Ferritic & martensitic steels: please help me to understand. . .

[starflex]

Hi there,

This is so stupid that is going to be embarassing.

I am trying to figure out when a steel is martensitic, or ferritic, or more precisely, when I want a particular final structure from a certain chemical composition.

I am aware of the two main families of elements that, added to the iron, change its behaviour:

- Ferritizing elements: Al, Fo, Si, V, Mo, W, Cr, C
- Austenitizing elements: Ma, Ni, Co.

Therefore, we can take two materials in our example: 1Cr 0.5Mo and 12Cr-1Mo-V.

The first one is ferritic, the second one is martensitic.

Now, my question would be: when the transition from Ferritic to Martensitic happens ? What is the percentage of elements that would let me guess that I want a martensitic structure at the end of the transformation (or, that in case of welding, would ring the bell "Hey, cool down to 100 Celsius before the PWHT to allow the martensitic transformation") ?

It's a stupid and awfully worded question, and I do apologize for that.

In other words: I have to study those 5, 10 , 20 materials I will be delaing with on regular basis (and that would cover 99% of my needs), knowing which-one-is-what (and that's fine); os is there a way to say that, once I have a certain percentage of X Y Z elements, I will be looking for a martensitic structure (plain words - I not taking into consideration at this stage the cooling rate).

Thank you in advance!

 

[cloa]

All practical steels can form martensite. Its just a matter of a minimum level of carbon and suitable cooling rate from an austenitic state- its just that the so called ferritic steels require an excessive cooling rate that is hard to do and would cause cracking and distortion and the austenitic steels require the steel to cool to a temperature far below room temperature rapidly to form martensite. The cooling rate is the key. There are formulas for estimating the martensitic cooling rate needed and martensitic transition temperature- try googling those.

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[mrfailure]

Not stupid at all. I often look up the steels to see which category they fall under. ASM has a good list in their handbooks (Desktop Edition as well as Volume 1 for steels) for classification purposes, but those are listed as AISI-type steels. I'm not sure if there is an equivalent handy reference for international steel classifications.

 

[metengr]

Your best bet would be to have a reference book, like "Stainless Steels for Design Engineers" which provides excellent information on types of stainless steels.

http://www.asminternational.org/mat...-/journal_content/56/10192/05231G/PUBLICATION

 

[EdStainless]

If you get the carbon low enough you can retain the ferritic structure, like in ferritic stainless grades, but these are not really steels then.
C is an austenite stabilizer, as is N.
You can make 12Cr-1Mo with low enough C so that it won't form any martensite.
Look at 410S and 410 as an example.

Once you add carbon to an iron based alloy it is martensitic, but how much martensite forms and what the structure looks like depends on both composition and thermo-mechanical history.

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P.E. Metallurgy, Plymouth Tube

 

[starflex]

Hi All,

Thank you so much for all your inputs. Much appreciated, and I do apologize again for the overly ignorant question.

A few clarificaitons, though.

I am not talking about stainless steels. Mainly, my concerns are steels for power industry (e.g. P2; P 5; P9; P91; P11; P12; P21, P22; X12 CrMoV5).

On general basis, depending on the chemical composition and the cooling rate, you can obtain different structures, or you have to do mandatorily the PWHT. Again, just an example, you want to do the PWHT in P22, as well as in X12CrMoV5. But, in the second one, you have to make sure to cool down below 100 Celsius first, to allow a complete martensitic transformation, before the PWHT (this from the pressure equipment code I am using).

Therefore: is there a general rule of thumb to say "yes, if you have this chem composition you have to do this, this, and that to achieve the desired structure"; do I just have to memorize 10/20 grandes of the most common steels (which is fine) and their behaviour, desired final structure, limits, etc, or what ?

Thanks again.

 

[metengr]

These are considered low alloy steels and can exist in quenched and tempered condition, normalized or annealed condition. Information on these alloys can easily be found in various codes and standards. You can search for this information on your own.

 

[EdStainless]

There are standard methods for approximating the Ms and Mf temperatures for these types of steels. That will tell you what the critical temperatures are.

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P.E. Metallurgy, Plymouth Tube