Heat treatment is a crucial process in metalworking that involves three main stages: heating, holding (or heat preservation), and cooling. In some cases, only heating and cooling are required. These steps are closely connected and must not be interrupted. Heating is a vital part of the process, and there are various methods used to achieve it. Traditionally, fuels like charcoal, coal, liquid, and gas were used, but with the development of technology, electricity has become a cleaner and more controllable option. Heat sources can be applied directly or indirectly through molten salts, metals, or even particles.
During heating, the workpiece is often exposed to air, which can lead to oxidation and decarburization—especially in steel, where the surface carbon content decreases. This negatively affects the surface properties of the treated parts. To prevent this, metals are typically heated in controlled or protective atmospheres, molten salts, or vacuum environments. Protective coatings or packaging methods can also be used to shield the material during the process.
The heating temperature is a critical parameter in heat treatment. Choosing and maintaining the correct temperature ensures the quality of the final product. The temperature varies depending on the type of metal and the desired outcome, but it is usually set above the phase transition temperature to create a high-temperature structure. Once the surface reaches the target temperature, it must be held for a certain time to ensure uniform internal temperature and complete microstructural transformation. This period is known as the holding time. In high-energy heating or surface treatments, the heating rate is very fast, so no holding time may be needed. However, chemical heat treatments often require longer durations.
Cooling is another essential step, and the method used depends on the specific process. The cooling rate is carefully controlled—slower for annealing, faster for normalizing, and even faster for quenching. Different types of steel may have different cooling requirements. For example, hardened steel can be cooled at a standard rate without issues.
Heat treatment is generally divided into three categories: overall heat treatment, surface heat treatment, and chemical heat treatment. Each category includes several sub-processes based on the heating medium, temperature, and cooling method. These variations allow for different microstructures and mechanical properties in the same metal. Steel, being one of the most commonly used materials, has a complex microstructure, leading to numerous heat treatment techniques.
Overall heat treatment involves heating the entire workpiece and then cooling it at an appropriate rate to alter its mechanical properties. It typically includes four basic processes: annealing, normalizing, quenching, and tempering. Annealing involves heating the workpiece to a specific temperature, holding it for a certain time, and then slowly cooling it to achieve equilibrium in the microstructure. Normalizing is similar but cools the workpiece in air, resulting in a finer structure and better machinability. Quenching involves rapid cooling after heating, which increases hardness but also makes the material brittle. Tempering follows quenching to reduce brittleness and improve toughness.
These four processes—annealing, normalizing, quenching, and tempering—are collectively known as the "four fires" in overall heat treatment. They are often combined, especially quenching and tempering, which are essential for achieving the right balance of strength and ductility. Some alloys undergo aging after quenching, where they are kept at a controlled temperature to enhance their mechanical or physical properties.
Deformation heat treatment combines mechanical deformation with heat treatment to achieve optimal strength and toughness. Vacuum heat treatment, performed under low-pressure or vacuum conditions, prevents oxidation and decarburization, ensuring clean surfaces and improved performance. It also allows for chemical treatments using infiltration agents.
Surface heat treatment focuses on altering the properties of just the surface layer. This is achieved using high-energy heat sources such as flames, induction currents, lasers, or electron beams. These methods allow for rapid heating of the surface without affecting the interior of the workpiece. Common techniques include flame hardening and induction hardening.
Chemical heat treatment changes the surface composition of the metal by introducing elements like carbon, nitrogen, or boron. This is done by placing the workpiece in a medium containing these elements and heating it for an extended period. Afterward, additional treatments like quenching or tempering may be applied. Key methods include carburizing, nitriding, and metal infiltration.
Heat treatment plays a key role in manufacturing mechanical components and tools. It enhances properties like wear resistance, corrosion resistance, and overall durability. It also helps improve the microstructure and stress state of raw materials, making them easier to work with in both cold and hot forming processes.
For example, white cast iron can be transformed into malleable cast iron through long-term annealing, significantly improving its plasticity. Properly heat-treated gears can last much longer than those without treatment. Additionally, low-cost carbon steel can be modified to mimic the properties of expensive alloy steels, making it suitable for applications that would otherwise require high-performance materials like heat-resistant or stainless steels.
Hexagon nuts (also known as finished hexagon nuts) are available in ASTM A563-A and are compatible with mild steel Bolts, such as ASTM A307, ASTM F1554 Grade 36, SAE Grade 2 and AASHTO M183. SAE Grade 5 and Grade 8 nuts are also available in finished style. Since hot-dip galvanizing usually increases the thickness of the threaded part of the fastener by 2.2 to 5 mils, the thread size of the galvanized hexagon nut is too large to compensate for the anti-corrosion coating on the bolt.
Chinese hex nut manufacturer Hongchuang Hardware provides hex nuts of various materials, such as carbon steel, stainless steel, alloy steel, titanium, etc.
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