I. Welding Process Assessment Concept
Welding process evaluation is a preliminary preparation for the entire welding operation. It is a process of testing and evaluating the accuracy of the proposed welding process for weldments and related products.
This includes the pre-weld preparation process, welding, testing and evaluation of results. The evaluation of the welding process is also an important process in production practice, which has prerequisites, objectives, results and limited scope.
Therefore, the evaluation of the welding process involves preparation for welding, welding of specimens, inspection of specimens, and determination of whether the welded joint of the specimen meets all technical performance indicators required in accordance with the welding process. proposed welding.
Finally, all welding process factors, welding data and test results accumulated throughout the process are organized into recommended and conclusive materials, forming a “welding process evaluation report”.
II. Importance of evaluating the welding process
The evaluation of the welding process is a critical link to guarantee the quality of welding of boilers, pressure vessels and pressure pipes. It is an indispensable part of the technical preparation work before welding boilers, pressure vessels and pressure pipes.
It is a mandatory project for the national quality and technical supervision agency to carry out engineering review.
It is a necessary measure to guarantee the correctness and rationality of the welding process, being an important guarantee to guarantee the quality of the welding and that the performance of the welded joints complies with the technical conditions of the product and the corresponding standard requirements.
Therefore, it is necessary to verify the correctness and rationality of the welding process through the corresponding experiment, that is, the evaluation of the welding process.
Welding process evaluation can also maximize welding production efficiency and minimize production costs under the premise of ensuring the quality of the welded joint, thereby achieving maximum economic benefit.
III. Objective of evaluating the welding process
- It serves as a technical document that must be followed in the production process of boilers, pressure vessels, pressure piping and equipment manufacturing, installation, maintenance and in teaching welder training.
- It is a fundamental link or important measure to be carried out in welding quality management.
- It is an important indicator that reflects the welding capacity and technical level of a unit.
- It is a project that must be carried out as stipulated by the relevant industry and national regulations.
4. Scope of Application of Welding Process Assessment
- Welding process evaluation is applicable to welding work of manufacturing, installing and maintaining steel equipment such as boilers, pipes, pressure vessels and load-bearing steel structures, as well as welder training and technical evaluation of welders. Welding process evaluation must be carried out prior to these works to determine the correctness of the proposed welding process.
- Welding process evaluation is applicable to welding methods such as shielded metal arc welding, tungsten inert gas welding, gas metal arc welding, flux-cored arc welding, gas welding and submerged arc welding.
- It is applicable to companies involved in manufacturing, installation or maintenance work.
- The evaluation of the welding process is targeted. The technical condition requirements of various products are different. If the product is a pressure vessel, the process evaluation test result must meet the standard requirements of the technical condition of the pressure vessel. If the product is a load-bearing steel structure, the process evaluation test result must meet the standard requirements of the technical conditions of the load-bearing steel structure. The main requirement of the welding process evaluation test is to meet the technical conditions of the product.
V. Characteristics of Welding Process Assessment
- Welding process evaluation aims to solve welding process problems under specific conditions for any steel material. It's not about selecting the best process parameters, but about providing a range of solutions that are generally acceptable to most people.
- Although welding process evaluation addresses performance issues under specific process conditions, it cannot resolve comprehensive quality issues such as stress relief, deformation reduction, and prevention of welding defects.
- The evaluation of the welding process must be based on the weldability of the raw materials. Reliable technical condition tests can guide production, thus avoiding the pitfall of using real products as test parts.
- During the evaluation of the welding process, human factors must be excluded. Do not confuse the evaluation of welding processes with the evaluation of the welder's skills. Personnel conducting the welding process evaluation must be able to distinguish whether defects are due to a problem in the welding process or a welder skill issue. If it is a skill issue, it should be resolved through welder training.
- The tests required by existing welding process evaluation procedures mainly involve mechanical tests at room temperature on welded joints. If a joint passes visual inspection, non-destructive testing, and room temperature mechanical testing, it is generally considered to have passed the welding process test. However, these results are not completely reliable for new types of steel in high-temperature and high-pressure pipelines in the energy industry. Additional tests, such as high temperature resistance tests, creep tests, and stress corrosion cracking tests, should also be considered.
SAW. Welding process evaluation procedure
Prepare and issue welding process evaluation task – Develop welding process evaluation plan – Weld and inspect test pieces – Prepare welding process evaluation report – Develop welding operation guide (or welding process card) based on the welding process evaluation report.
