Learn to weld heat-resistant steel

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Heat-resistant steel refers to steel that possesses both thermal stability and high strength at high temperatures. Thermal stability refers to the ability of steel to maintain chemical stability under high-temperature conditions (corrosion resistance, resistance to oxidation). Heat strength refers to the ability of steel to maintain sufficient strength under high-temperature conditions. The heat resistance is primarily ensured by alloying elements such as chromium, molybdenum, vanadium, titanium, and niobium; therefore, the selection of welding materials should be determined based on the content of these alloying elements in the base material. Heat-resistant steels are widely used in the construction of facilities in the petroleum and petrochemical industries. Those we come into contact with most often are pearlitic heat-resistant steels with low alloy content, such as 15CrMo and 1Cr5Mo. 1 Weldability of chromium-molybdenum heat-resistant steel: Chromium and molybdenum are the main alloying elements in pearlitic heat-resistant steel; they significantly enhance the metal’s high-temperature strength and oxidation resistance. However, they reduce the weldability of the metal, leading to a tendency for quenching in the weld zone and heat-affected zone. Cooling after welding in air results in the formation of hard and brittle martensite structure, which not only affects the mechanical properties of the welded joint but also generates significant internal stresses, thereby increasing the risk of cold cracking. Therefore, the main problem in welding heat-resistant steel is cracking, and the three factors that cause cracking are microstructure, stress, and hydrogen content in the weld; hence, it is particularly important to develop a suitable welding process. 2 Welding process for pearlitic heat-resistant steel 2.1 Beveling The beveling is typically carried out using flame or plasma cutting; preheating is necessary for cutting as well. After grinding, a PT test is performed to remove any cracks on the bevel. A V-groove is usually chosen, with a groove angle of 60°. From the perspective of preventing cracks, a larger groove angle is advantageous, but it increases the amount of welding required. The groove and the areas on both sides of it must be polished clean to remove contaminants such as oil, rust, and moisture (to reduce hydrogen content and prevent pores). 2.2 Alignment: Forced alignment should be avoided to prevent the generation of internal stresses. Since chromium-molybdenum heat-resistant steel has a tendency to crack, the degree of restraint on the weld during welding must not be too high, so as to avoid excessive stiffness. Especially when welding thick plates, it is necessary to avoid using struts, clamps, and fixtures that could hinder the free contraction of the weld. 2.3 Selection of Welding Methods At present, the commonly used welding methods for pipeline welding in our oil and petrochemical installation companies are TIG welding for the root pass and shielded metal arc welding for the filler and finish passes. Other welding methods include gas metal arc welding (MIG welding), CO2 gas shielded welding, electroslag welding, and submerged arc automatic welding. 2.4 Selection of welding materials The principle for selecting welding materials is that the alloy composition and strength properties of the weld metal should basically match those of the base material, or they should meet the minimum performance requirements specified in the product’s technical specifications. Furthermore, in order to reduce the hydrogen content, low-hydrogen alkaline electrodes should be used first. The electrodes or fluxes must be dried according to the specified procedures, and they should be taken as needed, stored in electrode insulation containers for easy access. The electrodes should not remain in the insulation container for more than 4 hours; otherwise, they need to be dried again. The number of drying cycles shall not exceed three, and detailed regulations exist for this in the actual construction process. When welding chromium-molybdenum heat-resistant steel by manual arc welding, austenitic stainless steel electrodes such as A307 can also be used; however, preheating is still required before welding. This method is suitable for cases where the welded parts cannot be heat-treated after welding. 2.5 Preheating Preheating is an important process measure for preventing cold cracks in the welding of pearlitic heat-resistant steels and for relieving stress. To ensure welding quality, preheating should be carried out, both during tack welding and the actual welding process, and a certain temperature range should be maintained. 2.6 Slow cooling after welding Slow cooling after welding is a principle that must be strictly followed when working with chromium-molybdenum heat-resistant steel; this must be done even during hot summer months. Generally, the weld area and the areas surrounding it should be covered with asbestos cloth immediately after welding, and small welded parts can be placed in asbestos cloth to cool down slowly. 2.7 Post-weld heat treatment Heat treatment should be carried out immediately after welding, with the aim of preventing the formation of delayed cracks, relieving stress, and improving the microstructure. 3 Welding precautions: (1) When welding this type of steel, preheating and slow cooling after welding are necessary, but the preheating temperature is not the higher the better; it is essential to strictly follow the requirements of the welding process. (2) For thick plates, multi-pass welding is recommended; the interpass temperature should be no lower than the preheating temperature. The welding should be completed in one go, with as few interruptions as possible. If an interlayer pause is required, thermal insulation and slow cooling measures should be taken, and the same preheating measures must be applied before welding again. (3) During welding, care should be taken to fill the arc crater, and the joint should be ground to remove arc crater cracks (thermal cracks). Moreover, the greater the current, the deeper the crater; therefore, it is necessary to strictly follow the specifications in the welding procedure sheet when selecting welding parameters and an appropriate welding heat input. (4) Construction organization is also a crucial factor affecting welding quality; coordination among different work teams is particularly important, in order to prevent the quality of the entire weld from being compromised due to poor coordination between subsequent processes. (5) Attention should also be paid to the impact of weather conditions. When the ambient temperature is low, the preheating temperature can be increased appropriately to prevent too rapid a drop in temperature, while also taking emergency measures such as protection against wind and rain. 4 Summary Preheating, heat retention, and post-weld heat treatment are essential process measures for welding chromium-molybdenum heat-resistant steel; all three are equally important and cannot be ignored. Any oversight at any stage can have serious consequences. Welders must strictly follow the welding procedures, and their sense of responsibility should be strengthened through proper guidance. One should not rely on luck; it is important to emphasize the seriousness and necessity of following the welding procedures. As long as we strictly adhere to these procedures during construction, ensure good coordination among different workers, and arrange the work processes in a proper manner, we can guarantee the quality of welding and meet all technical requirements.