High-Speed Steel Grades: M2, M35, M42 Composition and Cutting Performance

High-speed steels (HSS) are a family of highly alloyed tool steels whose defining characteristic is the ability to retain cutting hardness at the elevated temperatures generated during machining — a property termed red hardness or hot hardness. The three grades examined in depth here — M2, M35, and M42 — span the range from the universal general-purpose standard to cobalt-enhanced grades for difficult-to-machine materials, and together account for the overwhelming majority of HSS cutting tools in global production. Understanding their alloy chemistry, carbide microstructure, heat treatment responses, and performance envelopes is essential for any manufacturing metallurgist or tooling engineer specifying or applying these materials.

The Metallurgical Basis of High-Speed Steel Performance

High-speed steels are a subset of tool steels characterised by very high alloy content — primarily tungsten (W), molybdenum (Mo), chromium (Cr), vanadium (V), and, in cobalt grades, cobalt (Co) — combined with elevated carbon (0.80–1.10 wt%) to provide the carbon needed for carbide formation. The AISI designation system divides HSS into two families: the M-series (molybdenum base) and the T-series (tungsten base). M-series grades, particularly M2, have largely displaced T-series in modern practice due to lower raw material cost and equivalent or superior performance.

Carbide Types in High-Speed Steels

The microstructure of hardened and tempered HSS consists of a tempered martensite matrix containing four distinct carbide types, each with different crystal structure, composition, hardness, and role:

The distribution and volume fraction of these carbides depend critically on the austenitising temperature and time, which control how much carbide is dissolved into the matrix versus retained as undissolved primary particles. Undissolved primary carbides (primarily MC and some M₆C) provide wear resistance but do not contribute to secondary hardening or hot hardness. The dissolved carbide-formers (W, Mo, V, Cr, C) re-precipitate as fine secondary carbides during tempering, providing the secondary hardening peak that is the signature property of HSS. The martensite formation behaviour in HSS is complex due to the high alloy content stabilising austenite and generating substantial retained austenite fractions after quenching — a key reason triple tempering is necessary.

The Secondary Hardening Mechanism

Secondary hardening in HSS is the increase in hardness that occurs during tempering at 500–600°C after an initial decrease. It is caused by the precipitation of fine, coherent MC and M₂C carbides (nanometre-scale) within the martensite laths, driven by the high supersaturation of W, Mo, and V in the as-quenched martensite. These precipitates create a dense dispersion of obstacles to dislocation motion that raises yield strength and hardness significantly above the as-quenched matrix hardness contribution from martensite alone.

Secondary hardening peak location (approximate):
T_peak ≈ 540–560°C for M2, M35
T_peak ≈ 550–570°C for M42 (Co shifts peak higher)

Secondary hardening increment:
ΔHRC ≈ +4 to +8 HRC above the softening trough at 300-400°C

Retained austenite (as-quenched, typical):
M2:  15–25 vol%  (austenitised 1210°C)
M35: 18–28 vol%  (austenitised 1220°C)
M42: 20–30 vol%  (austenitised 1180–1210°C)

After triple temper at 560°C (3 x 1 hr):
Retained austenite: <3 vol% (stable for tool service)

Cobalt (Co) in M35 and M42 does not itself form carbides in HSS. Its role is to raise the A_c1 temperature (allowing higher austenitising temperatures without excessive grain growth), to increase the solubility of W and Mo in austenite at the austenitising temperature, and to slow the recovery of the dislocation substructure during service at elevated temperature. The net effect is a higher secondary hardening peak temperature and a greater retained hardness at 500–600°C, directly translating to improved tool life in high-speed cutting of difficult materials. Understanding the grain boundary phenomena underlying carbide precipitation and segregation is assisted by the Grain Boundaries Guide.

Chemical Composition of M2, M35, and M42

Source: ASTM A600 (Standard Specification for Tool Steel High Speed), current edition. The M2 and M35 grades differ only in cobalt content; the base W-Mo-Cr-V balance is identical. M42 is a distinct grade with significantly higher Mo and C and reduced W and V.

The Tungsten-Molybdenum Equivalence Principle

In HSS metallurgy, tungsten and molybdenum are largely interchangeable in their effects on hardenability, secondary hardening, and hot hardness, following an empirical equivalence of approximately 2 parts W = 1 part Mo by weight. This relationship arises from the similar atomic radii and carbide-forming tendencies of the two elements, and from the fact that both form M₆C carbides with similar thermodynamic stability. The M2 grade exemplifies this: its 6W-5Mo composition is approximately equivalent to a 6+10 = 16 “W-equivalent” content, closely matching the T1 grade (18W).

Tungsten-Equivalent (Weq) approximation:
Weq = W% + 2 × Mo%

M2:  Weq = 6 + 2×5 = 16  (vs T1 at 18W)
M35: Weq = 6 + 2×5 = 16  (same base, +5Co)
M42: Weq = 1.5 + 2×9.5 = 20.5 (highest Weq of the three)

Note: M42 achieves its highest hot hardness via high Weq + 8Co
despite lower nominal W content than M2.

