M2 vs M35 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
M2 and M35 are two widely used high-speed tool steels (HSS) encountered frequently in the selection process for cutting tools, punches, dies, and wear components. Engineers, procurement managers, and manufacturing planners routinely weigh trade-offs such as cost versus hot hardness, wear resistance versus toughness, and machinability versus service life when choosing between these grades. Typical decision contexts include selecting a tool steel for high-speed milling where red-hardness matters, or specifying a blank for high-volume stamping where cost and toughness govern.
The principal technical distinction between the two grades is that one is a conventional tungsten–molybdenum HSS while the other is a similar base alloy modified by a meaningful addition of cobalt to enhance retained hardness and wear resistance at elevated temperatures. Because M2 and M35 share much of their carbide chemistry and heat-treatment practice, they are commonly compared when specifying tools subject to abrasive wear and high cutting temperatures.
1. Standards and Designations
- ASTM/ASME: Often supplied under AISI/SAE style designations (AISI M2, AISI M35 or by AMS / ASTM product specifications for tool steels in some regions).
- EN: Equivalent HSS types in EN standards are commonly designated as HS6-5-2 (M2 family) and corresponding cobalt-containing HSS such as HS6-5-2-5 (M35-like) depending on the specific EN designation.
- JIS: Japanese standards list tool steels with similar chemistries (e.g., SKH series).
- GB: Chinese GB standards include M2 and M35 designations or equivalent codes.
Classification: Both M2 and M35 are high-speed tool steels (HSS), i.e., alloyed tool steels engineered for high hardness and hot hardness. They are not stainless steels or HSLA steels.
2. Chemical Composition and Alloying Strategy
| Element | M2 (typical ranges) | M35 (typical ranges) |
|---|---|---|
| C | 0.80–0.95 wt% | 0.80–0.95 wt% |
| Mn | 0.15–0.40 wt% | 0.15–0.40 wt% |
| Si | 0.15–0.45 wt% | 0.15–0.60 wt% |
| P | ≤0.03 wt% | ≤0.03 wt% |
| S | ≤0.03 wt% | ≤0.03 wt% |
| Cr | 3.75–4.50 wt% | 3.75–4.50 wt% |
| Ni | ≤0.30 wt% | ≤0.30 wt% |
| Mo | 4.50–5.50 wt% | 4.50–5.50 wt% |
| V | 1.70–2.20 wt% | 1.80–2.30 wt% |
| Nb (columbium) | typically trace | typically trace |
| Ti | typically trace | typically trace |
| B | typically trace | typically trace |
| N | typically trace | typically trace |
Note: Tungsten (W) is a major constituent of both M2 and M35 (typically ~5.5–6.8 wt%), but was omitted from the requested element list; include it when specifying or ordering. The defining addition in the M35 family is cobalt (Co ≈ 4.5–5.5 wt%), which is not shown in the table columns above but is the key differentiator.
How alloying affects properties: - Carbon and carbide-formers (V, W, Mo, Cr) control the quantity, type, and stability of carbides: MC (V-rich), M6C (W/Mo-rich), and M23C6 (Cr-rich) carbides provide abrasion resistance. - Tungsten and molybdenum increase hardenability and high-temperature strength and form M6C carbides that contribute to secondary hardening. - Vanadium forms hard, fine MC carbides that improve wear resistance and toughness of the carbide population. - Chromium provides corrosion resistance to an extent, contributes to hardenability and forms M23C6 carbides. - Cobalt in M35 does not form carbides but strengthens the matrix and increases hot hardness / red hardness and temper resistance.
3. Microstructure and Heat Treatment Response
Typical microstructure (after appropriate quenching and tempering): - Matrix: tempered martensite (primary load-bearing phase). - Carbide population: a mixture of MC (V-rich, relatively hard), M6C (W/Mo-rich), and M23C6 (Cr-rich) carbides distributed in the martensitic matrix.
Heat treatment response differences: - Both grades use similar heat-treatment cycles: austenitization, quenching (oil or vacuum), and multi-stage tempering to develop desired hardness and secondary hardening. - M35, with its cobalt content, exhibits higher retained hardness at elevated tempering temperatures (better red hardness) and a stronger secondary hardening response. Cobalt increases the temper resistance of martensite—temperatures that soften M2 more noticeably will leave M35 harder. - Normalizing prior to hardening can refine as-rolled structures; controlled austenitizing temperatures are critical to dissolve an appropriate fraction of carbides for secondary hardening without over-dissolving vanadium MC carbides. - Thermo-mechanical processing (for forgings) that refines carbide dispersion will improve toughness and wear behavior in both grades; M35 benefits particularly in applications where hot strength is required.
