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The maximum temperature of the heating furnace is 520 °C. The high-temperature medium pipes in the distillation reaction system are all made of P9 steel; there are a large number of such P9 pipes, with 2,311 welds in total. During the actual welding of P9 pipes, stress cracks, cold cracks, heat cracks, crater cracks, and delayed cracks can easily occur. Some of these cracks are difficult to detect and prevent, and once the equipment is put into use, they can cause significant economic losses as well as unpredictable and serious accidents. It is essential to prevent the formation of various types of cracks during the welding process. For the entire project, the welding quality of P 9 pipes is particularly important; it is a key aspect of pipe installation and also a challenging one. Based on the specific conditions at the site and through welding process evaluation, our company has developed appropriate welding and heat treatment processes. The project was completed successfully. 1 Analysis of the weldability of P9 steel: P9 steel is a high-strength martensitic heat-resistant alloy steel that exhibits good structural stability, high-temperature creep strength, resistance to oxidation and corrosion at high temperatures, as well as high-temperature fatigue strength. However, its weldability is poor; during welding, it is necessary to prevent the occurrence of cold cracks, delayed cracks, and reheat cracks. The chemical composition and mechanical properties of P 9 steel are shown in Table 1. 1.1 Cold crack sensitivity: The cold crack sensitivity of P9 steel is primarily influenced by three factors: the microstructure of P9 steel, which makes it susceptible to cold cracks ； During the post-welding martensite transformation, hydrogen remains in a supersaturated state within the martensite, resulting in a brittle structure in the weld and heat-affected zone ； Marshall transformation occurs after welding, increasing the structural stress at the joint. When the welding line energy is high, the interlayer structure becomes overheated, resulting in large grains, bainite structure, and network-like grain boundaries. These structures serve as sources of cracks, leading to the formation and propagation of cracks. To prevent welding cold cracks, appropriate welding procedures must be adopted, such as preheating, slow cooling after welding, post-heating, and heat treatment after welding. 1.2 Cold cracks in welds with low compatibility: When welding materials and base metals with low compatibility are used, cold cracks in the weld are likely to occur. To prevent this, it is necessary to increase the preheating temperature during welding, raise the temperature and duration of post-weld hydrogen removal treatment, or employ a welding method that utilizes welding materials with high compatibility, thereby eliminating cold cracks in such welds. 1.3 Hydrogen-induced cracks in low-sulfur steel: Since the actual sulfur content in P9 steel is below 0.005%, welding this type of low-sulfur martensitic heat-resistant steel, especially when welding different types of heat-resistant steels, easily leads to cold cracks in the weld heat-affected zone. It is necessary to increase the preheating temperature, extend the post-weld dehydrogenation treatment time, and carry out heat treatment immediately after welding; or else, a welding method with high material compatibility should be used to eliminate these weld cold cracks. 1.4 Softening of the welded joint HAZ and Type IV creep cracks: Welded joints of P9 heat-resistant steel exhibit softening in the weld heat-affected zone as well as a decrease in creep fracture strength. When these welded joints are heated in the temperature range from A1 to Ac3, significant changes occur in the properties of the grains in the weld HAZ; the room-temperature strength, creep fracture strength, and hardness all decline markedly. Type IV creep cracks are likely to occur during high-temperature operation, leading to premature failure of the welded joint under such conditions. 1. When welding P9 steel with arc crater cracks, using conventional methods to end the arc can easily lead to such cracks. During welding, it is possible to gradually reduce the current or use the interrupt-and-restart method to end the arc, ensuring that the arc crater is filled before moving to the edge of the weld pool to terminate the arc. 1.6 Stress cracks can be caused by misalignment of pipes, forced alignment, and welding defects, which lead to the accumulation of residual stresses. Additionally, due to the high structural stiffness and high restraint stresses of P9 steel, after the equipment is put into use, factors such as hydrogen diffusion, repeated changes in pipe temperature, repeated impacts from external forces, and the combination of these external forces with residual stresses can result in the formation of weld cracks that eventually lead to failure. It is essential to develop a proper welding process and strictly control the welding procedure; the welds should have smooth transitions, with no sharp corners or edges, in order to avoid stress concentration and thus ensure welding quality. From the above analysis, it can be seen that when welding P9 martensitic heat-resistant steel, preventing cold cracks and improving impact toughness are the main issues. 