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Welding procedure qualification is an important quality assurance activity for enterprises. In line with Chinese regulations and domestic conditions, China has adopted ASME’s \"Code for Pressure Vessels\" as a reference, and by incorporating relevant provisions from European standards, it has formulated the welding procedure qualification standard NB/T 47014 \"Welding Procedure Qualification for Pressure Equipment\", which is applicable to pressure vessels in China. We briefly outline the relevant requirements of ASME codes regarding the evaluation of welding processes for pressure vessels, in accordance with the provisions of Volume IX \"Welding and Brazing Evaluation\" of the U.S. ASME Boiler and Pressure Vessel Code, as well as the relevant sections of AWD 1.1 \"Code for Welding of Steel Structures\" applicable to ordinary steel structures. Principles for welding procedure qualification of boilers and pressure vessels. The necessity of carrying out welding procedure qualification for boilers and pressure vessels is determined, in principle, based on the important parameters of the welding process. Any significant deviation of a parameter from the specifications in the original welding procedure specification, or any change that exceeds the limits permitted by regulations, requires corresponding welding procedure qualification tests to be carried out. Welding procedure qualification items are classified by joint type, with the following three basic joint types serving as those for the qualification specimens; these encompass all the joint types that may occur in product designs. 1) Grooved fully penetrated butt joints can be used to evaluate all grooved fully penetrated butt joints and fillet joints, including grooved fully penetrated T-joints. 2) Grooved fully penetrated butt joints can be used to evaluate all grooved fully penetrated butt joints and fillet joints, including T-joints with grooved full penetration. 3) Grooveless fillet joints can be used to evaluate all grooveless fillet joints, including the fillet welds between nozzles and shells. Which welds require welding procedure qualification? In the manufacturing of boilers and pressure vessels, manufacturing regulations do not require weld procedure qualification for all welds in the product structure; only the following welds must undergo weld procedure qualification tests. 1) Various types of joints on welded pressure components. 2) Various types of joints on non-compressed load-bearing welded components, such as all permanent or temporary lugs connected to compressed components and the welds connecting the reinforcement plates. 3) The welds connecting non-compressed and essentially load-free components (such as attachments that increase the heat exchange surface area – fin plates, insulation support pins, etc.) to compressed components shall be determined in accordance with the following principles: If manual or mechanized welding methods are used, weld procedure qualification tests for fillet welds shall be conducted. If a fully automatic welding method is used, there is no need to conduct a welding procedure qualification. The rules for welding procedure qualification are determined based on the key parameters of the welding process. 1. Welding method: When switching from one welding method to another, welding procedure qualification tests must be conducted. Welding methods suitable for boilers and pressure vessels include: gas welding, shielded metal arc welding, submerged arc welding, gas metal arc welding, tungsten inert gas welding, plasma arc welding, electroslag welding, laser welding, electron beam welding, flash butt welding, induction heating pressure welding, resistance welding, thermite welding, gas pressure welding, inertia and continuous drive base metal welding, stud arc welding, and stud resistance welding. On the same weld of the actual welded part, if two or more different welding methods are used, or if different critical process parameters are applied, then weld procedure qualification can be carried out on the specimen by considering the base material and metal thickness for each welding method. The same set of procedure qualification specimens can also be welded using the combined joint method or welding process intended for the actual welds of the component. However, for each welding method or welding process, the thickness of the weld metal produced must be such that it allows for the preparation of the required tensile and bending specimens. For shielded metal arc welding, tungsten inert gas welding, gas metal arc welding, plasma arc welding, and submerged arc welding, or combinations of these methods, if the completed weld procedure qualification used specimens with a thickness greater than 13 mm, then the weld procedure qualification report for that method can be used together with the report for another welding method on the welds of the same actual workpiece, including the root pass. 2. Base metal categories There are a wide variety of base metals used in boilers and pressure vessels. If evaluation is carried out using the steel grade of the base metal or the material designation, the amount of work required for such evaluation is very large, and there is no need for it. To reduce such meaningless repeated evaluations, the ASME regulations in the United States classify the standard materials approved for use by their chemical composition, mechanical properties, and weldability. In other words, materials with similar alloy compositions, strength levels, and weldability are grouped together and assigned a P classification code. Within the same category of base metal, they are further grouped according to grades of strength and impact toughness, with the group number indicated after the classification number; for example, SA106‑A carbon steel belongs to Category 1, Group 1, and its classification code is denoted as P1‑1. The latest version of the ASME code classifies nearly 1,000 types of steel used in boilers and pressure vessels into 23 categories and 52 groups. This classification of base metal is based on a large amount of data from material weldability tests and welding process tests, as well as years of practical production experience. Therefore, for various base metal materials listed in the same category, if the other important parameters of the welding process specified in the proposed welding procedure are the same or within acceptable ranges, the welding procedure qualification reports for one type of base metal material can be applied