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Up Topic Welding Industry / Metallurgy / grain structure....
- - By PrairieDawg (*) Date 03-09-2008 03:02
I just have a quick question.

welding multiple passes would cause the grain structure of the welds underneath to grow larger or smaller?

I would say larger but I got into an argument with a friend over this.
Parent - - By 803056 (*****) Date 03-09-2008 15:53
In most cases the grain will become refined, i.e., smaller, because grains under stress (in the case of welding, there is always residual stress to be concerned with) will divide when subjected to the recrystallizing temperature.

However, that being said, thick weld beads may cool slowly enough that the grains of the previous weld bead lying within the HAZ may see some grain coarsening as the larger grains absorb small grains in an effort to increase the grain radius and reduce the free energy at the grain boundary. That is a special situation that can be avoided by using thinner weld layers that cool faster than thick layers.

Best regards - Al 
Parent - - By PrairieDawg (*) Date 03-10-2008 05:45
I was thinking that the grains would be larger because of what it says in my text. "Grain growth is a function of time and temperature. The higher the temperature and the longer the time, the larger the grains will be." So if you're welding multiple passes wouldn't that be increasing the heat dramatically thus creating a larger grain structure?
Parent - By 803056 (*****) Date 03-10-2008 12:38 Edited 03-10-2008 12:45
In the case of a weld, initial grain growth occurs as atoms attach themselves to preexisting grains at the boundary between the liquid metal and the still solid HAZ.  As the grains grow, there is less liquid until the weld is completely solidified and there is no more liquid. The faster the liquid cools, the more crystals nucleate and form in the liquid instead of simply attaching to the preexisting grains. The faster the puddle cools, the smaller the grains are, but more of them form. So, slow cooling results in fewer, but larger grains. Faster cooling results in more grains, but they are smaller.

Grain size reduction can occur if the larger grains are strained such as is the case with strain hardening. They break into smaller grains as the metal is raised to a temperature above the recovery temperature. There is always strain involved when the weld and adjacent base metals cool and contract. The uneven distribution of thermal gradients result in residual stresses which causes the grains to strain.

Large grains can become larger by absorbing much smaller grains if it will reduce the overall energy, but there has to be a driving force for this to occur. Temperature is often the driving force required. There is a limit to how large the grains can become because they will not absorb grains that are nearly the same size unless near liquidus temperatures are attained. This isn't the case unless the discussion is limited to the "mushy" zone adjacent to the liquid weld puddle. The time that the weld puddle is in the necessary temperature range is relatively short, so the grain growth is very limited unless extreme welding conditions are encountered.

In the case of welding, the initial formation of grains occurs as the weld solidifies. The size of the grains increases as the atoms in the liquid attach themselves to the preexisting grains. The size of the grain is dependent on the length of time the weld is in the liquid state. In the HAZ, the grains can coarsen or grow if they base metal is held at a temperature above the recrystallization  temperature and just below the melting temperature for relatively long periods of time as the larger grains devour the smaller grains. This is not the expected case if the welding technique was to employ multiple stringer beads and the interpass temperature was controlled.

However, if SAW was used, with slow travel speeds, high heat input, and very thick weld beads were deposited, the very slow cooling rates could in fact cause grain coarsening. Likewise, high interpass temperatures result in increasing the time at temperature of the weld and heat affected zones which could result in grains coarsening in the weld and some grain growth in the HAZ. The time  at temperature of concern are well into the austenizing temperatures for steel alloys. A temperature rise of a few hundred degrees isn't sufficient to cause recovery or recrystallization. Likewise the time required to initiate recrystallization is usually longer than that experienced when stringer beads are deposited.

At least, that's how I understand the situation.

Best regards - Al
Parent - - By GRoberts (***) Date 03-10-2008 17:15
I would tell your friend that it depends on what you are welding, if you are comparing the grain size of the HAZ or the weld, and if you are comparing multiple passes to single passes of the same weld volume (i.e. one pass twice as large as two passed, resulting in the same size weld), or the single pass and multiple passes being the same size (so the two pass weld is twice as large as the single pass weld when done.)

If you are welding austenetic materail (i.e. some stainless steels, nickel,etc), or ferritic material that does not transofrm, then limited recrystallization occurs, and the re-crystallization that does is because of strain, as Al mentions.  However, the larger the heat input, the larger the HAZ grains will be right next to the weld in the Coarse Grain HAZ (CG-HAZ).

