347 Stainless Steel: Composition, Mechanical Properties and High-Temperature Limits

347 Stainless Steel at a Glance

Type 347 is a niobium-stabilized austenitic stainless steel that combines the general corrosion behavior of a standard 18-8 grade with resistance to intergranular corrosion in welded and high-temperature service. The niobium addition is the defining feature of the grade. It reacts with carbon during solidification and subsequent thermal exposure to form niobium carbide, so chromium remains available to build the passive oxide film instead of being consumed by chromium carbide at grain boundaries. This data-sheet style summary sets out the composition, mechanical properties, temperature limits, corrosion performance and processing practice for the grade in a form that can be used directly for specification review.

Chemical Composition

Element	Content	Role
Chromium	18.0–20.0%	Passive film, oxidation and corrosion resistance
Nickel	9.0–13.0%	Austenitic structure and toughness
Niobium plus tantalum	10×C min, usually 1.0% max	Stabilizing element that forms niobium carbide
Carbon	0.08% max (347L: 0.03% max)	Kept low to limit carbide precipitation
Manganese	2.00% max	Austenite former and deoxidizer
Silicon	0.75% max	Melting and scale control
Phosphorus	0.045% max	Residual element control
Sulfur	0.030% max	Weldability and hot workability

Buyers should note that a mill certificate showing only chromium and nickel is incomplete. The niobium plus tantalum figure and the carbon content together define whether the stabilization ratio has been met, and they are the two values most often queried during incoming inspection.

Mechanical Properties in the Annealed Condition

Property	Typical Requirement
Tensile strength	515 MPa minimum
Yield strength (0.2% offset)	205 MPa minimum
Elongation in 50 mm	40% minimum
Hardness	Approximately 95 HRB maximum

The grade has good processability and weldability and can be strengthened by cold working, since it is not hardenable by heat treatment. Strength at elevated temperature is the reason the higher-carbon Type 347H variant exists; it is specified where creep and stress-rupture performance govern the design.

High-Temperature Performance

The useful long-term service range for 347 extends from cryogenic temperatures down to about minus 196 °C up to approximately 870 °C, with short-term excursions to about 980 °C tolerated in oxidizing conditions. Oxidation resistance is better than that of Type 304, and the grade maintains useful oxidation and carburization resistance in the 650 to 870 °C band. Because niobium carbides resist coarsening, the alloy holds creep strength better than titanium-stabilized grades during long dwell times, which is why furnace rolls, radiant tubes and turbine hardware are often specified in 347.

Corrosion Resistance

Intergranular corrosion resistance is the grade's principal advantage. Thanks to niobium stabilization, resistance remains strong even after long service or welding inside the sensitization range of 427 to 816 °C, and it is significantly better than Type 304 in that respect. General corrosion resistance is good in the atmosphere, in water, in most organic acids such as nitric acid and in weak alkalis.

Two limitations follow directly from the absence of molybdenum. Pitting and crevice corrosion resistance is similar to Type 304 and lower than Type 316. In strongly reducing acids such as hydrochloric acid, or in high-chloride environments, the grade is not adequate and a molybdenum-bearing or duplex stainless steel should be selected instead.

Heat Treatment, Surface Treatment and Service Restrictions

Solution treatment at 1020 to 1100 °C with rapid quenching optimizes corrosion resistance.

Annealing is applied where maximum ductility is needed before severe forming.

Polishing and passivation improve appearance and corrosion performance.

Avoid surface defects caused by niobium enrichment, which can act as corrosion initiation sites.

Do not apply 347 in strong reducing acids or high-concentration chloride service.

Welding is performed with matching filler metal using standard arc processes, and because the grade is already stabilized, welded joints retain their corrosion resistance without post-weld annealing. Controlled heat input and interpass temperature are still recommended to protect joint toughness.

Frequently Asked Questions

Q: What is the carbon content of 347 stainless steel?
Standard Type 347 is limited to 0.08 percent carbon maximum, while the low-carbon 347L version is limited to 0.03 percent maximum. Both grades still require niobium plus tantalum at a minimum of ten times the carbon content.

Q: What temperature range can 347 withstand?
Long-term service is generally quoted as minus 196 °C up to about 870 °C, with short-term exposure to around 980 °C acceptable in oxidizing conditions. Pressure-code allowable stresses are normally published only up to about 816 °C.

Q: Why is niobium added to 347?
Niobium preferentially combines with carbon to form niobium carbide, preventing chromium from forming chromium carbide at grain boundaries. That keeps chromium in solution and protects the heat-affected zone of welds against intergranular corrosion.

Q: Is 347 better than 316 for corrosion resistance?
No. Type 316 contains molybdenum and resists pitting and crevice corrosion better. Type 347 is superior specifically for intergranular corrosion resistance in welded and high-temperature service.

Q: What is the difference between 347 and 347H?
347H has a higher carbon range, which increases creep and stress-rupture strength for elevated-temperature pressure service. Standard 347 is preferred where welding and general corrosion resistance are the priorities.

Q: Can 347 be machined and formed easily?
Yes. The grade cold forms by bending and rolling with slightly lower ductility than Type 304, and it machines in the annealed condition using the heavy feeds and sharp tooling typical of austenitic stainless steels.