[PSUBS-MAILIST] Heads Question

Tue Sep 19 11:56:54 EDT 2017

Sean,
GMAW commonly is utilised in two primary modes , " Short Arc " and " Spray
Arc " .  "Short Arc" equals lower amperage versus "Spray Arc" equals higher
amperage . The amperage output and duty cycle of the welding machine
determines what mode you would be capable of achieving . Lower amperage ,
lower duty cycle power sources will usually only run effectively in the
"Short Arc" mode , some units in this class might be able to exhibit SA
characteristics but only for a short period of time due to their limited
duty cycle ( 10% - 20% ) at the higher amperage range necessary . Duty
cycle is usually delineated as how many minutes in a 10 minute window the
machine can maintain a rated amperage . "Short Arc" power sources are most
commonly found in the consumer to light commercial markets . The primary
determining factor being purchase price (a few hundred dollars to maybe a
couple of thousand for a multi purpose unit which could include smaw, gtaw
and gmaw capabilities. "Short Arc" is fine for doing sheet metal work ,
custom car or custom motorcycle work in light gauge metals .
The use of GMAW in industry would be primarily focused on the " Spray Arc
mode " . High amperages , much higher duty cycles ( at the least 60% to the
prefered 100% ) , high deposition rates , and very robust weld integrity on
thicker metals .  Of course all of this comes at a cost . I haven't checked
prices lately but the components I purchased to get in the "spray arc "
mode cost me somewhere around 6k . This is definitely the low end of the
price range in this category of welding , the sky is the limit when it
comes to equipping an industrial facility .
When I build another submarine I will weld the root and the hot pass with
GTAW ( TIG) and run the filler and cap passes with FCAW-G ( flux core with
a cover gas) also commonly known as " dual shield "   In spray arc mode of
course .  I have complete confidence in this process . The alternative of
course could be GTAW(TIG) root and hot pass and SMAW (stick) weld it out OR
if a person has a lot of time on their hands they could GTAW(TIG) weld the
joints completely out , root , hot , filler passes and cap .
To answer your question about  which is the biggest concern , embrittlement
or porosity . If you are welding with GMAW in the "short arc" mode then
embrittlement is the issue . If you are welding with GMAW in the "spray arc
" mode then porosity and blow thru on the root pass would probably be a
problem unless you did the root and hot pass first with TIG  . This is just
a cursory overview because Industry has a lot of options at their disposal
. Like using the Submerged Arc process with no gap for the root pass and
getting 100% penetration running at 600 to 1000 amps . Not something
someone can do at home in their garage ! Getting porosity on the root pass
while welding with GTAW ( TIG) on carbon steel is usually not a problem ,
Of course stainless steel is a completely different animal . Unless you
purge with inert gas oxidation and porosity is standard procedure .
Dan

On Tue, Sep 19, 2017 at 8:46 AM, Sean T. Stevenson via
Personal_Submersibles <personal_submersibles at psubs.org> wrote:

> Dan - I was under the impression that the issue with GMAW (MIG) was the
> risk of porosity due to ineffective purge (both purging the opposite side
> of the joint on the root pass, and back purging the puddle while filling),
> and that it is perfectly acceptable if 100% gas purge is ensured and low
> hydrogen electrodes are used? The process is used in lots of pressure
> vessel manufacturing, but perhaps the pros have better control over this?
>
> I like the idea of a GTAW (TIG) root pass. Doing the filling passes with
> SMAW (stick) seems tedious, particularly if there is a lot of filling to be
> done, but your message is making me think twice. Is the major problem the
> embrittlement, or the porosity?
>
> Sean
>
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