Welding process for KHR45A material in the radiant furnace tubes of ethylene cracking furnaces

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Author: Shen Dawei, Sinopec Third Company. Abstract: A weldability analysis and welding procedure qualification were conducted on the KHR45A material used for the radiant furnace tubes in ethylene cracking furnaces. A set of reasonable welding procedure measures was developed to guide welding operations, and these measures proved successful. Keywords: KHR45A ； Welding ； Process: Currently, the petrochemical markets at home and abroad are characterized by fierce competition in two areas: strategic crude oil reserve markets, and ethylene plant production centered around ethylene and its derivatives. Our country now has 3 ethylene production units with a capacity of 700,000 tons per year. To build an ethylene plant, it is first necessary to construct the core component of such a plant – the cracking furnace – and ensuring the welding quality of the radiant tubes in this furnace is particularly important. In previous ethylene cracking furnaces with a capacity of 300,000 t/year, the material used for the radiant tubes was generally Cr25Ni35; the welding process for such materials was relatively easy to master, and it was a well-established technique. However, during production it was found that furnace tubes made of this material have poor oxidation and carburization resistance, resulting in a short service life. To extend the service life of the furnace tubes, in subsequent ethylene cracking furnaces with a capacity of 700,000 tons per year, centrifugally cast tubes made of the high-chromium-nickel alloy Cr25Ni35 were gradually adopted to replace the initial Cr25Ni35 material. The KHR45A material is one type used in the radiant furnace tubes of pyrolysis furnaces. 1 Analysis of the welding properties of KHR45A material: KHR45A is a special alloy developed by Jiangsu Biaoxin Kubota Industrial Co., Ltd., featuring high temperature resistance, oxidation resistance, and carburization resistance. Its grade is Cr35Ni45Si2, and it contains various microalloying elements such as Nb, Al, Ti, Zr, etc. The chemical composition of this material is detailed in Table 1, while its mechanical properties are shown in Table 2. When the KHR45A material is used in the radiant furnace tubes of pyrolysis furnaces, the operating temperature is 1100°C, the operating pressure is 1.2 MPa, and the working media are primarily pyrolysis gasoline and saturated steam. It is easy to see from this that when welding this material, the high-temperature heat resistance of the weld joint should be given primary consideration. As can be seen from Table 1, the chemical composition of this material contains high levels of C and Ni, primarily to enhance its high-temperature resistance. Furthermore, this material also has a high silicon content, which is primarily intended to improve its oxidation resistance; meanwhile, KH can play a role in wetting the molten metal of the weld during the welding process. However, elements such as Si, Al, Ti, and Zr present in the KHR45A material also reduce its weldability to varying degrees. Therefore, when welding KHR45A material, the following three main problems arise: (1) Welding thermal cracking. KHR45A is a high-Ni austenitic heat-resistant steel, with a Ni content that can reach 45%. If an inappropriate welding process is used during construction, defects such as thermal cracks are likely to occur in the weld area and the areas surrounding it. This is especially true when the weld is repaired, as thermal cracks and other defects are more likely to appear in the base material within the heat-affected zone of the weld. The common main defect is weld solidification crack. The main reasons for the formation of thermal cracks are: ① The material has a low thermal conductivity and a high coefficient of thermal expansion; under the conditions of local heating and cooling during welding, large tensile stresses can develop in the welded joint during cooling, which is the primary cause of solidification cracks ； ②This material tends to form weld microstructures with strongly directional columnar crystals, which facilitates the segregation of harmful impurities such as S and P and leads to the formation of intergranular liquid layers, thereby increasing the likelihood of weld solidification cracks ； ③The alloying elements such as Si, Al, Ti, and Zr present in it can also form harmful eutectic interlayers due to their limited solubility, thereby causing thermal cracks in the joint. (2) Chemical composition segregation: Due to the high carbon content and alloy element content in KHR45A material, the liquid-solid phase region is large, resulting in severe segregation. The higher the C content, the easier it is for S and P impurities to precipitate during the primary phase transformation of the weld melt, and they accumulate at the grain boundaries, increasing the tendency for cracks. An increase in Si content makes it easier for silicate inclusions to form during welding, leading to intergranular segregation and thereby increasing the occurrence of solidification cracks. (3) For Y-shaped tube groove cracks in KHR45A material furnace tubes, there are two manufacturing processes: one is centrifugal casting of radial furnace tubes, and the other is static casting of Y-shaped tubes. Under centrifugal casting conditions, the presence of centrifugal force prevents many impurities from mixing into the material itself, resulting in a material composition with relatively few impurities and reducing the likelihood of issues such as layers or uneven surfaces. Under the static casting production process, due to the high presence of impurities, issues such as delaminations and thick layers in the Y-shaped tube material are quite severe. Microcracks can easily form in the welding grooves or in the base material itself. If strict inspections are not carried out prior to welding and defects in the base material are not thoroughly removed, it can cause significant problems in the welding process. Therefore, a proper welding process must be developed for welding KHR45A material, in order to minimize secondary repairs and re-welding, prevent the occurrence of welding thermal cracks as well as cracks in Y-shaped pipe fittings, and ensure welding quality. 