Kevin R. Cashen
Super Moderator
In the other threads in this series I have covered the different kinds of edges one can have on a knife, all with a valid claim to the title of “sharp” despite, their very different modes of cut. I would be remiss, here in the Heat Treating forum, not to cover one of the most profound effects that heat treatment can bring about in the developments of these edges. In the thread Tempering for different kinds of "sharp" I touched on the effect that carbides can have on the edge, but in this thread I will expand on how we can affect those carbide themselves in heat treatment.
When we exceed levels of around .8% carbon in steel we also exceed the iron's ability to hold extra carbon in solution. This extra carbon will concentrate to form carbide. In simple carbon steel this will be basic iron carbide (Fe3C), but in alloyed steels more complex carbides can occur with elements such as Cr, V, W, and others. But there are some factors that are worth remembering when heat treating any steel.
Not only is there a percentage of carbon that when exceeded will form carbide, but there is also a percentage of carbon in solution that will set a limit on how hard the steel can get. Most of us when hardening steel are quite happy with 65HRC as-quenched, why is this? Why not 69 or 70? Well, for the simple reason that once you reach .6% carbon in solution you start leveling off as you approach the limits of hardness you can obtain from carbon and, by the time you reach that .8% mark, adding more to the solution will actually work against you and begin to lower the hardness. This is why grain growth is actually one of the lesser things to worry about from overheating, as there are a number of bad things that will happen long before the grains begin to enlarge, and the main one is over-saturation of the austenite solution.
But that is another topic, for now we want to just remember the fact that you can control hardness in the heat to harden, almost as much as with tempering, and that there is only so much of the available carbon that you need in order to get that 65HRC you hope for. But, what I would like to talk about is what you can do with any carbon in excess of that needed for hardness. The extra is used in carbides to give you added effects, both beneficial and detrimental. As previously mentioned in other threads, abrasion resistance and hardness are not synonymous, and can be quite independent of each other. Once maximum hardness is obtained, abrasion resistance can be added on top of it with the extra carbide. But how we form and condition that carbide will have profound effects on the kind of edge we can create and how it will perform.
By controlling our hardening heat we can affect how much carbon is put into solution, and some steels will now offer different hardening heats in their spec sheets, one for greater hardness and a lower one for greater toughness. Ideally, we will only put enough carbon into solution to achieve our desired hardness, and leave the rest in well conditioned carbides. But what is “well conditioned?” For stable and consistent edge properties, finer carbides with an even distribution is desirable. This will allow you to get the finest of polished edges that will resist wear very uniformly. But what if we fall short of this condition?
Improper normalizing, poor annealing, and heavy cycling at undesirable temperatures, can result in larger, or segregated, carbide formations. This will have noticeable effects on the cutting edge. Fine polished edges many be difficult, to impossible, and hardness deviations of 20 points or more, microns apart, along the edge will drastically change how that edge breaks down in use.
Alloy banding or, in some steels, dendritic structures will provide areas of carbide concentration that will change edge performance, not necessarily for the better, but decidedly different. In the most extreme examples you can actually have bands of carbide so rich that it has robbed the surrounding material of maximum hardness, which can smear, wear, or even tear out in use. Such a blade will also cut soft fibrous materials quite aggressively, similar to serration, but it would be less than accurate to say it is holding a stable edge, and its cutting abilities would quickly wane on harder materials.
However, in heat treatment it may be a bit too late in the process to deal with carbides during the hardening operation. In its ability to control and condition carbide, normalizing is the greatest tool we have and is key to setting ourselves up for success in all subsequent operations. Normalizing has the ability to dissolve and redistribute carbide and determine how it will influence the final knife edge performance. It also has the ability to undo detrimental carbide conditions left from improper forging or annealing operations.
More than once I have had makers come to me desperate to know why the edge of their 1095 blades will not get sharp or stay sharp. Lab examination revealed the tell-tale microchipping effect. Their grain may even be quite good and the blade not overly hard, and yet the edge is plagued by brittle micro-divots. Metallography of the cross section quickly reveals that at some point, before hardening, all of the steels abundant carbon was put into solution and allowed to cool slow enough to gather it in the grain boundaries. Under the pressure of edge use, each grain, no matter how fine, is waiting to become a void in the edge as the brittle frame holding it in place lets go. This is just one condition that is quickly fixed with proper normalizing.
