Grade Origins & Standard Context
Both 27NiCrMoV15-6 and 26NiCrMoV14-5 were codified under the German SEW 555:1984 standard — “Steels for larger forgings for turbine and generator components” — developed from decades of operational data gathered across European power stations during the steam turbine expansion era of the 1960s through 1980s. The SEW 555 family was built around a single goal: through-hardened rotor integrity in very large cross-sections.
The two grades are not redundant. 27NiCrMoV15-6 carries higher nickel (~3.5–4.0%) and chromium (~1.5–2.0%), giving it a clear advantage in through-hardenability for the heaviest components. 26NiCrMoV14-5 sits leaner on alloying — more economical for medium-section rotors where extreme core properties are not the binding constraint.
Chemical Composition Deep-Dive
Both grades use a Ni-Cr-Mo-V system optimized for a Q&T martensitic microstructure. The key differentiator is nickel content: 3.50–4.00% Ni in 27NiCrMoV15-6 versus 3.20–3.70% Ni in 26NiCrMoV14-5, directly driving hardenability in heavy sections.
| Element | 27NiCrMoV15-6 (%) | 26NiCrMoV14-5 (%) | Metallurgical role |
|---|---|---|---|
| Carbon (C) | 0.22–0.30 | 0.22–0.29 | Primary martensite strength; controlled tightly for toughness |
| Silicon (Si) | 0.15–0.40 | 0.15–0.40 | Deoxidizer; slight solid-solution hardening |
| Manganese (Mn) | 0.20–0.40 | 0.20–0.40 | Low Mn minimizes temper embrittlement at slow cooling |
| Nickel (Ni) | 3.50–4.00 | 3.20–3.70 | Core hardenability; DBTT reduction — key differentiator |
| Chromium (Cr) | 1.50–2.00 | 1.20–1.60 | Hardenability; carbide stability; oxidation resistance |
| Molybdenum (Mo) | 0.40–0.60 | 0.35–0.55 | Solid-solution strengthening; temper embrittlement resistance |
| Vanadium (V) | 0.05–0.15 | 0.05–0.12 | Fine VC precipitation; creep strength; grain refinement |
| Phosphorus (P max) | ≤ 0.012 | ≤ 0.012 | Controlled to prevent grain boundary embrittlement |
| Sulfur (S max) | ≤ 0.008 | ≤ 0.008 | MnS inclusions reduce toughness; minimized for rotor integrity |
The significance of 0.3–0.4% additional nickel
In a forging weighing 15–30 tons, the core cools approximately 30–60× more slowly than the surface during quenching. Each 0.1% Ni addition shifts the ideal critical diameter (DI) by roughly 20–30 mm, making the 0.3–0.4% Ni premium significant for rotor diameters exceeding 800 mm.
Mechanical Properties Compared
Minimum guaranteed properties at center of forging, quarter-point test position per SEW 555:1984:
| Property | 27NiCrMoV15-6 | 26NiCrMoV14-5 | Advantage |
|---|---|---|---|
| 0.2% Proof stress Rp0.2 | ≥ 620 MPa | ≥ 580 MPa | 27NiCrMoV15-6 (+7%) |
| Tensile strength Rm | 760–900 MPa | 720–860 MPa | 27NiCrMoV15-6 |
| Elongation A5 (min) | ≥ 16% | ≥ 17% | 26NiCrMoV14-5 |
| Reduction of area Z (min) | ≥ 45% | ≥ 50% | 26NiCrMoV14-5 |
| CVN impact at +20°C (min) | ≥ 68 J | ≥ 54 J | 27NiCrMoV15-6 (+26%) |
| Hardness (typical) | 220–260 HB | 210–250 HB | Comparable |
Normalized property comparison
Through-Thickness Hardenability: The Critical Differentiator
For turbine rotor shafts in the 5–30-ton range, through-thickness hardenability is often the binding specification criterion. The center of a 1,200 mm diameter rotor cools approximately 30–60× more slowly than the surface during quenching. Insufficient hardenability means the core will contain bainite or mixed microstructures — delivering significantly lower strength, toughness, and fatigue life than specified.
