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Welded Stainless Machining

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  • Welded Stainless Machining

    4 inch stainless steel 316 i/2 inch wall tube with a 1/2 inch round plate welded on the end looks like a cup kinda. Now the problem im encountering is it seems to have gotten very tough from the welding. I have to machine the weld down to original diameter (4 inch) and drill a 1 5/16 hole through the plate welded on the other end. I have 48 of these to do. I own a bridgeport mill and also a 15 x 40 inch cinnci lathe. Any tooling recomendationd would be greatly appreciated. Normally i only machine 303 stainless this stuffs TOUGH. Thanx Mike

  • #2
    break out the carbide......sloooooww rpm.....we skip the coolant and use cutting oil.....keep it cool. With the carbide, try to avoid intermittent cuts, that will chip the corners in a hurry.

    Mark

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    • #3
      I bought a few sticks of stellite a while back, and I've found that very nice for doing stainless on the lathe. It will dull, but a touch to the grinder takes care of that. My experience has been that stellite is much more tolerant of intermittent cuts than say, carbide. It can also stand quite a bit of heat, which you can get in a hurry machining stainless. It is a bit on the expensive side, though.

      On another project, however, I noticed something different. I was building up material on a badly worn dog clutch by welding. The project is now done, and the results great, but I noticed that the consistency of the built up weld varied significantly with the weld power level used. When I cranked my cheap little MIG up all the way, I could lay down material, but the result was very difficult to machine and hard on tools. I believe the base material was probably cast iron, and I was building up using fluxless wire and tri-gas mix. The machining afterward was a tricky business, but I eventually got it done. If I ever do a job like that again, I'll use many more passes and lower weld power. The resulting buildup is still pretty strong, but not nearly has hard to machine afterward.

      Now, I haven't tried this using stainless, but perhaps it would also have hardening issues. IIRC, someone said here that at least some stainless steels are hardened and annealed in a way backwards from most metals. To harden, heat it up and cool gradually, and to anneal, heat it up and quench. (Note: I'm no metallurgist, and I don't even play one on TV) I do recall that, for a given variety of SS, you tend to get something that is either nice to weld, or nice to machine, but seldom both.

      Can someone straighten me out on this? Maybe all that is needed is a little annealing?

      -Mark
      The curse of having precise measuring tools is being able to actually see how imperfect everything is.

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      • #4
        Solution Annealing 316 SS

        Madman... The heat from the welding operation on this part has changed the grain structure. If you solution anneal it, 1950 degrees F for 30 minutes and quench in water, the grain will be in the best condition for machining. You will get chips instead of gum. The difference between your 303 SS and this 316 is that 303 has sulfur to make it more machinable and no moly, molybdenum. 316 SS has around 2% moly for increased corrosion resistance and toughness.

        If it is a small project you can take an acetylene torch and heat the part "cherry red" than drop in water, should help you machining with out the cost of a heat treat lot charge. Hope this helps

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        • #5
          Wow thanx

          Yeah Wow good info thanx a lot. I just qwas layed off from work manufacturing here in our area is DEAD. Im trying to make a living from my garage shop and you know it kinda sucks. But im not a quitter. Thanx a lot for your help. Mike

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          • #6
            If it was TIG welded beware. Tig welding often has little flakes of carbide in the starts and stops. These alloy with the base metal and form high alloy inclusions. Anything ferrous with loads of tungsten and carbon quenched quickly form little nodules of something like high speed steel.

            Mig and stick welding are free of this but may pose problems of their own. I suggest a quick and dirty local heating to 900F and an air cool to equalise and normalise the deposit and the HAZ.

            I suggest you do them in the lathe as it has a much more rugged spindle than any skinny ass R8 Bridgeport.

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            • #7
              If it was TIG welded, chances are what you have is chromium carbide precipitation. In common with all carbides, CC is HARD! I TIG quite a lot of 316 for small boat stuff, rails and such. Best way to avoid CC precipitation is to quench immediately after welding. This gets the HAZ rapidly through 'sigma phase' where carbide precipitation occurs. I was told this by my welding instructor (who usta build nuclear reactors) and also several welding inspectors. It has been borne out in practise. I have machined many welds in my little 9-20. HSS tool, 0 top rake, fairly high speed and flood coolant. Beautiful finish requiring little or no further treatment. Maybe a wipe with scotchbrite green.
              Rgds, Lin
              Just got my head together
              now my body's falling apart

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              • #8
                A little academic at this point, but to avoid chromium carbide precipitation in these welded parts use 304L or 316L. These are 'low carbon' versions. It will also preserve (more of) the corrosion resistance after welding.
                Tom

                Be careful what you wish for, you might just get it!

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