welding pipe bell to mild steel plate doesn't stick

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Hello,New member here, got a question.  I've been welding for decades, but no real education.  I sent out a welded assembly where I machined a hole in a 1/4" hot roll plate and welded one end of a threaded female pipe bell into the hole.  The plate is a base for a 5 ft. tall stand for working on taxidermy mounts.  When the customer put a 100 lb. elk head on the stand, the weld broke loose.  He said he welded it on the other side of the flange as well and it broke again.  When I mig welded it, it looked like it was burning in and did not have any noticeable porosity from oil, etc.  The machine is 300 miles away, so I don't have the luxury of inspecting the broken weld.The pipe bell, of course, is made in China and I don't know what steel they are using or if they are oil-quenching after it is forged.  Anyone got any experience with welding the standard pipe fittings?My alternative is to double layer the center of the plate and cnc machine the pipe threads into the plate, but it is more shop time than just cutting off a pipe bell end and welding it in place.  I looked at threadolet fittings, but cutting it down to the height I need would be a pain.I appreciate any input....Thanks,Don B.Last edited by dbren; 07-16-2009 at 06:51 PM.
Reply:Don, most all threaded pipe bells that have had experience with especially china made ones are all cast iron.  If the ones you are using are cast then that very well could be your problem.  How large a pipe are you using?  You can find steel threaded couplings.  I weld them in on irrigation systems all the time.  Perhaps you could cut a coupling in half and use it?  I'm not really under standing exactly how your stand is built.  I am assuming that an upright pipe will screw into the plate base.  I really could not see even considering going thru the trouble to CNC pipe threads into plates, besides if you double layer the plates you still only have 1/2 inch to thread. Before I did that I think I would just turn a bushing that slips over the pipe fairly snug, and drill a bolt hole thru the pipe and bushing and use a bolt to hold it in place.I'm a Lover, Fighter, Wild horse Rider, and a pretty good welding man......
Reply:That's my guess as well.....cast iron. That would explain it.It would be still together if it had been brazed instead of welded.Last edited by snoeproe; 07-16-2009 at 07:56 PM.JasonLincoln Idealarc 250 stick/tigThermal Dynamics Cutmaster 52Miller Bobcat 250Torchmate CNC tableThermal Arc Hefty 2Ironworkers Local 720
Reply:Don, It's tough to diagnose what happened with any great confidence...But after reading your post I'm wondering if this pipe fitting you used was actually cast iron.  If so, it could weld easily, but wind up being brittle.  You mention that it might be forged, do you know this for certain?  In my experience, a lot of black pipe fittings are cast.  Is this something you're successfully fabricated in the past?  Or a unique piece of work?You don't mention what type of filler metal, powersupply, equipment settings, and shielding gas you're using,and these could all be relevant details.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Hey y'all,Thanks for all the great input.  Y'all were absolutely right.  I was told that the cast fittings are a maleable steel.  Cheap.  I found some real forged fittings (3000# rating).  The steel is used for the weld-up jobs, so the threaded fittings are the same material.  I just finished using the first batch and it welds great.Thanks again!
Reply:Not only was the cast iron your problem that Chinese cast is the lowest of the low. It's like trying the weld a metal sponge who's pores are full of dirt. Bye the way aren't those forged fittings a bit pricey?Tough as nails and damn near as smart