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Harper John

Structure Of Martensite

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Hi,

 

I am making this post because despite having read many books on metallurgy, I still have difficulty in understanding martensite and whether it has something that resembles a granular structure and grain size. I have often been told that martensite does not have a grain size nor form grains, yet the literature is replete with references to grain size, especially when describing martensitic fracture surfaces.

 

I have read that martensite begins to from at the grain boundaries of austenite, grow inwards and a single plate can reach across the whole of the grain. However, can this plate, once it reached a grain boundary grow beyond it? If not, then surely the former austenite grain boundaries will determine the orientation and size of the martensite laths or plates, and thus effectively form grains of marteniste.

 

Another thought that has occurred to me is that carbides and impurities that have segregated at the austenitic grain boundaries will probably remain in place after the martensitic transformation, further demarcating areas of martensite from each other and lending it a granular character. Is this the case?

 

Now, not being a metallurgist, all this is mere supposition on my behalf and would greatly appreciate if someone here could either offer me a comprehensive explanation or else direct me to some literature that does so.

 

Thanking all who may care to answer,

 

Cheers

John

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Hi Harper,

As I understand things:

I would not call martensite a grain per se..its a body centered tetragolan crystal.

Lathe forms in lower carbon steels ..below about 0.6% carbon and plate forms mostly above 1%...the levels between can form a mixture...BUT this depends upon austenizing temp and chemistry as some elements can form carbides which can bind the carbon until very high temps are reached at which point that bond breaks and they will let go of the carbon...and unless you put the carbon into solid solution it can not form martensite in the first place.

Lathe is more needle like in appearance than plate.

Keep in mind that a single grain is composed of many cubic structures and therefore the body centered tetragonal martensite is not a single structure taking up the entirety of the grain.

It is also less dense and can act as a grain refiner on its own with multiple quenches.

 

There are a few new books out

G.B. Olson editor "Martensite" ASM 1992 is one

also there are a few technical articles out there.

 

Don Fogg had a video of martensite forming (got from Batson?) in a lab...interesting as you see nothing and then it is simply there...it forms very fast and to me it looked like it grew from one point. Maybe someone could put that on YOUTUBE?

 

I also recommend Vander Vort's metallographic work as he has some colored microscopy which shows interesting things.

 

I have more questions then you do....

 

Oddly enough Prof Geg Olson from Northwestern was in my shop a few weeks ago with four students and we were forging some of that special Questek Stainless that he designed.......if I had known I could have asked him your questions.

 

Too much to learn...

Ric

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Hi,

 

Thank yoiu for your replies. I have read Verehoven and a number of other books, but am none the wiser. My confusion is made worse by remarks or posts such as that of Mete, a metallurgist, some time ago in which he wrote: ".....When you reharden you will have fine grain austenite which in turn gives you fine grain martensite."

 

 

http://www.bladesmithsforum.com/index.php?ac...site.%5C%2Bmete

 

I really wish that a knowledgeable metallurgist would step forward and enlighten me.

 

Perhaps I am struggling with semantics, because I was told that grain boundaries are formed by colliding dendrites of a given orientation. Perhaps there are some virtual grain boundaries in martensite, formed by impinging plates or something similar, with the same net effect.

 

Cheers

John

Edited by Harper John

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Hi,

 

Thank yoiu for your replies. I have read Verehoven and a number of other books, but am none the wiser. My confusion is made worse by remarks or posts such as that of Mete, a metallurgist, some time ago in which he wrote: ".....When you reharden you will have fine grain austenite which in turn gives you fine grain martensite."

 

 

http://www.bladesmithsforum.com/index.php?ac...site.%5C%2Bmete

 

I really wish that a knowledgeable metallurgist would step forward and enlighten me.

 

Perhaps I am struggling with semantics, because I was told that grain boundaries are formed by colliding dendrites of a given orientation. Perhaps there are some virtual grain boundaries in martensite, formed by impinging plates or something similar, with the same net effect.

 

Cheers

John

 

John,

Does this muddy the water for you?

