I hope this don't sound stupid.

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When I was a kid my dad told me not to quench a weld.  I would make it form crystals.  I was probably thirteen and had a mind of my own and did it anyway.  The weld broke like glass in no time, before the project was even finished.  It had the most beautiful purple and silver crystalline structure in the area of the weld made up of mostly filler material. Why did the metal separate into geometrically similar shapes and layers when exposed to the cold quench evenly?   I think I do not understand crystals that well.  This may be a metallurgical question.
Reply:Normally when you're welding the cooling is slow enough that you get very large "grains" forming, large enough that they merge into the surrounding metal. So they're not really "grains" at all, just part of the larger work. When you quench a weld you're making the grains smaller and that makes the metal harder. Quenching is normally used to get exactly this effect. More rapid cooling stresses the metal so that instead of getting a smooth metal area it crystallises  along defined boundaries (small areas of lower density/different composition). With well quenched metal those boundaries are small and not fractured, but with very rapid quenching like you're doing things happen too fast for that so you get bigger crystals with fracture planes between them. With slower quenching smaller atoms migrate to the stress zones and fill them, so you get (say) nickel-heavy areas in the steel that have different metallurgy.Quenching metal is a whole art in itself and there's an extensive literature on the subject.I found useful results using this google search: http://www.google.com.au/search?hl=e...+defect+quench and a paper that seems useful (if you understand scientific language) here: http://www.msm.cam.ac.uk/phase-trans.../faulkner1.doc (or Google's HTML version: http://209.85.165.104/search?q=cache...lnk&cd=6&gl=au
Reply:Originally Posted by BeezerWhen I was a kid my dad told me not to quench a weld.  I would make it form crystals.  I was probably thirteen and had a mind of my own and did it anyway.  The weld broke like glass in no time, before the project was even finished.  It had the most beautiful purple and silver crystalline structure in the area of the weld made up of mostly filler material. Why did the metal separate into geometrically similar shapes and layers when exposed to the cold quench evenly?   I think I do not understand crystals that well.  This may be a metallurgical question.
Reply:Originally Posted by makoman1860Well if your talking Iron based materials....it goes something like this in a horribly oversimplified form:1-Most steels will have a pearlite structure of carbon and Iron 2-Heating the steel to the molten state shifts the structure to austenite3-Depending on the alloying elements and carbon content, the cooling rate after welding will determine if the structure returns to ferrite, bainite, martensite, or pearlite, or a combination of certain ones.The transformation back to pearlite from austenite takes the longest time, and therefore the slowest cooling rate, this transformation also happens at a higher temperature then the others. Therefore any austenite not given enough time to transform to pearlite, will become bainite, and what doesnt have time to form bainite......becomes untempered martensite. This transformation happens not only in the actual weld bead, but in the area of the HAZ that has reached the austenic teperature rage.Untempered Martensite is the casue of most fatigue and brittle fractures in low alloy and/or high carbon steels.Certain welding processes cause a much more rapid cooling rate then others and hence can cause problems if other precautions and procedures arent followed.Hope this made sense
Reply:[QUOTE=Moz;162263]Normally when you're welding the cooling is slow enough that you get very large "grains" forming, large enough that they merge into the surrounding metal. So they're not really "grains" at all, just part of the larger work. When you quench a weld you're making the grains smaller and that makes the metal harder. Quenching is normally used to get exactly this effect. More rapid cooling stresses the metal so that instead of getting a smooth metal area it crystallises  along defined boundaries (small areas of lower density/different composition). With well quenched metal those boundaries are small and not fractured, but with very rapid quenching like you're doing things happen too fast for that so you get bigger crystals with fracture planes between them. With slower quenching smaller atoms migrate to the stress zones and fill them, so you get (say) nickel-heavy areas in the steel that have different metallurgy.QUOTE]Moz, you have no clue what you are talking about!    Where did you get this?  Please read makoman1860's brief summary of carbon steel metallurgy, he has it right.  Here is a link you should study:  http://www.gowelding.com/met/carbon.htmSorry dude, but you're not doing anybody any good by posting a bunch of made up and unsubstantiated metallurgical fantasy.    Do your research, then get back with us.
Reply:Moko's explanation tells you exactly why welds made with 6010/6011 cannot take the vibration and stretch that welds made with 7018 and 7024 can. It's all in the cooling rate.The difference between art and craft is the quality of the workmanship. I am an artist.
Reply:Originally Posted by pulserMoz, you have no clue what you are talking about!  Where did you get this?
Reply:High speed cooling, Like running a small bead on large plate  or quenching a weld does not allow the hydrogen to escape.  It then becomes a high pressure spot inside the weld and pushes the metal apart.  SLOW cooling allows the hydrogen to migrate out of the weld.  That is why there is low hydrogen rod (7018).  This is also why we preheat bigger stuff.  It gets worse as the carbon content goes up in the plate.  This is also why a second pass will help slow down cooling and prevent cracking.  This is also why more small beads can be better than one large one.DavidReal world weldin.  When I grow up I want to be a tig weldor.
Reply:Well I didn't want anyone to disagree.  I thought the info was very good even if it applied to something else.  Bottom line is that since that day back then, when I finish welding something I sit it aside and do something else for about five minutes or until it's cool enough to touch.  It usually gives me enough time to take another drink of my beverage and ponder the next step in the project.  If I'm in a hurry I wait until it stops glowing and place it in front of my big fan to speed the cooling just a little but not too quickly.  I've had my tender parts astraddle some things at 85 MPH for long periods of time I've done this way in the build and haven't had a failure yet.  Course there is always the first time!
Reply:Originally Posted by David RHigh speed cooling, Like running a small bead on large plate  or quenching a weld does not allow the hydrogen to escape.  It then becomes a high pressure spot inside the weld and pushes the metal apart.  SLOW cooling allows the hydrogen to migrate out of the weld.  That is why there is low hydrogen rod (7018).  This is also why we preheat bigger stuff.  It gets worse as the carbon content goes up in the plate.  This is also why a second pass will help slow down cooling and prevent cracking.  This is also why more small beads can be better than one large one.David