Tool steel refers to a variety of carbon and alloy steels that are particularly well-suited to be made into tools (punches, dies, molds, tools for cutting, blanking, forming, drawing, steering and slitting tools). Their suitability comes from their distinctive hardness, resistance to abrasion and deformation, and their ability to hold a cutting edge at elevated temperatures. With a carbon content between 0.5% and 1.5%, tool steels are manufactured under carefully controlled conditions to produce the required quality. The presence of carbides in their matrix plays the dominant role in the qualities of tool steel.
They are generally grouped into two classes:
- Plain carbon steels containing a high percentage of carbon, about 0.80-1.50%
- Alloy tool steels, in which other elements (chromium, molybdenum, vanadium, tungsten and cobalt) are added to provide greater strength, toughness, corrosion and heat resistance of steel.
One of subgroups of tool steels is high-speed steels (HSS), which were named primarily for their ability to machine and cut materials at high speeds (high hot hardness). It is often used in power-saw blades and drill bits. This group of tool steels is described in a separate article.
Example of Tool Steel – A2 Steel
A2 tool steel is an air hardening, cold work steel of group A steels containing molybdenum and chromium. A2 steel contains 5% of chromium steel which provides high hardness after heat treatment with good dimensional stability. The carbon content in A2 tool steels is high. A2 offers good toughness with medium wear resistance and is relatively easy to machine. A2 tool steel can be used in many applications which require good wear resistance as well as good toughness. Typical applications for A2 steel:
- Forming dies
- Slitters
- Gauges
- Shear blades
- Blanking tools
- Punch dies
Hardness of Tool Steel – A2 Steel
Rockwell hardness of tool steel – A2 steel depends on heat treatment process, but it is approximately 60 HRC.
Rockwell hardness test is one of the most common indentation hardness tests, that has been developed for hardness testing. In contrast to Brinell test, the Rockwell tester measures the depth of penetration of an indenter under a large load (major load) compared to the penetration made by a preload (minor load). The minor load establishes the zero position. The major load is applied, then removed while still maintaining the minor load. The difference between depth of penetration before and after application of the major load is used to calculate the Rockwell hardness number. That is, the penetration depth and hardness are inversely proportional. The chief advantage of Rockwell hardness is its ability to display hardness values directly. The result is a dimensionless number noted as HRA, HRB, HRC, etc., where the last letter is the respective Rockwell scale.
The Rockwell C test is performed with a Brale penetrator (120°diamond cone) and a major load of 150kg.
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