Tool Steel

Tool steel is the high-carbon, alloyed steel of tools, dies and moulds, so named because it is the steel that is hardened to shape other materials. Where the carbon and alloy steels are the steels of structural parts, tool steel is the steel of the tooling itself — the die that stamps, the mould that shapes plastic, the punch that blanks — alloyed and heat-treated for the hardness, wear resistance and toughness such work demands. It is supplied soft, machined into shape, hardened and finished in its hard state, so a shop meets it as both the soft, machinable steel of the toolroom and the hard steel of the finished die. This entry sets out what tool steel is and how its common grades are machined and used.

The nature of tool steel

Tool steel is defined by what it becomes after hardening, and its chemistry is arranged for that end. It carries more carbon than structural steel — enough to harden to the levels a tool needs — and alloying elements chosen for the grade: chromium, vanadium, molybdenum, tungsten, each adding wear resistance, toughness or the ability to harden through the section. In its annealed supply state the steel is soft, so it can be machined, sawn and formed; it is then hardened by heat treatment — heated, quenched and tempered — to its working hardness, for the cold-work grades commonly around 58 to 62 HRC. The property that matters most is set by the grade: wear resistance for the die that must stamp a million parts, toughness for the tool that must take shocks, hot hardness for the red-hot die.

Machining tool steel before it is hard

The shop machines tool steel in its soft, annealed state, and the making of a die follows that fact. The steel arrives annealed — often spheroidised, its carbides gathered into spheres so it cuts easily — and in that state it is milled, turned, drilled and shaped like any medium carbon steel, though tougher and more abrasive, calling for sharp tools and firm workholding. The part is machined with allowance, because it will move a little in hardening and needs a finish in its hard state: the soft machining leaves stock for grinding. Once the part is hardened and tempered, it is finished by the processes that cut hard metal — grinding on the surface and cylindrical grinders, and for shaped profiles the spark erosion of EDM.

O1 and A2: the general cold-work steels

The cold-work tool steels are the everyday steels of dies and tools that work at room temperature, and O1 and A2 are its general grades. O1 is the oil-hardening steel: a plain, economical grade, quenched in oil to a hardness commonly in the high 50s and low 60s HRC, with good machinability annealed and good wear resistance. It is the general-purpose grade of the toolroom — the blanking die, the gauge, the small stamping tool. Its limit is that oil hardening distorts the part more than air hardening, so O1 suits tools simple enough to grind true afterwards. A2 is the air-hardening grade: alloyed with chromium and molybdenum, it hardens by cooling in air with less distortion than O1 at a similar working hardness, holding its size better through hardening. A2 is the modern general cold-work steel, chosen for the die and tool whose shape must not move, and for the longer-run stamping and forming tools.

D2: the wear-resistant die steel

Where a die must last, D2 is the steel, and its alloy is all about wear. D2 is the high-carbon, high-chromium cold-work steel — about 1.5 percent carbon and 12 percent chromium — whose hard chromium carbides give it wear resistance far beyond O1 or A2 over a long run. D2 hardens by air to the range of the other cold-work steels and keeps its edge and surface through long production runs, which is why it is the steel of the blanking, forming and trimming die that must run for tens of thousands of parts without re-sharpening. Its trade-offs mirror its wear resistance: D2 is harder to machine annealed than O1, more prone to distortion in hardening, and harder to grind in its hard state, so it is chosen where the run is long enough to justify its toolroom cost.

H13: the hot-work steel

When the tool works hot, H13 is the steel, and it is alloyed to keep its strength at temperature. H13 is the hot-work chromium-molybdenum-vanadium steel, the grade of the die-casting die and the forging die: tools repeatedly heated and quenched as they shape molten metal or hot stock, which must not soften, crack or wash away at the temperatures of their work. H13 is tempered to a hardness lower than the cold-work steels — commonly in the 40s and low 50s HRC, favouring toughness and resistance to heat-checking over wear resistance — and it keeps a useful hardness at red heat where O1 or D2 would soften. It is machined annealed like the others, and its dies are sunk by machining and finished by grinding and EDM in the hard state; its special property is that it does the whole job hot, casting and forging its thousands of parts while staying strong.

P20: the mould steel

For the moulds that shape plastic, P20 is the grade, and it is made to be machined as it is used. P20 is a chromium-molybdenum steel supplied pre-hardened — already heat-treated to a working hardness commonly in the range of 28 to 32 HRC — so the mould maker machines it directly to its finished shape, without a hardening operation afterwards. It is the steel of the injection mould: the cavity is machined, polished and textured in the pre-hardened steel, and the mould runs its production without further heat treatment, which would distort the cavity. Its moderate hardness is enough for the pressures and wear of plastic moulding, and its even, machinable structure takes the fine detail and mirror polish that mould cavities need. P20 is the mould maker’s steel: supplied ready, machined to the cavity and used as it comes, the quiet member of the family that does not need the furnace at all.

Choosing and using tool steel

Tool steel is chosen by the work the tool must do, and the choice follows the grade families. A tool that works cold and runs long needs wear resistance — D2; a tool that works cold and must hold its size through moderate runs needs the general grades — O1 or A2; a tool that works hot needs hot hardness and toughness — H13; a mould that must be machined ready needs pre-hardened steel — P20. In every case the shop’s relation to the steel is the same: machine it soft or pre-hardened, allow for its hardening if it is to be hardened, and finish it in the hard state by grinding and EDM, measuring its hardness on the scales this wiki treats. Tool steel is not a difficult material to machine in its working state — the shop simply does not machine it in that state, shaping the soft steel and finishing the hard.