How Martensite forms in low carbon SS like A743 gr. CA6NM? And why not then in Ferritic grades of SS

[Abhijeet242]

CA6NM typically contains Carbon not more than 0.06%. However, Ferritic steels contains carbon upto 0.12% yet, the steels are not hardernable.
What is the reason behind it?

 

[EdStainless]

1. C is an austenite stabilizer, a very powerful one.
2. Matinsite refers to a structure, not just the Fe-C phase

I can cold draw an ultra low C (0.012) 304L and form 5-10% martensite in the structure, I know this because of how magnetic it gets. Because the structure isn't very stable.

The ferritics that have over 0.09% C they are slightly hardenable, I have done this. It is just that with this little C the martensite is very lean and not as hard as you would expect so the effect is not very strong.

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P.E. Metallurgy, consulting work welcomed

 

[Dhurjati Sen]

Why not? Try austenitizing them, hold there and quench in salt baths.

DHURJATI SEN

 

[Abhijeet242]

Thank you for the response. May be i did not clarify it properly in my question. Actually I mean to say, we say that ferritic stainless steels like grades AISI 405,409,410,434 etc. These grades they contain low carbon and hence, as you said,are only slightly hardenable.

However, when we see, CA6NM, it is a martensitic stainless steel, and has carbon limited to 0.06% and yet can be hardened. So what makes CA6NM hardenable even if it has low carbon, even lower than ferritic stainless steel?

 

[kingnero]

CA6MN will react very different depending on the chemistry (especially C%). In large (= non-homogenous) castings, knowing what exactly you're working with is paramount.
I'm not sure of your values, I have seen CA6MN with C% well over .06% - edit, a quick google search confirms the max of 0.06% C. Material must've been out of spec then.

 

[SteveHooker]

Google Schaeffler diagram to see how chemistry affects the structure of stainless steels.

 

[arunmrao]

Maintain your carbon content in CA6NM around 0.03- 0.04% . Else, any amount of grain refiners or heat treatment. cannot help you meet the mechanical properties.

 

Answering your question fully requires a course in stainless steel physical metallurgy.
I will just say this: do not assume similarities between any two 400 series steels, and especially not with CA-6NM.
With all due respect, in welding and metallurgy, a little knowledge is a dangerous thing.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Martinos]

Hi, Abhijeet242.

In that case never mind carbon as such. It is mainly about chromium content, but also carbon and other elements are connected. There is some theoretic equation about stainless steel hardenability: