Process evaluation for corrosion-resistant surfacing of pressure vessels

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Objective: To obtain a surfacing process that yields surfacing metal with chemical composition meeting the specified requirements for corrosion resistance. Rule (1): The factors for evaluating the surfacing process using various welding methods include: surfacing layer thickness, base material, filler metal used for surfacing, welding position, preheating, post-weld heat treatment, gases, electrical properties, and technical measures. For details, refer to Table 16 of standard NB/T 47014. (2) Changing the surfacing welding method requires re-evaluating the surfacing process. (3) When using SMAW, SAW, GTAW, GMAW, and PAW, except that the evaluations for horizontal, vertical, and overhead welding positions are all applicable to the flat welding position as well, changing the welding position for which evaluation is successful requires re-evaluating the surfacing process. This is because the welding heat input required for surfacing at different welding positions varies, which results in different dilution rates of the weld metal and thus different chemical compositions of the weld metal; therefore, it is necessary to conduct a new surfacing process evaluation. (4) Re-performance of the welding procedure qualification is not required in the following situations: ① When the tubular test piece is fixed horizontally in the 5G position and the surfacing procedure has been qualified; it is suitable for flat welding, vertical welding, and overhead welding ； ②The surfacing process, which has passed evaluations for all welding positions including horizontal, vertical, and overhead welding, is suitable for all welding positions ； ③The surfacing process, which yields satisfactory results when the tubular test piece is fixed at a 45° angle (6G), is suitable for all welding positions. (5) The thickness of the base layer of the test piece, as well as the applicable range for the thickness of the base layer of welded parts, are shown in the table below (Table 17 of NB/T 47014). Images Evaluation Methods 1. Test piece shape Welded joints are divided into plate-type and tube-type; tube-type refers to pipes and rings. Tubular specimens can be surfaced welded on the outer or inner wall of the tube. 2. Specimen dimensions: (1) For plate-type surfacing specimens, the length and width shall be 150 mm or more; refer to Figure 14 of standard NB/T 47014. The length of the tubular surfacing test piece shall be 150 mm or more, and its minimum diameter shall meet the requirements for the number of specimens to be taken; surfacing can be carried out continuously around the circumference of the tube, as shown in Figure 15 of standard NB/T 47014. (2) The width of the surfacing layer is greater than or equal to 40 mm. Inspection requirements and result evaluation (1) The inspection items shall include: penetrant testing, bending test, and chemical composition analysis (when specified). (2) Penetrant testing can be carried out using the dye penetrant method or the fluorescent method; the testing procedures shall comply with the provisions of JB/T 4730, and no cracks are allowed in the test results. (3) The bending test is conducted in accordance with the test methods specified in Table 13 of standards GB/T 2653 and NB/T 47014, in order to evaluate the integrity and plasticity of the surfacing metal, the weld zone, and the heat-affected zone of the base material. Qualification criteria: After the bent specimen is bent to the specified angle, there shall be no open defects larger than 1.5 mm in the surfacing layer on the tensile face of the specimen, and no open defects larger than 3 mm within the weld line. (4) Methods for determining chemical composition: ① Direct measurement on the as-welded surface of the surfacing layer, or measurement by obtaining chips from the as-welded surface. ②It is determined on the machined surface after removing the as-welded surface layer, or by obtaining chips from the machined surface. ③Chip collection is determined by drilling horizontally from the side of the surfacing layer. The chemical composition analysis methods and acceptance criteria are specified in the relevant technical documents. (5) The minimum thickness for evaluating the surfacing layer is shown in Figure 17 of NB/T 47014. ①When measurements are taken on the as-welded surface, it is the distance a from the weld line to the as-welded surface; ② when measurements are taken on the machined surface after the as-welded surface layer has been removed, it is the distance b from the weld line to the machined surface; ③ when chips are collected by making horizontal drilling from the side for measurement, it is the distance c from the weld line to the edge of the drill hole.