M35 vs M42 – Composition, Heat Treatment, Properties, and Applications

Introduction

M35 and M42 are two widely used high-speed tool steels (HSS) that often appear as alternatives when engineers and procurement specialists specify cutting tools, drills, taps, and wear-resistant components. The selection dilemma typically centers on trade-offs among hot hardness and wear resistance versus toughness, machinability, and cost. In short, one grade emphasizes elevated-temperature hardness and wear resistance, while the other provides an advantageous balance of toughness and performance at lower cost.

Because both grades are members of the molybdenum/tungsten series of HSS and contain cobalt, they are compared often for demanding machining applications where tool life under heat and intermittent cutting matters. This article compares standards, composition, microstructure, heat-treatment behavior, mechanical properties, weldability, corrosion/surface protection, fabrication characteristics, typical applications, cost/availability, and final selection guidance.

1. Standards and Designations

• Common standards and designations for these grades:
• AISI/SAE: M35, M42 (often used in North America)
• DIN/EN: Often cross-referenced to HSxV or similar HSS codes (consult specific standard tables for exact cross-reference)
• JIS: Uses other HSS codes; check JIS G4305 and supplier literature
• GB (China): Listed under Chinese high-speed steels with local part numbers
• Classification:
• Both M35 and M42 are tool steels — specifically high-speed steels (HSS).
• They are alloy/tool steels rather than stainless steels or HSLA structural steels.

2. Chemical Composition and Alloying Strategy

The following table lists typical composition ranges for key elements. These are common manufacturing ranges used for specification and procurement; exact values vary by standard and producer.

How the alloying elements influence properties - Carbon: Establishes martensite hardness and carbide volume fraction; higher carbon increases as-quenched hardness but can reduce toughness and weldability. - Chromium: Adds temper resistance, hardenability, and corrosion resistance to a small extent. - Molybdenum and tungsten: Promote secondary hardening and hot hardness; molybdenum in M42 is especially high to support sustained hardness at elevated temperatures. - Vanadium: Forms stable vanadium carbides that improve wear resistance and edge retention. - Cobalt: Raises red (hot) hardness and thermal stability of the matrix; higher cobalt in M42 increases high-temperature performance at the expense of machinability and cost.

3. Microstructure and Heat Treatment Response

Typical microstructures - In both grades, the desired microstructure after appropriate heat treatment is a hard martensitic matrix containing a distribution of alloy carbides (Cr-rich, Mo/W-rich, and V-rich carbides). - M35: Martensite with uniformly distributed complex carbides (MC, M6C, M23C6). The presence of ~5% Co increases matrix stability and hot hardness without drastically changing carbide types versus M2/M35 family. - M42: Higher Mo and Co content increases the amount and stability of Mo-rich carbides and contributes to higher secondary-hardening response; the matrix resists tempering softening better at elevated temperatures.

Heat-treatment response - Normal sequence: preheat(s) → harden (high-temperature austenitizing) → quench (oil/air depending on size) → multi-stage tempering to achieve final hardness and toughness. - M35: Austenitize temperatures typically in the HSS range (e.g., roughly 1180–1220 °C, manufacturer-specific) followed by oil quench and tempering. Good combination of hardness and toughness when properly tempered. - M42: Requires careful austenitizing to dissolve requisite carbides for secondary hardening (temperatures are grade-specific). M42 benefits from controlled high-temperature tempering cycles to precipitate secondary carbides that confer exceptional hot hardness.

Thermo-mechanical processing - Forging and controlled rolling followed by proper normalization refine grain size and carbide distribution for both grades. M42’s higher alloy content benefits from microstructural control to avoid excessive retained austenite and to optimize carbide dispersion.

4. Mechanical Properties

Mechanical properties are strongly dependent on heat treatment and section size. The table below presents typical, heat-treated comparisons in qualitative and practical numeric ranges for hardness.

Interpretation - Strength and hot-strength: M42 typically delivers higher retained hardness and strength at elevated cutting temperatures due to higher Mo and Co and secondary carbide precipitation. - Toughness and ductility: M35 tends to be tougher and less brittle than M42 at similar hardness levels, making it preferable where shock or interrupted cuts are common. - Hardness: M42 generally achieves and holds higher hardness, particularly in high-temperature environments.

