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These three factors are the key points of die deformation

CLICK:285TIME:2018-11-27

Now, in die manufacturing, new technologies such as EDM, forming grinding and WEDM have been applied to better deal with the processing and heat treatment deformation of disordered dies. However, these new processes have not been widely used because of the constraints of various conditions. Therefore, how to reduce the heat treatment deformation of the die is still a very important issue.


Generally, the die requires high precision, and after heat treatment, it is not easy or even impossible to process and proofread. Therefore, even after heat treatment, even if the arrangement performance has reached the requirements, but if the deformation is too bad, it will still be abandoned because it can not be saved, which not only affects the production, but also constitutes an economic loss.


There is no comment on the general rules of heat treatment deformation. The following is a brief analysis of some factors affecting die deformation.


Effect of Die Data on Heat Treatment Deformation


The influence of data on heat treatment deformation includes chemical composition of steel and original arrangement.


From the data itself, the heat treatment deformation is mainly affected by the influence of composition on hardenability and Ms point.


When carbon steel is quenched by water-oil dual-liquid at normal quenching temperature, great thermal stress occurs above MS point; when it is cooled below MS point, austenite changes to martensite and arranges stress, but because of poor hardenability of carbon steel, the value of arranging stress is small. With its low Ms point, the plasticity of steel is poor and plastic deformation is not easy to occur when martensite arrangement is changed. Therefore, the deformation characteristics formed by thermal stress effect are retained and the die cavity tends to be shortened. However, if the quenching temperature advances (>850 C), the main effect may be due to the arrangement of stress, and the cavity tends to expand.


When making dies with low alloy East-West steels such as 9Mn2V, 9SiCr, CrWMn, GCr15, the quenching rules are similar to those of carbon East-West steels, but the amount of deformation is smaller than that of carbon East-West steels.


As for some high alloy steels, such as Cr12MoV steel, because of its high content of carbon and alloying elements and low Ms point, there are more retained austenite after quenching, which can counteract the volume expansion caused by martensite. Therefore, the deformation after quenching is appropriately small. Generally, when quenched by air cooling, air cooling and nitrate bath, The die cavity tends to expand slightly; if the quenching temperature is too high, the residual austenite content will be added, and the die cavity may also shrink.


If the die is made of carbon structural steel (such as 45 steel) or some alloy structural steel (such as 40Cr), because of its high Ms point, when the martensite at the beginning of the appearance is changed, the core temperature is still higher, the yield strength is lower, and there is certain plasticity. The instantaneous tensile stress on the center of the exterior is arranged so that the cavity tends to exceed the yield strength of the center. To swell.


The original arrangement of steel also has certain influence on quenching deformation. The original arrangement of steel referred to here includes the grade of dopants, the grade of band arrangement, the segregation degree of composition, the direction of free carbide distribution, and the different arrangement obtained by different pre-heat treatment (such as pearlite, tempered sorbite, tempered refractive body, etc.). For die steel, the primary consideration is carbide segregation, carbide shape and dispersion shape.


The effect of carbide segregation on quenching deformation of high carbon and high alloy steels (such as Cr12 steel) is particularly obvious. Because carbide segregation makes up of the uneven composition of steel heated to austenite, the MS points in different regions will be high or low. Under the same cooling conditions, austenite changes to martensite first and then, the specific volume of the modified martensite varies with the carbon content, and even some low-carbon and low-alloy areas may not get martensite at the base (but bainite, troostite, etc.), all of which will constitute the non-uniform change of parts after quenching. Shape.


Different carbide dispersion shapes (granular or fibrous) have different effects on the matrix expansion and contraction, so they also affect the deformation after heat treatment. Generally, the cavity expands along the direction of carbide fibers, which is more obvious; while it shrinks directly in the direction of fibers, which is not obvious. Some factories have made special rules for this purpose, and the cavity expands along the direction of carbide fibers. The location surface should be straight in the direction of carbide fibers in order to reduce the deformation of the cavity. When carbides are uniformly dispersed in granular form, the cavity will exhibit uniform expansion and contraction.


In addition, the arrangement before heat treatment has a certain effect on the deformation. For example, the original arrangement of spherical pearlite is less prone to deformation than that of flake pearlite after quenching. Therefore, the deformation of the rigid die, often after rough processing, a conditioning treatment, and then finishing and final heat treatment.


Effect of Mold Shape on Deformation


The effect of die shape on heat treatment deformation is still effected by thermal stress and stress arrangement in practice. Because the shapes of dies are various, it is still difficult to summarize the proper rules of deformation.


As for the symmetrical die, the deformation tendency of the cavity can be considered according to the size, shape and height of the cavity. When the wall of the die is thin and the height is small, it is easier to harden. At this time, the arrangement of stress may play a leading role. Therefore, the cavity tends to expand. On the contrary, if the wall thickness and height are large, hardening will not be easy. Thermal stress may play a leading role at the moment, so the cavity tends to shrink. What we are talking about here is the general trend. In production practice, it is necessary to combine the details of the parts.

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