Frank talk about martensite

[Matthew Gregory]

There are so many things I wish to know about martensite... why is it beneficial in a blade? How does it get formed? What does it look like?

Can anyone help?

 

[Sam Salvati]

Well, when one male martensite and one female martensite get together, a magical thing happens.......didn't you get this talk from your parents?(sorry couldn't resist).

 

[fitzo]

If there is going to be a question and answer section here, I would like to hear about:
1) What factors govern the formation of lathe versus plate marstensite in the 0.6-1% carbon range, what is the desirable formation, and why?
2) What are the effects of mixed martensite/pearlite formations on strength, edge holding, etc?

Those may be asked too early in the discussion, but i hope it comes up eventually.

 

[Kevin R. Cashen]

Gee Matt, how convenient it is for you to ask these questions. I may have some pictures but it will take a while for me to retrieve them.

Martensite is basically a supersaturated solution of carbon in iron in a configuration that is unnatural for room temperature. When normal room temperature ferritic phases are heated above Ac1 (1335F for the sake of this conversation) they will shift in their atomic stacking from a body centered cubic (bcc) to a face centered cubic (fcc) configuration. This new stacking has many more spaces for carbon atoms to occupy between the iron atoms thus it is capable of holding much more carbon in solution. This new carbon rich solution is called austenite. When austenite is slow cooled the arrangement will go back to bcc as the extra carbon comes back out of solution to form structures like pearlite, which makes for soft steel.

However if you cool austenite quick enough pearlite will not have time to form and the mechanisms by which the shift back to bcc will not occur at round 1000F when it normally would. Instead you will now be rapidly cooling metastable austenite that would normally have no business existing at this point so it is looking for something a little more stable to turn into. At a temperature known as Ms (martensite start), which is around 400F to 450F for many of the steels we work with, the cooling will strain the system enough that a body centered configuration will have to take over. At which point the austenite will have to deform in a tilting and shearing type deformation as a plane of new phase forms. But instead of being able to go back to bcc the trapped carbon will distort the stacking into a new arrangement called body centered tetragonal (bct). Bct is basically the bcc stretched along one of its axis into a rectangle instead of a cube, but is defined by a tetragonal shape within that rectangle.

This distortion of the stacking is what is responsible for the strength and even brittleness of martensite. The effect of all that strain makes any deformation of that metal much more difficult and in fact adding any more energy to a system that already has that much stored within it can easily results in failure. That is why a quenched blade that has yet to be tempered is so darned brittle.

The thing to always remember about martensite formation is that the shear type mechanism by which it is created is different than other transformations in that it is totally temperature dependant. The continuous lowering of temperature is what drive it, not time. If one stops cooling the whole process stops until cooling resumes, regardless of how long you hold it. Also since it is temperature driven, martensite forms very rapidly, almost instantaneously.

Martensite itself is made up of very straight and jagged looking sheets that when cross sectioned under a microscope looks like needles. It is said to appear “acicular” which means “needle like”. It forms in sheets or packets that can either lie in neat parallel rows or in chaotic jumbles that can impinge upon each other and looks a lot like broken glass under the microscope.

 

[silver_pilate]

GREAT explanation. The more I am able to delve into such technical information, the better I am understanding all of the details of our processes.

So, now that we've formed our martensite by continually cooling from Ms to a bit above room temperature, and we've got a stored-energy-rich material, we need to temper to prevent failure.

What happens to the martensite during tempering that relaxes the higher stress areas which may be prone to brittleness? And, if I remember correctly, I've read that tempering can actually promote continued martensite formation?? Am I recalling this right? If so, why is that?

--nathan

 

[jdm61]

Who's Frank?

 

[Kevin R. Cashen]

Fitzo, lathe martensite forms in steel that has less than .6% carbon while plate martensite occurs in steel with 1% carbon or more. Steel with carbon levels from .6% to 1% will have a mix of lathe and plate with increasing amounts of plate. At first it would appear that the carbon content alone could be directly responsible for the two morphologies but it appears that temperature at which it forms could be the key and since carbon content (and other alloying) determines Ms that is the connection.

Lathe martensite is very orderly and forms along habit planes that are said to be "rational" due to the orderly nature. Because of this it has more inherent toughness and is less prone to brittleness, this is why steels that contain .6% carbon or less have an instant advantage in toughness for large chopping blades. So when I suggest that 52100 may not be the best natural choice for a chopping bowie or camp knife, I am not just being a pin head and do have some logic to my reasoning

Plate martensite on the other hand forms in larger chaotic sheets with habit planes that have odd irrational angles. These large sheets can impinge upon each other at high angles and cause microfracturing. This makes plate martensite much more brittle, but the microfracturing can be controlled to a large extent by careful heat treating methods.

