Tool & High Speed Steel

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• Role of molybdenum
• Mining & processing
• Iron, steel and other alloys
  • Carbon steels
    • Metallurgy of Mo in carbon steels
    • Low carbon, low alloy, weldable structural steels
    • Alloyed and heat treatable steels
    • Special purpose steels
      • Maraging steels
      • Tool and high speed steel
      • High-temperature steel
    • Heat-treatable plate steel
  • Cast irons
  • Stainless steels
  • Nickel-based alloys
• Molybdenum metal, alloys and chemicals
• Applications by industry

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%)

Table: AISI-SAE tool steel grades

Defining property	AISI-SAE grade	Significant characteristics
Water-quenched	W	Molybdenum alloying optional
Cold-working	O	Oil-hardening, O6-0.3% molybdenum, cold-work steel used for gauges, cutting tools, woodworking tools and knives
	A	Air-hardening, low distortion during heat treatment, balance of wear resistance and toughness, all molybdenum alloyed - 0.15-1.8%
	D	High carbon, high chromium, 0.9% molybdenum, very high wear resistance but not as tough as lower alloyed steels
Hot-working	H	H1-H19 - chromium baseH20-H39 - tungsten baseH40-H59 - molybdenum base
Plastic moulding	P	Low segregation: reduced alloying of silicon, manganese and chromiumThrough hardenability: increased molybdenum and vanadium
High-speed	T	Tungsten base (today mostly replaced by M22)
	M	Molybdenum base
Shock resisting	S	Chromium-tungsten, silicon-molybdenum, silicon-manganese alloying, very high impact toughness and relatively low abrasion resistance
Special purpose	L	Low alloy, high toughness
	F	Carbon-tungsten alloying, substantially more wear resistant than W-type tool steel

Table: Typical alloying elements in tool steels and their effects

Alloying element	Advantages	Disadvantages
Chrome (Cr)	Hardenability, corrosion resistance, wear resistance	Lower toughness, poorer weldability
Cobalt (Co)	Heat resistance, temper embrittlement	-
Manganese (Mn)	Hardenability, strength	Thermal expansion
Molybdenum (Mo)	Hardenability, tempering resistance, temper embrittlement, strength, heat resistance, wear resistance	-
Nickel (Ni)	Yield strength, toughness, thermal expansion	-
Nitrogen (N)	Stress corrosion cracking resistance, work hardening, strength	Blue brittleness, aging sensitivity
Vanadium (V)	Wear resistance, tempering resistance	-

Cold-work steels

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.

Courtesy Peer Hansson, SSAB

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 steels

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.

Hot stamping die for automotive B-pillar component

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.

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	--	-	--	±	-	±

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

High-speed 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 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.

Typical compositions of selected high-speed steels (%)

Grade	C	Cr	Mo	W	V
T-1	0.75	-	-	18.0	1.1
M-2	0.95	4.2	5.0	6.0	2.0
M-7	1.00	3.8	8.7	1.6	2.0
M-42	1.10	3.8	9.5	1.5	1.2

Additions of 5-10% molybdenum effectively maximize the hardness and toughness of high-speed steels and maintain these properties at the high temperatures generated when cutting metals. Molybdenum provides another advantage: steels soften and become embrittled at high temperature when the primary iron and chromium carbides grow rapidly in size. Molybdenum, especially in combination with vanadium, minimizes this softening by causing the carbides to reform as tiny secondary carbides that are more stable at high temperatures. The largest use of high-speed steels is in the manufacture of various cutting tools: drills, milling cutters, gear cutters, saw blades, etc.

The useful cutting characteristics of high-speed steel have been further extended by applying thin, ultra-hard coatings. These coatings reduce friction and increase wear resistance, thereby increasing cutting speed and tool life.

The exceptional high temperature wear properties of molybdenum-containing high-speed steels are also ideal for applications such as automobile valve inserts and cam-rings.

Mill cutter (courtesy Boehler Edelstahl)