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#082 Fundamentals of Steel Materials for Dies (1) Properties of Steel Materials (Wear Resistance)

Category : Die Materials
June17, 2011

Steel materials for dies and punches are required to have various properties. To start with, let us understand them.

(1) Wear resistance

According to JIS, wear is described as "the phenomenon of metal surfaces that are moving relative to each other getting worn out due to the surfaces scratching each other or due to metallic adhesion". Wear resistance can be said to be the property in which such a phenomenon is difficult to occur.

(2) Factor affecting wear resistance

1) Effect of hardness

A big factor affecting wear resistance is "hardness". In general, the wear resistance increases as the material becomes harder. The wear resistance shows a big change bordering a Rockwell hardness value of around 40HRC. The amount of wear is larger below 40HRC, and the wear is small above that hardness value. However, it is not correct to assume that making the material hard by tempering it, because it is better that the internal residual stress in the steel material is low in addition to higher hardness of the material. This is the reason why tempering and annealing are done together.

2) Effects of the constituents

One of the main constituents of steel materials is carbon (C). Tempering becomes easy as the amount of carbon increases. If the amount of carbon exceeds 0.6%, the tempered hardness becomes almost constant. Although when the hardness becomes constant, the wear resistance does not become stable at that point, but the wear resistance increases further as the carbon content increases. In addition, the wear resistance increase as the elements such as W, Cr, V, Mo, etc., are added to the steel material.
 The amounts of addition of these constituents increase in the sequence of - carbon tool steel (SK), special tool steel (SKS), and die steel (SKD). Compare the constituents by checking the JIS standards, for example, and then you will know the difference.

3) Effect of the structure of the material

When a steel material is tempered, iron (Fe) and carbon (C) bond together and the material changes into a martensite. This martensite is effective for wear resistance. However, in high carbon steel or high alloy tool steel (SKD, etc.), not all the material is converted into martensite by tempering and annealing, and about 20 to 30% of the material remains as austenite. This residual austenite is not good for wear resistance.
As a method of converting the residual austenite into martensite, there is the sub-zero processing. In sub-zero processing, by cooling the steel material up to -60 to -80C, the residual austenite is converted into martensite.

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