How to maximize the life of the wire cutting die

At present, wire-cut electric discharge machines have been widely used in mold processing. Since wire-cutting machine processing is generally performed after heat treatment, the defects such as heat treatment deformation and surface decarburization are avoided. There are many mold manufacturing units, which usually assemble and use after a little grinding after wire cutting. Others are directly assembled and used without grinding, resulting in frequent chipping, breaking, and chipping. Even if the above phenomenon does not occur, the sharpening life of the die is not long. This article focuses on the stress state of the module surface after wire cutting and the methods to eliminate stress and improve its surface quality.

The stress state of the part surface after wire cutting

At present, many molds are processed by fast-moving wire-cutting machines. After wire-cutting, the surface roughness of the workpiece is Ra≥2.5μm, and the hardness distribution and internal stress state are very poor. During wire cutting machine processing, the current density in the discharge zone is as high as 10000A/mm2, and the temperature is as high as 10000℃～12000℃. The added medium liquid cools rapidly, resulting in the surface hardness of the cutting surface being only about 20HRC, while the hardness of the internal quenching layer is as high as 70HRC or more. After that is the heat-affected zone, and then the original hardness zone. What’s more serious is that the raw material is in a state of tensile stress due to quenching, and the thermal stress generated by wire cutting is also a tensile stress. The superposition of the two stresses can easily reach the strength limit of the material and cause micro-cracks, which greatly shortens the life of the die. Cutting cannot be used as the final processing procedure of punch and die. The hardness distribution of the cut section of the CrWMn material after wire cutting and the hardness change after tempering and aging. (Guide: Introduction to the material requirements for the manufacturing and processing of die stamping parts)

Measures to eliminate the stress generated by wire cutting

Grinding to remove the white layer

At present, most mold processing units use grinding to remove the 20HRC gray layer (that is, the white layer) of the surface layer after online cutting, and then assemble it for use. Although this can remove the white layer with low hardness, it does not change the stress state of the stress zone caused by wire cutting. Even if the grinding allowance after wire cutting is increased, the high hardness of the hard layer (up to 70HRC) makes it difficult to grind. Excessive grinding is easy to damage the geometry of the part. Therefore, the high hardness layer produced by wire cutting cannot improve the life of the die, because its brittleness is the root cause of cracks and chipping.

tempering treatment

After on-line cutting, grind off the white layer on the surface of the part, and then temper at 160℃～180℃ for 2h, then the high hard layer under the white layer can be reduced by 5HRC～6HRC, and the thermal stress generated by the wire cutting will also be reduced. , Thereby improving the toughness of the die and prolonging the service life. However, due to the short tempering time, the thermal stress is not completely eliminated, and the die life is not very satisfactory.

Grinding

Grinding after wire cutting can remove the low-hardness white layer and high-hard layer and improve the life of the die. Because the thermal stress generated during grinding is also tensile stress, superimposing with the thermal stress generated by wire cutting will undoubtedly aggravate the damage of the die. If the low-temperature aging treatment is carried out after grinding, the influence of stress can be eliminated, the toughness of the die can be significantly improved, and the life of the die can be increased. Because most of the dies with complex geometric shapes are processed by wire cutting, expensive coordinate grinders and optical curve grinders must be used to grind the dies with complex shapes. Generally, manufacturers do not have these two equipment, so it is difficult to promote.

Low temperature tempering after shot peening

Shot peening can transform the retained austenite in the wire-cut incision into martensite, increase the strength and hardness of the die, change the stress state of the surface layer, reduce the tensile stress, or even become a compressive stress state, causing cracks Difficult initiation and expansion, combined with low-temperature tempering to eliminate tensile stress in the quenched layer, can increase the life of the die by 10-20 times. Shot peening is limited by equipment conditions and the shape (inner surface) of die parts, and it is difficult to be widely used.

Low temperature aging treatment after grinding

After the wire-cut surface is polished, the high-hard layer has been basically removed, and then subjected to low temperature aging treatment (also called low temperature tempering treatment) at 120℃～150℃ for 5～10h, or 160℃～180℃ for 4～6h Low temperature tempering treatment. This can eliminate the internal tensile stress of the quenched layer, while the hardness is reduced slightly (the latter hardness is slightly reduced), but the toughness is greatly improved, the brittleness is reduced, and the die life can be increased by more than 2 times. This method is simple and easy to implement, the effect is very obvious, and it is easy to promote.

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