Heat treatment is a post-processing technique that alters the mechanical characteristics of steel to improve the forging performance. It is recommended that all-steel forgings may be heat treated.
After closed die forging, heat treatment is critical for achieving desirable stress relief, grain refinement, and enhanced mechanical and physical properties. After quenching and tempering the steel forgings, the required characteristics may be performed. The next sections will explain how steel is heat-treated and why it is necessary.
In order to attain the proper effect, the steel is heated to a specific temperature, perhaps as high as 2400°F, maintained there for a set period, and then cooled. While metal is heated, its complex form, referred to as the microstructure, changes due to its physical characteristics. The term soak time' refers to the amount of time the steel is heated. The duration of the soak has a significant effect on steel properties, as steel soaked for a long duration will exhibit microstructure changes that are distinct from metal soaked for a shorter duration.
After the soak period, the steel undergoes a cooling process that contributes to its final appearance. Steel may be rapidly cooled, a process called quenching, or slowly left to cool in the furnace to ensure the desired outcome is achieved. Various aspects that go into the formation of steel characteristics have a part in determining the soak temperature, soak time, cooling temperature, and cooling duration.
The temperature at which the steel is heated throughout the production process also affects the altered characteristics, and most steel may even be heated numerous times.
The typical heat-treating techniques for steel forging are as follows:
Annealing is an extensive word that refers to heating steel beyond its maximum temperature and then cooling extremely slowly and at a pace that results in a refined microstructure. Typically, cooling is sluggish during annealing. Annealing is most often used to soften steel to increase its machinability and create a more homogeneous microstructure.
Normalizing is a process carried out in a heat treating forge to ensure that the grain size of steel is consistent throughout. This process is referred to as grain refining, and it results in the creation of a more uniform piece of metal, thus increasing its strength and hardness. After heating a steel component over its critical temperature, it is air-cooled until it reaches room temperature.
Stress-reducing is a forging method that is used to eliminate or minimize internal tensions in steel. These stresses may be produced by various factors, from cold working to uneven cooling after forging. In forging, stress relief is often achieved by heating a metal below its lower critical temperature and then evenly cooling.
Quenching is the process of rapidly heating steel over its upper critical temperature and subsequently cooling it. Various cooling techniques may be used depending on the alloy and other factors, such as the trade-off between maximum hardness and cracking and deformation. Brine, polymer, freshwater, oil, and driven air all have different cooling rates. It is critical to utilize the correct quench medium since rapidly quenching some steels may result in cracking.
Tempering is efficient in relieving the tensions produced by quenching and lowering the hardness to a specific range. Additionally, it is used to ensure that particular steels satisfy specified mechanical properties.
There are different quenching media for heat-treating steel to its desired result.
Water: Water is an excellent medium for fast quenching when adequately agitated. On the other hand, water is corrosive to steel, and rapid cooling may sometimes result in deformation or fracture.
Salt Water: Saltwater is a more fast quench medium than plain water since the bubbles are readily burst and enable the component to cool rapidly. On the other hand, saltwater is more corrosive than plain water and must be promptly washed off.
Oil: When a slower rate of cooling is required, oil is utilized. Due to the very high boiling point of oil, the shift from the beginning to the end of Martensite formation is gradual, reducing the probability of breaking. Fumes, spillage, and sometimes a fire danger occur from oil quenching.
While there may be disputes over what makes one kind of heat treatment superior to another, specific procedures known as bad heat treatment can affect steel forging. Here are three ways bad treatment can affect steel forging.
If the steel microstructure is inconsistent from the start. This is especially critical for forging bladesmiths since forged knives will include a variety of microstructures. The steel should be uniformly normalized and annealed to ensure that the grain structure is homogeneous and the carbide structure is suitable for ultimate heat treatment. When the microstructure of a knife varies, the reaction to heat treatment varies as well.
Inadequate austenitizing temperature/time. If the steel is not adequately austenitized before quenching, residual ferrite may form. Because ferrite is soft and malleable, it prevents the steel from reaching its maximum hardness and reduces its strength.
Excessive heat/time used during austenitization. If the steel becomes too hot, grain growth is a probable result. Granules that are too large contribute to the grain's lack of toughness. Another consideration is an excess of carbon in the solution, which results in brittle "plate martensite." A further possible issue is an abnormally high retention of austenite. Increased austenitizing temperatures decrease the temperature at which martensite crystallizes to the point where it forms at or below room temperature.
Without heat treating metal, particularly steel, metal components for everything from aircraft to computers would either not work correctly or would not exist at all. In particular, non-ferrous metal components would be much weaker. Heat treatment is used to strengthen aluminum alloys, titanium alloys, bronze, and brass. Numerous metals are used in the manufacture of automobiles, aircraft, and other items that depend on solid metals for performance and safety.
Also, because heat-treated metals are often stronger than untreated metals, pre-treatment of metal components avoids corrosion, which means that costly metal parts will not need to be replaced as frequently or as frequently in the future. This results in the more economical and effective operation of equipment and the prevention of malfunctions.
Generally, the kind of transformation that occurs during heat treatment is determined by the temperature to which the steel material is heated, rate at which it is heated, the duration of the heating, the temperature to which the material is first cooled, and the rate at which it is cooled. In essence, the blacksmith must note this in the process of heat treating to achieve the desired result.
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