On-site construction heat treatment

At construction sites, heat treatment is generally carried out using flame heating, heat medium heating, or electric heating; electric heating is usually used for general equipment and pipelines where strict control over the heat treatment temperature is required. Flame heating and heat medium heating are suitable for the overall heat treatment of equipment such as spherical tanks, where high temperature requirements do not apply.

Post-weld heat treatment refers to the heat treatment carried out after welding to improve the microstructure and properties of the welded joint or to eliminate residual welding stresses. What is the purpose of post-weld heat treatment? Post-weld dehydrogenation treatment refers to low-temperature heat treatment carried out after welding is completed, before the weld has cooled below 100°C. The general standard is to heat to 200–350°C and maintain that temperature for 2–6 hours. The main purpose of post-weld hydrogen removal treatment is to accelerate the escape of hydrogen from the weld and heat-affected zone, and it is highly effective in preventing welding cracks during the welding of low-alloy steels. 　　During the welding process, due to uneven heating and cooling, as well as internal constraints in the components or external constraints applied, welding stresses always arise in the components after the welding is completed. The presence of welding stress in the components reduces the actual load-bearing capacity of the welded joint area, causes plastic deformation, and in severe cases, can lead to the failure of the components. 　　Stress-relief heat treatment involves subjecting the welded workpiece to high temperatures in order to reduce its yield strength, thereby relieving the welding stresses. There are two common methods: one is overall high-temperature tempering, which involves placing the welded part as a whole into a heating furnace, slowly heating it to a certain temperature, holding it at that temperature for a while, and then cooling it in air or within the furnace. This method can eliminate 80%-90% of welding stress. Another method is local high-temperature tempering, which involves heating only the weld area and its surrounding region before cooling it slowly, thereby reducing the peak value of welding stress and smoothing out the stress distribution, thus partially eliminating the welding stress. 　　In some alloy steel materials, the welded joints develop a hardened structure after welding, which deteriorates the mechanical properties of the material. Furthermore, this hardened microstructure may lead to joint failure under the influence of welding stresses and hydrogen. If the microstructure of the joint is improved after heat treatment, the plasticity and toughness of the welded joint are enhanced, thereby improving the overall mechanical properties of the welded joint. The common heat treatment methods are as follows: 1. Annealing – This involves heating the steel component to a temperature above its critical temperature (which varies depending on the type of steel; it is generally between 710–750°C, while for some alloy steels it can reach 800 or 900°C). The component is held at this temperature for a certain period of time before being cooled slowly. The purpose of annealing is: 1. To reduce hardness, thereby facilitating machining ； 2. Refine the grain structure and achieve a uniform microstructure in order to improve the mechanical properties of the steel blank, or to prepare it for subsequent quenching ； 3. Elimination of internal stresses II. Normalizing: The process of heating a steel piece above its critical temperature, holding it at that temperature for a certain period of time, and then cooling it in the air is called normalizing. The cooling rate for normalizing is faster than that for annealing, while the heating and holding times are the same as those for annealing. The purpose of normalizing is to refine the microstructure of low-carbon and medium-carbon steel parts as well as carburized parts, to increase strength and toughness, to reduce internal stresses, and to improve machinability. Normalizing is essentially a special form of annealing with similar objectives to annealing; the difference is that the cooling rate is faster than in annealing, which allows for shorter production cycles and makes it more economical. III. Quenching: The process of heating a steel piece above its critical temperature, holding it at that temperature for a certain period of time, and then rapidly cooling it in water, saltwater, or oil (with some materials being cooled in air) is called quenching. The purpose of quenching is to increase the hardness and strength of steel parts. For cutting tools, the main purpose of quenching is to increase their hardness, thereby ensuring the cutting performance of the tools as well as the wear resistance of die tools and measuring instruments. For components made of medium-carbon steel, quenching prepares the structure and properties for subsequent tempering, as high strength and high toughness cannot be achieved simultaneously after quenching; they are obtained only after tempering. There are many components such as gears and crankshafts, which, while in operation, are subject to both wear and impact forces; therefore, it is required that their surfaces have high hardness, while their cores possess good