SKH9 vs M2 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
SKH9 and M2 are two widely used high-speed tool steels employed for cutting tools, drills, taps, form tools, and wear-resistant components. Engineers, procurement managers, and manufacturing planners commonly confront the selection dilemma: choose based on regional standardization, subtle compositional differences, or supply-chain considerations versus specific performance targets such as wear resistance, toughness, or heat resistance.
The essential distinction for selection is that SKH9 is the Japanese standard designation and M2 is the American/International designation for a very similar tungsten‑molybdenum high-speed steel family. They are frequently compared because they occupy the same performance niche (general-purpose high-speed tool steel) and are often interchangeable in design—yet standard origin, specification tolerances, and vendor processing can influence the final choice.
1. Standards and Designations
- M2: Commonly referenced under AISI/ASM and ASTM/ASME-based specifications (AISI M2; ASTM: often captured in high-speed steels lists), widely used in North American and international supply chains.
- SKH9: Japanese Industrial Standard designation (JIS SKH9), used throughout Japan and many Asian supply chains; also accepted in many export markets.
- EN/ISO: Equivalent families in European norms are often designated as HS6-5-2-5 or similar tungsten‑molybdenum HSS grades—equivalence is approximate and depends on specific element ranges.
- GB (China): Chinese standards have their own designations but commonly provide direct chemical equivalents or cross-reference tables to SKH and M-series steels.
Classification: Both SKH9 and M2 are tool steels of the high‑speed steel (HSS) family—alloy steels specifically formulated for high hardness and red hardness (retention of hardness at elevated cutting temperatures). They are not stainless steels nor HSLA steels.
2. Chemical Composition and Alloying Strategy
Table: typical composition ranges (mass %) for SKH9 and M2. Note: the table lists the elements requested; tungsten (W) is an essential alloy element for these grades but was not one of the table columns—its typical content is stated beneath the table.
| Element | SKH9 (JIS) typical range | M2 (AISI) typical range |
|---|---|---|
| C | 0.85–1.05 | 0.85–1.05 |
| Mn | 0.20–0.50 | 0.20–0.40 |
| Si | 0.15–0.40 | 0.20–0.45 |
| P | ≤0.03 | ≤0.03 |
| S | ≤0.03 | ≤0.03 |
| Cr | 3.75–4.50 | 3.75–4.50 |
| Ni | — (trace) | — (trace) |
| Mo | 4.50–5.50 | 4.50–5.50 |
| V | 1.70–2.20 | 1.70–2.20 |
| Nb | — | — |
| Ti | — | — |
| B | — | — |
| N | trace | trace |
Important note: Both SKH9 and M2 also contain a substantial proportion of tungsten (W), typically in the range of approximately 5.5–6.75% (varies by producer). Tungsten plus molybdenum are the principal alloying elements that provide high hot hardness and wear resistance in this grade family. Minor alloying additions beyond the table (trace Ti, Nb, B) may be present in specific melts to control carbide morphology and grain size; such microalloying is vendor-dependent.
How alloying affects properties: - Carbon + strong carbide formers (W, Mo, V, Cr) produce hard intermetallic carbides that resist abrasive wear and allow tempering to high hardness. - Chromium and vanadium contribute to hard carbide populations improving wear resistance and red hardness. - Molybdenum and tungsten improve secondary hardening behavior and maintain hardness at elevated temperatures (cutting tool red hardness). - Silicon and manganese are present in small amounts for deoxidation and strength control; their levels also affect toughness and machinability.
3. Microstructure and Heat Treatment Response
Typical microstructures: - In the annealed condition both steels show a ferritic matrix with a network of primary and secondary carbides (complex carbides including M6C, MC types where M = W, Mo, V, Cr). - After austenitizing and quenching, the microstructure becomes martensitic with retained carbides; subsequent tempering produces secondary hardening due to precipitation of fine alloy carbides, which is critical for the high-temperature hardness of HSS.
Heat treatment response: - Austenitizing: Comparable austenitizing ranges are used for both SKH9 and M2; temperature and time control carbide dissolution and alloy distribution. - Quenching: Oil or forced-air quenching is common; both steels require controlled cooling to avoid cracking. - Tempering: Multiple tempering cycles at elevated temperatures produce secondary hardening. Tempering schedules determine final hardness (typical HRC in the 60s for cutting tools) and toughness balance. - Normalizing: Used to refine grain size and homogenize microstructure prior to final hardening; more common in shop practice than in final production for HSS. - Thermo-mechanical processing: Vendor practices such as vacuum-melting, electroslag remelting (ESR), or vacuum induction melting (VIM) improve cleanliness and toughness; producers may specify these melt routes.
Overall, SKH9 and M2 respond very similarly to standard HSS heat-treatment cycles; differences arise from minor compositional tolerances and steelmaking routes which affect carbide distribution and cleanliness.
4. Mechanical Properties
Table: comparative qualitative mechanical properties (typical, heat-treatment dependent)
| Property | SKH9 (typical after proper HSS heat treatment) | M2 (typical after proper HSS heat treatment) |
|---|---|---|
| Tensile Strength | High (HSS-class level; heat-treatment dependent) | High (comparable to SKH9) |
| Yield Strength | High (comparable to tensile behavior) | High (comparable to SKH9) |
| Elongation | Low to moderate (brittleness increases with hardness) | Low to moderate (similar) |
| Impact Toughness | Moderate to low (depends on carbide distribution and heat treatment) | Moderate to low (similar) |
| Hardness (service temper) | Typically in the high HRC range (tool steel HSS domain) | Typically in the high HRC range (tool steel HSS domain) |
Notes: - Numerical values for tensile, yield, and impact are extremely sensitive to the exact heat-treatment schedule, specimen condition, and carbide morphology; both grades are engineered for hardness and red hardness rather than ductility. - In practice, hardness (HRC) after appropriate austenitizing and multiple tempers is the most commonly specified property for cutting tools; both SKH9 and M2 can reach HRC in the low-to-mid 60s depending on tempering.
