Selection of Tool Steel for Hobs and Broaches
Hobs are rotary cutting tools used to generate gear teeth and other repeated forms, while broaches are linear multi-tooth tools that remove material in a single pass by means of progressively higher teeth. Despite this difference in motion, both are cutting tools that work under high localized stress, high friction, and high edge temperatures.
During cutting, plastic deformation in the shear zone and friction at the tool-chip interface generate substantial heat, and edge temperatures can approach 1000 °C in demanding operations. Broaches also carry high structural loads because many teeth engage sequentially, and the tool body must transmit large pulling or pushing forces, sometimes approaching 0.9 MN. Under these conditions, the main failure modes are crater wear, flank wear, edge chipping, depth-of-cut notching, and thermal softening. Tool steel selection, therefore, depends on which failure mode limits tool life first.
Selection Factors
For hobs, the main requirement is resistance to progressive wear. Because hobbing depends on maintaining tooth profile accuracy over long production runs, flank wear and crater wear usually matter more than impact resistance. In practical terms, this means wear resistance and hot hardness are normally more important than maximum toughness.
For broaches, the balance is tighter. Broaching teeth must resist wear, but they also face higher section loads, vibration, and the risk of local overload during entry or irregular cutting. If hardness is pushed too far without enough support from toughness, tooth chipping or breakage becomes a practical failure mode. For this reason, broach steels must combine good wear resistance with enough edge strength to survive real cutting loads.
Hot hardness becomes more important as the cutting temperature rises. When machining alloy steels, hard cast irons, or heat-resistant alloys, a steel that loses hardness at elevated temperatures will wear rapidly, even if its room-temperature hardness is high. In those cases, the upgrade path is usually driven by thermal softening rather than simple abrasion.
Recommended Tool Steels
AISI M2 Tool Steel | 1.3343 | SKH51
M2 remains the standard starting point for both hobs and broaches because it offers a practical balance of hardness, toughness, availability, and manufacturing cost. It is suitable when cutting conditions are stable, and the main target is dependable performance rather than maximum tool life.
Typical cutting hardness is 64–66 HRC. At this level, M2 provides adequate wear resistance for many general hobbing and broaching applications while still retaining enough toughness for serviceable edge strength and tool-body reliability.
Its limitation appears when wear becomes too rapid or when cutting temperature rises beyond what standard HSS can support efficiently. In that situation, M2 is usually replaced not because it fails immediately, but because it wears out too quickly to remain economical.
AISI M42 Tool Steel | 1.3247 | SKH59
M42 is used when hot hardness is the main requirement. With high cobalt content and hardness up to 69 HRC, it is well-suited to difficult-to-machine alloys, especially heat-resistant and high-strength materials, where cutting heat is the main cause of rapid edge deterioration.
Compared with M2, M42 offers better resistance to thermal softening. However, it does not provide the same margin of toughness, so it performs best under stable cutting conditions with limited shock and vibration and good alignment.
In practical selection terms, M42 is usually preferred when the main problem is loss of edge strength at temperature rather than tooth breakage from mechanical overload.
M3 (Class 2) and M4: Enhanced Wear Resistance
M3 Class 2 and M4 are logical upgrades when M2 loses edge accuracy too quickly through flank wear or crater wear. Their higher carbon and vanadium contents increase carbide volume and improve wear resistance, which makes them useful for longer production runs and more abrasive work materials.
M4 is especially relevant where wear is the dominant life-limiting factor and process stability is good. The gain is better resistance to edge rounding and abrasive loss, but the trade-off is reduced toughness compared with M2.
Powder metallurgy versions strengthen the case for these grades. Their finer and more uniform carbide distribution improves structural reliability and reduces the weakness associated with coarse carbide segregation. This is particularly valuable in large broaches, where both wear resistance and internal soundness matter.
T15: Premium Hot Hardness and Abrasion Resistance
T15 is selected when both abrasion resistance and hot hardness must be exceeded by M2 and M4. Its high tungsten, cobalt, and vanadium content makes it suitable for cutting high-tensile steels, hard cast irons, and aerospace alloys that generate high edge temperatures.
At 66–68 HRC, T15 can maintain cutting performance in applications where standard HSS grades soften too quickly. Its strength is not just high hardness, but the ability to retain that hardness better under heat.
Its limitation is that the grade is more difficult and costly to grind and process. That means it is justified when thermal load and abrasive wear are severe enough to shorten the life of M2 or M4, not when the job is simply a routine general-purpose cut.
ASP 30: High-Performance Powder Metallurgy Steel
ASP 30 is a premium powder metallurgy steel for severe service where both wear resistance and chipping resistance must be improved together. Its P/M structure minimizes carbide segregation and provides a more uniform microstructure, which supports better edge stability than conventional ingot HSS at similar hardness levels.
This makes ASP 30 suitable for demanding hobbing and broaching operations where standard high-speed steels suffer from combined wear and micro-chipping. In those applications, the value of ASP 30 comes not only from longer life, but also from more predictable performance and reduced risk of unstable edge failure.
Because of its cost, ASP 30 is generally justified where tooling downtime, regrinding frequency, or premature tool loss has a clear production penalty. It is not the default choice for routine work; it is the solution for operations where conventional HSS grades no longer provide stable economics.
Summary Table
| Tool Steel Grade | Typical Hardness | Primary Advantage |
| M2 | 64–66 HRC | Best general balance of toughness, wear resistance, and cost |
| M3 (Cl 2) / M4 | 64–67 HRC | Higher wear resistance for longer runs and more abrasive conditions |
| T15 | 66–68 HRC | Strong hot hardness and abrasion resistance for high-temperature cutting |
| M42 | Up to 69 HRC | High hot hardness for heat-resistant and high-strength alloys |
| ASP 30 (P/M) | 65–67 HRC | Premium wear and chipping resistance with improved structural uniformity |