If you are welding ferritic material that transforms to austenite upon heating, then there will be several HAZ regions with several different grain sizes.  The grain size will not be as large as the austenitic or non-transformable ferritic material due to grain refinement during transformation during cooling.  However, the single pass weld with higher heat input will still have a larger coarse-grain HAZ.  Subsequent passes will tend to refine the grain size of a previosly deposited weld bead though since the weld grains are generally larger than the coarse grains in a CG-HAZ.
Parent - - By PrairieDawg (*) Date 03-10-2008 18:09
I know there are alot of variables to this question but the main reason for me asking is this is going to be a question on my journeyman's test and they're probably not going to be adding in all these variables. The answers can be, get larger, get smaller, remain unchanged, and something else. So if you were to see this question on a test what would you pick?
Parent - By GRoberts (***) Date 03-10-2008 22:43
If I had to guess what the question was getting at, since it does say "welds underneath", my answer would be to get smaller since there is a lot of grain refining that happens when welding over subsequent weld beads in ordinary steels.
Parent - - By js55 (*****) Date 03-11-2008 13:50
I understand your dilemma. This situation seems to exist in many testing and certification environments. AWS is no excpetion. And as Al and Greg made clear it can be very complicated. It is, very complicated. The proper answer to your multiple choice question IMO would essentially be all three. Bigger, smaller, and no change, depending upon where in the cycle the particular HAZ location is, the temps involved the base metals, etc.
But of course, testing enviroments think they are keeping it simple and basic (or when they really get creative make a annoying attempt to try an make you think-which is actually synonymous with thinking like them)when what they are actually doing is forcing the tester to get inside the heads of the person that formed the question because the correct and most accurate answer is not available. So you are left with a guess. Or, you take a seminar so that you can memorize the pat answer like a monkey pullin a lever.
Parent - - By DaveBoyer (*****) Date 03-12-2008 06:46
       "like a monkey pullin a lever"
        There seems to be "trick questions" in many types of tests that seem to be in there solely to determine if You memorized the study guide.

         If You question the testing body about the not necessarily so answeres they will feed You the "That is what the experts say" line of crap. "THEY" the testing body, are not the experts, they just administer the test.

          Some of You who may take issue with these trick question & answer combinations are, in fact, the real experts.
Parent - - By PrairieDawg (*) Date 03-12-2008 09:44
The thing about this test in particular is if a question sounds funny to me I can always write it on this separate sheet of paper they give me what I think and why. So what I'm saying is I can challenge any question they ask me, and if I fail, I can pay my money for them to go over my sheet of paper and see if I have any valid arguments about any of the questions they asked me. If I make a valid point, they'll add what that question is worth to my percentage.

So when this question comes to me I'm going to answer "gets smaller" but I what do you guys think I should write down with it why thats right? It can only be long enough for me to remember (I have enough to remember already). Each question is worth 0.8% so I should probably make sure I'm going to get right the ones I do have beforehand.
Parent - - By js55 (*****) Date 03-13-2008 13:46
Doesn't any body else though, find the whole idea of question challenging rather curious?
Seems to me that if the questions are so ambiguous, or so poorly written, that challenging has become a standard procedure for remedy, that the testing process has a serious problem.
And given that ambiguity and interpretive environment, how can it be assumed that one would always recognize when a challenge is necessary?
In other words, you have to recognize ahead of time there is a problem. Thats the way the system works. But if you have truly recognized a problem you are most likely to be able to accomodate the problem, or at least account for it. In which case you are least in need of a challenge because you have just passed the most difficult hurdle.
This just sorta smacks of; well we know we may have written poor questions but we think we've found a way to cover our butts.
Parent - - By gwg (*) Date 03-14-2008 14:24
""I just have a quick question.

welding multiple passes would cause the grain structure of the welds underneath to grow larger or smaller?""

Simple answer, given no other information, multiple passes should = finer grains
Parent - By js55 (*****) Date 03-14-2008 14:50
I think your response is as valid as any response.  But, I also believe that once you reduce what is inherently a very complicated question to simplicity the response becomes arbitrary. And that is the problem I think with testing regimes (maintaining the context) that insist upon simplification of complicated phenomena. By saying that the simple answer is grain refinement you have completely eliminated the whole high temp HAZ wherein continued energy causes grain growth since there is no further phase change. Not invalid. It is most certainly true. Just in need of clarification. And with multiple passes high temp HAZ can compound.

And so a likely test scenario might be:
A) grain enlargment
B) grain refinement
C) Neither grain enlargment nor grain refinement
D) All of the above
E) None of the above
And even this question arrangement is somewhat messy.

AWS is very good at these types of questions. And the answer could very well be exactly as you say (choosing B). But given the choices, it seems as though greater knowledge of HAZ's (which would choose D) becomes a liability and not an asset. And therefore you need a seminar to tell you which simplified choice the testing fellers have chosen, or which lever to pull.
I say that it is all of the above and so no banana for me today.
Parent - - By ssbn727 (*****) Date 03-14-2008 22:59
Hi Jeff!