1. Write and issue welding process evaluation task
The main purpose of the task is to issue assessment tasks. Therefore, its main content should include: purpose of evaluation, evaluation indicators, evaluation items and qualification conditions of departments and personnel responsible for evaluation tasks.
(1) Determine Assessment Indicators
Technical indicators are determined based on theoretical knowledge of regulations and steel (weldability), etc. According to the “Welding Process Evaluation Procedure” DL/T869, the chemical composition and mechanical properties (strength, plasticity, toughness, etc.) of the weld metal should be comparable to or not lower than the lower limit of the welding material. base.
(2) Determine assessment items
Considering the actual work requirements of the project, cover the related items according to the scope of the regulation and determine the assessment items. The determination of the welding process evaluation items must consider the following aspects:
Steel:
(1) Classification of steel levels;
(2) Basic rules for steel quality levels under “evaluation”;
(3) Division of different types of steel. The meaning of welded joint of different types of steel is:
The classification of different types of steel welded joints is mainly divided into two categories: one is the one with the same metallographic structure but different chemical composition, such as the welding joint between low carbon steel and low alloy steel, both belonging to the perlite type of structure with small differences in physical properties but different chemical composition; the other category is those with different metallographic structures and chemical compositions and significant differences in physical properties, such as the welding joint between low-alloy pearlite steel and high-alloy martensitic steel or austenitic stainless steel.
The main characteristic of different types of steel welded joints is the uneven distribution of chemical composition, metallographic structure, mechanical properties and welding residual stress. The welding process must address these issues and adopt necessary technological measures to resolve them.
①Different A-type steel joints: one side of the welding joint is austenitic steel and the other is structured steel. Specific types include: A+M, A+B, A+P, and so on.
②M type different steel joints: One side of the welding joint is martensitic steel and the other is structured steel. Specific types include: M+B, M+P, and so on.
③Type B different steel joints: One side of the welding joint is bainitic steel and the other is pearlitic copper. There is only one type: B+P.
2. Assessment of test piece thickness
(1) Butt weld applicable to part thickness
①When the thickness of the evaluation test part is 1.5≤δ<8(mm), the applicable part thickness range is defined as: the lower limit is 1.5mm, the upper limit is 2δ, but not more than 12mm.
②When the thickness of the evaluation test part is 8≤δ≤40(mm), the applicable part thickness range is defined as: the lower limit is 0.75δ, the upper limit is 1.5δ. When the thickness of the evaluation test piece is greater than 40mm, the upper limit is not restricted.
(2) Fillet weld applicable to part thickness
The part thickness range applicable to the fillet joint thickness δ that was evaluated is the same as the butt joint thickness, but the test part thickness is calculated according to the following rules:
①The thickness of the plate-to-plate fillet weld test piece is the thickness of the core plate.
②The thickness of the tube-to-plate fillet weld test piece is the thickness of the tube wall.
③The thickness of the pipe seat fillet weld test piece is the wall thickness of the branch pipe.
In addition, for submerged arc welding, double-sided welding and small diameter thick-wall welding, etc., please check the regulations carefully and perform in accordance with the regulations.
3. Welding methods
Each welding method must be evaluated individually and cannot replace one another. If a combination of several welding methods is used for “evaluation”, each welding method can be “evaluated” individually or in combination.
The thickness of the weld metal for each welding method must be within the range of its own “judgement”. For example, if the root layer is welded by TIG welding (thickness 3mm), and the filling and covering processes are carried out by galvanized welding (total thickness 8mm) to evaluate the welding process (other conditions), this is considered as an evaluation of the combination of two welding methods. Approved welding methods are suitable for:
(1) Individual TIG welding:
The evaluated weld metal thickness is 3mm, with an applicable thickness range of (1.5~6)mm.
(2) Individual welding:
The evaluated weld metal thickness is 8mm, with an applicable thickness range of (6~12)mm. The Ds/Ws welding methods mentioned above can also be used separately for TIG welding and electronic welding after passing the evaluation and then combined. The “evaluation” of gas welding methods applies to the maximum thickness of the welded parts being equal to the thickness of the “evaluation” test piece.
4. Types of test pieces
(1) The approved process for “evaluating” flat specimens is applicable to tubular specimens and vice versa. However, several welding positions must be considered. For example, vertical flat welding can replace horizontal fixed pipe welding, and vertical flat welding can replace vertical pipe welding.
(2) The “evaluation” of butt joint specimens applies to corner joint specimens.
(3) The “evaluation” of full penetration specimens applies to non-full penetration specimens.
