The heat treatment process is arguably one of the most key steps in the process of making a tough, reliable knife. Without proper heat treating, the knife would probably fail at some point during its intended use and edge retention will suffer. Possible setbacks in using such a knife would include brittleness and inability to withstand abrasion and physical impacts, resist deformation due to thermal shock, withstand compression forces or even the inability to retain a usable edge.
What is Heat Treating?
Although the phrase heat treatment may suggest that this process involves the simple process of heating the knife’s blade, the cooling aspect is just as important in the process. In fact, this may be the most important aspect in the entire craft. The way you cool the blade after heating it will determine whether you will get the desired properties in the blade, negative properties, or none at all. Obviously, the temperature to which the steel blade is heated will also be an important factor in process.
It might seem surprising that knife steel, molded by first heating steel to its molten state, can achieve a varied range of physical characteristics through heat treatment. The reality is that heat treatment affects the steel’s physical and structural properties, without altering its chemical properties. Some of these physical changes are desirable and help suit the blade to specific uses. For instance, heat treating a knife steel to increase its hardness can improve the blade’s edge retention capabilities.
The Importance of Heat Treating Knife Steel
The most obvious reason for heat treating a knife’s steel blade is to increase its hardness. However, there are other important reasons to put a knife steel through heat treatment. Other reasons to heat treat a knife’s steel blade include the need to increase its strength, improve ductility, remove internal stresses or reduce the coarseness of the blade’s grains.
Methods of Heat Treating Blade Steels
Quenching involves heating the knife steel very quickly until it turns red hot and then cooling it very rapidly, which makes the blade hard. The chemical composition of the knife steel, which is dependent on the on the kind and quantity of alloys in the steel, determines how, and at what pace the blade has to be cooled.
Low alloy steels require fast cooling, in which case oil is more appropriate. Some alloys can do better in water cooling, while high alloy steels will just require air cooling. Air cooling achieves better uniformity, and poses less risk of deformation or cracking during the cooling process. An inappropriate cooling method can result in the blade shattering into pieces during the process. The quenching process results in a material referred to as martensite.
Usually, quenching is not a self-sufficient heat treatment method. Martensite, the result of the quenching process, is quite brittle and very likely to crack or shatter due to stress. Tempering is therefore necessary to reduce the brittleness of a quenched blade while still retaining the hardness the blade achieved through the quenching process.
Unlike the quenching process, in tempering, sudden cooling is not necessary, and should actually be avoided. The heating process during tempering should also be slow and deliberate, and should ideally last between 2 and 4 hours. The process is repeated a couple of times to ensure that the steel blade loses its brittleness. In general, this process makes the knife steel ductile, hence more appropriate for practical uses.
The purpose of this form of heat treatment is to reduce the size of the grains that make up the steel blade. In order to achieve this, the steel must be heated to a level that causes its microstructure to start changing. This improves the knife’s ability to withstand grounding and filing.
Effects that Heat Treatment can have on a Knife
Changes in Size
Heat treatment causes a knife blade to change its size by between 0.0005 and 0.002 inches per inch. The alloy content in the blade will play a role in this change, as well as how the heat treatment has been conducted. If these changes in size are not uniform, the blade will get deformed. Size changes resulting in shrinkage can be resolved by exposing the blade to cryogenic treatment.
Loss in Magnetic Abilities
The heat treatment process can sometimes reduce a blade’s magnetic abilities. Usually, this is as a result of an incomplete cooling process during the quenching stage, where the entire blade material does not turn into martensite. This problem can also be resolved by exposing the blade to low temperatures through cryogenic treatment.
The significance of heat treating a knife’s steel cannot be overemphasized. Without undergoing this critical process, a steel knife could not stand up to the demands of its intended practical uses either due to brittleness, softness, lack of strength, or inability to retain its sharpness over a meaningful period of time.