347 Stainless Steel: Niobium-Stabilized Grade for High-Heat Service

What Niobium Stabilization Does for Type 347

Type 347 is an austenitic chromium-nickel stainless steel stabilized with niobium. Its purpose is straightforward: to keep carbon away from chromium so that the protective passive film is not destroyed by carbide precipitation during welding or long exposure to elevated temperature. Niobium has a stronger affinity for carbon than chromium does, so it forms niobium carbide preferentially and leaves chromium in solid solution at the grain boundaries. The practical result is a welded structure that resists intergranular corrosion in the heat-affected zone without any need for post-weld solution annealing, which is why the grade appears so often in high-temperature piping, furnace hardware and welded process equipment.

The alloy is standardized as Type 347 in ASTM A240 for plate, sheet and strip, ASTM A213 and A312 for tube and pipe, and ASTM A182 for forgings, with the European equivalent designated EN 1.4550 (X6CrNiNb18-10). The higher-carbon Type 347H version is specified where improved creep and stress-rupture strength is required.

Chemical Composition and Stabilizer Ratio

Base chemistry is close to Type 304, with the decisive addition of niobium plus tantalum at a minimum of ten times the carbon content. That ratio guarantees enough stabilizer to combine with all available carbon, and it is the number that matters most when reviewing a mill certificate.

Element	Requirement	Notes
Chromium	17.0–19.0%	Passive film and oxidation resistance
Nickel	9.0–13.0%	Austenite stability
Niobium plus tantalum	10×C min, 1.00% max	Stabilizing element; 8×C min for Type 347H
Carbon	0.08% max	0.03% max for the low-carbon Type 347L
Manganese	2.00% max	Austenite former, deoxidation
Silicon	0.75% max	Melting and scale behavior
Phosphorus	0.045% max	Residual element control
Sulfur	0.030% max	Weldability and hot workability

The upper limit on niobium is just as important as the minimum: excessive niobium promotes the formation of iron-niobium intermetallic phases in service, which reduce ductility and toughness. Specifications that keep niobium plus tantalum near one percent or below give the best balance of stabilization and long-term stability.

High-Temperature Behaviour and Service Limits

Type 347 is used continuously up to about 816 °C, which is the temperature above which allowable stresses for the grade in most pressure-vessel and piping codes are no longer published. The alloy retains good creep and oxidation resistance through that range, and its niobium carbide particles coarsen slowly, so it holds its strength better than titanium-stabilized Type 321 during prolonged high-heat exposure. For short excursions the material tolerates temperatures approaching 900 °C without immediate damage, but design should always be based on continuous limits rather than on short-term capability.

In the 650 to 870 °C band the grade maintains both oxidation resistance and resistance to carburization, which is the reason it is used for furnace internals, radiant components and turbine hardware. Where the atmosphere is strongly reducing or carbon-rich, the niobium-stabilized structure resists carbon absorption better than unstabilized austenitic grades.

Corrosion Resistance and Where It Stops

The headline property is intergranular corrosion resistance. Because niobium carbide forms instead of chromium carbide, the alloy remains immune to sensitization even after long service in the 427 to 816 °C range or after welding. In welded assemblies this is a decisive advantage over Type 304, which can fail along grain boundaries in the heat-affected zone.

In general corrosion terms, Type 347 performs well in the atmosphere, in fresh water, in most organic acids such as nitric acid and in weak alkaline solutions. Two limits should be understood before selection. First, its resistance to pitting and crevice corrosion is similar to Type 304, because it contains no molybdenum, and therefore lower than Type 316. Second, in strongly reducing acids such as hydrochloric acid or in high-chloride environments, the grade is not sufficient and a molybdenum-bearing or duplex stainless steel should be chosen instead. Type 347 solves an intergranular problem; it is not a general-purpose corrosion upgrade.

Fabrication, Welding, Heat Treatment and Surface Condition

Type 347 is weldable with all standard arc processes and is normally joined with matching filler metal. Because the grade is already stabilized, welded joints do not need a post-weld anneal to restore intergranular corrosion resistance, although controlled heat input and interpass temperature are still recommended to protect toughness. The material cold forms and bends readily, with ductility somewhat lower than Type 304 but with higher strength at temperature, and it can be strengthened by cold working.

Where maximum corrosion resistance is required, a solution treatment at 1020 to 1100 °C followed by rapid quenching is applied; an anneal is used where maximum ductility is the priority. Polishing and passivation improve both appearance and corrosion performance, but surface defects associated with niobium enrichment should be avoided during finishing, since they can act as initiation sites. Standard mechanical properties in the annealed condition are a tensile strength of at least 515 MPa, a yield strength of at least 205 MPa and an elongation of at least 40 percent.

Typical Applications

Gas turbine components, combustor liners and exhaust hardware

Nuclear reactor piping, internals and pressure-retaining welds

Industrial furnace parts, radiant tubes, retorts and heat-treating fixtures

Chemical and petrochemical process equipment with welded joints at elevated temperature

Expansion joints, bellows and high-temperature fasteners

Superheater and heat-exchanger tubing in power generation

Frequently Asked Questions

Q: What defines the chemical composition of 347?
It contains 17 to 19 percent chromium, 9 to 13 percent nickel, 0.08 percent maximum carbon and niobium plus tantalum at a minimum of ten times the carbon content. The niobium forms stable carbides and prevents chromium depletion in weld zones.

Q: How does 347 perform in high-temperature environments?
It is used continuously up to about 816 °C with good creep and oxidation resistance, and its niobium stabilizer keeps it more stable than Type 321 in prolonged high heat, which suits furnace parts and turbine components.

Q: What are its corrosion resistance properties?
It offers excellent resistance to intergranular corrosion even after welding, outperforming Type 304 in welded structures, and resists general corrosion in mild chemicals and atmospheric conditions, though it is less resistant to chlorides than Type 316.

Q: How does 347 compare with 321 in fabrication?
Both weld with standard techniques, but 347 produces more stable welds for high-temperature service. It has good formability for bending and rolling, with slightly lower ductility than Type 304 but better strength at temperature.

Q: Does 347 need post-weld heat treatment?
No. Because the carbon is already tied up by niobium, welded joints retain their intergranular corrosion resistance in the as-welded condition. Solution annealing is applied only when maximum corrosion resistance or ductility is required.

Q: Where is 347 commonly used?
It is found in gas turbine components, nuclear reactor piping and industrial furnace parts, and in chemical processing equipment whose welded joints operate at elevated temperature.