Basic knowledge of welding procedure qualification

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1. Concept of welding procedure qualification: Welding procedure qualification is the preliminary preparation for the entire welding process. Welding procedure qualification is a testing process and evaluation of results carried out to verify the correctness of the welding procedures for the designated weldments and related products. It includes the processes of pre-welding preparation, welding, testing, and evaluation of the results. Welding procedure qualification is also an important process in production practice; this process has prerequisites, objectives, outcomes, and limitations. Therefore, the welding procedure qualification involves carrying out pre-welding preparations in accordance with the established welding procedure plan, fabricating welding test pieces and inspection specimens, and determining various technical parameters to assess whether the weld joints of these specimens possess the required performance characteristics. Finally, all the welding procedure factors, welding data, and test results collected throughout this process are compiled into conclusive and advisory documentation, resulting in a \"Welding Procedure Qualification Report\". 2. The significance of welding procedure qualification Welding procedure qualification is an important aspect in ensuring the welding quality of boilers, pressure vessels, and pressure pipelines. Welding procedure qualification is an essential component of the technical preparations prior to welding boilers, pressure vessels, and pressure pipelines. It is a mandatory item that quality and technical supervision agencies check during project inspections; it represents the necessary process to ensure that welding procedures are correct and appropriate. It also serves as an important guarantee for ensuring the quality of welded parts, as well as for ensuring that all properties of the welded joints meet the technical requirements specified for the product and relevant standard requirements. Therefore, it is necessary to verify the correctness and rationality of the welding process through corresponding experiments, namely welding procedure qualification. Welding procedure qualification can also help to improve welding production efficiency and minimize production costs while ensuring the quality of the welded joints, thereby achieving the greatest economic benefits. The purpose of the three welding procedure qualifications is to serve as technical documents that must be followed in the production processes such as manufacturing, installation, and maintenance of boilers, pressure vessels, pressure pipelines, and equipment, as well as in welder training and education. 2 are the key steps or important measures to be implemented in welding quality management. 3 is an important indicator reflecting the welding capacity and technical level of a unit. 4 are items that must be carried out as stipulated by industry standards and **relevant regulations. 4 Scope of application for welding procedure qualification 1 Welding procedure qualification is applicable to the welding work involved in the fabrication, installation, and maintenance of steel-based equipment such as boilers, pipelines, pressure vessels, and load-bearing steel structures. It is also used for welder training and the assessment of welder skills; welding procedure qualification must be carried out before these tasks are undertaken in order to determine the correctness of the proposed welding procedures. 2 Welding procedure qualification applies to welding methods such as shielded metal arc welding, tungsten inert gas welding, gas metal arc welding, flux-cored wire arc welding, oxyacetylene welding, and submerged arc welding. 3 Enterprises engaged in manufacturing, installation, or maintenance work. 4 Welding procedure qualification is tailored to specific needs; the technical requirements vary for different products. If the product is a pressure vessel, then the test results of its procedure qualification must meet the standards set for pressure vessels ； If the product is a load-bearing steel structure, the results of its process qualification tests must meet the standard requirements specified for the technical specifications of that load-bearing steel structure. The primary requirement for passing welding process qualification tests is to ensure that the technical specifications of the product are satisfied. 5 Characteristics of welding procedure qualification 1 Welding procedure qualification is aimed at resolving the welding process issues related to any given steel in specific conditions, rather than selecting the optimal process parameters; it operates within certain limits that are acceptable to most people. 2 Welding procedure qualification addresses the performance issues under specific process conditions, but it cannot resolve overall quality problems such as stress relief, reduction of deformation, and prevention of welding defects. 3 Welding procedure qualification should be based on the welding properties of the raw materials, and through reliable technical condition tests conducted as part of welding procedure qualification, production can be guided, thereby avoiding the drawback of using actual products as test specimens. 4 Human factors should be excluded during the welding procedure qualification tests, and welding procedure qualification should not be confused with welder skill assessment. The person in charge of conducting welding procedure qualification should be able to determine whether the cause of defects is a problem with the welding procedure or the welder’s skills; if it is a skill issue, it should be resolved through welder training. 5 The tests required according to the existing welding procedure qualification regulations are mainly mechanical tests of the welded joint at room temperature. That is, having passed the visual inspection, non-destructive testing, and room-temperature mechanical tests, it is generally considered to have passed the welding process test. For new steel grades for high-temperature and high-pressure pipes in the power industry, this result is not entirely reliable; tests such as high-temperature endurance tests, creep tests, and stress corrosion tests on the joints also need to be considered. 6 Procedures for Welding Procedure Qualification: Prepare and issue a welding procedure qualification task order – Develop a welding procedure qualification plan – Fabricate test pieces and inspect them – Prepare a welding procedure qualification report – Compile a welding operation instruction sheet (or welding procedure card) based on the welding procedure qualification report. 1 Preparation and issuance of the welding procedure qualification task order: The main purpose of this task order is to assign the qualification task; therefore, its key contents should include the purpose of the qualification, the evaluation criteria, the items to be evaluated, as well as the qualifications of the departments and personnel responsible for carrying out the qualification. (1) Determination of evaluation criteria: Various technical criteria are determined based on regulations and the theoretical fundamentals of steel (such as weldability). In accordance with the provisions of the \"Code for Welding Procedure Qualification\" DL/T869, it is required that the chemical composition and mechanical properties (such as strength, plasticity, toughness, etc.) of the weld metal should be comparable to those of the base material, or not lower than the minimum values specified for the base material. (2) Determination of evaluation items: Based on the actual operational requirements of the project, and in accordance with the scope of application of the relevant regulations, the relevant items are identified to determine the evaluation items. The determination of the items for welding process evaluation should be considered from the following aspects: Steel (1) Classification of steel grades ； (2) Basic provisions for steel grade classifications in “evaluation” ； (3) Classification of dissimilar steels: The term \"dissimilar steel welding joint\" refers to welds formed between steels of different types. These welds can be basically divided into two categories: in one category, the metals have the same microstructural type but different chemical compositions; for example, welds between low-carbon steel and low-alloy steel. Both of these types belong to the pearlitic microstructure category, and their physical properties differ only slightly, with the difference lying in their chemical compositions ； Another category consists of weld joints in which the metal microstructure and chemical composition differ, but the physical properties vary significantly, such as those of low-alloy pearlitic steel welded to high-alloy martensitic steel or austenitic stainless steel. The main characteristics of welded joints made from dissimilar steels are the unevenities in their chemical composition, microstructure, mechanical properties, and distribution of welding residual stresses. The welding process must therefore incorporate appropriate technical measures to address these issues. ①Class A dissimilar steel joints: The welded joint consists of austenitic steel on one side and steel with another microstructure on the other side. The specific types include 3 groups: A+M, A+B, A+P, etc. ②Class M dissimilar steel joints: One side of the welded joint is made of martensitic steel, while the other side is made of steel with a different microstructure; there are 2 groups of such types: M+B, M+P, etc. ③Class B dissimilar steel joints: One side of the welded joint is made of bainite steel. On the other side is pearlitic copper. The specific types are: only one group of B+P. 2 Determination of test piece thickness (1) Butt welds are suitable for weld pieces with a thickness of ① The test piece thickness is determined to be 1.5 ≤ δ