(4) The welding process approved by the “evaluation” of flat corner weld test pieces is applicable to the corner welds of tube and plate or tube and tube, and vice versa.
5. Welding materials
(1) Welding materials such as welding rods, wires and fluxes melt during the welding process and fuse into the weld metal in the form of filler metal. They are the main components of weld metal. Its selection and change can significantly influence the welding properties of the welded joint.
However, its variety makes it very difficult to “evaluate”. To reduce the number of evaluations and carry them out rationally, the selection of welding materials must follow the same principles as the selection of steel, divided by class level (see table in the procedure), to facilitate “evaluation”.
(2) For foreign welding rods, wires and fluxes, you can refer to related materials or carry out tests to confirm their compliance before use. Its chemical composition and mechanical properties should be similar to those listed in the national welding materials table. They can be classified at the corresponding class level and treated in the same way as domestic welding materials.
Welding rods, wires and fluxes not listed in the table of welding materials, if their chemical composition, mechanical properties and process characteristics are similar to those listed, may be classified at the corresponding class level and used. Those that cannot be classified must be “evaluated” separately.
(3) Welding rods and wires of each category must be evaluated separately. For those in the same category but at different levels, the higher level assessment is applicable to the lower level; between rods of the same level, those evaluated with acidic rods may be exempt from the basic rod evaluation.
(4) Changing the filler metal from solid wire to flux-cored wire, or vice versa.
(5) Changing the type of fuel gas or shielding gas, canceling the rear shielding gas.
(6) The selection of materials for welding dissimilar steels must follow the principles of DL/T752.
(7) For foreign materials, especially welding materials for high-alloy steel, you must fully understand the basic properties of the material. Some important indicators directly related to the product's performance must be verified through testing before use.
6. Diameter of tube test piece
General guidelines do not strictly dictate the “evaluation” of pipe diameters. Due to the wide variety of pipe specifications in the power industry, the following provisions have been made considering significant process variations:
(1) When “evaluating” specimens with an outer diameter of ≤60mm, and welding is done using the argon arc welding method, the process is applicable regardless of the outer diameter of the welded pipe.
(2) For other pipe diameters, the “evaluation” is applicable for welded pipe outer diameters ranging from the lower limit 0.5D0 to an unspecified upper limit.
7. Test piece welding position
The energy industry, taking into account the specific characteristics of the industry, has made specific provisions for the “evaluation” of welding positions and their applicability (see table in the guidelines). The following rules must also be followed in the following cases:
(1) In vertical welding, when the root weld changes from upward to downward welding or vice versa, a new evaluation must be carried out.
(2) For gas welding and tungsten electrode argon arc welding of pipes with a diameter of ≤60 mm, unless there are special requirements for welding process parameters, only horizontal pipes are generally “evaluated”, which is Applicable to all welding positions of the workpiece.
(3) During automatic welding of pipes in all positions, tubular specimens must be used for “evaluation”, and plate-shaped specimens cannot be replaced.
8. Preheating and intermediate temperature
When the preheating temperature of the evaluation specimen exceeds the desired parameters, a new evaluation must be carried out:
(1) When the preheating temperature of the evaluation sample decreases by more than 50°C;
(2) For welded parts requiring impact resistance, when the temperature of the interlayer increases by more than 50 ℃.
9. Post-welding heat treatment
(1) If in-process inspection is required and the test piece cannot be welded in one go, post-welding heat treatment must be carried out.
(2) The interval between post-welding heat treatment and completion of the welding operation must strictly follow the heat treatment specifications for various steels and comply with the provisions of DL/T 819 and DL/T 868. For example, P91 o Martensitic steel requires that after welding is completed, the weld must cool to 100℃ before the austenite turns into martensite, then the temperature is raised for post-welding heat treatment.
10. Welding specification parameters and operational techniques
When changes occur in welding specification parameters and operational techniques, the assessment must be redone based on the type of parameter or the process instructions must be changed.
(1) In gas welding, changes in flame characteristics;
(2) In automatic welding, changes in the distance between the conductive nozzle and the part;
(3) Change in welding speed greater than 10% of the evaluated value;
(4) Change from single-side welding to double-side welding;
(5) Change from manual welding to automatic welding;
(6) Change from multi-pass welding to single-pass welding, etc.
These points and other special conditions can be considered collectively to determine how to identify welding process evaluation items.
VII. Test Part Manufacturing and Inspection
1. The manufacturing of test specimens shall be conducted under effective supervision, strictly in accordance with the requirements and regulations of the process evaluation scheme.
