Inspection requirements for test pieces used in the welding procedure qualification of butt and fillet welds

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Key points for sampling and testing of mechanical property tests: Clause 2.2.6 of the \"Safety Technical Inspection Regulations for Fixed Pressure Vessels\" stipulates that: \"The welding materials used for the pressure-bearing components of pressure vessels must ensure that the mechanical properties of the weld metal meet the minimum values specified in the standards for the base material, and that the impact absorption energy meets the requirements specified in Table 2-1 of these regulations.\"； When necessary, other properties must also meet the corresponding requirements of the base material. To ensure that the mechanical properties of the weld metal meet the lower limits specified in the base material standards, it is essential first that the mechanical properties of the weld metal produced from the welding materials meet those lower limits. For welded pressure vessels, with the aim of ensuring their safe and reliable operation, it is required that the properties of the weld joints be no lower than those of the base material. The acceptance criteria for the mechanical property tests of weld procedure qualification specimens for butt welds are generally based on comparisons with the base material; these factors must also be taken fully into account in terms of sampling locations and testing methods, so as to keep them as consistent as possible with those of the base material. The mechanical property testing of welded specimens essentially involves evaluating the performance of the weld joints in those specimens; weld joints comprise the weld zone, the fusion zone, and the heat-affected zone. Among welding joints, the heat-affected zone is the most complex; there are few welding process parameters that can be controlled and adjusted, and its performance is often the lowest. It represents the weakest part of the welding joint and is the key area to be inspected in welded specimens. Toughness indicators are important mechanical properties of welded joints, and impact toughness testing is a sensitive testing method. By determining the impact toughness of welded joints, changes in welding processes can be identified; therefore, it is necessary not only to conduct impact toughness tests on the weld area but also on the heat-affected zone. Design standards for pressure vessels generally require that the performance of welded joints be no lower than that of the base material. The mechanical property testing of the base material involves sampling from specified locations in accordance with relevant standards, and the test results are typical, representative, and fair. Therefore, the sampling location for the mechanical properties of the weld procedure qualification specimens for butt welds should also be kept as consistent as possible with that of the base metal. It is not uncommon in actual production to complete a weld using one or more welding methods or one or more welding processes; the mechanical property testing of welding procedure qualification specimens must also take into account such combined welding scenarios. When welding procedure qualification specimens are prepared using combined welding, the weld metal and heat-affected zone resulting from each welding method and each welding procedure in the specimen shall be subjected to tensile testing, impact testing, and bending testing. The sampling locations for the mechanical property specimens used in the welding procedure qualification tests for butt welds, as well as the key inspection points, are intended to ensure that the mechanical properties of the welded joint meet the minimum values specified in the standards for the base material; the sampling locations should also be as consistent as possible with those of the base material. Inspection of butt weld specimens and samples 1. Inspection items for specimens: visual inspection, non-destructive testing, mechanical property tests, and bending tests. 2. Inspection requirements: Visual inspection and non-destructive testing results must show no cracks. 3. Mechanical property tests and bending tests: (1) Unless otherwise specified, the items for mechanical property tests and bending tests as well as the sample sizes shall comply with the provisions in Table 11 of standard NB/T47014. (2) When an impact test is specified, only steel and aluminum-magnesium alloy weld joints with a magnesium content exceeding 3% are subjected to Charpy V-notch impact tests, and for aluminum-magnesium alloy weld joints, impact specimens are taken only from the weld area. (3) When the test piece employs two or more welding methods (or welding processes), the tensile faces of the tension test specimens and bending test specimens shall include the weld metal and heat-affected zone from each welding method (or welding process) ； When impact testing is required, the weld metal and heat-affected zone of each welding method (or welding process) must be subjected to impact testing. (4) The dimensions of the tensile and bending specimens have allowable tolerances determined in accordance with relevant standards or technical specifications. 4. Sampling requirements for mechanical property tests and bending tests: (1) When sampling, cold working methods are generally used; when hot working methods are employed, the heat-affected zone must be removed. (2) It allows for the preparation of specimens by avoiding welding defects and imperfections. (3) Cold leveling of the specimen is permitted before removing the excess height. (4) The sampling locations for plate butt weld specimens and tube butt weld specimens shall be in accordance with Figures 2 and 3 of standard NB/T 47014. 