SKH55 (JIS G4403) is Japan’s M35-equivalent cobalt high-speed steel — the standard upgrade from SKH51 (M2) when cutting tool tip temperatures exceed what standard HSS can sustain. The 5% Co addition does not form carbides; instead it raises the secondary hardening response, pushing retained hardness at 600°C (1112°F) from SKH51’s ~58HRC to SKH55’s ~60–61HRC. This difference of 2–3 HRC points is sufficient to determine whether a cutting tool survives or fails when machining stainless steels, titanium alloys, nickel superalloys, or hardened steels where low thermal conductivity concentrates heat at the tool tip. This guide covers the cobalt mechanism in detail, the modified heat treatment required to exploit it, and the specific applications where the 30–40% price premium over SKH51 is justified by proportionally longer tool life.
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Heat Treatment
- The Cobalt Effect: Why Co Raises Red Hardness
- Cutting Tool Applications
- Common Mistakes
- SKH55 vs SKH51: When to Upgrade
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4403 | SKH55 | Japan | Reference |
| AISI/SAE | M35 | USA | ✅ Nearest Exact |
| EN ISO 4957 | 1.3243 / HS6-5-2-5 | Europe | ✅ Nearest Exact |
| DIN 17350 | S6-5-2-5 | Germany | ✅ Nearest Exact |
| BS 4659 | BM35 | UK | ✅ Nearest Exact |
| GB/T 9943 | W6Mo5Cr4V2Co5 | China | ✅ Nearest Exact |
The EN designation HS6-5-2-5 encodes the alloy: 6% W / 5% Mo / 2% V / 5% Co. This parallels SKH51 (HS6-5-2 = M2) with the Co addition appended. SKH55 and M35 are interchangeable in cutting tool applications; the EN 1.3243 composition is slightly tighter on some elements but performs identically in service.
2. Chemical Composition
| Element | SKH55 (JIS G4403) | M35 (AISI) | 1.3243 (EN ISO 4957) |
|---|---|---|---|
| C | 0.85–0.92% | 0.82–0.88% | 0.85–0.95% |
| Si | ≤ 0.45% | ≤ 0.45% | ≤ 0.45% |
| Mn | ≤ 0.40% | ≤ 0.40% | ≤ 0.40% |
| Cr | 3.80–4.50% | 3.75–4.50% | 3.75–4.50% |
| Mo | 4.50–5.50% | 4.50–5.50% | 4.50–5.50% |
| W | 5.50–6.70% | 5.50–6.75% | 5.50–6.50% |
| V | 1.70–2.20% | 1.75–2.20% | 1.80–2.10% |
| Co | 4.50–5.50% | 4.50–5.50% | 4.75–5.25% |
Source: JIS G4403:2015, AISI M35, EN ISO 4957:2018. The base composition is identical to SKH51/M2 (6W-5Mo-4Cr-2V) with 5% Co added. Cobalt does not form carbides — it dissolves entirely in the steel matrix, modifying how W and Mo participate in secondary hardening during tempering.
3. Mechanical Properties
| Property | SKH55 (Hardened) | SKH51 (Comparison) | Notes |
|---|---|---|---|
| Room-temperature hardness | 64–67HRC | 63–66HRC | 1–2HRC advantage for SKH55 |
| Hot hardness at 600°C | ~60–61HRC | ~57–59HRC | Key SKH55 advantage |
| Hot hardness at 650°C | ~54–56HRC | ~50–53HRC | SKH55 survives harder cuts longer |
| Charpy Impact (unnotched) | ~20–35 J (15–26 ft·lbf) | ~25–40 J (18–30 ft·lbf) | Co slightly reduces toughness |
| Transverse Rupture Strength | ~3000–3500 MPa (435–508 ksi) | ~3200–3800 MPa (464–551 ksi) | SKH55 slightly lower |
| Density | 8.16 g/cm³ (0.295 lb/in³) | 8.16 g/cm³ | Identical |
| Thermal conductivity | ~24 W/(m·K) | ~24 W/(m·K) | Identical |
The room-temperature hardness difference between SKH55 and SKH51 is modest. The decisive difference is retained hot hardness: at 600°C, SKH55 is approximately 2–3HRC harder — enough to maintain a sharper cutting edge through more material removal before the tool tip softens and begins to deform plastically (the moment cutting performance degrades rapidly).