interchangeably. For example, carbon structural steels such as SA106-B and SA-36 both belong to Class P-1. If the same welding methods, welding materials, and similar welding process parameters are used, and the thickness of the welded parts is within the range permitted by the process qualification standards, then the welding procedure for SA-36 steel can be prepared based on the welding process qualification report already completed for SA106-B steel, without the need to conduct another welding process qualification test for SA-36 steel. Table 3-7 lists examples of the classification groups for typical steel grades in Volume IX of the ASME Code, Welding Procedure Qualification. Table 3-8 specifies the general principles regarding the relationship between the base metal used in the procedure qualification specimens and the type of base metal being evaluated, when different types of base metals are welded together. Due to the significant differences between China’s material classifications and those in the United States, as well as the fact that no system for recognizing regulatory materials has yet been established, it is difficult to directly apply the material classification table listed in Volume 9 of the US ASME codes. If the design specifications require the use of materials in accordance with ASME codes, it is entirely possible to determine the necessity of conducting a welding procedure qualification based on the general principles for material classification listed in Table 3-8. When domestic standards are adopted, domestic standard materials can be classified and grouped in accordance with the aforementioned principles, by referring to the chemical composition and strength grades of similar steel grades specified in ASME codes. The welding procedure qualification standards listed above in China have all made attempts to classify domestic steel grades, but some of them are not comprehensive enough, and some do not conform to the classification principles. This is because classifying materials from the perspective of weldability assessment is a highly complex and meticulous task, and a large amount of basic data must be accumulated to achieve completeness and accuracy. At the current stage, for commonly used steel grades that have a large amount of experimental data available and years of production experience, relatively accurate classification and grouping are possible. For steel grades and materials for which the relevant data are insufficient and welding experience is lacking, it is required to conduct the necessary weldability tests prior to the welding procedure qualification. After obtaining sufficient experimental data, it is gradually classified. 3. Thickness of the base metal and thickness of the weld metal: The thickness of the base metal and the weld metal determines, to a certain extent, the performance of the joint. This is partly because, as the thickness increases, the strength properties of the base material itself change ； On the other hand, the cooling rate in the welded area is determined by the wall thickness of the joint; the greater the wall thickness, the faster the cooling rate, and the higher the strength properties of the joint. Therefore, for specific welding process parameters, there is a certain range of applicability for the thickness of the base metal or weld metal. The scope of application depends on the welding method used. For common welding methods such as shielded metal arc welding, tungsten inert gas welding, gas metal arc welding, and submerged arc welding, the evaluation results obtained from grooved butt joint test plates can be applied to product joints with a base metal thickness twice that of the test plate. The evaluation test results for fillet welds can be applied to all base metal thicknesses and all fillet weld sizes. In the following cases, the applicable range of the base metal thickness for welding procedure qualification is reduced to 1.1 times the thickness of the test plate. 1) For single-pass or multi-pass welds, when the thickness of each weld pass is greater than 13 mm. 2) When short-circuit transition GMAW is used and the thickness of the test plate is less than 13 mm. 3) When the temperature of the post-weld heat treatment of the test piece exceeds the upper critical transition temperature. For multi-pass welds in shielded metal arc welding, submerged arc welding, tungsten inert gas welding, and gas metal arc welding, when the joint thickness is greater than 20 mm, the range of base metal thickness applicable for welding procedure qualification is 1.33 times the thickness of the test piece. For unequal-thickness butt joints, the applicable range of base metal thickness for welding procedure qualification is determined as follows: 1) The thickness of the thinner component in the butt joint shall fall within the aforementioned applicable range. 2) In butted joints, for materials for which notch impact toughness is not required, the thickness of the thicker component has no limit. For materials requiring notched impact toughness, the thickness of thicker components should be within the aforementioned range. If the thickness of the welding procedure qualification test piece is greater than 38 mm, its maximum thickness may also be unrestricted. 4. Welding filler metal: In accordance with Volume 9 of the American ASME codes, welding filler metal is similar to the base metal, and can be classified by material category, alloy composition, strength grade, and type. Based on the type and standard number of the welding filler metal, the type of electrode coating, and the wire-flux combination, welding filler metals for carbon steel and alloy steel can be divided into 6 categories; welding materials for aluminum and aluminum alloys are classified into 4 categories; while welding materials for copper and copper alloys fall into 1 category. These are assigned F-No classification numbers, as detailed in Table 3-9. The welding filler metals for various steel grades can be classified into 12 categories as shown in Table 3-10, based on the chemical composition of the weld metal, and are designated with A-No classification codes. The classification of filler metals is similar to that of base metal, with the aim of reducing the workload associated with welding process evaluation. That is, welding filler metals of the same type can be used interchangeably. A weld procedure qualification report prepared using a welding filler metal belonging to a certain F-No or A-No is applicable to any welding filler metal within the same F-No or A-No category.