HDG fasteners vs Stainless steel fastners 4

[vjr0512]

In one of the Northsea Offshore project, as per NORSOK standard HDG fasteners shall be used. For one of the package, vendor has used Stainless steel SS316 fasteners. The application of the package is sea water filtration and the operating temperature is less than 50 deg c.

Client is not accepting to use SS fasteners and insists for HDG.

Is the SS bolts are not better than the HDG bolts for the marine atmosphere ? Any specific reason why it should not at all used? Can the usage of SS316 fasteners be technically justified with support?

Thanks to confirm please.

 

[OGMetEngr]

In a "salt-spray" environment, stainless steels (especially strain-hardened such as A4-80) are particularly prone to chloride-induced stress corrosion cracking. Bolting would be at the top of the list, as they are typically loaded to a substantial fraction of yield and are under constant tensile stress. Their concern is valid.

 

[vjr0512]

Thanks OGMetEngr for your reply..I agree with your explanation...but is it not that the stress corrosion cracking phenomena starts above about 60 Deg C? Here the service temp is well below that...Will it be of concern?

Thanks to clarify please

 

[OGMetEngr]

In a "salt-spray" environment, there can be accumulation of salts due to many wetting and evaporation cycles. This, along with the stress state of fasteners, is what makes this failure mechanism a possibility. I'd recommend reading the following for a view of the varying practices and rationale (or lack thereof) regarding this issue in oil and gas:

http://www.hse.gov.uk/offshore/ageing/stainless-steels-performance.pdf

 

[EdStainless]

In cold worked SS aqueous CSCC may require 60C, but there can be similar cracking that results from the concentration related to a splash or spray environment, and also cracking from organic chloramines (which will happen at room temp).

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[vjr0512]

Thanks OGMetEngr and Edstainless

 

[moltenmetal]

Thanks guys, I learned something today. The onset of stress cracking at temperatures as low as 20-30 C in ordinary austenitic stainless steels in a salt-concentrating evaporative environment is news to me.

 

[EdStainless]

The effect of chloride concentration and the impact of chloramines on CSCC is an interesting area.
There are a number of papers on the collapse of roofs of public swimming pools that provide insight to this subject.
This is not a concern in aqueous environments, but in cyclically moist environments things can change significantly.

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