1 . The influence of mold material
Mold material selection
A machine factory selected T10A steel to manufacture 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 matrix met the technical requirements, but the deformation was very significant and the matrix had to be discarded.
To make precise and complex dies with limited deformation, it is recommended to choose microdeformation steel such as air-hardened steel as much as possible.
The influence of mold material
Typically, Cr12MoV steel is considered a micro-deformation steel, which should have limited deformation.
Metallographic analysis of the matrix with excessive deformation showed a high amount of eutectic carbides in the matrix steel, which were present in strips and blocks.
(1) Causes of ellipse (deformation)
The presence of non-uniform carbides distributed in a specific direction in the matrix steel is the cause of severe deformation. The expansion coefficient of carbides is approximately 30% lower than that of the steel matrix structure.
During heating, 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. die to become elliptical.
(2) Preventive measures
① In complex and precision mold manufacturing, we should strive to select mold steel with minimal carbide segregation, even if it is not the cheapest option. We must avoid steel produced by small steel mills that have low quality materials.
② Matrix steel with significant carbide segregation must be forged properly to break the carbide crystal blocks and reduce the uneven distribution of carbides. This will also eliminate any anisotropy in the properties of the steel.
③ The forged die steel must be quenched and tempered to obtain a sorbite structure with uniform carbide distribution, 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 double refining treatment solution can be used to refine and evenly distribute the carbides. This will also round the edges and corners, reducing heat treatment deformation in the die.
two . 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 solve this, designers must take effective measures to ensure the manufacturability, rationality of structure and symmetry of the mold's geometric shape while maintaining the performance of the mold. This requires careful consideration during the design process.
(1) Try to avoid sharp corners and sections with different thicknesses
Designers should avoid sections, thin edges, and sharp corners with large thickness differences in mold design. Instead, smooth transitions should be implemented at the die thickness junction. This will effectively reduce temperature differences and thermal stress in the die section. Furthermore, the difference in microstructure transformation time and microstructure stress can be reduced by using transition fillets and cones.
(2) Enlarge the process bore appropriately
For molds that cannot guarantee a uniform and symmetrical cross-section, it may be necessary to modify the design by transforming non-through holes into through holes or adding additional process holes, as long as this does not affect the performance of the mold.
Molds with narrow cavities may become deformed after tempering. By adding two process holes during the design phase, the cross-sectional temperature difference during quenching can be reduced, resulting in less thermal stress and better deformations.
Increasing the number of process holes or converting non-uniform holes to through holes can also reduce the risk of cracking due to uneven thickness.
(3) A closed and symmetrical structure should be adopted as much 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 the ribs in general deformable slotted dies before quenching and then cut them after the process. This helps prevent R-deformation during quenching and improves the overall stability of the part.
(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 facilitate hot and cold processing and also reduce deformation and cracking.
When designing a combined structure, the following principles must be considered to ensure proper decomposition without affecting fit accuracy:
(1) Adjust the thickness to obtain a uniform cross section after decomposition.
(2) Decompose in areas where stress concentration occurs to disperse stress and prevent cracking.
(3) Match the frame with the process holes to make it symmetrical.
(4) Ensure convenience for cold and hot processing and assembly.
(5) Most importantly, ensure the usability of the framework.
Adopting an integral structure for large dies can make heat treatment difficult, leading to inconsistent cavity shrinkage after quenching. This can result in concave-convex edges, flat distortion, and difficulties in correcting these issues during future processing.
To address these challenges, the use of a combined structure is a suitable solution. After heat treatment, the structure can be assembled, ground and combined again. This not only simplifies the heat treatment process, but also effectively solves deformation problems.
3 . Influence of the matrix manufacturing process and residual stress
In factories, it is common to find that molds with complex shapes and high precision undergo significant deformation after heat treatment. Upon closer inspection, it is often found that the cause of this deformation is the lack of preheat treatment during 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 matrix after heat treatment.
