HSS High-speed steels:
Specification and application of high-speed steels
High-speed steels belong to the variety of tool steel grades, in which the cutting capacity, the hardness of the product, and the retention of the core are mainly assessed. They are used for the production of cutting tools, cutters, roughing and finishing knives, spiral drill bits, and in some cases for responsible machine parts, rings, bearings, spring elements of high-speed steel. As a result of similar but better tooling properties, they can be described as high speed tool steels.
They are characterized by very high hardness at high operating temperatures, while maintaining adequate core ductility, one of the larger abrasion resistance ranges from the rest of tool steels, and high durability and fatigue resistance. The highest assessed property of grade of high speed steels is the tempering resistance, which reaches the temperature of 600 ℃.
Chemical composition of high-speed steels
Among all tool steels, high-speed steels are the most expensive due to the high content of expensive alloying additives such as tungsten - W, cobalt - co, vanadium - V, not including Molybdenum - Mo, and chromium - Cr, of which is quite a lot compared to other steel.
All of these elements - apart from cobalt - form carbides in the steel arranged in the matrix, thanks to which the high-speed steels are characterized by all the features mentioned earlier. Omitted cobalt prevents the formation of grain growth in steel during heat treatment.
For high hardness responsible is, among other, Carbon - C, which in high-speed steels is within the range of 0.7-1.15%. As more carbide-forming additives are present in steel, this range of coal must be proportionally increased to maintain the amount of carbide at an appropriate level.
- Tungsten, forming with carbon hard, fine and evenly distributed carbides, is responsible for the high wear-resistance, and prevents the reduction of hardness of the steel during the high temperature, the so-called "red heat."
- Cobalt prevents high-speed steel from overheating during hardening, which increases the amount of carbide in steel and increases the melting point of the product during operation.
- Wanad also provides improved abrasion resistance and is good for fine-grained structure of the product. Improves resistance to high temperatures.
- Chrome improves steel hardening capacity and Molybdenum to some extent replaces Wolfram with improved abrasion resistance.
High-speed steels can be divided into high-speed cobalt steels - cobalt steels with vanadium additive, molybdenum or with addition of carbon - and noncobalt high-speed steels, tungsten high-speed steels, tungsten with molybdenum or carbon - where all the grades are referred to as high-speed ledeburitic steel.
Heat and mechanical treatment of high-speed steels
Heat treatment of high-speed steels is a series of slow and precise processes that consist of two-stage or even three-stage heating of steels to initial temperatures depending on the cross-section of the quenched element.
High speed steels delivered in softened state are characterized by very low thermal conductivity and therefore the heating process should be carried out slowly and accurately, due to the risk of material stress causing deformation and cracks during machining.
After initial mechanical treatment resulting in stress, the product should be cooled to a temperature of 600-650 ℃. After slow cooling, the product can be finished before tempering.
For larger cross-sections, the product is heated three times at an increasing temperature of 500 ℃-1050 ℃. For components with smaller and less complex dimensions, it is sufficient to heat twice. Preheating of materials is carried out in chamber furnaces and salt bath furnace (in SH430, SH960 solutions).
At temperatures of 1000-1100 ℃, M₂₃C, M₆C, and M₄C₃ carbides dissolve, which are responsible for the mechanical properties of the product after treatment. Hardening of high-speed steels occurs at temperatures of 1100-1300 ℃ depending on the type of product. To reduce the stresses, maintain ductility and hardness of the material, two or even three times tempering at temperatures of 520-600 ℃ is performed.
High-speed steel after machining has a lath martensitic structure. In addition to the described hardening of the element, other methods of heat treatment - thermo-chemical treatment, vacuum furnaces and protective atmosphere furnaces are available. These are just some of the other available processing methods, and with the development of the metallurgical technology, the number of methods of heat treatment may have increased.
The above-described high-speed tool steels are defined by the PN-86/H-85022 Standard, the Industry Standard BN-77/0631-05 and the PN-EN ISO 4957 Standard, according to which we provide:
- High-speed steel sheets according to PN-H-92130, PN-86/H-92139, PN-EN ISO 4957
- Forging and forged bars made of high-speed steel according to PN-94/H-93012, PN-79/H-94500, PN-EN ISO 4957
- Cold-rolled high-speed steel tapes according to PN-77/H-92330-00, PN-77/H-92330-01, PN-84/H-92331, PN-75/H-92335, PN-EN 10132,
- High-speed steel bars according to PN-94/H-93012. PN-75/H-93200, PN-72/H-93201, PN-72/H-93202, PN-EN 10058, PN-EN 10059, PN-EN 10060
- High-speed steel ground bars according to PN-86/H-93209, PN-76/H-93006, PN-EN 10278