304L Tubes for Welding and Fabrication Applications

304L tube is the low carbon version of 304 austenitic stainless steel. Its main selling point is that it can be welded without a post weld anneal while keeping good corrosion resistance, which is why it is so common in fabricated assemblies. 

Why 304L suits welding

Standard 304 SS Tube allows up to 0.08% carbon, while 304L Tubing is capped at 0.03%. When stainless steel is held in the 450-850°C range, as the heat-affected zone (HAZ) is during welding, carbon can combine with chromium to form chromium carbides at the grain boundaries. This is called sensitization. It leaves chromium-depleted zones that are prone to intergranular corrosion. The lower carbon in 304L leaves little carbon available to form these carbides, so welds in as-welded condition keep their corrosion resistance.

Seamless vs. welded tube

Seamless tube is preferred for high-pressure, high temperature or critical service. Welded tube costs less, comes in a wider range of sizes and lengths, and is usually fine for general fabrication, structures and low-pressure piping. Cold-drawn or bright-annealed welded tube has good dimensional consistency.

Fabrication and welding guidance

• Processes: TIG (GTAW), MIG (GMAW), and orbital TIG for tube to tube and tube to fitting joints are all common.

• Filler metals: ER308L is the standard match. Use ER316L if you need extra pitting resistance in chloride exposure.

• Heat input: Keep it low and use interpass temperatures below about 150°C to limit distortion and HAZ width. Austenitic grades expand about 50% more than carbon steel and conduct heat poorly, so distortion is a real concern.

• Purging: Use argon back-purge on tube welds to prevent "sugaring"  on the inside. Keep oxygen below ~50 ppm for sanitary or high-purity work.

• Cleanliness: Use dedicated stainless brushes and tools. Carbon steel contamination causes rust spotting.

• Post weld treatment: Remove heat tint by pickling, passivation or electropolishing to restore the passive chromium oxide layer.

• Forming: 304L bends, flares and swages well because of its high ductility, but it work-hardens quickly, so plan tooling and sequences accordingly.