Tool steels are specialized, high-quality carbon and alloy steels engineered to withstand the extreme demands of manufacturing. Their unique properties, such as hardness, resistance to abrasion, and the ability to maintain shape under high temperatures, make them indispensable for creating tools and dies used in cutting, forming, and shaping other materials. The American Iron and Steel Institute (AISI) classification system organizes these materials into six primary categories based on their chemical composition, heat treatment requirements, and specific application, tailored for optimal performance in environments ranging from ambient temperature stamping to high-speed machining.
Water-Hardening and Shock-Resisting Steels
Water-hardening steels represent the most basic and economical category of tool steels. They require rapid quenching in water to achieve maximum hardness, resulting in a Rockwell C hardness value in the low-to-mid 60s. This rapid cooling creates internal stresses that can lead to distortion or cracking, especially in complex shapes, and the steel only hardens on the surface for thicker sections. W-series steels are restricted to lower-temperature applications like hand tools, reamers, and cutlery, as they are prone to softening when exposed to moderate heat.
Shock-resisting steels are designed to absorb repeated impact without fracturing. These steels feature a lower carbon content than the water-hardening types, trading some maximum hardness for improved toughness. Alloying elements like vanadium, molybdenum, and silicon are added to enhance impact resistance. The S-series is the preferred choice for tools such as chisels, punches, and jackhammer bits, which must withstand sudden, high-stress loads in non-continuous production settings.
Cold-Work Steels
Cold-work steels are formulated for operations where the tool temperature remains below approximately 400°F, such as stamping, blanking, and cold-forming. This category is subdivided into three types, distinguished by their hardening process and alloy content, which determines their dimensional stability and wear resistance. Oil-hardening steels (O-series) use an oil quench during heat treatment, minimizing distortion and cracking risks, making them suitable for blanking dies and gauges.
Air-hardening steels, designated as the A-series, contain higher alloy levels, including chromium and molybdenum, allowing them to be hardened by cooling in still air. This slower cooling rate provides superior dimensional stability, meaning the tool maintains its precise shape after heat treatment with minimal warpage. These characteristics make A-series steels ideal for long-run dies and trimming dies where high accuracy is crucial.
The D-series steels feature the highest carbon and chromium content, often around 10% to 13% chromium. This high alloy content promotes the formation of hard, abrasive-resistant carbides, giving them exceptional wear resistance suited for severe abrasion applications like thread rolling and shear blades. Their complex composition makes them more challenging to machine compared to the O- or A-series.
Hot-Work Steels
Hot-work steels are engineered to withstand high temperatures, thermal shock, and abrasive wear encountered in processes like forging, extrusion, and die casting. These H-series steels must maintain strength and hardness above 600°F, a property known as hot hardness or red hardness. Their composition includes low carbon content for increased toughness, balanced with significant amounts of chromium, tungsten, or molybdenum to resist softening.
The H-series is chemically grouped based on its primary alloying element. All types are designed to resist thermal fatigue (heat checking). Chromium-based hot-work steels are used for die casting aluminum and magnesium due to their balance of toughness and resistance to heat checking. Tungsten and molybdenum-based types offer superior hot hardness and are employed in applications requiring continuous strength at the highest working temperatures, such as hot extrusion tooling and heavy-duty forging dies.
High-Speed Steels
High-speed steels (HSS) are designed for continuous, high-rate material removal operations where cutting friction generates intense heat. The defining characteristic of HSS is its ability to retain a sharp cutting edge and high hardness (typically 62–65 Rockwell C) even when the tip glows red-hot, a property termed red hardness. This capability allows for significantly faster cutting speeds than other tool steel grades.
The T-series is based on tungsten as the primary alloying element, with T1 being the original standard HSS containing approximately 18% tungsten. The M-series, or molybdenum-based HSS, has largely replaced the T-series due to its lower cost and often superior performance, offering a better balance of toughness and red hardness. Molybdenum steels, such as M2, are now the most common choice for manufacturing drills, taps, end mills, and other cutting tools used in high-production machining.
Special Purpose and Mold Steels
Mold steels, designated as the P-series, are tailored for plastic injection molding and zinc die casting applications. These steels require high polishability to achieve smooth part surfaces and corrosion resistance to handle various plastic compounds and cooling systems. P-series steels are often supplied in a pre-hardened condition and may be surface-treated through carburization to enhance wear resistance.
The L-series and F-series steels serve specialized functions. L-series steels are low-alloy types formulated for applications demanding exceptional deep hardening capability and high toughness, like certain aircraft parts. The F-series comprises carbon-tungsten steels that are water-hardened but contain alloying additions that provide improved wear resistance compared to the basic water-hardening steels.