5. Requirements for sampling and processing of tensile specimens: (1) The weld bead excess on the specimen should be removed by mechanical means so that it is level with the base metal. (2) For specimens with a thickness of 30 mm or less, tests shall be conducted using full-thickness samples. The thickness of the specimen should be equal to or close to the thickness T of the base material of the test piece. (3) When the testing machine is limited in capacity and cannot perform tensile tests on full thickness, the specimen can be sampled in uniform layers along its thickness direction; after dividing it into equal parts, the thickness of the resulting specimens should be as close as possible to the maximum thickness that the testing machine can handle. Testing of two or more equally divided specimen pieces replaces the testing of a specimen of full thickness. (4) The shapes of the tensile specimens are shown in Figures 4 to 7 of standard NB/T 47014. 6. Tensile testing and acceptance criteria: (1) When the base material of the specimens is of the same metal code, the tensile strength of each specimen shall be not less than the \"minimum value of the tensile strength specified in this standard for the base material\", rather than the previously required minimum value specified in the standards for the base material steel grade. ①The minimum specified tensile strength for the steel base material is equal to the lower limit of the tensile strength specified in its standards. ②For aluminum base materials of categories Al-1, Al-2, and Al-5, the minimum specified tensile strength is equal to the lower limit of the tensile strength specified for their annealed state. The minimum specified tensile strength for base metals of category Al-3 is given in Table 12 of NB/T47014. ③The minimum specified tensile strength for titanium base materials is equal to the lower limit of the tensile strength specified for its annealed state. ④The minimum specified tensile strength for copper base materials is equal to the lower of the tensile strength limits specified for its annealed state and those specified for other states. When the standard does not specify a lower limit value for the tensile strength of extruded copper in its annealed state, it can be determined as 90% of the lower limit value for tensile strength specified in the standard for the as-manufactured state, or it can be determined based on the results of experimental studies. ⑤The minimum specified tensile strength for nickel-based base materials is equal to the lower limit of the tensile strength specified in the standards for those base materials in their annealed state (for Ni-1 and Ni-2 categories) or in their solution-treated state (for Ni-3, Ni-4, and Ni-5 categories). (2) When the base materials of the test specimens are represented by two metal material codes, the tensile strength of each specimen shall not be lower than the smaller of the minimum tensile strengths specified in this standard for the two base materials. (3) If it is specified to use weld metal with a room temperature tensile strength lower than that of the base material, the tensile strength of each (piece) specimen shall be not lower than the minimum specified tensile strength for the weld metal. (4) If the above-mentioned specimen fractures on the base metal outside the weld or fusion line, its tensile strength value shall not be lower than 95% of the minimum tensile strength value for the base metal specified in this standard; this is different from the previous requirement that the minimum value for a single specimen must be no less than 95% of the lower limit specified in the standards for the base metal grade (95% for carbon steel, and 97% for low-alloy and high-alloy steels). (5) After stratified sampling, the acceptable criterion for the tensile specimens is no longer the average value; rather, the tensile strength of each specimen (sheet) must be not lower than the minimum value of the tensile strength of the base material specified in this standard. Since the base material specified in standard NB/T 47014 includes not only steel but also non-ferrous metal materials such as aluminum and copper. The base material refers only to the state before welding. After welding, the microstructure, composition, and properties within the fusion zone and heat-affected zone of the base material have all changed compared to their original state. Therefore, the material in the fusion zone and the welding heat-affected zone is no longer the material as it was before welding. The acceptable criteria for its tensile strength can no longer be based on the lower limit specified in the material standards; instead, they must be determined using the minimum tensile strength value for the base material as specified in NB/T 47014. For simplicity, the minimum tensile strength of steel base materials is set equal to the lower limit specified in the steel standards; whereas for non-ferrous metal base materials such as Al, Cu, and Ni, the minimum tensile strength is the value corresponding to the tensile strength of the material under annealed conditions. 7. Bending test specimens (1) The forms of the bending test specimens are shown in Figures 8 and 9 of the NB/T 47014 standard. (2) Requirements for processing bent specimens: The weld excess height on the specimens must be removed by mechanical means; the tensile surfaces of the face-bent and back-bent specimens should be made smooth, and there shall be no scratches or damage on these tensile surfaces. 