P91 welded pipe 1

[carletes]

Hello all! We are condidering the possibility of using welded alloy pipe A691 P91 Cl. 42 in a hot reheat system (30 bar, 575ºC) instead of semaless A335 P91 pipe. Manufacturer tell us that all its properties after normalized and tempering are the same than seamless pipe but we are not very sure. We have heard some things about creep properties of Heat Affected Zone that don't sound very well. Any comment or recommendation?

Best regards,

 

[davefitz]

Without the N+T the weld creep strength reduction factor is about 0.6.

The N+T needs to be conducted correctly to ensure this alloy has full assumed creep strength, including that the rate of cooldown from 1800 F be faster than -5 F/sec ( to be confirmed ) to ensure formation of martensite. The QC should include a micro-hardness traverse of the weld HAZ to confirm no soft zone exists after N+T, and other tests may be needed to ensure no cracking exists ( photo micrographs).

The weld area will still have different creep properties than the parent metal, since the weld electrode composition is different than theparetn metal, and ther is likely to be a detectable pre-existing shrink line at the ID if the weld. Usually large hot reheat pipes are large enough that a worker can crawl inside and grind away gross defects.

 

[carletes]

Dear davefitz,

So in your opinion a welded P91 pipe with a correct N+T can substitute a seamless pipe? I thought so, but, surprisingly, ASME B31.1 permits the use of A691 pipe for non-bolier external piping but not in the case of bolier external piping (ASME I)when the design conditions are the same. Strange?

Thanks

 

[davefitz]

From what I have seen of the shops that fab and weld P91 piping in the USA, there is a big question as to whether they could be expected to treat this material properly- there is a significant risk that the average shop would make some error.

Very few of these shops have a staff metalurgist that is familiar with P91, and simply following ASME rules is not adequate for P91. I would not use a creep weld strength reduction factor greater than 0.6 unless every aspect of the fab was reviewed and approved by a metalurgist that knows P91 inside and out.

 

[davefitz]

See the latest paper published by the ASME section II committee on materials " Issues of concern to ASME BPV committee TG on creep-strength enhanced ferritic steels , and remedies under consideration" by J Henry, M Gold(chairman), and J Tanzosh.

Basically , the ASME code had assumed that the users, fabricators, and engineers would retain in-house metallurgists that would assume technical responsibility for understanding how to fabricate these alloys. The corpoate downsizing of the 80's and 90's led to a dumbing down of the technical expertise, such that correct fabrication of P91 /T23/P911/P92/P122 is the exception, and not the rule.

New rules being proposed are:

a) inital qualifying heats submitted for code acceptance must inlcude samples at both ends of the alloying range ( lean and rich) for testing

b)redefining the normalizing and tempering temperatures ,, and define what operations will requrie a N+T ( ie ,hot bending)

c)define the PWHT temp based on Ni + Mn content

d) define max coldwork permitted without heat treatment

e)define a broad harness range that, if it is nto met, would then require further testing to ensure correct crystal sructure.

f)worry about SCC, and keep part dry between welding and PWHT

g)define a tempering parameter, and a range of acceptable values to qualify a part for hi-temp service. Implicit is the need to monitor and archive ALL time vs temp histories of the P91 parts during fabrication. This montoring by itself willbe a big improvement in QC- we normally see these cucrves provided if teh part is forged overseas, but it does not seem to be done by domestic foundries.

Basically, they will finally read the Mannesman + Vallourec P91 book and follow those recommendations ( which were available circa 1990).

 

[DSB123]

davefitz,
Yet again you have provided good info as regards P91 piping materials. I have a copy of the Mannesman+Vallourec P91 book and it is interesting reading. There seems to be so much that can go wrong with this material if QA is not 100% that it gives me real concern on a system that has recently been installed in our power station. One contractor wanted a modified A335 material with specific Al and Ni limits whereas the other Contractor did not and specified A335 material. Both had different heat treatment procedures (i.e. temperatures, soak times,etc). So we ended up with a system which is not consistent in its final condition but each says it's O.K.

Where can I get a copy of the paper you quote?

 

[davefitz]

dsb123:
The paper was presented at the summer 2004 PVP conference. The authors can send you a copy.