The below is found here:

"http://www.metallography.com/grain.htm

 

Complications--Grain Characteristics

 

Grain size measurement is complicated by a number of factors. First, the three-dimensional size of the grains is not constant and the sectioning plane will cut through the grains at random. Thus, on a cross-section we will observe a range of sizes, none larger than the cross section of the largest grain sampled. Grain shape also varies, particularly as a function of grain size. One of the earliest studies of grain shape was made by Lord Kelvin in 1887. He showed that the optimum space-filling grain shape, with a minimum surface area and surface tension, is a polyhedron known as a tetrakaidecahedron, which has 14 faces, 24 corners, and 36 edges. While this shape meets most grain criteria, it does not satisfy the required 120 degree dihedral angles between grains where three adjacent grains meet at an edge, unless the faces exhibit a minor amount of curvature. Another ideal grain shape, the pentagonal dodecahedron, agrees well with observations of grains, but is not a space filling shape. It has twelve five-sided faces. However, it must be recognized that we are sampling grains with a range of sizes and shapes. In most cases, the grains observed on a polished cross-sectional plane exhibit a range of sizes around a central mean and individual measurements of grain areas, diameters, or intercept lengths exhibit a normal distribution. In the vast majority of cases, we merely determine the mean value of the planar grain size, rather than the distribution. There are cases where the grain size distribution is not normal but bimodal, or "duplex." Also, our grain shapes can be distorted by processing procedures so that they are flattened and/or elongated. Different product shapes, and different processing procedures, can produce a variety of non-equiaxed grain shapes. This, of course, does influence our ability to measure the grain size.

 

Grain size measurement is also complicated by the different types of grains that can be present in metals, although their fundamental shapes are the same. For example, in body-centered cubic metals, such as Fe, Mo, and Cr, we have ferrite grains; in face-centered cubic metals, such as Al, Ni, Cu, and certain stainless steels, we have austenite grains. The grains exhibit the same shapes and are measured in the same way, but we must be careful in describing what kind of grains we are measuring. In the face-centered cubic metals, we may observe so-called twin boundaries within the grains (see sidebar on grain types). Aluminum alloys, however, rarely exhibit twins. When twins are present, they are ignored if we are trying to define the grain size. However, if we are trying to establish a relationship between microstructure and properties, for example, strength, we must consider twin boundaries as they influence dislocation movement, just as grain boundaries do. Hence, we must recognize the intent of the work being performed.

 

In heat-treated steels, it is recognized that the grain size of the product of the heat treatment, usually martensite, is not measured or cannot be measured. For low-carbon steel, the martensite forms in packets within the parent austenite grains. In high-carbon martensites, we do not observe any convenient structural shape that can be measured. In most cases, we try to measure the size of the parent austenite grains that were formed during the high temperature hold during the heat treatment. This is usually referred to as the "prior-austenite grain size" and it has been widely correlated to the properties of heat treated steels. The most difficult process here is the etching procedure needed to reveal these prior boundaries. Sometimes they cannot be revealed, particularly in low-carbon steels. In this case, it may be possible to measure the low-carbon lath martensite packet size, which is a function of the prior-austenite grain size."

 

 

Ric

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Hi Richard,

 

Many thanks for this lead.

 

In heat-treated steels, it is recognized that the grain size of the product of the heat treatment, usually martensite, is not measured or cannot be measured. For low-carbon steel, the martensite forms in packets within the parent austenite grains. In high-carbon martensites, we do not observe any convenient structural shape that can be measured. In most cases, we try to measure the size of the parent austenite grains that were formed during the high temperature hold during the heat treatment. This is usually referred to as the "prior-austenite grain size" and it has been widely correlated to the properties of heat treated steels. The most difficult process here is the etching procedure needed to reveal these prior boundaries. Sometimes they cannot be revealed, particularly in low-carbon steels. In this case, it may be possible to measure the low-carbon lath martensite packet size, which is a function of the prior-austenite grain size."

 

I think that the above gives me a clue. As I read it, it does not say that martensite does not have a grain size, rather that it is difficult or impossible to measure.