2 Welding Process for KHR45A Material 2.1 Welding Methods To minimize the occurrence of thermal cracks during the welding of KHR45A, it is necessary to use a welding process that results in relatively concentrated and low heat input, while also keeping the temperature between weld passes from rising too much. Therefore, tungsten inert gas welding or metal inert gas welding are suitable welding methods. Based on the actual conditions at the construction site, it was decided to use the manual tungsten inert gas welding method. 2.2 Selection of welding materials Choosing the correct and appropriate welding materials is the primary condition for completing welding tasks. Especially in the welding of special steel grades, it is particularly important to select the appropriate welding materials. When welding KHR45A material, welding wire of a grade equivalent to that of the base material must be used, and the alloy element content of the welding wire (including micro-alloying elements) should be similar to that of the base material. This ensures that the welding material and the base material have the same melting point, the same crystallization temperature, and the same coefficient of thermal expansion/contraction, thereby preventing defects such as poor adhesion of the weld metal and cracks in the weld seam. At the same time, to ensure the high-temperature mechanical properties of the weld, the C content of the welding material should be comparable to that of the base metal. The commonly used nickel-based welding materials (such as ERNiCr-3) have a low carbon content, which does not meet the requirements of radiation furnace tubes. Therefore, 45A welding wire (φ2.4mm) developed by Kubota Company and compatible with the KHR45A material was selected; this wire can match the chemical composition of the base metal and ensure the high-temperature mechanical properties of the weld, thereby guaranteeing welding quality. 2.3 Development of welding process: From the analysis of the weldability of KHR45A material, it is known that there are three main issues during the welding of this material. Therefore, when developing the welding process, it is essential to focus on ways to address these issues. First is welding thermal cracking. To address this issue, first, harmful impurities must be strictly restricted, and homogeneous filler metals should be used as much as possible for welding. At the same time, the groove should be thoroughly cleaned before weld joint preparation; if necessary, acetone can be used for cleaning the groove to prevent oil, impurities, and other substances from entering the weld metal and causing hot cracks. Secondly, by selecting appropriate welding materials and appropriately adjusting the alloy composition of the weld, efforts should be made to avoid the formation of a single-phase austenite structure. Third, the overheating of the weld should be minimized to prevent the formation of large columnar grains. During welding, a low heat input and thin weld beads should be used, with multi-layer and multi-pass welding employed. The temperature between passes must be kept below 100°C to prevent excessive growth of the intergranular structure in the weld and heat-affected zone, which could lead to a decrease in the high-temperature toughness of the welded joint. The next issue is the segregation of chemical components. In addition to the high carbon content in the KHR45A material being a cause of problems in this regard, another reason is the addition of a large amount of silicon to the material in order to improve its oxidation resistance (with w(Si) reaching around 1.8%). The increased silicon content makes it very easy for chemical composition segregation to occur during welding, thereby leading to welding hot cracks. The main measures to solve this problem are to adopt a proper welding process, control the heat input during welding, improve the first-pass success rate to reduce the need for rework, and avoid secondary overheating of the weld metal, thereby reducing the degree of segregation in the chemical composition of the weld. In welding operations, for welds that require repair, it is necessary to first determine the location of the defect; when removing the defect, the area of the opening should be minimized as much as possible. The larger the opening area, the larger the heat-affected zone resulting from patch welding, and the greater the tendency to develop thermal cracks; vice versa. When grooving is performed to repair weld defects, fine welding wire, low current, and multi-pass welding should be used. Third is to avoid microcracks in the base material of the Y-shaped pipe itself. Mechanically prepared grooves should be used as much as possible to avoid defects caused by heat exposure of the base material during flame cutting. Before joint welding, the groove must be carefully cleaned, and thorough visual as well as dye penetrant inspections must be carried out; the groove surface must be free of defects such as cracks, inclusions, and delamination. After the first layer of welding, a coloring penetrant inspection should be conducted again to ensure the welding quality. Visual inspection should be carried out between other weld layers, and the interlayer weld beads must be carefully cleaned. After welding is completed, a dye penetrant inspection should also be carried out on the weld surface and in the area surrounding the heat-affected zone to ensure there are no defects such as cracks. 