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Up Topic Welding Industry / General Welding Discussion / Interesting tensile pull textures
- - By MRWeldSoCal (***) Date 02-22-2016 22:57
Pulled these today, we knew there was some porosity and LOF but these had a nice odd look to them!  enjoy

------------->  http://imgur.com/a/vIuNx

J
Parent - - By 803056 (*****) Date 02-22-2016 23:49
Classic Fisheyes.

The samples should have been dry roasted for a while to rid it of diffusible hydrogen.

What welding process and electrode was used?

Al
Parent - - By MRWeldSoCal (***) Date 02-23-2016 00:08
Al-

ER70s-2.  It was welded by a track setup robot.  GMAW but it appears to be Short circuit.  The customer didn't tell us which one they only asked for information only.  we did bends, tensiles, and macro.  They  didn't think to far on baking then plates

J
Parent - - By 803056 (*****) Date 02-23-2016 00:59
Was this a rush job?

You will not typically see fish eyes if the sample sat around for a week or two before doing the tensile tests.

D1.1 has provisions for a low temperature "bake out" under certain conditions.

Al
Parent - - By mcostello (**) Date 02-23-2016 03:45
How can a gas "bake" out of a solid? Or is everything porous at some level?
Parent - By MRWeldSoCal (***) Date 02-23-2016 04:17
loose hydrogen can be baked out of metal due to how tiny the atom is.
Parent - By MRWeldSoCal (***) Date 02-23-2016 04:21 Edited 02-23-2016 04:23
It was kinda a rush it seems, I think they were doing their best to try and get an idea of there they were at.  You should have seen the welds :eek:

good to know about the bake out!

I thought it was lack of fusion because it was an off U-Groove, seems to buckle at the lower end into the weld

Jordan
- - By 803056 (*****) Date 02-23-2016 14:20 Edited 02-23-2016 16:29
Mcostello, you might enjoy reading some of the past discussions on the subject of diffusible hydrogen we've had.

Monatomic hydrogen (single atoms) can and does diffuse through the atomic lattice. The hydrogen collects at sites referred to as hydrogen traps. The hydrogen traps are often discontinuities, grain boundaries, etc.

This was something I found confounding when I was first introduced to hydrogen in welds, the affects, and preventive measures. The professor did a great job of putting the whole thing into context. As he put it - thick of yourself on a football field. The four largest beach balls you can find are positioned at the corners of the field. A small glass marble (like the one's kids used to play with) is located on the 50-yard line. All the empty space between the beach balls (representing iron atoms) and the small marble (representing the hydrogen atom) represents the space in the lattice structure. All that empty space gives the hydrogen plenty of freedom to move (diffuse) from one location to another.

The rate of diffusion is related to temperature. The higher the temperature, the greater the rate of diffusion.

Now, what happens to the hydrogen is interesting. Over time the hydrogen atoms diffuse to the surface and effuses (escape) into the atmosphere. In which case nothing bad happens. There are a couple of working hypothesis in the wonderful world of metallurgy and welding. One, and probably the oldest conjecture is some hydrogen can team up with other hydrogen atoms to become molecules. The molecules occupy a larger volume and exerts a stress on the atomic lattice. If the lattice lacks ductility, it can fracture. Another hypothesis is when there is an abundance of carbon, as in the case of a martensitic microstructure (iron saturated with carbon) the hydrogen is thought to combine with the carbon to form methane gas. The molecule of methane is much larger than the diatomic hydrogen, thus exerts a much greater stress on the bonds and tends to cause cracking (hydrogen assisted cracking, cold cracking, delayed cold cracking, under bead cracking, etc.).  

One can see the hydrogen effuse from the surface of  weld bead. There are examples of the experiments on You-Tube. I have a file that shows the hydrogen coming out of a weld bead. I will send it to you if you send me your e-mail address. I don't believe I can attach it to a post. I can attach a photograph of an earlier experiment I did for an AWS Section meeting several years ago. The interesting thing is the sample off gases (hydrogen) for upwards of twenty hours.

Hope that helps. This is exactly why the latest edition of D1.1 causes the hair on my neck to stand up on end. The statement that it is permissible to weld on surfaces with residual contamination such as water, oil, grease, etc. is a crock of bull crap. It flies in the face of common sense. I used to think that the members of that committee had an abundance of common sense. My faith in D1.1 is shattered like a skull hitting the pavement after falling from the twentieth floor of a high rise. What other clauses of D1.1 should the user ignore? Some of it, all of it, or just that one clause?

