This paper studies deformation status, causes of precision and complex molds, discusses measures to reduce, control deformation of precision and complex molds to improve quality and service life of mold products.

 

From simple perspective of selection of materials and heat treatment, a mold factory chose T10A steel to manufacture more complex molds with widely differing cross-sectional dimensions and requiring less deformation after quenching, with a hardness of 56-60HRC. After heat treatment, hardness of mold meets technical requirements, but mold has a large deformation and cannot be used, causing mold to be scrapped. Later, mold factory used micro-deformed steel Cr12 steel, hardness and deformation of mold after heat treatment met requirements. Preventive measures: To produce molds that are sophisticated and complex, require less deformation, try to use micro-deformed steel, such as air-quenched steel.

A factory sent a batch of Cr12MoV steel to manufacture more complicated molds. Molds had round holes of Φ60mm. After mold was heat-treated, some of mold round holes appeared oval, causing mold to be scrapped. In general, Cr12MoV steel is a micro-deformed steel and should not show large deformation. We performed metallographic analysis on severely deformed molds, found that mold steel contained a large amount of eutectic carbides, was distributed in bands and blocks.

This is because of presence of uneven carbides distributed in a certain direction in mold steel. Expansion coefficient of carbides is about 30% smaller than that of steel's matrix structure. It prevents inner holes of mold from expanding when it is heated, and prevents inner holes of mold from shrinking when it is cooled, causing uneven deformation of inner hole of mold, which makes round hole of mold appear oval.

 

① When manufacturing precision and complex molds, try to choose mold steel with small carbide segregation, do not plan cheap, and choose steel with poor material produced by small steel mills.
② For die steel with severe segregation of carbides, reasonable forging should be performed to break carbide ingots, reduce level of uneven distribution of carbides, and eliminate anisotropy of performance.
③ Tempering heat treatment should be performed on forged mold steel to obtain uniform, fine, and dispersed sorbite structure in carbide distribution, thereby reducing deformation after heat treatment of precision and complex molds.
④ For molds with large size or cannot be forged, solid solution double thinning treatment can be used to make carbide fine, uniformly distributed, and rounded corners, which can reduce heat treatment deformation of mold.

Some mold materials and steel materials are very good, often because mold structure design is irrational, such as thin edges, sharp corners, grooves, abrupt steps, wide thickness, etc., resulting in large deformation after mold heat treatment.

Because thickness of mold is uneven or there are sharp rounded corners, thermal stress and tissue stress of mold are different during quenching, which causes volume expansion of each part to be different, and mold is deformed after quenching.

When designing mold, in order to meet actual production needs, thickness different of mold should be minimized, structure should be asymmetry. At thick and thin junction of mold, structural design such as smooth transition should be adopted as much as possible. Machining allowance is reserved according to deformation rule of mold, so that mold will not be scrapped due to mold deformation after quenching. For molds with particularly complicated shapes, in order to achieve uniform cooling during quenching, a combined structure can be used.

It is often found in factories that some molds with complex shapes and high precision require deform large after heat treatment. After careful investigation, it is found that molds have not undergone any pre-heat treatment during mechanical processing and final heat treatment stages.

Superimposed residual stress during machining and stress after quenching increase deformation of mold after heat treatment.

(1) After roughing and before semi-finishing, a stress relief annealing should be performed, that is, (630-680)℃ * (3-4)h furnace cooling to 500℃ or less and air cooling, 400℃ * (2- 3)h destressing treatment can also be used.
(2) Reduce quenching temperature and reduce residual stress after quenching.
(3) Air cooling (classified quenching) using 170oC oil.
(4) Use of isothermal quenching process can reduce residual quenching stress.
Above measures can reduce residual stress of mold after quenching, and mold deformation is small.

 

Deformation of mold after heat treatment is generally considered to be caused by cooling, which is incorrect. For molds, especially complex molds, correctness of machining process often has a greater impact on deformation of mold. It can be clearly seen from the comparison of some mold heating processes that heating rate is fast and often produces large deformation.

Any metal must expand when heated. Because when steel is heated, temperature unevenness of each part in same mold (that is uneven heating) will inevitably cause expansion inconsistency of each part in mold, thereby forming internal stress due to uneven heating. In temperature below transformation point of steel, non-uniform heating mainly generates thermal stress. If heating temperature is not uniform, non-uniformity of structural transformation will also occur, which will cause structural stress. Therefore, the faster heating speed, the greater temperature difference between surface of mold and center, the greater stress, and the greater deformation caused by heat treatment of mold.

When heating complex molds below transformation point, they should be heated slowly. Generally speaking, deformation of vacuum heat treatment of mold is much smaller than heating and quenching of salt bath furnace. With preheating, primary preheating (550-620oC) can be used for low alloy steel molds; secondary preheating (550-620oC and 800-850oC) should be used for high alloy steel molds.