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Analysis of Cracking in Joints When Welding Pearlite Heat-Resistant Steel with Austenitic Welding Materials – Nie Zhenhai, Maintenance Company of Qilu Petrochemical Company. September 1989. Abstract: Some equipment and pipelines in the petrochemical industry are made of pearlite heat-resistant steel. In the past, due to limitations in on-site construction conditions, some organizations often used austenitic stainless steel electrodes for welding. This type of welded joint often develops cracks during use. This article analyzes these specific cases, identifying the causes of the damage, inspection methods, and improvement measures. In petrochemical plants, pearlitic heat-resistant steel is often used for pressure vessels and pipelines where the temperature ranges from 400 to 500°C. This material possesses sufficient strength and oxidation resistance at high temperatures, as well as good resistance to sulfur corrosion and hydrogen corrosion, making it an excellent and economical material. However, since welding such materials requires the use of heat-resistant steel electrodes with chemical compositions and high-temperature resistance properties identical to those of the base material, and the welding process using such electrodes is quite demanding – requiring preheating before welding as well as heat treatment to eliminate residual welding stresses – due to limitations in on-site conditions, many organizations prefer to use austenitic electrodes for welding assemblies or for repairing defects. These joints often develop cracks during welding and after being in use for a period of time (several years). Table 1 shows the cracking occurrences in the austenitic joints of some pearlitic heat-resistant steel pressure vessels and pipelines at Qilu No.1 Fertilizer Plant. Equipment and pipeline names, temperature, material specifications, crack conditions, and time of discovery: Waste heat conversion boiler air vessel – 225°C, 12CrMo+16Mn; diameter 25 mm. Longest crack: 900 mm, depth: 15 mm. Seventh year. Medium-temperature shift furnace – 410°C, 13CrMo4; diameter 443 mm. Seven cracks were detected via radiography; the largest ones were visible to the naked eye. Under repair. Methanation furnace – 400°C, 15CrMo; diameter 26 mm. Eight cracks were detected via radiography; the largest ones were visible to the naked eye. Under repair. Pipelines from the fifth to the second stage of the convection section – 500°C, 13CrMo4; diameter Φ273×8. Visible thermal cracks occurred during welding. Pipelines from the sixth to the second stage of the convection section – 510°C, 12CrMo; diameter Φ219×8. Cracks ran along the weld line and eventually led to rupture. Seventh year. Pipeline from the waste heat conversion boiler to the medium-temperature shift furnace – 12CrMo; diameter Φ377×10. Cracks formed along the weld line and caused penetration. 2–4 years old. Welding characteristics and causes of cracks: I. Welding characteristics of pearlitic heat-resistant steel. Pearlitic heat-resistant steel is a low-alloy heat-resistant steel that contains various alloying elements such as chromium, molybdenum, vanadium, tungsten, and niobium. Its weldability is similar to that of low-carbon quenched and tempered high-strength steel. The main problems are hardening in the area near the weld seam and cold cracking. This type of steel has a high tendency to harden; it undergoes martensitic transformation when cooled to lower temperatures. Therefore, when welding this steel, if the cooling rate is high, hardened microstructures are most likely to form, which reduces the plasticity of the weld and heat-affected zone and increases their brittleness, often leading to cold cracks. To prevent cold cracking, preheating before welding and post-weld heat treatment are necessary. Welding should be carried out continuously without interruption, and the workpiece must be kept at a temperature above the preheating temperature throughout the welding process; a high heat input rate should also be used. II. Welding characteristics of austenitic stainless steels The main welding defect in austenitic stainless steels is hot cracking, especially single-phase austenitic stainless steels have a greater tendency to experience hot cracking. To prevent the formation of thermal cracks, the following measures should be taken: use a lower welding heat input (low voltage, low current, high welding speed), maintain a low interpass temperature by waiting for the previous weld layer to cool before applying the next layer, and employ narrow weld beads with no wand movement in order to reduce weld overheating and enhance resistance to thermal cracks. III. Welding characteristics of dissimilar steels Welding of dissimilar steels generally refers to the welding of stainless steel with low-carbon steel or ordinary low-alloy steel. When welding pearlitic heat-resistant steel with austenitic welding materials, although the base metals on both sides are of the same