Tool steel

What defines the tool steel family?

Tool steel is high-carbon steel alloyed with chromium, molybdenum, tungsten, vanadium and, in some grades, cobalt. These elements form hard carbides during solidification. The carbides withstand the temperatures and stresses involved in cutting, forming and shaping other materials. Each grade requires heat treatment. The certificate analysis indicates the potential properties, while hardening and tempering determine the resulting properties.

The AISI letter groups classify tool steels by service rather than composition. W identifies water-hardening grades. O identifies oil-hardening grades. A identifies air-hardening grades. D identifies high-carbon high-chromium grades. S identifies shock-resisting grades, H identifies hot work grades, M and T identify molybdenum-based and tungsten-based high speed steels, P identifies plastic mould steels, and L identifies low-alloy special purpose grades. The letter therefore indicates service conditions before chemical composition.

The Laxcon Steels grade reference lists 88 tool steel grades with their full composition. The table below carries every one of them that has a cross-standard equivalent on record.

Wear resistance, toughness and dimensional stability

Tool steel selection involves a balance between wear resistance, toughness and dimensional stability. Increasing one of these properties generally reduces at least one of the others.

Wear resistance depends on carbide volume fraction, hardness and size. AISI D-2 contains 1.4 to 1.6 percent carbon and 11.0 to 13.0 percent chromium. Its large volume of hard chromium carbides supports long-run blanking and forming die applications. The grade is comparatively brittle. A thin D-2 punch can chip under conditions in which a tougher grade would deform.

Tough grades contain less carbon and carbide and retain more alloy in solution. AISI S-7 contains 0.45 to 0.55 percent carbon, 3.0 to 3.5 percent chromium and 1.3 to 1.8 percent molybdenum. It is a shock-resisting grade used for chisels, punches and impact-loaded tooling. Its edge retention is lower than that of a high-carbon grade.

Dimensional stability depends on the quench represented by the letter groups. W-1 is a nearly plain carbon, water-hardening steel. Its severe quench can distort or crack a tool. O-1 contains sufficient alloy to harden in oil and undergoes less movement. A-2 contains 4.75 to 5.5 percent chromium with molybdenum and vanadium. It hardens in still air and has the lowest distortion of the three groups. A finished die cannot be straightened after hardening. Distortion can therefore determine grade selection before hardness.

Cold work, hot work and high speed steels

Cold work and hot work grades differ according to the operating temperature of the tool. The W, O, A and D series provide wear resistance and edge retention at ambient temperature. They soften when the tool becomes hot in service.

Hot work grades retain hardness at elevated operating temperatures. They also resist thermal fatigue caused by repeated heating and cooling of the die surface. H-13 is the reference grade in this group. It contains 0.32 to 0.45 percent carbon, 4.75 to 5.5 percent chromium, 1.1 to 1.75 percent molybdenum and 0.8 to 1.2 percent vanadium. The low carbon content supports toughness, while the alloy content supports hardness at temperature. Applications include die casting dies, extrusion tooling and forging dies.

High speed steels apply the same principle to cutting tools. M-2 contains 4.5 to 5.5 percent molybdenum, 5.5 to 6.75 percent tungsten, 3.75 to 4.5 percent chromium and 1.6 to 2.2 percent vanadium. It retains hardness at the temperature reached by a cutting edge during high-speed operation. The M series is molybdenum based, while the T series is tungsten based. Highly alloyed M grades containing cobalt serve the most demanding cutting applications.

The P series and maraging steels fall outside these cold work, hot work and high speed divisions. P-series grades are plastic mould steels. P-20 contains 0.28 to 0.4 percent carbon and 1.4 to 2.0 percent chromium. It is the most used grade in the series and is supplied pre-hardened. This condition permits machining and polishing of a mould cavity without subsequent heat treatment. Maraging steels are not conventional tool steels. They contain around 18 percent nickel with cobalt and molybdenum, while carbon is limited to about 0.03 percent. They gain strength by ageing iron-nickel martensite rather than through carbon and undergo very little distortion during the process.

ASTM specifications and designations

Three ASTM specifications define the composition ranges associated with the letter groups. ASTM A681 covers alloy tool steels, including most A, D, H, O, S, P and L grades. ASTM A686 covers carbon tool steels in the W series. ASTM A600 covers high speed tool steels in the M and T series. Some alloy additions are optional within these specifications. For example, the L-6 entry permits 0.20 to 0.30 percent vanadium but does not require it. Two bars designated L-6 can therefore differ in behaviour.

Questions about tool steel grades

Tool steel equivalents across standards

Basis "verified" means every value in that row was checked against the governing standard on 2026-08-18; "inherited" means it came from the previous website and has not been checked. The complete mapping table is on the equivalents reference.

Where the tool steel family is not the right answer

Tool steel is unsuitable where corrosion resistance is required. High-chromium grades retain most of their chromium in carbides rather than in solution. They do not form a passive film, and hardened dies can rust. Martensitic stainless grades or precipitation hardening stainless grades are used when moulds or tools require corrosion resistance.

Tool steel is also unsuitable for structural or load-bearing parts. At working hardness, these steels have very little ductility. They fail by cracking rather than bending. Through-hardening alloy steels are intended for such applications.

Tool steel is unsuitable when a drawing does not specify hardening and tempering. The properties that distinguish the grades result from this treatment rather than from the untreated bar. Selection based only on maximum hardness is also unsuitable. Distortion during hardening and insufficient toughness in service cause more tool failures than wear.