The Engineering Problem That Demanded a New Steel
The section size effect is the fundamental metallurgical challenge in large turbine rotor forging: at cross-section diameters above 500 mm, the centre of the forging cools too slowly during quenching to transform to a suitably strong and tough microstructure using conventional Cr-Mo alloy steels — making a specially engineered high-hardenability grade essential.
Between 1960 and 1980, the global power generation industry experienced one of its most aggressive expansions in scale. Steam turbine units that had been built at 100–200 MW in the 1950s were growing toward 600 MW, and ambitions for 1,000 MW units were already on the drawing boards of engineering houses in Germany, the United Kingdom, the United States, and Japan.
This posed a specific, technically severe problem for materials engineers: rotor forgings had to grow with the turbines. A 600 MW low-pressure (LP) steam turbine rotor might span 10–12 metres in length. Its cross-sectional diameter at the disc regions could exceed 1,500 mm — and critically, the mechanical properties required for reliable service had to be uniform across the full diameter, not merely at the surface.
The problem was not that existing steels lacked strength at the surface. The problem was that, at section thicknesses above 500 mm, conventional Cr-Mo steels could not maintain their strength, toughness, and ductility uniformly all the way to the centre of the forging.
The section size effect — the central challenge of large turbine rotor metallurgyThis phenomenon occurs because the centre of a thick forging cools more slowly than its outer surfaces during the quench step of heat treatment. Slow cooling rates in the centre produce coarser microstructures, reduced yield strength, and most critically, degraded impact toughness. For a high-speed rotor operating under 10⁸+ fatigue cycles at elevated temperature, a weak centre is not a margin issue — it is a catastrophic failure mode.
The answer that German metallurgists converged on through the late 1960s and into the 1970s was a quaternary alloy system — one that used four separate strengthening mechanisms to maintain through-hardening response even at the slow cooling rates occurring at the centre of a 650 mm or larger cross-section. That system became codified under the designation 26NiCrMoV8-5, material number 1.6931 per DIN EN 10027-2, and governed by SEW 555 (1984) published by the German Steel Federation (Stahl-Eisen-Werkstoffblatt).
Today, custom 1.6931 (26NiCrMoV8-5) forged bars, rings, and turbine rotor shafts remain the primary product specification for large steam turbine rotor applications globally. Understanding why requires examining the alloy's design logic from first principles.
The Alloy Architecture of 26NiCrMoV8-5
26NiCrMoV8-5 is a low-alloy, high-hardenability Ni-Cr-Mo-V steel containing approximately 0.26% carbon, 1.8–2.2% nickel, 1.4–1.8% chromium, 0.25–0.45% molybdenum, and 0.05–0.15% vanadium — each element chosen to solve a specific aspect of the large-section hardenability and toughness problem.
Why Nickel Is the Critical Differentiator
Nickel at 1.8–2.2% expands the austenite stability range and suppresses the pearlite and bainite transformation nose in the continuous cooling transformation (CCT) diagram. Even at cooling rates as low as 0.5 °C / sec — the rate occurring at the centre of a 650 mm diameter forging during quench — the steel transforms predominantly to martensite or lower bainite rather than coarse upper bainite or pearlite.
Nickel is the only major hardenability-enhancing element that simultaneously improves toughness. Carbon, manganese, and chromium all reduce toughness when added in the quantities needed for hardenability. This is the key to 26NiCrMoV8-5's lasting commercial dominance: nickel does the heavy lifting for through-hardening without sacrificing the impact energy the rotor needs to survive 30+ years of fatigue and thermal cycling.
The Role of Molybdenum in Temper Embrittlement Resistance
Large forgings are quenched and tempered at 580–660°C for many hours. During slow cooling through the embrittlement range of 400–550°C, impurity elements — principally phosphorus, tin, and antimony — can segregate to grain boundaries and cause catastrophic intergranular embrittlement. Molybdenum at 0.25–0.45% directly suppresses this grain-boundary segregation mechanism, which is what makes the grade reliable over decades of service in turbines subject to repeated start-stop thermal cycling. The mechanism is well-established in metallurgical literature: Mo suppresses the grain-boundary segregation of phosphorus and other tramp elements that cause intergranular embrittlement during slow tempering.
