Mechanical property for carbon steel operating in creep range 565°C~650°C

[ags1]

Need to evaluate creep damage in carbon steel (SA 516 Gr 70/SA 106 Gr B) at 650°C short term exposure (~150Hrs@ 650°C, balance life@ 565°C in total life of 100000 Hr).ASME IID gives allowable stresses up to 550°C but doesn't permit use beyond 538°C. Similarly thruogh API 579 FFS-1, table F.2, F.4 we can drive UTS and yeild but that too up to 593°C and then how to drive allowables stress out of yield and UTS, ASME IID stress base is good for operation at normal temperature but not under creep...and 593°C is any way more than 550°C which is end as per IID for CS to be used under creep.

Further, for pipe (A 106 gr B) or even cyclinder made of plates (SA 516 Gr 70), can I use B31.3 Appendix V for evaluation of creep even though vessel is designed as per Div 1. or can I mix i.e. Allowable as per ASME IID and criteria as per B31.3 Appendix V. However I am having again difficulty here of getting allowable stresses at such high temperature as B31.3 Table-A1 gives stresses up to 593 °C and ASME IID up to 550°C only. We know in IID while looking at the italics figure that beyong which temperature it is under creep but in B31.3 there is no clarity i.e. beyond which temp point stresses are time dependent. In addition, even u wish to extrapolate...how many values to be picked up (from the netire time dependent property points) as picking of entire or just two adjacent value gives different results...how to conclude even on extrapolation.

Over and above, the passing criteria is 1% Creep damage in 100000 Hrs !!!! after doing all the guess work and extrapolation with B31.3 -Appendix V...not even close to it...damage value what i am getting is is around 133.6%.

Please share if anybody has any clue on this. Pointwise responses will really help. If you need more clarity, please write to me, will clarify.

Also advise the difference between Level-1screening curves provided in API 579 FFS-1 for carbon steel, Carbon steel graphitized and c-0.5Mo. When and what to use if material is SA 516 Gr 70 and SA 106 Gr B.

 

[metengr]

Forget the white board calculations, you have a situation where carbon steel material has no business being operated in this temperature regime.

For proper evaluation, you need to remove actual samples and have a metallurgical evaluation performed to determine the condition of the pipe microstructure, hardness and thickness testing. Yes, you have serious concerns related to graphitization, followed by spherodization and creep damage especially at girth weld regions. These would be the regions I would concentrate on for the above damage mechanisms.

 

[ags1]

Thanks for the response on this. Just to clarify,the query is about the design of Internals inn new equipments. referring API 579 as it is referred for creep evaluation.
Equipment will be refractory lined but internals will be exposed to high temperatures as stated in my query. The Internals whch needs to be designed at these high temperature are cylones and internal steam/air rings (This is in reactor). In regenerator, Internals are of SS304H but there are further challenges as design is 840°C and short term exposure is 900°C.
What I am working is input provided for design. So exploring how to approach the solution. I inderstand your concern as material expert, but I do see the possibility of CS/CS-Graphitised/CS-Mo Level-1 screening curves which is extrapolated my give solutioons. What is your opinion on the use of these curves.

 

[metengr]

Ok, your original inquiry sounded like this was an evaluation for an in-service carbon steel material. Regardless, for the application....

but internals will be exposed to high temperatures as stated in my query. The Internals whch needs to be designed at these high temperature are cylones and internal steam/air rings (This is in reactor)

...., I would look at Cr-Mo steels (minimum Grade 22 or even Grade 91, which are better suited for this type of temperature (565 deg F).

Having carbon steel at this elevated temperature regime will degrade and result in creep deformation that could interfere with form and function within the reactor internals.

 

[TGS4]

If you are intent on operating carbon steels (our any material for that matter) in the creep regime, and you don't know how to evaluate that, stop. There are experts available to assist you (You will have to pay them...). There are far more things going on than just can be captured with an allowable stress.

 

[ags1]

Thanks and really appreciate for the replies made. Yes I also do concerned about use of CS at these temperature. However just to investigate in details, pls advise me on the following which are extended part of my original query;

1.Advise the difference between Level-1screening curves provided in API 579 FFS-1 for carbon steel, Carbon steel graphitized and c-0.5Mo. When and which curve shall I use if I am using material as SA 516 Gr 70 or SA 106 Gr B.

2. Can I use SS304H if I have design internal temp. of 840°C and short term exposure (150Hrs) of 900°C.

Just to emphasize on my query; Matrial and design conditions are suggested by Licensor. Now since he onl made this as input to us..beleive there must be some reasons. Just exploring before I regret.

 

[metengr]

1.Advise the difference between Level-1screening curves provided in API 579 FFS-1 for carbon steel, Carbon steel graphitized and c-0.5Mo. When and which curve shall I use if I am using material as SA 516 Gr 70 or SA 106 Gr B.

The SA 516 and SA 106 would fall under the creep curves and graphitization curve for carbon steel.

You realize that API 579 is not a construction code and is for in-service assessment.

Re 304H, you are above the maximum permitted service temperature range for ASME B&PV Code at 840 deg C.

 

[XL83NL]

When you mention 'exploring how to approach the solution', yet dont see the problem, and
want to extrapolate curves, and
dont take advise from some of the experts that have already provided valuable input,
Youre misging the point.

 

[ags1]

Thanks metenger for taking out time to respond, just few more points to be clarified;

on the Level 1 screening, when or beyond which tmeperature graphitisation curves shall be referred.
Can the same MOC i.e. SA 516 Gr 70 or SA 106 gr B cam be used by adding Mo or will it change the designation of material. What is C-0.5Mo grade/Level-1 screening curve and when it is being used.

