welding 4140 to mild

[marmon]

we are having a problem welding 4140 square bar to mild plate. we have tried 9598 and similar but with no success. the part will be in operation for a month or so then split. the 4140 is 2x2 sq.bar and a 5" x 1-1/4 flat bar is welded perpendicular to it so the 5" f.b. is welded to the 12' 4140 bar. If anyone has experiance making this stick please fill me in.

 

[metengr]

Are these fillet welds or other?
What is the heat treatment condition of the 4140?
Use E7018 H4 weld rod or equivalent if using another welding process other than SMAW.
Apply a local preheat of 350 deg F. Depending on weld size, you might need to stress relieve.

 

[unclesyd]

Where is the connection breaking, the flat bar or the 4140 bar?
What is the load on the flat bar?

 

[marmon]

its annealed 4140 its breaking at the 4140 and they are fillet welds. the load is unknown and would be hard to calculate

 

[metengr]

Ok. Per my original post, use a local preheat of 350 deg F and E7018 H4 weld rod.

 

[unclesyd]

I would add a little to metengr's and that is to complete the weld in session and if possible cove the weld either with glass cloth or have bucket of vermiculite and cover the weld and rod to allow slow cooling.

If the electrode recommended above gives you any trouble you may want to try 312 SS rod, commonly used as a proprietary universal welding rod.