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- Sep 9, 2003
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A couple of weeks ago we had a discussion involving heating and soaking and dissolution of carbides or lack thereof in 5160. In that thread I posted a picture to help explain things that was mislabeled and I actually did not have proper permission to publish images of that steel, although I do the metallography I never use material gathered from other peoples products without complete clearance to do so, so I had to remove that image from my server and thus from the thread.
However, I actually have much better examples of what we were discussing in my files, I just needed time to find them and discuss things. Now that I have returned from a rather involved presentation I had to give this weekend, I have the time to share some more with you and address those issues again in a clearer way and cover many topics that have been touched on since. And this time with images that are free and clear.
The people who make the steel we work with thoroughly work out the best ways to get the most potential out of them. As bladesmiths we trust them to make our steel yet often seem to doubt they know anything about treating and working it. The assumption that just because a piece of steel is in the shape of a blade that none of the properties or principles apply is erroneous at best and downright arrogantly blind at worst. Often the reason that later numbers on the spec sheets dont apply is because we didnt pay any attention to the previous numbers. We totally ignore the recommended heat and soak times and then try to say that the books are wrong when we get different results and the tempering temperatures dont match up later on.
One cant pick and choose which recommended procedures they are going to follow or ignore and expect to have all the results match up.
Lets start with 5160 again. If done the way the spec sheets recommend we will heat to 1525F and soak for 1 hour per inch of thickness. Now I will dispense with any discussion about the nonsense that this could cause grain growth since we have put enough nails in that coffin allow it to rest in peace, anybody new to this and needs convincing can do a search on the topic and find all the old opinions and assumptions blown apart with undeniable facts.
The reason we need to go higher with 5160, than say 1095, is mostly due to the fact that is has less than .8% carbon which means will consist of 75% pearlite (finely segregated iron and carbon) grains, and 25% ferrite (low carbon iron) grains. When we heat this to the critical temperature (there are formulas to account for the alloying that will give you around 1370F for A1 in 5160), the pearlite will begin to dissolve, fairly quickly but not certainly not the instantaneous change the Iron carbon equilibrium diagram would theoretically suggest. But anyhow the pearlites time is now over and we need to worry about that darned ferrite (low carbon iron). That is what the upper critical temperature (Ac3) line is all about. From Ac1 to Ac3 we are putting more and more carbon into that iron to make austenite out of it. When we cross Ac3 we will have accomplished this, but there will still be plenty of carbon trapped in carbides to pull into the mix if we want good hard steel (martensite) when we quench. Yes carbides! Even in 5160, because that pearlite was made up of ferrite and cementite (iron-carbide) with a good amount of chromium carbides thrown into the mix. In 1095 you can make all austenite and leave many of those carbides in solution because with .95% carbon you have more than enough to spare and dissolving even more could even be a problem! But with 5160 you are right on the threshold of maximum hardness to begin with leaving too much undissolved carbide will result in very wimpy martensite. Thus you need to go to 1525F and hold it there to let the heat do its job if you want to get the most out of the steel.
To those who just dont want these facts to mess with their comfortable beliefs my word for it should not be enough, for anybody for that matter, so I present the following:
This is 5160 lamellar annealed (what you get from heating to critical and stuffing in vermiculite, wood ash or your forge) and then cycled several times a low heat (just under or just at nonmagnetic) and then heated to 1414F (non magnetic) for 5 minutes and quenched.
Here you will see very wimpy martensite (low carbon) that is in the areas that managed to go into solution. If you look you can actually see the interface where the pearlite lamellae dissolved together and formed austenite solution. But more interestingly you will still see the ghostly remnants of the lamellae in the form of lines of individual undissolved carbides, looking like stings of black beads in the martensite.
This sample took considerable effort to break, but snapped as if it were fully hard. It cut neat groves down the side of, and ruined, a brand new Nicholson file in just two high pitched passes. Anybody how has ever done a file test would have passed the steel with flying colors. This past weekend some the top names in bladesmithing passed it around an skated a file on it, before I revealed that it was only 47 Rockwell C.
