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  • #16
    Originally posted by Jim-Tx View Post
    It never strengthens a weld to grind it either.
    USUALLY using the word "never" will make you wrong.

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    • #17
      Grinding a weld flush gets a lot of debate, with lots of 'always' and 'never'. Several people already said it depends on circumstances, and I will throw my support in, as well.


      Some of the factors (NOT and exhaustive list)

      Often in the 'real world', it isn't necessary to grind welds, and so it is undesirable due to the associated cost (time, materials, etc), unless cosmetics require it. Many (most) of the jobs hobby welders do fall into this catagory.

      Occasionally, the reinforcement is desirable (many seal welds, which are often essentially laid on the surface, with little burn in, by intent, for example), and grinding is not desirable.

      Grinding IS desirable for many situations. If the toe of the weld isn't blended smoothly, or there is excessive reinforcement, the stress riser at the weld toe may lead to problems such as cracking later, especially under cyclic loading (heavy vibration, repeated impact loading, etc.) In these situations, welds are routinely ground smooth and flush, to eliminate the stress riser. Another case where grinding may be required is corrosive environments, for a variety of reasons including that regions under higher stress are subject to certain types of corrosion at a greatly increased rate compared to nearby material, small pits/irregularities act as corrosion cells for several types of corrosion (due to a variety of factors...), and many of the corrosion resistance enhancing treatments are not effective where the surface isn't smooth.

      Some weld techniques may use finish grinding as an essential part of the process, such as temper bead welding, where the heat input is casrefully controlled such that each pass properly tempers the pass(es) underneath, reducing or eliminating the need for post-weld heat treatment. The last layer may be removed, as it was laid to condition the metal underneath, and is not, itself, in the desired condition.

      There are also some cases where grinding may be detrimental for other reasons. For example, if controlling distortion is important, the removal of reinforcement may allow a weld to move enough (by changing the distribution of trapped stress) to increase distortion.


      There are plenty of other cases both where grinding is desirable or required, and where grinding is undesirable. As a generality, grinding flush will rarely be detrimental, and will often improve properties, but most of the time isn't needed if the design takes into account that grinding won't be done.
      I may not be good looking, but I make up for it with my dazzling lack of personality

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      • #18
        Wow, lots of great info. Thanks for all the responses (although a few I feel were totally sarcastic). I did grind down these welds and you can't tell where the weld is...in other words it looks like one piece of metal. I hope this means it was good? Anyway, I appreciate the helpful info.

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        • #19
          I grind down my welds and I do it for a host of reasons. Prime is that I am not the prettiest welder in the world. I have some dead nerves in my r shoulder and arm that do not let me hold as steady as I would like. But my welds are good strong welds.

          When I was learning to weld I was taught to grind them down to get a look-see into the weld itself. That was good advice as it helped me develop my welding technique. I don't grind down 100% but I do most of them that are visible.

          On the question of whether or not its stronger to leave or makes it weaker if ground down? Albeit somewhat simplistic and not universal in application consider this:

          If you join together 2 pieces of 1/2 in plywood with a 2x4 does that make the plywood stronger?
          Last edited by SavageSunJeep; 10-19-2008, 09:49 PM.
          Don
          Scottsdale, AZ
          www.SavageSun4x4.com

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          • #20
            I guess I stand corrected. Grind away!
            Jim

            Miller MM 210
            Miller Dialarc 250P
            Airco 225 engine driven
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            Lots of other tools and always wanting more

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            • #21
              Grinding Welds

              I worked for the Air Force as a depot level aircraft engineer at Tinker AFB, OK in the late 70s in the jet engine overhaul area. We did the repairs on engines for the KC-135 tankers, B-52, C-141 transport, several fighters and the F-111 Fighter bomber. All but the F-111 engines were "big, black, and heavy" stainless steel and nickle alloys and were pretty tolerant of abuse. The Pratt & Whitney TF30 on the F-111 was one of the first of the new generations of the lightweight "tinfoil" engines made mostly from titanium everywhere but in the hot sections. We lost two airplanes in short order in England and I was involved in the accident investigation. In both planes, a high pressure case had come apart and had blown out the side of each plane.

              We finally traced the cause of the failures to a little old lady in the polishing and deburring shop who decided on her own that the welds along a major seam of the cases weren't pretty enough and she was grinding them down and polishing them. The grinding had weakened the cases to the point where they wouldn't stand up to the internal pressures during high speed, low level flight.

              Steve in Central TX

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              • #22
                My welding instructor told me that my welds were ugly but I had great penetration. The welds were as strong as the 2 pieces I was sticking together.
                I don't care for the "never" and "always" answers. In my world of Information Security, the answer is mostly "It depends". With the exception of safety, I believe that answer holds true with welding/grinding.

                RC

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                • #23
                  A lot depends on the thickness of the metal being welded for grinding back.

                  I welded a lot of 19mm (3/4") X 1.5mm wall square tube for some projects I made, and I grind back to make it look good, but I use a plasma welder, (much like a Tig welder), where you fuse the joint material together and add filler etc.

                  A lot of people welding thin square tube with a stick welder just dab the rod to the tube in short spurts, hoping not to burn through, and rely on a fairly pronounced amount of weld to hold the joint together......to grind back in that case would be daft.

                  Welding 2 pieces of 1/4" thick steel together means you have to vee the weld out first, so you have at least penetrated down far enought to leave some weld in the joint when you greind back.

                  Thin stuff like 2mm sheet is OK to grind if you can see the other side, but when it's hidden, grinding is a bit iffy.

                  Here's a dodge when you want to weld thin wall square tube and make it strong and look neat afterwards...... bend the walls of the tube inwards slightly at 45 deg for 1mm at the end where the weld is to be.....this will allow you to push the end of one tube into the end of the other tube for about 2mm or 3mm and give you something to weld on without blowing through.....and when you grind back there will always be weld in the joint no matter what.

                  You'll never get a full depth weld joint on thin tubing without actually blowing through, and if you don't blow through you aren't full depth.....any grinding means you have a half thickness at the join.....that goes for butt welding and 90 deg side on welding too.

                  You can prove your weld strength by butt welding two pieces of thin wall and then grinding back flush......put the piece in the vice and apply a pull test to the length....it should not break at the weld.....but 100% of the time it will break at the weld.
                  Ian.

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