Calculating transition temperatures

[GRoberts]

I don't know for sure where the equations came from, but the ones we have apparently used here for a while that seem to work good on a variety of steel alloys are: (temperature in F)

Ac1 = 1333+40Si-25Mn-26Ni+42Cr
Ac3 = 1570-323C-25Nn+80Si-32Ni-3Cr
Ms = 1042-853C-60Mn-30Cr-30Ni-38Mo
Mf = Ms-483

Grampi1 in an earlier post provided a very interesting link that had some equations on a Nippon steel website as follows: (temperature in C)

Ac1=750.8-26.6C+17.6Si-11.6Mn-22.9Cu-23Ni+24.1Cr+22.5Mo-39.7V-5.7Ti+232.4Nb-169.4Al-894.7B

Ac3=937.2-436.5C+56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr+38.1Mo+124.8V+136.3Ti-19.1Nb+198.4Al+3315B

Ms=521-353C-22Si-24.3Mn-7.7Cu-17.3Ni-17.7Cr-25.8Mo

The two sets of equations get somewhat different results in some leaner low alloy steels, but not as significant differences for some higher alloy (but still low alloy) steels. The Nippon steel equations do not seem to work for Martinsitic stainless steels either.

So the question is, which equations are best for which types of alloys? Are there other equations that work better for a particular alloy family? Does anyone know where the first set of equations originated and what range of composition they were intended to be used for?

 

[sdjvar]

Hi GRoberts,
I am new in this area and I read your Thread but I am not sure what's the transition temperature? Is it the melting temp and pouring temp. I am trying to find way to calculate the melting of any base alloy with different or variable elements. Is there such a formula that you can use to do so? Thank you for your reply.
Sonny Tran

 

[Metalguy]

Sdjvar,

No, not melting/pouring, but the temps. where various phases of steel change during heating and cooling.

The equations are interesting, but I can't add to them per the orig. request.

 

[tano]

Dear Groberts: the meanings of these temperatures are:

Ac1: the temperature at which austenite begins to form during heating.
Ac3: The temperature at which transformation of ferrite to austenite is completed during heating.
In both cases the letter c is due to the french word chauffage.
Ms: temperature at which austenite begins to transform during cooling.
There are many equations from different authors and with different scopes(of chemical compositions). All of them are obtained through numerical regression.The work of K.W. Andrews is old but still very intersting. It gives the equations for all these temperatures: K. W. Andrews,JISI, 1965, 203, (7), July, 721-727.

My best regards.

 

[NickE]

sdjvar-

I think you may be trying something that is quite hard. much like the equations in this thread (and the carbon equivalence equations in a thread below), Empirical studies and regression analysis may be the only way .

for one or two alloying elements there are ways to calculate the phase diagram from thermodynamic properties such as enthalpy, entropy, heat of fusion, etc. As long as you know these properties, and can fit them to the proper calculus. (my thermo is a little rusty or Id be able to give some equations but I have done the above once.)(theres also some stuff you need to include with soulubility too....)

I think that with lots of different elements though trying to get an equation bsed on thermo theroy would be worthy of quite a prize.

nick


(GOOD LUCK)


Nick
I love materials science!

 

[GRoberts]

I'm sure that the equations were obtained by regression (or more recent ones by neural networks). I'm definately not trying to break new ground here, just wondering if there were other equations out there already that people wanted to share that might work better for certain alloys. How is it determined how to heat treat a non standard alloy if the important critical temperatures aren't known? Or how do you determine these important temperatures on non-standard alloys without equations or lots of experimentation?

 

[sdjvar]

Thank you everybody. I have tried to come up with some equationsbut when I test them the results never came out correctly. If the equation work on one alloy then it will not work on different alloy if the content of the element change or included in the alloy (such as W, Ti).
Thank you for all the information.
SDJVAR