SKH9 vs M2 – Composition, Heat Treatment, Properties, and Applications

Introduction

Engineers, procurement managers, and manufacturing planners commonly face the choice between two very similar high‑speed tool steels for cutting, forming, and wear‑resistant tooling: a JIS‑designated high‑speed grade and the widely referenced AISI/SAE grade. The selection dilemma typically centers on supply specification and standard compliance, as well as processing and post‑weld requirements—tradeoffs between availability, heat‑treatment practice, and secondary processing cost often drive the decision rather than major differences in basic performance.

Both grades are high‑speed steels designed for hot‑worked cutting tools and cold/hot machining applications. They are frequently compared because their nominal chemistries and resulting properties overlap closely: one is provided under a Japanese national specification and the other under North American/ international tool‑steel conventions. In practice the deciding factors are standards conformity, heat‑treatment instructions, and form availability rather than substantial metallurgical differences.

1. Standards and Designations

• AISI/SAE/ASTM: AISI/SAE M2 (common international reference for this type of high‑speed steel; often covered in ASTM tool steel data sheets).
• JIS: SKH9 (Japanese Industrial Standard for an equivalent high‑speed steel).
• EN/DIN: Comparable EN/DIN designations exist (commonly presented as HS6‑5‑2 or similar EN tool‑steel grades; exact EN label varies by country and specific alloy family).
• GB (China): Chinese standards list tool steels with similar chemistries (different numbering/labels).
• Classification: Both SKH9 and M2 are high‑speed tool steels (HSS), i.e., tool/alloy steels formulated for red‑hardness, wear resistance, and toughness at elevated temperatures. They are not stainless steels.

2. Chemical Composition and Alloying Strategy

The following table lists typical composition ranges for each grade. Values are presented as typical ranges; actual certified analysis should be obtained from the mill or supplier for procurement and acceptance testing.

Notes: - Both grades are alloyed with relatively high W and Mo for hot hardness and complex carbide formation; V promotes hard, stable MC‑type carbides that improve wear resistance. - Minor elements (Nb, Ti, B, N) are usually at trace levels or not deliberately added except in specialty variants (e.g., powder‑metallurgy or modified HSS). - The alloying strategy emphasizes high hardenability and stable carbides rather than corrosion resistance: high W/Mo + Cr + V yields complex M6C/M2C and MC carbides for wear resistance and red hardness.

3. Microstructure and Heat Treatment Response

Typical microstructure: - Both SKH9 and M2 develop tempered martensite matrices containing a distribution of hard carbides: tungsten‑ and molybdenum‑rich complex carbides (often described as M6C/M2C types) and vanadium‑rich MC carbides. Carbide size and distribution depend strongly on melting, solidification, and forging/rolling practice.

Heat treatment behavior: - Austenitizing: High‑speed steels require high austenitizing temperatures to dissolve the requisite carbide fraction and obtain the desired matrix carbon in solution. Typical austenitizing ranges for this family are high (commonly in the neighborhood of $1180^\circ\text{C}$ to $1250^\circ\text{C}$), but exact temperatures are taken from supplier heat‑treatment charts. - Quenching: Oil quenching or gas quenching is used to form martensite; due to high alloy content, retained austenite can be significant and is often controlled by cryogenic treatment if lower retained austenite is required. - Tempering: Multiple temper cycles (two or three) at intermediate temperatures (often in the $520^\circ\text{C}$–$600^\circ\text{C}$ range, depending on target hardness) stabilize the matrix and precipitate secondary carbides. Tempering schedule determines final hardness vs. toughness tradeoff. - Normalizing/annealing: For machining or forming, a soft anneal (holding to a temperature to spheroidize carbides, slow cooling) is used to reduce hardness and improve machinability.

Differences: - Metallurgically the two grades respond very similarly. Differences observed in practice usually stem from specific processing (melting route, forging/rolling, anneal/normalizing schedules) and from vendor heat‑treat recommendations rather than intrinsic compositional contrast.

4. Mechanical Properties

Mechanical properties for high‑speed steels depend so strongly on heat treatment that numbers are best quoted as typical ranges for common conditions. The table below shows representative ranges for annealed and for hardened & tempered conditions.