toughness. At this point, surface quenching can be used to meet the aforementioned requirements. Surface quenching involves rapidly heating the surface of a workpiece to a quenching temperature (at which point the temperature inside the metal is still relatively low), and then immediately spraying water on the surface of the workpiece to cause rapid cooling. This way, the requirement for a hard surface and tough core can be achieved. When surface heating is required, an oxy-acetylene flame, high-frequency current, or medium-frequency current can be used for heating. IV. Tempering: The process of heating a hardened steel piece to a temperature below its critical point, holding it at that temperature for a certain period of time, and then cooling it in air or oil is called tempering. The purpose of tempering is to eliminate the brittleness and internal stresses resulting from quenching, to adjust the microstructure, and to improve the plasticity and impact toughness of the steel parts. For tools, it is to retain hardness while minimizing brittleness as much as possible. For parts, this is done to improve toughness, but it inevitably leads to a decrease in hardness. V. Quenching and tempering: High-temperature tempering after quenching is called quenching and tempering. The purpose of tempering is to endow steel parts with high toughness and sufficient strength, thereby granting them excellent overall mechanical properties. Many important components such as the spindle, lead screw, gears, etc., are subjected to quenching and tempering treatment. Quenching and tempering is generally carried out after the mechanical processing of parts; it is also possible to quench and temper the forged billets or the roughly processed blanks before proceeding with mechanical processing. VI. Aging Treatment: The treatment method employed to eliminate the internal stresses generated during the manufacturing of blanks, in order to prevent or reduce deformation caused by these internal stresses, is called aging treatment. There are two types of aging treatment: natural aging and artificial aging. Natural aging involves first performing rough machining on the surface of the part to be processed, and then leaving it outdoors for a certain period of time ； Or suspend the workpiece (such as a screw rod) for several days to gradually reduce its internal stresses. Natural aging yields good results, but the process is long and inefficient. Artificial aging involves heating the parts to 100–160°C after low-temperature tempering but before final machining, holding them at that temperature for 10–40 hours, and then cooling them slowly. Artificial aging is efficient, but it incurs certain costs. VII. Chemical Treatment: Chemical treatment is a heat treatment method that alters the chemical composition of the surface layer of steel, thereby changing the structure and properties of that surface layer; it differs from conventional heat treatment methods. 1. Carburizing of steel: The process of introducing carbon atoms into the surface of a steel part is called carburizing. Carburizing is used for low-carbon steel and low-alloy steel (0.1-0.25% C); steel with a carbon content higher than 0.3% is rarely used for this purpose. After carburizing and quenching, the steel parts possess high surface hardness (HRC=60-65) and wear resistance, while maintaining high toughness in the core. Some impacted wear-resistant parts, such as shafts, gears, cams, piston pins, and other similar components, are mostly carburized. 2. Nitriding of steel: The process of introducing nitrogen atoms into the surface of steel parts is called nitriding. Nitriding is commonly used for medium-carbon alloy steels containing elements such as aluminum, chromium, and molybdenum. After nitriding, steel parts can have their surface hardness, wear resistance, corrosion resistance, or fatigue strength improved. This method is commonly used for important parts such as bolts, nuts, pins, etc. 3. Cyaniding of steel: The process of simultaneously diffusing carbon and nitrogen atoms into the surface of steel is called cyaniding. Cyaniding is applicable not only to medium-carbon steel, low-carbon steel, or alloy steel parts. It can also be used for high-speed steel cutting tools. The surface hardness and wear resistance of steel parts treated with cyanidation can both be improved. VIII. Blackening treatment: The process of heating metal parts in a solution containing high concentrations of alkalis and oxidants to induce oxidation, thereby forming a magnetic film of iron oxide on the metal surface, is called blackening treatment. Blackening treatment is a type of oxidation treatment method; its main purpose is to prevent rust on the metal surface, enhance the appearance and luster of the metal surface, and eliminate the stress generated during the quenching process. Blackening treatment is mainly applied to carbon steel and low-carbon alloy tool steel. Due to the influence of materials and other factors, the film color of the blackening layer varies from blue-black, black, red-brown, to brownish-gray; its structure is relatively dense, with a thickness of around 0.6–0.8 micrometers

Hello, may I ask how high can these methods generally reach in terms of temperature? For carbon steel PWHT, it’s also around 621 degrees, right? What method should be used? Can PWHT be performed on stainless steel on-site?