SA516 gr 70 graphitization 3

[asifraza0]

Hello All:

I have a question regarding the temperature limit of SA516 gr.70 for graphitization
I'm operating a reactor made from SS304 but has a body flange and blind flange made from SA516 gr.70. The operating temp of the reactor is 450 deg c - 465 Deg C
ASME section VIII, part D, section II, Para A-240 states that prolonged temp in the range of 800 Deg C or more can lead to microstructural deterioration including graphitization.
Questions:
1) what is the onset temp of graphitization? does it start at 800 Deg F and increases with increasing temperatures?
2) what are the risks using SA516 gr.70 between 450 deg C - 465 Deg C
3) what is the predicted life of the component once graphitization starts?
4) What are the signs once graphitization starts?
5) Any real life applications and failures?

Thanks in advance for all your help
Araza


Asif Raza

 

[metengr]

The replies below are by no means complete. Graphitization is complex and cannot be easily quantified into a typical damage mechanism.

1) what is the onset temp of graphitization? does it start at 800 Deg F and increases with increasing temperatures?

For carbon steel, such as SA 516 gr 70, the temperature which graphitization may occur is 425 deg C and above. Note, this does not imply graphitization will happen only that the risk increases.

2) what are the risks using SA516 gr.70 between 450 deg C - 465 Deg C

There is a risk. It depends on actual temperature exposure in service, past forming strains, and aluminum/silicon content.

3) what is the predicted life of the component once graphitization starts?

It depends on the type of graphitization which occurs, for example random nodules in the base metal or graphite nodules in the heat affected zones of weld locations.

4) What are the signs once graphitization starts?

Nothing, you need to perform field metallography to possibly detect it.

5) Any real life applications and failures?

Yes, sudden unexpected, brittle type cracks which form or creep failure because of loss of carbon content in forming nodules versus remaining in solution to provide creep strength.

 

[weldstan]

As metengr stated, you are definitely at risk of graphitization with SA-516 and I would never had selected it for that service. Whereas graphitization is time at temperature dependent as well as chemistry, expected life cannot be determined. A better selection would have been Carbon-1/2 Mo alloy.