Interpretation: Neither grade is categorically "stronger" in tensile terms when prepared with equivalent heat treatment; the key differentiator is carbide distribution and steelmaking cleanliness, which can slightly favor one producer’s processing route. Toughness is typically a trade-off with hardness; careful tempering is used to achieve the needed balance.
5. Weldability
Weldability for high-speed steels is generally poor relative to low-alloy steels because of their high carbon content and strong carbide-forming elements which increase hardenability and susceptibility to cracking.
Useful predictive formulas (qualitative interpretation only): - Carbon equivalent (IIW) is often used to estimate cold cracking susceptibility: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - The Pcm formula is another weldability index: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - Both SKH9 and M2 have relatively high $CE_{IIW}$ and $P_{cm}$ values due to C, Cr, Mo, V and W (W also increases hardenability but is not in these specific formulas). Consequently, expect high preheat requirements, low interpass temperatures to avoid thermal shock, and a need for post-weld heat treatment (PWHT) to relieve stresses and temper martensite. - Typical practice: avoid welding high-speed steels when possible. If welding is necessary, use controlled preheat, low heat-input processes, matching or specialized filler metals, and post-weld tempering to restore toughness. Alternatively, braze or mechanically join components where feasible.
6. Corrosion and Surface Protection
- Both SKH9 and M2 are not stainless steels. Corrosion resistance in atmospheric or aqueous environments is limited compared with stainless grades.
- Typical protection strategies: oiling, painting, phosphating, or galvanizing where appropriate; for cutting tools, corrosion is normally controlled by lubrication and storage rather than coatings.
- PREN (Pitting Resistance Equivalent Number) is not applicable because these are not stainless grades: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Use of hard coatings (TiN, TiAlN, CrN, DLC) is common to improve surface wear and reduce corrosion exposure in cutting operations; coatings also increase performance at elevated temperatures.
7. Fabrication, Machinability, and Formability
- Machinability: In the annealed condition both materials machine relatively easily; in the hardened condition they are abrasive due to carbides. M2 is commonly used as a tool steel and has well-documented grinding and finishing practices; SKH9 behaves very similarly.
- Grinding and finishing: Diamond or CBN grinding wheels are often required for hardened HSS, and coolant control is critical to avoid thermal cracking.
- Formability/bending: As with most tool steels, cold forming is limited; hot forming is possible but requires controlled thermal cycles to avoid carbide precipitation issues.
- Surface finishing: Polishing and coating adhesion behave similarly for both grades; coating processes benefit from clean substrate and controlled surface prep.
8. Typical Applications
| SKH9 (typical uses) | M2 (typical uses) |
|---|---|
| General-purpose cutting tools in Japanese/Asian-specified tooling (drills, taps, milling cutters, reamers) | General-purpose cutting tools in North American/International tooling (drills, taps, milling cutters, reamers) |
| Forming and cold-work tools where HSS properties are required and JIS specification is mandated | High-performance cutting tools and machine-tool tooling where ASTM/AISI/ISO reference to M2 is specified |
| Wear-resistant components where HSS-grade red hardness is needed | Broad HSS applications across multiple industries (aerospace, automotive tooling, dies) |
Selection rationale: - Choose based on specification precedence (customer or contract requires JIS or AISI/ASTM), supply-chain sourcing, or specific supplier process claims (e.g., ESR, VIM for cleanliness). - Both grades are selected for the same operating envelopes: high-speed cutting, elevated-temperature strength, and wear resistance.
9. Cost and Availability
- Relative cost: Both grades are positioned similarly in price as mainstream high-speed steels. Small differences in cost stem from regional availability, tungsten and molybdenum market prices, and steelmaking route (ESR/VIM product premiums).
- Availability by product form: Bars, blanks, and pre-hardened tooling stock in both SKH9 and M2 are widely available from specialty steel suppliers. Regional preference influences stocking: M2 is more ubiquitous in North America and Europe, SKH9 more commonly stocked in Japan and parts of Asia.
- Lead time and premium: Vendor-specific processes (clean steel, remelted variants) are often more expensive but deliver improved toughness and performance.
10. Summary and Recommendation
Table: concise comparison
| Attribute | SKH9 | M2 |
|---|---|---|
| Weldability | Poor (high hardenability; requires careful procedures) | Poor (similar) |
| Strength–Toughness balance | High hardness and wear resistance; toughness depends on processing | High hardness and wear resistance; toughness depends on processing |
| Cost | Comparable; regional variances may apply | Comparable; regional variances may apply |
Conclusion and practical guidance: - Choose SKH9 if your procurement or production chain follows JIS/Japanese standards, if tooling or replacement parts are specified by JIS designations, or if local suppliers provide SKH9 with traceability and vendor-specific processing (ESR/VIM) that meet your toughness requirements. - Choose M2 if your drawings, specifications, or industry standards call for AISI/ASTM/ISO M2, if you require suppliers aligned with North American or international sourcing, or if you need broad vendor availability and interchangeability with M2 references.
Final note: Metallurgically SKH9 and M2 occupy the same HSS family and are functionally equivalent for most applications when produced and heat-treated to comparable standards. The decisive factors are specification conformity, vendor processing (cleanliness and remelt methods), and heat-treatment controls that determine carbide distribution, toughness, and final tool performance.