321 vs 347 Stainless Steel: Stabilized High-Temperature Alloys Compared

What Makes a Stainless Steel Stabilized?

321 and 347 belong to the 18/8 austenitic family (about 18% chromium and 9-13% nickel) and solve a specific problem. When ordinary 304 is heated in the range of roughly 425-815 °C - during welding or in service - carbon can combine with chromium at grain boundaries to form chromium carbides, leaving the adjacent metal chromium-depleted and vulnerable to intergranular corrosion. Stabilized grades add an element that bonds with carbon even more readily than chromium does: titanium in 321, niobium in 347. The carbon is locked up as stable carbides, so the chromium stays in solid solution and the material keeps its corrosion resistance after welding and in service.

Chemical Composition Compared

Composition ranges per ASTM A240 (wt%):

Element	321 / UNS S32100	347 / UNS S34700
Carbon, max	0.08	0.08
Manganese, max	2.00	2.00
Phosphorus, max	0.045	0.045
Sulfur, max	0.030	0.030
Silicon, max	0.75	0.75
Chromium	17.00-19.00	17.00-19.00
Nickel	9.00-12.00	9.00-13.00
Titanium	5 x C min; 0.70 max	Not specified
Niobium (Columbium)	Not specified	10 x C min; 1.00 max

The stabilization ratios are written into the standard: titanium must be at least five times the carbon content and no more than 0.70%; niobium must be at least ten times the carbon content and no more than 1.00%.

Mechanical Properties (Annealed, ASTM A240)

Property	321 / S32100	347 / S34700
Tensile strength, min (MPa)	515	515
Yield strength, min (MPa)	205	205
Elongation, min (%)	40	40

At room temperature the two grades are essentially equivalent, which is why material selection between them is decided by service temperature and loading rather than by datasheet strength.

High-Temperature Performance: Creep and Scaling

Both grades resist scaling in oxidizing atmospheres to about 900 °C. The important difference appears in creep: above about 800 °C, 347 develops better creep strength because niobium carbides remain finely distributed and stable at high temperature, providing more effective precipitation strengthening than titanium carbides. For this reason 347 is preferred for pressure-containing components such as heater tubes and high-temperature reaction vessels. 321 remains widely used in exhaust systems, thermal oxidizers and expansion bellows, where good fabricability and resistance to cyclic thermal loading are valued.

Welding and Fabrication

Both grades weld readily with conventional austenitic stainless steel practice, and neither requires post-weld heat treatment. For 347 joints, filler metal of matching niobium-stabilized composition (ER347) is typical. 321 is often welded with the same ER347 filler, or with ER321 where a matching titanium-stabilized deposit is required, because titanium oxidizes easily in the weld pool. Heat input should be kept moderate and interpass temperature below about 150 °C to limit carbide precipitation and distortion.

Typical Applications

321: aircraft exhaust manifolds and stacks, thermal oxidizers, expansion bellows, furnace components below about 900 °C, and chemical or petrochemical piping at moderate temperature.

347: refinery heater tubes, high-temperature reaction vessels, steam superheater and reheater tubing, boiler components, and welded assemblies that see sustained service above 800 °C.

Frequently Asked Questions

What is the main difference between 321 and 347?

The stabilizing element: 321 uses titanium (5 x C minimum, 0.70% maximum), while 347 uses niobium (10 x C minimum, 1.00% maximum). The nickel range also differs slightly: 9.00-12.00% for 321 and 9.00-13.00% for 347.

Which grade has better creep strength at high temperature?

347. Above about 800 °C, niobium carbides provide stronger precipitation strengthening than titanium carbides, giving 347 better creep resistance in long-term high-temperature service.

Can 321 be substituted for 347?

For service below about 800 °C and where creep is not the governing criterion, 321 can often be substituted. For pressure parts operating above 800 °C, 347 is the safer choice because of its superior creep strength.

Do 321 and 347 resist intergranular corrosion after welding?

Yes. Both are designed so that titanium or niobium combines with carbon, preventing chromium depletion at grain boundaries during welding or service in the sensitization range of approximately 425-815 °C.

What filler metal is used for welding 321 and 347?

347 is typically welded with matching niobium-stabilized filler (ER347). 321 is often welded with the same ER347 filler, or with ER321 where a matching titanium-stabilized deposit is required.

What is the maximum service temperature of 321 and 347?

In oxidizing atmospheres both resist scaling to about 900 °C. For load-bearing parts the practical limit is lower because creep strength drops with temperature; design codes should be consulted for pressure applications.