New ASME Code case for P91/T91 ??? 1

[MJCronin]

to all...

Can someone describe the new code case I have heard so much about for heat treatment of ASME P91/T91 components ?

How will this affect the market for new high temperature boilers/HRSGs and how might it affect possible future litigation that I sense will occur in the near future ?

Any "war stories", tall tales, secrets or even rumors that can be shared ?

-My opinion only....

-MJC

 

[metengr]

Yes. The new ASME Code Case deals with an upper limit for PWHT of Grade 91 material to avoid over tempering or forming new martensite by exceeding the lower critical transformation temperature. The upper temperature limit is 1470 deg F.

In the event of exceeding the maximum temperature, the component would require either re-heat treatment or if it is a localized problem, the affected material is removed and replaced with material that is properly heat treated. I listed the Code Case number a few weeks ago, but I don’t have access to it at the moment.


I see no impact on market conditions.

No affect on the market. The reputable manufactures that know the pitfalls of Grade 91 will continue to treat this material with respect and buyers that want to cut corners by using in-experienced fabricators will continue to do so. Litigation? Go ask your friendly lawyer.

 

[stanweld]

metengr,
Have they taken into account the %Ni + %Mn content of base metal and weld filler metal with regard to the 1470F max PWHT Temp? Will they be addressing heat treatment after bending? If not they ought to; just one man's opinion.

 

[metengr]

stanweld;
I didn't have a copy of the Code Case in front of me at the time I posted. I have a copy, here are the specifics related to revisions to Table PW-39;

Note 3). If % Ni + Mn content is less than 1.5%, but greater than or equal to 1.0%, the maximum PWHT temperature is 1450 deg F

If the % Ni + Mn is less than 1%, the maximum PWHT temp is 1470 deg F

Note 4). If a portion of the component is heated above the PWHT temperature stated above, one of the following actions shall be performed;

a). the component in its entirety must be renormalized and tempered

b). If the maximum holding temperature above is exceeded, but does not exceed 1470 deg F, the weld metal shall be removed and replaced.

c). the portion of the component heated above 1470 deg F and at least 3" on the either side of the overheated zone must be removed and be renormalized and tempered or replaced.

d). the allowable stress shall be that for Grade 9 at the design temperature provided that the portion of the component heated to a temperature greater than that allowed above is reheat treated within the temperature range specified above.

No specific mention of heat treatment after bending/forming.

 

[stanweld]

metengr,
Just reviewed the 05 Addenda to ASME Section I. All has been defined therein as you stated. It permits raising the PWHT from 1425F max. to a maximum of 1470F provided the %Ni +%Mn <1.0% or 1450F if < 1.5%. Min. PWHT is 1350F.

 

[stanweld]

metengr,
Further to my review of the 05 Addenda, I noticed that Table PW-11 had deleted the > 850F temperature limits for RT. Was this a misprint?

 

[metengr]

stanweld;
Yes, I believe you’re correct, this is a misprint. I don't recall seeing any proposed changes regarding this item.

 

[davefitz]

The case can be read at <cstools.asme.org> , case BC05-1483 , section I paragraph PW-39 , page 96.1 ( rev A 05).

See note 4 for the revisions. However, as I read table PW-39, it may only reference P91 sections that are overheated in the welding process, and might not strictly apply to the hot bending overheats and associated material property degradation.

To close that loophole, it might be neccesary to transfer the same notes in note 4 to the ASTM spec SA-335.

 

[stanweld]

I doubt that SA-335 is the place, since it is incumbent on the manufacturer to meet the material specification's requirements. Section I needs to address hot and cold bending similarly to B31.1, which in turn should be amended to address the specific parameters required for both hot and cold bending of P/T91.

