Frank talk about martensite

mete said:
The bcc crystal [ferrite] has 9 atoms in the basic crystal and the fcc [austenite] has 14 atoms. Carbon is not free floating. It is called an interstitial alloying element as it fits between iron atoms rather than replacing them [substitutional element]. ....Atoms combine to form crystals.Crystals combine to form grains.Grains grow by absorbing adjoining grains [this requires grain boundary movement].....Bainite is a diffusion process unlike the martensite shear process. There are two types , upper and lower.Bainite requires quenching to the bainite transition temperatures and holding it until bainite transition is complete. This can take a very long time with some steels which make them unsuitable for bainite.While there has been much hype about bainite for blades ,the benefits are not great enough to bother with it....For a complete discussion of bainite , my listing of the Cambridge U above includes a book you can download ,no charge, all about bainite , some 400+ pages !!!
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That was good stuff Mete. Because of my past work experience I know how to properly heat treat steel and have done it for years but I never fully understood the how & why in the scientific sense. My pea brain has had a really hard time grasping or should I say, fully understanding, the actions that take place in many of these explanations. Especially when the talk turns to atoms and how they stack up, form, change, etc. I've tried, I mean I've really tried to understand what I'm reading and just couldn't seem to visualize the process. Kevin you are a god send and do an incredible job with your explanations and diagrams but for some reason there was always something that didn't tie it all together for me. Mete you just helped make things click with this simple explanation of the order, atom > crystal > grain. I would bet Kevin has mentioned it in past text but due to me being bedazzled with information overload I must have missed it. I just want to thank both of you guys for repeatedly explaining this stuff. I mean how many times have you both explained this in the past few years. Over and over again and you're still explaining it. I guess it just takes some guys longer than others to " get it " :rolleyes: . I've been on a thank you kick in a few threads as of late to many of the guys here that unselfishly provide answers to the same old questions. No kidding, I really mean this, I'm not trying to be a suck a** :o I am just blown away, humbled and overwhelmed....everyday, by the talent and brilliant minds that are here. Thanks for the patience :)
 
Nathan the Machinist said:
Dear Kevin,

What is bainite?
Are there different kinds of bainite?
Why would someone want bainite?
How do you make bainite?
Can I make bainite in all steels, or just some?
Do you have a technique to make a martensitic edge and a bainite spine, and would there be any benefit to that?

Thank you,
Nathan
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Oh now why did you have to go a ruin a great martensite discussion by bringing up the "B" word? :thumbdn: To be honest I have always been a martensite kind of guy and I would rather discuss the latest about Britney Spears than bainite as I see them often on the same level;)
 
bladsmth said:
Thanks kevin and mete. That was one of the simplest technical explanations I've see.

I've used the Toy box example to explain it to those who can't wrap their heads around crystal shapes:( this is an analogy, and not 100% exact.

Remember when you got a box of BLOCKS and BALLS to play with, they came neatly arranged in the BOX in nice rows, 50 blocks and 50 balls, with a plastic cover on the box (This play set was called the eutectic box of iron and carbon). The box is the steel, the blocks are the iron , the balls are the carbon. After playing with the blocks and balls (forging),and thus destroying the plastic cover, you try to put them back into the box.....they won't fit any more. They are all jumbled up, and take up too much space.So Daddy Kevin shows you a neat trick..... the blocks are hollow, and that there is a secret door on each block that opens up and you can stick a ball in it. The magic word to open the block up is Ac1, (1335F). When the doors open up the balls all roll into the blocks.This makes them fit into a much smaller space in the box. Now, if you just set the box down and go to bed, the blocks and balls don't really like being crammed into such a little space in the box, and when you aren't looking they line back up into rows of balls and rows of blocks.(This happens with slowly cooling the steel , and is annealing.) Now that is fine (fine pearlite,to be exact), but every time you move the box, they dump out easily (soft). You can't take the box to Grandpa mete's house because they fall out in the back seat ( won't hold up to wear). So Daddy Kevin tells you a second secret word, Ms (450F). When you put the toys away,you say this word, and the secret doors open up with all the balls getting between the blocks ( quick cooling), with a ball touching on every side of a block. This way they are packed so tightly against each other and the box walls ,that they don't fall out of the box. But there is still a slight problem, they are squeezed in so tight the box could explode if you toss it in the back seat of the car too hard (steel breaks easily). Daddy Kevin says to give the box a few gentle taps (tempering), allowing the blocks and balls to find a slightly more comfortable arrangement that still stays in the box (hard) but won't explode if you drop the box (break easily).
Now,Daddy Kevin never misses a chance to teach you a little science, so he has you look close at the blocks and balls. See, there are still only 50 of each, but some blocks are just touching other blocks (iron) while others are in tight little clusters with the balls wedged between the blocks (carbides). This is how Daddy Kevin wants you to keep your box of iron and carbon. When you get bigger he will show you how to deal with having more balls than blocks ( hyper-eutectoid), and what happens when you have other shapes, like triangles, added to the toy box (alloy steels).
Now go to bed and have sweet dreams of forging. (Some of you may need therapy after reading this).

Stacy

jdm61 - Surely you know who Frank is?
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The words "daddy Kevin" conjures up so many images and none of them are good:D:barf:
 
Mark Williams said:
I'm not buying that statement.