 

Since I made my previous post, I looked in Google and found any number of references to martensitic grain size, but always without being specific as to what exactly is being measured or talked about.

 

Now, I am more inclined to ask what happens to the austenitic grain boundaries after martensitic transformation has occurred, the "prior-austenite grain size" mentioned in that quote - Even more so, if retained austenite is present. Of course, it is also possible that in practical parlance, "martensitic grain size" refers either to the size of the lpate, packets, or the prior austenitic grain size.

 

Cheers

John

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Hi Richard,

 

Many thanks for this lead.

 

 

 

I think that the above gives me a clue. As I read it, it does not say that martensite does not have a grain size, rather that it is difficult or impossible to measure.

 

Since I made my previous post, I looked in Google and found any number of references to martensitic grain size, but always without being specific as to what exactly is being measured or talked about.

 

Now, I am more inclined to ask what happens to the austenitic grain boundaries after martensitic transformation has occurred, the "prior-austenite grain size" mentioned in that quote - Even more so, if retained austenite is present. Of course, it is also possible that in practical parlance, "martensitic grain size" refers either to the size of the lpate, packets, or the prior austenitic grain size.

 

Cheers

John

 

John,

I think what you will find is that martensite will be described as a structure withing prior austenite rather than the grain size of its own. As it is less dense and is actually a distrotion of the prior austenite it takes up more space and displaces any prior austenite grains as it forms. In doing so it may be difficut to see the past austenite grian size.

as to retianed austenite...this is where you can determine past grins size I would think.

Also there may be intermatalics and carbides which can form along past grain boundaries.

 

However,

I do not think marteniste has a grain size per se, but rather is a crystaine structure which forms withing austenite grains.

 

Ric

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John,

 

I am not a metallurgist but, the way that I keep it straight in my mind, not necessarily correct, just what works for me. The steel exists as an array of crystals of either body (pearlite & cementite) or face centered cubes (austenite & martensite). Crystals are an orderly array of atoms hooked together into a kind of three dimensional lattice. The crystals form in the steel and are all jumbled together as though someone packed sand into a container. The size of the crystals that form are what is refered to as "grain" i.e. a fine grained steel is composed of many, small crystals while coarse grained steel would contain fewer, and larger, crystals. "Grain" is a poor choice of word because it is so open to misinterpretation.

 

~Bruce~

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We do the best we can with the terms that have evolved (or we inherited) and "grain" is one of them. It can and does mean different things in different contexts. All meaningful metallurgical discussions I have ever had (or read) refer to "prior austenite grain size", and you go from there.

 

I understand the desire to get a definitive clarification on this, but I am not sure that there is one.

 

In general, the smaller the prior austenite grain size was, the tougher (less prone to fracture in impact) the martensite formed from it will be. It does not necessarily translate into the best cutting edge all the time though. As with all things steel "it depends" on what the application is, what sort of structure is optimal, and I do not think that we yet have all the answers. The devil is in the details. ;)

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Hi Howard,

 

I have ever had (or read) refer to "prior austenite grain size", and you go from there.

 

Indeed, but the question still remains why? Is is because the prior austenite grain boundaries remain, albeit invisible to normal etchants? I have heard of etching processes that are supposed to reveal it in martensite. But even so, is it the prior grain boundary that is revealed, or remaining impurities that collected there during austenization?

 

And what is that "graininess" that we see when we break a quenched martensitic bar that has been overheated during austenization? Does the fracture follow the prior austenite grain boundaries? And if so, why? Is it because the boundary is still there, or is it because impurities collected there act on the same way as say, perforations on a postal stamp?

 

To my mind there is another question that we must answer first: Can austenite laths/plates shoot across austenitic grain boundaries as they form? If not then the grain boundaries must remain as demarcators of the newly formed areas of martensite. But if on the other hand martensite plates can traverse grain boundaries, then it is a whole new ball game.

 

I think that we need the input of a metallurgist with a strong grounding in solid state physics.

