Tool Steel | The Four Types of Steel (Part 4)


This article is the fourth in a four-part series on the different types of steel. Read Part 1.

Tool steel contains various amounts of tungsten, molybdenum, cobalt and vanadium to increase the heat resistance and durability of the metal. This makes them ideal when used for cutting and drilling.

Tool Steel is divided into these six groups:  water-hardening, cold-work, shock-resisting, high-speed, hot-work, and special purpose. The determination of a choice from the group will depend on; the cost effectiveness, working temperature, required surface hardness, strength, shock resistance, and toughness requirement. If there is a more severe demand requirement such as extreme temperature, increased abrasiveness, high corrosion, or excessive loading – higher alloy content (and carbides) may be required.

Tool Steel Code Class Attributes
W Water hardening
O Cold worked Oil hardening
A Cold worked Air hardening
D Cold worked High Carbon, High Chromium
S Shock resisting
H Hot worked H1 to H19 are Chromium based
H Hot worked H20 to H39 are Tungsten based
H Hot worked H40 to H59 are Molybdenum based
M High Speed Molybdenum based
T High Speed Tungsten based
P Plastic mold
L Special Purpose Low Alloy
F Special Purpose Carbon / Tungsten based

Tool Steel

Water-hardening class – Named from its essential property of having to be water quenched. This grade of tool steel is essentially plain high carbon steel. It is commonly used because of its low cost.

Cold-work classes – Is a group of three tool steels: oil-hardening, air-hardening, and high carbon-chromium. The steels in the group have high hardenability and wear resistance, with average toughness. Typically they are in the production of larger parts or parts that have a minimum distortion requirement when being hardened.

  • Both Oil quenching and Air-hardening both reduce the distortion and higher stress caused by the quick water quenching.Because of this they are less likely to crack.
  • D-grade of tool steel in the cold-work class can contain approximately 10% to 13% chromium. This type of tool steel does retain its hardness at increased temperature (425 °C / 797 °F max).  The most typical applications for this type of tool steel is in forging dies, die-casting die blocks, and drawing dies.

Shock-resisting class – This class has high shock resistance and good hardenability. It is designed to resist shock at both low and high temperatures. It also has a very high impact toughness and relatively low abrasion resistance.

High speed class

T-type and M-type tool steels are used for cutting tools when strength and hardness must be retained at high temperatures.

Hot-working class – H-group tool steels were specifically developed to maintain strength and hardness while exposed to prolonged elevated temperatures.

Plastic Mold & Special purpose classes:

  • P-Code (Plastic Mold Steel) – Designed to meet the needs of zinc die casting and the special requirements of plastic injection molding dies
  • L-Code – A low alloy special purpose tool steel.
  • F-Code – Water harden-able / more wear resistant than W-type tool steel.
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