2. Preventive measures
To reduce the residual stress and deformation of the matrix after quenching, the following measures can be taken:
(1) Conduct a stress-relieved annealing process once at a temperature of (630-680)°C for (3-4) hours with oven cooling to 500°C or 400°C for (2-3 ) hours, between rough machining and semi-finishing machining.
(2) Lower the quenching temperature to reduce the residual stress after quenching.
(3) Quench the matrix in oil at 170°C and let it cool in air (quenching in stages).
(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 the heating process
1. Influence of heating rate
The common belief that deformation of a die after heat treatment is caused by cooling is incorrect.
In reality, appropriate mold processing technology, especially complex molds, has a greater impact on mold 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 of any metal expands when heated
When steel is heated, the non-uniform temperature of each part in the same mold (i.e., uneven heating) will result in non-uniform expansion, leading to internal stresses caused by uneven heating.
Below the steel transformation point, thermal stress is mainly 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 rates 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 must be gradually heated below the phase transition temperature.
In general, mold distortion during vacuum heat treatment is significantly lower compared to a salt bath oven.
For low alloy steel dies, a 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 increasing the quenching temperature is crucial to ensuring high die hardness. However, actual production experience shows that this is not a suitable method.
For complex matrices, the normal heating temperature is employed for both heating and quenching. The heat treatment deformation that occurs after heating at the maximum allowable temperature is much greater compared to the minimum allowable temperature.
(1) Causes of deformation
As is widely known, increasing the quenching temperature leads to an increase in the grain size of steel. This is because a larger grain size increases hardenability, resulting in greater stress during quenching and cooling.
Furthermore, as most complex dies are made from 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 die deformation after heat treatment.
(2) Preventive measures
To meet the technical requirements of the mold, it is important to select a suitable 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 linked to the type of steel and its quality. Selection should be made based on mold performance requirements, taking into account the precision, structure and size of the die, as well as the nature, quantity and processing method of the material to be processed.
For parts without deformation and precision requirements, carbon tool steel can be used to reduce costs. For parts prone to warping and cracking, alloy tool steel with higher strength and 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, even though the material cost may be higher. This will solve warping and cracking problems, resulting in a long-term economical solution.
It is also important to tighten the inspection and management of raw materials to prevent cracking during heat treatment due to defects in raw materials.
Formulating reasonable technical specifications (including hardness requirements) is a crucial step in preventing warping and cracking during quenching. Local hardening or surface hardening can meet the usage requirements, and general hardening should be avoided whenever possible.
For entire tempering 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 the 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 considered the only technical specification in the design of the selected steel. This is because the highest hardness is often measured in a small sample size limited, which can differ significantly from the hardness that can be achieved in a larger full-size mold.
The quest for greater hardness often requires an increase in the cooling rate during quenching, which can result in increased deformation and cracking. Therefore, specifying higher hardnesses depending on the technical condition can pose challenges for heat treatment, even for small molds.
In conclusion, the designer must establish reasonable and feasible technical specifications based on the intended use and selected steel types. Additionally, the hardness range associated with temper brittleness should be avoided when determining hardness requirements for selected steel grades.
1. Causes of deformation
Alloy steels, such as Cr12MoV steel, generally have a significant amount of austenite retained after quenching. Different steel structures have varying specific volumes, with austenite having the lowest specific volume, which is the main cause of volume reduction in high-alloy steel matrices after low-temperature quenching and tempering.
The specific volume of various steel structures decreases in the following order: martensite, quenched sorbite, pearlite and austenite.
2. Preventive measures
(1) Properly reduce the quenching temperature
As mentioned previously, higher quenching temperatures result in a greater mass of retained austenite. Therefore, selecting the appropriate quenching temperature is crucial to reducing mold shrinkage. To meet the technical requirements of the mold, the overall performance of the mold must be considered and the quenching temperature must be reduced appropriately.
(2) Increase quenching temperature
The data shows that the retained austenite content of Cr12MoV steel tempered at 500°C is half that of steel quenched at 200°C. Therefore, the tempering temperature must be increased while still meeting the technical requirements of the die. In practice, the deformation of a Cr12MoV steel matrix 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 austenite residual mass and minimize deformation and size changes during stable use. Therefore, cryogenic treatment should be used for precision and complex matrices.