Vanadium: Grain Refinement and Secondary Hardening
At only 0.05–0.15%, vanadium forms nanoscale vanadium carbide (VC) precipitates during tempering that pin austenite grain boundaries during forging and austenitising, keeping grain size fine. A finer prior-austenite grain size translates directly to improved impact toughness and fatigue crack growth resistance — critical properties for a rotor that may accumulate 10⁸+ loading cycles over its service life. Vanadium also provides secondary hardening, allowing target strength to be reached without increasing carbon content, which would compromise toughness.
What SEW 555 Actually Requires
SEW 555 (1984) — Stähle für größere Schmiedestücke für Bauteile von Turbinen und Generatoren — is the governing German Steel Federation specification for 26NiCrMoV8-5 and related turbine-duty grades. It specifies chemical composition limits, minimum and maximum mechanical properties on the finished heat-treated forging, and critically, requires that mechanical test specimens be taken from mid-radius and core positions — not surface layers — to verify through-section uniformity.
- Tensile strength (Rm): 830 – 980 MPa
- Yield strength (Rp0.2): 690 – 830 MPa (position-dependent within section)
- Elongation (A5): ≥ 14%
- Reduction of area (Z): ≥ 45%
- Charpy impact energy (KV) at room temperature: ≥ 54 J (longitudinal)
- Charpy impact energy (KV) at −20 °C: ≥ 35 J (transverse)
- Brinell hardness: 248 – 302 HBW; surface-to-core spread ≤ 30 HB
- Test specimen sampling position: Mid-radius and core — not from surface layers only
- Heat treatment condition: Quenched and tempered (QT) — supplier must document full heat treatment record
The sampling position requirement is the defining feature of SEW 555. A supplier that meets surface properties but cannot demonstrate uniform core properties has not met the standard. For procurement engineers: always require that mechanical test certificates state the sampling position per SEW 555. A certificate that only shows surface hardness is insufficient for a code-compliant turbine rotor forging.
Grade Comparison: 26NiCrMoV8-5 vs. Competing Grades
The NiCrMoV turbine steel family comprises several grades differentiated primarily by nickel content, which determines through-hardening capability and therefore the maximum cross-section diameter at which uniform core properties can be guaranteed. 26NiCrMoV8-5 (1.6931) represents the cost-optimised solution for the 300–600 MW turbine class with section diameters up to approximately 650 mm.
| Grade | Material No. | Ni Content | Rm (MPa) | Max Section Ø | Primary Application |
|---|---|---|---|---|---|
| 26NiCrMoV8-5 | 1.6931 | 1.8 – 2.2% | 830 – 980 | ≥ 650 mm | LP/HP rotors, 300–600+ MW |
| 26NiCrMoV11-5 | 1.6948 | 2.6 – 3.2% | 880 – 1,030 | ≥ 800 mm | Very large LP rotors, generator shafts |
| 26NiCrMoV14-5 | 1.6957 | 3.4 – 3.8% | 930 – 1,080 | ≥ 1,000 mm | Ultra-large rotors, 1,000+ MW class |
| 30CrMoV9 | 1.7707 | None | 800 – 950 | ≤ 300 mm | Smaller turbine parts, high-strength bolting |
| ASTM A470 Class 8 | — | ~2.0% | ≥ 862 | ≥ 600 mm | US-market turbine rotors; near-equivalent to 1.6931 |
The Ni content gradient across the family is intentional engineering logic. As rotor cross-sections grow beyond 650 mm, the centre cooling rate during quench drops below the point where 1.8–2.2% Ni can guarantee a martensitic core. Increasing nickel to 2.6–3.2% (26NiCrMoV11-5) or 3.4–3.8% (26NiCrMoV14-5) extends the hardenability window. For most 300–600 MW turbines — the global operating fleet's largest single segment — 26NiCrMoV8-5 remains the technically proven, cost-optimised specification.
Why the Forging Process Is Inseparable from the Steel
Specifying 26NiCrMoV8-5 chemistry is necessary but insufficient. The forging process — steelmaking cleanliness, forging ratio, temperature control, and post-forge NDT — determines whether the alloy delivers its designed properties or retains the coarse, segregated cast structure that causes in-service failure.
Steelmaking Cleanliness
EAF + LF + VOD steelmaking is required to reduce hydrogen to <1.5 ppm, sulphur to <0.005%, and phosphorus to <0.010% — levels that prevent hydrogen embrittlement and temper embrittlement in thick sections. Standard EAF steel without VOD degassing is not acceptable for large turbine rotor grades.
