Causes and preventive measures of heat treatment d

2022-07-31
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Causes and preventive measures for heat treatment deformation of precision molds

causes and preventive measures for heat treatment deformation of precision molds: the deformation status and causes of complex precision molds are studied to explore measures to reduce and control the deformation of complex precision molds, so as to improve the quality and service life of mold products

I. Influence of die material:

1. Material selection of die: considering the simplicity of material selection and heat treatment, a machinery factory selects T10A Steel to manufacture more complex dies with wide difference in cross-section size and small deformation after quenching, and the hardness is required to be HRC. After heat treatment, the die has a long service life at the same time, and the hardness meets the technical requirements, but the die is deformed greatly and cannot be used, resulting in the scrapping of the die. Later, the plant used micro deformed steel Cr12 Steel, and the hardness and deformation of the die after heat treatment met the requirements. Preventive measures: therefore, micro deformation steel, such as air quenching steel, shall be selected as far as possible for manufacturing precision and complex molds requiring small deformation

2. influence of mold material: a factory sent a batch of complex Cr12MoV steel molds with ¢ 60mm round holes. After the mold heat treatment, some of the round holes of the molds appeared ellipse, resulting in the scrapping of the molds. Generally speaking, Cr12MoV steel is a micro deformation steel and should not have large deformation. The metallographic analysis of the severely deformed die shows that the die steel contains a large amount of eutectic carbides, which are distributed in strips and blocks

(1) causes of die ellipse (deformation): This is due to the existence of uneven carbides distributed in a certain direction in the die steel. The expansion coefficient of carbides is about 30% smaller than the matrix structure of the steel. It prevents the expansion of the inner hole of the die during heating and the shrinkage of the inner hole of the die during cooling, resulting in uneven deformation of the inner hole of the die and ellipse of the round hole of the die

(2) preventive measures: ① when manufacturing precision and complex molds, try to select the mold steel with small carbide segregation instead of cheap, and select the steel with poor material produced by small steel plants. ② Die steels with severe carbide segregation should be properly forged to break carbide crystal blocks, reduce the grade of uneven carbide distribution and eliminate the anisotropy of properties. ③ The forged die steel should be subject to quenching and tempering heat treatment to obtain a fine and dispersed sorbite structure with uniform carbide distribution, so as to reduce the deformation of precision and complex dies after heat treatment. ④ For the dies with large size or unable to be forged, the solid solution double refinement treatment can be adopted to refine and evenly distribute the carbides and round the edges and corners, so as to reduce the heat treatment deformation of the dies

II. Influence of die structure design: some die materials and steel materials are very good. Often, the die structure design is unreasonable, such as thin edges, sharp corners, grooves, abrupt steps, and great thickness differences, resulting in large deformation of the die after heat treatment

1. Causes of deformation: due to uneven thickness or sharp rounded corners at all parts of the die, the thermal stress and structural stress at various parts of the die are different during quenching, resulting in different volume expansion at various parts, resulting in deformation of the die after quenching

2. Preventive measures: when designing the mold, under the condition of meeting the actual production needs, try to reduce the wide gap between the thickness of the mold and the asymmetric structure. At the junction of the thickness of the mold, try to adopt the structural design such as smooth transition. According to the deformation law of the die, the machining allowance shall be reserved to prevent the die from being scrapped due to the deformation of the die after quenching. 1. The impact energy shall be accurate. For the mold with particularly complex shape, the feeding structure can be adopted to ensure uniform cooling during quenching

III. Influence of mold manufacturing process and residual stress: it is often found in the factory that some molds with complex shape and high precision requirements have large deformation after heat treatment. At present, there are no specific provisions on the depth of tooth path in China. After careful investigation, it is found that the molds have not been subject to any pre heat treatment during machining and the last heat treatment

1. Cause of deformation: the superposition of residual stress during machining and stress after quenching increases the deformation of the die after heat treatment

2. Preventive measures: (1) stress relief annealing shall be conducted after rough machining and before semi finishing machining, i.e. () ℃ × () H furnace cooling to below 500 ℃ outlet air cooling, or 400 ℃ × (H) stress relief treatment. (2) Reduce quenching temperature and residual stress after quenching. (3) Quenching oil 170 C outlet air cooling (step quenching) is adopted. (4) Quenching residual stress can be reduced by isothermal quenching process. The above measures can reduce the residual stress and the deformation of the die after quenching

IV. influence of heating process of heat treatment:

1. Influence of heating speed: the deformation of the mold after heat treatment is generally considered to be caused by cooling, which is correct because the electric cored preheating device is not used in the extrusion unit. For moulds, especially for complex moulds, whether the processing technology is correct or not often has a great impact on the deformation of moulds. It can be clearly seen from the comparison of heating processes of some moulds that the heating speed is fast and often produces large deformation

(1) cause of deformation: any metal will expand when heated. As the steel is heated, the uneven temperature (i.e. uneven heating) of each part in the same mold will inevitably lead to the inconsistent expansion of each part in the mold, thus forming the uneven internal stress of heating. At the temperature below the transformation point of the steel, uneven heating mainly produces thermal stress, and uneven heating beyond the transformation temperature will also produce the isochronous transformation of the structure, which will produce the structure stress. Therefore, the faster the heating speed, the greater the temperature difference between the surface and the center of the die, the greater the stress, and the greater the deformation of the die after heat treatment

(2) preventive measures: the complex mold should be heated slowly when it is heated below the phase change point. Generally speaking, the deformation of the mold in vacuum heat treatment is much smaller than that in salt bath furnace. Preheating is adopted, and primary preheating (c) can be adopted for low alloy steel dies; Secondary preheating (C and C) shall be adopted for high alloy diamond mold

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