The four common phenomena in the heat treatment process

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To improve their performance, metal materials are often enhanced through heat treatment processes. However, during such treatment, various phenomena can occur that prevent the metal from being used properly after treatment, or that alter its properties. Therefore, it is important to be aware of the common phenomena that arise during metal heat treatment and of the damage they can cause. 1 Overheating phenomenon: Excessive heating during heat treatment can easily lead to the enlargement of austenite grains, thereby reducing the mechanical properties of the parts. 1) General overheating: Excessively high heating temperatures or prolonged exposure to high temperatures, which leads to the coarsening of austenite grains, is referred to as overheating. Large austenite grains lead to a reduction in the strength and toughness of steel, an increase in the brittle transition temperature, and an increased tendency to deformation and cracking during quenching. The cause of overheating is either out-of-control furnace temperature readings or overheating due to improper mixing (often resulting from a lack of understanding of the manufacturing process). The overheated structure can be refined by annealing, normalizing, or multiple high-temperature temperings, allowing it to austenitize again under normal conditions. 2) Fracture morphology inheritance: When steel with overheated structure is reheated and quenched, although the austenite grains can be refined, coarse granular fracture patterns may still appear. There are many theoretical controversies regarding the inheritance of fracture patterns. It is generally believed that excessively high heating temperatures cause impurities such as MNS to dissolve into austenite and accumulate at grain boundaries. During cooling, these inclusions precipitate along the grain boundaries; consequently, under impact, fractures tend to occur along the boundaries of coarse austenite grains. 3) Geneticity of coarse microstructures: When steel parts with coarse martensite, bainite, or wurtzite microstructures are re-austenitized by heating them slowly to conventional quenching temperatures, or even lower, their austenite grains remain coarse. This phenomenon is known as structural geneticity. To eliminate this genetic tendency toward coarse microstructures, intermediate annealing or multiple high-temperature tempering treatments can be employed. 2 Over-sintering: Excessively high heating temperatures not only cause the austenite grains to become larger, but also lead to oxidation or melting at the grain boundaries, resulting in weakened grain boundaries; this phenomenon is known as over-sintering. The properties of steel deteriorate severely after over-sintering, and cracks form during quenching. Overheated tissue cannot be restored and must be discarded. Therefore, it is necessary to avoid overheating in the work process. 3 Decarburization and oxidation: When steel is heated, the carbon in its surface layer reacts with oxygen, hydrogen, carbon dioxide, water vapor, and other elements present in the medium (or atmosphere), resulting in a decrease in the carbon concentration on the surface. This process is known as decarburization. After quenching, decarburized steel exhibits reduced surface hardness, fatigue strength, and wear resistance; moreover, residual tensile stress forms on the surface, which can lead to the development of network-like cracks. When heated, the reaction between the iron and alloys, as well as various elements, in the surface layer of steel with oxygen, carbon dioxide, water vapor, and other substances present in the medium (or atmosphere) to form an oxide film is known as oxidation. After oxidation at high temperatures (usually above 570 degrees), the dimensional accuracy and surface finish of workpieces deteriorate. Steel parts with poor hardenability of their oxide films are prone to develop soft spots after quenching. Measures to prevent oxidation and reduce decarburization include: coating the workpiece surface, sealing and heating with stainless steel foil, using a salt bath furnace for heating, heating in a protective atmosphere (such as purified inert gases with controlled carbon levels in the furnace), and using a flame combustion furnace (to make the furnace gas reducing). 4 Hydrogen embrittlement: The phenomenon of reduced plasticity and toughness in high-strength steels when heated in a hydrogen-rich atmosphere is known as hydrogen embrittlement. Workpieces affected by hydrogen embrittlement can have this issue eliminated through dehydrogenation treatments such as tempering or aging. Heating in a vacuum, a low-hydrogen atmosphere, or an inert atmosphere can also prevent hydrogen embrittlement. In modern continuous heat treatment furnaces, quenching is followed by timely tempering, and oxygen removal can be carried out during this tempering process. Based on current usage patterns and statistical data, products treated in continuous controlled atmosphere heat treatment furnaces generally do not suffer from hydrogen embrittlement.