The higher Mo content in M42 (9.0–10.0 wt%) versus M2 (4.5–5.5 wt%) is the primary driver of M42’s superior hot hardness. Mo is approximately twice as effective as W per unit weight in solid-solution strengthening of the martensite matrix, which directly contributes to elevated-temperature hardness retention beyond what cobalt alone would provide.

Heat Treatment of High-Speed Steels

Preheating

Due to the low thermal conductivity of HSS (approximately 20–25 W/m·K, compared to 50 W/m·K for carbon steel) and the extreme austenitising temperatures required, direct charging into a hot furnace creates massive thermal gradients that cause cracking in all but the simplest geometries. A two-stage preheat is mandatory:

First preheat: 450–500°C, hold 20–30 min per 25 mm of section. This removes residual stresses from machining or prior operations and allows uniform heating to begin.

Second preheat: 850–870°C (slightly above A_c1), hold 20–30 min. At this temperature carbide dissolution begins slowly and the piece reaches thermal uniformity before the final rapid transfer to the hardening furnace.

Austenitising (Hardening Temperature)

Quenching

HSS tools are quenched by one of three methods, in order of decreasing cooling rate:

Salt bath quench (martempering): The most common method for complex-geometry tools. The workpiece is transferred to a salt bath at 500–600°C (above M_s), held for temperature equalisation (typically 3–8 min), then air cooled to room temperature. This interrupted quench minimises distortion and thermal shock while achieving sufficient cooling rate to suppress pearlite and bainite formation. Given the deep hardenability of HSS, the cooling rate in a salt bath at 550°C is adequate for sections up to 75–100 mm.

Oil quench: Used for simpler geometries and smaller sections. Faster cooling rate than salt bath; higher distortion and cracking risk for complex tools.

Air quench: Used for very large sections or intricate thin-wall tools where oil and salt bath risks are unacceptable. Adequate due to very high hardenability of HSS.

Triple Tempering

Every HSS grade must receive a minimum of three separate tempering cycles. This is not optional — single or double tempering leaves unacceptable levels of retained austenite and untempered fresh martensite that will cause dimensional instability and brittle failure. Each temper is typically 1 hour at temperature plus adequate heating time.

The cryogenic treatment of HSS tools (cooling to −70 to −196°C after quench, before tempering) is practiced by some manufacturers to more completely convert retained austenite and improve dimensional stability. For tools with stringent dimensional tolerances, cryogenic treatment between the quench and first temper can reduce retained austenite to <1 vol%, significantly improving repeat grinding accuracy over tool life.

Mechanical Properties and Performance Data

The inverse relationship between hardness and toughness is clearly visible: M42, with the highest hardness, has the lowest transverse rupture strength and Charpy toughness. This is why M42 is unsuitable for heavily interrupted cuts (milling hard materials, broaching) in the conventionally produced form — powder metallurgy M42 partially recovers toughness through finer carbide distribution and eliminates the large primary carbide clusters that act as fracture initiation sites in wrought M42.

The hardness testing methods article details the Vickers, Rockwell, and microhardness scales used for quality verification of HSS tools, including the conversion tables applicable when comparing HRC values from hot hardness testing to room-temperature HRC values. Impact testing methodology for tool steels is covered in the Charpy impact test article.

Grade-by-Grade Technical Profile

M2: The Universal High-Speed Steel

M2 is the most widely produced and specified HSS grade globally, accounting for approximately 80% of all HSS tool production by volume. Its balanced 6W-5Mo-4Cr-2V chemistry delivers a combination of hot hardness, wear resistance, grindability, and toughness that no single alternative grade matches across the full range of cutting applications. M2 is the reference against which all other HSS grades are judged.

The 2 wt% vanadium content in M2 provides abrasion resistance through MC carbide particles while remaining grindable with conventional aluminium oxide (Al₂O₃) grinding wheels. Higher vanadium grades (M3 type 2 at 3 wt% V; T15 at 5 wt% V) offer superior wear resistance but mandate CBN or SiC grinding wheels and command substantially higher grinding costs in tool manufacture and resharpening.

M2 Typical Applications

Twist drills, jobber drills, step drills, taps (all sizes), reamers (hand and machine), end mills, slot drills, form tools, milling cutters (plain and side-and-face), gear hobs, broaches, saw blades, and threading dies. M2 is the standard specification for drills and taps per ISO 521, ISO 529, and DIN 338/340.

M35: Cobalt-Enhanced General Purpose Grade

M35 is produced by adding 4.5–5.5 wt% cobalt to the M2 base composition. The metallurgical effects are: (1) the A_c1 temperature rises by approximately 30°C, permitting slightly higher austenitising temperatures and greater carbide dissolution without grain growth penalty; (2) the martensite M_s temperature is marginally lowered, slightly increasing retained austenite in the as-quenched condition; (3) the secondary hardening peak temperature rises by approximately 10°C; and (4) the hot hardness at 540°C improves by approximately 2–3 HRC equivalent over M2 of the same base heat treatment.

