Tool and die steel

Family overview

Tool and die steels are used for tools and dies that cut, machine or press other materials into shape. This page covers cold-work alloy tool steel (SKD11) and high-speed tool steel (SKH51, SKH55). Cold-work die steel is alloyed to resist wear and compressive deformation at around room temperature, and high-speed steel is alloyed to keep its hardness when cutting heat raises the temperature of the cutting edge. The two groups emphasize different properties and are treated as separate families, but both can be candidates where applications overlap, such as cold-forming punches.

In both groups, hardening and tempering produce a hard martensitic matrix, and carbides of chromium, tungsten, molybdenum and vanadium dispersed in it provide wear resistance. More carbide generally helps against wear but lowers toughness and makes machining and grinding harder. The grade is therefore often chosen after confirming whether the tool or die reaches the end of its life through wear, through chipping (broken edges) or cracking, or through softening under heat. A hardness figure alone is not enough to judge this balance.

Tool steel as bought is in a different condition from tool steel in service. The bar is used in an annealed, soft condition so that it can be machined, and the working hardness is produced afterwards by heat treatment. The hardness of the annealed bar and the working hardness of the tool or die in use are therefore separate values, and the working hardness depends on the heat-treatment conditions.

Role of the main elements

• Carbon (C): Sets the hardness of the martensitic matrix and combines with chromium, tungsten, molybdenum and vanadium to form carbides. Both groups are higher in carbon than structural steels, and as carbide increases, wear resistance improves while toughness tends to fall.
• Chromium (Cr): In SKD11 it forms a large amount of chromium-based carbide, which raises wear resistance and hardenability. Much of the chromium is tied up in carbides, however, so the steel is not rust-free like stainless steel, and rust protection is often applied in storage and use. In high-speed steels its main role is to secure hardenability.
• Molybdenum (Mo): Raises hardenability and resistance to temper softening (hardness does not fall easily even after tempering). In high-speed steels it forms, together with tungsten, the carbides that sustain hot hardness, and it can replace part of the tungsten.
• Tungsten (W): Sustains hot hardness in high-speed steels. It dissolves into the matrix when the steel is heated to a high temperature, precipitates as fine carbide during tempering to cause secondary hardening, and part of it remains as hard carbide that contributes to wear resistance.
• Vanadium (V): Forms hard vanadium-based carbide that raises wear resistance and restrains grain coarsening. In exchange, grinding tends to become harder.
• Cobalt (Co): In cobalt-added high-speed steels such as SKH55 it acts to raise hot hardness. Toughness tends to fall in exchange, so this is weighed together with the chipping risk of the tool.

Heat treatment and delivery condition

• Delivery condition: Tool steel bars are commonly machined in the annealed condition. The surface condition (machined or ground surface and so on) and whether the bar is cut to length change the machining allowance and the operations that follow, so the delivery condition is specified together with the grade. The hardness limit of the annealed condition is confirmed against the applicable standard.
• SKD11 hardening and tempering: Its high hardenability lets thick sections harden with relatively mild cooling, and it is known for small distortion on hardening. The tempering conditions change the balance of hardness, toughness and dimensional stability, and tempering is sometimes repeated, or sub-zero treatment added, to reduce the retained austenite left after hardening.
• High-speed steel hardening and tempering: The steel is heated to a high temperature to dissolve a large part of the alloy carbides into the matrix, cooled, and then tempered several times. Hardness is secured by secondary hardening, in which fine carbides precipitate during tempering, and repeated tempering reduces retained austenite.
• Decarburization and surface: If carbon leaves the surface during heating to a high temperature (decarburization), surface hardness drops and tool life can be shortened. The decarburized layer is often removed within the machining allowance, and during heat treatment it is limited by vacuum heating or atmosphere control.
• Distortion and machining allowance: Heat treatment changes size and shape, so the stock carries a machining allowance and the dimensions are finished by grinding or electrical discharge machining after heat treatment. A size tolerance alone does not control form such as straightness or roundness, so the required form conditions are specified separately in the order.
• Structure and carbide distribution: Coarse or clustered carbides can lower toughness and grindability. This tendency is stronger in bars with a large cross-section, so the structure (carbide distribution) is sometimes made an inspection item.

Grades

Composition range

Confirm the composition range against the applicable standard.

Typical applications

• Cold-forming and shearing dies (SKD11): press punches and dies, blanking, piercing and trimming dies, and cold-forging and heading dies, which cut or press material into shape at around room temperature
• Wear-resistant rolls, blades and parts (SKD11): forming rolls, slitter knives, gauges and guides, where wear decides service life
• Cutting tools (SKH51, SKH55): drills, taps, reamers, end mills, milling cutters and broaches, whose cutting edges become hot
• Punches and heading and drawing dies (SKH51, SKH55): cold-forming punches, heading dies and drawing dies that take wear and impact together
• Cutting that needs hot hardness (SKH55): tools whose cutting edge becomes hot because the cutting speed is high or the workpiece is hard to machine

Products covering these grades

• High-speed steel (HSS) rodSKH51 · SKH55 · M2 · M35
• Die & tool steelSKD11