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Impact of Normalizing Temperature on High-Strength Bainitic Steel Pipe

Understanding the influence of normalizing temperature on the microstructure and properties of high-strength bainitic steel pipe is crucial for optimizing its performance in various applications. In this...

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Causes of Cracking of Welding Seams of Power Station Boiler Tubes (Part Two)

Posted: 05/24/2021 10:25:39 Hits: 58

1.5 Metallographic inspection
Cut 3 pieces of metallographic samples along the base material of the leaking pipe, the largest opening and crack tip of the leakage port from the cross section. The etchant is the nitric acid alcohol solution with 4%. The metallographic samples are observed by the Axio Observer Alm metallurgical microscope. The metallographic structure of the base metal and welding seams on both sides of the leaking part is ferrite plus pearlite; the spheroidization is level 3, and the metallographic structure is normal, as shown in Figures 3 and 4.

Figure 3 Metallographic structure of base metal

Figure 4 Metallographic structure of welding seams

The cracks are mainly distributed in the heat-affected zone of the welding seam. The cracks originate from the outer surface of the pipe. The grains near the cracks are coarse and continue to expand to the inner surface along the original austenite grain boundaries, showing the characteristic of typical reheat cracks, as shown in Figure 5.

Figure 5 The morphology of the crack tip

1.6 Scanning electron microscope topography inspection
After ultrasonic cleaning the sample at the largest leaking part in alcohol, the fracture was observed with a ZEISS EVO 18 scanning electron microscope. As shown in Figure 6, the fracture morphology is shaped like crystal sugar, which is a typical intergranular cracking morphology. A large amount of Fe oxide exists on the surface of the fracture after EDS detection and analysis, which is mainly caused by high-temperature oxidation after leakage. No other foreign matter was found.

Figure 6 SEM morphology of fracture

2. Analysis and discussion
12Cr1MoVG steel is pearlitic low-alloy heat-resistant steel with good thermal strength and durability. It is generally used to make heating surface pipes with wall temperatures less than 570℃, headers, steam pipes, and large forgings of boilers with wall temperatures less than 555℃, etc. Relying on the solid solution strengthening of Cr and Mo alloy elements, a combination of the contained V and carbon to form VC and other carbide dispersion strengthening, it can maintain a high structure, structural stability and long-lasting strength. The steel has good welding performance, but has a certain tendency to crack under certain conditions. Cr-Mo (-V) heat-resistant steel is mainly composed of Cr and Mo, both of which are carbide forming elements and have precipitation strengthening effect. As the content of Cr and Mo increases, the tendency to reheat cracks increases. The addition of V in Cr-Mo steel will significantly increase the tendency of reheat cracking. If the following situations happen, such as great restraint stress of the welding seam, low preheating temperature, great welding heat input, and improper heat treatment parameters of post welding, it is easy to form reheat cracks. From the macro inspection results of the leaking part, there is no obvious swelling of the pipe diameter at the burst, and the edge of the burst is rough without overheating. The analysis result of chemical composition shows that the composition of the sample provided for inspection meets the technical requirements of GB/T 5310-2017. The hardness and tensile of the bursted pipe end meet the technical requirements of relevant standards for 12Cr1MoVG. The metallographic structure of the base metal and welding seams on both sides of the bursted pipe end is ferrite plus pearlite; the metallographic structure is normal, and there is no obvious sign of aging. The crack originates from the outer surface and is mainly distributed in the welding heat-affected zone. The grains near the crack are coarse, cracking along the original austenite grain boundary and expanding to the inner wall, which is a typical reheat crack. After investigation, the bursted pipe end of the welding seam was the on-site installed welding during the heating renovation of the No. 2 boiler in 2009. During the construction, excessive force was used for connecting valves and the tube was a clamping tube with high stress due to the narrow space. After high-temperature operation for a long time, external surface cracks are formed at the welding fusion line, which continues to expand and cause leakage.

3. Suggestions
3.1 The main reason for the leakage of the reheater tube is the improper welding process. Under high-stress conditions and during long-term operation, reheat cracks are generated from the fusion line area on the surface of welding seams, and further expansion leads to leakage.

3.2 In order to prevent leakage caused by similar reheat cracks, it is necessary to avoid excessive force for connection, reduce the welding stress and optimize the welding process. If the preheating temperature is appropriately increased, the preheating temperature before GTAW welding is controlled at 150℃ to 200℃; before SMAW welding, the preheating temperature is about 250°C. The temperature between layers is less than or equal to 300℃, and the heat preservation effect is strengthened. The stress relief treatment after welding should be carried out in time, and the heating speed and holding temperature should try to avoid the temperature range of 500℃ to 700℃, where 12Cr1Mo V steel is sensitive to reheat cracks. The holding time should be appropriately extended according to the wall thickness.

3.3 In combination with maintenance, use penetration or magnetic powder to inspect the welded seam of the reheater tube. If cracks are found, the tube shall be cut and treated in time.

4. Preventive measures
The reliable heat exchange equipment should be replaced to avoid leakage. Chemically clean the boiler to remove grease on the waterside. In order to prevent the risk of cracking of the boiler caused by the swelling of the boiler, and the recurrence of similar accidents, the following suggestions are hereby proposed:
(1) When designing the boiler, the boiler design company should try to avoid direct contact of the drum to the frame and high-temperature flue gas, so as to reduce the risk of excessively high temperature of the drum.
(2) The user of the boiler should strengthen the water treatment of the boiler. Strictly carry out the water quality test. The water treatment index must meet the relevant standards to ensure that the boiler runs without scale or thin scale.
(3) Strengthen professional knowledge training, safety and responsibility for operators of boilers and water treatment personnel, and master the methods and techniques of sewage discharge to achieve scientific sewage discharge.
(4) Strengthen management; shut down the boiler regularly, and conduct detailed inspections of the inside of the boiler. The abnormal situation found during the use of the boiler should be reported in time. Scientifically analyze the problem, and hidden dangers should be eliminated in time.

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