03-14-2018, 08:28 PM
Now its getting good!
The Mohs scale is, imho, too imprecise to use in a discussion of knife materials. Vanadium carbide is harder than aluminum oxide and silicon carbide. However, cubic boron nitride and diamond are harder than vanadium carbide. The abrasives will cut the material around them, and will abrade the carbides to some degree, but not efficiently. The carbides can be cut efficiently with cbn and diamond. However, diamond has a drawback. It doesn't work well at high speed. Uncooled diamond on steel will degrade by diffusion into the steel.
Verhoevens book has many pictures of steels and carbides as examples. Sizes range from sub micron to 50 microns or more in the various steels. CPM/powder processed steels have carbides in the 3 to 6 microns. Carbide type plays a role in size, but processing is the largest influence. Chromium carbides are generally regarded as larger than vanadium carbides. That is not true, particularly when they are in the same alloy. Its widely believed because molecules of Cr23C6 and Cr7C3 are bigger than VC. However, carbides are rarely molecular in size.
Some steels in the heat treated condition have no carbides. I have to elaborate now. There are what are known as primary carbides or undissolved carbides. These are present in the steel even when heated to the hardening temperature, thus the undissolved designation. These are the type of carbides most often discussed. They make the biggest difference in wear resistance.
Then there are tempering carbides. These are extremely small, single digit nanometer size in some cases and dimensions. They can also be extremely hard, with tungsten carbide tempering carbides harder than vanadium carbide. These form when quenched steel is tempered. Tempering carbides are pretty much unavoidable. They are generally not considered for wear resistance, as the volume % is fairly small in knives and the small size reduces their effectiveness for such purposes.
Carbide volume % varies widely as well, depending on steel. Steels with less than about 0.6 weight % carbon will not have much if any undissolved carbides when heat treated. Low alloy and plain carbon steels above 0.6 wt % carbon have increasing amounts as carbon increases, up to 20 volume % or a bit more. Common CPM/powder steels will have about 15% by volume, give or take a point or two. Some will go higher, up to 20-25 % in some steels. These are on the border of what is practical for knives.
Wall of text to be continued.
The Mohs scale is, imho, too imprecise to use in a discussion of knife materials. Vanadium carbide is harder than aluminum oxide and silicon carbide. However, cubic boron nitride and diamond are harder than vanadium carbide. The abrasives will cut the material around them, and will abrade the carbides to some degree, but not efficiently. The carbides can be cut efficiently with cbn and diamond. However, diamond has a drawback. It doesn't work well at high speed. Uncooled diamond on steel will degrade by diffusion into the steel.
Verhoevens book has many pictures of steels and carbides as examples. Sizes range from sub micron to 50 microns or more in the various steels. CPM/powder processed steels have carbides in the 3 to 6 microns. Carbide type plays a role in size, but processing is the largest influence. Chromium carbides are generally regarded as larger than vanadium carbides. That is not true, particularly when they are in the same alloy. Its widely believed because molecules of Cr23C6 and Cr7C3 are bigger than VC. However, carbides are rarely molecular in size.
Some steels in the heat treated condition have no carbides. I have to elaborate now. There are what are known as primary carbides or undissolved carbides. These are present in the steel even when heated to the hardening temperature, thus the undissolved designation. These are the type of carbides most often discussed. They make the biggest difference in wear resistance.
Then there are tempering carbides. These are extremely small, single digit nanometer size in some cases and dimensions. They can also be extremely hard, with tungsten carbide tempering carbides harder than vanadium carbide. These form when quenched steel is tempered. Tempering carbides are pretty much unavoidable. They are generally not considered for wear resistance, as the volume % is fairly small in knives and the small size reduces their effectiveness for such purposes.
Carbide volume % varies widely as well, depending on steel. Steels with less than about 0.6 weight % carbon will not have much if any undissolved carbides when heat treated. Low alloy and plain carbon steels above 0.6 wt % carbon have increasing amounts as carbon increases, up to 20 volume % or a bit more. Common CPM/powder steels will have about 15% by volume, give or take a point or two. Some will go higher, up to 20-25 % in some steels. These are on the border of what is practical for knives.
Wall of text to be continued.