Strength of weld

By Lawrence (*****)

Date 05-23-2008 06:41

One example of differing strengths for the same size weld made by two different processes would be to look at different penetration profiles in fillets.

GMAW (spray), SMAW, and FCAW will all have differing depths of penetration in any given fillet weld size.. The deeper penetrating process (FCAW in this case) will be stronger for that given weld size.

The same is true for GMAW (spray) vs MCAW... The metal core will penetrate deeper into a fillet than GMAW so it produces a stronger weldment even though the weld "size" may be the same.

Engineers use this sort of data to choose which process will be best suited for a particular project and what weld sizes will be required on the drawings.

By Boon

Date 05-23-2008 10:37

Hi Lawrence,
So penetration profiles in fillets determine the strength in a given weld size. How about comparison in depths of penetration between GTAW and SMAW processes which we are working with currently.
For the same welding process, would variations in current (assuming 130A vs 160A, both within range of WPS) also affect the depths of penetration?
Regards
Boon

By tom cooper (**)

Date 05-23-2008 12:17

L-
This question fascinates me - I twice tried to find data that confirms strengths vary as a function of penetration AND how CJP joint geometry affects total strength. There is no published data for this on the earth. Is a double bevel CJP stronger than a CJP V-groove? Or a CJP U-Groove? or any other CJP design? Then the question must be asked if one is stronger than another in tension? or in bending? or in some other combined stress situation? How much stronger is a weld having, for instance, .030" penetration as compared to the same weld having .050" penetration? Is the welder assured that he can deliver a weld having .050" penetration if asked to do so? How would he/she know? Depth of penetration is not a typical PQR variable, so how would any one know unless they accomplish "info only" macro's. All the welder can do and all the weld procedure is expected to establish is that the resulting weld provides the full strength of the base material (unless for some special purpose the weld is intended to be the point of failure). And this is regardless of the process. There are no analytical stress equations that address penetration depth as a variable of strength. This could not even be modelled in FEA unless we had assurity of the % of base metal to weld metal dilution.

On the other hand, the total heat input of the weld process used to gain deeper penetration may adversely affect material properties such as tougness, if not dealt with in some form of stress relief or other pwht.

I wish such data did exist. If you find some, please let me know.
Thanks.

By Lawrence (*****)

Date 05-23-2008 14:07

Tom,

Your questions are way out of my league.

My observations were directed to fillets and the fact that increased penetration in a fillet will produce more fusion at the faying surfaces, and therby a stronger weldment.

The strength of processes in groove welds??? I guess they would be determined by charpy's and tensiles.

And the difference in groove configuration?? V vs U Double vs Single... Way out of my league.

But it would be interesting to know.

So many variables... and so many favorable and unfavorable outcomes... Cooling rates, gas mixtures for FCAW, position, Gun angle... Alan covered most of this....

High level engineering stuff.

By aevald

Date 05-23-2008 07:15

Hello Boon, to add only slightly to Lawrence's spot-on reply. GTAW welding can be influenced by shielding gas type and composition(argon, helium, and argon/helium mixes) as well as the grinding preparation angle of the tungsten. So when considering similar weld sizes done with the GTAW process, these variables can affect the penetration characteristics relative to visible weld size and thus possibly influence weld strength. Additionally if you throw aluminum welding into the mix, many of the arc control characteristics of the newer GTAW inverter machines can further affect penetration characteristics, in particular, hertz control capability which has the ability to focus the arc and provide deeper penetration. Just a few more things to consider. Best regards, aevald

By pipewelder_1999 (****)

Date 05-23-2008 13:04

I had heard from a reputable source recently that GTAW will be more likely to have higher impact properties because of the reduced number of very small inclusions. I did not realize that and have yet to find any backup data that supports that other than it coming from a PE that is a welding engineer. It would be interesting to see a comparison of "All weld metal" properties of similar tensile strength weld deposits made from various processes. 7018, 70SX GTAW, 71T1, 70Sx GMAW.

