Martensite: Five Facts

I don't know if this is the place to ask but however:While american books refers steel up to 1.7%C,a russian materials science book defines it up to 2.14% of carbon.Why is that?-

Great question. Short answer russian book is correct at approx 2%by weight. Longer answer: Below 2% upon heating, a single homogenous phase (austenite)is formed which permits the metal to be worked and formed. Above 2% upon heating you dont get a single uniform solid solution. Instead we get either austenite plus cementite orferrite and cementite. These leave the metal difficult to work- brittle and hard. But it can be easily "cast" into shape. So they are differentiated from steels by name as cast irons rather than steel.

This discussion is just for iron carbon alloys, only iron and carbon.

in the us we seldom use ultra high carbon steels,(above say 1.5 wt % C) although some superelastic steels are coming into commercial use. Look at the iron iron carbide equilibrium diagram and the steel and iron zones designated at the bottom, and you should figure this out.

Milo

Reviewing some old posts...

An excerpt from your comment;

formed which permits the metal to be worked and formed.

Keeping in mind that as you work the metal, it becomes work hardened. Some people call it metal crystallization, that's not correct..

I recall my professor in Physical Metallurgy compared it to Clint Eastwood movie for a 'A Fistful of Dollars ".

When he was pounding that piece of metal he wasn't forming it, he was work hardening it and then strapped to his chest by his heart.

Hey Milo

Thinking about axe blades, and the decline in quality in recent times, and wondering which step in the process (or material composition) is likely making the difference.

The older axes - I had two given to me by my father - are very hard and tough. They don't chip easily at all, you could chop tree roots out of the ground and the rocks didn't make a dent in em. And they retained their sharpness better as well.

Whereas the new blades are easily chipped and don't hold their sharpness either when working on wood.

What happens if the tempering isn't ideally followed as in your diagram? I mean, if the temperature is not right for the alloy, or isn't done for long enough? Any other reason why axe blades today are not what they used to be?

Could be chemistry difference, production difference, or as you suggest problems with heat treat.

The old axe was probably forged from a billet that was rolled out of a bloom that was rolled out of a big honkin ingot. So there was a lot of grain refinement between reheats and reduction in section. and it was probably hand ground by a guy who paid attention to the condition of the wheel and didnt letbthe heat of the grinding decarburize the edge or create a touch of untempered martensite by the heat of grinding.

Todays axe is hopefully forged and is certainly out of billet cast steel meaning fewer reheats and less reduction in rolling. Less grain flow means less toughness to me. Its not just grain size, imho, its also work imparted.

It could be many other things some of which you mentioned, but i am thinking that using billet cast steels, less hot work, perhaps not even forging, and automatic machine grind are all possible contributors to the differences you havevexperienced.

milo

Thanks. Very interesting.

I have a so called litmus test for any hammer or axe I buy.........I hit it and listen to whether it 'sings' or not. (Caution: avoid any that break into "I've been working on the railroad")

On the other hand I have a 2lb axe head that was walloped by lightning. This baby not only sings but stays sharp far longer than normal.

I don't know how I missed this from you Milo.

In my physical metallurgy classes, this was a very fundamental basic to understand reinforced with quite a few labs with different carbon content.

I recall the professor talking about the martensitic grain structure resembles a pile of straw.

Carbon. Carbon. Carbon.

I'll bet they did invest in more than a couple of lab sessions to make sure you got this right.

The martensite is acicular and randomly oriented. Straw is as good a word picture as any to describe this.

Milo

Carbon is your friend. We couldn't move forward till we understood it.

Thats what I remember most...... that and cooling curves on alloys in phase diagrams

11.7.13 Martesite is a metastable structure from the transformation of austenite through transient phenomenon called quenching. Martensite is the bulwark of the hard materials industry. Not all autenite gets transformed into martensite in the first go, especially in the high alloy steels. This retained austenite and as quenched martensite are a pain in the neck. More conversion of retained austenite to martensite and de-stressing untempered martenite is achieved through multiple tempering cycles. Cryogenic treatments are also increasingly becoming popular for reducing retained austenite. But tempering post cryo is always recommended. Best practice is to sandwich cryo between multiple tempering cycles.