 

 

Hi Norris,

 

....a fine grained steel is composed of many, small crystals while coarse grained steel would contain fewer, and larger, crystals. "Grain" is a poor choice of word because it is so open to misinterpretation

 

 

Decades ago I did a short course on basic metallurgy, before venturing onto other completely unrelated studies. From what I remember, the basic unit crystal lattice size is a constant for steel. Grain boundaries form where crystal lattice groupings of different orientation collide. The grain size in turn is determined by the numbers of rows that can grow with identical orientation. But not being a metallurgist, I stand to be corrected.

 

Cheers

John

Edited by Harper John

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John,

 

I am not a metallurgist but, the way that I keep it straight in my mind, not necessarily correct, just what works for me. The steel exists as an array of crystals of either body (pearlite & cementite) or face centered cubes (austenite & martensite). Crystals are an orderly array of atoms hooked together into a kind of three dimensional lattice. The crystals form in the steel and are all jumbled together as though someone packed sand into a container. The size of the crystals that form are what is refered to as "grain" i.e. a fine grained steel is composed of many, small crystals while coarse grained steel would contain fewer, and larger, crystals. "Grain" is a poor choice of word because it is so open to misinterpretation.

 

~Bruce~

 

 

Bruce,

Martensite is body centered tetragonal.....not face centered.

 

Ric

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And what is that "graininess" that we see when we break a quenched martensitic bar that has been overheated during austenization? Does the fracture follow the prior austenite grain boundaries? And if so, why? Is it because the boundary is still there, or is it because impurities collected there act on the same way as say, perforations on a postal stamp?

 

To my mind there is another question that we must answer first: Can austenite laths/plates shoot across austenitic grain boundaries as they form? If not then the grain boundaries must remain as demarcators of the newly formed areas of martensite. But if on the other hand martensite plates can traverse grain boundaries, then it is a whole new ball game.

 

I think that we need the input of a metallurgist with a strong grounding in solid state physics.

Cheers

John

 

John,

Most fractures happen at/along grain boundaries because the energy levels are less between grains then within grains. BUT again we use the term grain when it does not really apply, but rather it is the common usage (much like folk talk about tempering when they really mean quenching).

 

As to martensite (I think you meant that rather than austenite in the "answer first" above) shooting across austenitic grain boundaries...yes it does to a degree because it is less dense and the distortion it takes when it forms moves it outside the austnitic lattice........see Krause's book on the "metallurgy of steels".

 

Ric

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The causes for the fracture following the prior austenite grain boundaries can vary a great deal. "It depends" on the chemistry of the steel, and precisely what happened to it (it's thermal history, and in some cases, it's mechanical history as well). In modern practice liquid commercial steels that we are all most familiar with, the carbon and alloy content play a larger role in determining the path of the fracture than the mechanical history. With more "traditional" methods, the mechanics play a larger role. Which alloying elements are present, and which impurities (unwanted alloying elements, notably P & S, but others qualify sometimes).

 

Many things can and do collect at the austenite grain boundaries. Much of the time that is where the fracture plane lies, to the best of my knowledge.

 

I have always been fascinated with the fracture surface, and failure analysis is a favorite mind game, when I get the chance. Lots of good stuff done in the 50's by IHC and other machinery producers in this area. Not much of it has to do with high carbon levels like we use in blades, however.

 

I don't know any further how to answer the question. :unsure:

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Hi Howard and Richard,

 

Thank you all for your explanations.

 

I found this links that gives a good description of martensitic microstructure (pgs14-15): file:///C:/papers/Martensite_5.ppt#306,15,Microstructures

 

More food for thought.

 

Cheers

John

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Bruce,

Martensite is body centered tetragonal.....not face centered.

 

Ric

I do not want to mislead anyone here and apologize for my error.

 

Ric,

 

Hopefully I can learn something here. When the steel heats up and reaches austenitic, it goes through a phase change moving from a body-centered (one carbon atom in center of iron box) to face-centered (6 carbon atoms, one in center of each face of the iron box) structure. Correct? It is my understanding that when the steel is quenched, it "freezes" the face centered structure but, under strain and therefore distorted into a tetragonal form instead of a cube. It seemed to make sense to me that the increase in volume of martensite vs. non-martensite comes from the uptake of carbon from the grain boundaries and into the iron/carbon matrix. If my understanding is lacking in some way please help me understand!

