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Accidents and solutions for die steel
According to the statistics of the mold accident investigation, it is found that in the accidental form of the tool or mold, the three major categories of damage, dimensional change, deformation, and wear account for the majority, among which the heat treatment, which is one of the three major causes, accounts for the vast majority. From the recent tendency, there are cracks caused by wire-cutting related to the method of die machining; premature die damage caused by overload or high speed related to the method of die use; Commonly caused by molds, due to the accident caused by the built-in cartridge heaters; or due to the thermal crack caused by the use of water-soluble release products in die casting molds.
(1) Damage:
Damage usually occurs when the load exceeds the material allowable stress, or when the load stress plus the tensile residual stress exceeds the allowable stress.
Because the mold steel has poor ductility in general conditions, and it rapidly breaks without rapid deformation due to plastic deformation, so-called brittle failure, it is necessary to fully consider the stress concentration when calculating the allowable stress of materials in the design. problem. In addition, the fatigue strength is also the same, and the notch effect needs to be taken into consideration. However, since the mold steel is like a structural steel, the value of the mechanical characteristics is not clear and it is difficult to actually calculate the allowable stress, but when selecting the mold steel, designing and setting the heat treatment conditions, it is necessary to use basic accurate data as much as possible.
There are many reasons for the damage in manufacturing, such as quenching and cracking, and the reasons for quenching and cracking are many. Perhaps it is an unexpected reason. Therefore, careful attention must be paid. In addition, even if quench cracking is prevented, if there is a residual tensile stress after the heat treatment, this will correspond to the destruction of the stress portion, and therefore it is also necessary to reduce the residual stress. In particular, it will play a decisive role in solving the problem of damage caused by processing methods or the use of overload.
(2), size change, deformation:
There are two kinds of dimensional change and deformation: deformation by heat treatment and deformation in use (bulging or bending). Due to the recent development of surface immersion hardening at high temperatures, special attention has been paid to heat treatment deformation. This is because the surface hardened layer is as thin as 5 to 10 microns and cannot be subjected to a correction process after the surface treatment.
In the case of a hot working mold or a plastic working mold whose shape is complicated and whose accuracy needs to be maintained, if the heat treatment strain is large, correction processing may not be performed. However, due to the low hardness of most of the molds, pre-hardened steels can be used.
However, dimensional changes and deformations, including deformation in use, basically involve the selection of mold steels, the design of heat treatment conditions, and the consideration of material orientation (discharge).
(3), wear:
The wear depends on the choice of tool steel and the hardness of the tool steel.
Conceptually speaking, since these are properties that are contrary to toughness, it is necessary to accurately grasp the conditions of use, and the balance between wear resistance and toughness should be taken into consideration when selecting the die steel and determining the heat treatment conditions.
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