why do you heat treat a knife
Before you even start up the forge for the quench, prepare your work area by laying out all the tools and equipment you will need for the process. Make sure you have your container lid and a few sets of pliers. Make sure you have a BC fire extinguisher (the kind that puts out grease/oil fires) nearby. Mentally rehearse all of the steps you need to make. The most important detail is that the knife enters the oil at or above critical temperature. The sequence I use is to heat the blade until it becomes non magnetic, then stick it back into the forge and push/pull the blade's full length through the heat once more, making sure the piece is evenly colored. When the blade comes out of the forge, you should move it into the oil slowly enough not to splash oil, and quickly enough not to lose significant heat. Be prompt about it. Dunk the hot blade into the oil,
onto the regulator block for approximately 15 seconds.
After the 15 seconds, submerge the entire blade into the oil beside the block. Note: The reason the regulator block is used instead of simply filling the container with enough oil that the blade rests 1/3 to б its width against the bottom of the container is so that the blade can be fully submerged after the edge hardening. Laying the blade on its side without a regulator block to fully submerge it risks warping due to one side of the blade being cooled faster than the other. When the bubbling and hissing of the oil subsides (around 45 seconds, time is not critical here) pull the blade out and allow it to cool in open air. Put the lid back on your quenching container to smother any flames. To reduce the brittleness, the material is tempered, usually by heating it to 175350`C (347662`F) for 2 hours, which results in a hardness of 5363 HRC and a good balance between sharpness retention, grindability and toughness.
Tempering should be carried out within a reasonable time after hardening, preferably within an hour or so. It is of vital importance that the blade should be allowed to cool to room temperature before tempering is started. The transformation to martensite will otherwise be interrupted and the hardening results may be impaired. A higher tempering temperature will yield a somewhat softer material with higher toughness, whereas a lower tempering temperature will produce a harder and somewhat more brittle material, as shown by the figure below. A camping knife or a survival knife, for example, may be tempered at 350`C (662`F) so that it will be able to withstand rough handling without breaking.
On the other hand, if the knife is expected to keep a sharp edge, it can instead be tempered at 175`C (347`F) for maximum hardness. Tempering temperatures below 175`C (347`F) should be used only in exceptional cases, when extreme demands are made on high hardness, since very low tempering temperatures will result in a very brittle material. Similarly, tempering temperatures above 350`C (662`F) should be avoided, since this could give rise to brittleness and reduced corrosion resistance. Note that if the tempered blade is exposed to temperatures above the tempering temperature (e. g. during grinding), the properties of the knife will be impaired. Correctly performed hardening will result in a good balance between hardness, toughness and corrosion resistance of the finished knife blade.
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