Carbon Steel 516 Grade 70 at 850 Application (Casing) 1

[niravpshah]

Folks,

We are running into dilema and need some practical solution. We understand that carbon steel above 800 degree causes graphitization and in long service hours failure may occur.

We want to quatify the life and looking for some practical way to assess life at 850F.

Any experience??

Thanks,
Shah

 

[waskillywabbit]

For SA-516-70, according to ASME code "use of this material at this temperature is not current practice." I'd suggest looking for another material that will take your process requirements.

Brian

 

[M1Can]

Generally carbon steel would not be used for any critical application above 750F. Having said this I have seen A36 casing used for up to 1000F but a greatly reduced allowable stress as creep rupture becomes an issue over 750F. ie at 850F the allowable for A36 is about 1/3 of the cold strength.

If you look at the ASCE handbook

"The Structural Design of Air and Gas Ducts for Power Stations and industrial Boiler Applications"

They have some notes concerning material selection vs temperature. An appropriate material selection is A242 Type 1 (Corten).

M1 Engineering
m1engineering.ca

 

[metengr]

The best way to quantify service life at this temperature for carbon steel is to evaluate creep rupture strength and creep deformation rate. What are your specific stress levels and application?

Depending on the stress level, at 850 deg F you will have some finite life up until either excessive deformation occurs or you reach the creep rupture life.

 

[jte]

niravpshash-

metengr pointed you in the right direction. In Section II-D you'll find notes G10, S1, and T2 at the end of Table 1A which echo the above comments. IIRC the basis for the Section IID allowable stresses limited by note T2 (stress values in italics) is a 20 year life but you should check that in the basis for establishing allowable stresses. Beyond 20 years, you could take a creep rupture approach and evaluate the life of the component in accordance with API-579 appendix F.7.

jt

 

[niravpshah]

Friends,

I really appreciate your response. As you mention that this is not the right material for this application and I dont disgree with that. But the stress calculation at that temperature shows that the stresses are lower than the allowable. And for some reason we can't change the material. So We want to assess the life. I got the following suggestion but I wasnt sure on,
API-579 appendix F.7." Could someone shed some light where can I find that?


"Beyond 20 years, you could take a creep rupture approach and evaluate the life of the component in accordance with API-579 appendix F.7."


Thanks,
Shah

 

[metengr]

Yes. Go to the API web site below, and order a copy.

http://www.techstreet.com/info/api.tmpl

This is a comprehensive and complex fitness for service standard. Make sure that you understand what is required, and seek additional technical expertise, if necessary.


What is your application?

 

[waskillywabbit]

I would be interested to know as to why you can't change the material? And as metenr asked above what the application is?



Brian

 

[niravpshah]

Brian

We have made the several casings for combustor and then later on realized that our worst case temperature was 850 instead 750.

So I compared the stresses at 850 allowable so they still meet since we had good safety factor.

Also wanted to know,

"Does ASME has any section detailing on creep calculation"?

"Is there any reference for life calculation for graphitization?

Thanks guys,

Shah

 

[metengr]

I might be able to help you. What is your calculated stress level for the combustor? I do have access to some creep deformation and creep rupture data for this material.

Graphitization can occur at service temperatures at or above 850 deg F with some level of threshold stress. This damage mechanism requires many tens of thousands of operating hours at elevated temperature exposure to either spheroidize and/or graphitize carbon steel. Keep in mind that you are right at the lower end of the graphitization temperature range, so if aluminum was used to de-oxidize the steel, it will exhibit increased susceptibility to graphitization over time. If silicon was used to de-oxide, there will be less susceptibility. This material will, however, spheroidize and creep at 850 deg F.

So, my question to you is how much creep deformation can you sustain with this combustor over time (2%, 5%, etc) I can estimate a creep rupture life at 850 deg F provided you can supply me with a stress level.

 

[niravpshah]

metengr,

Thanks for your candid response. I really appreciate it. I agree with you 100% about all the things you have said.

I was going though one of the technical reference by J. R. Foulds And R. Vishwanathan. And they talk exactly what you have said.

The thing is, I want to lay out a method to assess the life and then pass on to the design engineer. So I am really looking for the data rather than the answer to one question.

My questions are,


"Does ASME has any section detailing on creep calculation"?

"Is there any reference for life calculation for graphitization?

Thanks,

Shah

 

[metengr]

The answers to your questions;

1. Yes. Please review Section II, Appendices 1-100, see Table 1-100.

2. Nothing that is reliable. Graphitization is an unpredictable damage mechanism, and can occur at random locations within a component. It results in the formation of graphite nodules that locally depletes the metal from carbon. The carbon depleted regions of the metal begin to suffer from local creep damage on a microscopic scale. These regions eventually link up to form macrocracks and eventual failure.

 

[M1Can]

For what it is worth the following is my experience with the design of casing at elevated temperature:

1) Up to 800 to 850F use carbon steel
2) In the range of 800 to 1000F use carbon steel or alloy steel based on economics steel cost weight customer preference etc
3) Above 1000 F alloy steel
4) ASME requirements do not strictly cover the design of casing but used as a guide.
5) Allowable Stress based on creep rupture criteria in the temperature ranges discussed here

The criteria is listed in Appendix 1, ASME section II, this may not be up to date but used to be
lesser of
i) average stress produce creep .01% per 1000 hrs
ii) 2/3 average stress cause rupture at 100 000 hrs
iii) 80% minimum stress cause rupture at 100 000 hrs

Based on the above the allowable stress for carbon steel was 825F-7.8 ksi 1000 F-2.2 ksi ,

6) Graphitization was never a design consideration and I am not sure why , but would be interested in anyones comments.


The above was based on experience with a previous employer that was one of the major suppliers of OEM boilers and auxilliary equipment.





M1 Engineering
m1engineering.ca

 

[metengr]

M1Can;
The answer to comment (6) is mentioned in my previous post. Your previous employer should have been aware of the dangers of graphitization especially if they fabricated boiler components. They probably thickened the wall of the combustor to reduce service stresses to compensate for the reduced material strength.

Your statement "In the range of 800 to 1000F use carbon steel or alloy steel based on economics steel cost weight customer preference etc" is rather interesting. We, like most owners of power boilers, have rather tight material specifications that would not allow the use of carbon steel to be exposed to a metal temperature of 850 deg F. This would be considered marginal design.

For those customers that have no in-house material specifications, they would be replacing combustors at a higher rate with the use of carbon steel. Good for your previous employers business.

 

[niravpshah]

M1Can & metenger,

Thanks guys for your posts. This is exactly what I was looking for.

I just went through Apendix 1 of section II.

Are guys are aware of any creep curve for carbon steel in any ASME section?

Thanks again for perfect answers.

Shah

 

[metengr]

Shah;
The ASME SA-516 pressure vessel plate is designed for moderate to low temperature service. I checked my sources, and it is highly unlikely that you will find any creep deformation/rupture data for this material above 700 deg F. It is produced to a fine grained practice, which results in a degradation of toughness and creep strength properties upon exposure to elevated temperatures. There is limited elevated temperature data showing the reduction in toughness upon exposure to PWHT time @ temperature, and actual high temperature tensile data. At 850 deg F for this material, you see a drop in tensile and yield strength values from ambient temperature (80 to 60 Ksi UTS, and 55 Ksi to 32 Ksi).

I have actual creep data for SA-515 Grade 70 plate material, and others (that I know unclesyd would appreciate from USS) that are more suitable for elevated temperature service.

 

[niravpshah]

metengr,

Thanks for your response. You are a great person and greater engineer.

Thanks,

Shah