2.4 Welding Procedure Qualification: Based on the selected welding materials and the established welding procedures, a welding procedure qualification guide is prepared to guide the conduct of such qualification. 2.4.1 Evaluation Criteria The standard applicable to welding procedure qualification is SH J509--1988 \"Welding Procedure Qualification for Petrochemical Projects\". 2.4.2 Materials for Evaluation 2.4.2.1 Base Material The KHR45A base material used for welding process evaluation is a centrifugally cast tube with dimensions of Φ120.8mmX6.4mm; its chemical composition is shown in Table 3, while its mechanical properties are listed in Table 4. 2.4.2.2 Welding materials: The welding material used for evaluation is 45A wire with a diameter of φ2.4 mm; the metal chemical composition is shown in Table 5. 2.4.3 Welded joint: The joint type is a butt joint with a V-groove, and the welding position is vertical and fixed. A schematic diagram of the connector is shown in Figure 1. 2.4.4 Shield gas The shield gas is argon, with a flow rate of 10~12 L/min on the front side and 15~25 L/min on the back side. 2.4.5 Welding process parameters: DC positive polarity, cerium-tungsten electrode, φ2.5mm. The temperature between the paths is kept below 100°C. The welding process parameters are shown in Table 6. 2.4.6 Non-destructive testing: Color penetrant inspection was carried out on the groove, root pass, and cover pass welds; they passed the test. Radiographic inspection was also performed on the welds, and they passed as well. 2.4.7 Tensile test 2.4.7.1 Samples of 19.96mm×4.36mm were cut according to the evaluation criteria for a tensile test at room temperature; fracture occurred at the weld line, with σb=575MPa. 2.4.7.2 Another specimen with dimensions of 6.4 mm × 10 mm was taken and subjected to high-temperature tensile testing at 1075°C; the fracture occurred in the base material, with a σb value of 81 MPa. 2.4.8 Metallographic examination: Base metal: austenitic matrix, dendriticly distributed carbides, σ-phase. Heat-affected zone: austenitic matrix, dendriticly distributed carbides, σ-phase. Weld seam: austenitic matrix, equiaxed or elongated network of carbides. 3 Practical Welding Applications: In 2001, during the renovation of the E-BA-2101 cracking furnace at Shanghai Petrochemical with an annual ethylene production capacity of 700,000 tons, appropriate welding procedures were developed taking into account the characteristics of on-site welding of KHR45A radiant furnace tubes. Detailed welding operation instructions were prepared based on successful welding procedure evaluations, in order to guide the welding process. At the same time, in light of the welding defects that are prone to occur with this steel grade, welders were specially trained in targeted welding skills, and only after passing the examinations were they allowed to carry out welding work. Furthermore, during the welding of the furnace tubes, all work related to building the furnace structure and installing the lining is suspended, in order to prevent debris and impurities from the lining material from affecting the welding quality and to create a favorable welding environment for the furnace tube welding. In welding production, the welding process parameters can be adjusted as follows: current of 80~110A, voltage of 10~12V, and welding speed of 8~15cm/min. In addition to strictly following the qualified welding process, the following aspects also require close attention: (1) When aligning the weld seams, the position of the joints must be adjusted properly, ensuring that the misalignment of the inner walls is no more than 0.5 mm. (2) When welding the base layer, argon shielding should be provided on the back side, and the argon flow rate must be strictly controlled. Generally, when starting to fill with argon, the argon flow rate is increased appropriately to ensure that all the air in the furnace tube is completely expelled ； During welding, the argon flow rate should be appropriately reduced to prevent depressions in the back side of the weld due to the blowing effect of the argon. (3) During the welding process, multi-layer and multi-pass welding is employed; after each layer is welded, a dedicated person should use a thermometer to measure the temperature, ensuring that the temperature between passes remains below 100°C (ideally between 60–70°C). (4) Welders with high welding skills and a strong sense of responsibility should be assigned to weld materials of Cr35Ni45, in order to minimize the need for weld repairs. By adopting the above welding processes and construction control measures, a remarkable achievement of 100% first-pass welding success rate for the radiant furnace tubes in the pyrolysis furnace was obtained. 4 Conclusion As long as the correct welding materials are selected in welding production, reasonable welding processes are employed, and the welding process is strictly controlled to minimize secondary repairs and patch welding, the problem of welding thermal cracks in KHR45A high-chromium-nickel material centrifugally cast furnace tubes can be completely resolved