Table 2 — The High Alloy Content of Type 309L and 312 Makes Them Suitable for Joining Stainless Steel to Carbon Steel Ni Si C Mn Cr FN WRC-92 N Mo 309L 13.4% 0.4% 0.02% 1.8% 23.2% 10% 0.05% 0.10% 312 8.8% 0.4% 0.10% 1.6% 30.7% the most common stainless steel base material, but no one offers a Type 304 electrode. If I am supposed to match the filler metal to the base metal, what do I use to weld Type 304 stainless? To weld Type 304 stainless, use Type 308 filler, as the additional alloying elements in Type 308 will better stabilize the weld area. However, 308L is also an acceptable filler. The “L” designation after any type indicates low carbon content. A Type 3XXL stainless has a carbon content ≤ 0.03%, where standard Type 3XX stainless can have a maximum carbon content of 0.08%. Because a Type L filler falls within the same classification as the non-L product, fabricators can, and should strongly consider, using a Type L filler because lower carbon content reduces the risk of intergranular corrosion issues — Fig. 1. In fact, the authors contend Type L filler would be more widely used if fabricators simply updated their procedures. Fabricators using the gas metal arc welding (GMAW) process may also want to consider using a Type 3XXSi filler, as the addition of silicon improves wet out — Fig. 2. In situations where the weld has a high or rough crown, or where the weld pool does not tie in well at the toes of a fillet weld or lap joint, using an Si-type GMAW electrode can smooth the weld bead and promote better fusion. If carbide precipitation is a concern, consider a Type 347 filler (see next to last question), which contains a small amount of niobium. How do you weld stainless steel to carbon steel? This situation occurs in applications where one portion of a structure requires a corrosion-resistant exterior face joined to a carbon steel structural element to lower cost. When joining a base material with no alloying elements to a base material with alloying elements, use an overalloyed filler so that the dilution within the weld metal balances or is more highly alloyed than the stainless base metal. For joining carbon steel to Type 304 or 316, as well as for joining dissimilar stainless steels (Table 2), consider a Type 309L electrode for most applications. If a higher Cr content is 34 WELDING JOURNAL / JULY 2016 Fig. 3 — Because of different expansion rates, distortion from warping must be compensated for to a greater extent when welding carbon steel to austenitic stainless. Fig. 4 — To avoid hot cracking, most standard austenitic filler metals are designed to solidify with a small amount of ferrite. This picture shows 309L weld metal with a small amount of ferrite (dark color) in an austenitic matrix.
Welding Journal | July 2016
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