Forging Ratio ≥ 3:1
A minimum area reduction of 3:1 from ingot to forging is required to break down the as-cast dendritic structure, close porosity, and establish a wrought microstructure with directional grain flow and uniform properties through the cross-section. Lower ratios leave casting defects that NDT cannot reliably detect.
Temperature Window
Forging must be conducted within 1,150 °C to 850 °C to achieve adequate plastic deformation and recrystallisation without hot-shortness cracking above the upper limit or adiabatic shear bands below the lower limit. Reheating intermediate to large forgings risks surface decarburisation that compromises fatigue properties.
Ultrasonic Testing
EN 10228-4 requires full-volume UT scanning of alloy steel forgings for turbine applications. Large rotor forgings are typically tested to Quality Class 4 (the most stringent standard class), with additional focused UT in high-stress disc regions per the turbine OEM specification.
For buyers sourcing 26NiCrMoV8-5 forgings: the steelmaker's in-house capabilities matter as much as the mill certificate. A vertically integrated supplier controlling steelmaking, forging, heat treatment, and NDT under one quality system eliminates the supply-chain discontinuities — change of ingot supplier, subcontracted heat treatment, third-party UT with different calibration blocks — that create quality risk in rotor forgings. Jiangsu Liangyi operates its own EAF + LF + VOD steelmaking line alongside 2,000T–6,300T open-die presses. View our complete 26NiCrMoV8-5 forging manufacturing capabilities.
Heat Treatment: Unlocking the Grade's Full Potential
26NiCrMoV8-5 forgings are delivered in the quenched and tempered (QT) condition, following a four-stage heat treatment sequence: dehydrogenation anneal, austenitising and quench, tempering, and mechanical property verification from core and mid-radius test prolongations per SEW 555.
Step 1 — Dehydrogenation Anneal (620–650 °C)
Before hardening, large forgings are soaked at 620–650 °C for extended periods — typically 1 hour per 25 mm of effective section thickness, often exceeding 24 hours for sections above 500 mm diameter. This drives out hydrogen absorbed during steelmaking and forging (principally from atmospheric moisture during reheating). Hydrogen remaining in the steel at levels above ~2 ppm causes delayed, subcritical crack growth — hydrogen-induced cracking that may not manifest until weeks after delivery. This step is mandatory for section sizes above 300 mm and is routinely skipped by under-equipped suppliers to reduce cycle time and energy cost.
Step 2 — Austenitising and Quench (840–900 °C)
The forging is heated to 840–900 °C, held at temperature for time sufficient to fully homogenise the austenite chemistry through the full cross-section (typically 1 hour per 25 mm of effective section plus a core soak allowance), then immediately quenched — water or controlled-concentration polymer solution — in a tank large enough to achieve rapid, uniform cooling at the core. A properly sized quench tank for a 650 mm section diameter requires 110–200 m³ capacity with forced circulation to maintain cooling medium temperature within ±5 °C and prevent vapour film formation that would impede heat transfer at the forging surface.
Step 3 — Tempering (580–660 °C)
The as-quenched forging is tempered at a temperature selected to achieve the target strength level per SEW 555. Soak times for a large rotor section (600+ mm effective section) can exceed 30 hours to ensure temperature uniformity within ±5 °C at the core. This is where vanadium carbides nucleate and grow, providing secondary hardening and grain pinning. Controlled cooling from tempering temperature through the embrittlement range (400–550 °C) is specified by some end users as an additional precaution against residual temper embrittlement — particularly important for sections that could not be cooled rapidly during quenching.
Step 4 — Mechanical Property Verification
Specimens are cut from sacrificial test prolongations machined onto the forging specifically for this purpose, at sampling positions defined by SEW 555: typically at one-quarter radius, mid-radius, and core. All required properties — tensile strength, yield strength (0.2% proof stress), elongation, reduction of area, Charpy V-notch impact energy at RT and at −20 °C, and Brinell hardness — must be recorded and certified before the forging is released for machining. Any property falling outside the SEW 555 range triggers a non-conformance requiring re-heat treatment or scrap disposition.
