Mold heat treatment deformation is one of main defects of moulding manufacturing process. For some precision mold, which are often scrapped due to heat treatment deformation, controlling deformation of precision mold has always been a key issue in heat treatment production.
It is well known that during heat treatment, especially in quenching process, due to temperature difference caused by inconsistency of heating and cooling rates of various sections of mold section, combined with unequality of structural transformation, volume of each section of mold is not uniform and tissue transformation is uneven, which causes "organizational stress" and thermal stress caused by temperature difference between inside and outside of mold. When internal stress exceeds yield limit of mold, it causes mold deformation.
Therefore, reducing and controlling deformation of precision molding is an important research topic for heat treatment workers.
In this paper, we will study deformation causes of precision and complex mold, and explore measures to reduce and control deformation of precision mold to improve quality and service life of mold products.

 

From a simple consideration of material selection and heat treatment, a mechanical factory chooses T10A steel to manufacture a more complex mold with a large difference in cross-sectional dimensions and requires less deformation after quenching. Hardness requires 56HRC-60HRC. After heat treatment, hardness of mold meets technical requirements, but mold deformation is large and cannot be used, resulting in scrapping of mold. Later, this plant adopted micro-deformed steel Cr12 steel, hardness and deformation of mold after heat treatment can meet requirements.
Therefore, to manufacture precision and complex mold requiring less deformation, it is necessary to use micro-deformed steel, such as air-quenched steel.

A factory sent a batch of Cr12MoV steel for more complex mold, molds have ¢60mm round holes, after mold heat treatment, some mold round holes appear elliptical, resulting in mold scrap.
In general, Cr12MoV steel is a micro-deformed steel and should not exist large deformation. We have carried out metallographic analysis of severely deformed molds and found that mold steel contains a large amount of eutectic carbides and is distributed in a strip shape or a block shape.

This is because of presence of uneven carbides distributed in a certain direction in die steel. Expansion coefficient of carbide is about 30% smaller than that of steel. When heated, it prevents inner hole of mold from expanding. When cooling, it prevents inner hole of mold from shrinking, causing uneven deformation of inner hole of mold, and oval of mold is elliptical.

1 When manufacturing precision molding, try to choose mold steel with less segregation of carbides. Do not use cheap steel.
2 For die steel with serious segregation of carbides, reasonable forging should be carried out to break carbide ingots, reduce grade of uneven distribution of carbides, and eliminate anisotropy of properties.
3 Forged die steel shall be subjected to quenching and tempering heat treatment to obtain a uniform, fine and dispersed carbide structure of carbide, thereby reducing deformation of precision complex mold after heat treatment.
4 For molds with large size or forging, solid solution double refining treatment can be used to make carbides refine and evenly distributed, edges and corners are rounded, which can reduce heat treatment deformation of mold.

Some moulding materials and steel materials are very good, often because of unreasonable design of mold structure, such as thin edges, sharp corners, grooves, abrupt steps, thickness and other disparities, resulting in large mold deformation after heat treatment.

Due to uneven thickness or sharp rounded corners of mold, different thermal stress and tissue stress between various parts of mold are caused during quenching, resulting in different volume expansion of each part, which causes mold deformation after quenching.

When designing mold, in order to meet actual production needs, mold thickness and asymmetrical structure should be minimized, structural design of smooth transition should be adopted as much as possible at thick and thin junction of mold. According to deformation law of mold, machining allowance is reserved, and after quenching, mold is not scrapped due to mold deformation.
For a mold having a particularly complicated shape, in order to make cooling uniform during quenching, a joint structure can be employed.

 

It is often found in factory that some molds with complex shapes and high precision require large deformation after heat treatment. After careful investigation, it was found that mold was not subjected to any pre-heat treatment in mechanical processing and final heat treatment.

Residual stress during machining process and stress after quenching are superimposed, which increases mold deformation after heat treatment.

(1) After roughing and semi-finishing, a stress relief annealing shall be carried out, that is, (630-680)℃ × (3-4) h furnace cooling to 500℃ or less air cooling, 400℃ × (2- 3) h stress relief treatment.
(2) Reduce quenching temperature and reduce residual stress after quenching.
(3) Using quenching oil 170℃ oil cooling (classified quenching).
(4) Quenching residual stress can be reduced by austempering process.
By adopting above measures, residual stress after quenching of mold is reduced, and mold deformation is small.

Mold deformation after heat treatment is generally considered to be caused by cooling, which is not true. Molds, especially complex mold, have a great influence on mold deformation. Comparison of some mold heating processes shows that heating speed is faster and tends to produce larger deformation.

Any metal should be expanded when heated. Because temperature of parts in same mold is uneven when steel is heated, it will inevitably cause inconsistency of expansion of various parts in mold, thereby forming an internal stress due to uneven heating. At temperatures below phase transition point of steel, uneven heating mainly produces thermal stress, which is unevenly heated beyond phase transition temperature, and also causes unequality of structural transformation, which causes both tissue stress. Therefore, the faster heating rate, the greater difference in temperature between surface of mold and core; the greater stress, and the greater deformation caused by heat treatment of mold.

When complex mold is heated below phase change point, it should be slowly heated. Generally speaking, mold vacuum heat treatment deformation is much smaller than salt bath furnace heating and quenching. For preheating, one-time preheating (550℃ - 620℃) for low alloy steel mold; secondary preheating (550℃ - 620℃ and 800℃ - 850℃) for high alloy steel mold.

In order to ensure that mold reaches a higher hardness, some manufacturers believe that it is necessary to increase quenching heating temperature. However, production practices have shown that this is not appropriate. For complex mold, heating is quenched at normal heating temperature. Heat treatment deformation after heating at upper limit temperature is much greater than heat treatment at lower limit temperature.