4. Heat Treatment
Annealing
Heat to 850–900°C (1562–1652°F) at ≤ 50°C/hr, hold 3–4 hours, furnace cool at ≤ 20°C/hr to 600°C, then air cool. Target: ≤ 269HBW. Pre-machining to near-net-shape is performed in this condition.
Hardening
Two-stage preheat: 450–500°C (842–932°F) → 850–900°C (1562–1652°F), then austenitize at 1180–1210°C (2156–2210°F) for 2–5 min (dependent on section size). The austenitizing temperature is at the high end of the HSS range — necessary to dissolve sufficient W, Mo, and Co into austenite for maximum secondary hardening response. Over-soaking above 1220°C risks grain growth; under-soaking below 1170°C leaves too many undissolved carbides.
Quench in salt bath (540–580°C, 1004–1076°F) for 3–5 min, then air cool; or oil quench; or high-pressure gas quench (≥ 10 bar nitrogen). Salt bath quenching produces the most consistent microstructure and is preferred for production tooling.
As-quenched hardness: typically 64–66HRC. Retained austenite content: 20–35%, which must be transformed and tempered in subsequent cycles.
Tempering: Triple Temper Mandatory
| Cycle | Temperature | Duration | Purpose |
|---|---|---|---|
| 1st Temper | 550–570°C (1022–1058°F) | 1–2 hr | Transforms retained austenite; precipitates M₂C carbides (secondary hardening peak) |
| Cool | To below 60°C (140°F) | — | Allows freshly formed martensite to cool before next cycle |
| 2nd Temper | 550–570°C (1022–1058°F) | 1–2 hr | Tempers martensite from 1st temper; converts additional retained austenite |
| Cool | To below 60°C (140°F) | — | — |
| 3rd Temper | 550–570°C (1022–1058°F) | 1–2 hr | Final tempering of any remaining untempered martensite; achieves stable microstructure |
Final hardness after triple temper at 550–570°C: 64–67HRC. The secondary hardening peak for SKH55 occurs approximately 5–10°C lower than SKH51, so tempering temperature optimization matters: tempering at 540°C instead of 560°C typically adds 0.5–1.0HRC of final hardness, at a slight toughness cost.
5. The Cobalt Effect: Why Co Raises Red Hardness
Cobalt’s mechanism in HSS is indirect — it acts as a matrix modifier, not a carbide former:
- Increases carbide dissolution at austenitizing temperature: Co raises the solubility of W and Mo in austenite. More W and Mo dissolve into solution at 1200°C, leaving fewer undissolved primary carbides and loading the matrix with more alloying elements available for secondary hardening
- Finer, more numerous M₂C carbide precipitation: When tempered at 550–570°C, the Co-enriched matrix precipitates finer and more uniformly distributed M₂C (W,Mo)₂C carbides. This finer precipitation produces a sharper secondary hardening peak and higher peak hardness than SKH51
- Higher coarsening resistance: The M₂C carbides in a Co matrix are more resistant to Ostwald ripening (coarsening) at elevated cutting temperatures. This is the direct source of improved red hardness — the precipitates that maintain hardness remain fine and coherent for longer at 600–650°C
6. Cutting Tool Applications
| Work Material | Why SKH55 Over SKH51 | Typical SKH55 Tool |
|---|---|---|
| Austenitic stainless steel (SUS304, SUS316) | Work hardening creates high cutting forces; low thermal conductivity → high tool tip temp | Drills, taps, end mills, reamers |
| Titanium alloys (Ti-6Al-4V) | Extremely low thermal conductivity (7 W/m·K); 80% of cutting heat goes into tool, not chip | Drills, end mills, form tools |
| Nickel superalloys (Inconel 718, Waspaloy) | Abrasive carbides + work hardening + high cutting temp combination | Drills, form tools, broaches |
| Hardened steel (30–45HRC) | High hardness requires tool to maintain sharp edge at elevated contact stress/temp | Taps for hardened holes, form tools |
| Duplex stainless (SUS329J3L) | High strength + work hardening requires sustained cutting force at elevated tip temp | Drills, taps |
| High-silicon aluminum alloys | Abrasive Si particles wear cutting edge; Co HSS resists abrasion better at elevated speeds | Reamers, form tools (long run) |
For standard carbon and alloy steels (S45C, SCM440), cast iron, and aluminum below ~300 m/min cutting speed, SKH51 provides equivalent performance. The SKH55 premium is only recoverable against difficult-to-cut materials where tool life improvement is measurable.