Mixed pearlite/martensite configurations are very common in simple steels. The chances are if you are working with a 10XX series, W1 or W2, you will have some pearlite in your martensite. However it must be stressed that this is a problem if you want maximum hardness. Pearlite get softer as the spacing of its lamellae gets coarser, but when it is very fine pearlite can be deceptively strong and hard, just nowhere near as strong and hard as martensite. And when you have made pearlite that portion of the steel is lost to anything else so every percentage of pearlite you have is that much less martensite you have, and if it is on the edge you blade has lost that much of its potential. Martensite with pearlite mixed in will have the pearlite in clusters in the prior austenite grain boundaries so you will get martensite surrounded by patches of softer pearlite. The effect of this in a file test would be to have the file skate like nothing is wrong, like skating something along the top of pieces of glass imbedded on playdoe, as opposed to glass stuck in concrete as it could be. Also a mixed structure of pearlite and martensite will not be as tough as a well tempered pure martensite structure. So unless you are looking to intentionally put pearlite above the edge as in a hamon, pearlite in your martensite really should be avoided whenever possible.

 

[Kevin R. Cashen]

GREAT explanation. The more I am able to delve into such technical information, the better I am understanding all of the details of our processes.

So, now that we've formed our martensite by continually cooling from Ms to a bit above room temperature, and we've got a stored-energy-rich material, we need to temper to prevent failure.

What happens to the martensite during tempering that relaxes the higher stress areas which may be prone to brittleness? And, if I remember correctly, I've read that tempering can actually promote continued martensite formation?? Am I recalling this right? If so, why is that?

--nathan

Tempering allows some of the carbon to slip out of solution to form super fine tempering carbides, this allows the distorted bct arrangement to relax into the more stable bcc that the steel (iron) would normally assume. If there is retained austenite this process will destabilize the austenite by depleting it of carbon and allowing to transform to martensite upon cooling, making yet another temper a good idea.

 

[fitzo]

Tapadh leat! I'll have more, just have to think some to make it orderly.

This could be a long, enlightening thread! Kind of you to play Shell Answer Man!

 

[mete]

Martensite starts out as a tetragonal crystal [elongated cube] with carbon atoms wedged into the side of the crystal. On tempering carbon atoms come out of the crystal to form carbides. As they do the crystal becomes shorter and shorter, eventually becoming a cube of ferrite.....If you want to be overwhelmed - www.msm.cam.ac.uk/phase-trans/2002/martensite.html While this has an enormous amount of info you may find sections that answer some of your questions without reading everything. There are also videos and even videos of lectures !!

 

[steelshaper]

SO let me know if I have this wrong but The basic iron crystal has a set number of iron atoms. Heat causes the iron crystal to change shape (ac1). Because the iron crystal basicly expands, carbon atoms that are disolved (also because of heat) and free floating along side the iron crystals move into the space betwen iron atoms.

My question is what is the relationship between grain growth and Iron crystals. Is it that the iron crystals combine? As in fine grain martensite would be made up of singular tetragonal crystals but coarse grain would mean the crystals are combined to form larger crystals? Or do I have it all wrong?

 

[Kevin R. Cashen]

...Because the iron crystal basicly expands, carbon atoms that are disolved (also because of heat) and free floating along side the iron crystals move into the space betwen iron atoms.

Actually this is where most folks get it wrong unless they have studdied the metallurgy, there is not an expansion at Ac1 there is a contraction. Fcc is actually a denser mode of stacking so while it does open up more spaces for carbon it takes up less sapce than bcc. Steel contracts and becomes denser when it forms austenite and expands drastically when it forms matensite.

...My question is what is the relationship between grain growth and Iron crystals. Is it that the iron crystals combine? As in fine grain martensite would be made up of singular tetragonal crystals but coarse grain would mean the crystals are combined to form larger crystals? Or do I have it all wrong?

The iron crystals combine to form one larger grain, larger grains grow at the expense of smaller grains, that is why it is best not to have a mixed grain size of very different sizes. I would rather have one grain size larger with them all being the same size than having them mixed like this.

 

[Kevin R. Cashen]

Martensite starts out as a tetragonal crystal [elongated cube] with carbon atoms wedged into the side of the crystal. On tempering carbon atoms come out of the crystal to form carbides. As they do the crystal becomes shorter and shorter, eventually becoming a cube of ferrite.....If you want to be overwhelmed - www.msm.cam.ac.uk/phase-trans/2002/martensite.html While this has an enormous amount of info you may find sections that answer some of your questions without reading everything. There are also videos and even videos of lectures !!

Wow! mete that is fantastic, that is the first photo I have ever seen of Adolph! If folks read at the link you have posted they will get more than they could here or in that little article I wrote. I will be sitting down to read it this afternoon.

 

[Nathan the Machinist]

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

 

[mete]

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 !!!

 

[Troop]

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 !!!

Robert,
I thought each crystal is a "grain". In John Verhoeven's new book, "Steel Metallurgy for the Non-Metallurgist", Ch.1, pg.4 (Summary of Major Ideas)
2. "Each crystal is called a grain."
Please "Un-Confuse" me.
Thanks
- Mitch

 

[Karl B. Andersen]

Who's Frank?

Oh, c'mon, you know Frank!

 

[Karl B. Andersen]

Thanks for that link, Mete. That IS some phenominal stuff!

 

[Stacy E. Apelt - Bladesmith]

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?

 

[Mark Williams]

So when I suggest that 52100 may not be the best natural choice for a chopping bowie or camp knife, I am not just being a pin head and do have some logic to my reasoning

.

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.