When we exceed levels of around .8% carbon in steel we also exceed the iron's ability to hold extra carbon in solution. This extra carbon will concentrate to form carbide. In simple carbon steel this will be basic iron carbide (Fe3C), but in alloyed steels more complex carbides can occur with elements such as Cr, V, W, and others. But there are some factors that are worth remembering when heat treating any steel.
Not only is there a percentage of carbon that when exceeded will form carbide, but there is also a percentage of carbon in solution that will set a limit on how hard the steel can get. Most of us when hardening steel are quite happy with 65HRC as-quenched, why is this? Why not 69 or 70? Well, for the simple reason that once you reach .6% carbon in solution you start leveling off as you approach the limits of hardness you can obtain from carbon and, by the time you reach that .8% mark, adding more to the solution will actually work against you and begin to lower the hardness. This is why grain growth is actually one of the lesser things to worry about from overheating, as there are a number of bad things that will happen long before the grains begin to enlarge, and the main one is over-saturation of the austenite solution.
But that is another topic, for now we want to just remember the fact that you can control hardness in the heat to harden, almost as much as with tempering, and that there is only so much of the available carbon that you need in order to get that 65HRC you hope for. But, what I would like to talk about is what you can do with any carbon in excess of that needed for hardness. The extra is used in carbides to give you added effects, both beneficial and detrimental. As previously mentioned in other threads, abrasion resistance and hardness are not synonymous, and can be quite independent of each other. Once maximum hardness is obtained, abrasion resistance can be added on top of it with the extra carbide. But how we form and condition that carbide will have profound effects on the kind of edge we can create and how it will perform.
By controlling our hardening heat we can affect how much carbon is put into solution, and some steels will now offer different hardening heats in their spec sheets, one for greater hardness and a lower one for greater toughness. Ideally, we will only put enough carbon into solution to achieve our desired hardness, and leave the rest in well conditioned carbides. But what is “well conditioned?” For stable and consistent edge properties, finer carbides with an even distribution is desirable. This will allow you to get the finest of polished edges that will resist wear very uniformly. But what if we fall short of this condition?
Improper normalizing, poor annealing, and heavy cycling at undesirable temperatures, can result in larger, or segregated, carbide formations. This will have noticeable effects on the cutting edge. Fine polished edges many be difficult, to impossible, and hardness deviations of 20 points or more, microns apart, along the edge will drastically change how that edge breaks down in use.
Alloy banding or, in some steels, dendritic structures will provide areas of carbide concentration that will change edge performance, not necessarily for the better, but decidedly different. In the most extreme examples you can actually have bands of carbide so rich that it has robbed the surrounding material of maximum hardness, which can smear, wear, or even tear out in use. Such a blade will also cut soft fibrous materials quite aggressively, similar to serration, but it would be less than accurate to say it is holding a stable edge, and its cutting abilities would quickly wane on harder materials.
However, in heat treatment it may be a bit too late in the process to deal with carbides during the hardening operation. In its ability to control and condition carbide, normalizing is the greatest tool we have and is key to setting ourselves up for success in all subsequent operations. Normalizing has the ability to dissolve and redistribute carbide and determine how it will influence the final knife edge performance. It also has the ability to undo detrimental carbide conditions left from improper forging or annealing operations.
More than once I have had makers come to me desperate to know why the edge of their 1095 blades will not get sharp or stay sharp. Lab examination revealed the tell-tale microchipping effect. Their grain may even be quite good and the blade not overly hard, and yet the edge is plagued by brittle micro-divots. Metallography of the cross section quickly reveals that at some point, before hardening, all of the steels abundant carbon was put into solution and allowed to cool slow enough to gather it in the grain boundaries. Under the pressure of edge use, each grain, no matter how fine, is waiting to become a void in the edge as the brittle frame holding it in place lets go. This is just one condition that is quickly fixed with proper normalizing.
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