Grains, Carbides, and You

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me2

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I've noticed recently what seems to be a fair amount of confusion between carbides and grains. In some instances the terms get used interchangeably and, to me at least, this creates considerable confusion, as I use specific definitions for each, and swapping them around when talking about them gives me a head ache. So, for my benefit, and that of others who may be trying to read some metallurgical/materials science texts to expand their knowledge, 2 unofficial definitions, followed by hopefully easy to grasp analogies.

Grains: these are the individual crystals (yes, metals are typically crystalline) of the metal. All the atoms stack in repeated patterns with the same orientation. For room temperature iron, the pattern is that of many small cubes stacked over and over, though not just simple cubes. Each cube has an iron atom in it's center, so it's like a box with a surprise iron atom inside. The grains will fit and stick together, similar to the individual little beads in styrefoam, or like bubbles in soapy water. However, each grain is oriented a little differently from it's neighbors, and the border between the 2 is the grain boundary.

Linked below, in the second micrograph next to the paragraph labeled cementite, is a picture of 1095. The grains are clearly visible, outlined in white for us by Mr. Cashen. This is annealed 1095, cooled very slowly from high temperature.

http://www.cashenblades.com/metallurgy.html


Carbides: these are the combinations of iron, or other elements, bonded to carbon in the steel. These essentially are ceramic particles in the steel matrix. They have various sizes, crystal structures, and characteristics. They are similar to the fruit pieces mixed in Jello, where the Jello is the steel matrix and the fruit pieces are carbides.

The following link shows a micrograph of D2. The carbides are the clear irregular shapes. Note some of the carbides are roughly 20 microns in size (slightly less than 0.001". A sharp edge is between 20 and 100 times thinner, which illustrates the issues with large carbides, low edge angles, and high sharpness. The grains are visible as well, though less distinct. They are roughly 30-50 microns across.

http://www.google.com/imgres?hl=en&...0&ndsp=19&ved=1t:429,r:1,s:0,i:88&tx=97&ty=76

I'd like to put in a work here about carbide volume, as it is another term I've seen cause some confusion. Carbide volume is expressed as a percentage, and indicates how much total carbide is present in a give amount of steel. So, in a steel with 5% carbide volume, a 100 cubic inch sample would have 5 cubic inches of carbide in it. This becomes important to understand when one deals with the CPM steel. These typically have a high carbide volume. However, one of the advantages of the CPM process is to break up and distribute the carbides. There is the same amount of carbide vs. a non-CPM version of the steel, they're just smaller and more evenly spread out. Picture the Jello and fruit example. There is one can of fruit in 5 cans of Jello. Now take all the fruit out and dice it into 1/8" cubes and put it back. The same amount of fruit is present, it's just smaller pieces and, hopefully, not a single spoonful of Jello will be without fruit, because it is more evenly spread throughout the Jello.

Great effort is put into making these 2 things as small as possible. Some blade smiths go to great effort to reduce the grains to as small as possible. Others go to great effort to make the carbides as small as possible. These 2 things are somewhat at odds, as higher temperatures reduce carbide size by dissolving them, similar to higher temperature allowing water to dissolve more salt. However, higher temperatures make grains bigger, so a balance must be reached.

I would like to repeat something Kevin has said many times, because it bears repeating. Control of the carbide size and location is at least as important as control of grain size. Put the carbides in the wrong place and forget to move them, and I don't think it's an exageration to say you can get a piece of 1095 or 52100 with a hardness in the mid 20's HRc with the impact toughness of a piece at 60 or greater, just a couple dozen foot pounds, a small fraction of what it should be at that hardness.

The picture of Kevin's 1095 above illustrates this condition. Unfortunately, the white outlines, while convenient, are cementite (iron carbide). Though this is annealed, this structure will result in relatively brittle, yet very soft steel. Excellent for illustrating grains, but not for making knives. In this picture, all the grain boundary carbide should be removed by later heat treatments for the best blade results.


So, to recap, carbides and their sizes are different from grains and their sizes. They are related, but are not the same. Large grains with extremely small carbides are possible, as are very fine grains with large and inconveniently located carbides. Neither is desirable, but the latter is potentially much worse, IME. Fortunately, the size of both is pretty easily controled with good heat control and an understanding of what you want to achieve.
 
Indeed. This is all a source of confusion. I know Kevin and many others have discussed it here and elsewhere, but it's good to bring it up to get the old timers to think about it and clear things up for the new members. I was mainly thinking of one or 2 times I've seen posts about sharpening that said something to the effect of you need as fine a carbide size as possible for a sharp edge, so be sure to do the grain refining properly. It sure sounded like the two were being used interchangably, but maybe I just misread the post.
 
The confusion between the crystalline grain and the anisotropic grain flow seems to be very wide spread and has lead to some very heated arguments pertaining to "grain refinement". There still seems to be a great deal of unnecessary debate and controversy on this and how the anisotropic grain may apply to knifemaking from a structural standpoint.

It takes a good deal of basic foundational understanding to decipher much of the literature surrounding these topics,... and even then, it still may be a little bit confusing.

Example:

http://www.tunersgroup.com/forging.html
 
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Yep, and even when one tries to read something like that, there is a level of understanding necessary before reading it. In any case, the above should help for anyone having trouble visualizing grains and carbides. A lot of my reason for posting this is there is a great deal of emphasis on grains size and refinement, which is important, but much less is mentioned on carbide distribution, and refinement. Carbide size is discussed often, but it gets mashed in with grain size and sometimes gets jumbled.
 
