321 Stainless Steel Grade Comparison Table: Chemistry and Properties

Why Titanium Is Added to Grade 321

Grade 321 is a titanium stabilized austenitic stainless steel. Titanium is added at a minimum of five times the carbon content and not more than 0.70 percent, so that it combines with carbon to form stable titanium carbides. Titanium carbide is far less soluble than chromium carbide, which means carbon is held within the grains instead of migrating to grain boundaries and precipitating as chromium rich carbide during welding or elevated temperature exposure. That single mechanism is what prevents intergranular corrosion and knife line attack in the sensitization range of roughly 425 C to 850 C.

Grade 347 works on the same principle using niobium and tantalum at a minimum of ten times the carbon content and up to 1.10 percent. Both stabilized grades are specified where welded components must operate hot for long periods without losing corrosion resistance at the weld.

321 Stainless Steel Grade Comparison Table

Grade	UNS	Carbon max	Chromium	Nickel	Molybdenum	Stabilizer	Typical service
304L	S30403	0.030 percent	18.0 to 20.0	8.0 to 12.0	None	None	General corrosion, low chloride
316L	S31603	0.030 percent	16.0 to 18.0	10.0 to 14.0	2.00 to 3.00	None	Chloride and acid service
321	S32100	0.08 percent	17.0 to 19.0	9.0 to 12.0	None	Titanium, 5 times carbon minimum	Welded high temperature service
347	S34700	0.08 percent	17.0 to 19.0	9.0 to 13.0	None	Niobium plus tantalum, 10 times carbon minimum	Power, boiler and refinery service
310S	S31008	0.08 percent	24.0 to 26.0	19.0 to 22.0	None	None	Oxidation resistance above 900 C

The table shows the trade off clearly. 321 and 347 are less alloyed than 316L and do not resist chlorides as well, but they tolerate welding followed by high temperature exposure, which 304L and 316L cannot do without post weld solution annealing. Grade 310S is reserved for the highest temperature duties.

High Temperature Strength and Oxidation Limits

Tube supplied to ASTM A213 TP321 has a minimum tensile strength of 515 MPa, a minimum 0.2 percent proof strength of 205 MPa and a minimum elongation of 35 percent in the annealed condition. Those values sit above 304L and reflect the strengthening contribution of titanium carbide. In continuous service 321 is normally used up to about 800 C, and in intermittent service up to roughly 900 C.

321: continuous service about 800 C, intermittent about 900 C

347: continuous service about 870 C with slightly better creep resistance

304L: limited by carbide precipitation above about 425 C in welded sections

316L: better chloride resistance, continuous service about 800 C

310S: oxidation resistance to about 1100 C, lower strength and higher cost

Oxidation behaviour is similar to other 18-8 grades; the advantage of 321 lies in structural stability rather than in scale resistance. Where heavy scale or sulphidation is expected, a higher chromium grade is required.

Welding and Fabrication of 321

321 welds readily by TIG, MIG, plasma and resistance methods. Titanium is a strong deoxidizer and is partly lost in the weld pool, so filler metal selection normally falls to a niobium stabilized ER347 consumable, which provides the same stabilization effect in the deposit. Resistance to solidification cracking is good, and no post weld heat treatment is required.

Interpass temperature should be controlled to 150 C maximum, and heat input kept moderate to limit distortion in thin wall sections. Heat tint left by welding must be removed by pickling and passivation, because the chromium depleted oxide layer is the main corrosion risk in a fabricated assembly. Forming behaviour is close to 304, although the titanium addition lifts room temperature strength slightly and increases springback during bending.

Surface Treatment and Typical Applications

Common finishes are pickling and passivation, a drawn or hairline finish, and mirror polishing to Ra not exceeding 0.2 micrometre. Finish choice follows the duty: hygienic surfaces take a fine mechanical polish, while architectural work accepts a uniform hairline grain.

Heat exchanger and condenser tubing to ASTM A213 TP321 and ASTM A249 TP321

Expansion bellows, flexible joints and exhaust components subject to thermal cycling

Furnace internals, radiant tube supports and high temperature ductwork

Food processing machinery, dairy equipment and architectural decoration

Refinery and petrochemical piping that is welded and operated above 500 C

Frequently Asked Questions

Q: Why is titanium added to 321 stainless steel?
Titanium combines with carbon to form stable titanium carbides, which stops carbon from precipitating as chromium carbide at grain boundaries and therefore prevents intergranular corrosion after welding.

Q: How do I choose between 321 and 304L?
Choose 321 when a welded part must serve long term above about 500 C or in a carbon containing environment; choose 304L or 316L when the main risk is chloride attack and the service temperature stays moderate.

Q: Should I use 321 or 316L for seawater?
Use 316L. Its 2 to 3 percent molybdenum gives roughly 24 to 26 pitting resistance equivalent, while 321 has about 18 to 20 and is not intended for chloride rich media.

Q: Can 321 stainless steel be welded?
Yes. A niobium stabilized ER347 filler is recommended, no post weld heat treatment is needed, and the interpass temperature must be kept at 150 C or below.

Q: What surface treatments are available?
Pickling and passivation, drawn hairline finish and mirror polishing to Ra not exceeding 0.2 micrometre, chosen to suit hygienic, architectural or high temperature duties.

Q: Is 321 available as seamless tube and pipe?
Yes, it is produced as seamless and welded tube to ASTM A213 TP321 and ASTM A249 TP321, and as pipe to ASTM A312 TP321 for high temperature welded systems.