Tool and high-speed steels
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:
- Cold-work tool steels (Mo ≤1.8%)
- Hot-work tool steels (Mo ≤3.0%)
- Plastic mould steels (Mo ≤1.3%)
- High-speed tool steels (Mo ≥7%)
AISI-SAE tool steel grades
||Molybdenum alloying optional
||Oil-hardening, O6-0.3% molybdenum, cold-work steel used for gauges, cutting tools, woodworking tools and knives
||Air-hardening, low distortion during heat treatment, balance of wear resistance and toughness, all molybdenum alloyed - 0.15-1.8%
||High carbon, high chromium, 0.9% molybdenum, very high wear resistance but not as tough as lower alloyed steels
||H1-H19 - chromium base
H20-H39 - tungsten base
H40-H59 - molybdenum base
||Low segregation: reduced alloying of silicon, manganese and chromium
Through hardenability: increased molybdenum and vanadium
||Tungsten base (today mostly replaced by M22)
||Chromium-tungsten, silicon-molybdenum, silicon-manganese alloying, very high impact toughness and relatively low abrasion resistance
||Low alloy, high toughness
||Carbon-tungsten alloying, substantially more wear resistant than W-type tool steel
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:
- Good tempering properties
- Sufficient thermal stability
- High hot toughness
- High resistance to wear at elevated temperatures
- Good thermal fatigue resistance
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 (1.2344) 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.
Development of a hot stamping die steel (1% molybdenum, 0.2% vanadium, 0.04% niobium) effect of altering alloy composition (reduced silicon, chromium, nickel) and key resulting properties (arrows).
Plastic mould steels
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:
- Economic machinability or cold-hobbing ability
- Smallest possible distortion upon heat treatment
- Good polishing behavior
- High compressive strength
- High wear resistance
- Sufficient corrosion resistance
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 1950s, 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.