Interpretation: - Both SKH9 and M2 can be hardened to high HRC values typical of HSS (60–67 HRC), with corresponding very high tensile strength and low ductility. - Annealed material is machinable and significantly tougher/more ductile than the hardened condition. - Differences in strength and toughness between SKH9 and M2 in equivalent heat‑treated conditions are generally small; practical performance will reflect carbide distribution and heat treatment execution.

5. Weldability

High‑speed steels with relatively high carbon and alloying content are intrinsically poor to challenging to weld. Two commonly used compositional weldability predictors are:

• Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• Pcm (Dai): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Both SKH9 and M2 produce relatively high $CE_{IIW}$ and $P_{cm}$ values because of their carbon, chromium, molybdenum, tungsten, and vanadium contents. This indicates high hardenability and susceptibility to cold cracking and formation of brittle martensite in the HAZ. - Practical consequences: welding HS tool steels requires strict preheat, controlled interpass temperature, use of matching/compatible filler materials, and post‑weld tempering. For critical tooling, welding is often avoided in favor of brazing, mechanical joining, or full rebuild machining of replacement blanks. - If welding is necessary, consult supplier welding procedure specifications and perform qualification welds with full post‑weld heat treatment.

6. Corrosion and Surface Protection

• Neither SKH9 nor M2 is a stainless alloy. Corrosion resistance is limited to that provided by the Cr content but is insufficient for exposure to aggressive environments.
• Common protective strategies: painting, plating, vapor‑deposition coatings (TiN, TiAlN), nitriding, shot peening, or galvanizing for non‑tool applications. For cutting and forming tools, surface engineering (coatings, surface hardening) is the primary protection and wear extension method.
• PREN is not applicable to these non‑stainless tool steels, but for clarity the stainless corrosion index is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Use corrosion‑resistant grades (stainless tool steels or other alloys) when corrosion resistance is a primary requirement; otherwise protect HSS by coatings and controlled storage.

7. Fabrication, Machinability, and Formability

• Machinability: In the annealed (soft) condition, both grades are reasonably machinable for roughing and finishing operations. In hardened condition, grinding and EDM are typical methods for shaping.
• Cutting and grinding: High alloy content and hard carbides increase abrasive wear on cutting/grinding tools; use diamond or CBN where appropriate for high productivity.
• Formability: Cold forming is limited; hot forming or machining blanks is standard practice. Bending/forming of annealed stock is feasible but springback and cracking risk increase with carbon/alloy content.
• Surface finishing: Polishing and coating adhesion benefit from controlled carbide distribution and good heat‑treatment practice.

8. Typical Applications

Selection rationale: - Both grades are chosen for wear resistance and hot hardness. Choose based on local availability, required certification (JIS vs AISI/ASTM), and whether downstream vendors expect material in one specification versus the other. - For extreme wear or specialized properties, consider powder‑metallurgy HSS or carbide substitutes.

9. Cost and Availability

• Cost drivers: tungsten and molybdenum market prices, production method (conventional cast vs powder metallurgy), and certification/traceability requirements.
• Availability: M2 is globally established and widely stocked by international distributors; SKH9 is common in regions where JIS is the procurement norm and may be favored by domestic mills.
• Product forms: both are available as bars, blanks, and tool‑steel plate; PM‑M2 (powder metallurgy) variants command a price premium but offer improved toughness and uniform carbide distribution.

10. Summary and Recommendation

Summary table (qualitative):

Recommendation: - Choose SKH9 if you require compliance with Japanese national standards, are procuring within supply chains that specify JIS designations, or if the supplier provides tailored heat‑treatment and certification that fit your process. - Choose M2 if you need a widely recognized international grade with broad downstream availability, standard AISI/ASTM documentation, and easier sourcing from global distributors. M2 is often the preferred choice when cross‑border procurement or multi‑source supply is important.

Closing note: Metallurgically SKH9 and M2 are essentially equivalent high‑speed steels; performance differences in service are typically driven by heat treatment, carbide control, manufacturing route, and surface treatment rather than by a fundamentally different chemistry. For critical tooling applications, obtain mill certs, require supplier heat‑treatment instructions, and qualify the actual batch performance with hardness, microstructure, and, where necessary, toughness testing.