5. Weldability

Weldability of high-speed steels is limited compared with mild steels. Key factors are carbon content, hardenability, and microalloying.

Common weldability indices (illustrative): - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (for general weld cracking risk): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation - M35: Moderate carbon and moderate cobalt; preheating, controlled interpass temperatures, and post-weld heat treatment are required to avoid cold cracking and to restore toughness. Welding consumables and procedures tailored for HSS should be used. - M42: Higher carbon and higher alloy content (especially Mo and Co) increase hardenability and susceptibility to weld cracking and hard, brittle HAZ microstructures. Welding M42 is more challenging and often discouraged unless critical; if welding is required, strict preheat, low heat input, and tempering cycles are mandatory. - In both cases, brazing or mechanical joining are commonly preferred over fusion welding for critical HSS parts.

6. Corrosion and Surface Protection

• Neither M35 nor M42 is a stainless steel; corrosion resistance is modest and mainly derived from Cr content.
• Typical surface protection for non-stainless tool steels:
• Protective coatings: TiN, TiCN, AlTiN (physical vapor deposition) to improve wear and reduce adhesion.
• Chemical/galvanic coatings: not typical for cutting tools but possible for tooling fixtures—galvanizing is rarely used on HSS cutting tools.
• Paints and passivation: more applicable to housings and non-cutting components.
• PREN (pitting resistance equivalent number) is not applicable to these non-stainless tool steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• For cutting environments where corrosion may accelerate wear, specify corrosion-resistant coatings or consider stainless tool steels only where corrosion plus wear performance justifies the trade-offs.

7. Fabrication, Machinability, and Formability

• Machinability:
• M35: Machinability is moderate for HSS; presence of cobalt reduces machinability somewhat but provides toughness and hot hardness.
• M42: Lower machinability than M35 because of higher cobalt and molybdenum plus higher carbon—expect tougher cutting conditions for shaping before final heat treatment.
• Grinding and finishing: Both grades require diamond or CBN grinding for hardened-state finishing. M42’s hardness requires more robust abrasive systems and slower removal rates.
• Forming and bending: As tool steels, both are not readily cold-formed in the hardened state. Hot working or forging in annealed condition is common; final shaping usually completed before hardening.
• Surface finishing: Coatings (PVD/CVD) commonly applied post-heat-treatment to improve wear and lubricity.

8. Typical Applications

Selection rationale - Choose M35 when a balance of toughness, reasonable hot hardness, and lower cost is desired; ideal for interrupted cuts and applications where occasional shock loads occur. - Choose M42 when tool life at high cutting speeds and elevated temperatures is the priority, particularly for continuous high-speed machining of difficult materials or where regrinding frequency must be minimized.

9. Cost and Availability

• Cost: M42 is generally more expensive than M35 due to higher alloy content (notably cobalt and molybdenum) and more demanding processing. Expect higher per-kilogram and per-tool costs for M42.
• Availability: M35 is widely available in standard forms (round bars, flats, tool blanks). M42 is available but may have more limited stock sizes and lead times; many suppliers provide M42 primarily for high-performance tooling markets. Coated and finished tooling in both grades is common from specialized toolmakers.

10. Summary and Recommendation

Choose M35 if... - You need a cost-effective HSS with good toughness for interrupted cuts, tool life is important but not dominated by high-temperature wear, and moderate hot hardness suffices. - You anticipate welding, repair, or onsite modifications and want relatively easier post-weld restoration.

Choose M42 if... - Your primary requirement is superior edge retention and hot hardness at high cutting speeds or in heavy-duty production where elevated temperatures and abrasion dominate tool wear. - Longer in-service life between regrinds is critical and the budget supports a higher initial tooling cost.

Final practical note Specify the grade along with expected heat treatment condition, component section size, and intended application environment. For critical tools, ask suppliers for heat-treated samples, recommended temper cycles, and coating options. Such details often have proportionally larger influence on in-service performance than small composition differences alone.