Tool Steels

Tool Steels' defining properties include resistance to wear, stability during heat treatment, strength at high temperatures, and toughness. To develop these properties, tool steels are always heat treated. Because the parts may distort during heat treatment, precision parts should be semifinished, heat treated, then finished. Tool steels are classified into several broad groups, some of which are further divided into subgroups according to alloy composition, hardenability, or mechanical similarities.

• Type W - Water-hardening, or carbon, tool steels rely on carbon content for their useful properties.

• Type S - Shock-resisting tool steels are strong and tough, but not as wear resistant as many other tool steels.

• Types O, A, and D Cold-work tool steels include oil and air-hardened types are often more costly but can be quenched less drastically than water-hardening types. Type O steels are oil hardening; Type A and D steels are air hardening (the least severe quench), and are best suited for applications such as machine ways, brick mold liners, and fuel-injector nozzles. The air-hardening types are specified for thin parts or parts with severe changes in cross section -- parts that are prone to crack or distort during hardening. Hardened parts from these steels have a high surface hardness; however, these steels should not be specified for service at elevated temperatures.

• Type H - Hot-work steels serve well at elevated temperatures.

• Types T (tungsten alloy) and M (molybdenum alloy) - High-speed tool steels make good cutting tools because they resist softening and maintain a sharp cutting edge at high service temperatures.

• Type L - A special-purpose, low-cost, low-alloy, tool steel often specified for machine parts when wear resistance combined with toughness is important.

• Type F - Carbon-tungsten alloys (Type F) are shallow hardening and wear resistant, but are not suited for high temperatures or for shock service.

• Type P - A mold steel are designed specifically for plastic-molding and zinc die-casting dies.