What prevents you from sharpening your knife well?
In some cases, when sharpening a knife, the sharpener faces incomprehensible difficulties. Despite a good sharpener, a set of expensive abrasives, and proven skills, it is not possible to sharpen the knife to the desired sharpness. Steel some kind of "does not accept" sharpening, and the change of abrasives does not give results. Previously overlooked factors are revealed, several of them can not be predicted, they manifest themselves only in the process of sharpening. First of all, it is the steel itself, its chemical composition, its manufacturing methods, methods of heat treatment, and finally the quality of the abrasive used.
1. Chemical composition of steel
One of the most important factors for creating the sharpest and most stable cutting edge on the knife is the chemical composition of the steel. And the most important factor affecting the quality of steel and the quality of sharpening is the negative influence of sulfur and phosphorus. These elements in the composition of steel should be as little as possible. Even the smallest amount of phosphorus in steel makes it fragile, reducing the impact strength of the metal. Sulfur in turn causes the formation of iron sulfide, which greatly increases the brittleness of steel. It causes the formation of hot cracks during heat treatment, which make the steel prone to color out. The best steels contain no more than 0.015% sulfur and up to 0.025% phosphorus, but such parameters are only possible in very expensive knives. Most knives are of average quality, have steel with a significantly large amount of these substances, and exceeding their content to 0.2%, will not allow you to sharpen the knife qualitatively, the steel will "crumble".
2. Steel production technology
Another factor affecting sharpness is the technology of steel production. In this case, powder technologies can make it possible to obtain a more finely dispersed and more uniformly distributed carbide phase. This is initially a different quality of steel compared to "regular" steels. So sharpening good steel based on good powder can give the maximum sharpness and durability of the cutting edge, exceeding the results of any other steel.
An important aspect in the production of steels is their melting. Steel produced in electric arc furnaces will differ from the same composition of steel produced in induction furnaces. Also, steel produced in an electroslag remelting furnace will differ in its qualities from steel produced by other methods.
3. Heat treatment
The heat treatment process is extremely important for the quality of steel and the properties it acquires. It is a combination of heating, holding, and cooling operations for solid metal alloys. The result is a steel with the specified properties, it changes the internal texture and structure. And here the key role is played by such factors as: the heating time of the steel, the type of furnace used, the size of the workpieces, their location in the furnace, and the holding time. Heat treatment itself can consist of many phases:
1) Annealing, the purpose of which is to obtain an equilibrium structure of steel.
2) Quenching, during which the steel is heated to a temperature above the critical one, kept at this temperature for some time and cooled.
3) Release needed to relieve internal stresses. As a result of steel tempering, the material becomes more plastic, while reducing the strength.
4) Normalization, the process of technology close to quenching.
5) Cryogenic treatment is a heat treatment at ultra-low temperatures - 153 degrees, necessary for steel hardening.
All these processes must be carried out in strict compliance with the established technological rules. Even the smallest mistakes can lead to a change in the quality of steel and not being able to sharpen it to the maximum. "Over-dried steel" will get stained, "not quenched enough" will not retain sharpness.
4. No harmful effects associated with the "memory effect" of steel
Another factor that affects the achievement of maximum sharpness is the absence of harmful effects in the production process that can affect the technological memory of steel.
The shape memory effect is the phenomenon of returning to the original shape when heated, which is observed in metals after pre-deformation. It is related to the fact that each metal and alloy has its own crystalline grid, with a given architecture and size. But under the influence of changes in temperature and pressure, the crystalline grid is rearranged. This process is called "polymorphic transformation", which occurs during the heat treatment of steel. As a result, a phase with a new crystalline grid is formed — martensite, after which the method of grid rearrangement is called "martensitic transformation".
This polymorphic process assumes that the ordered movement of molecules or atoms in the crystal causes a modification of their location in relation to each other. If the technological process is disrupted, processes related to the memory effect may be initiated at this point. After some time, the damaged steel itself loses its properties, such as sharpness. A well-sharpened knife stored on a shelf suddenly stops cutting. This effect is most often seen in the case of Damascus steel, especially made by "back-yard" methods without technological control. It will be almost impossible to achieve a stable sharpness on such steel.
5. Use of high-quality abrasives
Of course, the maximum sharpness can not be achieved without the use of the highest quality abrasives. At the same time, it must be taken into account that the abrasives must be suitable for specific steel. In particular, it is important to understand that the use of even diamond and CBN bars is not a universal method of sharpening. For example, the use of CBN bars for sharpening carbon and Bulat steels can give a negative effect. It will be expressed in the color of the cutting edge. Such steels should be sharpened with softer abrasives based on carborundum (silicon carbide) or electrocorundum (aluminum oxide). The quality of the abrasives themselves can also vary greatly depending on the manufacturer, which can save both on the abrasive powder and on the components of the bond. The result may be deterioration of the abrasive during operation, leaving uneven rough drawings on the bevel, etc.