2. There must be a dedicated person carefully recording each step during the welding process, and a parameter recorder capable of saving the recorded data must be equipped. Records must be properly preserved for review.
3. Inspection items must be complete, conducted in accordance with relevant regulations.
Key inspection items include:
(1) Inspection of the appearance of the weld seam: The remaining height of the weld metal should not be less than the original material, the depth and length of the undercut should not exceed the standard, and there should be no cracks, unfused areas, inclusions slag, arc pits or porosity on the weld surface.
(2) Non-destructive testing of weld seams: Radiographic inspection of tubular specimens shall be carried out in accordance with the requirements of DL/T821, and the weld quality shall not be lower than the level II standard. Non-destructive testing has no correlation with the mechanical properties of the welded joint, but understanding welding defects in “evaluation” is very necessary. Furthermore, one should consider avoiding these areas when cutting specimens. Therefore, it must be included in the inspection items.
(3) Tensile test (dimensional samples):
① The remaining height of the sample is mechanically removed and leveled with the original material.
② Sample thickness: Full thickness samples can be used when the thickness is less than 30mm. If the thickness is more than 30mm, it can be processed into two or more pieces of samples.
③ The tensile strength of each sample should not be less than the lower limit of the original material.
④ The tensile strength of different steel samples should not be lower than the lower limit of the original material on the lower side.
⑤ When two or more samples are subjected to a tensile test, the average value of each group of samples should not exceed the lower limit of the value specified by the original material.
(4) Bend test:
① Bending specimens can be divided into transverse (back) face bending, longitudinal (back) face bending and transverse face bending.
② When T is less than 10, T = t; when T is greater than t, t = 10. The width of the sample: 40, 20, 10 (unit: mm).
③ The remaining height of the sample is mechanically removed, the original surface of the original material is retained, and the bottom cut and weld root notch cannot be removed.
④ The defect on the transverse lateral curvature surface should be regarded as the tensile surface.
⑤ The three main factors that affect the bending test are: the ratio of the width to the thickness of the sample, the bending angle and the diameter of the bending axis. The bending test method of regulation SD340-89 and related provisions does not correspond to the elongation of the material itself. Therefore, the elongation of the outer surface of the bending sample exceeded the lower limit of elongation specified for some steels, which is not entirely reasonable.
For a more reasonable determination of plasticity in the bending test, the new regulation stipulates that the bending test method must be conducted in accordance with the Metal Bending Test Method GB/T232.
The bending test conditions are specified as follows: the sample thickness is less than equal to 10, the bending shaft diameter (D) is 4t. The distance between the supports (Lmm) is 6t+3 and the curvature angle is 180 degrees.
For steels with a specified lower elongation limit of less than 20% under standard and technical conditions, if the bending test is not qualified and the measured elongation is less than 20%, it is permitted to increase the diameter of the bending axis for the test.
After bending to the specified angle, there shall be no cracks greater than 3 mm in length in any direction on the tensile surface of each part of the sample, within the weld and the heat-affected zone. Cracks on the edges are excluded, but cracks caused by slag inclusions must be accounted for.
(5) Impact test: For pressure and load-bearing components, as long as they meet the conditions of the impact sample, they must undergo an impact test. Therefore, it must be carried out when the following conditions are met:
① If the welding thickness is not sufficient for sampling (5x10x5mm), it may not be necessary.
② When the welding thickness is greater than or equal to 16mm, an impact test is required, 10x10x5mm.
③ Evaluation passing standard: The average value of three samples shall not be less than the lower limit specified by relevant technical documents, and one shall not be less than 70% of the specified value.
(6) Metallographic examination: The tubular corner joint must not have two inspection surfaces in the same cut.
(7) Hardness test: The hardness of the weld seam and heat-affected zone shall not be less than 90% of the hardness value, shall not exceed the Brinell hardness of the original material plus 100HB, and shall not exceed the following specifications :
When the total alloy content is less than 3%, the hardness should be less than or equal to 270HB;
When the total alloy content is 3~10, the hardness should be less than or equal to 300HB;
When the total alloy content is greater than 10, the hardness should be less than or equal to 350HB;
For P91, 220~240 steel is ideal.
(8) The preparation, cutting and evaluation of the above samples must be conducted in accordance with relevant standards.
(9) After the inspection, a formal report shall be issued by qualified personnel.
(10) Inspection procedures and requirements must comply with regulations.
