Fusion vs. Penetration

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I read, and hear, folks talk about fusion and penetration.Are these interchangeable terms, or is my take on the terminology correct/incorect.To me, penetration is the ability of the rod, combined with amperage to actually dig into the metal when welding a joint.  More, or less, assuring that everything reaches a certain temp. and is the strongest joint possible.I have less of an understanding of fusion.  I think it means either of 2 things.....1.  Actual melting of the parent metal to form a bond seperate, and in addition to, the material deposited by the rod.2. Or, a good bond between the material deposited by the rod, and the parent metal without the necessity of actually digging into the steel that's being welded.What little I know about the subject leads me to think that when blacksmiths used to weld, it was actually fusion combined with pressure supplied by the hammer.  Almost like forging I guess.I mostly use low penetration rod when building something, and it's a question I've always been concerned about."Any day above ground is a good day"http://www.farmersamm.com/
Reply:farmersamm,Fusion is more like bonding. The ability to tie in to previously deposited weld and/or base metal. To fuse the two together. It sounds like you understand penetration. A good visual experiment for this is to weld a pass on a plate, cut it. Then lightly sand the cut surface and soak in muriatic acid for a few minutes(following all safety requirements). The weld pass will be distinguished, showing the depth of penetration. Also known as a macro etch test.
Reply:Originally Posted by qaqcfarmersamm,Fusion is more like bonding. The ability to tie in to previously deposited weld and/or base metal. To fuse the two together. It sounds like you understand penetration. A good visual experiment for this is to weld a pass on a plate, cut it. Then lightly sand the cut surface and soak in muriatic acid for a few minutes(following all safety requirements). The weld pass will be distinguished, showing the depth of penetration. Also known as a macro etch test.
Reply:Now for the second question.On a fillet weld I used to concentrate on the outer edges of the weld in order to make the pass as straight as possible.  The welds looked ok, and I haven't had a failure so far.Then I started to worry about the root.  Was I getting enough heat into the root?  So I started to try to get heat into the root as well as the edge of the weld.  The welds don't look as good as before.  I'm trying to get heat into 3 places at once, both edges, and the root.I guess, as a reference point, I like to stick the rod into the puddle to the point of feeling it hit the steel, or slag. Don't know which.  Cast iron, I like to hold a longer arc.Is the puddle of sufficient temp. to create a good joint without trying to stick the rod into the root on every swing?"Any day above ground is a good day"http://www.farmersamm.com/
Reply:I'll start with; "I'm unqualified to answer this question."  But I've read; "pause at the edges, pass across the root". I don't want to spread internet BS, but this is something I've seen more than once. Shoot me if you must. 9-11-2001......We Will Never ForgetRetired desk jockey. Hobby weldor with a little training. Craftsman O/A---Flat, Vert, Ovhd, Horz. Miller Syncrowave 250
Reply:Originally Posted by farmersammNow for the second question.On a fillet weld I used to concentrate on the outer edges of the weld in order to make the pass as straight as possible.  The welds looked ok, and I haven't had a failure so far.Then I started to worry about the root.  Was I getting enough heat into the root?  So I started to try to get heat into the root as well as the edge of the weld.  The welds don't look as good as before.  I'm trying to get heat into 3 places at once, both edges, and the root.I guess, as a reference point, I like to stick the rod into the puddle to the point of feeling it hit the steel, or slag. Don't know which.  Cast iron, I like to hold a longer arc.Is the puddle of sufficient temp. to create a good joint without trying to stick the rod into the root on every swing?
Reply:Hello farmersammFusion - Full metallic bond between weld and base material via melting and solidification.Lack of fusion - Where this has not occurred at the fusion line between WM and BM.Penetration - Degree of weld within the BM,( measured from the surface of the BM), in square butt welds / fillet welds..........or penetration as in a"root bead" deposited from the outside of hollow sections, eg: pipe, etc, to achieve full thickness / full strength welds. The height of the root bead, ( measured from the ID surface), is referred to as amount of "penetration".Defect - Excess penetration in the case of root bead heightCheers
Reply:Originally Posted by farmersammNow for the second question.On a fillet weld I used to concentrate on the outer edges of the weld in order to make the pass as straight as possible.  The welds looked ok, and I haven't had a failure so far.Then I started to worry about the root.  Was I getting enough heat into the root?  So I started to try to get heat into the root as well as the edge of the weld.  The welds don't look as good as before.  I'm trying to get heat into 3 places at once, both edges, and the root.I guess, as a reference point, I like to stick the rod into the puddle to the point of feeling it hit the steel, or slag. Don't know which.  Cast iron, I like to hold a longer arc.Is the puddle of sufficient temp. to create a good joint without trying to stick the rod into the root on every swing?