Does anyone have any information related to the above ?

Thanks

Gerald

By Stephan (***)

Date 05-23-2008 13:18 Edited 05-24-2008 09:26

Hmmm...

Gerald, I beg your forgiveness, since I just flew over this thread and have no particular answer on your - as usual - excellent question.

But... this appears to grow up to a very interesting topic!

Best regards,
Stephan

By js55

Date 05-23-2008 13:37

If I may add a proviso here. To design by the strength differential of processes (and it is more impact toughness related as was stated due to non metalic inclusions with fluxed processes, than actual tensile strength which is highly variable-in other words, variances in chemistry is likely to have a much more profound influence than penetration patters) is a very sophisticated and detailed process and requires heat lot control of materials, control wof WPS parameters, and chemical limitations to work. For the great majority of designs the minimum specified yields or tensiles based upon a material specification has more repeatability and reliability, and is much more common. And is the very reason that material specifications exist.
In other words, if an SMAW, GTAW, SAW, FCAW, GMAW or whatever says 70ksi min then go with that. For the great majority of applications strength differentials may be interesting but are academic.

By js55

Date 05-23-2008 13:40

As for non metallic inclusions and the impact toughness differentials of welding processes based upon fluxes or lack thereof, etc, there are volumes of TWI Technical Reports on the subject that confirm. To my knowledge they did the most work on this subject.

By Stephan (***)

Date 05-23-2008 13:58

Jeff,

"In other words, if an SMAW, GTAW, SAW, FCAW, GMAW or whatever says 70ksi min then go with that. For the great majority of applications strength differentials may be interesting but are academic."

Wise words...

And as I guess, the weld metal deposit's strength may rather not be the subject of matter.

To my best knowledge the consumable(s) or filler-material(s) respectively, is/are produced to provide rather "higher" or "better" mechanical properties compared with the base-material(s).

Exceptions (e.g. ultra-high-strength steels) may prove the rule.

What is rather quite more interesting is what you have described so excellently in your post:

"To design by the strength differential of processes (...) is a very sophisticated and detailed process and requires heat lot control of materials, control wof WPS parameters, and chemical limitations to work."

Great!

If you don't mind that I say so, but this should point us exactly in the right direction.

Best regards,
Stephan

By aevald

Date 05-23-2008 14:56

Hello Gerald, I believe another factor could be included into this conversation, dilution. Depending upon the base materials and certainly the filler metals the type of dilution that could take place between the various processes could have a fairly significant effect upon the final weld metal deposit and possibly differences that further apply to weld strength and other properties. I am basing these thoughts on the consideration that low penetration processes with quick-freezing characteristics can produce less dilution and therefore end up with different properties where the penetration is deep and the weld-metal set up times are extended. I believe you can well include time-at-temperature factors as well considering the heat-sensitivity of many of the various metals in use today. Basically I'm sort of saying that certain processes are better suited for the joining of particular materials. Just a bit more to consider. Best regards, Allan

By pipewelder_1999 (****)

Date 05-23-2008 15:03

I agree. The GTAW process can often cause much higher dilution than other processes because of the variable rates at which filler material can be added. I also faintly recall something I read indicating that the arc efficiency of the process can lead to energy inputs that actually vary from the standard calculated heat input using amps/volts/travel speed. In addition the polarity of the process may affect this.

I will have to dust off a few chapters I haven't read for awhile.

By Bob Garner (***)

Date 05-23-2008 16:14

Allow me to chime in here as a structural engineer. We typically design welds to a minimum strength such as the 70 ksi mentioned. We, as engineers, don't pay too much attention to the welding variables; these we leave up to the person preparing the W.P.S. All we care about is the minimum strength for our designs to work. We rely on welders being certified to make the welds correctly and welding inspectors to assist in this process.

When we do siesmic stuff, then we get involved in specifying minimum Charpy's. But the weld chemistry, penetration, etc., we rely on you guys. We really have the easy part; you have to work all the variables.

Bob Garner