STAINLESS Q&A BY DAMIAN J. KOTECKI Q: We welded the flange of a 304 stainless steel pipe fitting (female threads), after hammering the flange to approximately fit the tank curvature, to the cylindrical surface of a 304 tank. Unfortunately, the fitting was not aligned correctly, so the pipe inserted into the fitting does not take off from the tank in the correct direction. The welder suggested that we could locally heat the fitting and the tank, then thread a pipe into the fitting and bend the fitting and tank shell slightly to get the correct direction for the pipe. Is this a good idea? A: There is some logic in the welder’s suggestion. Locally heating the fitting and tank shell will reduce the yield strength of these materials, making them easier to bend. But things are not so simple. Assuming that the fitting and tank are really 304, not 304L, sensitization needs to be taken into account. Welding the fitting to the tank shell will already have somewhat sensitized the fitting and tank shell. So both could be damaged by intergranular corrosion, depending upon the severity of the corrosive environment inside and outside the tank. See the November 2007 Stainless Q&A column for details of sensitization and intergranular corrosion. Local heating, improperly applied, can severely sensitize the tank shell and fitting. The sensitization temperature range for welding is generally considered to be 900° to 1600°F (480° to 870°C). But localized heating is a much slower process so that the minimum sensitization temperature can be reduced to 840°F (400°C). If that temperature is exceeded anywhere during local heating, there must be a transition in temperature to the cold tank shell so there will be a considerable region of the tank shell that will be sensitized. Water quenching cannot prevent that and may cause distortion, which could make the pipe alignment problem worse, not better. I have to assume that the extent of sensitization you already have is acceptable, but local heating above 840°F is likely to make it much worse. If the extent of sensitization already present due to welding is acceptable and you don’t want to make matters worse, then local heating needs to be limited to a peak temperature anywhere in the tank shell and fitting to about 750°F (400°C) to provide some margin of safety. Some significant reduction of yield strength occurs when 304 is heated, which would be helpful in your proposed bending operation. I AWS PROFESSIONAL PROGRAM ABSTRACT SUBMITTAL AT FABTECH 2014 Atlanta • Nov. 11-13, 2014 Submission deadline: Mar. 29, 2014 The AWS Professional Program is an annual three-day program of sessions on technical topics featuring the most recent welding research and best practices in manufacturing and construction from around the world. Submit your Professional Program abstract online by March 28 at http://awo.aws.org/professional-program-abstract-form or contact Martica Ventura at (800) 443-9353 ext 224 (mventura@aws.org) ® 20 JANUARY 2014
Welding Journal | January 2014
To see the actual publication please follow the link above