Quality Assurance and Certification Requirements
Large turbine rotor forgings require a comprehensive inspection and certification regime because the consequences of in-service failure — unplanned outage, turbine destruction, personnel hazard — are catastrophic. For 26NiCrMoV8-5 forgings supplied to European turbine OEMs, EN 10204 3.1 is the baseline certificate; EN 10204 3.2 (with named third-party inspector witness) is typically required by end users and insurance underwriters.
| Inspection / Certification | Standard | Scope |
|---|---|---|
| Material Test Certificate 3.1 | EN 10204 3.1 | Chemical analysis + all mechanical properties, certified by manufacturer's authorised quality representative |
| Third-Party Witness Certificate 3.2 | EN 10204 3.2 | All 3.1 data, countersigned by a buyer-nominated independent third-party inspector (TPI), arranged and paid for by the buyer |
| Ultrasonic Testing Report | EN 10228-4 / SEW 555 | Full-volume UT; acceptance quality class per turbine OEM or buyer specification (typically Class 3 or 4) |
| Magnetic Particle Inspection | EN 10228-1 | Surface and near-surface discontinuity detection on all accessible forging surfaces |
| Dimensional Inspection Report | Customer drawing / ISO GPS | Full dimensional verification of all forging outline dimensions and tolerances per agreed drawing |
| Hardness Survey | SEW 555 / customer specification | Brinell hardness at multiple defined positions; verifies surface uniformity and compliance with the 248–302 HBW window |
Confirm at the quotation stage — not after order placement — whether the supplier can host TPI witness inspections at their facility. Suppliers who must transfer forgings to a third-party inspection house introduce additional handling risks and scheduling delays that can significantly extend lead time on a large rotor forging.
Why 26NiCrMoV8-5 Has Remained Irreplaceable for 50+ Years
Despite 50 years of materials science advancement, 26NiCrMoV8-5 remains the dominant specification for LP and IP steam turbine rotor forgings in the 300–600 MW class because no alternative alloy has demonstrated a superior combination of through-section hardenability, impact toughness, temper embrittlement resistance, and cost at this section size range.
An Optimised Balance
At operating temperatures up to 450 °C and section diameters of 300–650 mm, the Ni-Cr-Mo-V composition window of 26NiCrMoV8-5 represents a true engineering optimum. Dozens of composition variants have been explored since the 1970s; none have improved the balance sufficiently to displace the established grade.
Industrial Ecosystem
Five decades of production have built a validated manufacturing ecosystem — specialised forging manufacturers, qualified heat treaters, calibrated UT inspection teams, and turbine OEM material qualifications — specific to this grade. Switching to an alternative incurs re-qualification costs with no guaranteed return.
Regulatory Track Record
Power generation equipment is subject to pressure equipment directives (PED 2014/68/EU), turbine OEM type-approval systems, and insurance underwriter specifications. 26NiCrMoV8-5 is already qualified in all these systems; qualifying a new material requires years of field data that no candidate material can yet provide.
In turbine materials engineering, an optimised system that works reliably for 50 years is vastly preferable to a theoretically superior system that introduces unknown failure modes. 26NiCrMoV8-5 is not still the standard because the engineering community lacks imagination — it is still the standard because it remains the right engineering answer for its application class.
Principle reflected across major turbine OEM material specifications worldwideSourcing 1.6931 Forgings: What Buyers Should Know
When sourcing 26NiCrMoV8-5 (1.6931) forging parts, buyers should verify eight key supplier capabilities: integrated steelmaking with VOD, press capacity, quench tank sizing, furnace temperature uniformity, in-house NDT, EN 10204 3.2 witness inspection capability, reference forging documentation, and multi-country export experience.
- Integrated steelmaking: Supplier must operate EAF + LF + VOD melting in-house. Ingot purchased from a separate steelmaker eliminates the supplier's ability to guarantee hydrogen and residual element control — the factors that most directly determine thick-section toughness.
- Open-die press capacity: Minimum 2,000T is required for quality large-section forging; 6,300T preferred for forgings above 10,000 kg to complete the forging sequence without intermediate reheats that risk decarburisation.
- Quench tank volume: 110–200 m³ with forced circulation pumps. Smaller tanks heat up too quickly during quenching and fail to achieve the required core cooling rate for thick sections.
- Furnace temperature uniformity: ±5 °C documented uniformity across the working zone; full thermal logging records provided with each heat treatment batch.
- In-house NDT laboratory: UT, MPI, and hardness testing must be performed in-house by permanently employed, qualified personnel — not subcontracted to a third-party inspection house whose scheduling is independent of the forging manufacturer.
