Stop Starts

Date 05-23-2009 12:20 Edited 05-23-2009 12:33

Welcome Maty;

Clearly your are doing something right if the plates bent without breaking.

Most welders have developed their own techniques over the years. In some cases the technique prove to be unacceptable when subjected to testing.

I've never been a big advocate of GMAW (MIG/MAG) using the short circuiting mode of transfer because it is a "low heat" process prone to fusion type defects. That is the process typically utilizes the lower end of the useful voltage range, the lower end of the wire feed range (low amperage), and when welding in the vertical position, it uses the upper end of the travel speed. That isn't to say it can't be performed successfully, but it does mean the welder has to pay close attention to the welding parameters and the manner in which the arc in manipulated.

I'm not sure I understand how you manipulate the welding arc from your description. I've seen situations where the welder used "spray transfer" parameters, but intermittently pulled the "trigger" thereby extinguishing the arc momentarily as he welded. He described it as "pulsed welding". Few of us would agree that the technique produces what is truly pulsed transfer. I have attached a photograph where the welder did what I described. In essence, what the welder produced is overlapping "tack" welds which were not accepted.

It sounds like you are using what I would call a "whipping" technique, that is, moving the arc upward a short distance and then back into the molten puddle to control the fluidity of the weld puddle. Each movement upward will permit the puddle to cool slightly thereby controlling the tendency to "drip" downward or to become excessively convex. A small amount of whipping is fine, but excessive movement can result in incomplete fusion in the root. Let's face it, welding is a balancing act where all the variables have to be controlled in order to obtain acceptable results.

Vertical downward progression on anything other than thin materials is a gamble. The house always wins in that the welder will not get complete fusion in the root on a consistent basis unless his skills are extraordinary. Few welders can perfect the technique and pass the requisite welder performance tests. Most codes discourage the use of downward vertical progression with any welding process. Notice I didn't say they prohibit the use of downward progression. They simply establish close controls that discourage most companies from using it.

It's good to have you join us.

Best regards - Al

Attachment: GMAWVerticalMay2009.pdf (111k)

By dbigkahunna (****)

Date 05-23-2009 15:44 Edited 05-23-2009 15:51

I have to take umbrage with your statement as it pertains to SMAW, "Vertical downs- Very nice finish when doing architectural handrails but it is known as the weakest weld, i belive we dont give much credit for the vertical down welding process".
A good DH weld is just as strong as a UH weld. A poor DH weld is just as weak as a poor UH weld. A 7010 weld with DH progression is just as strong as a 7010 weld with UH progression, all other variables being the same. There may be engineering/economic issues where the use of a UH progression is preferable to DH, but the weld strength is not.
The issue comes between the shoulder and the holder not between the stinger and the puddle.
BABRT's

By MatyBoilermaker Date 05-23-2009 20:52

Thanks for that picture but i will try to get a pic of my V-Up as soon as i can that picture was a little bit of what i describe but the weld overlaps each-other so you dont really realise thats its peformed by stop starts- and i have not used pulse before i am a boilermaker by trade im just intrested in the welding bit since in my working career i like to to think my teachers were the best in australia i belived they showed my the right way of doing things .
thanks guys for ya help i will try to get them pics asap.

By 803056

Date 05-24-2009 03:48

If you're referring to pipe welding I agree to a point. From my vantage point it would appear that the greatest use of downward vertical progression is in the cross country pipeline industry. The acceptance criteria listed in API 1104 is amongst the least stringent when it comes to common discontinuities that are considered to be defects by those working in the structural steel or ASME pressure vessel or piping industries.

Downward progression inherently results in low heat input regardless of the welding process due to the high travel speeds needed to keep ahead of the molten weld puddle and slag. That isn't to say it can't be done, but it is recognized by the AWS Structural Codes, ASME B&PV Codes, and military welding standards as a technique prone to problems for all but the best of welders.

The comments are not meant to be derogatory, just a simple observation and comparison of various welding standards.

