12-05-2017, 07:52 PM
Mr. Jan, do you have any references for the size effect you mention above? I have not heard it discussed in terms of dislocation pile up on the interior grain boundaries. In relation to the burr, I'm not sure it applies if the burr is only several 0.1ths of a micron, but I have not seen anyone measure a large burr for thickness. It's typical for the grains to be bigger than that, and typical grain size in inexpensive kitchen knives is likely a dozen or more microns.
In terms of why do the burrs bend back and forth, that's what thin materials do. Think of fibers in fiber glass. They have 0% ductility, being glass, but can flex back and forth without damage, though for them it has to be a relatively large radius. The thinner the burr, the more it can flex back and forth. How many times can you fold a piece of paper back and forth before it tears on it's own, not with you pulling it apart? Now apply that to something that has a tensile strength at least an order of magnitude greater, maybe 20 to 50 times greater. It's really a wonder we ever get rid of burrs in the first place.
Some types of steel and heat treatment combinations will have burrs that are very rigid and break off without much effort. Others will have burrs that hang on like grim death. On steels I know are in the latter category, I either cut them off and start over, or try not to form them in the first place. Usually, I don't know until it's too late, so I end up doing both.
In terms of why do the burrs bend back and forth, that's what thin materials do. Think of fibers in fiber glass. They have 0% ductility, being glass, but can flex back and forth without damage, though for them it has to be a relatively large radius. The thinner the burr, the more it can flex back and forth. How many times can you fold a piece of paper back and forth before it tears on it's own, not with you pulling it apart? Now apply that to something that has a tensile strength at least an order of magnitude greater, maybe 20 to 50 times greater. It's really a wonder we ever get rid of burrs in the first place.
Some types of steel and heat treatment combinations will have burrs that are very rigid and break off without much effort. Others will have burrs that hang on like grim death. On steels I know are in the latter category, I either cut them off and start over, or try not to form them in the first place. Usually, I don't know until it's too late, so I end up doing both.