Discussion of Chloride induced SCC of CF8M 2

[valvesarefun]

Hello All,

I am wondering if the ferrite present in most CF8M castings helps it to resist chloride induced SCC better than it's wrought counterpart 316SS. I am thinking that its probably too small of a ferrite to austenite ratio to matter, or if it does make a difference then the difference is negligible, but I am interested in getting an authoritative opinion on the subject.

Best Regards,

VRF

 

[mcguire]

The effect is beneficial and significant. Fontana, Met Prog, Vol 80, shows that the threshold stress for SCC increases from about 5 ksi to 15 ksi for CF8M with 2% ferrite versus 316. With 15% ferrite it rises to about 35 ksi.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[valvesarefun]

Dr. Mcguire,

Is the critical pitting temperature also increased for CF8M as opposed to wrought 316? If so, is the increase appreciable?

I've read that foundries like to shoot for around 8% max ferrite due to the decrease in hot tear deformities, is there a functional disadvantage to shooting a little higher? As I understand, it is possible to have maybe as much as 20% ferrite and still remain within the specification for CF8M if your elemental composition is properly balanced.

It seems like if you alloy on the high side of the specification, you could almost end up with a product very similar to lean duplex. at least as far as PREN is concerned.

Best Regards,

-VRF

 

[EdStainless]

The offset to this is that the ferrite is a leaner chemistry and is susceptible to pitting attack.

I believe that a big part of the benefit is that the higher ferrite material will tend to have lower residual stress levels.

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Plymouth Tube

 

[mcguire]

As EdStainless says more ferrite doesn't mean less pitting. It doesn't help that aspect at all.

If you want minimal pitting with 8% ferrite, which is a good target, reduce sulfur and keep your nitrogen up. Nitrogen helps equalize the pitting resistance between the austenite and ferrite. To do this it ought to be at least 0.10%. I see no value to any sulfur from a corrosion standpoint, 0.001% is plenty.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[valvesarefun]

Edstainless,

Thanks for your reply.

In that case why are the duplex grades so resistant to pitting? Don't they have ferrite content of like 35-65%? And aren't the elements which promote ferrite formation also the elements which enhance pitting resistance?

Not arguing your point here, just trying to better understand.

Also could you briefly explain what causes residual stress, as it pertains to CF8M?

Best regards,

VRF

 

[EdStainless]

In a duplex the ferrite phase has similar pitting resistance to the austenite (mainly driven by Cr and N).
In an austenitic the ferrite phase ends up lean. You can get selective pitting initiation at the ferrite or even galvanic driven corrosion between the A and F.

The stress issue is similar to using ferrite in SS to minimize cracking in welds. The phase change from A to F has a volume change. This helps reduce stresses from solidification, in both welds and castings.
In most SCC cases residual stresses are far greater than service stress.

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