basic steel repair

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a friend of mine that is also a home brewer has a nice brew set up that is made from angle and square tubing (normal mild steel). he has been using it for a little over a year and some of the GTAW welds are starting to crack along their edges. i know the guy that built his brew setup, he was my CWI when we worked together doing aircraft work. so needless to say i know the weld is not the problem but design or wire selection was. weld joint is a butt done with 70s-2. here is where the question starts, the parallel cracks are on the joints closest to the 300,000 btu natural gas burners so they see some heat cycles. material is probably reaching ~400* and then cooling sometimes to below freezing within an 8 hour period. what is the best way to repair these cracks? would a stainless wire like 309L work best with it's much higher ductility or would one of the chromium wires work better? any ideas are more than welcome.
Reply:I'm not sure which wire would work best for that application, but our oven molds made from tigg'd aluminum at my old production job used to crack at the welds often and my boss was a very good aluminum tig welder so I know the problem wasn't his welding ability or filler selection.   The constant heating/cooling cycling is just very hard on stuff.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:The welders skill is not the problem, but I would say the welding rod could be, just look at all those steel welded wood stoves that get red hot and then cool, somtimes to sub zero temps, they last for decades with no cracked welds, I dont know what the weld rod is but mabe contacting a manufacture or even lincoln that makes rods could advise what to use..
Reply:Could be the filler, which I kind of doubt, or the joint design.  IF the joint is highly restrained, the thermal cycling, even the low number of cycles you're describing, could cause fatigue cracking in the welds.It's all speculation without photos and added information, but you might consider this before you conclude that the filler metal or welder is the culprit.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Sounds like the failure is in the heat affected zone along the toe of the weld.I recently saw this same type of failure in about 30 welds on the angle steel railing around a Big Tex tandem axle trailer.  Almost every joint was cracked through the full thickness along the weld toe line.  This is kind of puzzling because "mild steel" should be low enough in carbon to not excessively harden (and become brittle) in the HAZ.  All I can figure is that the steel is actually somewhat hardenable, making the HAZ brittle, and that stresses of heating and cooling in your case, and the stresses of flexing and twisting on the trailer, were enough to cause the fracture.In the case of the brew system stand, if the steel is being heated by the burner, and then sometimes rapidly quenched by a boil over (I'm a brewer too) or by washing down with the hose, that could lead to embrittlement and thermal stress and cracking.I think the 309 filler idea may help, because as you say it is ductile, and thus it may "give" enough when stressed to prevent failure in the HAZ.  Anouther idea is to preheat and post heat to slow cooling after rewelding an help prevent hardening.  Again, this is an odd problem that should not be occurring in mild steel, and mild steel should not require preheat/postheat.Another idea is to make this part of the frame out of much thicker steel to better handle the thermal stresses.
Reply:Originally Posted by A_DAB_will_doCould be the filler, which I kind of doubt, or the joint design.  IF the joint is highly restrained, the thermal cycling, even the low number of cycles you're describing, could cause fatigue cracking in the welds.It's all speculation without photos and added information, but you might consider this before you conclude that the filler metal or welder is the culprit.
Reply:Whoops, the internet strikes again.  I wasn't trying to imply that you rushed to judgement or anything.  That was just bad communication on my part.To be clear, the welds that are cracking are butt joints in angle iron.  Are they 90° corners, mitred at 45°?  If not, what is the exact joint design?I'm digging for more info because I don't think just rewelding the joints with a different filler metal is going to get the job done.  If  these are cracking due to thermal fatigue cycling, I think pulser was right when he suggested reinforcing the joint.Mild steel doesn't harden, and thermal cycling from room temp up to 400°F isn't going to affect the weld metallurgically at all.  Welding the same joints with 309L or any other SS filler metal isn't going to alter the stress distribution.  Toe line cracks suggest high stress concentrations; which can cause failures sound welds.I think you're going to need to add some kind of gusset or reinforcing plate over the joints that are cracking in order to distribute the stresses applied by heating and cooling over a wider area.What form that reinforcement should take depends on the joint configuration... Originally Posted by quasifirst off i was trying to point every one away from the weld quality and the person that did it. i would think being a CWI in an aircraft shop would have explained that the weld was sound. but apparently people got confused. their is definitely some tension locked up in this thing. the burner is mounted on a square frame within another square frame and the outer one is box tubing the inner one with the cracks is angle. i would love to explore redesigning the layout but it is just not practical. the cracks are around the hot liquor tanks that to my has never boiled over and he doesn't have much of a reason to wash the thing down so the quenching is highly unlikely.
Reply:Are well placed expansion joints feasible?
Reply:I work with ss all the time, and when in doubt I use Hastalloy W. I work at an aerospace shop, and the ams# for the wire is AMS 5786. I know that this wire is a very "elastic" filler and resists cracking on most types of steel, mild or stainless.