SCC in molten salt

[CWEng]

Carbon steel seems to be an acceptable material for molten nitrate salt piping <750F (Sandia labs solar power Design basis document). However, a cracked weld in a small bore pipe was sent to a lab who evaluated it as SCC (and I would agreed from the analysis provided). What conditions might be present in molten salt that would contribute to SCC in carbon steel?

 

[CWEng]

Forgot to say: Welds were also not stress relieved. Is that required/standard practice for CS welds in molten nitrate/salt service?

 

[Ron]

Did the failure occur at or near the weld?

 

[EdStainless]

In clean molten salt systems you would not expect corrosion, but over time various impurities and decomposition products collect and they can cause nasty surprises. I seriously doubt that this was classical SCC.
Most of these systems have been built with Cr-Mo grades in the piping, ferritic SS in the HX, and Ni alloys in the furnaces. Plain carbon steels will have two problems, first corrosion from trash in the system, and then lots of corrosion products further trashing up the system.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[CWEng]

The failure was only in the weld metal itself. There didn't appear to be any in the base metal.

The cracks were completely through wall, and had quite a bit of secondary branching like classic SCC in the weld metal. Near the ID of the pipe was intergranular, and closer to the OD it switched to transgranular, but still with branching.

I have little information on the use of this particular piping, but it was not the main salt transport piping, but a secondary system.

 

Would love to see a micrograph or two.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[CWEng]

I'd have posted them initially, but we don't own the photos. I doubt it would be permitted if I asked permission due to the situation. We're just trying to glean some information from this failure to see what we can do to mitigate it for our products (the weld in question was another companies weld in the same system). It doesn't seem like there is much out there in the literature about it I can find.

 

[EdStainless]

So this was a non-PWHT fabrication weld?
These systems see a lot of thermal stress from cycling.
Unless there were a lot of these cracks or significant signs of corrosion in the area I would worry about thermal stresses first.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[CWEng]

EdStainless- yes these were (2) non-PWHT fabrication (pipe butt welds). Thermal cycling result in fatigue (low cycle) cracks from my understanding? These cracks definitely had significant branching (in the weld metal only) and intergranular at least to start with. Wouldn't a thermal cycling/fatigue crack be relatively straight and unbranched during at least the initiation phase and most of the growth phase until its overloaded?

 

[EdStainless]

I wonder if the stresses would be high enough to drive a different cracking mode, we are talking about stresses above the yield.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[CWEng]

The metallurgical analysis did determine that all the fracture was brittle in nature. No ductile fracture was observed in either crack. To me, that would point to fractures that were not from over-stressing, as he steel would have been at or above the melting temperature of the salt.

Speaking of stresses from thermal cycling. If that is a contributor to cracking, would PWHT help with that similar to how it does with high cycle fatigue? I don't recall if there is much difference in the beneficial effect of PWHT in high vs low cycle fatigue. Is PWHT standard practice for carbon steel piping in molten salt? It seems like it would be for at least the P11 used for some systems if not the CS. I'm not aware of what piping code they use, so there could be exemptions for carbon steel and even for P11. But I don't think I'd want to use the exemptions at least for P11 if I was a plant owner.

 

[EdStainless]

I am sure that the system that I worked with was all in T-11 or T-12.
They wanted to keep the wall as light as possible so that the piping was more flexible, because they know that stress from thermal expansion was a big issue.
I recall that they were also checking the salt for trace contaminants, among them Fe.
If your fractures were brittle that leads me to believe that they happened at lower temps maybe, is there any chance of moisture in the system during a shut down?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy