Tool steel is a family of high-carbon, high-alloy steels designed for cutting tools, dies, moulds and wear parts. Its superior hardness and wear resistance are exactly what makes it one of the most difficult materials to cut, drill, mill or saw. The difficulty is not a manufacturing defect; it is a direct consequence of the chemistry and heat treatment that give tool steel its working performance.
Why Tool Steel Resists Cutting
Four factors combine to make tool steel hard to machine:
High carbon content. Most tool steels contain 0.5% to 1.5% carbon, and some cold-work grades exceed 2.0%. Carbon is what allows the steel to reach 58-65 HRC after hardening, and it also forms the hard carbide particles that resist the cutting edge.
Strong carbide formers. Chromium, molybdenum, vanadium and tungsten are added to increase hardenability, wear resistance and temper resistance. Vanadium and tungsten carbides are harder than the cutting edges of ordinary high speed steel tooling.
Delivery condition. The same grade can be soft and workable or extremely difficult to cut depending on whether it is supplied annealed or hardened. Machining cost can differ by a factor of three or more between the two conditions.
Heat treatment response. Quenching and tempering, not the base chemistry alone, generate the high hardness and the residual stresses that blunt tools and distort thin sections.
The table below summarises the common AISI/SAE tool steel families covered by ASTM A681 and the hardness levels normally encountered in supply and in service. Values are typical ranges and are quoted for guidance when planning cutting operations.
| Family | Typical grades | Annealed hardness (typical) | Working hardness (typical) |
|---|---|---|---|
| Water-hardening | W1, W2 | 170-220 HB | 60-66 HRC |
| Oil-hardening | O1, O2 | 190-220 HB | 57-62 HRC |
| Air-hardening, medium alloy | A2, A6 | 210-240 HB | 57-62 HRC |
| High-carbon, high-chromium cold work | D2, D3 | 210-255 HB | 58-62 HRC |
| Shock-resisting | S1, S7 | 190-230 HB | 54-58 HRC |
| Hot-work chromium | H11, H13 | 190-230 HB | 48-52 HRC |
| Molybdenum high speed | M2, M35 | 220-250 HB | 62-65 HRC |
All figures refer to the steel in the annealed, spheroidised condition for the annealed column. Grades with higher alloy content and more massive carbide networks, such as the D and M families, are consistently the most difficult to cut.
Annealed Versus Hardened Condition
In the annealed condition the structure is a soft ferrite matrix carrying spheroidised carbide particles. This condition is specifically produced so that the material can be machined, and it can normally be turned, milled and drilled with coated carbide tooling at moderate speeds. Hardness in this state usually falls between 170 HB and 255 HB, which places tool steel in the same broad machining category as a low-alloy engineering steel rather than an exotic alloy.
Once the part is hardened, the situation changes completely. Hardened tool steel commonly sits between 45 HRC and 65 HRC. At these levels conventional high speed steel tooling fails within seconds, and cutting is only practical with tooling whose hardness exceeds that of the workpiece. Red hardness also becomes a consideration for high speed steel grades such as M2 or T1, which retain useful hardness at temperatures approaching 600 degrees Celsius and therefore resist the softening effect of cutting heat.
Cutting and Machining Methods That Work
Effective processing of tool steel follows a simple principle: perform as much material removal as possible before hardening, then use the correct technology for the remaining stock.
Annealed machining. Turning, milling, drilling and tapping with TiAlN-coated carbide inserts, rigid fixturing, positive rake geometry and generous coolant supply. Take substantial feeds rather than dwelling in the cut to avoid surface work hardening.
Turning and milling above 45 HRC. Cubic boron nitride (CBN) inserts, or ceramic inserts for continuous cuts at high surface speed with small depth of cut. Machine rigidity is the limiting factor more often than insert grade.
Wire electrical discharge machining (WEDM). The standard route for hardened dies, punches and complex profiles. It cuts any hardness level and leaves a heat-affected recast layer that must be removed by a finishing skim pass where fatigue or impact performance matters.
