July 2011 Archives

Below we introduce materials including non-steel materials.

(1) Pre-hardened steel

These are mold materials developed for plastic molding and used for dies of presses. These have not been stipulated in JIS standards, and the materials used are those developed under the brand names of the manufacturers. These materials have reasonable hardness and can be machined. The material properties are adjusted to obtain a hardness of about 40 HRC, and precipitation hardened types (see note [*]) are used.
Pre-hardened steel is used frequently for punches and dies for blanking or bending in the case of low volume production. In the case of large volume production, pre-hardened steel is used for punch plates, backing plates, etc. This material is commonly used because it can be machined and is convenient because it can be used as is without heat treatment.

* Precipitation hardening:
This is hardening caused by the precipitation of certain constituents when a supersaturated solid solution is heated to an appropriate temperature.

(2) Flame hardened steel

Flame hardened steel is used for dies for the blanking, drawing, or bending of automobile components. This is a material in which surface hardening can be done easily by flame hardening.
Normally, while it is necessary to harden (total hardening) the die to the necessary hardness after it has been machined, this step can be simplified in the case of flame hardened steel, and although the strength is lower than the steels that require total hardening, it is used widely in the case of some specific applications since the time required for preparing dies can be shortened.

(3) Aluminum bronze

Aluminum bronze is a copper-based alloy. It can withstand high press pressures, has superior wear resistance, and high galling resistance. This material is also called HZ alloy (Hitachi Shipbuilding), AMPCO metal (USA). This is used frequently in the dies for drawing or for forming hard to form materials such as SUS or titanium, etc.

(4) Cast iron

Cast iron is used very frequently for die sets, or the dies forming medium to large sized products, or the dies for drawing, etc.

4-1. Gray cast iron (JIS G 5501)

This is also called ordinary cast iron, has excellent casting properties, and is used frequently for die sets, holders, and punches and dies of medium sized products.
The JIS symbol is FC, and its types have been stipulated from FC100 to FC350 with the number increasing in steps of 50. Among these, FC200, FC250, and FC300 are being used for dies.

4-2. Spherical graphite cast iron (JIS G 5502)

This is also called ductile cast iron, and is used when the strength of gray cast iron is considered insufficient. In gray cast iron, carbon precipitates inside the form of flakes inside a matrix of ferrite or pearlite. In contrast, magnesium has been added to the melt so as to make the precipitated carbon have the shape of spherical nodules. By making the precipitated carbon have a spherical shape, this material has achieved a high strength close to that of steel, and also has excellent ductility, rigidity, and wear resistance.
The JIS symbol is FCD, and its types have been stipulated from FCD350 to FCD800. Among these, FCD450, FCD500, and FCD600 are being used for dies.

High speed tool steels are frequently abbreviated in Japanese as "Hice". The symbol for these materials is SKH.
High speed tool steels can be based on Tungsten (W) or on Molybdenum (Mo). These are specified in JIS G 4403.

About 18% of tungsten is added in tungsten-based high speed tool steels, and they do not contain any molybdenum. These steels come in the types of SKH2, SKH3, SKH4, and SKH10. Since steels of this series have high wear resistance, they are used very frequently in cutting tools, etc.
About 5% of molybdenum and about 6% of tungsten are added in molybdenum - based high speed tool steels. These steels come in the nine types of SKH51 to SKH59.
Since steels of this series have a high toughness, they are ideally suitable for press dies that are subject to shock. SKH51 (old symbol SKH9) is the type used most frequently.

The tempered hardness of high speed steels is more than 63 HRC. The tempered hardness of the die steel SKD11 is about 62 HRC. High speed steel is used very frequently for small diameter punches, etc., in which case the wear resistance and toughness of die steels are somewhat insufficient.

Powdered high speed tool steel

Similar to the method of manufacturing ordinary tool steel, high speed tool steel is manufactured by solidifying molten material.
A uniform and fine structure of powdered high speed tool steel is prepared by first finely powdering the molten material, and solidifying that powder. By doing this, the wear resistance, toughness, and fatigue resistance all are increased. A sintered hardness of about 64 to 70 HRC is obtained.
These steels are used when using high speed die steels are not satisfactory. JIS has standardized powdered high speed tool steel in SKH40 (molybdenum based). A large number of manufacturers use originally developed steels.

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.

Among the materials constituting dies, these are the materials that are mainly the materials that form the structure.

(1) Rolled steels for ordinary structures (SS materials)

This is a material used most commonly as the base material for molds, punches, and dies. The very frequently used type is SS400. This is usually referred to as the SS material.

The number in the material name SS400 indicates the tensile strength. SS400 has a tensile strength of 400 to 510N/mm².

Since this is a soft material with tenacity, and there is also the opinion that it is difficult to process because during hole drilling the cutting chips (cut pieces) connect to each other in the form of a long spiral.

(2) Carbon steel for machine structures (SC materials)

These materials are also used in a manner similar to SS materials. The SC materials come in varieties from S10C to S58C. The number here expresses the amount of carbon (C) contained in the steel material. The amount of carbon ranges from 0.08% to 0.61%. When the amount of carbon exceeds 0.61%, the material then becomes carbon tool steel (SK materials).

The interesting fact about this material is that its mechanical properties have not been indicated. Only some reference values are given. The reason for this is that the tensile strength stipulated in JIS is the test result for a material of 25mm diameter. This is a consideration for the fact that the conditions change in the case of large structures.

Various materials were being used earlier in the standards. For example, they were S35C, S45C, S50C, S55C, etc. At present, due to standardization of plates, it has been standardized to use either S50C or S55C. The reason why the steels with larger numbers have remained is that the materials with more carbon content are stronger.

From the point of view of ease of machining, these materials are considered to be easy to use since the cutting chips do not become long as in the case of SS materials and the handling of scrap becomes easier. This is due to the difference in the carbon content. In the case of SS materials, there is no specification of the amount of carbon content (extremely small).

Repetitive load acts on the parts such as punches, etc., in press forming operations. Repeated load becomes the cause of fatigue failure. Fatigue failure can be linked to fatigue strength.
The causes related to fatigue strength are the following:

	1.	1.Hardness after thermal refining (after obtaining a hardness of about 45HRC)
	2.	Eliminating sharp corners (rounding or chamfering)
	3.	Making the surface smooth.

(1) Relationship with tensile strength

The fatigue strength is almost directly proportional to the tensile strength of the material. There is a difference between hardened material and raw material. The fatigue strength is about 50% of the tensile strength in the case of the raw (unhardened) material, and is about 45% in the case of the hardened material. This fact is not dependent on the material itself.

(2) Relationship with hardness

Even regarding the hardness of the material, the fatigue strength is higher for a harder material. The fatigue strength becomes the highest for a material that has been thermally refined to a hardness of 45HRC.

(3) Surface condition

Even if the tensile strength and the hardness are suitable, the fatigue strength decreases if the surface condition of the parts such as punches, etc., is bad. It is better to finish the surface to as good a state as possible.

(4) Shape

In the case of a shape with depressions and projections, the corners of depressions should not be made square but should be rounded. The effect is larger as the radius of curvature R is made larger.

(5) Residual compressive stress

Fatigue failure is generated from the surface of a part. Therefore, it is possible to increase the fatigue strength by making the surface (surface layer) hard and, in addition, if a residual compressive stress is present. It is very effective to carry out shot peening treatment of the surface in which the material surface is hit with steel ball or glass beads at a high speed.