| Grade | Estimated DI (Grossmann) | Practical max. section for full core Q&T |
|---|---|---|
| 27NiCrMoV15-6 | ~950–1,100 mm | Up to ~1,200 mm dia. / 30 t |
| 26NiCrMoV14-5 | ~800–950 mm | Up to ~900 mm dia. / 15–18 t |
Heat Treatment Parameters
Both grades are supplied in the quenched and tempered (Q&T) condition.
| Stage | 27NiCrMoV15-6 | 26NiCrMoV14-5 |
|---|---|---|
| Austenitizing temperature | 840–870°C | 830–860°C |
| Hold time | ≥ 1 hr per 25 mm section | ≥ 1 hr per 25 mm section |
| Quench medium | Water or polymer (forced circ.) | Water or polymer (forced circ.) |
| Tempering temperature | 570–640°C | 560–630°C |
| PWHT limit (weld repair) | ≥ 30°C below original temper | ≥ 30°C below original temper |
| Stress relief (post-machining) | 550–580°C / ≥ 2 hr | 540–570°C / ≥ 2 hr |
Complete temperature time-history records for all heat treatment cycles are retained as part of the EN 10204 3.1 inspection documentation package for every forging shipped by Jiangsu Liangyi Co., Limited — see the full product specifications and forging size capabilities for bars, rings, and rotor shafts. Car-bottom furnaces accommodate single pieces up to 30 tons and 6,000 mm in length, with independently controlled zones and calibrated type K/N thermocouples.
High-Temperature Creep Behavior
Steam turbine rotor sections operate at 300–420°C depending on stage and plant type. At these temperatures, dislocation climb drives time-dependent plastic deformation (creep) as a service life consideration over multi-decade operation.
| Creep parameter | 27NiCrMoV15-6 | 26NiCrMoV14-5 |
|---|---|---|
| 1% creep limit at 300°C / 10⁵ hr | ~480 MPa | ~440 MPa |
| 1% creep limit at 350°C / 10⁵ hr | ~340 MPa | ~300 MPa |
| Recommended max. service temp. | ≤ 420°C | ≤ 400°C |
Fracture Toughness & Ductile-to-Brittle Transition Temperature
For turbine rotors undergoing repeated thermal cycling, the ductile-to-brittle transition temperature (DBTT) is a critical safety parameter. A rotor cold-started with a pre-existing crack at metal temperatures below the DBTT risks brittle fracture.
| Parameter | 27NiCrMoV15-6 | 26NiCrMoV14-5 |
|---|---|---|
| Typical DBTT (forging center) | −20 to −40°C | −10 to −25°C |
| K₁c fracture toughness (typical) | ~120–150 MPa√m | ~100–130 MPa√m |
| CVN at 0°C (typical, center) | ≥ 54 J | ≥ 40 J |
The shift in DBTT with nickel addition follows a well-established relationship: each 1% Ni addition depresses the 50% fibrous fracture transition temperature by approximately 20–25°C in martensitic rotor steels. The 0.3–0.4% Ni premium in 27NiCrMoV15-6 therefore yields a reproducible 7–10°C DBTT benefit, compounding favorably with the improved core hardenability effect in thick sections.
