How to weld heat-resistant steel?

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Heat-resistant steel refers to steel that exhibits both thermal stability and high-temperature strength under high-temperature conditions. Thermal stability refers to the ability of steel to maintain chemical stability under high-temperature conditions (corrosion resistance, resistance to oxidation). Thermal 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 based on the content of these alloying elements in the base metal. 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, causing a tendency for quenching in the weld zone and heat-affected area. Cooling after welding in air leads to 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 usually carried out using flame or plasma cutting; preheating is necessary for cutting as well. After grinding, a PT test is performed to eliminate any cracks on the bevel. Typically, a V-groove is used, with a groove angle of 60°. From the perspective of crack prevention, a larger groove angle is preferable; however, this increases the amount of welding required. Meanwhile, the groove and both inner and outer sides must be thoroughly ground to remove contaminants such as oil, rust, and moisture (to prevent hydrogen absorption and porosity). 2.2 Alignment requirements: Forced alignment must be avoided to prevent the generation of internal stresses. Given that chromium-molybdenum heat-resistant steel has a relatively high tendency to crack, the restraint on the weld during welding should not be excessive, so as to avoid creating excessive stiffness. Particularly when welding thick plates, the use of tie bars, clamps, and fixtures that impede the free contraction of the weld should be minimized. 2.3 Selection of welding methods Currently, the commonly used welding method for pipeline welding by our petroleum and petrochemical installation units is TIG welding for root pass, followed by shielded metal arc welding for filling and capping. Other welding methods include metal inert gas welding (MIG welding), CO2 gas shielded welding, electroslag welding, and submerged arc automatic welding, etc. 2.4 Selection of welding materials. The principles for selecting welding materials are that the alloy composition and strength properties of the weld metal should essentially be consistent with the corresponding indicators of the base material, or they must meet the minimum performance criteria specified in the product’s technical requirements. 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 storage containers for easy access. They should not be left in those containers 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 performing manual arc welding on chromium-molybdenum heat-resistant steel, austenitic stainless steel electrodes can also be used, such as the A307 electrode. However, preheating is still required before welding. This method is suitable for situations where the welded components 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, and a certain temperature range should be maintained, both during spot welding and the overall welding process. 2.6 Post-weld slow cooling. Post-weld slow cooling is a principle that must be strictly adhered to when welding Cr-Mo heat-resistant steels; this must be done even during hot summer days. Generally, immediately after welding, the weld and its vicinity should be covered with asbestos cloth. Small welded components can be placed inside asbestos cloth for slow cooling. 2.7 Post-weld heat treatment Post-weld heat treatment should be carried out immediately; its purpose is to prevent the occurrence of delayed cracks, relieve stresses, and improve the microstructure. 3 Welding precautions (1) When welding this type of steel, measures such as preheating and slow cooling after welding are necessary. However, a higher preheating temperature is not always better; it is essential to strictly adhere to the welding procedure requirements. (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 interruptions avoided as much as possible. For interlayer pauses, thermal insulation and slow cooling measures should be taken; likewise, the same preheating measures must be applied before welding resumes. (3) During welding, attention should be paid to filling undercuts; the joints should be ground and any undercut cracks (hot cracks) removed. Moreover, the greater the current, the deeper the weld crater; therefore, it is necessary to strictly follow the welding procedure specification when selecting welding parameters and an appropriate welding heat input. (4) Construction organization is also an important factor affecting welding quality; coordination among different trades is particularly crucial to prevent the entire weld from losing its quality due to poor transition 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 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 tasks in a logical manner, we can guarantee the quality of welding and meet all technical requirements.