type, considering the overall weld material, it exhibits characteristics similar to welding dissimilar steels. When welding dissimilar steels, issues such as weld dilution, low-plasticity zones in the fusion zone transition layer, carbon migration, and stress differences in the joint occur; all of these have a significant impact on the weldability and service life of the joint. (1) When dilute pearlitic steel is welded using austenitic electrodes, the low content of alloying elements in the pearlitic steel results in the dilution of the alloy composition of the weld metal. This leads to insufficient amounts of austenite-forming elements in the weld, which can cause the formation of brittle martensitic structures. As a result, the quality of the joint is degraded, and there is a risk of cracks forming. (II) When welding dissimilar steels with a low-plasticity transition layer in the fusion zone, there is a narrow low-plasticity zone adjacent to the fusion line in the austenitic weld metal; the width of this zone is generally 0.2–0.6 mm, and its chemical composition and microstructure differ from those of the rest of the weld. Its formation is due to the significant difference in composition between pearlitic steel and austenitic filler metal; at the edge of the molten pool, the temperature of the liquid metal is lower, its fluidity is poor, the residence time of the liquid is short, and the mechanical stirring it experiences is also weak. The presence of a low-plasticity zone significantly reduces the impact toughness of the joint, and it can cause cracking in the weld seam under the effects of welding stress and restraint stress. As shown in Figure 1 and Figure 2. Figure 1 shows cracks at the fusion line resulting from the repair of the converter weld. Figure 2 also shows cracks at the fusion line resulting from the repair of the converter weld. (III) Carbon migration in the fusion zone: During the welding process, and especially during post-weld heat treatment and high-temperature operation, carbon diffusion and migration occur near the fusion line between different steel types. Carbon diffuses from the pearlitic steel side through the fusion line toward the austenitic weld, resulting in a decarburized layer on the pearlitic steel side near the fusion line, where ferrite is formed and the material softens; meanwhile, carburization occurs on the adjacent austenitic weld side, forming a highly hard black carburized layer. The carbon in the carburized layer precipitates in the form of chromium carbides, thereby hardening the material. As shown in Figure 3. Middle fusion zone, upper fusion zone. Figure 3: Micrographic image of the fusion line at the converted waste boiler air pocket, ×250. Due to the different deformation resistances of the carburized layer and the decarburized layer, stress concentration occurs, which results in a decrease in the high-temperature strength, plasticity, and corrosion resistance of the joint, as well as an increase in brittleness. This leads to cracking along the fusion line at high temperatures, as shown in Figures 4, 5, and 6. Figure 4: Colored photo of the crack at the fusion line of the converted steam drum (84.6). Figure 5: Colored photo of the crack site after grinding. Figure 6: Reproduced X-ray image of the crack shown in Figure 4. (IV) Stress differences in welded joints: Pearlite steels and austenitic steels have different linear expansion coefficients; within the temperature range of 20–600°C, most pearlite steels have coefficients ranging from 13.5×10-6K-1 to 14.5×10-6K-1, while austenitic steels have coefficients ranging from 16×10-6K-1 to 18.5×10-6K-1. When welding dissimilar steel joints made of these two materials, large stresses are often generated; the metal near the weld and the fusion line is subjected to tensile stress, while the metal slightly farther from the fusion line is under compressive stress. Tensile stress can cause the joint to fracture at the boundary between the weld and the base material. When such a welded joint is heated to high temperatures, welding stress can be reduced through a relaxation process; however, during the subsequent cooling process, new residual stresses inevitably arise due to the differences in the thermophysical properties of the steel plate and the weld metal. Therefore, post-weld heat treatment cannot eliminate residual stresses; it can only cause a redistribution of these stresses, which is different from welding the same type of steel. Such welded joints generate significant thermal stress when operating at high temperatures. Pearlitic steel is more prone to oxidation than austenitic welds, which may lead to the formation of notches; under thermal stress, fatigue cracks can occur on the side of the pearlitic steel. When the joint is in a state of thermal fatigue, the resulting alternating stresses are more dangerous, as they can cause the joint to fail within a short period of time. As shown in Figures 7 and 8. Figure 7: Cracking at the fusion line of the air duct (79.11). Figure 8: Cracking at the fusion line of the tubes in the converter off-gas boiler converted to a medium-tempereature furnace (82.6). The tube welds in these two locations are situated at the junctions of two sets of 90° elbows, and cracks occur on a short time scale due to thermal stress and additional bending stress. (IV) Formation of welding hot cracks is due to the completely opposite welding requirements of pearlitic steel and austenitic steel: the former requires a larger heat input, while the latter requires a smaller one. When pearlitic heat-resistant steel is welded using austenitic welding materials, cracks are very likely to occur if proper considerations are not taken. In on-site repair welding, austenitic welds are used due to limitations in heat treatment conditions. To prevent hardening and cold cracking in the zone near the weld seam of pearlitic heat-resistant steel, especially in containers with greater thickness, a higher heat input is often used; as a result, hot cracks frequently occur in the welded portion of stainless steel. Figures 8 and 9 show typical examples of such thermal cracks. Figure 9: X-ray photo of thermal cracks resulting from improper repair of a medium-temperature converter (1980); weld between 13GrMo44 and Ö302; cracks in 13GrMo44. Figure 10: Cracks in the gas collection pipes from section 5 to section 1 of the convection furnace (schematic, 1981.10). Characteristics and inspection methods of cracks in welded joints of dissimilar steels. I. Characteristics of cracks: 1. The crack widths are relatively large; some of these cracks are visible to the naked eye. 2. Thermal cracks that occur during welding can appear in both the weld area and the fusion line, with no distinct pattern. As shown in Figures 1, 2, 9, and 10. 3. Cracks that occur after use at high temperatures mostly form at the weld line and extend inward along it. This type of crack is widespread and can occur in every weld under the same conditions. As shown in Figures 7 and 8. II. Inspection Methods 1. Since the width of such cracks is relatively large, X-ray radiography can generally be used for inspection. 2. If X-ray testing is difficult to carry out, ultrasonic testing can be used as a reference. Although the weld itself has an austenitic structure and is difficult to inspect using conventional pulsed ultrasonic testing methods, since the base material has a pearlitic structure and cracks often occur along the fusion line, inspection can be carried out as a reference. 3. Since cracks that occur after use at high temperatures first appear on the inner surface, internal surface coloring inspection and macroscopic inspection are effective methods. 4. Since the weld structure is austenitic, conventional ultrasonic testing cannot detect internal defects in the weld, while X-ray testing has a high rate of missed defects due to limitations in the direction of radiation, sensitivity in detecting cracks, angles at which cracks can be detected, and the detection efficiency. Conclusion 1: By analyzing the welding characteristics and defects associated with the use of austenitic weld materials for the aforementioned pearlitic heat-resistant steels, it can be seen that although using austenitic weld materials is a simple method for welding such steels on-site, the presence of dangerous defects such as cracks significantly reduces their service life; this lifespan is even shorter when subjected to high additional stresses. 2. Due to the metallic structure characteristics of austenitic welds, inspection poses certain difficulties, resulting in a high rate of missed defects; this can allow dangerous defects such as cracks to remain in the welds, posing a significant threat to the safe use of containers and pipelines. Therefore, conditions should be created to use heat-resistant steel electrodes and corresponding welding processes, while trying to avoid the use of joints made from different types of steel. 3. The main challenges in welding pearlitic heat-resistant steel on-site lie in the pre-welding preheating and post-welding heat treatment conditions. In the past, the heat treatment equipment was complex, and it was difficult to ensure the required process parameters, which is why this approach was not widely adopted. In recent years, many domestic manufacturers have begun producing high-efficiency heaters suitable for use on-site, which are bendable and modular. There are track-type, finger-type, rope-type, etc., which are highly suitable for the heat treatment of pressure vessels and pipelines on-site. Therefore, when welding such steel materials in the future, it is necessary to improve welding heat treatment equipment and use heat-resistant welding materials in order to enhance the performance of the welded joints, extend their service life, and prevent the occurrence of dangerous defects and accidents. Papers from the 6th Welding Conference of Shandong Province, September 1989; Papers from the 1st Welding Conference of East China, Shanghai, October 1989