I had asked ASME to post the paper on their website at <

http://www.asme.org>

, but no reply yet on that front.

 

[athomas236]

I tried today to find the paper in davefitz post dated 30 Dec 04 on the ASME web site but could not find it, so probably not there yet.

I read the second paragraph of davefitz post dated 30 Dec 04. If ASME did make the assumption as stated then this clearly was a serious mistake not only for US based suppliers and fabricators but also for the fact that ASME is an internationally recognised national code that is widely used around the world.

At the moment my company is invoved with 3 projects involving 10-12 HRSGs that use P91 extensively. On one project we are about to start site welding of main steam piping in P91 which leaves me wandering what to do about this potential problem.

athomas236

 

[davefitz]

athomas236:
You can get the paper from the authors, or thru the ASME via the papers presented at the 2004 PVP conference. Apparently Eng-tips will not allow posting of e-mail addresses or faxes anymore, but the first author works for Alstom in Chatanooga Tennesee, the second works at Gold Metallurgical servicees,LLC in North Benton, Ohio, and the third author at B+W in Barberton, Ohio.

Our P91 saga continues. Another major P91 faiure detected 2 days ago, a 12" hot reheat riser in the boiler setting. Apparently the fabricator did not meet sectionI fig PG 42.1- the max slope at a weldolet thickness transition should not exceed 30 degrees, but they use a 45 degree thickness transition between a 12" P91 nozzle and 12"x 22"F22 weldolet nozzle saddle.Failure was similar to an earlier failure at the same plant that had a crack int he weld between the P91 HP main steam pipe and the 1.25Cr 1 Mo V turbine stop valve. In both cases, a better desig would have included a PF91 transition piece.

Personally, I think the ASME wording on fig PG-42.1 should be reworded to require transition pieces when the 2 components have disparate creep strengths. I am sure a review of all ASME records would show that the rules implied in that figure were based on the use of similar strength materials and the figure cannot be correctly used in the case of dissimilar metals.

 

[athomas236]

davefitz,

Thanks for the information, as you say the saga continues.

What I have done today is request a copy of the T91/P91 book from Vallourec and Mannesman and asked our library to get a copy of the Summer 2004 PV&P proceedings from the British Lending Library so I can read the paper you mentioned.

Our site staff have also been forewarned of the sensitivity of P91 and asked to provide details of the extent of welding supervision on site and copies of the weld, PWHT and QA/QC procedures.

I have an action to provide them with more details of the potential problem by early next week so its all getting a bit hectic.

 

[athomas236]

Gentlemen,

For your information I have now received the weld procedure spec and procedure qualification record brief details of which are shown below. I am not a welding engineer or metallurgist but I have tried to include information that seems to be important based up on various threads and posts.

Weld procedure covers the welding of SA335-P91 to SA335-P91 upto 56mm thick and has been prepared in accordance with ASME 9, 1 and B31.1.

Min preheat temp = 204C
Max interpass temp = 330C
Preheat temperature is maintained during welding

PWHT temperature range 710-750C
If thickness <= 50mm hold time is min of 2 hours. If thickness > 50mm hold time is min of 3 hours
Note says that continuous or special heating where applicable should be recorded.

Filler metal
AWS class ER90S-B9(solid) for first GTAW weld runs and E9016-B9 for other SMAW weld runs.

No hardness measurements specified.

The procedure qualification was made on a 28mm thick pipe with base and filler metal as procedure specification.

Preheat temp = 202C
Max interpass temp = 274C
Preheat temperature is maintained during welding

PWHT temperature range 718-720C with hold time of 2 hours 10mins. Temperature history was recorded.

Tensile ans bend tests described as satisfactory and Brinell hardnesses were:

Base 103 to 143
HAZ 122 to 177
Weld 159 to 183

Naturally I would pleased to receive any comments.

athomas236

 

[athomas236]

Gentlemen,

Thanks to the good advice from davefitz I have been able to obtain copies of the draft abstract and paper called Issues of concern to ASME B&PV Committee TG etc.

athomas236

 

[davefitz]

athomas236:
Is there specified a requirement to cool the weld to 100C prior to PWHT ( this is to ensure complete transformatin to martensite prior to PWHT ?