 

[davefitz]

Stanweld;

It seems the manufacturer of the base pipe does supply the SA-335 pipe, in straight sections , with proper material properties as per SA-335. The hot bending fabrication error occurs at the downstream foundry that uses a hot bending machine to fabricate the bends, and many of these foundries appear to be ignorant of the need to N+T the entire section of piping to correct the sections that were overtempered ( ie, zones adjacent to the local 1900 F induction heated zone).

Generally, what seems to happen is the EPC vendor obtains competitive quotes from several foundries for the bent els, and selects the lowest bidder. The bent els are then purchased , under the generic spec of SA-335, but without addending to the purchase spec a QC requirement to perform a hardness traverse of the bent piping or mandating a N+T. It often happens that the lowest cost foundry doen't have a staff metallurgist, and being unaware of the P91 material degradation during overtempereing will merely follows code stated requirements and would only add the neccesary N+T if stated in the purchase spec from the EPC vendor. As I read B31.1, the responsibility for correct fabrication procedures is the "designer's", not the foundry.

Until adequate corrections are made to the code and ASTM specs, it will be neccesary for the EPC specification, and the ultimate customer's specification, to explicitly require the N+T for hot bent P91 els and/or addend the same wording as Section I is proposing in note 4 of case BC05-1483.

Other equally common and equally damaging errors are (a) insufficient rate of cooling following N+T- it must be cooled faster than -9 F/min to avoid ferrite formation ,and (b) the max permitted thickness transition angle may not exceed 30 degrees at the OD in the vicinity of butt welds- this is explicitly defined in the codes, but lately been ignored by reputable designers, to the detrement of the end user.

 

[MJCronin]

davefitz....

Thank you for putting your finger on what I believe to be an important EPC responsibility in the design and fabrication process...

You stated:

"It often happens that the lowest cost foundry doen't have a staff metallurgist, and being unaware of the P91 material degradation during overtempereing (sic).. merely follows code stated requirements and would only add the neccesary N+T if stated in the purchase spec from the EPC vendor. As I read B31.1, the responsibility for correct fabrication procedures is the "designer's", not the foundry."

I agree....and this exact point will be examined in a courtroom in the near future. The impending slew of Type IV cracking failures and the resultant disruption of revenue will cause a lot of heartburn in the owners suite.

With today's frequent churning of EPC power plant personnel and a gutless management structure that tries to "pin" any PE that was associated with the design, fabrication or testing of a failed component, it should be no surprise to anyone what will happen.

My initial question was meant to solict details and opinions about the extent of the problem and to establish if it was even reasonable to conclude that the EPC vendor's would have known about this heat treatment and Code issue as it was developing.

My opinion only...

-MJC

 

[davefitz]

MJC:

If by PE you mean professional engineer, then the legal issues with the EPC's can become even more convoluted than you indicated.

It appears it has also happend that EPC vendors have claimed that some of their staff PE's had reviewed the design documents, when in fact the staff engineer had never seen the documents. The lack of any requirement to add the PE stamp or signature on contract documents based on the EPC's filing with the state PE board that a PE was on their staff is a huge loophole that allows unreviewed work to enter the marketplace.

 

[DSB123]

MJC,
You seem to be suggesting that their is a pending legal situation regarding Type IV cracking found in some equipment. Can you expand as I'm interested in anything to do with P91 failures.


Regards

 

[stanweld]

davefitz,
From my experience, the OEM and the Engineer-designer/Procurer for the EPC company bear greatest responsibility for most of the problems encountered with P91. They seldom addressed the PWHT, dissimilar materials welding and hot/cold bending post heat treatment requirements for the alloy. Futhermore, the attention payed to shop surveillance was often non-existent or when in place, the inspector had little or no guidance.

 

[davefitz]

stanweld:
B31.1 is clear on the responsibility. For power piping , Paragragh 129.3"heat treatment of bends and formed components" states:

"If hot bending or forming is performed , the material shall recieve a full anneal, normalize and temper, or tempering heat treatment as specified by the designer"

"If a postbending or postforming heat treatment is to be perfromed, the designer shall fully describe the procedure to be used".

