Classification of heat treatment

I’m not an expert in heat treatment, but I’ll explain it briefly to see if that can help clarify things! 1. Heat treatment for restoring mechanical properties refers to the heat treatment carried out in order to restore the original properties of a material that has suffered significant damage to its mechanical properties as a result of machining. 2. What is heat treatment to improve mechanical properties? It refers to heat treatment carried out in order to optimize mechanical properties further, 3. Stress-relief heat treatment is intended to remove the internal stresses generated during the processing process, and it is often necessary to carry out this treatment!

Heat treatment to restore mechanical properties is carried out after product processing is complete, especially when the heat treatment state of the product is altered as a result of hot stamping or welding processes; such heat treatment is necessary to restore the product to its original heat treatment state； Heat treatment to improve mechanical properties is a treatment applied to a material after machining, in order to fully utilize its mechanical properties. This is the most common and widely used heat treatment method, including quenching, tempering, quenching and tempering, carburizing, and other chemical heat treatments ； Heat treatment to eliminate thermal stress is a type of heat treatment carried out after machining or welding, in order to remove the internal stresses that arise from the uneven changes in the material’s internal structure caused by previous processing steps; such stresses can pose risks to the performance of the component. Methods used for this purpose include annealing and normalizing. Regarding the example you mentioned, I personally believe that only one normalizing heat treatment is necessary; as long as a reasonable processing procedure is established based on the usage requirements, the desired results should be achieved. To verify the effects of heat treatment, before actual heat treatment, samples were taken from key areas such as the cylinder body, head, and pipe connections (after welding) to undergo simulated heat treatment. The simulated samples were analyzed and inspected to observe the results.

Processes of metal heat treatment: Heat treatment processes generally include three steps: heating, holding at a constant temperature, and cooling; sometimes there are only two steps, namely heating and cooling. These processes are interconnected and must proceed continuously. Heating is one of the important processes in heat treatment. There are many heating methods for metal heat treatment; initially, charcoal and coal were used as heat sources, and later liquid and gas fuels were employed. The use of electricity makes heating easy to control and causes no environmental pollution. These heat sources can be used for direct heating, or for indirect heating through molten salts or metals, as well as floating particles. When metals are heated and the workpieces are exposed to air, oxidation and decarburization often occur (that is, the carbon content on the surface of steel parts decreases), which has a very adverse effect on the surface properties of the parts after heat treatment. Therefore, metals should generally be heated in a controlled or protective atmosphere, in molten salts, or in a vacuum; they can also be protected during heating using coatings or packaging methods. Heating temperature is one of the important process parameters in heat treatment processes. Selecting and controlling the heating temperature is a key issue in ensuring the quality of heat treatment. The heating temperature varies depending on the metal material being treated and the purpose of the heat treatment, but it is generally raised above the phase transition temperature to obtain a high-temperature microstructure. Furthermore, the transformation requires some time; therefore, once the surface of the metal workpiece reaches the desired heating temperature, it must be held at that temperature for a certain period of time to ensure that the temperatures inside and outside are equal and that the microstructure undergoes complete transformation. This period of time is known as the holding time. When high-energy-density heating and surface heat treatment are used, the heating rate is extremely fast, and generally there is no holding time, whereas the holding time for chemical heat treatment is usually longer. Cooling is also an essential step in the heat treatment process; the cooling methods vary depending on the process, with the main focus being on controlling the cooling rate. Generally, the cooling rate for annealing is the slowest, that for normalizing is faster, and that for quenching is the fastest. However, different steel grades also have varying requirements; for example, air-hardening steels can be hardened at a cooling rate similar to that used for normalizing. Metal heat treatment processes can be broadly divided into three categories: overall heat treatment, surface heat treatment, and chemical heat treatment. Depending on the heating medium, heating temperature, and cooling method, each major category can be further divided into several different heat treatment processes. The same metal can exhibit different microstructures, and thus different properties, when subjected to various heat treatment processes. Steel is the most widely used metal in industry, and its microstructure is also the most complex, which is why there are numerous types of heat treatment processes for steel. Overall heat treatment is a metal heat treatment process in which the entire workpiece is heated and then cooled at an appropriate rate to change its overall mechanical properties. The overall heat treatment of steel generally includes four basic processes: annealing, normalizing, quenching, and tempering. Annealing involves heating the workpiece to an appropriate temperature, maintaining that temperature for a duration that varies depending on the material and size of the workpiece, and then cooling it slowly. The purpose is to bring the internal structure of the metal to an equilibrium state, thereby achieving good mechanical and functional properties, or to prepare the structure for further quenching. Normalizing involves heating the workpiece to an appropriate temperature and then cooling it in air. The effect of normalizing is similar to that of annealing, except that the resulting microstructure is finer. It is commonly used to improve the machinability of materials, and sometimes it is employed as the final heat treatment for parts with less stringent requirements. Quenching involves heating a workpiece and holding it at that temperature before rapidly cooling it in a quenching medium such as water, oil, or other inorganic salts and organic aqueous solutions. After quenching, the steel parts become harder, but they also become more brittle. To reduce the brittleness of steel parts, the quenched steel parts are held at an appropriate temperature above room temperature but below 650°C for an extended period of time before being cooled; this process is known as tempering. Annealing, normalizing, quenching, and tempering are the “four key processes” in overall heat treatment. Among them, quenching and tempering are closely related and are often used together; neither can be omitted. “The “four fires,” depending on the heating temperature and cooling methods, give rise to various heat treatment processes. This post was last edited by szftduanhaihua on 2009-4-6 16:21.]