 

[athomas236]

davefitz

I am still studying the paper and at the same time preparing a document for our site team to use in discussions with the contractor. When I have finished I will post a summary in this thread.

At the moment I have not yet identified such a requirement.

regards

athomas236

 

[athomas236]

davefitz,

With regard to your question of 14 Jan 05, there is a statement at the end of the section on PWHT that says.

"In is anticipated that action regarding rules that would require cooling to some minimum temperature below the pre-heat temperature prior to PWHT will be considered when more definitive data is available."

athomas236

 

[davefitz]

athomas236:
I think the report is trying to be diplomatic instead of technically rigorous. Sme fabricators find that there is a savings in labor by conducting PWHT immediately following welding and do not cool down the piece prio to PWHT. Also, some folks interpret the current code rules as requiring maintenance of preheat unitl after PWHT is completed. IN both cases it is embarassing to admit the past code rules are not optimum for these alloys; they should be cooled down to 100C prior to pwht in order to assure that all (95% +) magnetite is formed prior to PWHT , the alternative is to cause magnetite to form after PWHT and that new magnetite would not be tempered.

Also, for that report to suggest that increased testing of as-built ,suspicious , in place pipework (ie, the entire inventory of P91 piping since 1986) would not be prudent sort of suggests an ostrich philosophy. I guess it takes a major accident to wake some folks up.

 

[athomas236]

Further to my post of 14 Jan 05, see in the following posts some notes I have made on P91.

The notes are in 7 sections as follows.

1. Introduction
2. General
3. Code acceptance of ne materials
4. Hot bending
5. PWHT
6. Cold bending
7. Stress corrosion cracking

regards

athomas236

 

[athomas236]

1. Introduction

This note reviews some of the problems of using P91 (X10CrMoVNb9-1) and the methods that could be used to overcome some of these problems where these are available. Although this note concentrates on P91 material, the problems discussed are equally applicable to other creep enhanced materials such as P92 (X20CrMoV12-1).

The information in this note is taken from:

(a) the draft of a paper presented at the 2004, ASME Piping and Pressure Vessel Conference (called the ASME paper in this note)
(b) the T91/P91 Book published by Vallourec and Mannesmann
(c) ASME B31.1, Power Piping
(d) British/European piping code BS EN 13480-Part 4
(e) engineering websites.

 

[athomas236]

2. General

Grade 91 is a 9%Cr, 1%Mo, V ferritic steel with creep enhanced properties. The material was developed in the USA in the late 1970’s and was included in the ASME Code in 1983/1984.

Unlike carbon and low alloy steels, P91 depends for its elevated creep strength on achieving and maintaining a specific microstructure. This specific microstructure is created by the transformation to martensite during cooling. Any event during manufacture, erection or operation that disrupts this microstructure will compromise the integrity of the material and prevent it from achieving the creep properties upon which the Code allowable stresses are based. In such cases the premature failure of such components is unavoidable.

In each of the following sections each of the problems with P91 materials is reviewed together with possible solutions, where these are available.

 

[athomas236]

3. Code acceptance of new materials

(a) The problem

Any material supplier that wants a new material included in the Code has to provide details of the mechanical properties from three batches of the material and make recommendations regarding the specified compositional ranges of the major alloying elements.

For creep enhanced materials, the microstructure is sensitive to changes in composition. So if the specified ranges are wider than those of the batches upon which the mechanical properties are based then it is possible for a subsequent batches of material to fall within the specified ranges but not have the mechanical properties upon which the Code allowable stresses are based.

Although not mentioned in the ASME paper, it is clear that the information provided by the material supplier will refer to the parent material itself and not to any weld that may be made in the parent material. So any degradation in base material properties caused by welding may not be recognised in the Code allowable stresses. The same would apply if the weld filler metal has poorer creep properties than the parent metal.

(b) The solution

The solution proposed in the ASME paper is that Code specified compositional ranges should not be significantly wider than the three batches upon which the Code allowable stresses have been determined.

The paper does not state the compositional ranges of the three batches of P91 on which the Code allowable stresses are based. Without this information, it is not possible to establish if the composition of the P91 pipes currently being installed at ??? project are in accord with those on which the Code allowable stresses are based.