Crack problem

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I have been givin the task of tig welding a solid ss slug into a 3 inch round mild steel tube ,the slug has been machined so that it is the same od as the tube so a small weld is required so the tube can be covered with rubber after completion. The problem is my tig welds keep cracking when they cool , so far i have tried pre-heating and also several different filler metals and sizes and also no fill at all Pure tungsten and argonneed the parts done soon as it is a hot job can anyone help ?
Reply:Are you sure its MS your trying to weld too?Exactly what filler have you used?Pics?
Reply:309 mild steel308 is the other i believe I am reasonably sure that it is mild steel
Reply:No pics as this is a problem at work and i am home now
Reply:Can you describe the joint prep and fit-up?
Reply:Are you welding a bat?Shake and Bake
Reply:Different expansion / contraction rate for the 2 dissimilar metals, if you can't shrink fit it you'll have to figure out a pre / post heat rate.
Reply:309L is the correct filler metal.  If you're running a fillet all the way around the joint, then I'm with old skool.  Mild steel has 2x the thermal conductivity/coefficient of thermal expansion of stainless steel.Your highly restrained joint is tearing itself apart as the joint cools.  Try pre-heating the assembled pieces to 250-300F and then cooling by wrapping in insulation after welding.  Or, if you have an oven or furnace you can slow cool the part by stepping the heat down slowly over several hours.  This will reduce the thermal stresses created by localized heating from welding, followed by rapid cooling.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Did someone say crack problem ??I think the title to the post was misleading, or I read it wrong "Any day above ground is a good day"http://www.farmersamm.com/
Reply:I see the crack but I don't see the problem... A few weldersA lot of hammersA whole lot of C-clamps
Reply:Originally Posted by borkshopPure tungsten and argon
Reply:Originally Posted by borkshopI have been givin the task of tig welding a solid ss slug into a 3 inch round mild steel tube ,the slug has been machined so that it is the same od as the tube so a small weld is required so the tube can be covered with rubber after completion. The problem is my tig welds keep cracking when they cool , so far i have tried pre-heating and also several different filler metals and sizes and also no fill at all Pure tungsten and argonneed the parts done soon as it is a hot job can anyone help ?