I. Those that restore mechanical properties: tempering, partial annealing. II. Those that improve mechanical properties: quenching and tempering, carburizing, nitriding, blueing, blackening. III. Those that relieve stress: annealing

What was said upstairs is absolutely correct. I guess what the original poster meant was: how can such a container undergo so many heat treatments? Isn’t that too much? Actually, that’s not the case. The purpose of each heat treatment process is different. For example, the heat treatment aimed at restoring mechanical properties is carried out because changes occur internally in the steel or material during the manufacturing process, such as eutectic transformations. Mechanical heat treatment is used to improve the way molecules are arranged and organized internally, thereby enhancing mechanical properties. On the other hand, adding other elements and alloys serves to strengthen the metal and improve its resistance to oxidation and strength; so these are distinct approaches. Eliminating thermal stress is also a way to restore mechanical properties. The simplest method is tempering; further, combining tempering with quenching results in quenching and tempering, which serves the same purpose but with better effects – it eliminates residual stresses, thereby maintaining the integrity of the metal’s molecules during intense deformation processes and ensuring high strength, as well as preventing changes such as bending deformations

Heat treatment cannot be performed multiple times, as doing so too often can easily cause the carbon and alloying elements inside to be damaged.

1. Laser heat treatment technology: Laser heat treatment is a high-tech process for rapid local surface quenching, which is primarily used to strengthen the surface of components. It can increase the surface hardness, wear resistance, strength, and heat resistance of metal materials and parts, while allowing the core of the parts to retain good toughness. As a result, the mechanical properties of these parts feature excellent wear resistance, high impact toughness, and high fatigue resistance. The use of laser heat treatment technology can significantly improve product quality and double the service life of products. Laser heat treatment has significant advantages over traditional quenching processes: the surface hardened layer has a fine and uniform structure, is not restricted by the shape of the part, can achieve a hardness of up to HRC60, and its thickness can reach up to 4 MM. 2. Laser cladding technology: A laser-based process that deposits a layer of material with specific physical properties on the surface of a designated workpiece, in order to improve its surface properties or restore the original geometric dimensions of the part. High-performance materials can be optionally used to modify the substrate surface, resulting in a cladding structure that is fine-grained, free of microdefects, and features high metallurgical bonding strength. Laser cladding technology can also be used to repair holes and cracks on the surface of materials, restoring the geometric dimensions, morphology, and properties of damaged workpieces. 3. The application of laser processing technology imposes no restrictions on the geometric dimensions of mechanical parts’ shapes; laser processing can be carried out wherever the laser can reach ； There are no restrictions on the base material; steel, iron, non-ferrous metals, and certain types of cemented carbides can all be repaired ； The metal surface can be modified to enhance the physical properties of parts such as wear resistance, corrosion resistance, and heat resistance, thereby extending the service life of these parts and reducing production costs. This post was last edited by jyszyx on 2009-4-17 11:58]

In this case, a single heat treatment is sufficient; the heat treatment intended to improve mechanical properties causes the structure to transform into austenite when heated above the A3 line, thereby eliminating internal stresses

Yes, each type of heat treatment serves a different purpose! ! Actually, sometimes factories combine them in order to save costs; strictly speaking, this is not acceptable. However, if done this way, the performance won’t be that good, but everything else will be fine! ! After all, cost is also very important; for heat treatment, it just involves those four heating steps, which are used repeatedly! Hehe

Based on my experience, the claim that it cannot be quenched multiple times comes from practices imported from abroad; in reality, I don’t think there’s any problem with that – at high temperatures everything is lost anyway, so it can indeed be quenched multiple times! !