I've never seen photos of air bubbles in a piece of steel before. I don't think that happens too often with rolled steel.
 
I've never seen photos of air bubbles in a piece of steel before. I don't think that happens too often with rolled steel.

It just so happens that I have some:

This is a 1-½ inch square sample 1/8 inch thick of average quality 1095, I etched in sodium bisulfate.

This pic shows the top grain. The scratches from sanding run perpendicular to the direction of the grain flow created by the rolling process, diagonally (in this pic) left to right. The direction of the grain flow runs diagonally top to bottom.

DSCN6170.jpg


This pic shows the end grain.

DSCN6174.jpg


This pic shows the side or edge grain.

DSCN6175.jpg


This pic shows all three.

DSCN6171.jpg


This clearly illustrates the anisotropy of the steel,... strongest against the top grain, weakest against the end grain, and in between the two from the side grain.

It also shows that the different surfaces have differing degrees of corrosion resistance.

I think it is obvious that this steel wouldn’t be the best choice for a stock reduction knife, and a “cleaner" steel would be more appropriate for that. However, with proper forging this piece could be dramatically improved and comparable, by pushing the side grain across the point of a blade (or the end grain) and squeezing the top grain down towards the edge so that it runs more parallel to the bevel, also refining the grain along the edge.

This is not to say though, that the so called “clean” steels couldn’t benefit from forging closely to shape. They can and the same basic principals apply. The problem is that most of your high alloy, air hardening, corrosion resistant steels or stainless steels etc., tend to be “hot hard” and “red short”,… and don’t lend themselves well to the forging process in your average type blacksmith shop set up. However, they are forged with modern high tech equipment in the aerospace industry, because of the higher strength to weight ratio that proper forging offers.
 
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What does that all have to do with explaining the difference between carbides and grains? I see a new thread coming.
 
Tai, I could definitely see why forging that piece would definitely improve the quality of the blade that was produced from it.
Thank you for posting.
 
I think the pores and flaws may be slightly exaggerated by the etching, but it does serve as a useful illustration.
 
Sorry me2,... just adding some info., about another aspect of the "grain" and source of confusion.
 
If you want to see air bubbles in steel try some home brewed crucible or bloom steel that doesn't have any deoxidisers in it. I've seen pictures of crucible steel that was so porous, big air bubbles, that the maker had to scrap it after doing all the work necessary to make it.

Another point of confusion is that often you read about cementite and carbides when cementite is a carbide. It's just a bit of a special case because it is an iron based crystal like the matrix of the steel and carbides of other metals are things added to the mix, so to speak. It's a bit like discussing cementite, ferrite, and pearlite when pearlite is formed from cementite and ferrite. They're the same thing, in a way, but at the same time different.

What it all boils down to is the necessity of knowing the alloy that you are dealing with.

Doug
 
… about the same time you think you understand it,… is about the same time you should realize, you don’t have a clue! LOL

Good thread...
 
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I need an asprin. But this does make for good reading. I admire you guys for your depth of knowledge.
 
It’s really just a boat load of simple, basic, well established material/metallurgical "facts" put together. However, once we get to that point and think we understand it,.. It should inspire us to ask better questions, become even more curious.

“How might this all apply to knifemaking", and help us make "better" knives, etc.?

Lots of possibilities...
 
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Confusing? I still have to read over my replies when it comes to heat treating to make sure that my answer matches the temperatures in use and what steel alloy is being discussed.

The latest that I ran into had to do with the standard advice that hypoeutectic steel only needs to be heated long enough to to make sure it's at the same temperature throughout before quenching. Then someone asked about 6150 and someone else who had used it said that it needed a ten minute soak at temperature. Then I ran across data for S5, which has about 0.6% carbon and is somewhat close to 6150, and it was recommended that it be give a minimum of a 30 minute soak, even for less than 1" thickness. Also the austinizing temperature had a big effect on as quenched hardness as well as whether or not it's quenched on oil or water. These general rules that we run into regarding steel selection and heat treating only take you so far. There is no substitute for knowing the alloy that you are using and the actual assay is nice to know too.

Doug
 
Good stuff, me2! As far as disolving carbides, you mention a higher heat. Would longer soak times at austinizing temp help shrink carbide size, without a major effect on grain growth?

Zeb
 
I dont think so. For a given temperature, there is a limit to the amount of carbon the austenite can hold. Once that limit is reached, the carbides won't dissolve any more, as the carbon doesnt have anywhere to go. It depends on the type of carbide. Also, longer soak times at proper temperature generally wont cause grain growth issues. You've probably seen Kevin ' s pictures of O1 soaked for hours in an oven with no grain growth issues. There were issues, they just weren't grain growth.

This seems like a good time to bring up steels with carbides that wont dissolve. The highly alloyed steels will have carbides that form from liquid. These are called ledeburitic steels. This happens in plain carbon steels too, but with that much carbon they become cast irons.
 
What does that all have to do with explaining the difference between carbides and grains? I see a new thread coming.

Very good point me2, I have noticed several discussion lately inexplicably pulled into this same forging to shape thing. While it is any body's privilege to offer their thoughts as they see it relating to the thread topic, and that is an area of personal discretion, to a point, I would point out that this is the heat treating forum. There is another forum on Knifedogs called "Hot Metal" devoted to the forging of steel and its effects where not only would this reoccurring topic be most appropriate it would also get the most attention and deserved discussion.
 
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