Sinker EDM. Used for blind cavities, sharp internal corners and deep ribs in hardened moulds and dies, with graphite or copper electrodes.
Abrasive waterjet. Cold cutting process with no heat-affected zone; suitable for plate, but taper increases with thickness.
Laser cutting. Practical on thinner sections and in the annealed condition; edge quality and dross control become difficult as thickness and hardness increase.
Grinding, sawing and abrasive cut-off. Surface and cylindrical grinding provide the final dimensions and finish after hardening; aluminium oxide wheels suit annealed and low-alloy grades, while cubic boron nitride wheels are preferred for high speed steel and hardened parts.
Practical Parameter Guidance
The following starting ranges are commonly used in production. They should always be verified against the actual grade, hardness and machine capability.
| Operation | Tooling | Typical starting condition |
|---|---|---|
| Turning, annealed alloy tool steel | Coated carbide (P25-P35) | 60-120 m/min, feed 0.2-0.3 mm/rev |
| Milling, annealed cold work steel | Coated carbide, positive rake | 80-150 m/min, climb milling, air or flood coolant |
| Turning, hardened 45-60 HRC | Mixed ceramic or CBN | 60-150 m/min, depth of cut below 1 mm |
| Milling, hardened above 55 HRC | Solid CBN or ceramic | High speed, shallow passes, rigid setup |
| Profile cutting, hardened | WEDM | Rough pass plus two or more skim passes |
Choosing the Right Supply Condition
The cheapest way to deal with the cutting difficulty of tool steel is to plan it into the order. Three approaches are common in industrial practice:
Buy annealed. Rough machine with a 0.3-0.5 mm allowance, harden and temper, then finish by grinding or EDM. This is the conventional route for dies and punches.
Buy pre-hardened. Pre-hardened mould steels at approximately 28-34 HRC can be machined directly with carbide tooling and then polished, avoiding a second heat treatment and the distortion that comes with it.
Buy near net shape. Plate, bar or hollow sections cut close to final size reduce the volume of difficult material that must be removed at all.
Distortion control matters as much as cutting speed. Because tool steel hardens through the section, quenching generates internal stresses that can move dimensions long after machining is complete. Stress-relieving before finish machining, and in some cases a double temper, is standard practice for precision components.
Frequently Asked Questions
Q: Is tool steel harder to cut than mild steel?
Yes. Even in the annealed condition, tool steel has higher hardness, more carbides and greater work-hardening tendency than mild steel, so cutting speeds are lower and tool wear is higher. After hardening, mild steel tooling cannot cut it at all.
Q: Can fully hardened tool steel be machined?
Yes, but only with tooling or processes that do not depend on a sharp steel edge, such as CBN and ceramic inserts, grinding, electrical discharge machining or abrasive waterjet cutting.
Q: Should tool steel be annealed before machining?
Whenever the geometry allows. Annealing to a spheroidised structure at 170-255 HB reduces cutting forces, improves surface finish and lengthens tool life substantially compared with machining a hardened or partly hardened part.
Q: Which tool steel grades are the easiest to machine?
The lower-alloy oil-hardening and shock-resisting grades, such as O1 and S7, are generally the most forgiving. The high-carbon high-chromium cold work grades and the high speed steel grades are the most demanding because of their carbide content and working hardness.
Q: Does the cutting method affect final tool performance?
It can. Thermal cutting and EDM leave a recast or oxidised layer that may crack under impact loading, so a skim pass, grinding or stress relief is recommended on functional surfaces after thermal and electrical processes.
Q: Why does quenching make tool steel so hard?
Heating above the transformation temperature dissolves carbon and alloying elements, and rapid cooling traps them in a distorted martensitic structure. This structure, refined by tempering, produces the hardness, wear resistance and dimensional stability expected of a finished tool.