Steel: solution, mech mixture, compound? All of the above?

Hardenable normalized steel is a heterogeneous alloy consistig of ferrite (solid solution) and cementite (compound).

When hardened, it is martensite (solid solution).

There are always additional inclusions (compounds) present.

HTH

Michael Dahms

Steel is an alloy of iron and carbon. The carbon is in solid solution in iron. Hope this is the simplest explanation.

Simple but wrong. man cementite

Michael Dahms

Congratulations for having decided that you need more expertise than a bunch of "fans" who can't decide the definition of "steel", and who can't bring themselves to look it up.

It is composed of compounds (iron carbides) and mixtures at the atomic level(dissolved carbon in ferritic or austenitic matrix of iron) and mixtures at the microscopic level (the iron carbides and iron atomic mistures forming things like the Pearlite regions in steel).

You might appreciate this reference site for many things metallurgical and in more or less fairly plain English.

Just by reading a few things at this site, you may find out how misinformed some "fan" posters can be.

Careful, you might find that you like actually knowing about the technologies.

Jim

That's just about the most belittling comment I've ever heard on a news group, mostly due to the fact that it didn't appear induced. Do I sense an underlying issue?

What do you mean "fans"? I work for a heavy stamping die maker in the automotive industry and many other folks on those newsgroups are cutting metal for money as I type this message...

I think that's the issue. I don't understand how some of the carbon is in solution, and some is in a compound. Or perhaps it's both at once. I'm used to molecules having a specific number of one atom and a specific number of another. However, I don't know very much about metallic bonding at all, so it's just my ignorance.

Thank you. Very interesting.

Indeed! And what about the rest of the posters on this *professional* group that gave the wrong answer?

I've always thought that once you see one mistake on a print, you don't trust the rest...

Not sure where _that_ came from, but OK.

Regards,

Robin

Some of the people are "fans" of materials as some people are "fans" of baseball.

Some people are "players" of either.

If the choice of the term "fans" irritates your rectum, you are listening with the wrong end.

Players come in all skill and knowledge levels. Some get booed, sometimes.

Absolutely correct, and in simple terms I attempted to explain it.

Do you understand that salt is dissolved in water? And if you have enough salt in water, some of it is dissolved and some of it is just a physical mixture (the undissolved part).

There are many other examples that can be given.... including dirt and water where some parts of the dirt dissolve into the water, and other parts of the dirt swirl around as a mixture of fine particles. If you fish on most rivers, you can see this as you fish.

Or that ethyl alcohol can be dissolved in water? The salt breaks down into ions of sodium and chlorine. The alcohol literally dissolves or mixes as an intact molecule.

If yes, then you should have no great difficulty in metals being dissolved in another metal in the molten state.

Most of the understanding of metal mixtures in the solid state comes from looking at chemically etched metals with microscopes and other instruments, making some measurements, and then *thinking* about the meaning of what it is that you saw. If you have done only the "big picture" stuff of working with the metal mixtures with your hands, machines and naked eyeballs, you have decreased possibility to readily grasp what metal mixtures (alloys) are all about because you have no experience approaching a fundamental behavior level.

There is something in the textbooks besides artfully arranged ink on paper.

Hi -

Steel is a bit more complicated than a chemical compound made of molecules.

A simple model to think about might be coffee where you have stirred in some sugar. (ie you have a solution of sugar in coffee). Then keep adding sugar until there is so much that no more can dissolve. Now you have got a mixture of two phases - one phase is sugary coffee, the other phase is wet sugar. Sugar is present in both phases. The wet sugar phase can have a number of different forms, ie small crystals or big crystals, etc, depending exactly on how you mixed it in.

To apply the model, mild steel as you would normally use it consists of two phases. alpha-iron (carbon dissolved in iron) and cementite (a compound basically iron carbide). Carbon is present in both phases. The cementite phase can have a number of different forms depending exactly on how the steel has been made - and the physical form of the two phases is a major factor in determining the properties of the steel.

Cheers

---Ian---

Now, is the percentage of cementite within a mass of steel always going to be the same after a certain amount of carbon has been added? 0.83% is the only number that sticks out but that doesn't seem consistant with what I have read. I know at somewhere around 2% the steel super saturates and becomes cast iron...?

What about in an austinitic state? Is the entire chunk of steel then austinite (beyond its critical temp., based on percentage of carbon)? And what about after quenching.

I think these questions are moving more into the text book range now, but I'm still interested in the cementite vs. iron/carbon solution issue.

Thanks for the answers. I always find material easier to remember when I understand what's going on.

Regards,

Robin

No. It is not. It depends on heat treatment. Only after very low cooling from the austenite range, there is a linear correlation between carbon and cementite contents.

Yes.

Martensite.

Michael Dahms

BTW: Please stop top-posting and full-quoting!

HTH

Michael Dahms

Michael,

I did not top post. Ian Kemp quoted some of my post at the top of his message, and then he quoted the rest of my message below his, including the part he had already quoted.

After posting thousands of messages on various newsgroups over the past 5 years, I have gotten into the habit of not looking below someone's sig at the bottom if there is basically more than two blank lines. I just assume there's nothing there.

If you look back, I did bottom post and I did trim the quoted portion of the message to which I was responding.

Anyway, thanks for the answers.

Regards,

Robin

At a given carbon level, and assuming you're not quenching to martensite, you will have a set percentage of cementite (Fe3C) present. The distribution and size of the cementite can vary depending upon cooling rate,etc, but the amount present will be fixed due to the set amount of carbon that can be dissolved into solid solution (0.005%?), and the set ratio of Fe and C in Fe3C. This can be calculated using the Tie Line and Lever Law using an Fe-C binary phase diagram.

This is only valid for low cooling rates. At higher cooling rates, a higher contet wil remain (metastable) in ferrite.

Michael Dahms

In the book "Model Engineering--A Foundation Course" by Peter Wright, on page 47 while discussing the hardening of carbon steel, it is written:- Steel which has 0.87 per cent of carbon is known as eutectoid steel. Its structure comprises thin particles of cementite alternating with similar particles of ferrite, in the ratio of 87 per cent ferrite to 13 per cent cementite. This particular structure is known by the name Pearlite.

I have done a calculation of the amount of carbon in Pearlite and it appers to be 0.43 per cent insteaad of 0.87 per cent. I present it here and ask for comments:-

Cementite is Fe3C. The atomic weight of iron is roughly 56 and the atomic weight of carbon is roughly 12 (the actual differences are very small). Thus, the ratio of carbon to iron in cementite is

C/Fe = 12/168

Thus the proportion of carbon in cementite is 12/180 = 1/30

Because the proportion of cementite to ferrite is 13/87, we have

Fe3C = 0.13

so C = 0.13/30 = 0.0043333... =0.43333... per cent

this is roughly half what the author says.

If my calculation is wrong, why is it wrong? If it is correct, I shall apprise the author accordingly.

Regards

Sian:

Right here at

"Thus the proportion of carbon in cementite is 12/180 = 1/30"

Acually 12/180 = 1/15, at least if you're working in base 10.

Pittsburgh Pete

DISCLAIMER

We don't believe what we write, and neither should you. Information furnished to you is for topical (external) use only. This information may not be worth any more than either a groundhog turd, or what you paid for it (nothing). The author may not even have been either sane or sober when he wrote it down. Don't worry, be happy.