P91 Welding HAZ Question

[rn01]

For hot induction bending of P91 piping, the induction heated zone will produce an adjacent zone where intercritical temperatures will be experienced. In this case, the entire spool must be re-normalized and re-tempered in accordance with latest code cases. If P91 piping with improper N+T at a bend is discovered after installation, the entire section of piping in question must be cut out for re-treatment or replacement.

Numerous publications have noted that the outerboundary of the HAZ for P91 welds will also experience the intercritical temperature range during the welding thermal cycle. This weakened zone is where "Type IV" creep failure occurs. Normalizing and tempering may be the only way to completely eliminate the effects of a weld heat-affected zone but this approach can not be used for field welded joints since localized N+T would not be acceptable.

It appears that the zone experiencing intercritical temperatures during bending is rather narrow. Is the HAZ area experiencing intercritical temperature during welding much smaller and less impacted than the zone from the induction bending process and, as a result,less of a concern?

 

[metengr]

rn01;
Are you familar with induction bending? Yes, the heat zone applied by inducation heating coils is rather narrow, but you have to consider that when the induction bending is performed it is performed over the entire radius of the fitting, meaning you have many of these narrow intercritical HAZ's that must be eliminated by a subsequent N&T HT. This is the concern in the industry that suspect fittings were not subjected to a N&T after forming or past fabrication bending operations.

I mentioned to you before that any post fabrication bending operation MUST be subjected to a N&T to assure bulk properties are consistent for Grade 91 material.

The narrow HAZ from girth welding can be tolerated because it is only a single band, not multiple bands, away from the weld region.

 

[rn01]

Metengr,

Thank you for the response. Sorry, I didn't phrase my question properly. It is my understanding that you can not locally N+T P91 because you will still be left with an intercritical temperature HAZ at the outside of the treated induction heated zone. That HAZ would be created by the local N+T treatment. So if you eliminate all but the outer HAZ's during a local N+T, I gather those remaining areas are still more of an issue than the narrow HAZ from girth welding.

 

[metengr]

rn01;

It is my understanding that you can not locally N+T P91 because you will still be left with an intercritical temperature HAZ at the outside of the treated induction heated zone.

Local N&T is not really practical because of the temperatures required (1900-1950 deg F) for proper normalization, and the hold time that is required at 1900-1950 deg F. The N&T heat treatment is best done in a furnace, and in this case the entire component would be treated, not a local area or region.

So if you eliminate all but the outer HAZ's during a local N+T, I gather those remaining areas are still more of an issue than the narrow HAZ from girth welding.

I believe that with my response above you should have a better feel for what it takes to perform a N&T heat treatment.

.....those remaining areas are still more of an issue than the narrow HAZ from girth welding.

No. The intercritical HAZ can be formed from welding or if you did not reach a proper austenitization temperature for normalization. In reality, the material does not care about the source of the heat welding or otherwise.

 

[davefitz]

Also, the rate of cooling following Normalization must be accelerated faster than -9 F/min, or else the entire bend may have an unacceptable amount of ferrite. We have seen cases where the narrow band of overtempered material was worsened by subjecting the entire piece to a normalization followed by a slow cool. As opposed to a narrow band of overtempered material, the entire piece was non-martensitic.

 

[rn01]

davefitz,

Thank you for your response. For heavier pipe walls, say 3 inch and greater, can you still maintain a 9 degree/min cooling rate without creating other issues due to the differential temperature across the material?

 

[stanweld]

Heavier induction bends can be cooled appropriately. You may need to accellerate cooling using fans, water spray or water quench to do so.

As davefitz stated, faster cooling rates than the 9 F/min are highly recommended. Remember that cooling from the ID surface will be slower than from the OD when cooling in still air.

 

[davefitz]

rno1:

You are correct, a 3" wall thickness is approx the max that can be cooled from one side at -9F/min and not incur residual stresses from exceeding yield stress during cooldown.

I could not find published data on the austenitic properties as the P91 is cooled from 1850 F, but I estimate them as follows ( at 1500 F):
E= 20. E 6 psi
thermal exp coef = a = 8.1 E-6 in/in F
yield stress = 20 ksi
therma diffus= TD= 0.177 ft2/hr
poisson ratio = 0.33

based on the procedure of the TRD 301 annex 1, after the cooling process has proceeded for a length of time that implies allparts are cooling at the same rate V ( F/min) ( time greater than 3 * s^2/TD, s= wall thickness) , the so called "quasi staionary period, we have :
thermal stress between OD and mid wall =
=Ea (Tmw-Tod/ ( 1-poisson) ( perfect cylinder, SCF=1)

This will reach yield stress at a DT = 80 F= Tmw-Tod

The relationship between ramp rate V, wall thickness s, thermal diff TD, and DT is, during the quasi-staitonary period :
DT=Z*V*s^2 / TD, where Z approx = -0.177 for a dia ratio=1.13

For DT=80, V=-9*60 F/hr , I find s = 3.2 "

For wall thickness greater than 3" , it would be neccesary to cool form both sides ( Id and OD) via an oil quench.

The mathematical basis for these estimates can be foudn in Timoshenko, or also in "Instationare Warmespanungen in Hohlzylindern" , W. Albrecht, Konstruction 18 (1966) heft 6 pp 224-231.

 

[metengr]

The 3" or greater wall thickness that may require liquid quenching to achieve the necessary cooling for 100% martensite transformation is directly from the ASME material specification.