Another Hydrogen Bakeout Questions ASME Sect 1 work

[rmillercwi]

Regarding common steels such as ASME P1-P5 materials, as I understand it the intent of performing a hydrogen bakeout is to dry out the weld.

The process is primarily performed on welds using the SMAW weld process is it not?

The bakeout is of less importance using weld processes using the GTAW method exclusively is it not?


To ASME, some welds require PWHT and some do not. In varous procedures, I see a trend towards hydrogen bakeout and PWHT being used in the same sentence as if to imply that if a weld did not require PWHT that the same weld would not require a bakeout.

This does not make sense and believe this to be an inaccurate statement. I see the hydrogen bakeout and PWHT as being two entirely different things.

I am currently reviewing a spec for a contractor that has to weld qty 26- 8" 2.25cr elbows inside an HRSG. Half of those require PWHT (sch 120) and half of those do not (sch 100). The weld joints subject to PWHT are (to procedure) are going to go thru a bakeout process at some point after welding and prior to stress relieving. Those weld joints that are not subject to PWHT will not go thru a bakeout at all.

It seems to me that you either do the bakeout promptly after welding or you don't do it at all. What is relevant for sch 100 pipe should be just as relevant (or irrelevant) to perform on a sch 120 pipe. As an owners rep, I am having trouble buying into this.

Using good rod (Lincoln H4) and have adequate electrode drying controls at least documented.

Questions:

How important is the bakeout on 1.25 cr materials (or P1,P5a for that matter) for welds that will NOT be PWHT'd and/or will surely have a delayed PWHT?

What is the point of doing a delayed bakeout? I see none here whatsoever.

How would you perform a bakeout if you did not have electrical resistance heating equipment available? Cant see a guy sitting on top of these welds with a weed burner for two hrs but maybe so?

 

[weldstan]

The pupose of the hydrogen bake out (dehydrogenation) treatment is not to "dry out the weld." It's purpose is to allow hydrogen to diffuse from the weld and thus mitigate the potential for hydrogen assisted cracking, which is more likely to occur in alloy steels where martensite is formed during cooling. Hydrogen (as H20) is more prevalent in flux coated/cored electrodes and submerged arc fluxes. Hydrogen is dissociated through the arc into hydrogen and oxygen and thus transferred into the weld. The longer the delay in the bake out, the more ineffective it will be.

In general for piping, a dehydrogenation heat treatment is not needed when using low hydrogen welding processes for the low alloy steels in question; however, when welding heavy walls where cooling is rapid (essentially quenching to the preheat temp) with resultant martensite formation together with high residual welding stress, it is advisable to provide an intermediate heat treatment (dehydrogenation as a minimum) to prevent delayed cracking.

 

[metengr]

Agree with the response from stanweld.

How important is the bakeout on 1.25 cr materials (or P1,P5a for that matter) for welds that will NOT be PWHT'd and/or will surely have a delayed PWHT?

Not important IF low hydrogen electrodes are used and properly stored for use, and for joint thickness 2" or less.

What is the point of doing a delayed bakeout? I see none here whatsoever.

If you don't follow the previous response (low hydrogen and thickness).

How would you perform a bakeout if you did not have electrical resistance heating equipment available? Cant see a guy sitting on top of these welds with a weed burner for two hrs but maybe so?

The bake out would/should not be performed using conventional torch heating.

 

[Guest102023]

I also agree, but disagree with classifying a hydrogen bake-out as a heat treatment. Yes it involves both heat and a treatment but that doesn't necessarily make it a 'heat treatment' (the way the Code talks about heat treatment, unless there is some clause I have yet to encounter that says so). Of course we all call it that in colloquial language.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[wegm]

rmillercwi,
As stated in AWS D10.10 hydrogen bake out is performed to remove hydrogen from material prior to manufacture, fabrication or repair activity. So the process is used not only after welding or repair welding but also in some cases before welding when the base metal contains hydrogen due to service exposure and intents to avoid hydrogen cracking after completion of the welding. Hydrogen bake out is not an ASME (or other stds) requirement but it comes form welding engineer's evaluation based on good engineering practice. Some standards give recommendations for the process like AWS D10.10 suggests for carbon and low alloy steels hydrogen bake out to be performed at a temperature at least 316 degree C for 2 hours per inch of thickness with 2 hours minimum. Since post weld hydrogen bake out depends on the thickness of base metal, I suppose that's the reason the welding engineer has specified such a process only for the sch120 elbows. If low hydrogen welding consumable used hydrogen bake out is not necessary for alloy steels up to 2.25Cr-1Mo. Hydrogen bake out shall be performed immediately after welding in order to avoid delayed cracking.
Please have a look at this link:

http://www.weldinguide.com/gdanastasiadis/viewtopic.php?f=2&t=443

regards
wegm

 

[rmillercwi]

Thank you all so much for the feedback. I have perhaps a dumb question. Have been around this type of fabrication since 1988. We have always used low hydrogen electrodes when anything called for the SMAW process. My understand is that the entire family of 7018/8018...9018...9018b9 electrodes are all classed as low hydrogen.

I have read many times that as long as low hydrogen electrodes/diligent use of rod ovens/ proper baking..... were used, the risk of hydrogen related cracking is low. My question is using low hydrogen electrodes as apposed to what other types of electrodes? I have yet to see any fabricator in my industry use anything but the electrodes detailed above with the exception of some contractors using 6010/6011 to fill an open root on some low pressure carbon steel piping.

 

[metengr]

My understand is that the entire family of 7018/8018...9018...9018b9 electrodes are all classed as low hydrogen.

Incorrect. You need to specify low hydrogen by using the suffix designator, like E7018 H4 or H8.

 

[rmillercwi]

Thanks metengr. To clarify, standard E7018 without the supplemental H4/H8 designator is not considered a low hydrogen electrode?

 

[metengr]

rmillercwi;
It can be listed as "low hydrogen" because you have low hydrogen coverings, with some having low moisture absorbing capability on SMA weld rod offered in SFA 5.5, and 5.1. However, there is a note in the electrode covering classification table which indicates an electrode can be further classified by meeting specified diffusible hydrogen content by designation. To me, an electrode covering classification is much different in comparison to weld rod that is tested to meet a particular diffusible hydrogen content.

What I am saying is don’t be misled into thinking you procure low hydrogen electrodes by simply purchasing them to SFA 5.5 or even 5.1. I always require electrodes to be purchased to a hydrogen designator to ensure low hydrogen for welding.

 

[pandithan]

last digit ends with 5,6 &8 are low Hydrogen Electrode only

 

[masotheuri357]

Dear Colleagues ,
The "Hydrogen Bake Out" is a special heat treatment with multiple variations based on several situations susch as:
Operating and Service Conditions, Type and thickness of material, etc.
We know that the Welding Activities produces high temperatures with severe temperature gradients, and rapid cooling and solidification, which increases the residual stresses, grain size, and brittleness in the welding area and the surrounding base material (HAZ), which in the presence of H2 can create a path to HIC; then the likelihood of HIC can be reduced through dehydrogenation heat treatment (DHT).
Again, based on the analysis and evaluations of the differents situations the DHT is not necessary in some cases, however can be mandatory in for some cases, this is independant of the welding process to be used.
In the Petrochemicaland oil refineries the DHT is mandatory for any material (excepting the SS) that was on H2, NH3 service condition, the intention of the DHT on theses cases is to remove service-induced atomic hydrogen prior to starting any weld.
The Welding Research Council (WRC) in their Bulletin 452 explains a little more...