8. Bend test and acceptance criteria: The bend test for welded joints is conducted in accordance with the test methods specified in GB/T 2653 and Table 13 of NB/T 47014, to determine the integrity and plasticity of the welded joints. (1) The maximum thickness of the bent specimen is 10 mm for steel, nickel, and certain aluminum alloys, and 3 mm for certain aluminum and copper alloys. (2) The results of the bending test are related to the surface processing roughness; only when the surface processing quality is consistent can the test results be compared ； In addition to the base metal categories listed in items 1 to 4 of Table 13 of standard NB/T 47014, for base metals whose specified lower limit for the elongation after fracture A is less than 20%, if the bending test specified in item 5 of Table 13 fails and the actual measured value is also less than 20%, it is permissible to increase the bend radius and conduct the test again; in this case, the bend radius shall be equal to S(200–A)/(2A), where A is the specified lower limit for elongation after fracture multiplied by 100, and the distance between the supports shall be equal to the bend radius plus (2S+3) mm. (3) The bending angle shall be measured when the specimen is under load. (4) During the lateral specimen bending test, the weld metal and heat-affected zone shall be completely located within the bent portion of the specimen ； During combined evaluation, a side-bending test should be conducted. (5) Qualification criteria: After the bend test specimen of the butt weld is bent to the specified angle, there shall be no single open defect with a length greater than 3 mm in the weld area or the heat-affected zone on its tensile face, in any direction. Open defects at the corners of the specimen are generally not considered, but the length of open defects at the corners caused by lack of fusion, inclusions, or other internal defects shall be taken into account. When two or more specimens are used, each specimen shall meet the above requirements. 9. Impact test and acceptance criteria: (1) The sampling location for the impact specimen is shown in Figure 10 of standard NB/T 47014. The longitudinal axis of the specimen should be perpendicular to the weld axis, while the axis of the notch should be perpendicular to the surface of the base material. For specimens taken from the weld area, the axis of the notch should lie on the center line of the weld; for specimens taken from the heat-affected zone, the distance k between the axis of the notch and the intersection of the specimen’s longitudinal axis with the fusion line should be greater than 0, and the notch should pass through as much of the heat-affected zone as possible. (2) The form, dimensions, and testing methods of the impact specimen shall comply with the provisions of GB/T 229. When the size of the test piece makes it impossible to prepare a standard specimen (with a width of 10 mm), small impact specimens with widths of 7.5 mm or 5 mm should be prepared in sequence. (3) The impact test temperature shall not be higher than the impact test temperature specified in the steel standard. (4) The average value of the impact absorption energy for each zone of steel welded joints, taken as a group of 3 standard specimens, shall meet the requirements specified in the design documents or relevant technical documents, and shall not be lower than the values given in Table 14 of NB/T 47014. At most one specimen may have an impact absorption energy that is lower than the specified value, provided that it is still not lower than 70% of the specified value. (5) For aluminum-magnesium alloy base materials with a magnesium content exceeding 3%, the test temperature shall not be higher than the lowest designed metal temperature of the pressure-bearing equipment. The average value of the impact absorption energy for three standard specimens in the weld area shall meet the requirements specified in the design documents or relevant technical documents, and shall not be less than 20 J. At most, one specimen may have an impact absorption energy below the specified value, provided that it is not less than 70% of the specified value. (6) The impact energy values for small-scale impact specimens with a width of 7.5 mm or 5 mm are 75% or 50% of those for standard specimens, respectively. (7) Impact temperature test: The temperature for impact tests, as commonly referred to, is “room temperature,” and there is no standard-defined range, which is non-standard. The concept of “room-temperature impact test” is introduced in GB/T 229 “Metallic materials — Charpy pendulum impact test method”. This standard stipulates that “for tests where a test temperature is specified, the test shall be conducted within a range of ±2°C of the specified temperature.” If not specified, the room temperature shock test is conducted within the range of 23°C ± 5°C.” Standard NB/T 47014 requires that the impact test temperature for welding procedure qualification specimens shall not be higher than the temperature specified in the steel standard ； For aluminum-magnesium alloys with a magnesium content exceeding 3%, it is not only reasonable but also practical to set the test temperature at or below the minimum design metal temperature of pressure equipment. For welds in which the base materials are steels such as 20R, 16MnR, Q245R, or Q345R (where the steel standards specify an impact test temperature of 0°C), and for which a welding procedure qualification for butt welds has been conducted in accordance with JB 4708-2000, with the impact tests carried out at “normal temperature” or “room temperature”; provided that the absorbed impact energy meets the acceptance criteria stipulated in NB/T 47014, there is no need to weld additional specimens for impact toughness testing in order to conduct the 0°C impact test on the welded joints. In such cases, the original welding procedure qualification remains valid. The applicable temperature range is greater than or equal to 0°C. This is because the impact absorption energy of the welded joints of such steel grades remains largely unchanged across temperatures from 0°C to 35°C, with no significant difference. However, if the design documents or user requirements stipulate that impact tests at temperatures below 0°C must be conducted for the welding procedure qualification, then such tests must be carried out as required.