With regard to stainless steel welding, what is creep resistance?
Creep is the phenomenon of continuing plastic strain over long periods of time at loads below the yield point. Creep only happens at high temperatures (around 900 degrees F and higher for stainless steels). Certain stainless steel weld metals have higher resistance than others to creep at high temperatures. For example, 308H (0.04 to 0.08 percent C) has higher creep resistance than 308L (0.04 percent C maximum), meaning that 308H can sustain a given load for a longer time than 308L, or it can withstand a higher load than 308L for the same length of time. But eventually, both will fail under load at high temperature.

Creep failure may take years to occur. Applications typically limited by creep can often be found in power plants that are designed to operate at high temperatures without creep failure for periods up to 30 years.

We are looking to weld alloy 20, and would like to know what filler to use and any pre or post weld heat treatment recommendations.
Alloy 20 is an austenitic stainless steel that is very sensitive to solidification cracking, so it is important to use the "low residual" version of the matching filler metal - AWS A5.9 classification ER320LR. You should not preheat, and you should keep the weldment cool: The maximum interpass temperature should not be more than 300 degrees F. You should use what might appear to be excessive filler metal to make a convex bead, and overfill craters. Avoid any concave beads as they are very likely to crack. No post-weld heat treatment (PWHT) is advisable.

We have a job that we have to weld for some engineering changes. It is for an Asian automobile manufacturer, and the steel we are welding is a cast iron referred to as FC250. I am not having any luck at all welding this material. We have tried TIG welding with no pre-heat and with a pre-heat of 400 degrees F. We have tried several specialty stainless filler metals, but we are still not getting a good weld. The weld we are making is multi-pass. Can you help?
The FC250 specification can be found in JIS G 5501 standard for Grey Iron Castings. There is no specified composition, but the fact that it really is a grey cast iron is very helpful. The right electrode can be either a 55 percent nickel filler if you are making a multi-layer repair and are not machining the weld, or our a 99 percent nickel filler if you have to machine the first layer of weld metal. You will need preheat and interpass temperatures of about 400 degrees F. You should preheat the entire casting, not just the weld area, and you should cover the entire piece with insulation after welding and let it cool very slowly.

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