M35 is marketed under various proprietary names: Cobalt HSS, HSCo, HSS-Co5, Cobalt 5, and Gold Point in different markets. The gold or yellow titanium nitride (TiN) PVD coating commonly applied to cobalt HSS tools serves both as a performance coating (reducing friction and improving chip flow) and as a visual identifier — though the coating composition varies by supplier and should not be relied upon to identify the base steel grade.

M35 Typical Applications

Drills and taps for stainless steels, heat-resistant alloys, titanium, copper alloys, and hardened steels up to 42 HRC. End mills for stainless and hard steels. Reamers for difficult-to-machine materials. M35 is specified when M2 tools show accelerated flank wear or edge breakdown under the conditions described — it is not a universal upgrade, as the marginal cost premium is not justified for standard carbon steel machining.

M42: High-Cobalt Super High-Speed Steel

M42 is a fundamentally different alloy design from M2/M35: higher C (1.05–1.15 wt%), dramatically higher Mo (9–10 wt%), reduced W (1.15–1.85 wt%), reduced V (0.95–1.35 wt%), and 8 wt% Co. This chemistry maximises the tungsten-equivalent and cobalt content to achieve the highest secondary hardening and hot hardness of any standard HSS grade, at the cost of reduced toughness and grindability.

The high Mo content of M42 creates a predominantly M₆C carbide microstructure based on (Fe,Mo)₆C and (Fe,W,Mo)₆C rather than the (Fe,W)₆C typical of W-heavy grades. These Mo-rich M₆C carbides dissolve slightly more easily than W-rich M₆C at the austenitising temperature, providing better matrix enrichment per unit temperature — which is why M42 can be austenitised at the lower end of the HSS range (1180–1210°C) while still achieving exceptional hot hardness.

M42 Typical Applications

End mills, drills, and taps for nickel-based superalloys (Inconel 718, Waspaloy, Hastelloy), titanium alloys (Ti-6Al-4V), hardened steels (35–55 HRC), cobalt alloys, and hard cast irons. Saw blades for aerospace structural materials. Form tools for interrupted cuts in hard steels where controlled chip load and sharp edge retention are critical. PM-M42 is strongly preferred over wrought M42 for these applications.

Powder Metallurgy HSS

Conventional wrought HSS is produced by arc or induction melting followed by ingot casting, homogenisation, and multi-step hot working. During ingot solidification, carbides segregate to interdendritic regions, forming large, unevenly distributed primary carbide particles and bands visible in the final wrought product. These carbide bands reduce transverse properties, create preferential fracture paths, and cause non-uniform hardness after heat treatment.

PM-HSS eliminates these problems: the liquid metal is gas-atomised into spherical powder particles (typically 50–150 μm), each solidifying as a microsegregation-free rapidly-solidified droplet. The powder is consolidated by hot isostatic pressing (HIP) at 1100–1150°C into fully dense billets with carbide particle sizes of 1–3 μm uniformly distributed throughout the matrix, compared to 5–30 μm carbide clusters in wrought material.

The practical benefits of PM-HSS include: transverse rupture strength 20–40% higher than wrought equivalents; grinding ratio (material removed per unit wheel wear) 30–60% improved for vanadium-rich grades; ability to incorporate alloying additions beyond the segregation limits of conventional casting (e.g., T15 at 5 wt% V, ASP2052 at 8 wt% V). PM grades are designated by prefix in some systems (CPM-M4, CPM-Rex, ASP series) and carry a 3–5× price premium over wrought HSS that is justified in high-value tooling applications.

Cutting Tool Coatings on HSS

Physical vapour deposition (PVD) coatings are routinely applied to HSS tools to extend tool life, reduce cutting forces, and improve chip control. Coatings must be applied below the HSS tempering temperature to avoid softening the substrate:

Coating adhesion to HSS depends critically on surface preparation (grinding, polishing, degreasing) and the substrate hardness — fully hardened M42 at 69 HRC provides better coating support than under-hardened M2 at 58 HRC. The coating layer (2–5 μm typical) does not change the base metal properties but significantly reduces the coefficient of friction at the tool-chip interface, reducing cutting temperatures and extending the regime within which HSS outperforms uncoated carbide on grounds of edge toughness and cost.

International Equivalents and Standard Cross-Reference

The EN HS designation encodes W-Mo-V-Co content in sequence. HS6-5-2 = 6W-5Mo-2V; HS6-5-2-5 = 6W-5Mo-2V-5Co; HS2-9-1-8 = 2W-9Mo-1V-8Co. This naming system is immediately informative for composition identification, in contrast to the AISI M/T numbering system which conveys no compositional information directly. Correlation with the iron-carbon phase diagram fundamentals helps understand why the higher alloy content of HSS shifts the eutectoid to lower carbon and why the large primary carbide volume fraction in HSS (20–30 vol%) places the alloy composition well above the hypereutectoid region of the eutectoid reaction baseline. The pearlite colony growth and bainite microstructure articles provide context on the transformation products that must be suppressed during HSS quenching. The annealing and normalising article covers the softening treatments applied to HSS bar stock before machining into tool blanks.