Definitions

oil hardening

Tool steel

Tool steel refers to a variety of carbon and alloy steels that are particularly well-suited to be made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion, their ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures (red-hardness).

With a carbon content between 0.7% and 1.4%, tool steels are manufactured under carefully controlled conditions to produce the required quality. The manganese content is often kept low to minimise the possibility of cracking during water quenching. However, proper heat treating of these steels is important for adequate performance, and there are many suppliers who provide tooling blanks intended for oil quenching.

Tool steels are made to a number of grades for different applications. Choice of grade depends on, among other things, whether a keen cutting edge is necessary, as in stamping dies, or whether the tool has to withstand impact loading and service conditions encountered with such hand tools as axes, pickaxes, and quarrying implements. In general, the edge temperature under expected use is an important determinant of both composition and required heat treatment. The higher carbon grades are typically used for such applications as stamping dies, metal cutting tools, etc.

Tool steels are also used for special applications like injection molding, because the resistance to abrasion is an important criterion for a mold that will be used to produce hundreds of thousands of parts.

AISI-SAE Grades

The AISI-SAE grades of tool steel is the most common scale used to identify various grades of tool steel. Individual alloys within a grade are given a number; for example: A2, O1, etc.

AISI-SAE Tool Steel Grades
Defining property AISI-SAE grade Significant characteristics
Water-hardening W
Cold-working O Oil-hardening
A Air-hardening; medium alloy
D High carbon; high chromium
Shock resisting S Tungsten base
High speed T Tungsten base
M Molybdenum base
Hot-working H H1-H19: chromium base
H20-H39: tungsten base
H40-H59: molybdenum base
Plastic mold P
Special purpose L Low alloy
F Carbon tungsten

Water-hardening grades

W-grade tool steel gets its name from its defining property of having to be water quenched. W-grade steel is essentially high carbon plain-carbon steel. This type of tool steel is the most commonly used tool steel because of its low cost compared to other tool steels. They work well for small parts and applications where high temperatures are not encountered; above 150 °C (300 °F) it begins to soften to a noticeable degree. Hardenability is low so W-grade tool steels must be quenched in water. These steels are rather brittle.

The toughness of W-grade tool steels are increased by alloying with manganese, silicon, and molybdenum. Up to 0.20% of vanadium is used to retain fine grain sizes during heat treating.

Typical applications for various carbon compositions are:

  • 0.60—0.75% carbon: machine parts, chisels, setscrews; properties include medium hardness with good toughness and shock resistance.
  • 0.76—0.90% carbon: forging dies, hammers, and sledges.
  • 0.91—1.10% carbon: general purpose tooling applications that require a good balance of wear resistance and toughness, such as drills, cutters, and shear blades.
  • 1.11—1.30% carbon: small drills, lathe tools, razor blades, and other light-duty applications where extreme hardness is required without great toughness.

Cold-working grades

Grade-O refers to oil hardening and grade-A refers to air hardening. These tool steels are used on larger parts or parts that require minimal distortion during hardening. More alloying elements are used in these steels, as compared to water-hardening grades. These alloys increase the steels' hardenability, and thus require a less severe quenching process. These steels are also less likely to crack and are often used to make knife blades.

D-grade tool steels contain between 10% and 18% chromium. These steels retain their hardness up to a temperature of 425 °C (800 °F). Common applications for these grade of tool steel is forging dies, die-casting die blocks, and drawing dies.

Composition

O-1 steel contains 0.90% carbon 1.0%-1.4% manganese, 0.50% chrome, 0.50% nickel, and 0.50% tungsten. It is a very good cold work steel and also makes very good knives. A-2 steel contains 1.0% carbon, 5.0% chromium, and 5.0% molybendum. D-2 steel is a semi-stainless alloy containing 1.5% carbon and 11-13% chromium; additionally it is composed of 0.45% manganese, 0.03% phosphorus, 0.02% sulfur, 0.30% nickel, 1.10% vanadium, 0.7% molybendum, and 0.60% silicon. Quantities of minor ingredients vary slightly with manufacturer. D2 is very wear resistant but not as tough as lower alloyed steels. widely used for shear blades, planer blades and industrial cutting tools, sometimes used for knives.

Shock resisting grades

S-grade tool steel are designed to resist shock at both low and high temperatures. A low carbon content is required for the necessary toughness (approximately 0.5% carbon). Carbide-forming alloys provide the necessary abrasion resistance, hardenability, and hot-working characteristics.

High speed grades

T-grade and M-grade tool steels are used for cutting tools where strength and hardness must be retained at temperatures up to or exceeding 760 °C (1400 °F). M-grade tool steels were developed to reduce the amount of tungsten and chromium required.

T1 (also known as 18-4-1) is a common T-grade alloy. Its composition is 0.7% carbon, 18% tungsten, 4% chromium, and 1% vanadium. M2 is a common M-grade alloy.

Hot-working grades

H-grade tool steels were developed for strength and hardness during prolonged exposure to elevated temperatures. All of these tool steels use a substantial amount of carbide forming alloys. H1 to H19 are based on a chromium content of 5%; H20 to H39 are based on a tungsten content of 9 to 18% and a chromium content of 3 to 4%; H40 to H59 are molybdenum based.

Special purpose grades

  • P-grade tool steel is short for plastic mold steels. They are designed to meet the requirements of zinc die casting and plastic injection molding dies.
  • L-grade tool steel is short for low alloy special purpose tool steel. L6 is extremely tough.
  • F-grade tool steel is water hardened and substantially more wear resistant than W-grade tool steel.

Other tool steels

  • Silver steel is a common tool steel in the UK that is roughly equivalent to drill rod in the US.

References

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