One of my greatest frustrations is the old well it works fine for me line when it is used to deny heat treating facts. My question has always been how do you know it works fine and too often the standard answer has been it skates a file so I know it got hard.
Here is another image of the pearlite being broken down into spheroidal structures before going into solution.
However, I actually have much better examples of what we were discussing in my files, I just needed time to find them and discuss things. Now that I have returned from a rather involved presentation I had to give this weekend, I have the time to share some more with you and address those issues again in a clearer way and cover many topics that have been touched on since. And this time with images that are free and clear.
The people who make the steel we work with thoroughly work out the best ways to get the most potential out of them. As bladesmiths we trust them to make our steel yet often seem to doubt they know anything about treating and working it. The assumption that just because a piece of steel is in the shape of a blade that none of the properties or principles apply is erroneous at best and downright arrogantly blind at worst. Often the reason that later numbers on the spec sheets dont apply is because we didnt pay any attention to the previous numbers. We totally ignore the recommended heat and soak times and then try to say that the books are wrong when we get different results and the tempering temperatures dont match up later on.
One cant pick and choose which recommended procedures they are going to follow or ignore and expect to have all the results match up. Lets start with 5160 again. If done the way the spec sheets recommend we will heat to 1525F and soak for 1 hour per inch of thickness. Now I will dispense with any discussion about the nonsense that this could cause grain growth since we have put enough nails in that coffin allow it to rest in peace, anybody new to this and needs convincing can do a search on the topic and find all the old opinions and assumptions blown apart with undeniable facts.
The reason we need to go higher with 5160, than say 1095, is mostly due to the fact that is has less than .8% carbon which means will consist of 75% pearlite (finely segregated iron and carbon) grains, and 25% ferrite (low carbon iron) grains. When we heat this to the critical temperature (there are formulas to account for the alloying that will give you around 1370F for A1 in 5160), the pearlite will begin to dissolve, fairly quickly but not certainly not the instantaneous change the Iron carbon equilibrium diagram would theoretically suggest. But anyhow the pearlites time is now over and we need to worry about that darned ferrite (low carbon iron). That is what the upper critical temperature (Ac3) line is all about. From Ac1 to Ac3 we are putting more and more carbon into that iron to make austenite out of it. When we cross Ac3 we will have accomplished this, but there will still be plenty of carbon trapped in carbides to pull into the mix if we want good hard steel (martensite) when we quench. Yes carbides! Even in 5160, because that pearlite was made up of ferrite and cementite (iron-carbide) with a good amount of chromium carbides thrown into the mix. In 1095 you can make all austenite and leave many of those carbides in solution because with .95% carbon you have more than enough to spare and dissolving even more could even be a problem! But with 5160 you are right on the threshold of maximum hardness to begin with leaving too much undissolved carbide will result in very wimpy martensite. Thus you need to go to 1525F and hold it there to let the heat do its job if you want to get the most out of the steel.
To those who just dont want these facts to mess with their comfortable beliefs my word for it should not be enough, for anybody for that matter, so I present the following:
This is 5160 lamellar annealed (what you get from heating to critical and stuffing in vermiculite, wood ash or your forge) and then cycled several times a low heat (just under or just at nonmagnetic) and then heated to 1414F (non magnetic) for 5 minutes and quenched.
Here you will see very wimpy martensite (low carbon) that is in the areas that managed to go into solution. If you look you can actually see the interface where the pearlite lamellae dissolved together and formed austenite solution. But more interestingly you will still see the ghostly remnants of the lamellae in the form of lines of individual undissolved carbides, looking like stings of black beads in the martensite.
This sample took considerable effort to break, but snapped as if it were fully hard. It cut neat groves down the side of, and ruined, a brand new Nicholson file in just two high pitched passes. Anybody how has ever done a file test would have passed the steel with flying colors. This past weekend some the top names in bladesmithing passed it around an skated a file on it, before I revealed that it was only 47 Rockwell C.
One of my greatest frustrations is the old well it works fine for me line when it is used to deny heat treating facts. My question has always been how do you know it works fine and too often the standard answer has been it skates a file so I know it got hard.
Here is another image of the pearlite being broken down into spheroidal structures before going into solution.