I proposed that the Certification Committee not use D1.1:2015 for the CWI examination. My proposal wasn't accepted. Maybe because I wasn't there to make my case. One must not throw stones at the sacred calf or jeopardize the golden goose.

Best regards - Al
Parent - - By MRWeldSoCal (***) Date 02-23-2016 17:14
I am going to be doing this for my beginners SMAW class.  That article was so good I printed it and made a small note book.  Read it 5 times since haha.

Thanks Al

Jordan
Parent - - By 803056 (*****) Date 02-23-2016 18:02 Edited 02-23-2016 18:05
I'm happy to hear you found it useful.

Word of caution: If you try this for your class, use 6010 (best) or E7018 that has been left out for several days in a moist area. Do not use 7018-H8, 7018-H4 or any with the R or M suffix if you want to produce hydrogen.

As soon as the weld is terminated, cool it quickly in cold water, knock off the slag, wire brush, and dry as quickly as possible. Once it is dry, drop it carefully in baby oil. The bubbles should start in five minutes or so.

Maybe you should try welding on a surface that has residual water, or oil, or grease using an E7018-H4R to see if hydrogen is introduced into the weld pool. Some people will suggest the arc will burn off the contamination and not influence the amount of hydrogen introduced. I am not so sure that is the case.

Best regards - Al
Parent - - By MRWeldSoCal (***) Date 02-23-2016 19:02
I was going to set up a few scenarios.  (1) 6010 as is, (1) 7018 left hovering over a bowl of water, (1) and one 7018 fresh out of the over and have three glasses set up and do them all in order. 

I found those nearly endless pages of info very interesting, Its part of my lesson.

J
Parent - By pipewelder_1999 (****) Date 02-24-2016 03:09
Or maybe run a bead over some stuff as allowed by D1.1!
Parent - - By CWI555 (*****) Date 02-27-2016 04:27
With the average arc around ~6500F, more than 3/4 of the water molecules will disassociate into mostly H, H2, O, O2, and OH. I can't say as I understand how at least some of that doesn't end up in the puddle at the moment of solidification.
Parent - - By 803056 (*****) Date 02-27-2016 12:11 Edited 02-27-2016 12:14
You are correct with regards to the disassociation of the hydrogen bearing molecules. It all has to do with the solubility of the various gases at the high temperatures of molten metal. As the metal cools and solidifies, the solubility of the gases plummets. Not all the gas effuses into the atmosphere. Nitrogen largely manifest its presence as porosity, oxygen reacts with the deoxidizers and is carried off with the slag in the case of SMAW, SAW, or FCAW, or as simple silicon oxides and silicates in the case of GTAW or GMAW. Oxygen can also combine with carbon to form carbon monoxide and manifest itself as porosity if there is insufficient deoxidizers present. Hydrogen, consisting of a single electron and a single proton, is small enough to stay in solution in the solidified carbon or low alloy steel at a much lower concentration. As already mentioned, it is able to diffuse through the atomic lattice and effuse to the atmosphere, collect in hydrogen traps, or cause the more damaging delayed cold cracking.  

I was reading up on the subject a couple of days ago in preparation of updating one of my courses. I found some information regarding the potential amount of hydrogen that can be introduced into the solidified weld. With some editorial liberties:

SMAW using E6010/E6011 - 50 to 65 ppm (56 to 67 ml/100g of weld)

SMAW using E7018 - 3-9 ppm (3.4 - 10 ml/100g of weld)

Source: Handbook of Structural Welding  John Lancaster / Abington Publishing

Best regards - Al
Parent - By CWI555 (*****) Date 02-27-2016 16:32
Thanks for that reply and sourcing. Another interesting rabbit to chase down a hole.

Regards,
Gerald
Parent - - By phinojosar (*) Date 02-23-2016 21:04
Thank you very much for the information Al, by the way great article on February Inspection Trends!

This kind of experience is very useful for a new CWI like myself.

Best regards.
Parent - - By 803056 (*****) Date 02-23-2016 23:36
Thanks for the compliment phinojosar. That's what the forum and Inspection Trends is all about, helping each other. The problem is keeping the articles in IT short. Its tough to cover all the bases in a short article.