7. Common Mistakes
8. SKH55 vs SKH51: When to Upgrade
Work material is difficult-to-cut: austenitic stainless (SUS304/316), titanium alloys, nickel superalloys, or hardened steels above 30HRC. Or when SKH51 tool life is marginal — fewer than 100 holes per drill regrind, taps failing under 20 holes, high scrap rate from tool failure in the cut. The 30–40% Co HSS price premium is typically recovered at 1.5× or better tool life improvement in these materials.
Work material is standard: carbon steel, alloy steel (SCM440), cast iron, aluminum, and most plastics. At cutting speeds below the temperature threshold where red hardness matters (~200 m/min for steel), SKH51 provides equivalent service life. SKH55’s higher cost is not recovered. For brittle work materials where tool failure is chipping rather than thermal wear, SKH51’s slightly higher toughness is preferable.
Maximum simultaneous demands on wear resistance, toughness, and hot hardness: high-volume production taps for stainless, precision reamers for titanium, form tools for Inconel. PM-HSS grades (HAP40, ASP2052) provide both Co-level red hardness and significantly improved toughness from the fine, uniform carbide distribution achievable only through powder processing — at a higher cost premium than Co-HSS over standard M2.
9. FAQ
Q: Is SKH55 the same as M35?
Yes. JIS G4403 SKH55 and AISI M35 share the same W-Mo-Cr-V-Co composition framework. EN 1.3243 (HS6-5-2-5) is the European equivalent. Material certified to any of these standards performs identically in cutting tool applications. The designations differ by standard system only.
Q: Can SKH55 be used for cold-work dies?
Technically yes — SKH55 hardens to 64–67HRC and provides excellent wear resistance. In practice, it is rarely used for dies because the cost premium over SKD11 (D2) is not recovered at room-temperature service conditions where red hardness provides no benefit. SKD11 at 60HRC is the standard die steel specification; SKH55 is reserved for applications where cutting tool tip temperatures require the Co addition.
Q: What is the difference between SKH55 and SKH57?
SKH57 (M42) contains approximately 8% Co versus SKH55’s 5% Co, plus slightly higher carbon and vanadium. This pushes hardness to 67–69HRC and hot hardness even further, at additional cost and a further reduction in toughness. SKH57 is used only for the most extreme difficult-to-cut materials (titanium alloys, Inconel, hardened steel above 45HRC) where SKH55 is still insufficient.
Summary
- SKH55 = M35 = EN 1.3243: standard M2 (SKH51) base composition + 5% Co for improved red hardness
- Co addition raises hot hardness at 600°C by ~2–3HRC relative to SKH51 — sufficient to prevent tool-tip softening in difficult-to-cut materials
- Triple temper at 550–570°C is mandatory — two cycles leave excess retained austenite that causes dimensional changes and chipping in service
- Austenitize at 1180–1210°C — higher than standard HSS grades, necessary to dissolve Co-enhanced W/Mo for maximum secondary hardening response
- Premium is justified for stainless steel, titanium alloys, nickel superalloys, and hardened steels; not recoverable for standard carbon/alloy steels
- For maximum simultaneous wear resistance and toughness, consider powder-metallurgy HSS (HAP40, ASP2052) over Co-HSS
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