It is well known that the higher quenching heating temperature, the larger grain size of steel, and the larger grain size can increase hardenability, the greater stress generated during quenching and cooling. Furthermore, since complex mold is mostly made of medium-high alloy steel, if quenching temperature is high, amount of retained austenite in structure increases due to low Ms point, mold deformation after heat treatment is increased.

In the case of ensuring technical conditions of mold, heating temperature is reasonably selected, and the lower limit quenching heating temperature is selected as much as possible to reduce stress during cooling, thereby reducing complicated heat treatment deformation.

 

Some high-alloy die steels, such as Cr12MoV steel mold, length, width and height of mold of after quenching and low-temperature tempering. This is caused by excessive amount of retained austenite after quenching of mold.

Because alloy steel (such as Cr12MoV steel) contains a large amount of retained austenite after quenching, various microstructures in steel have different specific volumes, and austenite has the smallest specific volume. This is main reason for shrinkage of high alloy steel mold after quenching and low temperature tempering. Specific volume of various microstructures of steel decreases in following order: martensite - tempered sorbite - pearlite - austenite.

(1) Properly reduce quenching temperature. As quenching heating temperature described above is higher, amount of retained austenite is larger, so selecting an appropriate quenching heating temperature is an important measure to reduce shrinkage of mold. Generally, in the case of ensuring technical requirements of mold, it is necessary to consider comprehensive performance of mold and appropriately reduce quenching heating temperature of mold.
(2) Some data show that after quenching of Cr12MoV steel mold, tempering at 500℃ is less than half of retained austenite content at 200℃ tempering. Therefore, under premise of ensuring technical requirements of mold, tempering temperature should be appropriately increased. Production practice shows that deformation of mold of Cr12MoV steel mold tempering at 500℃ is the smallest, but hardness is not much reduced (2HRC~3HRC).
(3) Cold treatment after quenching of mold is the best process to reduce amount of retained austenite. It is also the best measure to reduce dimensional change of mold for stable use. Therefore, complex and precision mold should generally be cryogenically treated.

Heat treatment deformation of mold is often manifested after quenching and cooling. Although there are various factors, influence in cooling process cannot be ignored.

When mold cools below Ms point, steel undergoes a phase change. In addition to thermal stress that is caused by inconsistent cooling, there is also a structural stress due to unequality of phase transition. The faster cooling rate, the more uneven cooling, the greater stress generated, the greater mold deformation.

(1) Under premise of ensuring hardness requirements of mold, pre-cooling should be used as much as possible. For carbon steel and low-alloy mold steel, it can be pre-cooled to black at corners (720℃~760℃). For steels with relatively stable austenite in pearlite transformation zone, it can be pre-cooled to about 700℃.
(2) Use of graded cooling quenching can significantly reduce thermal stress and microstructure stress generated during quenching of mold, and is an effective method to reduce deformation of some complex mold.
(3) For some precision mold, austempering can significantly reduce deformation.

Mold deformation after quenching, no matter what method is adopted, deformation is unavoidable, but for precision molding with strict control of deformation amount, following methods can be used for control.

For complex and precision mold with low basic hardness requirements and high surface hardness requirements, tempering heat treatment can be carried out after roughing of mold, and low temperature nitriding treatment (500℃ ~ 550℃) after finishing. Due to low mold nitriding temperature, there is no phase change of matrix structure, furnace is cooled to room temperature, cooling stress is also small, and mold deformation is small.

For complex and precision mold, if hardness requirements are not too high, pre-heat treated pre-hardened steel can be used to pre-heat mold steel (such as 3Cr2Mo, 3CrMnNiMo steel) to reach hardness at the time of use (lower hardness is 25HRC~35HRC, higher hardness is 40HRC~50HRC), and then mold is processed and no longer heat treated, thus ensuring precision of precision mold.

For complex precision mold, age hardening steel can be used. For example, PMS (1Ni3Mn2CuA1.Mo) steel is a new type of ageing die steel. After 870℃ solid solution quenching, hardness is about 30HRC, which is convenient for machining. After mold is formed and and then subjected to aging heat treatment at about 500℃, higher hardness of 40HRC~45HRC can be obtained, mold deformation is small, and only polishing treatment is needed, which is an ideal steel for complex and precision mold.

 

Causes of deformation of precision molding are often complicated, but as long as we master deformation law and analyze causes, use of different methods to prevent mold deformation can be reduced and controlled. In general, heat treatment deformation of precision molding can be prevented by following method.
(1) Reasonable material selection. For complex precision mold, material should be made of micro-deformed die steel (such as air-quenched steel). Die steel with serious carbide segregation should be reasonably forged and heat-treated. For larger and unforgeable die steel, solid solution double refining heat treatment can be carried out.
(2) Design of mold structure should be reasonable, thickness should not be too disparate, shape should be symmetrical, deformation law should be mastered for larger mold deformation, and machining allowance should be reserved. For large and precision mold, a combined structure can be adopted.
(3) Precise heat treatment of precision and complex mold to eliminate residual stress generated during machining.
(4) Reasonably select heating temperature and control heating speed. For precision molding, slow heating, preheating and other balanced heating methods can be adopted to reduce heat treatment deformation of mold.
(5) Under premise of ensuring hardness of mold, try to use pre-cooling, staged cooling quenching or warm quenching process.
(6) For complex and precision mold, vacuum hardening and quenching after cryogenic treatment should be used as far as possible.
(7) For some precision molding, pre-heat, aging heat, quenching and tempering heat treatment can be used to control precision of mold.
In addition, correct heat treatment process operations (such as plugging, tying holes, mechanical fixing, proper heating methods, correct selection of cooling direction of mold and direction of movement in cooling medium) and reasonable tempering heat treatment process are also effective measures to reduce deformation of precision molding.