Hot Pass with E6010 Electrodes

[Leinster]

Can someone explain to me what the exact function of the hot pass is after using an E6010 electrrode for the root run. I have heard a number of different theories but most of them seem to point at reducing the possibility of hydrogen cold cracking as the E6010 electrodes release a lot of hydrogen when burned.

I have read the statement that to "reduce the likelihodd of hydrogen cold cracking use a hot pass technique to eliminate susceptible microstructure by means of self tempering effect, reduce hydrgen content by allowing hydrogen to enfuse from the weld area". Can someone please explain how the hydrogen is enfused from the weld by putting a hot pass in straigth after the root run?

 

[SJones]

From the Lincoln publication Welding Pressure Pipelines & Piping Systems:

The purpose for the hot pass is to melt and float out the wagon tracks left after the root pass

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[Ron]

Hydrogen absorption and diffusion tend to concentrate in the cooler portions of the weld. The hot pass moderates this and causes the diffusion from the initial absorption to occur in more weld volume (reduces the density of the hydrogen occurrence), thus reducing the propensity for concentrated embrittlement and thus cracking.

As SJones noted, this also tends to obliterate wagon tracks which also lead to root cracks.

 

[Duwe6]

The "hot pass is to melt and float out the wagon tracks left after the root pass" only works it the root is remelted. Most pipe welders do not remelt their roots. However, the difference between 6010 and 7018 for the Hot-Pass seem to be acedemic only. I've been looking for any effects, either "in the field" or in literature. So far, have been unsuccessful in finding anything that shows any difference.

 

[Kiwi2671]

IMHO the purpose of the "hot pass" is fourfold.
1 Fill the weld bevel.
2 Melt and float out the wagon tracks left after the root pass. (as per SJones)
3 Release trapped gases (as per Ron)
4 A significant other is that a large majority of pipeline welds are done in a clamp and under some sort of stress. A root bead and hotpass are generally the minimum amount of weld that must be done before the crane / sideboom etc can be released and the weld can then be left to completely cool and be filled and capped at a later time / date.

Duwe6,
With all due respect I have to disagree with you regarding "wagon tracks.
The term hotpass is not really applicable to E7018 electrodes (or even vertical up with a 6010)as it is really only a second run. The hotpass (or second run) can be at the same amps as the third, fourth, fifth etc. A GTAW / 7016 / 7018 or 6010 vertical up root will produce a flat or concave root.
E6010 vertical down produces a very convex bead with thin lines of slag on either side.
Some welders grind the lump off and use a mild hotpass amperage whilst others do minimal grinding and use a very high amperage.
Anything from 50 - 150 amps higher than the root pass is used. The hotpass is done with a skipping motion and you can actually watch the wagon tracks being burnt out as you are welding.
Hope that helps,
Regards,
Kiwi

 

[Leinster]

Thanks for your replies....

Ron,
If the initial absorption is essentially being diluted by the hot pass but what about procedures in which the hot pass and all remaining runs are also with E6010 electrodes. In this situation surely you would essentially be adding more hydrogen each run. Please could you advise

Thanks

 

[Ron]

The hydrogen does not come from the electrode. It comes from humidity absorption in the coating flux and from ambient conditions. Control those and you control the hydrogen...for instance, heat the electrodes in a rod oven before use...you'll see a lot less of a hydrogen issue.

 

[Kiwi2671]

Ron,
As you are probably well aware putting cellulose electrodes in an oven is a big no no.
Cellulose electrodes are manufactured with a specific moisture content so there is always going to be hydrogen absorbed into the weld metal.
Regards,
Kiwi

 

[Ron]

Oops! Kiwi you're right... wasn't thinking. You're right about the rod oven, but a little care in storage doesn't hurt to keep the humidity down. Even though the cellulose coating is less susceptible to moisture, if you get surface condensation (not a common problem, but does occur), it's another source. Otherwise, not much to do there except dilute and diffuse.

 

[gtaw]

Cellulose, like any hydrocarbon, produces hydrogen as it decomposes in the presence of the welding arc. The cellulose of the electrode covering is a major source of atomic hydrogen in the weld pool and the completed weld.

The hydrogen effuses into the atmosphere over time, but it tends to collect in what are termed hydrogen traps, i.e., point defects, grain boundaries, etc. of the individual grain structure of the steel. There are several hypothesis of the exact mechanism of delayed cracking associated with hydrogen. You can pick the one or two that strike your fancy.

I like one that I recently heard that stated the hydrogen combines with excess carbon typically associated with martensitic microstructures to form methane gas. The methane in turn exerts tremendous pressure of the surrounding atomic lattice structure and develops small cracks if the lattice structure lacks sufficient ductility to accommodate the methane. This theory seems to correlate with the amount of cracking associated with hydrogen introduced into the weld pool while depositing weld somewhat better than some of the other theories.

Back to the electrodes used with SMAW, the low hydrogen electrodes utilize a limestone based covering that reduces the amount of hydrogen introduced into the weld puddle by the flux covering. However, the benefit of a low hydrogen electrode is lost if it is not stored properly to control the amount of moisture absorbed by the flux covering. Think of what happens when you leave a bag of cement in a garage, cellar, or on a car port where it is subjected to high ambient moisture for a length of time. In short order it becomes a "bag-O-brick" due to the hygroscopic nature of the cement powder (a limestone derivative).


Best regards - Al

 

[Leinster]

Many thanks for all the input.... Very interesting stuff!!

So how does putting in a hot pass allow hydrogen to diffuse out of the root run especially if the hot pass is an E6010 which produces a lot of hydrogen in the weld?

 

[Leinster]

Ok, so how the hydrogen diffuses out of the weld pool is unclear however I have another query...

What is the minimum temperature you should allow the root run get to using an E6010 electrode before putting in the hot pass? I have heard 350 degrees celcius but I feel this seems a little high.

 

[Kiwi2671]

Leinster,
350 degrees C is ridiculous.
The minimum temperature before putting in the next pass is your preheat temperature.
The maximum temperature before depositing the next pass(interpass temperature) is as per your WPS,
Regards,
Kiwi

 

[Mr168]

I think you'll find the interpass temperature of a 6010 hot pass to be less of a diffusible hydrogen/material properties issue, and more of a "we just blew a hole the size of Kansas in our root pass" issue.

As Kiwi mentioned, give them a minimum preheat temperature, give them a maximum interpass temperature, and let the welder do his thing.