30CrMoNiV5-11 vs 26NiCrMoV11-5: Which Turbine Rotor Steel Should You Choose?

Two Ni-Cr-Mo-V forging grades, two different design philosophies. One is built for heat; the other for toughness. Here is how to pick the right one for your rotor, disc or shaft — without over-spending or under-engineering.

Also written as: 30CrMoNiV5-11 / 30CrMoNiV5.11 / 1.6946  ·  26NiCrMoV11-5 / 26NiCrMoV11.5 / 1.6948

If you are specifying a forged rotor, disc, valve spindle or heavy shaft for a turbine or generator, you have almost certainly seen both of these grades on a drawing or in a supplier quote. They look similar on paper — both are quenched-and-tempered low-alloy steels with chromium, molybdenum, nickel and a little vanadium — and both serve the same families of rotating components. That surface similarity is exactly why so many design and procurement engineers ask us which one to use.

The honest answer is that they are not interchangeable, and swapping one for the other without checking the service envelope can either waste money or shorten component life. The difference comes down to a single trade-off baked into their chemistry: chromium-and-vanadium for high-temperature creep strength, versus nickel for toughness and deep hardenability. Everything else follows from that one choice.

Meet the two contenders

30CrMoNiV5-11 (material number 1.6946) is a chromium-molybdenum-vanadium creep-resistant low-alloy forging steel defined in the German SEW 555 specification for large turbine and generator forgings. 26NiCrMoV11-5 (material number 1.6948) is a high-nickel Ni-Cr-Mo-V low-alloy forging steel defined in EN 10083-3, used where toughness and deep hardenability matter most.

Side-by-side: chemical composition

The numbers below tell the whole story. Watch two columns in particular — nickel and chromium. They move in opposite directions, and that single inversion is the root cause of every performance difference that follows.

Notice what the metallurgist did here. In 30CrMoNiV5-11, the extra chromium and the deliberate vanadium addition form fine, thermally stable carbides that pin dislocations and refuse to coarsen at red heat — precisely what you want for creep resistance. In 26NiCrMoV11-5, the priority is different: nickel does almost nothing for creep, but it dramatically improves hardenability and pushes the ductile-to-brittle transition temperature downward, so the steel stays tough even in a thick rotor body or during a cold winter start-up.

Side-by-side: mechanical & thermal behaviour

A practical decision guide

Set the data sheet aside for a moment and answer one question first: what actually limits your component — temperature, or size and fracture safety? Then follow the matching branch below.

When neither one is the answer

If your design steam temperature climbs above about 580 °C — for example in an ultra-supercritical plant — both grades run out of creep strength, and you should be looking at a 9–12% chromium martensitic steel such as X12CrMoWVNbN10-1-1 instead. At the other end, if the part never sees more than ~450 °C and creep is irrelevant, a general-purpose grade like 34CrNiMo6 is often all you need, at lower cost. Specifying a premium turbine grade for a mild-duty shaft is a common and avoidable over-spend.

What this means for forging and inspection

Both grades are deep-hardening, vanadium- or nickel-rich alloys, so both demand the same disciplined production route: clean melting with vacuum degassing to keep phosphorus and sulfur low, a generous forging reduction ratio to refine grain and close internal voids, and tightly controlled quench-and-temper cycles. Heavy 26NiCrMoV11-5 rotors are commonly given a double temper to fully stabilise the microstructure and beat reversible temper embrittlement, which its higher nickel makes it more sensitive to. For 30CrMoNiV5-11, the tempering window is chosen to balance creep strength against toughness without softening the carbides that do the high-temperature work.

Whichever grade you specify, the part should leave the shop with 100% volumetric ultrasonic testing, magnetic-particle inspection of machined surfaces, full mechanical testing from representative locations, and an EN 10204 inspection document — a 3.1 certificate signed by the manufacturer, or a 3.2 certificate countersigned by a buyer-nominated independent inspector — covering chemistry, heat-treatment records and trace elements. (These are per-shipment material certificates, not company accreditations.) For a rotor that will spin for 30 years, the paperwork is part of the product.

Glossary of key terms

Creep rupture strength

The stress that causes a material to fracture after a fixed time at high temperature, typically quoted for 100,000 hours. It is the dominant design limit for hot turbine rotors.

Hardenability

The depth to which a steel can be hardened by quenching. High nickel (as in 26NiCrMoV11-5) lets large cross-sections harden fully to the core.

DBTT (ductile-to-brittle transition temperature)

The temperature below which a steel fractures in a brittle manner. Nickel lowers the DBTT, improving cold-start and low-temperature safety.

Temper embrittlement

A loss of toughness caused by tramp elements segregating to grain boundaries during slow cooling or long service at 350–575 °C. Controlled by low P/S and, for heavy rotors, double tempering.

Q&T (quenched and tempered)

The standard heat-treatment condition for both grades, giving a tempered-martensite structure that balances strength and toughness.

Frequently asked questions