Both 321 and 347 are austenitic 18Cr/9Ni stainless grades stabilized against sensitization in the 425-815 °C (800-1500 °F) carbide-precipitation range, with a peak carbide-formation rate near 675 °C (1250 °F) [S1]. The functional difference is the stabilizer element: 321 uses titanium at a minimum of 5 × (C+N), and 347 uses niobium (columbium) at a minimum of 8-10 × C, which dictates weld behavior, peak service temperature, and post-weld heat-treatment requirements [S1][S5].
Specifying the wrong grade costs corrosion resistance or weldability: 304 and 304L suffer chromium-carbide precipitation in the HAZ, 321 is hard to weld with matching filler because titanium does not transfer across an arc, and 347 is the standard filler for joining 321 parent plate [S5]. The decision matrix below lines the two grades up against temperature ceiling, weld consumable availability, post-weld annealing needs, and cost.
Chemistry and Stabilization Mechanism
Nitrogen must be counted against the titanium budget in 321, because titanium nitride is highly stable and consumes titanium that would otherwise stabilize carbon; niobium in 347 is not subject to the same nitrogen penalty, which is one practical reason 347 is favored where residual N is uncontrolled [S1]. Both grades rely on the stabilizer forming a more stable carbide than chromium carbide, drawing carbon out of solution and preventing the chromium-depleted grain-boundary zones that cause intergranular attack [S1][S3].
High-Temperature Ceiling and Sensitization Range
Type 321 is rated for continuous service up to about 900 °C, combining scaling resistance, phase stability, and post-exposure aqueous-corrosion resistance [S5]. Type 347 bar is published with a maximum service temperature of 1500 °F (816 °C) for the stabilized condition, and is described as excelling from cryogenic temperatures up to that ceiling [S3].
The shared sensitization window, 425-815 °C (800-1500 °F), is where chromium carbide precipitation is fastest and where unstabilized 304 fails in the HAZ after welding or stress relief [S1][S3]. Above 1040 °C (1900 °F) chromium carbides redissolve, so a re-anneal at that temperature followed by rapid cooling can reverse sensitization, but the same reheat into 675 °C for stress relief will re-sensitize an unstabilized 18/8 grade [S1].
Welding, Filler Selection, and Post-Weld Heat Treatment
Titanium does not transfer well across a welding arc, so 321 is not recommended as a welding consumable; 347 (niobium-stabilized) is the standard filler for joining 321 base metal and is the consumable of choice where post-weld annealing is impractical [S5]. 347 base metal is also weldable in heavy sections without post-weld annealing, because the niobium carbides keep chromium in solution through the HAZ thermal cycle [S3].
For assemblies where the part will see service above 500 °C, 304L's lower hot strength disqualifies it even though it is more available and cheaper, which pushes the spec back to 321 or 347 [S5]. When dissimilar-metal joints to carbon or low-alloy steel require a 675 °C stress relief, a stabilized austenitic grade on the stainless side of the joint is mandatory to avoid re-sensitization [S1].
Mechanical and Physical Properties at a Glance
347 bar is independently published at minimum tensile 75 ksi (515 MPa) and yield 30 ksi (205 MPa), matching the A240 plate numbers [S3].
Physical constants for annealed 321 are 8027 kg/m³ density, 193 GPa elastic modulus, mean CTE 16.6 µm/m/°C (0-100 °C) rising to 18.6 µm/m/°C at 0-538 °C, thermal conductivity 16.1 W/m·K at 100 °C, and specific heat published for the 0-100 °C band [S5]. General corrosion resistance of 347 is comparable to 304 in most environments, with the decisive edge inside the chromium-carbide precipitation range [S3].
Application Mapping: Where Each Grade Wins
321 dominates fabricated non-welded or lightly welded components in chemical-process equipment, refinery piping, thermal-oxidizer parts, bellows, and high-temperature heat-exchanger shells where service approaches 900 °C but no heavy welded build-up is needed [S5][S6]. Common strip and tube uses include exhaust manifolds, furnace parts, and expansion joints where titanium stabilization is sufficient and the cosmetic finish is not a priority (321 does not polish well and is avoided for decorative trim) [S5].
347 is the default for heavy welded assemblies in the 427-816 °C band, petroleum-refinery and chemical-process pressure equipment, aerospace high-temperature welded components, and any welded section that will not get a post-weld anneal [S3][S5][S6]. For creep- and stress-rupture-loaded service above ~540 °C, 347H (higher C, 0.04-0.10%) is the upgraded pick over standard 347 because the higher carbon improves high-temperature strength [S3][S5]. 347H should not be confused with 321H, which is a separate high-carbon variant on the titanium-stabilized side.
Decision Matrix: 321 vs 347 by Selection Criterion
On peak continuous service temperature, 321 reaches ~900 °C, 347 is conservatively rated to 816 °C (1500 °F), and 347H extends creep strength above that band [S3][S5]. On weld consumable availability, 347 is widely stocked as filler wire and is the AWS-standard match for 321 base metal, while 321 filler is essentially unavailable and 321 base metal is welded with 347 filler [S5].
On post-weld heat treatment, both 321 and 347 are explicitly described as not requiring post-weld annealing, versus 304L which still needs control of HAZ thermal history [S3][S5]. On polishability and decorative use, 321 is unsuitable, which alone rules it out for architectural trim and food-grade polished surfaces [S5]. On cost, 321 typically runs cheaper than 347 because titanium is the less expensive stabilizer and 321 is the higher-volume commodity grade, so 321 wins on price when the service envelope and weld schedule allow it.
Limits, Failure Modes, and Common Mis-Specs
The most common mis-spec is selecting 321 as a welding consumable, which produces a weld pool stripped of titanium and leaves the joint sensitized; the fix is to weld 321 base metal with 347 filler (or use 347 base metal throughout) [S5]. A second frequent error is assuming 304L is interchangeable with 321 above 500 °C; it is not, because 304L's lower hot strength will creep under sustained load at temperatures where 321 still holds dimension [S5].
A third trap is using the 5 × C minimum titanium rule without accounting for nitrogen, which leaves the steel under-stabilized and the chromium-carbide protection incomplete; specifying the 5 × (C+N) form per ASTM A240 avoids this [S1][S5]. Finally, 347 should not be casually substituted where the maximum service temperature sits at or above 900 °C, where 321's scaling resistance and phase stability are documented to outperform it [S5]. For broader context on stainless selection, see the stainless steel overview reference and the related 316L vs 317L chloride-pitting comparison; for high-temperature tube and pipe applications, the stainless pipe reference covers matching product forms.
For component-level specifications, see high voltage tester.