2 P9 Steel Pipe Welding Process 2.1 Pre-welding Preparation (1) Before starting pipe welding work, organize the welders to receive technical training so that they master the welding methods, procedures, operational requirements, and quality control standards; conduct welding skill tests to ensure that they obtain the appropriate welding qualifications. (2) Carry out welding procedure qualification tests, and based on these results, develop welding WPS or WWI documents ； (3) The groove at the joint edge is machined using a grooving machine; it is a single V-shaped groove with blunt edges, and the groove angle a is 30 degrees. ～35. The groove gap c is 3–4 mm, and the root thickness P is 1–1.5 mm. (5) Thoroughly remove oil, rust, moisture, burrs, etc. within a range of 20 mm on both sides of the base metal surface inside and outside the groove, as well as from the surface of the welding wire, until a metallic luster is revealed. Penetrant testing shall be conducted on the groove surface; no defects such as cracks are allowed ； (6) The welding rod is dried at 350~C to 400~C for 2 hours; when in use, it is placed in a insulated container at 100~C to 150~C and taken out as needed. If the welding rod is used for more than 4 hours, it should be re-baked, and re-baking shall not be performed more than twice. 2.2 The welding method selected is GTAW for root welding, followed by SMAW for filling and finishing; argon is used as a shielding gas during pipeline welding. 2.3 Selection of welding materials: The welding materials must meet the requirements regarding the chemical composition, mechanical properties, weldability of P9 pipeline material, as well as the requirements related to the welding process and media. Alkaline low-hydrogen welding materials should be used. For TIG welding, the PP·TIG—R 71 wire produced by Shanghai Electric is used, while for shielded metal arc welding, the R 707(E9Mo-15) electrode manufactured by Zhuzhou Xiangjiang is employed. When welding materials are received, it is necessary to carefully check the certificates of conformity and quality certificates. A dedicated storage staff member shall be responsible for storing the welding materials, cleaning the welding wires, drying the electrodes, issuing and collecting the welding materials, as well as keeping proper records. 2.4 Preheating before welding: Since P9 steel is a martensitic steel that is highly sensitive to hydrogen, it is necessary to strictly control the hydrogen content and its diffusion during welding. This involves carefully controlling the preheating temperature and the temperature between layers, in order to reduce the temperature gradient in the welded part and weld seam, thereby preventing the formation of cold cracks and reheat cracks. The higher the welding preheating temperature, the lower the tendency for welding cold cracks; however, excessively high preheating temperatures can also contribute to the formation of welding cracks. The correct selection of welding preheating temperature is very important for preventing welding cold cracks. For root welding using TIG welding, due to its very low hydrogen diffusion content, the preheating temperature before welding is 150°C to 200°C; for MIG welding, the preheating temperature before welding is 250°C to 300°C. Both sides of the weld must be heated uniformly, with the heating width being no less than 3 times the wall thickness. Electric heating is used, and thermocouples are employed for temperature measurement. 2.5 Welding process parameters The welding process parameters are shown in Table 2. The interpass temperature should not be lower than the welding preheating temperature, but it should also not be too high; excessively high interpass temperatures can reduce the impact toughness of the weld metal. The interpass temperature should be maintained between 200°C and 300°C, with quality control personnel using handheld infrared thermometers to measure this temperature in order to keep it within the appropriate range. 2.6 On-site Welding Process Control 2.6.1 Double-layer TIG welding for root pass The reason for using the double-layer TIG welding method for the root pass is as follows: P9 steel is a highly alloyed steel, and argon shielding is necessary during TIG welding; both the root pass welding and the subsequent layers of welding can cause oxidation on the back side of the metal ； When using TIG welding to apply a root pass, a thin weld layer can easily lead to burn-through, affecting the quality of the root weld ； When a layer of TIG welding is used as a base coat, the strength of the weld is insufficient, and cracks are likely to occur under external forces. The double-bottom welding process was employed to ensure the continuity of argon filling and thus guarantee the quality of the weld. Note that when performing root welding, it is not possible to proceed as one would with welding steel; the wire feed must be uniform, and the force used for feeding the wire should not be such that it pushes the wire out at the root area. Otherwise, this can easily result in unmerged wire ends in the root weld. Additionally, when performing root welding on large-diameter thick-walled pipes, one person welds while another monitors the root welding from the opposite side. 2.6.2 During the multi-layer, multi-pass welding of P9 steel, the molten iron in the weld pool has high viscosity and poor fluidity ； Using small-scale welding can easily lead to defects such as slag inclusions and lack of interlayer fusion. Therefore, to avoid the formation of large defects, multi-layer and multi-pass welding with small weld bead sizes is employed, with each weld bead having a thickness of no more than 4 mm. This not only helps to control the welding heat input, but also allows the subsequent weld beads to refine the grains resulting from the heat treatment of the previous layers, thereby improving the overall properties of the welded joint. The R 707 welding rod produces a lot of spatter, has poor slag removal properties, and the slag is difficult to remove. Each layer of weld must be cleaned thoroughly, especially at the joint areas and on both sides of the weld; these areas need to be polished clean using a grinder. 