Can CA-6NM be considered a 400 series stainless? 1

[jimhokie]

I'm in the middle of a dispute regarding the corrosion resistant nature of CA-6NM. A specification says that external surfaces of corrosion resistant parts shall not be painted, referring to a note that defines (for the purposes of this spec) what materials are considered to be corrosion resistant. The note states: "Corrosion resisting materials are as follows: CRES (except 400 series), nickel-copper alloy, copper-nickel alloy, nickel-chromium-iron alloy, brass, bronze, gunmetal, titanium, copper, NI-MO-CR (Alloy 600) and NI-CR-MO-CB (Alloy 625)." Based on this definition, is CA-6NM, Class A per ASTM-487 corrosion resistant, or does it fit the exception for 400 series?

We are aware that CA-6NM will develop a light protective oxide coating, but the issue comes down to whether the spec as described above requires it to be, or excludes it from being painted. I'm hoping someone with a stronger metallurgical background that either of us can easily resolve this. Thanks for any insight that someone may be able to provide!

Jim

 

[metengr]

jimhokie;
CA6NM is considered to be part of the family of cast martensitic (4XX series) stainless steel. The UNS for this grade is S41500. So, despite some reference to this being a corrosion resistant grade of stainless steel in literature, according to the specification referenced above it does not meet the list of corrosion resistant alloys mentioned.

 

[jimhokie]

Thank you for that input. Is there a formal definition for what constitutes "400 series"? I thought 400 series typically had more carbon and little or no nickel, whereas CA6NM has ~4% Ni to improve corrosion resistance. Is it fair to say that as 400 series alloys go, CA6NM has better corrosion resistance? (Although this may be a moot point given the spec's definition of corrosion resistant materials.)

 

[salmon2]

4XX could be ferritic and martensitic stainless steel. Their microstructure could be BCC ferrite or tempered martensite. Their corrosion resistance are considered worse than 3XX which has a FCC austenite of microstructure.

The corrosion resistance of stainless steel is considered from Cr content, not from Ni. Therefore, 4% of CA6NM may not help corrosion but certainly will help low temperature Charpy and hardenability.

 

[metengr]

Is there a formal definition for what constitutes "400 series"?

http://www.matweb.com/search/datasheettext.aspx?matguid=8f5fb31be2b04951a782b7070ab4a4d0

 

[metengr]

jimhokie;
The CA6NM martensitic stainless steel contains Ni to improve weldability and toughness, over the standard 410 martensitic stainless steel. I have worked with both alloys in large steam turbines. Both will corrode similarly in wet steam, the only difference being the CA6NM can be weld repaired with less risk of cracking.

 

[jimhokie]

Thanks for all the info. I think that answers my question pretty well!