Hardening and Tempering Knife Blades: Part 2
The first process of heat treatment is called normalizing. During the knife making process stress would be created in different portions of the blade. Some of the areas might be ultra hard while some areas of the blade might be soft.These differential stresses in the blade can affect the finishing and the hardening and tempering of the steel might not occur in an uniform manner. Thus you might end up with a blade that might have weak spots and can break under strain. During normalizing the blade would be carefully and evenly heated and cooled slowly, thus removing the stresses that might have been built up in the blade during the forging process.
Hardening is the process used to add strength to the steel blade. Here we should understand that we can only harden a steel with enough carbon in its alloy using the regular hardening methods. To harden steel that contains too less carbon, one should use a case-hardening process that adds carbon to the steel. The idea behind hardening the steel blade is to produce a uniform structure within the blade. This removes the differential stresses and removes the difference in hardness in the different parts of the blade. In the hardening process, the steel is heated to a certain ideal temperature (which varies based on the particular steel alloy) and then it is cooled rapidly by immersing into a cold fluid which might be water, special oil or salt bath. In some cases the cooling is done by air alone without quenching in any liquid. When steel is heated to a particular temperature, it transforms into "austenite", and when it is rapidly cooled, it becomes "martensite", which is the hardest form of steel. In this form the steel is extremely hard, and would be difficult to use. It would have a good edge, but is very fragile and can shatter easily.
Tempering is the next process in the heat treatment cycle. Tempering steel is required to provide it with a correct balance of strength and flexibility. In this process the steel is reheated to add toughness and also to make it slightly soft and flexible. During tempering the steel is heated to a lower temperature compared to hardening and is allowed to cool down more slowly, in most cases naturally. This is a final chance to adjust any distortions in the blade and generally a uniform hammering would be given to the blade starting from either ends. The tempering process softens the steel and makes it more pliable. Tempering of steel is a complex process and requires expertise to identify the correct time and temperature for the different alloys of steel. Sometimes oxidation takes place and the steel will begin to show colors during heating. Although this can be used to judge the tempter of steel, it should be kept in mind that the color is affected by many factors including the polish on the steel if any, the alloy elements present and the period of the tempering cycle. A very careful control of the time and temperature of the cycle will give very good results from the tempering process. As each type of steel responds differently based on its alloy combinations, it is better to test with sample pieces to find the best combination of time and temperature. Tempering relieves the stresses in the steel formed during the hardening process and gives the knife smith a chance to control the hardness of the steel based on the purpose of the knife.
Many knifesmiths double tempter steel as a effective practice. The first tempering cycle would be for about an hour and in some cases, fresh umtempered martensite might be created. The second tempering cycle would be used to temper any such freshly created martensite.
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