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#086 Fundamentals of Steel Materials for Dies (5) Steel Materials Used in Dies (Tool Steels)

Category : Die Materials
July15, 2011

These are materials constituting molds and are the materials used in the main parts mainly of punches, dies, strippers, and packing plates, etc.

(1) Carbon steels for tools (SK materials)

The amount of carbon content of SK materials is 0.6 to 1.5%. The SK materials are available in various types, from Type 1 to Type 7. SK1 has a carbon content ratio of 1.3 to 1.5% while SK7 has a carbon content ratio of 0.6 to 0.7%. The carbon content ratio of the material decreases from SK1 to SK7. By the way, when the carbon content ratio becomes less than or equal to 0.6%, the material becomes a material for machine structures (SC material).
In press dies, SK3 and SK5 are used very often.
The method of their use in press dies is for punches and dies for small production volumes.
Since the hardness of SK materials is weak towards heat, even after sufficient hardening, it is not possible to expect long life when used for items that generate machining heat such as in press blanking operations. Because of this, rather than using these materials for punches and dies, it is very common to use these materials as supplementary part such as packing plates, etc.

(2) Alloy tool steels (SKS, SKD)

Alloy tool steels are SK materials whose characteristics have been modified by adding special elements such as tungsten (W), chromium (Cr), molybdenum (Mo), and vanadium (V), etc.

1) SKS Materials

SKS3 is used in press molds. These are materials obtained by adding Cr and W to SK materials. The amount of deformation due to hardening is about 1/2 of that of SK materials. Although at present deformation due to heat treatment is no longer of any concern since their machining is done by wire cutting electric discharge machining after hardening, this was of great concern earlier.
These materials are used in the punches and dies for medium to small production volumes. Apart from that, these materials are also used in strippers or punch plates that require hardening.

2) SKD Materials

These materials are called die steels. These are materials obtained by adding Cr, Mo, and V to SK materials. The material SKD11 is used very frequently in press molds. These materials can also be said to be the mainstream materials for punches and dies. These materials are used in the molds for medium to large production volumes. The deformation due to heat treatment is still smaller than the SKS materials.
The deformation is small during wire cutting electric discharge machining, and ease of machining is also a reason why these materials have become the mainstream materials for molds. Since the deformation during wire cutting electric discharge machining is still smaller when high temperature tempering is done (at about 500 to 550 ℃) rather than the normal tempering (about 180 to 200 ℃), high temperature tempering has come to be used more often. However, since the hardness decreases (to about 58HRC), there was also the drawback that the tool life becomes poor. Manufacturers of steel materials are developing materials in which a hardness of 60HRC can be maintained even after high temperature tempering. However, since the heat treatment conditions of these materials have changed finely, unless care is taken, the expected heat treatment characteristics may not be obtained.

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