 

~Bruce~

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Bruce, this board is for learning and the vast majority of people are very tolerant and incouraging to those of us who are still trying to put the pieces together to form an understanding of what is going on. When a piece of hot steel comprised of face centered crystals is quenched it can trap the crystals in that form if it happens fast enough. That is known as retained austinite. Retained austinite is not stable at "cool" temperatures and will eventually convert to untempered martinsite. That is, as was explained to me, why we do multiple temperings, to convert the retained austinite to untempered martinsite and then temper the fresh martinsite.

 

Another thing about austinite is that there is much more room between the iron atoms in the crystaline matrix to hold carbon atoms. I think the max is 77 points of carbon but all my reference material is still in storage so don't hold me to that, but it's close. The body centered iron crystal, ferrite, can only hold less than 2 points of carbon within it's matrix. When austinetic steel is quenched, some of this excess carbon is caught within the matrix of the body centered crystal elongating the structure and putting the atomic bonds between the iron atoms within the crystal under stress. This body centered crystal with the trapped carbon is martinsite and it's formation is good, bad, and indifferent. It is good in that it makes the steel hard. It is bad in that it can make to steel too hard. The atomic bonds can be so stressed by the retained carbon within it's structure that these bonds can break, as in the dreaded "tink", just sitting on the workbench. Tempering allows some of this carbon is go out of solution and reduce the stress on the atomic bonds. The movement of carbon within steel is a factor of time and temperature with temperature being the most significant, thus we search for that sweet temperature that will produce the desired amount of hardness for the intended application. The martinsite crystal is also larger than the ferrite crystal with the same number of iron atoms. That's why ferrite on one edge of the blade and martinsite on the other side can turn a straight blade into a curved blade, which can be good or bad depending upon whether or not you wanted this to happen and if the blade survives it happening.

 

I hope this helps. If I got something wrong, my appologies to all concerned. Someone will post the corrections and we both will learn.

 

Doug Lester

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Hi Harper,

As I understand things:

I would not call martensite a grain per se..its a body centered tetragolan crystal.

Lathe forms in lower carbon steels ..below about 0.6% carbon and plate forms mostly above 1%...the levels between can form a mixture...BUT this depends upon austenizing temp and chemistry as some elements can form carbides which can bind the carbon until very high temps are reached at which point that bond breaks and they will let go of the carbon...and unless you put the carbon into solid solution it can not form martensite in the first place.

Lathe is more needle like in appearance than plate.

Keep in mind that a single grain is composed of many cubic structures and therefore the body centered tetragonal martensite is not a single structure taking up the entirety of the grain.

It is also less dense and can act as a grain refiner on its own with multiple quenches.

 

There are a few new books out

G.B. Olson editor "Martensite" ASM 1992 is one

also there are a few technical articles out there.

 

Don Fogg had a video of martensite forming (got from Batson?) in a lab...interesting as you see nothing and then it is simply there...it forms very fast and to me it looked like it grew from one point. Maybe someone could put that on YOUTUBE?

 

I also recommend Vander Vort's metallographic work as he has some colored microscopy which shows interesting things.

 

I know Greg Olson. I am working with Questek on a couple of projects - like the special corrosion resistant steel for landing gear applications and one for wind turbine grades.....

 

I have more questions then you do....

 

Oddly enough Prof Geg Olson from Northwestern was in my shop a few weeks ago with four students and we were forging some of that special Questek Stainless that he designed.......if I had known I could have asked him your questions.

 

Too much to learn...

Ric

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Different types of Martensite

Scott,

Is the first photo some of Van der Vort's color microscopy? Some of the pictures of this type can be really interesting...I do not know what I am looking at most of the time, but its interesting.

 

Ric

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Hi,

 

Since my last post, I had the following explanation given:

 

Upon quenching the original austenitic grain boundaries remain but are not visible (under the microscope) because the usual etchant nital does not reveal them. Sometimes, if precipitates accumulate at the boundaries, the outline of the former austenitic grains can be seen.

