Tool steels are used for working, cutting, and forming metal components, moulding plastics, and casting dies for metals with lower melting points than steel. Accordingly, tool steels need high hardness and strength combined with good toughness over a broad temperature range.
The microstructure of all tool steels is based on a martensitic matrix. Molybdenum additions in tool steels increase both their hardness and wear resistance. By reducing the critical cooling rate for martensite transformation, molybdenum promotes the formation of an optimal martensitic matrix, even in massive and intricate moulds that cannot be cooled rapidly without distorting or cracking. Molybdenum also acts in conjunction with elements like chromium to produce substantial volumes of extremely hard and abrasion resistant carbides. Increasing physical demands on tool steels result in an increasing molybdenum content. Depending on their application, tool steels are classified into:
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Cold-work tool steels are tool steels used for forming materials at room temperature or at slightly raised temperatures (~ 200°C). Specifically, tools for blanking metallic and non-metallic materials, including cold-forming tools, are manufactured from these steels.
Fundamentally, cold-work tool steels are high carbon steels (0.5-1.5%). The water-quenched W-grades are essentially high carbon plain carbon-manganese steels. Steel grades of the O series (oil-hardening), the A series (air-hardening), and the D series (high carbon-chromium) contain additional alloying elements that provide high hardenability and wear resistance as well as average toughness and heat softening resistance.
The four major alloying elements in such tool steels are tungsten, chromium, vanadium, and molybdenum. These alloys increase the steels' hardenability and thus require a less severe quenching process with a lower risk of quench cracking and distortion. All four elements are strong carbide formers, also providing secondary hardening and tempering resistance.
Hot-work tool steels are tool steels used for the shaping of metals at elevated temperatures. Their principal areas of application include pressure die casting moulds, extrusion press tools for processing light alloys, and bosses and hammers for forging machines. The stresses encountered here are cyclical, often with abrupt temperature changes and recurring mechanical stresses at high temperatures. Hot-work steels must constantly endure tool temperatures above 200°C during use. To achieve optimum performance, hot-work tool steels require the following properties:
Cycle times applied in plastic injection moulding, pressure die casting or press hardening (hot stamping) can be reduced considerably by increasing the tool steel’s thermal conductivity, which significantly raises productivity. Heat conductivity is influenced by several material parameters such as microstructure, defects, and alloying elements.
Armco iron is nearly pure iron with a low defect density and high heat conductivity in the order of 70-80 W/mK. Compared to Armco iron, traditional hot-work steel such as H13 (1.) has much lower heat conductivity in the range of only 20-30 W/mK. This reduced thermal conductivity is due to high lattice distortion and defect density of the (tempered) martensitic microstructure as well as to a substantial content of alloying elements. All these characteristics interact with phonons, electrons, and magnons as the “vehicles” of heat transport.
Since all hot-work steels have a defect-rich martensitic microstructure, the difference in optimizing heat conductivity lies in the alloying composition. When in solid solution, alloying elements can cause local lattice distortion (size misfit vs. iron), modify the electronic structure, and/or have influence on magnetism. Generally, heat conductivity is reduced as the alloy content increases. Looking at individual elements in a solute state, nickel, chromium, and silicon were found to negatively influence heat conductivity. The effects of vanadium and molybdenum appear less detrimental. After tempering, the amount of solute vanadium, chromium, and molybdenum decrease by carbide precipitation, which diminishes their negative effect on heat conductivity.
Effect of alloying element on properties of hot-work steel Property Si Mn Cr Mo Ni V Wear resistance - - + ++ - ++ Hardenability + + ++ ++ + + Toughness - ± - + + + Thermal stability + ± + ++ + ++ Thermal conductivity -- - -- ± - ±Tools for processing plastics are mainly stressed by pressure and wear. According to the type of plastic, corrosive conditions can prevail in addition to stresses. The type of plastic and processing method define the key requirements in addition to those generally valid to hot-work steels:
When tool steels contain a combination of more than 7% molybdenum, tungsten, and vanadium, and more than 0.60% carbon, they are referred to as high-speed steels. This term describes their ability to cut metals at “high speeds”. Until the s, T-1 with 18% tungsten was the preferred machining steel. The development of controlled atmosphere heat treating furnaces then made it practical and cost effective to substitute part or all the tungsten with molybdenum.
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Materials that are used to make tools should understandably be particularly hardy, and tool steel fits the bill. “Tool steel” as a term refers to carbon and alloy steels that are strong enough to be made into tools that can work on wood, plastic, and other metals in various processes, like stamping and forming, to name just a couple. Hand tools, drills, cutters, and bits are more often than not made from tool steel, as are larger items like machine dies and plastic extruding machinery. There are several different types of tool steels that are categorized into grades (and there are a few sub-grades, too), so to figure out what each grade is good for, keep reading. We’ll also cover what exactly is in tool steel and how it’s made.
Tool steels are hard, tough, and wear-resistant metals that won’t soften at high temperatures. They’ll typically have 0.7–1.5 wt% carbon in them, but some can have anywhere from 0.2–2.1 wt%. Although a higher carbon level will make the steel stronger and more hardenable, it will also make it brittle and less easy to weld. When cold-worked, tool steels are around 60/62 HRC on the Rockwell C hardness scale, but could range between around 58/64 HRC (some have been known to reach 66). Tool steel can withstand heat treatment, with the specific temperature being dependent on its exact composition. Here’s what it looks like before it’s made into tools:
In addition to carbon, tool steels will have other elements to improve their strength and change their properties according to what exactly the tool is needed for. For instance, nickel or cobalt can be added to give the metal extra strength and high-temperature resistance, while adding carbide former made from different combinations of iron-based alloys, like tungsten, vanadium, chromium, and/or molybdenum, can make it more wear-resistant.
Although the main method for making tool steel is via electric arc furnaces (EAF), this is not the only way. Below we’ll cover the processes used to make it.
EAF: Recycled steel scrap is melted and purified in a furnace, with the alloying elements mixed in until the composition is just right. Chemicals are added to keep oxidation at bay and remove impurities, and it’s then poured into a ladle (an oversized bucket with a spout). It’s then put into giant ingot molds and carefully cooled.
Electroslag refining (ESR): This process melts the metal extremely slowly for a smooth and non-porous surface.
Powder metallurgy: The metal powders are pressed and sintered until they’re solid and dense.
Annealing: To make the steel easier to work with and less brittle, its molecular structure can be changed with annealing, which is heating it at a steady and high temperature for a certain amount of time before cooling it right down again.
Hot or cold drawing: This process is used to make smaller or uniquely shaped tools with high tolerances. As these steels aren’t very ductile, several passes at temperatures of up to °C are needed, but you’ll only be able to go over it once (and lightly) with cold drawing to prevent breakage.
As promised in the intro, here’s a look at the different types of tool steel grades which, as you’ll see, differ in both composition and characteristics.
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