- EN 10204 3.2 witness inspection capability: The supplier must be willing and logistically equipped to host buyer-nominated TPI witness inspections at their facility without requiring forging transfer.
- Reference forging documentation: Request certificates from comparable large-section 26NiCrMoV8-5 forgings previously supplied, showing section dimensions, test position, and all SEW 555 properties.
- Export documentation competence: Delivery to 50+ countries confirms familiarity with export packing, heavy-forging logistics, and international payment and documentation requirements for high-value industrial goods.
Jiangsu Liangyi has manufactured 1.6931 (26NiCrMoV8-5) forging parts since 1999, supplying power generation, oil & gas, and marine customers in more than 50 countries. Our Jiangyin facility combines in-house EAF + LF + VOD steelmaking, 2,000T–6,300T open-die presses, 1 M and 5 M ring rolling mills, and a fully equipped NDT laboratory — covering every stage from liquid steel to certified finished forging under a single ISO 9001:2015 quality management system.
All 26NiCrMoV8-5 turbine rotor forgings are supplied with EN 10204 3.1 certificates as standard. EN 10204 3.2 certification is available when buyers arrange third-party witness inspection. Third-party inspection can be arranged with internationally recognised organisations such as TÜV, DNV, Bureau Veritas, or SGS, coordinated by the buyer.
26NiCrMoV8-5 (1.6931) — Frequently Asked Questions
26NiCrMoV8-5 (material number 1.6931) is primarily used for large steam turbine rotor forgings in the 300–600+ MW power generation class. It is also used for generator shafts, seamless rolled rings for turbine disc stages, and other heavy-duty rotating components requiring uniform mechanical properties through cross-sections exceeding 500 mm in diameter. The grade is governed by SEW 555, the German Steel Federation standard for steels for turbine and generator forgings.
The material number for 26NiCrMoV8-5 is 1.6931 per DIN EN 10027-2. The grade is also commonly written as 26NiCrMoV85 or 26NiCrMoV8.5. Its nearest US equivalent is ASTM A470 Class 8.
26NiCrMoV8-5 forgings are governed by SEW 555 (1984), published by the German Steel Federation (Stahl-Eisen-Werkstoffblatt). Full title: Stähle für größere Schmiedestücke für Bauteile von Turbinen und Generatoren (Steels for Larger Forgings for Components in Turbines and Generators). SEW 555 specifies chemical composition, mechanical properties tested at mid-radius and core positions, hardness limits, and required heat treatment conditions.
Per SEW 555, 26NiCrMoV8-5 (1.6931) forgings in the quenched and tempered condition must meet: tensile strength (Rm) 830–980 MPa; yield strength (Rp0.2) 690–830 MPa; elongation (A5) minimum 14%; reduction of area (Z) minimum 45%; Charpy KV at room temperature minimum 54 J (longitudinal); Charpy KV at −20 °C minimum 35 J (transverse); Brinell hardness 248–302 HBW with surface-to-core spread ≤ 30 HB.
The primary difference is nickel content and achievable section size. 26NiCrMoV8-5 (1.6931) contains 1.8–2.2% Ni and is used for turbine rotor sections up to approximately 650 mm diameter in the 300–600 MW class. 26NiCrMoV11-5 (1.6948) contains 2.6–3.2% Ni and provides superior through-hardening for cross-sections above 800 mm in the 600–1,000 MW class. Both are governed by SEW 555. The higher nickel content increases material cost but is necessary to maintain uniform core properties in larger sections.
26NiCrMoV8-5 forgings require four sequential heat treatment steps: (1) Dehydrogenation anneal at 620–650 °C (extended soak per 100 mm of section); (2) Austenitising at 840–900 °C followed by water or polymer quench; (3) Tempering at 580–660 °C for up to 30+ hours for large sections; (4) Mechanical property verification from core and mid-radius test prolongations per SEW 555.
Jiangsu Liangyi Co., Limited, based in Jiangyin, Jiangsu Province, China, has manufactured 26NiCrMoV8-5 (1.6931) forging parts since 1999. ISO 9001:2015 certified. In-house EAF + LF + VOD steelmaking, 2,000T–6,300T open-die presses, 1M and 5M ring rolling mills, full NDT laboratory. EN 10204 3.1 MTC issued as standard; EN 10204 3.2 available when buyer nominates and arranges a third-party inspector (TPI). Exports to 50+ countries. For full product specifications, composition tables, and pricing — see our 1.6931 forging parts product page.