SS 304L stainless steel for welded pipework and vessels

When austenitic stainless steel is held between about 425 °C and 860 °C, carbon moves to the grain boundaries and combines with chromium to form chromium carbides. The strip of metal next to each grain boundary loses chromium, and with it some of its corrosion resistance. This is called sensitisation.

Welding heats a band on each side of the joint through exactly that range. In thin sheet the time is short and the effect is usually small. In thicker plate, multi-pass welds or slow-cooled sections, the heat-affected zone can become sensitised. Put that joint into an acidic or chloride-bearing process and it can suffer intergranular corrosion, sometimes called weld decay.

By capping carbon at 0.030%, 304L leaves too little carbon to form harmful amounts of carbide in the time a normal weld takes. The heat-affected zone keeps its chromium where it belongs. No solution anneal is needed after welding, which is a big saving on large vessels and long pipe runs that cannot go into a furnace.

That is why 304L is the usual choice for welded tanks, process pipework, dairy silos and pharmaceutical utility lines. In nitric acid service it is also preferred over standard 304.

Carbon adds strength to austenitic steel, so taking it out costs you some. ASTM A240 sets 304L minimum yield at 170 MPa and tensile at 485 MPa, against 205 MPa and 515 MPa for 304. For most fabricated equipment this makes no practical difference. For pressure design it does, because the code allowable stresses for 304L are lower.

At high temperatures the gap matters even more. For sustained service above roughly 500 °C, designers usually move to 304H or a stabilised grade such as SS 321, because low-carbon grades lose creep strength.

Much of the stainless pipe, sheet and plate made today is dual-certified as 304/304L. The steel is melted with carbon at or below 0.030%, so it meets the 304L chemistry, and it also reaches the higher 304 strength minimums. One heat, both grades on the certificate.

If your drawing calls for 304L, dual-certified material is fully acceptable. If it calls for 304, dual-certified material also meets it. We will tell you at quotation whether the lot on offer is single or dual certified, and the test certificate will show both the carbon value and the tensile results.

Both are low-carbon grades, so both handle welding well. The deciding factor is chloride. If the process fluid, cleaning chemicals or surrounding air carry chlorides, 316L's molybdenum gives a real margin against pitting and crevice corrosion. That includes brines, salted foods, coastal sites, chlorinated water and many disinfectants. If the service is clean water, air, mild chemicals, nitric acid or dry products, 304L does the job at lower cost.

A practical approach many plants take is 316L for product-contact lines and 304L for utilities, structural frames and non-contact parts. If you are not sure which side of the line your application falls on, send us the fluid and temperature details and we will tell you what customers typically use for that service.

304L is a fabrication grade, so pipe and flat products are where you will see it most. We supply seamless and ERW pipe and tube, sheet, strip and plate, along with bar, wire, flanges and fittings for completing welded assemblies. Sizes are confirmed at enquiry.

For welding, matching 308L filler is the normal choice. Keep interpass temperatures reasonable, clean off heat tint after welding and pickle or passivate if the joint will see corrosive service. The low carbon helps, but it does not excuse sloppy cleaning.

For hygienic or pharmaceutical lines in 304L, mention the internal finish you need when you enquire.