Cracks & Corrosion in the Heat-affected Zone of DSS Flange (Part One)

Abstract: Aiming at the leakage during the testing of the seawater pipeline system of a nuclear power plant, from the construction to the testing process, the multi elimination methods were used to analyze the corrosion. By analyzing the properties of the material, distribution of stress, and the medium environment of the material, it is concluded that the crack and corrosion are stress corrosion, which meets the three elements of stress corrosion: ① The material is sensitive to the material. ② The materials and the structure used in some areas of the system are different, resulting in the effect of structural stress and load stress. ③ Operate in the corrosive medium, that is, seawater. In this way, stress corrosion cracking conditions are achieved, and penetrating cracks appear and cause the leakage of the medium. Since the structure and the medium environment are stable, this problem can be solved by replacing the flange made of non-sensitive duplex stainless steel with quality that meets the requirements of the standard. The analysis method of this crack and corrosion can be used for reference by others.
 
The welding seam between the reverse flange side (forging flanges in S32750) and the pipeline in QA2 S32750 of the 4-channel pump outlet of the seawater power plant at a nuclear power plant, with a quality level belonging to the Russian standard nuclear grade IIIc, and a specification of φ508×5.54 mm with a butt joint, using φ1.6 mm, grade ER2594 stainless steel wire and TIG welding. It is qualified after testing by non-destructive inspection. After the contractor completed the testing of the pump unit (seawater being injected into the pipeline), the project was handed over to the owner. When testing the pump, the testing department found that there are different degrees of cracking for the base metal at the edge of the welding seam on the reverse flange side of the pump outlet of the four channels. After removing the short pipe connecting the pump and the flange on site, it was found that not only the base metal cracks but also partial pitting corrosion and perforation existed on the flange side. The defects of the four channels and four welding seams were very serious. By analyzing the joint material, component force, corrosive medium and temperature, the cause of the crack is found by exclusion analysis, which can be used for reference by other manufacturers.
 
1. Causes and analysis
In order to find out the cause of the defect, from the construction to the testing, a comprehensive inspection of the influencing factors was carried out, and the root cause of the defect was found through the exclusion method. The specific investigation is as follows:

(1) Qualifications and skills of personnel

After verification, the welder has the HAF603 welder qualification certificate and the welder project qualification meets the requirements. The welder qualification certificate is valid for 6 months and should be renewed by the National Nuclear Safety Administration, which meets the requirements of HAF603.

(2) Welding procedure qualification and data package

After consulting relevant papers, welding elements such as line energy have an impact on the structure and performance of welded joints of pipelines in S32750. After verification, the welding process qualification of this system is qualified and meets the requirements of Russian standards. The welding data package has been approved. At the same time, the system where the on-site pipeline is located uses this process to complete 115 welding. There are no cracks and corrosion in other welding seams, and the welding process meets the standard requirements.
 

(3) Base metal and welding consumables

By checking the documents provided by the pipe base metal and welding material manufacturers, the quality documents of the pipe base material and welding material are complete and meet the requirements of the design documents. By checking the documents provided by the flange manufacturer, it is found that the hardness test and ferrite content test were missing. In addition, comparing the flange raw materials with the manufacturer's inspection data, the manufacturer's strength test data was low.
 
The chemical composition and mechanical properties of forging flanges, pipes, and welding materials were tested, and the results are shown in Table 1 and Table 2.
 
Table 1 Chemical analysis and related physical and chemical inspection of flanges, pipes and welding materials

 
Table 2 Mechanical properties, physical and chemical inspection of surfacing metal

 
Test items for forging flanges in S32750 (ASME SA-182): chemical analysis, normal temperature tensile, ferrite content tests and Brinell hardness.

Test items for pipes in S32750 (ASME SA-790): chemical analysis, normal temperature tensile, ferrite content tests, Rockwell hardness, intergranular corrosion, pitting, pitting equivalent and tests for bending properties.

Test items for welding wire ER2594 (ASME SFA5.9): chemical analysis, normal temperature drawing, ferrite, intergranular corrosion and pitting. Through the above tests, it is found that the ferrite content of the pipeline does not meet the requirements, and the comprehensive performance of the duplex stainless steel cannot be achieved; there are doubts about the material.