This is a repost of a thread I started on some other forum. I've seen some of the same confusion here, so I thought I'd post it. I'll check and make sure the links work.
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. Kevin Cashen, ABS Master Smith. 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 and clusters 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 20-40 microns across.
http://www.metalravne.com/selector/help/testing/lm.html
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 Cashen 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.
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. Kevin Cashen, ABS Master Smith. 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 and clusters 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 20-40 microns across.
http://www.metalravne.com/selector/help/testing/lm.html
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 Cashen 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.
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