316/316 L bolts and nuts - Sensitization 1

[Felipe255]

First of all, im new here and english is not my main linguage, im sorry for any kind of grammatic mistakes.

Anyway, my question is about 316 L/316 nuts and bolts.
I work as intern on a company that builds fiberglass storage tanks.
And for some flanges they use low carbon stainless steels (304 L or 316 L) bolts and nuts.

My first question is: the low carbon increases the resistence against sensitization, but what is the point for low carbon on nuts and bolts? The way they are manufactured can make they reach that specific temperature range that might form Cr23C6?

Second question, why should i use 316L bolt instead of 316 bolt for a flange made of fiberglass? They never get contact with the specific stored product, for exemple 30 % hydrochloric acid, moreover, a mechanical engineer told me the suited bolt for this exactly situation would be a plastic bolt. What is the reason behind this?

Again, im sorry for any kind of grammatic mistakes.

 

[MagBen]

If not to exposure to high temperature concerning sensitization, and no welding is involved, you can treat 316 and 316L identically, although 316L may have a very little better general corrosion resistance, while a very little lower strength.

 

[Felipe255]

So What does give 316 L a better general corrosion resistence compared to 316 without considering sensitization effect? Molibdenium content is almost the same for both of them.

And another question: Can the manufacturing process of nuts and bolts introduces a sensitization effect on S.S nuts and bolts?

 

[Dhurjati Sen]

Dear Felipe255,

Agreed that the bolts are never in contact with HCl but there would be HCl fumes around while loading / unloading and other operations. So L grade is used for greater resistance to corrosion (SCC and not sensitisation).

And yes, ideal bolt material would be fibre glass, but again those are yet to be manufactured.

Regards.

DHURJATI SEN
Kolkata, India

 

[TugboatEng]

To answer the question about stainless bolts in the flanges, I believe stainless fasteners should be used in all plastic applications because steel bolts swell from corrosion and that will typically crack plastic components.

 

[EdStainless]

Filipe, some cautions about using SS fasteners.
They can come in various strengths, some are cold worked for higher strength, some are annealed and are very soft. In some environments this can have an impact on corrosion.
Some have cut threads and some have rolled threads. Rolled threads are to be preferred.
SS will tend to gall (stick, cold weld) to it self when clean and under stress.
You need to use an anti-siese compound. You need to make sure that it is one that is compatible with the FRP and the chemicals being used. At low temperatures ones containing Cu of often used, at higher temps they use Ni. But the base grease is also important.

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

 

[TugboatEng]

It's too bad Nitronic 60 isn't a standard material for nuts.

 

[EdStainless]

TBE, If I could get 2205 bolts and Nit 60 nuts I would be very happy.

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

 

[TugboatEng]

I work with one manufacturer that figured out they can use carbon steel studs and capped nuts for externally corrosive environments. It's gosh darn brilliant and I hate sharing their intellectual property but it needs to be common knowledge.