We are welding 304 stainless steel with Gas Tungsten Arc Welding (GTAW) and Gas Metal Arc Welding (GMAW) with a ER308L filler metal. Our problem is that every once in a while we get a little rust in the weld zone. Are we using the correct filler rod? What could be causing the rust to occur?
What you are seeing is the difference in corrosion resistance between the low temperature and high temperature oxides. Low temperature oxides have very good resistance to corrosion while oxides formed at high temperatures, such as those found during welding, do not have good corrosion resistance. To restore the corrosion resistance after welding you need to pickle the surface using a nitric-hydrofluoric acid solution (HNO3 6 percent to 25 percent plus HF 0.5 percent to 6 percent) followed by a warm water wash. A good reference for this procedure can be found in ASTM A380. This treatment will remove the high temperature oxide and the chromium depleted region, allowing the surface to reform the low temperature oxide layer.

Keep in mind that personnel involved in the pickling operation must use appropriate personnel protection including face shields, rubber gloves and rubber protective clothing. Adequate ventilation and personnel access controls must be maintained in the areas in which these chemicals are in use and for the entire time they are in use. Additionally, the rinse water and residual acids must be properly disposed of, in accordance with federal, state and local safety and water pollution regulations.

When welding 316 stainless steel to low alloy chrome-moly steel (1.25 Cr – .5 Mo type steel) some information indicates the joint should not be used at temperatures greater than 500 degrees C. Why is this so?
There is some merit to the recommendation for service where creep is an issue if a stainless steel filler metal is used. In the past, ER309 or ER309Mo filler metals were used for this application. The phenomenon that occurs at high temperatures in such joints is that the carbon in the Cr-Mo steel heat affect zone (HAZ) is attracted to the chromium in the stainless weld metal, so the carbon will diffuse out of the HAZ at high temperature and form chromium-carbides along the fusion boundary. The result is a carbon-depleted zone in the HAZ along the fusion boundary. This carbon depleted zone has poor creep resistance, so premature creep failure could occur there. The popular solution is to use nickel-base alloy filler metal such as ERNiCr-3 or ENiCrFe-2. Those filler metals greatly reduce the tendency for carbon to migrate out of the Cr-Mo steel base metal and, as a result, the creep life of the joint is increased.

We are trying to weld Inconel alloy 600 to 304 stainless steel. We are using the Gas Tungsten Arc Welding (GTAW) welding process and an ER308 filler rod. The two pieces are about .062 in. thick and we have chamfered both. The problem is that we are getting cracks on the 304 side of the weld joint. The more we weld the more cracks occur. Can these two materials be joined by welding?
I believe your problem is the choice of filler metal. The ER308 filler metal tends to crack when diluted with the Inconel 600. I suggest you change your filler metal to AWS A5.14 class ERNiCr-3. It will cost more, but should eliminate the cracking.

This column is sponsored by Penton and the Lincoln Electric Co., Cleveland. Dave Barton is a senior welding engineer in the Application Engineering Group of The Lincoln Electric Co. He oversees welding procedure development for both new technology and existing products, performs failure analyses for customers, and serves as a consultant on welding application problems. Barton has been with Lincoln Electric for 21 years. Send your questions for Mr. Barton in care of WDF by e-mail to: askdav@penton.com