SS321 vs SS347: High-Temperature Performance Comparison and Selection

Two Stabilized Grades, One Purpose

Grades 321 and 347 belong to the stabilized austenitic stainless steel family. Both contain a deliberate addition that reacts preferentially with carbon, keeping chromium in solid solution and preventing the chromium-carbide precipitation at grain boundaries that causes intergranular corrosion after service in the sensitisation range. The difference is the stabilising element: 321 uses titanium, while 347 uses niobium (columbium). Their equivalents are UNS S32100 / EN 1.4541 and UNS S34700 / EN 1.4550 respectively.

Chemical Composition per ASTM A240

ASTM A240 specifies the following limits for plate, sheet and strip. Single values are maximums unless shown as a range.

Element	321 (UNS S32100)	347 (UNS S34700)
Carbon	0.08 max	0.08 max
Manganese	2.00 max	2.00 max
Silicon	0.75 max	0.75 max
Phosphorus	0.045 max	0.045 max
Sulfur	0.030 max	0.030 max
Chromium	17.0-19.0	17.0-19.0
Nickel	9.0-12.0	9.0-13.0
Stabiliser	Ti 5 x (C+N) min, 0.70 max	Nb + Ta 10 x C min, 1.00 max

Both grades share the same chromium range and similar nickel content, so their general corrosion resistance in aqueous media is close. The selection between them is driven by welding, by the exact high-temperature environment, and by the form in which the material will be used.

Corrosion Resistance in the Critical 427-816 °C Range

The range from about 800-1500 °F (427-816 °C) is where unstabilised stainless steels sensitise: chromium carbides precipitate at grain boundaries and the adjacent metal becomes chromium-depleted, so the steel corrodes intergranularly. Both 321 and 347 resist this, but not identically. Grade 347 is generally regarded as superior in the 427-816 °C range, particularly in strongly oxidising media, because its niobium carbides are more stable and remain finely distributed at these temperatures. Grade 321 performs well but can show reduced resistance during very prolonged exposure in that critical range, when titanium carbides coarsen and free titanium is consumed. For long-life equipment operating continuously inside 427-816 °C, 347 is therefore the safer specification.

Welding Behaviour: Why 347 Is the Filler for Both

There is a fundamental difference in how the two stabilisers behave in welding. Titanium does not transfer well across a high-temperature welding arc: it is oxidised and lost in the arc column, so a 321 filler rod would not deliver effective stabilisation to the weld metal. Niobium, by contrast, transfers reliably across the arc. For this reason grade 347 is the standard filler metal for welding both 321 and 347 parent material, and it is widely used to join 321 components. Post-weld annealing is not required for either grade in most services, because the stabilised heat-affected zone remains resistant to intergranular corrosion.

Thermal Cycling and Applications

Where components experience repeated heating and cooling, grade 321 has a well-established record, particularly in aerospace. Its titanium-stabilised structure handles thermal cycling well, and the grade is a standard choice for aircraft exhaust systems, engine bellows and ducting that cycle between ambient and operating temperature. Grade 347 is preferred where maximum carbide resistance is required inside the sensitisation range, and it is often specified as filler metal or for components in corrosive high-temperature chemical service. Typical shared applications include expansion joints, heat-exchanger tubing, furnace parts, boiler superheater components and chemical processing equipment up to about 900 °C.

FAQ

Q1. Which is better at high temperature, 321 or 347? For corrosion resistance inside the 427-816 °C sensitisation range, 347 is generally better. For thermal-cycling resistance and aerospace exhaust applications, 321 has the stronger service record. Above about 900 °C, both lose strength and higher-alloy grades such as 310 should be considered.

Q2. Why is 347 used as filler for welding 321? Titanium is lost across the welding arc, so 321 filler would not stabilise the weld. Niobium transfers reliably, so 347 filler gives a stabilised weld matching the parent material.

Q3. What is the difference between stabilization elements? 321 is stabilised with titanium; 347 with niobium plus tantalum. Both prevent chromium-carbide precipitation, but niobium carbides are more stable during prolonged exposure in the 427-816 °C range.

Q4. Do 321 and 347 require post-weld heat treatment? No. Both are stabilised, so the weld heat-affected zone resists intergranular corrosion without solution annealing in most services.

Q5. What are the equivalents of 321 and 347? 321 is UNS S32100 / EN 1.4541; 347 is UNS S34700 / EN 1.4550. Both are covered by ASTM A240 for flat products and by tube and pipe specifications such as ASTM A312 and A213.