For section I boiler components, the problems with P91 have been generally well addressed by the boiler OEM's, with the possible exception of the max slope of 30 degrees at thickness transitions and use of transition pieces.

For B31.1 components, the bubble of activity of building combined cycle plants in the period 1998-2003 led to assigning the design responsibilities for the P91 piping to unqualified ,newly graduated engineers and designers that had no exposure to the metallurgical quirks of P91. They didn't know what they didn't know, and had no clue that they needed to even question the adequacy of code requirements. ( I venture to add that most did not even read the code).

 

[metengr]

As a side note to this entire discussion; Type IV creep failures in fabricated Grade 91 components are not entirely attributed to improper heat treatment during fabrication. In certain cases, Type IV cracking is a final outcome of Grade 91 components that are sub-critically post weld heat treated AND are subjected to thermal/mechanical bending stresses in service. The European Creep Collaboration Committee has performed extensive research on the threshold thermal/mechanical bending stresses that will trigger Type IV creep damage. I would bet most designers are not even aware of this information.

So, let the buyer, designers, etc., beware AND understand that Codes and Standards used in design provide minimum requires for safe operation. For boiler external piping under ASME Section I administration, ASME B31.1 technical requirements prevail, which is good.

Regarding boiler headers and appurtances not covered under ASME B31.1, yes, Section I does not specifically describe fabrication process detail as B31.1. I would venture to guess that headers which have developed incipient Type IV creep damage over the years pose less of a safety risk in comparison to external piping. I would agree that Section I should have a comment or two, similar to B31.1.

It is unfortunate that the ASME Boiler and Pressure Vessel Code committees over the years have become stacked with fabricators and consultants with small fry users like myself as a minority. Until more users get involved and express concerns related to changing the Code, nothing more will happen.

 

[davefitz]

I just realized that the code case referred to above might be impossible to implement during field construction of P91 components.

The type IV creep damage is caused by overtempering ( weld reheat aging) of the HAZ- some narrow band of parent metal adjacent to the weld line will be overheated above 1425 F during the welding process. If we literally follow the code case note 4, the weld plus a 3" band of metal on either side of the weld must be removed , but the process of replacing the section via welding simply repeats the same defect. The alternative of N+T the entire piece cannot be accomplished when the entire piece is the entire main steam gtrnasfer pipe.

If it is true that the weld process overtempers the HAZ at a temp above 1425 F, then that particular overheat would need to be excluded in the case where the weld is a butt weld done in the field.

 

[DSB123]

davefitz,
I know its a while since you posted but I've just read your post regarding the cooling rate as being 9 Deg F/min as a minimum. This has got me a little worried as the pipe spec provided by a Contractor stated the cooling rate after PWHT as 100 Deg C/hour. I wonder if you could give your thoughts on this cooling rate (This rate is identified as being cooling down to 400 Deg C with no rate below that)

Regards

DSB123

 

[davefitz]

DSB123:

The required cooling rate is at least as fast as -9F/hr in the temperature range of 1900F to 1400F, to avoid formation of ferrite. This is known from TTT temperature-time-transformation curves for this alloy. They were originally developed in the late 1970's ( I am looking for my copy of the original and will advise later) and a similar copy is presented in the Mannesman Vallourec booklet "the P91 book".

As different crystal structures are formed, they change the volume of the melt. By monitoring the volume or dilatation of the melt, they can infer which crystal has formed. This is confirmed by later metalurgi8cal analysis and hardness testing.

For this reason, the ASTM specs for P91/F91 alloys requires forced air cooling for thinner walled parts and oil quench for thicker walled parts. We discovered this problem when the Mom and Pop foundry used on our project had erroneously covered the part with inisulation to provide a slow cooldown, and it was all ferrite and as soft as a fresh baked doughnut.

 

[davefitz]

Correction- -9F/minute.