Admit it Kevin...You are just jealous of the
superduper-high performanceness of 52100 that has been heated to some erronious temp with torch and quenched a gazillion times. Each quench continually giving successively higher levels of hardness and ductility. :p
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Mark I think you just miss that point that when you mix 1%Carbon with 1.6% Chromium in your iron it magically becomes more MYSTERIOUS than normal steel:D Mere mortals have a hard time fathoming it!;)
 
Troop , the individual crystal ,just 9 atoms in a bbc crystal ,stacks up and that group of crystals ,all lined up, is called a grain. While we can't normally see an individual crystal we can see the grains either at 1000X in a microscope or looking at a fracture surface. We measure grain size [ASTM methods] using those two methods....I think the statement should have been "each group of crystals is called a grain".....The visual method is one makers should be using with experiments with HT to actually see which of your methods gives smaller grain size. Large grains give you weak and brittle.
 
BCC:

BCC.jpg


FCC:

FCC.jpg



Martensite:

martensite3.jpg


Lathe martensite in 5160:

lathe1.jpg

lathe2.jpg


Plate martensite in 1095:

plate1.jpg

plate2.jpg
 
mete said:
Troop , the individual crystal ,just 9 atoms in a bbc crystal ,stacks up and that group of crystals ,all lined up, is called a grain. While we can't normally see an individual crystal we can see the grains either at 1000X in a microscope or looking at a fracture surface. We measure grain size [ASTM methods] using those two methods....I think the statement should have been "each group of crystals is called a grain".....The visual method is one makers should be using with experiments with HT to actually see which of your methods gives smaller grain size. Large grains give you weak and brittle.
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Thanks, Mete. Calling an individual crystal a "grain" sounded a little fishy to me.
- Thanks
 
Kevin R. Cashen said:
Oh now why did you have to go a ruin a great martensite discussion by bringing up the "B" word? :thumbdn: To be honest I have always been a martensite kind of guy and I would rather discuss the latest about Britney Spears than bainite as I see them often on the same level;)
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OH OUCH, sorry about those questions at Ashokan Kevin:(.
 
Thanks guys for patiently going over the same stuff again and again and again.........
It may be silly but the thing that kept tripping me up was; I was not sure if we were talking about steel at the atomic level. It makes more sense now. Also alot of the basic terms I have not heard since I was in high school taking chemistry.

Thanks
 
mete, I can't believe how great that link is!!!! There's some wonderful images (and even animated quicktime movies!) showing the location of carbon atoms in the crystal structures that really helped me get a grip on what's going on...

THANK YOU!!!!!!
 
I've always leaned more towards lathe martensite for most blades. There's something about the micro fractures and chaotic structure of the plate martensite that bothers me.

Are the micro fractures just inherent in the plate martensite structure, or are they a direct result of the stress from quenching,... or both? How can we minimize the micro fracturing in heat treating and what practices should we avoid?
 
Keeping the grain size udner control will reduce the size of the plates and thus their ability to microfracture, gentler quenches also help. Most folks new to knifemaking get the idea that quench has to be the fastest cooling possible, the faster the better, when the reality is all you have to do is be faster than pearlite formation (perhaps upper bainite but if your quench can beat pearlite the chances are you will have that licked as well). Speed is only one quiliaty in a quenchant to look at and often the only defense I hear about a arpticualr quench medium is "it is fast enough for me", once again if fast was all that mattered why doesn't everybody just use brine? If faster is better why not quench A2 in water mixed with jet dry and dish soap?

The autotempering effects of marquenching can also act as a buffer to the microfractures. Of course one could also adjust you temperature and soak time to control how much carbon is put into solution and reduce the ammount of plate martensite, but this would require exacting controls in austenitizing. I believe I have a micrograph of 52100 that had decarb and you can see the martensite change from plate to lathe as you get closer to that lower carbon skin.
 
I guess it really has a lot to do with what type of knife you are making and what your preferences are, in relation to higher and lower carbon steels. Most of the time I'm going to be tempering my blades back fairly well to stress relieve them etc. A hardness of between about 58-60 is about as high as I like to go most of the time. So, why use way more carbon content than I need?

I also wonder about the longevity or "life" of the tool. Couldn’t "high performance" in the short run also equal a shorter life in the long run of the tool in many cases?
 
I still don't get what tempered martensite is. If carbon is coming out of the BCT and forming carbides, wouldn't that mean BCC ferrite and iron carbide-cementite?
 
Tai Goo said:
I guess it really has a lot to do with what type of knife you are making and what your preferences are, in relation to higher and lower carbon steels. Most of the time I'm going to be tempering my blades back fairly well to stress relieve them etc. A hardness of between about 58-60 is about as high as I like to go most of the time. So, why use way more carbon content than I need?

I also wonder about the longevity or "life" of the tool. Couldn’t "high performance" in the short run also equal a shorter life in the long run of the tool in many cases?
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I could be wrong on this but one reason is an increased amount of carbides. Carbides icrease edge holding in a way. As the martensite is worn the carbides are left as tiny teeth. The more extra carbon in the mix the more tiny teeth.
 
Hardheart the martensite wants to get back to bcc but when tempering you dont let it. Only some of the carbon excapes the crystals. Leaving martensite that is not as stressed out.
 
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