Thermal Expansion Coefficient of Type 304 Stainless

Austenitic stainless expands about a third faster than carbon steel at room temperature, faster still as it heats, and conducts heat a third as well, and that combination is the source of most stainless fabrication grief: steep local gradients producing large strains. Weld distortion in thin stainless sheet is notorious for exactly this reason, and stainless pipe runs need proportionally more expansion capacity than an equivalent carbon steel layout.

Where the mismatch bites hardest is at joints between families. A stainless liner in a carbon steel vessel, or stainless tubes in a carbon steel tubesheet, accumulates differential movement of about 3.7 µm per metre per kelvin at room temperature, and more as both heat up: over a 6 m exchanger and a 200 °C swing that is at least 4.4 mm of relative motion, which must be absorbed by floating heads, expansion bellows or deliberate flexibility. Ferritic stainless grades (409, 430) are the exception: they expand at close to carbon steel rates, about 11 µm/(m·K), which is one reason they are used for automotive exhaust in contact with steel.