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- [21] Q: We fabricate a lot of carbon steel pipes. Occasionally we get a job that uses carbon steel pipes lined with stainless steel. Sometimes the liner is 304L, sometimes 316L. Since the pipes have a relatively small diameter, they can only be welded from the outside. In preparing the joint, we peel back the carbon steel to expose the stainless in the root area, then weld the root and one or two more layers with gas tungsten arc welding (GTAW) using 309L (for a 304L liner) or 309LMo (for a 316L liner) filler metal, and argon backing gas. We finish the joint with 309L or 309LMo covered electrodes or flux core. This seems like a waste of expensive filler metal. After we get the stainless steel root pass in place, why not finish with carbon steel filler metal?WELDING JOURNAL, 2018, 97 (03) : 22 - 23Kotecki, Damian J.