347H vs 347

[Eduardo1982]

Hey guys.
Does anyone is familiar with 347H Stainless Steel? I have read that this material is highly recommended to work in solar industry (high temperature and working with molten salts). Actually I wish to know why this 347H is recommended instead of 347. Is there any literature that talks about corrosion rates in molten salt baths for such materials?
Any info I will appreciate.

 

[TugboatEng]

What flavor of salt? These materials would not be acceptable for chloride salts.

 

[Eduardo1982]

TugboatEng
Molten salts are typically made up of 60% sodium nitrate and 40% potassium nitrate.
Working temperatuve range about 350-560 °C.

 

[EdStainless]

Most of the work that I recall used ferritic SS for this, such as 430.
347H can be tough to work with. Since it has high C it much have higher Nb, and that can welding difficult.

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P.E. Metallurgy, consulting work welcomed

 

[Eduardo1982]

EdStainless
If I remember well , austenitic SS has higher creep properties over ferritic SS, so I think this could be a reason why 347SS could be used.

http://www.pms-c.com/wp-content/uploads/2012/04/Atlas_Technical_Handbook_rev_July_20101.pdf

 

[EdStainless]

Eduardo,
Yes, that would be the case at the top end of the temp range.
But at even slightly lower temps the ferritic will be stronger.
And the lower thermal expansion (almost 50%) greatly reduces stresses with thermal cycling.
This is important since these plants cycle from cool to hot every day.
There are also higher C grades of ferritic that may be suitable.
In the past I have supplied 625 for solar absorber tubes (annealed in air and the oxide left on them) and 439 for heat exchangers (sort of steam generators).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed