Critical Cooling Rate and Quenching Steel

It smell Homework all arround ...............

The iron-iron carbide equilibrium diagram would be a good place to start to understand what it is that they want you to know in question 1.

Look up words eutectoid, hardenability, and phase transformation.

2. Doesn't specify oil, water or air quenchant, section diameter, nor the chemical makeup/alloys. If the data given contains a clue as to those things, I'm missing it on a saturday night.

And it reads suspiciously like homework.

milo

The iron-iron carbide diagram is NOT the place to look at in this situation. As some know, that is an equilibrium diagram. Quenching in water from austenite will induce martensitic transformation, which is out of equilibrium. Fe-Fe3C diagram will help you as much as scratching a wooden leg.

As well, the TTT digrams are mostly used for isothermal treatments. What you need to find is the CCT (continuous cooling transformation) diagrams, specific for the steels you are looking for. The CCT diagram looks like TTT, but the curves are displaced.

You do need to look up 2 diagrams- the phase diagram of steel (T vs. %C) and the TTT diagram. The first one shows that 0.2% steel cools through the ferrite+austenite phase while the 0.8% steel is a eutectoid composition. So, the latter's microstructure has a band like appearance. In quenching, in general, steels become harder and more brittle. In a fast water quench of 0.8% steel, it remains autenitic until the Ms line is reached and then transforms to Martensite between Ms and Mf lines. The critical cooling rate can be calculated by taking the slope of the cooling line which is tangent to the nose of the TTT curve. However, I think that water quenching of eutectoid steel will be much faster and won't be tangent to the TTT nose. Right, Milo?

No. Wrong (Fe-Fe3C would have some value for the 0.2%C, but not for 0.8%). The cooling conditions make the Fe-C diagram unusable here (at least for 0.8%C). And the TTT curves don't really apply. See my answer above.

It does smell like homework, but at least don't lead him on the wrong way.

Dear mr Rabbit.

I pointed the original poster to the iron Ironcarbide equilibrium diagram so that they would figure out that the .8 C special case in the problems was in fact the eutectoid composition. Before running a marathon, perhaps one should master walking, or standing on ones own two feet. Seeing no assurance that the conditions of the questions were understood, (difference s between .2 and .8C, we tried to teach the poster to understand the differences and special conditions that were being studied. And the Iron Iron carbide equilibrium diagram was the place to START.

Had the original question mentioned the eutectoid composition, we would have responded differently.

Thanks for your posts. Clarification for sensemaking is always welcome here.

milo

Milo,

You are correct in your teaching intentions. However, the kid wants his homework done. He's supposed to know already about the Fe-Fe3C (or Fe-C) diagram already, but he didn't take the time to read, or he paid no attention in class.

Now, the eutectoid composition wouldn't make a difference in this case. The answer (for his level) would be the same as for 0.6 (for example) or 0.8%C. The type and distribution of martensite plates would change, but I doubt they got that far deep.

Thanks to all!

what I understand is that on quenching 0.8%C steel when heated to autenitic phase and then quenched, it will start to convert into martensite at Ms till Mf with retained austine. retained austenite will be converted to martensite on colling the steel to sub- zero temperature. Also from the TTT diagram, if .08%C steel is cooled (and not quenched) will get transformed to pearlite, which is the eutectiod of steel. And if 0.2%C is cooled will pass through the Austenite-ferrite phase first and then finally will be transformed to alpha ferrite. Please correct me if I am going wrong somewhere.

Now again I am not understanding what will happen to 0.2%C steel on quenching. I cannot interpret this from TTT diagram which is for equilibrium conditions only. Will it also get transformed to martensite? In other words what will be effect of carbon content in quenched steels?

Is there any formula for calculating the critical cooling rate of steels?

Thanks in advance

This should help:

http://www.springerlink.com/content/g11611mp8k2240m1/

milo

You are still going to get some hardening effect on 0.2%. That's it. Off you go to the library. Pay more attention in class, or go back and ask the professor who's ruined your weekend by torturing you with this insufferable homework.

Dear members,

Thank you very much for your concern in the subject. I am sorry for not to understand why most of the members have the intention of my question being a home work instead of explaining me the actual phenomenon (if any body knows) which I want to understand. If I were being a college student, it was better for me to ask my Professor rather posting my question here. Anyway no more comments from dear members are needed because I don't want the members to waste their time in solving my homework. Thanks again and goodbye.

1. Solving your homework is YOUR own personal responsibility, not ours. If you don't put in the effort to do your own homework, how are you ever going to learn anything? Furthermore, do you expect your mother to clean you up every time you take a dump? Surely not, do you? So why then should you expect other people to do your homework for you when it's your own personal problem to solve? Do you expect others to do your job for you when you begin working? If so, then why should you be holding the job at all? Why not let you go in favor of a more competent person?
2. Sadistic jerks like me may also either mock you publicly here, or even deliberately give you wrong answers so that you'll fail your assignment in order to teach you a lesson about the importance of doing your own homework.

Do you understand now why you should NEVER write in here to cheat on your homework?

Dear memebers

A good informative article is here http://www.substech.com/dokuwiki/doku.php?id=iron-carbon_phase_diagram. Please check

Thanks