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304 is the default stainless grade for most plate work, and for good reason: it is available, predictable and reasonably priced. But there is one service condition where 304 quietly fails while the drawing still says "stainless": continuous heat in the 425 to 815 °C range.

Hold 304 in that band and carbon combines with chromium along the grain boundaries, precipitating chromium carbides and leaving the adjacent metal short of chromium. That is sensitization. The plate keeps its room-temperature strength and looks untouched, but the chromium-depleted band corrodes quickly in service—and next to a weld it shows up as knife-line attack.

321 answers this with titanium. Titanium has a stronger affinity for carbon than chromium does, so titanium carbides form first and the chromium stays in solution where it protects the steel. The standard requires titanium at not less than five times the carbon content, which is why 321 is called a stabilized grade.

The temperature limits are the practical part. 321 is normally used for continuous service up to about 800 °C and intermittent service up to 925 °C. 304 can be used above 425 °C in many codes, but only where sensitization does not matter—its corrosion resistance is what fails first, not its strength.

Typical places where 321 is the right call: exhaust systems and manifolds, heat exchanger tube sheets, furnace and kiln parts, expansion joints, annealing fixture frames, superheater supports. Where 304 is still enough: decorative work, food contact, ambient chemical service, and anything that never passes 400 °C.

The cost is real. 321 runs roughly 10 to 15 percent above 304, machines less freely because of titanium inclusions, and welds with a slightly stiffer puddle. In exchange, thin sections usually need no post-weld solution annealing—that is the point of stabilization.

What to put on the purchase order: 321 to ASTM A240 or EN 10028-7 1.4541, titanium to carbon ratio of 5 or more, carbon 0.08% maximum, an intergranular corrosion test to ASTM A262 Practice E for critical service, and a mill certificate with heat number. If chlorides are also in the picture, 316Ti keeps the molybdenum and the stabilization together.

We supply 304, 304L, 321 and 316Ti stainless steel plate with EN 10204 3.1 certificates, cut to your sizes.