HEATER TUBE GRAIN SIZE ASTM LIMITS

[integridadmecanica]

Dear colleagues,

I am performing a remaining life assessment of a heater tube made of in Carbon steel A 106, the normal temperature is around 320ºC so is not in the creep range but methallographic inspection has shown an ASTM grain size growth from 12 to 10. I would like to know if someone knows what the ASTM G size limit is? 6 or 5? in wich grain size start to appear important degradation?

Regards

 

[metengr]

There is no specific grain size requirement for material supplied under this ASTM specification because this material is for elevated temperature service. Also, as you indicated, provided you operate under expected design temperature conditions, this material will not be exposed to time dependent properties (creep).

With that said, the slight change in grain size observed could be from original tube fabrication where a slight amount of cold work resulted in grain growth after exposure to many thousands of hours at 320 deg C. This is typical.

I would be more worried about wall thickness wastage or corrosion in service.

 

[integridadmecanica]

Dear metengr,

Thanks for your reply. But corrosion velocity is not a problem (it is too low, Remaining life is more than 35 years)

I trying to look for an ASTM G size where consider damage could be considered. If trying to calculate the lifetime of heater tubes with Creep theory, the life of tubes is infinity, and it is not true because a grain growth is ocurring. My question is if i left the tube for thousand hours operating in this condition at what Grain Size ASTM rise before a change need to be performed?

Regards

 

[metengr]

integridadmecanica;
Grain size will not be the limiting variable for remaining life because you have no creep exposure. Yes, over time, there will be grain growth until all of the residual stress or cold work is eliminated. Your original tensile properties will be slightly lower. However, this will not effect long term performance. We have economizer tubes in several of our aged boilers that have been in service for over 50 years with no problems.

Focus your efforts on NDT, not microstructural.

 

[blacksmith37]

A GS of 10- 12 sounds suspicious for A106. Not that it is a problem for your application.