Reply:Originally Posted by chrispc66Hello farmersammFusion - Full metallic bond between weld and base material via melting and solidification.Lack of fusion - Where this has not occurred at the fusion line between WM and BM.Penetration - Degree of weld within the BM,( measured from the surface of the BM), in square butt welds / fillet welds..........or penetration as in a"root bead" deposited from the outside of hollow sections, eg: pipe, etc, to achieve full thickness / full strength welds. The height of the root bead, ( measured from the ID surface), is referred to as amount of "penetration".Defect - Excess penetration in the case of root bead heightCheers
Reply:Thanks for the text book answer, I was simply referring to defects wrt fusion and penetrationeg:Lack of pen, (LP)excess pen (XP)orlack of fusion (LOF)lack of root fusion (LR)Lack of side fusion (LS)etc, etcIncidently, a discontinuity is a defect, albiet a compliant one in accordance with the applied acceptance criteria. Just have a look at any RT report for any weld, usually plenty of compliant defects interpreted, hence "discontinuity"Cheers
Reply:A discontinuity is only a defect when it exceeds the maximum allowable tolerances.  Up to that point, it is still a discontinuity.  The term "defect" is only used when it fails.  You can have discontinuities in a passing weld.  If you have defects, then it is a failing weld.
Reply:There are no compliant defects. That is why there are the two different words. What they are noting on the report is the presence of a discontinuity, some codes require that acceptable discontinuities be noted. A discontinuity that is larger than allowed by the applicable code then becomes a defect. It's like calling a pipe and a round tube the same thing.
Reply:Thank you!  That is basically what I was trying to say.
Reply:I'll add some comment to this.Fusion: joining or combining at the atomic level, produced via heat and/or pressure. No filler required no arc required. (explosion welding, friction/spin welding, forge welding)Penetration: depth of fusion. Someone mentioned thinking of welding as gluing: it is not! In gluing the bond is at the surface, and all the parts joined never share an atomic connection.  In welding the bond created is at the atomic level. Meaning that atoms of the various parts to be joined share a newly created bond (sharing electrons {or protons or neutrons or some kind of "trons"} as welder I have reached the limit of my knowledge of atomic bonding.)The blacksmith with his hammer flux and forge create as good or better of a weld as any modern arc weld process. The act of applying pressure (hit with BFH) forcibly drives the atoms at the joint into fusion, creating the weld. The blacksmith has a great advantage over the modern arc welder in that he (the smith) will reheat the welded part many times after the weld and during the heating (rather just after when the part is pulled from the forge) a sound weld is glaringly evident with color of the part, (nonfusion shows as a black line bisecting the joint.) With thin enough material one can see nonfusion in the interior of a welded part (seen as a dark spot in the otherwise glowing orange surface) Remember the best certification one can acquire, is via a BFH. With this certification YOU know all about your welding abilities.TJ
Reply:I often prefer a long lever or hydraulic pressure instead of a BFH.  Fusion is getting (some) of one part to melt into the melted part of the other part and letting the molten zones mingle and then cool and solidify into one.Penetration is how deep you get.  Sometimes because of the joint geometry or process limitations, joint prep is done to enable the correct penetration to occur.  On thick material, you pretty much have to do some joint prep to enable the weld to physically get to the desired depth.  You physically can't get a rod (or wire, or filler, etc) down deep enough to get fusion in thick pieces without it.  Again within process limitations.As said above, welding is not gluing!  When welding, you are melting some of the parts to be joined as well as the (possible) filler material.  The best laid schemes ... Gang oft agley ...