Regarding your question, I agree with the view of the poster on the 3rd floor. However, when it comes to two heat treatments, these are actually necessary for some materials; for example, certain tool and die steels require more than two quenching processes, while others need more than three tempering processes.

Heat treatment to restore mechanical properties: After processing, especially following hot stamping or welding, the heat treatment state of the product is altered and its mechanical properties change; therefore, heat treatment is required to restore it to its original heat treatment state； Heat treatment to improve mechanical properties: After machining, heat treatment is applied to the material in order to fully utilize its mechanical properties. This is the most common and widely used heat treatment method, including quenching, tempering, quenching and tempering, carburizing, and other chemical heat treatments ； Heat treatment to eliminate thermal stress: After machining and welding, internal stresses are generated due to the uneven changes in the internal structure of the workpiece caused by previous processing steps, which pose risks to the use of the workpiece. Heat treatments such as annealing and normalizing are employed to eliminate these effects. Dehydrogenation treatment: refers to the treatment required for containers such as liquefied petroleum storage tanks to remove hydrogen (mainly considering H2S). For the situation you described, it is recommended to perform heat treatment just once – a full-scale heat treatment. Specific requirements shall comply with the capacity regulations.

I. Types of annealing 1. Complete annealing and isothermal annealing. Complete annealing, also known as recrystallization annealing, is commonly referred to simply as annealing. This type of annealing is mainly used for castings, forgings, and hot-rolled profiles made from various carbon steels and alloy steels with hypoeutectoid compositions; it is sometimes also used for welded structures. It is generally used as the final heat treatment for parts that do not require heavy processing, or as a preliminary heat treatment for certain parts. 2. Spheroidizing annealing is mainly used for hypereutectoid carbon steels and alloy tool steels (such as the steels used in the manufacture of cutting tools, measuring instruments, and molds). Its main purpose is to reduce hardness, improve machinability, and prepare for subsequent quenching. 3. Stress-relief annealing: Stress-relief annealing is also known as low-temperature annealing (or high-temperature tempering). This type of annealing is used primarily to eliminate residual stresses in castings, forgings, welded parts, hot-rolled products, and cold-drawn items. If these stresses are not eliminated, they will cause the steel part to deform or develop cracks after a certain period of time, or during subsequent machining processes. II. During quenching, the most commonly used cooling media are brine, water, and oil. Workpieces quenched in salt water can easily achieve high hardness and a smooth surface, and are less prone to the formation of soft spots that do not harden; however, they are prone to severe deformation or even cracking. Using oil as a quenching medium is only suitable for quenching certain alloy steels with high stability of supercooled austenite or small-sized carbon steel workpieces. III. Purposes of steel tempering 1. To reduce brittleness and eliminate or minimize internal stresses. After quenching, steel parts have high levels of internal stress and brittleness; if not tempered in a timely manner, these parts may deform or even crack. 2. To achieve the mechanical properties required by the workpiece, quenching results in a high hardness but also high brittleness. In order to meet the different property requirements of various workpieces, the hardness can be adjusted through appropriate tempering, thereby reducing brittleness and obtaining the desired toughness and ductility. 3. Stabilize the dimensions of the workpiece. 4. For certain alloy steels that are difficult to soften through annealing, high-temperature tempering is often used after quenching (or normalizing) to cause the carbides in the steel to aggregate appropriately, thereby reducing its hardness and facilitating machining.

Heat treatment to restore mechanical properties: After processing, especially following hot stamping or welding, the heat treatment state of the product is altered and its mechanical properties change; therefore, heat treatment is required to restore it to its original heat treatment state； Heat treatment to improve mechanical properties: After machining, heat treatment is applied to the material in order to fully utilize its mechanical properties. This is the most common and widely used heat treatment method, including quenching, tempering, quenching and tempering, carburizing, and other chemical heat treatments ； Heat treatment to eliminate thermal stress: After machining and welding, internal stresses are generated due to the uneven changes in the internal structure of the workpiece caused by previous processing steps, which pose risks to the use of the workpiece. Heat treatments such as annealing and normalizing are employed to eliminate these effects. Dehydrogenation treatment: refers to the treatment required for containers such as liquefied petroleum storage tanks to remove hydrogen (mainly considering H2S). For the situation you described, it is recommended to perform heat treatment just once – a full-scale heat treatment. Specific requirements shall comply with the capacity regulations.

This belongs to surface alloying treatment