Casting stainless steell / forged stainless steel body valve welding ends 1

[viki100]

Hi, all..
I have problem, that supplier offers me a cryogenic ball / gate valves ASME class 300 / 600 (BW welding ends) body / bonnet made of casting CF8M. When I asked them, CF8M, doesn't problem with sensitization (intergranular corrosion, postweld heat treatment) cast stainless steel after welding for cryogenic application.
They confirmed that CF8M/ 316SS (with Charpy impact test) is ok, even for the body welding ends. Yes, we have good experience the CF8M.
I think that, to my knowledge, good engineering practice should be used CF3M / 316L SS (carbon below 0,035 %) instead CF8M / 316SS for cryogenic applications, and not only.
Many manufacturers of valves (the cryogenic valves) offer valves made of CF8M for welding.
According to ASME B16.34 ASTM A351 CF8M and CF3M are the same group materials 2.2.
Could you help me in this topic, explain to me about it.

Thanks a lot.
Viki

 

[hsbcn]

Actually, CF8M is quite common in cryogenic valves. Is your only concern the carbon content?

I work with valves in exotic alloys at The Alloy Valve Stockist. This is our blog, and this is me on Twitter and LinkedIn. Let's connect!

 

[moltenmetal]

Corrosion resistance of CF3M and CF8M in the welded condition are different, but I'm having a hard time imagining a cryogenic application where that would matter. Again, I think people mistake the reason for using L grades of stainless steels, thinking it has something to do with "weldability", i.e. your ability to deposit a strong, crack-free and ductile weld. The straight grades and even the high carbon H grades, forged or cast, are perfectly weldable in that respect, but they do differ in the corrosion resistance of the HAZ after welding due to chrome carbide depletion in the HAZ.

 

[jwhit]

If corrosion resistance is also an issue, the L grade would be the better choice. The higher C content of CF8M makes it a little easier to keep the ferrite content low since C is an austenite former. Ferrite in the austenite can go thru a ductile/brittle transition and reduce notch toughness(impact strength). Either grade should work down to -320f/-196C without issue.

 

[viki100]

Liquid nitrogen and liquid natural gas LNG aren't a corrosive, but mercury is dangerous. Valve with flanges made of CF8M is good, but welding is problem. The ferrite content in CF3M and CF8M are similar. Higher impact strength at low temperature has CF3M than CF8M (to - 196 deg C). Both grades casting CF8M and CF3M are very well weldability.

 

[jwhit]

It is relatively easy to control the amount of ferrite in these alloys by custom tuning the critical elements to achieve the ferrite needed. One issue that can develop when ferrite is very low(<2%) is hot shortness. It isn't unusual to run into hot tearing that doesn't occur with higher ferrite levels, weldability can also be affected. In many cases, a post weld solution anneal is a good idea to restore HAZ corrosion resistance, especially with CF8M.