9cr Bakeout 2

[rmillercwi]

Have done an extensive search and cant find my answer 100%. Doing some consulting at a plant for something un-related and got roped into a P-91 welding issue.

Plant is in an extended outage and has qty 8- 4" weldolets to a 16" sch 80 pipe to install. All 9cr materials. Was asked to ok the contactor procedure. Giving it a quick look, nothing jumped out at me. Right pre-heat/interpass (400/600)temps, right fillers(ER90S-B9 root, 9018-B9H4), rod ovens,electrical resistance heating, cooling to below 200 before stress, 1375-1425 soak temp, slow cooling under insulation....

The only thing that stood in my way was the contractor wanting to perform a 600 f bakeout for 2 hours prior to dropping it to below 200. I know its a good idea but there was going to be no delayed PWHT and the repair effort is being performed inside a building. Field conditions very similar to shop conditions. It will go from pre-heat to weld to cool to stress all in one seamless operation. In this case the bakeout did not seem necessary.

I didn't recall ever seeing a shop welded 9cr joint go thru a bakeout. Reviewed various weld procedures and didn't find a one that listed the requirement. Re-read Sperkos and Jeff H article from back in 05 and saw no mention of it there either.

Stumbled across Eurowelds article from November 01 and various articles elsewhere and many do recommend a 600f bakeout in all cases where the joint has utilized a flux process.

Starting to question my judgement and hoping I did the right thing.

Your opinion of this matter would be appreciated

 

[metengr]

I would concur; there is no need for a separate bake-out step for Grade 91 CSEF steel provided the PWHT is being done immediately after welding. Use of low hydrogen electrodes really reduces the risk of delayed cracking.

 

[weldstan]

The dehydrogenation treatment is unwarrented in your case. It does nothing, because the dehydrogenation process will occur during the immediately conducted full PWHT after allowing the weld to cool below the martensite finish temp, as you are doing.

 

[Guest102023]

I agree that preheat and strict moisture control of consumables has more significant benefits than bake-outs. This is also done by process selection: each has its intrinsic range of deposited H. Straight to PWHT is good practice, especially with the newer exotics like P91*.

* After cooling to a low enough temperature to ensure full transformation from austenite where applicable.

Having latterly become more involved in the fabrication of low alloy refinery and power plant steels, I have observed that 'bake-out' among contractors appears to have gained status as a kind of heat treatment, and also that PWHT will correct everything that was done wrong during welding, including inadequate preheat. Neither are true.

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

 

[WeldUK]

I agree with the other replies. For steels with a Cr content greater than 9%, failures due to stress corrosion cracking have been reported for as-welded components waiting to be postweld heat treated after welding (even if no load was applied!). However, if PWHT will be immediately carried out, post-heating is not necessary.

If PWHT cannot be immediately carried out, a bakeout will mitigate but not eliminate the risk of stress corrosion cracking, therefore, PWHT shall be carried out as soon as possible and careful handling is required to keep the components dry and to avoid excessive loading.

Reference: my companys's experience and EPRI report 1012748 on CSEFs, unfortunately this has been revised and is not available for free download anymore.

 

[wegm]

rmillercwi,
The post-weld hydrogen bake out in such a case depends basically on the base metal thickness and since you are to weld 4" weldolets to a 16" pipe, I suppose that the base metal thickness is much more higher than 15 mm and thus post-weld hydrogen bake out is recommended.
In addition I would like to add that the CSEF (creep strength enhanced ferritic steels) properties of P91 are affected by the accuracy of PWHT and its correct performance and thus improper PWHT may result in loosing CSEF properties. Please have a look at this link that attaches a PWHT curve for such a case:

http://www.weldinguide.com/gdanastasiadis/viewtopic.php?f=2&t=381

regards
wegm

 

[pradipgoswami]

Hi RMiller CWI,

Performing a 600 F bakeout for 2 hours prior to dropping temperature to below 200F, is "NOT ADVISABLE". Grade -91 steel delivers the best meachnical and high temperature Creep Strength when the microstructure is prediminatly martensitic.So it requires a straight cooling to 200F right from the welding temperatures and holding it for 1hr/ inch of thickness for complete martensitic transformation to be over. A prolonged hold @ 600 Deg F would delay the martensitic transformation and may make the final micrsturcture as "bainitic" which is not advisable for best operating conditions of the equipments.

Any bake-out if required should be done after "completion of martensitic transformation" and before PWHT. The intent of bake -out is to delay the "PWHT". However if Low Hydrogen "H4" grade electrodes are used, with a good control of Interpass Temperature, Bake-Out may be minimized or eliminated.

See below the extract from API-RP-938B:-Use of 9Cr-1Mo-V (Grade 91) Steel in the Oil Refining Industry

If welds are to be cooled down to ambient after welding and before PWHT, a postweld bake-out may be of critical importance, especially for thick-walled components when presence of residual hydrogen is of concern. One example of postweld bake-out process includes soaking the weld joint and at least 75 mm ( 3 in.) on each side of the weld, at 320ºC (~ 600ºF) for a minimum of 20 or 60 minutes for thin and thick wall components respectively. Then the weldment is wrapped
with insulation and allowed to cool below 93ºC (200ºF). This process facilitates hydrogen diffusion from the weld joint.


Thanks.

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario, Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299

 

[pandithan]

Hydrogen bake out is not required if the weldment is taken for PWHT immediate, but bring down to 80 degree Celsius and hold it before u take up for SR