The ambiance is intense. A three-judge panel judges four bladesmiths’ works; the winner will be chosen. The presentation includes a countdown timer, ringing metal, and flame blasts. One competitor seeks to outsmart their rivals through experience, while another turns to their understanding of materials science and metal heat treatment.
Without leaving the couch, viewers may learn the ins and outs of the bladesmith craft by watching the well-known US television series “Forged in Fire.” Knowing how to properly harden the metal that will be hammered into a precise and resilient blade is one of a professional bladesmith’s trade secrets. The performers in the show frequently face an impossible challenge on this stage. More tools than only the hammer and anvil are required of a bladesmith. The heated blank dipped into the liquid is the most exciting part of the performance. This is the hardening, which determines the quality of the surface, the characteristics of the steel, the shape of the blade, as well as the final assessment of the judges. After all, if a mistake is made, the metal can bend, crack or remain too soft.
Origins of hardening
Up until the middle of the 19th century, it was thought that blacksmithing or hammering alone determined the quality of steel and steel products. Dmitry Konstantinovich Chernov, a Russian scientist, didn’t discover and demonstrate that high-quality steel is a product that has undergone heat treatment, including hardening, as a result of which changes occur in the metal that could have been mistaken for magic in the Middle Ages until 1866–1868 while he was studying the metal of defective guns.
Steel has a crystalline structure subject to environmental influences and forms several stable crystal lattices. Martin Heinrich Klaport, a German chemist, initially identified this characteristic, known as crystal polymorphism, using calcium carbonate as an example in 1798.
Chernov developed this idea in terms of steel. He listed the four Chernov’s points, or crucial temperatures, as a, b, c, and d. When they are, during cooling or heating of the solid steel product, the phase state and structure of the steel change. The science of metal heat treatment was developed due to Chernov’s findings. Following that, metallurgy started to use a more scientific approach and relied less on the knowledge collected by earlier generations.
How hardening affects steel
Steel has a crystalline polymorphism structure at the molecular level. The words “many” and “form” in Ancient Greek are where the word “polymorphic” comes from. In this instance, a specific temperature causes the steel’s crystal lattices, which can differ significantly from one another, to change into one another. A polymorphic transformation is what this is. Additionally, completely different phase components might emerge under various cooling conditions (accelerated versus gradual), and the steel’s structural makeup changes after hardening or undergoing other heat treatments. The qualities of steel are affected by this process. The structures and phases produced are known as austenite, martensite, ferrite, cementite, pearlite, and so forth, depending on the kind and intensity of the heat effect. These are essentially sophisticated definitions of physical and chemical terminology.
In plain English, hardening is heating a steel product to a high temperature and then rapidly cooling it to diminish its ductility and toughness. The substance becomes robust, challenging, and brittle. All this occurs while the steel is still solid, not when it has reached the melting point.
Here is how it appears in use. For instance, a cold blank for a future knife or drill is heated to a temperature only slightly above the critical point at which the same polymorphic alteration of the crystal lattice occurs. For a predetermined period, the metal is maintained at that temperature. To lock in the altered crystal structure, the blank is then rapidly cooled in water, a salt solution, or oil (depending on the degree of steel alloying and the needed characteristics). The steel product is simultaneously subjected to internal stress, which may result in early failure.
Due to this, tempering is a process that is typically applied to the hardened part. A steel product is heated to relatively low temperatures during this technological procedure and then cooled in the air or a furnace. It aids in keeping the steel’s strength while reducing its brittleness.
Steel hardening methods
The environment in which steel hardening occurs is one of the critical variables. The pace of cooling depends on the medium selected, which may be salt or oil solutions, specific polymer solutions, or water. Each of these media has a particular capacity for cooling, so if you choose the wrong one, the product will either not harden or, on the other hand, experience excessive pressures from the rapid cooling, which will cause the material to break. Therefore, specialized quenching liquids should be utilized for each alloy, such as oil for alloyed steels and water for carbon steels.
Steel can be hardened using various techniques, such as stream hardening, step hardening, interrupted hardening employing two media, and more. The initial steel grade, the desired final properties, and the amount of surface area that needs to be hardened are only a few variables that influence the chosen process.
For instance, the cutting edge, known as the Hamon, is quenched in Japanese katana swords. To do this, the bladesmiths coat a blade that hasn’t been hardened with a special clay, wash it off the edge, and then set the sword. Experiments have also been conducted. For instance, the katana blade’s volumetric effect, known as a double Hamon, was achieved by combining hemp oil with green tea instead of water.
Steel hardening defects and imperfections
But the process of hardening steel is very delicate. The product must be heated to the proper temperature. Experienced blacksmiths typically use the color of the steel’s surface to visually determine the temperature at which it is heating. The product won’t have the desired properties if the artisan makes a mistake. On an industrial scale, specialized equipment such as pyrometers, thermocouples, and other inspection tools are used to regulate the heating temperature.
What might go wrong? Perhaps the steel isn’t tough enough. Low heating temperatures, brief holding periods, or slowly lowering temperatures are the culprits here. An annealed and re-quenched component can fix such a flaw. Increasing brittleness is a result of overheating. Additionally, it can be fixed by quenching again and annealing (normalizing).
Metal burning may happen when a steel product is heated to temperatures just below the melting point. This leads to a flaw that cannot be fixed and makes the metal exceedingly brittle. Inadequate hardening can also cause decarburization, oxidation, deformation, and cracking. Uneven parent metal structure or temperature change rate are two factors that might result in such flaws. After all, a change in the crystal structure (from austenite to martensite) results in a 3% increase in volume.
Therefore, customized tables and color schemes prevent or minimize such flaws. Additionally, continuous-cooling transformation diagrams allow one to choose a specific steel grade to ascertain the ideal heat treatment circumstances to produce the desired structure.
A careful study of metallurgy is required because of the enormous number of peculiarities that might occur during the heat treatment of steel products, which must be considered in the technological process. Even on television, it is abundantly evident that individuals who wish to get the most outstanding results in a situation with limited resources would benefit significantly from knowledge and expertise.
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