Carbide Precipitation in A-376 tp 347H stainless steel 2

[crivas]

Hi,

We have found in a furnace tubes made out of A-376 Tp 347H stainless steel that there have been in some of them a massive carbide precipitation on the grain borders.
I am not an expert in materials, could someone tell me at which temperature this phenomena will occur most likely. Which is the temperature limit for this material. We follow the skin temperature of the tube and this is around 500 to 550 ºC. This is also and all furnace around 30 years old. Could this be due to aging of the material, I mean is this a process that it occurs suddenly, or is a matter of time.

Thank you very much for your help
Carlos

 

[metengr]

We have ASME SA 213 TP347H tubing in our superheaters - not exactly the same application because SA 376 applies to central station service.

We have also performed periodic evaluations of TP347H boiler tube material that has been in service for over 200,000 operating hours, and have seen the same phenomenon where you have grain boundaries decorated with chromium carbides. This is normal with long term exposure to elevated temperature service.

Your stated metal temperatures of 500 to 550 deg C are fine for this material. The maximum temperature limit for this material is near 575 deg C because above this temperature, you see a significant decrease in allowable stress.

 

[EdStainless]

This is the long term aging that you would expect in this material. If the material was originally stablized correctly there should have been a dispersion of carbides in the structure, a little heavier along the grain bound.
With high temp service the carbides will grow, and the ones along the GBs will grow the most. As you reach the upper end of the temp range you might even see the carbides in the grains disolving over time. This is a primary/secondary carbide kinetics issue.
The only problem with this situation is that there is a reduction in ductility, so treat them carefully, and keep running.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[crivas]

Thank you for your answers,

As I understands this is due to aging and long periods exposed to high temperature. Could you tell me where I can find some publications about this issue. I wouuld like to have more information about the temperatures that the material wihtstand and possible effects of the carbide precipitation, regarding the integrity of the tubes and the security of the plant.
Carlos

 

[metengr]

The best source on high temperature behavior and properties of stainless steels is the following;

Handbook of Stainless Steel by Peckner and Bernstein

http://www.getcited.org/?PUB=101695282&showStat=Ratings

This book contains specific information on the oxidation behavior, creep/stress rupture and mechanical properties of 321H and 347H austenitic stainless steel material at elevated temperatures.

You can search the internet or the Ni Development institute web site for various technical papers on austenitic stainless steels.