and controlled.

In general, the following methods can be used to prevent heat treatment deformation in precision and complex molds:

(1) Selecting Appropriate Materials

For precision and complex dies, micro deformation die steel with good material properties (such as air-quenched steel) should be selected. For die steel with significant carbide segregation, reasonable forging and quenching and tempering heat treatment should be performed. For larger die steel or die steel that cannot be forged, solid solution double refining heat treatment can be used.

(2) Reasonable Mold Structure Design

The mold structure design should be reasonable, with symmetrical shape and not excessively wide thickness. For molds with significant deformation, the deformation laws should be understood and machining allowances should be reserved. For large, precise, and complex molds, a combined structure can be used.

(3) Eliminating Residual Stresses during Machining

To eliminate residual stresses during machining, heat treatment should be performed in advance for precision and complex dies.

(4) Appropriate Heating Temperature Selection

The heating temperature should be selected reasonably and the heating speed should be controlled. Slow heating, preheating, and other balanced heating methods can be used to reduce heat treatment deformation in precision and complex dies.

(5) Appropriate Cooling Process

On the condition of ensuring the hardness of the die, pre-cooling, step cooling quenching, or warm quenching processes should be used as much as possible.

(6) Vacuum Heating Quenching and Cryogenic Treatment

Where possible, vacuum heating quenching and cryogenic treatment after quenching should be used for precision and complex dies.

(7) Pre-Heat Treatment, Aging Heat Treatment, and Nitriding Heat Treatment

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 proper operation of heat treatment processes (such as hole plugging, hole binding, mechanical fixation, appropriate heating methods, correct selection of cooling direction and movement direction in cooling medium, etc.) and reasonable tempering heat treatment processes are also effective measures for reducing the deformation of precision complex molds.

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Shane
Author

Shane

Founder of MachineMFG

As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.

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