 

Within the austenitic grains `packets' of martensite form. These packets are often incorrectly referred to as "grains" as martensite does not form true grains.

 

The smaller the austenitic grain size, the smaller the `packets' of martensite formed.

 

If a freshly quenched piece of steel is fractured and "granularity" is observed, as say when doing a triple quench to refine the "grain" size, what we see is one of these two:

 

- The outline of the said `packets' of martensite; And or

 

- if the steel was allowed to precipitate other constituents at the grain boundaries, then these having become the weakest regions, the fracture follows the former asutenite grain boundaries rather than the interface of the adjacent `packets' of martensite.

 

I cannot vouch for the veracity of this explanation as I have not been able to confirm it from independent sources in full, so I post it with some reservations, though it makes as much sense to me as anything else suggested.

 

Cheers

John

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FCC and BCC aren't as different as I first believed. Positions of atoms do change, but not as significantly as I thought when looking at classic diagrams of single cubes of each structure.

 

If you draw out 3D representation of a series of cubes of FCC on 2D paper, then you will notice that you can link up the atoms in an alternative manner to give what looks like BCC. In reality, these BCC wire frames are not exact cubes, but are stretched in one axis. Now, squash and pull these BCC cubes until they are true cubes - equal in all 3 axis, and you have achieved something similar to the transition from austenite to ferrite (without any single atom having to be moved very far).

 

It isn't like atoms make huge leaps from the middle of the cube to the face. Rather they adjust slightly, and then you can draw cubes in a different way, slightly offset from the originals.

 

Martensite is the half-way house, where the FCC is trying to get to BCC but ends up not quite making it. You can't make an even cube in all 3 axis by linking up atoms in the resulting structure. The best you can do is draw a slightly elongated cube (so metallurgists don't call it cubic!).

 

Hunt around in Cambridge university metallurgy website (do it! there are some excellent lectures that anyone can understand) and somewhere, maybe in some attached powerpoints, there are some simple diagrams that show this elegantly.

 

The martensitic transformation travels at the speed of sound. It's like a chain reaction that is just waiting for the first event to happen, or a shockwave. No single atom has to move very far - just like how sound travels through a material. I guess, the martensite plates are formed in a single go as a result of a single shockwave?

When there is carbon in the mix, the Ferrite transition cannot happen until carbon has diffused out the way - right out the way - not just the slight adjustment required in the iron atoms but a big migration of Carbon right out of the matrix. This is much slower.

 

As austenite cools, forces acting on the atoms change, and mount up until an atom changes position, and starts the chain reaction. Once some atoms change position, then some of the forces are relieved so there is not the same pressure for the remaining un-transfomed atoms to move. . . until the temperature drops again. This is why you get MS and MF and a range of martensite percentages in between.

 

So, if my understanding is correct, I would guess that there isn't a lot of breakthrough of martensite plates across austenite boundries - I can imagine some, but the boundary would surely be an obstacle.

And, retained austenite will retain the original austenite boundry.

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Hi Jhobson,

 

A very informative post.

 

So, if my understanding is correct, I would guess that there isn't a lot of breakthrough of martensite plates across austenite boundries - I can imagine some, but the boundary would surely be an obstacle.

 

 

Years ago, I saw a very elegant diagram that illustrated how martensite formed, by distortion of the crystal lattice, going from face centered cubic to body centered tetragonal - Wish I could find it somewhere...<sigh>

 

From what I understand, and someone please correct me if I got it wrong, grain boundaries form because crystal clusters (dendrites) of differing orientation clash. If this is correct, and am not saying that it is, then how could a single martensite plate form across a boundary, given that in such a plate the crystal orientation has to be the same?

 

Cheers

John

Edited by Harper John

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If this is correct, and am not saying that it is, then how could a single martensite plate form across a boundary, given that in such a plate the crystal orientation has to be the same?

I guess that was my point, but made a little more succinctly.

 

btw, dendrites are formed from solidification process. Grains can be formed from other processes.

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