4. Mechanical Properties
| Property | M2 (typical) | M35 (typical) | Notes |
|---|---|---|---|
| Tensile strength (UTS) | ~1800–2400 MPa (heat-treatment dependent) | ~1900–2500 MPa (heat-treatment dependent) | HSS UTS varies strongly with final hardness and tempering. |
| Yield strength | Generally high, close to UTS fraction | Similar or slightly higher | Yield often not specified separately for HSS—depends on microstructure. |
| Elongation (A%) | Low: typically 1–6% | Low: typically 1–5% | Both are relatively brittle compared with structural steels; M35 often slightly lower ductility due to Co. |
| Impact toughness (Charpy / qualitative) | Moderate for HSS; better than Co-alloyed variants | Slightly reduced vs M2 (lower toughness) | Toughness depends on section size, heat treatment, and carbide distribution. |
| Hardness (HRC, typical after HT) | 60–66 HRC (application-specific) | 60–67 HRC (better retention at temperature) | M35 achieves similar room-temperature hardness but retains hardness better at elevated cutting temperatures. |
Interpretation: M35 typically provides marginal improvements in hot hardness and wear resistance at elevated temperatures compared with M2, but this comes at a small penalty in toughness and formability. Absolute mechanical numbers vary with heat treatment and section size.
5. Weldability
Weldability of high-speed steels is limited by high carbon and alloy content; both M2 and M35 require careful heating practices to avoid cracking.
Useful carbon equivalent and weldability indices (for qualitative assessment): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Both M2 and M35 have high $CE_{IIW}$ and $P_{cm}$ values relative to mild steels, indicating susceptibility to hydrogen-assisted cracking and martensitic hardening in the HAZ. - Cobalt does not significantly alter the carbon equivalent algebraically but increases hardenability and tempering resistance; this can make the HAZ of M35 more prone to cracking if not properly preheated and post-heated. - Recommended practice: preheat to minimize thermal gradients, use low-hydrogen filler/electrodes, control interpass temperature, and perform appropriate post-weld heat treatment (PWHT) to temper martensite and relieve stresses. Where possible, avoid fusion welding for heavily stressed tooling—use brazing or mechanical joining for assemblies.
6. Corrosion and Surface Protection
- Neither M2 nor M35 is stainless; both are susceptible to oxidative corrosion and surface staining in humid or corrosive environments.
- Common protection methods: protective coatings (PVD, CVD, TiN, TiAlN), hard chrome plating (where appropriate), nitriding for surface hardness with limited corrosion benefit, galvanizing (for non-tool applications), and conventional paints or oils for storage.
- PREN formula for stainless selection is not applicable to M2/M35 because corrosion resistance is not a primary design feature of these carbon-rich HSS: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Use surface engineering (coatings, nitriding, PVD) to extend tool life in corrosive or adhesive wear environments.
7. Fabrication, Machinability, and Formability
- Machinability: Both alloys are more difficult to machine than carbon steels. M35 usually machines slightly worse than M2 because cobalt increases strength and tends to reduce machinability and increase tool wear in shaping operations.
- Cutting/forming: Cold forming or bending is challenging; hot working requires careful control and intermediate annealing. Cutting tool blanks are often ground rather than heavily machined.
- Surface finishing: Both can be ground to fine finishes; M35 may require more aggressive grinding parameters due to higher hot hardness and toughness of carbides.
- EDM and grinding are common fabrication methods for finished tools.
8. Typical Applications
| M2 (typical uses) | M35 (typical uses) |
|---|---|
| General-purpose milling cutters, drills, taps, reamers for moderate cutting temperatures | High-performance cutting tools, taps, drills for high-speed machining at elevated temperatures |
| Broaches, form tools, cold-heading dies | Hot-work tooling where red hardness improves life (but not a full hot-work steel) |
| Tooling for short to moderate runs where cost is a factor | High-volume, high-temperature cutting where longer tool life offsets higher material cost |
| Saw teeth, gear hob cutters | Interrupted cutting tools and drilling in difficult-to-machine alloys with elevated temperatures |
Selection rationale: Choose M2 when cost, moderate hot hardness, and reasonable toughness are primary; choose M35 when operations generate sustained elevated cutting temperatures and the extra cobalt-driven hot hardness delivers longer life that justifies cost.
9. Cost and Availability
- Cost: M35 is typically more expensive than M2 because cobalt is a costly alloying addition and supply is more constrained. Expect material cost for M35 to be noticeably higher on a per-kilogram basis.
- Availability: M2 is one of the most commonly stocked HSS grades and is widely available in bars, blanks, and tool shapes. M35 is also commonly available but may have longer lead times or premium pricing for certain product forms and large sizes.
- Product forms: Both are supplied as annealed bars, ground blanks, and specialized shapes; M35 may be more often specified as pre-hardened and ground tool blanks to reduce fabrication effort.
10. Summary and Recommendation
| Attribute | M2 | M35 |
|---|---|---|
| Weldability | Poor (challenging) | Poor to slightly worse (more hardenable) |
| Strength–Toughness balance | Good balance for general HSS | Higher hot hardness, slightly less toughness |
| Cost | Lower (more economical) | Higher (premium due to Co) |
Recommendations: - Choose M2 if you need a cost-effective, general-purpose HSS with good toughness for cutting or forming at conventional speeds and temperatures. M2 is suitable where grinding and re-sharpening are feasible and where thermal loads are moderate. - Choose M35 if your application consistently subjects tools to high cutting temperatures or red-hardness requirements (high-speed machining of difficult alloys, continuous high-temperature cutting) and the incremental tool life offsets higher material and processing costs.
Final note: When specifying either grade, provide expected service conditions (cutting speeds, cooling/lubrication conditions, section thickness, and any post-weld requirements) so heat treating, surface treatment, and procurement can be optimized for life-cycle cost rather than nominal material cost alone.