— Jiangsu Liangyi Co., Limited Engineering Team, based on published SEW 555-era rotor steel literatureWeldability & Weld Repair Considerations
Neither grade is weld-friendly. Both are high-carbon-equivalent steels developed for mechanical performance. In-service weld repair is sometimes commercially justified, making comparative weldability relevant.
| Parameter | 27NiCrMoV15-6 | 26NiCrMoV14-5 |
|---|---|---|
| IIW carbon equivalent (CE) | ~0.92–1.05 | ~0.88–0.98 |
| Recommended preheat | ≥ 200°C | ≥ 175°C |
| Interpass temperature limit | ≤ 250°C | ≤ 250°C |
| Cold-crack risk level | High (CE ~1.0) | Slightly lower |
| Weld process required | TIG/MIG, H4 or lower hydrogen | TIG/MIG, H4 or lower hydrogen |
| PWHT temperature | ≥ 580°C; ≤ (T‑temper − 30°C) | ≥ 560°C; ≤ (T‑temper − 30°C) |
26NiCrMoV14-5’s slightly lower CE makes it marginally more forgiving in weld repair, requiring lower preheat and offering reduced cold-crack sensitivity.
Application Mapping
| Application | 27NiCrMoV15-6 | 26NiCrMoV14-5 |
|---|---|---|
| LP steam turbine rotors (>15 t) | Preferred | Marginal — section-dependent |
| HP steam turbine rotors (<10 t) | Over-specified | Well-suited |
| Generator rotor (>20 t) | Preferred | Insufficient core hardenability |
| Gas compressor discs (medium) | Acceptable | Preferred — cost-optimal |
| Combined-cycle IP rotors (>380°C) | Preferred — creep margin | Caution at upper temperature |
| Large turbine bolts / studs | Acceptable | Common specification |
| Cold-climate installation (DBTT) | Preferred — lower DBTT | Review cold-start procedure |
The Specification Decision
Higher alloy content is justified
- Forging weight exceeds 15–18 tons
- Rotor diameter exceeds 900–1,000 mm
- Service temperature at or above 380°C
- Cold-start at ambient below 0°C is routine
- DBTT specification requires ≤ −25°C
- Design life requirement exceeds 30 years
- Fracture mechanics demands K₁c > 120 MPa√m
Leaner alloy is adequate
- Forging weight below 12–15 tons
- Rotor diameter below 800–900 mm
- Service temperature below 380°C
- Weld repair is likely in service life
- Budget sensitivity is a real constraint
- Medium-section generator or compressor disc
- Lead time flexibility allows standard stock grades
Grade-by-application quick reference
Use when reviewing a specification or RFQ for turbine rotor components.
| Scenario | Recommended grade | Primary rationale |
|---|---|---|
| 30-ton LP rotor, 1,100 mm dia. | 27NiCrMoV15-6 | Core hardenability, CVN, service life |
| 8-ton HP rotor, 600 mm dia. | 26NiCrMoV14-5 | Adequate properties, cost-effective |
| 20-ton generator rotor | 27NiCrMoV15-6 | Guaranteed core hardenability |
| Compressor disc, 500 mm dia. | 26NiCrMoV14-5 | Optimal cost-performance ratio |
| IP rotor, 400°C, cold-start plant | 27NiCrMoV15-6 | Creep margin + DBTT safety |
| Weld repair anticipated in service | 26NiCrMoV14-5 | Lower CE, lower cold-crack risk |
Frequently Asked Questions
What is the main difference between 27NiCrMoV15-6 and 26NiCrMoV14-5?
The primary difference is nickel content and the resulting through-thickness hardenability. 27NiCrMoV15-6 contains 3.50–4.00% Ni and 1.50–2.00% Cr, versus 3.20–3.70% Ni and 1.20–1.60% Cr for 26NiCrMoV14-5. This gives 27NiCrMoV15-6 a superior ideal critical diameter (DI ~950–1,100 mm vs ~800–950 mm), higher minimum yield strength (620 MPa vs 580 MPa), and higher CVN impact energy at the forging center (≥68 J vs ≥54 J at +20°C), making it the preferred grade for rotor forgings exceeding 15 tons or 900 mm diameter.
Which standard covers both grades?