2.6.3 Argon shielding inside the tube: To prevent oxidation at the root of the welds in P9 steel, it is necessary to provide local argon shielding inside the tube during the root welding process using TIG welding for P9 steel. The argon purging protection area is centered on the axis of the groove; sponge blocks are used to seal off areas 250–300 mm on each side. Argon is introduced from one end, and both sides of the welded joint are sealed with soluble paper to create an airtight chamber. An argon supply tube is then inserted through the gap in the groove for purging (a lighter can be used to test the chamber; if ignition fails, it indicates that the chamber is properly sealed). For the first pass of root welding, the argon flow rate should be maintained at 20–30 L/min, while for the second pass, it should be set at 10–20 L/min. 2.6.4 Welding environment: To ensure that the welding environment remains unaffected, measures against rain, wind, and moisture are taken, such as erecting a work shed ； Minimize welding outdoors; stay updated on weather forecasts to avoid welding in adverse weather conditions ； The heat treatment unit is powered separately to prevent sudden power outages on site and ensure that the heat treatment process continues without interruption. During the welding process, unless layer welding is required due to process or inspection specifications, the welding should be completed in one continuous sequence without interruption ； If welding is interrupted during the pipeline welding process due to certain reasons (such as power outages, strong winds, rain, etc.), measures such as post-heating and slow cooling must be taken as required. Before proceeding with welding, an inspection should be carried out; only after confirming that there are no cracks in the weld seam can preheating and heating be performed in accordance with the original welding procedures before continuing with welding. 2.6.5 Post-heating dehydrogenation treatment: If heat treatment cannot be carried out immediately after welding, post-heating dehydrogenation treatment should be performed right away. The specific method involves heating the weld to 300°C–350°C and maintaining that temperature for 1 hour, after which it is cooled slowly to room temperature. 2.6.6 Post-weld heat treatment: To minimize welding residual stresses and achieve good overall properties, high-temperature tempering at 760°C to 770°C is employed. The heating and cooling rates should not exceed 160°C/h; the holding time is 3 hours. After cooling to 300°C, it can cool naturally. Welding heat 3: Quality status P 9. Pipe welding was completed in its entirety from July 15, 2011, to October 10 of the same year. P9 steel welded joints are prone to delayed cracking. After heat treatment for 24 hours, 100% radiographic inspection was carried out on all welds; a total of 2311 welds were inspected, with a first-pass qualification rate of 99.1%. The non-qualified welds were mainly those with large variation in parameters and those located in positions where it was difficult to achieve proper alignment (a total of 21 welds). Twelve welded joints were randomly selected for spectral analysis; the chemical composition of the welds fully meets the requirements. To ensure that there are no issues with the welds of P9 steel after the installation is put into use, and as a responsibility to the owner to reassure them, all welds on the P9 steel pipes were subjected to ultrasonic and dye penetrant testing after pressure testing but before operation. Three welds were found to have delayed cracks, and repairs were carried out promptly; the success rate of the first attempt at repair through non-destructive testing was 100%. After heat treatment, the hardness of the welds was sampled at a rate of 20% in accordance with standard specifications; 599 weld joints were tested. The average hardness of the weld area was 250 HB, while the average hardness of the heat-affected zone was 200 HB. The pass rate for these hardness tests was 100%. 4 Precautions: Practical experience has shown that P9 steel alloy has a high alloy content and poor weldability, making it prone to cold cracks and delayed cracks. Therefore, special attention should be paid to the following points: (1) Use low-hydrogen welding electrodes, and these electrodes must be baked at a constant temperature of 350°C to 400°C for 1 to 2 hours as specified in the instructions, to ensure their dryness ； (2) Multi-layer and multi-pass welding with low wire energy is adopted, while a double underbead welding process is used for large-diameter thick-walled pipes ； (3) When extinguishing the arc, use the decaying current method to fill the arc crater ； (4) Due to the poor deoxidizing ability of the welding rod, each weld layer must be thoroughly cleaned, and the interlayer temperature must be strictly controlled ； (5) It is necessary to ensure a smooth transition at the weld, avoiding the formation of edges and sharp corners, thereby reducing stress concentration ； (6) Strictly follow the regulations for pre-welding preheating, post-welding hydrogen removal treatment, and post-welding heat treatment, and strictly control the temperature and time of preheating and heat treatment. 5 Conclusion Through the joint efforts of all members of the Jin’ao project team in our unit, as well as strict oversight by the supervisors who ensured full monitoring of the welding process for the P9 steel pipes, the project was successfully completed. The welds of Pipe P 9 in the 100t/a delayed coking unit of Hubei Jin’ao Technology Chemical Co., Ltd. have passed all types of inspections (visual inspection, non-destructive testing, spectroscopic analysis, hardness testing) as well as hydrostatic testing; all parameters related to these welds meet the specified requirements, indicating excellent weld quality. Practice has proven that the welding and heat treatment processes mentioned above are feasible for P9 pipe welding. Due to the poor weldability of P9 steel, in field construction it is necessary to develop appropriate welding procedures and control measures based on the actual on-site conditions, and to maintain oversight throughout the process to ensure their correct and effective implementation