6 . Influence of cooling medium and cooling method
Deformation that occurs during heat treatment of dies is often visible after quenching and cooling. Although there are several factors that contribute to this, the impact of the cooling process cannot be neglected.
1. Causes of deformation
When the matrix is cooled below the MS point, phase transformation in the steel occurs. This leads not only to thermal stress caused by uneven cooling, but also to 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.
2. Preventive measures
(1) Use pre-cooling whenever possible
When ensuring the hardness of the die, pre-cooling should be used as much as possible. For carbon steel and low alloy steel, it can be pre-cooled until the corners are black (720-760°C). For steels with subcooled austenite stable in the pearlite transformation zone, precooling can be done up to about 700°C.
(2) Adopt step cooling quenching
The step cooling quenching method is an effective way to reduce deformation in some complex matrices by significantly reducing thermal stress and microstructure stress during the quenching process.
(3) Use Austempering
Austempering can significantly reduce deformation in some complex, precision dies.
7 . Improve heat treatment process and reduce die deformation by heat treatment
It is impossible to completely eliminate deformation in a die after quenching. However, the following methods can be used to control deformation in complex, precision molds:
(1) Select an appropriate heating temperature
When ensuring hardening, the lowest possible quenching temperature should be selected. However, for high-carbon alloy steel matrices (such as CrWMn and Cr12Mo steel), increasing the quenching temperature to reduce the MS point and increasing residual austenite can be used to control quenching deformation.
Furthermore, the quenching temperature of high-carbon steel dies with large thickness can be increased to avoid quenching cracks. For matrices prone to warping and cracking, stress relieving annealing must be performed before tempering.
(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 normally required.
(3) Appropriate cooling mode and cooling medium
Whenever possible, pre-cooling quenching, staged quenching and staged cooling should be selected. Pre-cooling quenching is effective in reducing deformation in thin or thin dies. It can also reduce deformation in dies with large thickness differences to a certain extent.
For molds with complex shapes and significant differences in cross-section, staged hardening is recommended. If high-speed steel is quenched at 580-620°C, quenching deformation and cracking can be avoided.
(4) Correctly carry out extinguishing operations
To ensure the most uniform cooling of the mold, the correct method of quenching the part in the middle must be selected. The workpiece should enter the cooling medium in the direction of minimum resistance, and the slower cooling side should be moved toward the liquid. As soon as the mold cools below the MS point, the movement must be stopped.
For example, in case of uneven thickness in the mold, the thickest part should be tempered first. To reduce heat treatment deformation on workpieces with large section changes, process holes, reinforcing ribs and asbestos fillings can be added to the holes.
For workpieces with concave and convex surfaces or through holes, the concave surface and hole must be tempered upward to release bubbles into the through hole.
8 . Conclusion
The cause of deformation in complex and precision 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 complex and precision molds:
(1) Selection of appropriate materials
For complex and precision dies, microstrain die steel with good material properties (such as air-hardened steel) should be selected. For steel matrices with significant carbide segregation, reasonable forging, quenching and tempering heat treatment should be carried out. For larger die steels or die steels 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 must be understood and machining tolerances must be reserved. For large, precise and complex molds, a combined structure can be used.
(3) Elimination of residual stresses during machining
To eliminate residual stresses during machining, heat treatment must be carried out in advance for precision and complex dies.
(4) Appropriate selection of heating temperature
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 hot quenching processes should be used as much as possible.
(6) Vacuum heating quenching and cryogenic treatment
Whenever possible, vacuum heat quenching and cryogenic treatment after quenching should be used for precision and complex dies.
(7) Heat pretreatment, aging heat treatment and nitriding heat treatment
For some precise and complicated dies, preheat 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 fastening, appropriate heating methods, correct selection of cooling direction and direction of movement in the cooling medium, etc.) and processing processes Reasonable quenching heat treatment are also effective measures to reduce the deformation of complex precision molds.