Both grades are governed by SEW 555:1984 — “Steels for larger forgings for turbine and generator components.” This standard defines chemical composition, minimum mechanical properties tested at the quarter-point position at the forging center, and applicable heat treatment conditions. EN 10204 defines the inspection documentation requirements for turbine-grade forgings. Jiangsu Liangyi Co., Limited issues EN 10204 Type 3.1 mill test certificates as standard. Type 3.2 (third-party witnessed) is available on request.
For what forging weight should I specify 27NiCrMoV15-6?
27NiCrMoV15-6 is recommended for turbine rotor forgings exceeding approximately 15–18 tons or 900–1,000 mm in diameter. Its estimated ideal critical diameter (DI ~950–1,100 mm) ensures more than 90% martensite at the center in heavy sections. For components below this threshold, 26NiCrMoV14-5 provides adequate hardenability at lower alloy cost.
What is the maximum service temperature for 27NiCrMoV15-6?
27NiCrMoV15-6 is recommended for service temperatures up to approximately 420°C. At 350°C over 100,000 hours, its 1% creep limit is approximately 340 MPa — superior to 26NiCrMoV14-5 at approximately 300 MPa under the same conditions. 26NiCrMoV14-5 should be limited to approximately 400°C maximum continuous service temperature.
Can 27NiCrMoV15-6 be welded or weld-repaired?
27NiCrMoV15-6 must never be welded in the as-forged (non-heat-treated) condition. After full Q&T treatment, weld repair is technically feasible but demanding: preheat of at least 200°C, TIG or MIG with H4 or lower hydrogen consumables, interpass temperature limited to 250°C, and PWHT at minimum 580°C — kept at least 30°C below the original tempering temperature. Original heat treatment records must be confirmed before specifying any PWHT setpoint.
What certifications does Jiangsu Liangyi Co., Limited hold for these forgings?
Jiangsu Liangyi Co., Limited holds ISO 9001:2015 certification as its core quality management credential. EN 10204 Type 3.1 mill test certificates — issued by our own authorized quality representative — are provided as standard with every shipment. EN 10204 Type 3.2 certificates, which require countersignature by an independent third-party inspector, are available when customers arrange witness inspection through agencies such as TÜV Rheinland, Bureau Veritas, SGS, Lloyd’s Register, DNV, or ABS. Third-party inspection is welcomed and fully supported at our Jiangyin facility.
Does Jiangsu Liangyi Co., Limited manufacture 27NiCrMoV15-6 forgings?
Yes. Jiangsu Liangyi Co., Limited manufactures custom 27NiCrMoV15-6 open die forgings and seamless rolled rings from 30 kg to 30 tons, including turbine rotor shafts, discs, and flanges. Production is fully in-house from EAF+LF+VOD steel melting through 6,300-ton hydraulic presses, computer-controlled Q&T heat treatment in car-bottom furnaces up to 6,000 mm length, and 5-axis CNC finish machining. Contact: sales@jnmtforgedparts.com | Tel: +86-13585067993
Conclusion
27NiCrMoV15-6 and 26NiCrMoV14-5 are complementary grades within the SEW 555:1984 family, each engineered for a different segment of the turbine rotor size and duty envelope.
27NiCrMoV15-6 is the grade of choice for the largest and most demanding applications — where through-thickness hardenability for sections up to 1,200 mm diameter, high CVN core impact energy (≥68 J at +20°C), a lower DBTT (typically −20 to −40°C), and long-term creep resistance above 380°C combine to justify its higher alloy content. 26NiCrMoV14-5 serves medium-section applications below approximately 12–15 tons and 800–900 mm, where its hardenability is adequate and where lower CE or cost optimization makes it the rational specification choice.
The critical error to avoid: applying 26NiCrMoV14-5 to sections that exceed its hardenability capability. Use the decision matrix in Section 10 and the hardenability data in Section 4 as your primary specification tools.
📧 Email: sales@jnmtforgedparts.com
📞 Phone / WhatsApp: +86-13585067993
🌐 Website: www.jnmtforgedparts.com
📍 Address: Chengchang Industry Park, Jiangyin City, Jiangsu Province, China