Proposal to repair 1.25Cr-1/2 Mo with INCONEL 3

[mcisajp]

We have a proposal for the contractor to have a joint of 1.25Cr-1/2 Mo, line to vessel in reformed gas with hydrogen service, that showed a defect in Rx after PWHT to be repair with Inconel 82 and avoid a new PWHT.

After removing the transverse crack the dig area is 1x3/4 " with depth of 9 mm. Weld is to fill with TIG and Inconel 82 3/32" , preheating at 150 C and covering with mineral fiber after weld is completed. After cooling down do X-ray again and put in service, without a new PWHT...

Is there any comment on this procedure?

 

[Carburize]

It might work if the contractor uses a temper-bead (half-bead) technique. I would have them run a test sample to see what hardness the procedure develops on the HAZ.

 

[metengr]

Clarification - half bead is not the same as temper bead. In fact, I would not recommend half bead - we will be deleting reference to half bead from the NBIC book in Part RD. Despite it’s original use in ASME Section XI, it is no longer recommended in boiler and pressure vessel alternative weld repairs.

 

[mcisajp]

What will be a god value of hardness to look for....

 

[metengr]

You need to carefully review your service conditions to determine if there is a maximum hardness requirement. It could be that you need to remain at or below 22 HRc for vessel material weld heat affected zones (HAZ's) to reduce susceptibility to hydrogen attack. For steam service that is benign in terms of corrosion or hydrogen attack, I have approved heat affected zone hardness of 28 HRc with this base material using the temper bead method.

You have one other option, that is local PWHT using the recommended method in the NBIC, see Part RD-1100. This is commonly referred to as the "bulls eye" method. Again, you need to evaluate service conditions for possible elevated temperature damage mechanisms.

 

[stanweld]

If the defect was in a newly made weld to ASME B31.3, I would recommend that the Contractor repair the weld with the original welding procedure. The cost of an additional welding procedure qualification would more than outweigh the cost of an additional PWHT. Also there would be no added stress at the dissimilar metals interface during cyclic temerature operation due to the coefficient of expansion mismatch.

If this is a repair to an existing, in-service weld, it should prove quite effective if made per metengr's recommendations with temper bead technique. We have made this type of repair a number of times with complete success on P4 materials up to 2 1/2- nches thick, including meeting the B31.3 hardness requirements in the HAZ. Depending on the base/weld material thickness, you may wish to increase the preheat.

 

[weldtek]

Metengr,
Can you expound on the half bead and temper bead techniques and the reasoning for accepting one over another?

 

[Rich2001]

Here is a link on Temper Bead Welding Technique

http://www.gowelding.com/met/temper.htm

Vita sine litteris mors est.

 

[metengr]

Weldtek;
The half-bead technique was the original welding concept for elimination of PWHT on P-No1 and P-No 3 base materials. It was used extensively during the 1970's-1980's, and was referenced in the National Board Inspection Code at that time. The half-bead method is very cumbersome to implement and really provides no added benefit in comparison to the current temper bead technique, which is now the preferred method and is endorsed by the NBIC.

The problem with the half-bead method was the amount of grinding involved to remove approximately 50% of the first weld layer. This was very time-consuming and difficult to enforce, but it did result in the tempering of the base metal HAZ from subsequent weld passes.

When it was discovered that similar tempering could be achieved using the temper bead technique mentioned above, most if not all had abandoned the half bead method.

Currently, I am on a task group for the NBIC subcommittee to revise Part RD and align it with the wording in the recent edition of the ASME B&PV code, Section IX, which has now provides guidance for the qualification of a temper bead welding procedure.

 

[weldtek]

Rich2001 & Metengr,
Thanks for the enlightenment.

 

[schrijen]

schrijen

I think that based on the proces conditions the material choice is based on resistance against Nelson Hydrogen Attack . Is this the case than is welding without PWHT not possible because your missing the stable mix carbides in the HAZ. For the resistance against Nelson Attack you need a minimum of 5% Cr in the carbides and this is not possible without PWHT. Welding without PWHT means you end up with the Nelson Attack resistance of normal carbon steel in the HAZ. So after maybe 10.000 hours the attack (decarburisation of the cementite and small cracks on the grainbounderies caused by methane formationwill start)
Choosing nickel based fillers is possible but does not change my view with respect to the HAZ. The HAZ in fact does not "know" which kind of welding filler you use !!

My experience is that welding of this material is possible without PWHT for steam services etc. and by using a heat input of apprx. 15KJ/cm and a preheat of 225-250 degr.C you will end up with a max. hardness in the HAZ of 250 HV.
The best choice is always welding with matching fillers in stead of nickel based fillers. I also do not understand the reason for it because also the yield strength of the nickel based fillers are twice at high as the yield strength of the base material. This means the residual welding stress will be the same because the lowest yield strength of the components is responsable for the residual stress.

 

[metengr]

schrijen;
As we have stated in the posts above, temper bead like any weld repair method must be carefully evaluated, and your statement regarding exposure to hydrogen was my concern, as well. If you need to achieve a base metal HAZ of 22 HRc this will not be achievable with the temper bead method for this base material.

Your second paragraph however, I would take exception. Austenitic filler materials are commonly used to absorb solidification (thermal/mechanical) stresses between the weld metal and base metal during and after weld repair. Despite the strength differences, the austenitic filler metals have a greater ability to absorb thermal/mechanical stresses during welding because of their increased ductility.

 

[schrijen]

metengr

I am concerned about the two types of hydrogen problems that can occur in the syngas-environment.

Normally syngas evironment means resistance against Nelson attack and therefore this material choice.
Creating the nescessary carbides only by half bead or temperbead or whatever is not absolutely not possible.
Yurioka et all have done a lot of investigations about it because everybodey wants to delete PWHT after welding if it is possible.
So PWHT is always nescessary.

If the hardness as you mentioned is below 250Hv normally there are no hydrogen embrittlement problems during start up or shut down of the installations as long as the plastic deformations is below 2%.
As you know, in fact hydrogen embrittlement can also occur at lower hardnesses if there is a lot of plastic deformation because hydrogen embrittlement is simply blocking of the dislocation movement. The only thing is the harder the microstructure the easyer is goes when deformation occurs.

The results and conclusions of a lot of investigations of the Batelle Institute about metallurgical-mechanical-thermal modelling and measuring of residual welding stresses during fabrication,welding etc. are not in line with your comments.

 

[mcisajp]

TO EVERYBODY PARTICIPATING:
Thanks to everyone for your enlighten and active participation on this topic.

Repair was done with inconel 82, 3/32" rod , preheating at 300-350 F , insulating the external side of the weld and finally flame heating for 5 to 10 minutes and then covering the weld with insulation to cool down. Final results of the repair show less than 210 BHN in the HAZ and adjacent area of the weld. QC was done with MT ( to account for the 1.25 Cr- 1/2 Mo in the adjacent area), PT and RX with good results. The contractor presented a qualify procedure for P4 and also one for a P5 up to a thickness of 0.47".

Thanks to all again