Cryotreatment Of Blades
Heat treatment of steel using liquid nitrogen (cryoprocessing) has become one of the main trends in the knife industry in the last decade. This is primarily due to the widespread use of modern powder steels, many of which maximize their qualities when using this technology.
Cryo-treatment makes it possible to achieve an increase in hardness, wear resistance and strength as a result of the transformation of residual austenite into martensite, as well as increasing the impact strength and wear resistance of steels due to an increase in the number of carbides. Thus, cooling immediately after quenching to cryogenic temperatures, changes the properties of steel always in a certain specified direction, because it excludes martensite tempering. This makes it possible to obtain significantly higher hardness of quenched steel, practically unattainable by other heat treatment methods. When considering steel cooling in terms of physical processes, there is an accelerated attainment of martensitic steel structure in the transition from austenitic. Martensite has high hardness and a general set of properties required for cutting tools. Cooling allows to reduce the volume of austenite in high carbon content steel by up to 6 times. It also helps to reduce residual stresses that occur during heat treatment.
"Austenite — is one of the structural components of iron-carbon alloys, a solid solution of carbon (up to 2%) and alloying elements in the gamma iron modification. It is nonmagnetic, characterized by moderate hardness, reduced elasticity, significant strength and toughness."
"Martensite — is the main structural component of hardened steel; it is an organized supersaturated solid solution of carbon in α-iron of the same concentration as the initial austenite."
Cryo-treatment is used in the last stage of the heat treatment of steel. The main task of this treatment is the thermal hardening of the metal, due to a directed change in its structure and properties. Technologically, this process is divided into three main steps:
cooling of the blank to the desired temperature;
holding the steel in the vessel for a specified time at a specified temperature;
slow heating to room temperature;
Most often, containers with liquid nitrogen or special low-temperature chambers equipped with an electronic control system (cryogenic processor) are used for cryogenic treatment. A cryogenic processor is a unit that produces cooling to temperatures below -150° C, at a fairly uniform and low rate. It is necessary to prevent a sharp thermal impact on the blank, leading to the deterioration of the metal structure. The first commercial processor was developed in the late 1960s. The total cycle of steel processing in modern cryoprocessors takes about 72 hours. The first 24 hours are used to reach the optimum temperature for the blank, the second 24 hours are used to keep the blank at this temperature, and the remaining time is used to slowly return to room temperature. There are cryoprocessors in which it is possible to reach extreme temperatures, and they are able to perform metal tempering in addition to cryogenic processing.
However, most knife manufacturers do not have access to such high-tech devices, limiting themselves to the use of special vessels. For this purpose, so-called "Dewar vessels" are used, which are designed for long-term storage of substances at elevated or lower temperatures. A constant temperature is maintained in these vessels by passive methods at the expense of good thermal insulation and internal processes in the stored substance. Originally, the "Dewar vessel" was a glass flask with double walls, from the space between which the air was pumped out. In modern vessels, the inner and outer parts are made of aluminum or stainless steel. In this case, the inner and outer vessels are connected by a common neck. To reduce heat loss, the inner vessel is covered with additional thermal insulation. Modern "Dewar vessels" have low evaporation losses. Cryoprocessing time in such vessels differs significantly from cryoprocessors. Most often a temperature of about -198°C is used. Cold treatment begins immediately after quenching. The blank is held in the vessel for 2 hours, then about the same amount of time is needed to cool it to room temperature.
Cryoprocessing of knife steels began to be used by craftsmen several decades ago. They developed, through experiments, different modes for the most popular knife steels at that time. In particular, cryoprocessing of D2 steel blades was actively used in the United States. This method did not give a significant increase in hardness, but it allowed accelerating the overall heat treatment process. Various high-speed steels subjected to such cooling also showed similar results.
In today's era of widespread use of powder steels, cryoprocessing has become very popular throughout the world. For example, the common S30V steel increases its hardness by about 1.5 HRC after cryo-treatment, while retaining all of its performance characteristics. Similar results are obtained with the popular Elmax steel. The latest generation "powders," so-called "super steels" such as S390, M398, S125V, etc., are already mandatorily cryo-treated.