Reply:i guess fusion is melting/mixing  together, and penetration  would be how deep  bm melted..i dont believe there is any atomic sharing of" trons."  .welding is a physical process, other than the undesired oxides,  nochemical changes occur,.none desired..
Reply:Thanks AllI am interested in what's actually goin' on inside the weld.  It's something I've started to be concerned about.  I guess, I'm trying to do a better job, not just cosmetically, but mechanically.Either that, or I'm just gettin' old and worryin' more."Any day above ground is a good day"http://www.farmersamm.com/
Reply:OK, SO I SAID GLUE.  SUE MEJust couldn't come up with a better term for my conceptJust an idiots way of trying to understand the process"Any day above ground is a good day"http://www.farmersamm.com/
Reply:Originally Posted by farmersammThanks AllI am interested in what's actually goin' on inside the weld.  It's something I've started to be concerned about.  I guess, I'm trying to do a better job, not just cosmetically, but mechanically.Either that, or I'm just gettin' old and worryin' more.
Reply:(Ain't spellin' that long handle of yours)Man, that's it in a nutshell.  I should of thought of that instead of tryin' to get fancy.My greatest concern has been when welding a thick piece of stock to lighter material.  I have to keep the heat down to keep from blowin' thru the thin stuff, and it seems like the thicker stock acts as a big ol' heatsink."Any day above ground is a good day"http://www.farmersamm.com/
Reply:Originally Posted by farmersamm(Ain't spellin' that long handle of yours)Man, that's it in a nutshell.  I should of thought of that instead of tryin' to get fancy.My greatest concern has been when welding a thick piece of stock to lighter material.  I have to keep the heat down to keep from blowin' thru the thin stuff, and it seems like the thicker stock acts as a big ol' heatsink.
Reply:Originally Posted by farmersamm(Ain't spellin' that long handle of yours)
Reply:Indeed."Real power is an arcair hand torch with a piece of 3/4 inch carbon and 1500 amps.  You feel like Zeus throwing a lightning bolt."
Reply:Originally Posted by farmersamm(Ain't spellin' that long handle of yours)Man, that's it in a nutshell.  I should of thought of that instead of tryin' to get fancy.My greatest concern has been when welding a thick piece of stock to lighter material.  I have to keep the heat down to keep from blowin' thru the thin stuff, and it seems like the thicker stock acts as a big ol' heatsink.
Reply:When welding thick-to-thin, the thick piece doesn't -act- like a heatsink.It -IS- a heatsink.You have to get enough heat into the thick piece to sufficiently melt some material.  Then you kind of dance the arc and puddle to the thin piece and get enough heat into that one to sufficiently melt it as well, and you get the molten zones of all the pieces and the puddle to flow and turn into one.The zen of welding, making the many become one.The good news when working on thick-to-thin is that you only have to get a puddle deep enough on the thick piece to match the thickness of the thin piece.Example, you have a piece of 1/4 inch thick steel and are trying to attach a piece of 1/8 inch steel.  You only have to melt into the thick piece by 1/8 inch and melt the thin piece into that molten zone.  The best laid schemes ... Gang oft agley ...I think what I've done is worry myself into some bad habits.I used to do pretty much what everyone has discussed, then started reading about the problem of insufficient root penetration.  Then I went crazy and started to try to put heat into every part of the damn weld.  With an emphasis on stickin' straight into the root somehwere in the process.I think that sometimes getting ahold of tech reading material can be a bad thing if you don't have someone to discuss it with before you come to the wrong conclussion.This has been a really good thread for me, and I hope others who may not be too sure of what really is happening inside the puddle learn from it too.I've been doing this stupidity for about a year, and I haven't been too thrilled with the results.  I need to go back to the way I was doing things.If the weather clears sometime soon I have to start making an implement to push trees with.  It'll be a good oppurtunity to post some closeups of welds.Idiots aren't born, they're self made"Any day above ground is a good day"http://www.farmersamm.com/
Reply:I agree with the terms posted above which mention penetration as a depth measure, and fusion as a bond.   Originally Posted by weldrwomnA discontinuity is only a defect when it exceeds the maximum allowable tolerances.  Up to that point, it is still a discontinuity.  The term "defect" is only used when it fails.  You can have discontinuities in a passing weld.  If you have defects, then it is a failing weld.