As you say, "A good DH vertical is as strong as a UH weld." And there's the rub. A good DH vertical is a rarity in most applications involving the thicker materials used to fabricate structural steel or pressure vessels. The acceptance criteria for those industrial sectors are more stringent than for cross country pipeline, thus discontinuities such as incomplete fusion, incomplete joint penetration, crater cracks that are rejected by AWS, ASME, and military standards are acceptable per API 1104 within their prescribed limitations.

Every welding application has acceptance criteria that is somewhat unique to the application. Not all acceptance criteria are the same, some being more tolerant of weld discontinuities than others. Perhaps the most stringent of those codes I mentioned would be the bridge code, followed by the structural code, and then different ASME codes and the military standards. Each in its own right has proven to be sufficiently stringent for their specific application.

The other point that can't be lost is welders that have been employed by an industrial sector have adapted their techniques to produce welds that meet the requirements imposed. I find it interesting that few pipe welders can pass the structural plate test without some practice and few structural welders can pass the open root pipe tests without practice to hone those skills. I find it even more interesting that few welders can pass the T-fillet break on their first attempt even though they have passed the grooved plate or pipe test previously. Again, there is nothing derogatory intended by my the comments, they are simply statements of my observations backed by forty years experience as a welder (both pipe and plate using multiple processes and base metals) and inspector.

Best regards - Al

By welderbrent (*****)

Date 05-28-2009 14:48

Gentlemen, I must agree with both of you, especially since I don't think there is a 'major' dispute going on the subject.

Having worked as a 'Boilermaker' in the employ of several companies in field erection of storage tanks and pressure vessels, one notable one being CBI, I know that many vertical welds on butt joints 8-10' in length were completed using 7018 with downhill progression.

I also believe that it is mainly in the training and competence of the operator/weldor as to rather the completed weld will only pass RT, if even that, or would pass a bend test.

In my limited experience I believe way too many of these welds are run too cold and do not have the needed penetration required to consistently pass bend tests. And if not run too cold, they still do not have the required heat input because they are run too fast for the volt/amp setting they are using. Many not run too cold are run employing improper technique and are still not satisfactory as to soundness.

7018 will 'burn through' it's own slag to a limited degree. When running heat range, speed, and proper technique combined together in a proven manner you will end up with the quality of completed weld to pass either test. You also can pass any test without a 'perfect' cleaning of the previous pass. None of this is to say that one should scrimp on weld prep. The cleaner the better. Especially in pipe or pressure vessel applications. Tanks are generally not held to the same extreme quality.

I personally believe this to hold true on the running of GMAW short circuiting as well. The advantage there being that there is no slag to contend with.

I have passed tests with both processes that were both RT and bent. I would love to see how they compare when run through the entire test process for establishing the PQR. How do the DH runs hold up to tensile, Charpy, and the rest? I'm sure they are not as good as the UH but are they as good as the parent metal? Would have to be to pass the tests. If so, then they are 'good enough' so to speak. The main point of my question here was how do they 'compare'?

I concur with Al that GMAW-S is not one of my preferred processes to see fabricators using. Most of the people using it don't have the training required to accomplish the task to the needed quality. They also have never seen their work come apart to realize how critical every aspect of the operation is in the short circuiting mode. Thus, in their view, 'It looks good, therefore, it must be good.' If every shop using it had their employees regularly weld a sample and witness the outcome of a bend test I think we would see a larger degree of satisfactory application of the process. Apply that to downhill 7018 as well.

Oh, and people need to realize the limitations of certain processes, rod/wire sizes, etc. 3/32 7018 run on 2" thick material without pre-heat is not going to give satisfactory results. It applies to wire sizes and processes employed as well. Everything has it's limitations. The first and main limitation is the knowledge, training, experience, and proficiency of the welding operator.

Having said all that, I say to the original poster, Practice, Practice, Practice. Then, Practice some more. If you don't have a bender available, stick your completed coupons in a vise and bend them with a hammer. At least you will find out if you are able to consistantly BOND two pieces of metal together, not just STICK them together.

Have a Great Day, Brent

By 803056

Date 05-28-2009 18:08

Well put.

Best regards - Al