1. Material selection of dies: Considering the simplicity of material selection and heat treatment, a machinery factory selected T 10A steel to manufacture complex dies. The cross-sectional dimensions of the dies are different, and the deformation after quenching is small, and the required hardness is 56-60HRC. After heat treatment, the hardness of the die meets the technical requirements, but the die is deformed greatly and cannot be used, resulting in the die being scrapped. Later, the factory adopted micro-deformation steel Cr 12, and the hardness and deformation of the die after heat treatment met the requirements. Precautions: Therefore, micro-deformation steel, such as air-quenched steel, should be selected as far as possible for molds with complex manufacturing accuracy and small deformation.
2. Impact of mold material: A factory has issued a batch of complicated molds made of Cr 12MoV steel, all of which are round holes < 60mm. After heat treatment, some round holes of the mold appear ellipse, which leads to the scrap of the mold. Generally speaking, Cr 12MoV steel is a micro-deformation steel, and there should be no big deformation. Metallographic analysis of severely deformed die shows that there are a large number of * * crystalline carbides in die steel, which are distributed in strips and blocks.
(1) The reason for the ellipse (deformation) of the die is that there are carbides in the die steel which are unevenly distributed in a certain direction. The expansion coefficient of carbide is about 30% smaller than that of steel matrix structure. When heating, the inner hole of the mold is prevented from expanding, and when cooling, the inner hole of the mold is prevented from contracting, resulting in uneven deformation of the inner hole of the mold and elliptical round hole of the mold.
(2) Precautionary measures: ① When manufacturing precision and complicated dies, die steel with less carbide segregation should be selected as far as possible, and steel with poor material produced by small steel mills should be selected. ② Die steel with severe carbide segregation should be forged reasonably to break carbide grains, reduce uneven distribution of carbide and eliminate anisotropy of performance. ③ Forging die steel should be quenched and tempered to obtain sorbite structure with uniform, fine and dispersed carbide distribution, so as to reduce the deformation of precision and complex die after heat treatment. (4) For large or unforgeable dies, double refining treatment of solid and solution can be adopted to make the carbide distribution uniform and smooth, so as to reduce the heat treatment deformation of the die.
Second, the impact of mold structure design: Some molds are very good in material selection and steel materials, often because the mold structure design is unreasonable, such as thin edges, sharp corners, grooves, abrupt steps, and uneven thickness. , resulting in large deformation of the mold after heat treatment.
1. Causes of deformation: Due to uneven thickness or sharp rounded corners in various parts of the die, the thermal stress and organizational stress between various parts of the die are different during quenching, resulting in different volume expansions of various parts after quenching and deformation of the die.
2. Preventive measures: When designing the mold, under the condition of meeting the actual production needs, try to reduce the thickness disparity and structural asymmetry of the mold, and try to adopt structural design such as smooth transition at the junction of mold thickness. According to the deformation law of the die, the machining allowance is reserved, so that the die will not be scrapped because of the deformation after quenching. For molds with particularly complicated shapes, in order to make them cool evenly during quenching, adhesive structure can be adopted.
Third, the mold manufacturing process and the influence of residual stress: It is often found in factories that some molds with complex shapes and high precision are deformed greatly after heat treatment. After careful investigation, it was found that the mold did not undergo any preheating treatment during processing and final heat treatment.
1. Deformation reason: The superposition of residual stress during machining and stress after quenching increases the deformation of the die after heat treatment.
2. Preventive measures: (1) After rough machining and before semi-finishing, stress relief annealing shall be conducted once, that is, the furnace shall be cooled to below 500℃ within (630-680) × (3-4) h, and then discharged for air cooling, or 400℃×2-3)h for stress relief treatment. (2) Reduce the quenching temperature and the residual stress after quenching. (3) adopt quenching oil 170? Oil cooling (step quenching). (4) Adopting isothermal quenching process can reduce quenching residual stress. By taking the above measures, the residual stress of the die after quenching can be reduced and the deformation of the die is small.
Fourth, the influence of heat treatment heating process:
1, the influence of heating speed: the deformation of the mold after heat treatment is generally considered to be caused by cooling, which is incorrect. Mold, especially complex mold, the correctness of processing technology often has a great influence on the deformation of mold. It can be clearly seen from the comparison of heating processes of some molds that faster heating speed often leads to greater deformation.
(1) Deformation reason: Any metal will expand when heated. When steel is heated, the temperature of each part in the same mold is uneven (that is, uneven heating), which will inevitably lead to inconsistent expansion of each part in the mold, thus forming internal stress due to uneven heating. At the temperature below the transformation point of steel, uneven heating mainly produces thermal stress, and when it exceeds the transformation temperature, uneven heating will also produce non-isochronous microstructure transformation, which will not only produce structural stress. Therefore, the faster the heating speed, the greater the temperature difference between the mold surface and the core, and the greater the stress and deformation of the mold after heat treatment.
(2) Precautions: When the complex mold is heated below the transformation point, it should be heated slowly. Generally speaking, the deformation of mold in vacuum heat treatment is much smaller than that in salt bath furnace. ? Adopt preheating, one-time preheating (550-620? c)； Secondary preheating (550-620? C and 800-850? C) carry out the test.