What Is Incoloy 27-7Mo? UNS S31277

Ask a corrosion engineer to name the moment a stainless steel stops being "ordinary," and the answer usually points to molybdenum. The super-austenitic grade 27-7Mo lives at that moment. Designated UNS S31277, it is an advanced stainless steel built around roughly seven percent molybdenum — enough to leave conventional 6% Mo grades behind and, in many aggressive environments, to close in on far more expensive nickel-base alloys. This guide breaks down what the alloy is made of, how it behaves, where it earns its keep, and why it has become a go-to choice for critical forged components.

1. What Alloy 27-7Mo Is Made Of

A typical heat of 27-7Mo carries about 27% nickel, 22% chromium, 7.2% molybdenum, and 0.34% nitrogen, with roughly one percent copper and the balance iron. The generous nickel content, working with nitrogen, locks in a fully austenitic microstructure that stays stable and tough. Chromium and molybdenum form the frontline defence against localized attack, while nitrogen sharpens pitting resistance and adds strength at the same time.

The controlled, limiting composition specified for the grade is shown below.

2. Why 7% Molybdenum Changes the Game

Localized corrosion — pitting and crevice attack — is what kills most stainless components in chloride service. The best single predictor of that resistance is the Pitting Resistance Equivalent Number (PREN), calculated from chromium, molybdenum, and nitrogen content. For UNS S31277 the PREN lands near 43, above standard 316L and, thanks to its higher molybdenum and nitrogen, ahead of typical 6% Mo super-austenitics on crevice performance.

Every extra percent of molybdenum buys resistance in the exact places corrosion likes to hide — inside crevices, under deposits, and at the bottom of a growing pit.

In the lab metrics engineers actually specify, the alloy resists pitting to a critical pitting temperature above roughly 85 °C and crevice attack to a critical crevice temperature near 45 °C under standard ferric-chloride testing. Its nickel and nitrogen content also give strong resistance to chloride stress-corrosion cracking — a failure mode that quietly ends the life of many lower-nickel stainless steels in warm brine.

The elements divide the acid work between them. Molybdenum, copper, and nickel handle reducing conditions, while high chromium covers oxidising media. That balance is why 27-7Mo performs so well in mixed acid streams — the messy, real-world chemistries where a material must fight oxidising and reducing attack at once.

3. How Does Alloy 27-7Mo Compare to Other Alloys?

The quickest way to place UNS S31277 is to line it up against the grades it competes with — from workhorse 316L up to a nickel-base benchmark. Molybdenum content and PREN track closely with localized-corrosion resistance.

PREN and Mo figures are approximate, are published typical values for each grade, and depend on the formula and actual heat chemistry; always verify against the mill certificate for a given lot.

The takeaway: UNS S31277 occupies the sweet spot between mainstream 6% Mo super-austenitics and the nickel-base alloys. It offers a meaningful step up in molybdenum and nitrogen over typical 6Mo grades, yet stays well below the price and lead time of nickel-base alloy 625 — which is exactly why specifiers reach for it when a 6Mo grade is marginal but a nickel alloy is overkill.

4. Mechanical & Physical Properties

Corrosion resistance would mean little if the alloy were weak or hard to shape. 27-7Mo delivers an unusually good balance of strength and ductility for a corrosion-grade stainless, thanks partly to its nitrogen, and keeps useful strength as temperatures climb.

For pressure equipment, allowable design stresses for ASME Boiler and Pressure Vessel Code construction are addressed under ASME Code Case 2458 — a materials code case that defines design values for the grade, not a manufacturer certification. The grade is also covered by familiar product standards — bar and forging stock to ASTM/ASME A/SA 479, forgings and fittings to A/SA 182, plate and sheet to A/SA 240, and pipe and tube to the A/SA 213, 249, and 312 series.