As for the hydrogen experiment, several people have expressed problems to get the experiment to work as advertised. Follow my instructions with regards to quickly cooling the hot sample in water as soon as the bead is terminated. Use steel samples about three to four inches long. It should work just fine.

Al
Parent - - By Paladin (***) Date 02-24-2016 13:38
Since the weld needs to be quickly cooled, for the experiment to work, then I would assume that the hydrogen is "baking off" when allowed to cool slowly.

This baking off. What temperature and how long is it done? Is this normal in text coupons? Where is it done?

Now I will wonder if I need to "bake" my simple little straps for API 1104.
I know, "you can drive a truck thru 1104" and I've never failed one,  but no need to take chances, right?

If the test samples the OP posted had baked them, would the fish eye not be there?

I'm having a little trouble getting my head around the "aging of steel". Is it stronger a week after welding than one minute after after cooling off?

Thanks,
Floyd
Parent - By PhilThomas (**) Date 02-24-2016 19:11
The quick cooling causes the hydrogen to (largely) stop diffusing until you can cut the sample and put it in the baby oil.  This is very similar to the process used to do an AWS diffusible hydrogen test when the manufacturer wants to assign an electrode with one of the "-Hx" designations.

The hydrogen will diffuse at basically anything above room temperature, but the higher the temperature, the faster the diffusion.

Since the hydrogen diffuses over time and is based on temperature, it is somewhat normal in test coupons.  If the OP had kept the samples at 300F for a few hours, or at room temp for a few days, the fisheyes (which can be thought of as a gathering of hydrogen trying to work its way out) would likely not be there.

As to "aging of the steel" - I guess you could say it is less likely to crack as time passes.  I'm not sure the steel itself it actually "stronger" on a micro level.  On a macro level - especially for high strength steels - the weld in total could exhibit lower tensile test strength due to a combination of susceptible microstructure and the presence of hydrogen which is still within the steel.

I love the marble, beachball and football field analogy.  It makes clear the concept of a fancy metallurgical term - interstitial - which is a big word which means "can move about between the atoms in the steel"

In steels, there are a couple other interstitial elements which, when present in sufficient quantities, can also cause problems:  nitrogen and boron.  But more on them on another day!
- - By 803056 (*****) Date 02-24-2016 19:55 Edited 02-24-2016 20:07
Hello Paladin;

Actually your hydrogen "bake off" reminds me that there is a procedure called a "Hydrogen Bake Out". The completed weldment is heated to at least 550 degrees F and held at temperature for around six hours. The steel expands when heated, increasing the distance between the adjacent atoms and allowing the hydrogen to escape more easily.

I am sure the Ideal Gas Law comes into play. As the weldment is heated, the partial pressure of the gas increases and "forces" the hydrogen to effuse more readily into the atmosphere that is at a lower pressure relatively speaking.

So, you have a couple of mechanisms at play, all encouraging the hydrogen to leave the atomic lattice and reduce the probability of delayed cold cracking.

While aluminum may age naturally and gain strength, I don't recognize it as a strengthening mechanism in carbon and low alloy steels. In the case of aluminum, the strengthening mechanism requires the aluminum to be heated to a high temperature to allow the alloying elements to go into solution and form a supersaturated solution when the aluminum is quenched. The supersaturated system is not stable. At room temperature (or slightly above) the solubility decreases (compared to the solubility at high temperature) and the excess alloying elements form small clumps within the atomic lattice (interstitial) thereby straining and distorting the lattice. Anything that distorts the atomic lattice makes the metal stronger and harder.

A similar mechanism is used to harden and strengthen precipitation hardenable stainless steel. The mechanism is triggered by heating the quenched stainless to get the supersaturated alloying constituents to clump within the atomic lattice. Starting to sound similar to the age hardened aluminum?

Best regards - Al
Parent - - By 803056 (*****) Date 02-27-2016 12:39
Here's a link to good demonstration I found on you tube. Professionally done!

Al

https://www.youtube.com/watch?v=Wjz8eh3uxkU
Parent - By Stringer (***) Date 02-27-2016 15:35
I blame the robot.
Parent - By Northweldor (***) Date 02-27-2016 17:46 Edited 02-27-2016 21:59
This film, more than half a century old, also posted by Bill Bruce, illustrates the diffusion even more vividly. Amazing photography for the time!

Edit: Sorry, here's the link:

https://www.youtube.com/watch?v=bv9ApdzalHM
Up Topic Welding Industry / General Welding Discussion / Interesting tensile pull textures

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