Please share your experience as I am very much keen to know.

 

[ags1]

@metengr (Materials)
Will appreciate your reponse on my last post.

 

[metengr]

avedhesh

Can the same MOC i.e. SA 516 Gr 70 or SA 106 gr B cam be used by adding Mo or will it change the designation of material.

Reply; No, one cannot indiscriminately add alloying elements because the designation would change, hence the creation of Cr-Mo alloys, which I mentioned are better selections.

What is C-0.5Mo grade/Level-1 screening curve and when it is being used.

The C-1/2Mo steel is a separate steel designation known as SA 204 for plate versus carbon steel SA 516 plate. This curve cannot be used for Sa 516 material material selection.

 

[metengr]

avedhesh;
First, for SA 516 material and SA 106 Gr B material, the allowable stress values in ASME Section II, Part D, Table 1 are listed to only 1,000 deg F or 538 deg C. For ASME B&PV Code design, you cannot or are not permitted to use this material above 1,000 deg F or 538 deg C service temperature because the committee's position is that above this temperature, the carbon steel material is unsafe for long term service. Not permitted means not permitted for design purposes.

For SA 106 Gr B material, a similar restriction is placed at 1,000 deg F (538 deg C) in Section II, Part D, Table 1.

In my opinion using the above materials even for reactor internals would result in very limited service life (mechanims such as plastic collapse or local creep deformation that renders the internals useless in operation) resulting in repeated costs to replace the internals. I would not recommend these materials for your design.

 

[TGS4]

I am going to give you the same advice here that I gave you when you posted this question on LinkedIn: you need to get an expert in the field of high temperature design to assist you.

The advice that you have been given by metengr is some bare minimum recommendations.

 

[ags1]

@metengr (Materials)
Please check my query again ..it is on how to work out allowable stress and what is the base of stresses given for the material in italics /creep range temperature in IID.
For a moment, please ignore my original query and pls try to answer my query below each consideration for the allowable stress;

(1) 100% of the average stress to produce a creep
rate of 0.01%/1000 hr;
(2) 100Favg% of the average stress to cause rupture
at the end of 100,000 hr;

What is average stress here ? Is it membrane which is same across the section under evaluation ?

(3) 80% of the minimum stress to cause rupture
at the end of 100,000 hr. Stress values

What is minimum stress here ?

 

[metengr]

ags1;
No, it is not membrane stress in service. The average stress and minimum stress values above refer to stresses used in standardized creep and stress rupture test data which are submitted as part of a data package to approve material for inclusion in Section II, Part D. Do not confuse allowable stress values with stress values from laboratory creep and stress rupture test results.

So, the average stress is an average of stresses from creep test data. Minimum stress is the minimum stress to result in stress rupture laboratory testing.

 

[ags1]

Thansk so much for answering as I could never guess the meaning here.
My other query is on design life criteria..

Usually you must have seen the design criteria stating " 1% creep in 100000Hrs". What I understand if equipment is operating under creep range and may have faced one or more operating stresses i.e. pressure,loads and Temperatures; the sole or cululative damage when estimated for total 100000 Hr then cumulative damage life fraction shall nor exceed the value of 0.01. Am I making correct interpretation here?

Now My second query is, generally equipment life/service is asked for 20 years and it is lesser in case of reactors with removabale or replacebale internals say 15 years. Being specific to reactors with removable internals, if I am limiting induced stresses (designing accordingly) so that only 1% creep damage should occur in first 100000 Hrs (11.41Years), is it very conservative to assume balance creep damage which is 99% will occur in balance years (3.59 years).

What If I design it for rupture i.e will limit induced stresses so that the cululative damage in 100000 Hrs should be closer to <1 (100%) or some margin suitably can work out so that rupture should occur at the end of 15 years. If wish to design for 15 years before rupture then will limit stresses so that cululative damage should be closer to 1(<1) and rupture shall theoritically happen at the end of 15 years of desired service life.

If I give you desired service life of 15 years, what would you choose to design i.e. 1% creep in 100000 Hrs or ill design for rupture (100% creep) in 15 years.

I don't know how you are taking my queries..but I am new to this practical creep design and believe there is no one better than you who can answer my all queries.

 

[TGS4]

X% creep does not mean X% life fraction. It means x% creep strain.

I'm not sure that you have a sound technical foundation in creep (high-temperature elasto-visco-plastic deformation). You would be best to start with finding some textbooks on creep. In secondary creep, damage is essentially linear with time-temperature, however in tertiary creep, damage increases exponentially in time-temperature.

I suspect that your employer has asked you to do the design of some high temperature internals. But, they have not provided you with the training or experience to do so. You wouldn't be the first one to have this happen to them - it happens all of the time. I applaud your efforts to do some independent learning, but as of right now you are dangerous under-qualified/knowledgable and under-experienced to be doing this type of work. Please tell your employer that your knowledge and experience are insufficient to do this work, and that they need to hire an expert to do this for them. You can ask that the expert teach you what and how as part of the work - I often do that with my clients.

Also, find out if your local university has a graduate-level engineering course on creep - such a thing may only be taught every two or three (or more) years, so make sure to perform an extended search. And then find textbooks on the topic and read read read.

 

[metengr]

ags1;
TGS4 has given you very sound advice. I am not a creep design or analysis expert by any means, I do know materials, failure mechanisms and ASME/NBIC codes and standards with emphasis in Power Generation. Follow the advice by TGS4.