Don't even get me started on what you just astutely observed because, I'll just get misinterpreted all over again ;) ;) ;) I do however, completely agree with your observation that a procedure is even put in place to challenge test questions as a form of "CYA" for the test writers even though all they have to do is to reject the challenge without giving any reason for doing so in the first place to the person challenging the question :( :( :( I'm also in complete agreement with your opinion regarding the lack of confidence in the test writers own ability to write questions that are readily understandable which is probably (Unless I'm convinced otherwise) why these folks insert the "Challenge the question" procedure in the first place!!!

If there are any folks out there that have challenged questions on let's say - the AWS CWI exam for example... Has anyone ever received a response explaining to them that they either agreed or disagreed with the challenged question or questions at issue??? Please respond because my curiosity has widened as a result of this issue. I know that I'll be catching flack on this one but, oh well... Discussions are fun aren't they???

Parent - By CWI555 (*****) Date 03-15-2008 02:21

I've challenged several questions from various organizations. Never got an answer or for that matter a response on any of them.
There are organizations that claim to be testing for a given thing in general, but 80 percent of a specific test is oriented towards one industry only.
Never mind that the industry in question doesn't represent the majority of the work and technicians who perform it.
Those are just the state side organizations, it gets worse in Europe.

Parent - - By CWI555 (*****) Date 03-14-2008 14:37
"like a monkey pulling a lever"

Aptly put. AWS, API, ASNT, TWIC, JSNDI, EFNDT, and about any other recognized testing scheme are all about the same in this regards.

"(or when they really get creative make a annoying attempt to try an make you think-which is actually synonymous with thinking like them)"

Very true. As much as it's really unfair to the person being subjected to it, They are all more or less the same. It seems to be more a case of learning how to take the test, than being tested for what you actually know.

Parent - - By js55 (*****) Date 03-14-2008 14:52
"learning how to take the test, than being tested for what you actually know"
WOW. Gerald took all my yap and broke it down to its simplest and most clear terms. Wish I had done that.
Parent - - By ncdotcwi Date 03-15-2008 13:18
I find this discussion on grain structure intriguing and interesting.  I just busted a welding contractor for welding overhang brackets and Stay In Place form support angles (stitch welded 3-12) in 50+ degree temps on the top of a 2" thick compression flange on a welded steel girder without using any preheat.  The wheels are turning in my head as far as what is going on with the grain structure and the potential for future cracking after this bridge is put into service.  Any comments on what can be expected due to the possibility of martensite forming and what kind corrective action might be taken???
Parent - By 803056 (*****) Date 03-16-2008 04:54
I've challenged questions on many examinations I've taken.

Usually the challenged question is "debated" only if it means the difference between passing or failing. Some organizations look at the number of people answering the question correctly versus the number that answers the questions incorrectly. If too many people get the question wrong, it is pulled or revised before being used again.

I've also participated in developing questions for use on national examinations and believe me, it isn't an easy task. A question that is reviewed and agreed upon by the committee can be reviewed a second time a month or two latter and get tossed because it doesn't read the same during the second review by the same people. That being said, multiple reviews by several people usually improves the quality of the question and eliminates most of the ambiguity.

The English language is not as precise as we would like to believe. Seven people reading the same text can interpret the question several different ways. What looks clear and concise to the writer isn't as clear to the reader (or to the writer several days later).

There are times when there is no correct answer provided for a test question. Those are the types of questions I challenge. If the challenge is well written and explains how or why the reader determined there was no correct answer listed, credit will be awarded if a correct answer is given by the challenger. If the challenge doesn't provide a sound basis for the answer offered by the challenge, it is rightfully set aside.

I can't remember receiving a response to any of my challenges other then when I took the SCWI examination the first time way back when. I challenged a lot of questions on that particular examination. I received my SCWI, so I assumed some of the questions I challenged made the difference between passing and failing.

As for a response to ncdotcwi; you should know that an answer to that question is not meaningful unless you provide us with more specific information. What is base metal specification and grade?. What is the size of the weld? How many weld passes were deposited to develop the required weld size? What were the welding parameters, i.e., heat input? What was the classification of the electrode? This is a good natured slap on the back of your head because your screen name implies you are a CWI. Any answer without all the proper information is simple conjecture and generalizations.

Best regards - Al
Parent - By gwg (*) Date 03-18-2008 02:15
The formation of martensite is probably low risk in your stated welding conditions because the hardenability of structural steels (A 36), not the micro-alloy type but conventional, would require more significant cooling rates and high alloy content to achieve the formation of martensite in the base metal HAZ. A more serious problem is delayed cracking caused by hydrogen.

As far as corrective action, I would have the suspect welds subjected to a wet fluorescent MT just to be sure in the event the lack of preheat and surrounding conditions resulted in hydrogen that could have been introduced into the weld region.
Parent - - By Jim12 (**) Date 03-25-2008 19:32
Parent - By hogan (****) Date 03-25-2008 20:16
i would guess M270
Up Topic Welding Industry / Metallurgy / grain structure....

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