5. Key Advantages at a Glance

Put together, the chemistry and mechanics give 27-7Mo a distinctive value proposition:

6. Where Alloy 27-7Mo Is Used

The alloy shows up wherever chloride- and acid-driven corrosion set the design limit. Its breakout application is flue gas desulfurization: in the wet, low-pH, chloride-laden environment of an FGD scrubber on a high-sulfur coal plant, 27-7Mo outlasts many alternatives. From there it spreads across heavy industry.

In each of these, components are rarely simple — flanges that must seal, discs and shafts that must carry load, rings and tube sheets that must hold pressure. These are parts where a forged structure, with its refined grain flow and freedom from casting defects, is the difference between long life and early failure.

Why forged 27-7Mo, specifically

Forging aligns the metal's grain with the shape of the part, improving fatigue life and soundness compared with cast or machined-from-plate equivalents — a real advantage for pressure-containing and load-bearing hardware. The grade is supplied as forging stock precisely so engineers can specify these near-net, high-integrity shapes. If you need it as a finished component, our Incoloy 27-7Mo (UNS S31277) forging parts page is the place to order — it details the forged rings, flanges, discs, shafts, bushings, tube sheets, and sleeves Jiangsu Liangyi produces to order, supplied with material test reports and any third-party inspection you arrange, under our ISO 9001-certified quality management system.

7. Frequently Asked Questions

What is Incoloy 27-7Mo (UNS S31277)?

It is an advanced 7% molybdenum super-austenitic stainless steel with about 27% nickel, 22% chromium, 7.2% molybdenum and 0.34% nitrogen. The high alloy content gives it corrosion resistance beyond 6% molybdenum grades, approaching several nickel-base alloys in many aggressive media.

What is the chemical composition of UNS S31277?

Typically ~27% Ni, 22% Cr, 7.2% Mo, 0.34% N and ~1% Cu, balance iron. Limiting ranges are Ni 26.0–28.0, Cr 20.5–23.0, Mo 6.5–8.0, N 0.30–0.40 and Cu 0.50–1.50, with carbon held to 0.020 max.

What is the PREN of alloy 27-7Mo?

About 43, driven by its chromium, molybdenum and nitrogen content. Combined with roughly 7.2% molybdenum, it resists pitting and crevice corrosion better than conventional 6% molybdenum super-austenitic grades.

Is 27-7Mo a stainless steel or a nickel alloy?

It is a super-austenitic stainless steel. With about 27% nickel it sits at the very high-alloy end of stainless steels and bridges the gap toward nickel-base alloys, delivering much of their corrosion performance at a lower price point.

How does it compare with 6Mo super-austenitics and nickel-base alloy 625?

UNS S31277 carries about 7.2% molybdenum versus roughly 6% in typical 6Mo grades (such as UNS S31254), plus higher nitrogen, giving stronger crevice and pitting performance. In many chloride and mixed-acid environments it approaches nickel-base alloys such as UNS N06625, making it a cost-effective alternative when those alloys would be over-specified.

Can alloy 27-7Mo be welded and machined?

Yes. It is readily welded by common arc processes (GTAW, GMAW, SMAW, SAW, PAW) with suitable matching or over-alloyed fillers, and it machines much like other super-austenitic stainless steels — best worked in the solution-annealed condition with rigid setups and positive feeds.

What product forms is the alloy available in?

Standard forms include plate, sheet, rod, bar, wire, pipe, tube, and forging stock. For forged components it is supplied as forging stock and converted into rings, flanges, discs, shafts, bushings, sleeves, and tube sheets.

8. The Bottom Line

Alloy 27-7Mo (UNS S31277) is what happens when a stainless steel is engineered to punch above its class. Seven percent molybdenum, backed by high nickel, chromium, and nitrogen, delivers a PREN of about 43, strong resistance to pitting, crevice attack, and chloride cracking, and a dependable balance of strength and formability — all at a cost below the nickel alloys it often replaces. For the harshest corners of pollution control, marine, chemical, and oil-and-gas service, it remains one of the smartest material choices available, and in forged form it brings the added integrity those critical parts require.