1. Concept of welding procedure qualification

Welding procedure qualification is the preliminary preparation for the entire welding work. The welding procedure qualification work is the test process and result evaluation carried out to verify the correctness of the welding procedure of the proposed weldment and related products.

It includes the process of pre-welding preparation, welding, testing and result evaluation. Welding procedure qualification is also an important process in production practice. This process has a premise, a purpose, a result, and a limited scope. Therefore, the welding process qualification should be based on the proposed welding process plan, including pre-welding preparation, welding test pieces, inspection test pieces, and determining whether the welded joints of the test pieces have various technical indicators of the required performance. Finally, the accumulated experience of the whole process Various welding process factors, welding data and test results are compiled into conclusive and recommended information to form a "welding process assessment report".

2. Characteristics of welding process qualification

1> Welding process qualification is to solve the welding process problems of any steel under specific conditions, rather than selecting the best process parameters. It has a certain range and is acceptable to most people.

2> Welding process evaluation is to solve the performance problems under specific process conditions, but it cannot solve the overall quality problems involved in eliminating stress, reducing deformation, preventing welding defects, etc.

3> The welding process evaluation should be based on the welding performance of the raw materials and guide production through reliable technical condition tests for welding process evaluation, avoiding the disadvantage of using actual products as test pieces.

4> Human factors should be eliminated during the welding procedure qualification test process, and welding procedure qualification and welder skill qualification should not be confused. The person in charge of the welding procedure qualification should be able to distinguish whether the cause of the defect is a welding process problem or a welder's skill problem. If it is a skill problem, it should be solved through welder training.

5> The tests required by the existing welding procedure qualification regulations are mainly normal temperature mechanical tests of welded joints. That is, if it has passed the appearance inspection, non-destructive testing and normal temperature mechanical test, it is generally considered to have passed the welding process test. For new steel types for high-temperature and high-pressure pipelines in the power industry, this result is not completely reliable, and high-temperature endurance tests, creep tests, stress corrosion and other tests of joints must also be considered.

3. Procedure for welding procedure qualification

1. Prepare and issue welding procedure qualification task sheet

The main function of the mission statement is to issue assessment tasks. Therefore, its main content should be: assessment purpose, assessment indicators, assessment items, and qualifications of the departments and personnel undertaking the assessment tasks.

(1) Determination of evaluation indicators

Various technical indicators are determined based on the regulations and theoretical basic knowledge of steel (weldability). According to the provisions of the "Welding Procedure Qualification Regulations" DL/T869, the chemical composition and mechanical properties (strength, plasticity, toughness and other indicators) of the weld metal are required to be equivalent to the base metal or not lower than the lower limit of the corresponding specified value of the base metal.

(2) Determination of assessment items

According to the actual work requirements of the project, the relevant coverage of the project shall be done according to the applicable scope of the regulations, and the projects to be evaluated shall be determined. The items for welding procedure qualification should be determined from the following aspects:

Steel

(1) Classification of steel products;

(2) Basic provisions for steel grades in “assessment”;

(3) Classification of dissimilar steels The meaning of dissimilar steel welded joints is:

The steel combinations of dissimilar steel welded joints are basically divided into two categories: one is the welded joints of the same metal structure type but different chemical compositions, such as low carbon steel and low alloy steel, they are all pearlite structure types, and the physical properties are quite different. Small, only the chemical composition is different; the other type is the metal structure type and chemical composition are different but the physical properties are quite different, such as the welded joint of low alloy pearlite steel and high alloy martensitic steel or austenitic stainless steel.

The main characteristics of dissimilar steel welded joints are: the welded joints formed have inhomogeneities in chemical composition, metallographic structure, mechanical properties and welding residual stress distribution. The welding process needs to take necessary process measures to address these problems. be solved.

① Class A dissimilar steel joint: One side of the welded joint is austenitic steel, and the other side is other structural steel. The specific types are: A+M, A+B, A+P, etc. 3 groups.

②M type dissimilar steel joint: One side of the welded joint is martensitic steel, and the other side is other structural steel. The specific types are: M+B, M+P, etc. 2 groups.

③Type B dissimilar steel joint: One side of the welded joint is bainitic steel. The other side is pearlitic copper. The specific types are: B+P only one group.

2. Evaluate the thickness of the specimen

(1) Butt welds are suitable for the thickness of the weldment

① When the thickness of the evaluation test piece is 1.5≤δ<8 (mm), the applicable range regulations for the thickness of the weldment are: the lower limit is 1.5mm, the upper limit is 2δ, and not greater than 12mm.

② When the thickness of the evaluation test piece is 8 ≤ δ ≤ 40 (mm), the applicable range for the thickness of the weldment is: the lower limit value is 0.75 δ and the upper limit value is 1.5 δ. When the thickness of the evaluation test piece is greater than 40mm, the upper limit is not limited.

(2) Fillet welds are suitable for the thickness of the weldment

The evaluated thickness δ of the fillet joint is applicable to the same range of thickness of the weldment as the thickness of the butt joint, but the thickness of the test piece is calculated according to the following provisions:

①The thickness of the plate-to-plate fillet weld specimen is the thickness of the web.

②The thickness of the pipe-sheet fillet weld specimen is the pipe wall thickness.

③The thickness of the pipe seat fillet weld test piece is the thickness of the branch pipe wall.

In addition, the regulations for double-sided submerged arc welding, small diameter and thick wall, etc. must be carefully checked and implemented according to the regulations.

3. Welding method

Various welding methods should be "qualified" individually and should not be substituted for each other. "Assessment" takes the form of a combination of more than one welding method, in which each welding method can be "assessed" individually or in combination. The weld metal thickness of each welding method during application should be within the applicable scope of the respective "qualification". For example: use argon arc welding to weld the root layer (thickness 3mm), electrode arc welding filling and cover process (total thickness 8mm) for welding process qualification (other conditions). This belongs to the combined assessment of two welding methods. In addition to being effective in combined processes, the qualified welding process is also applicable to:

(1) Argon arc welding alone: The thickness of the weld metal is assessed to be 3mm, and its applicable thickness range is (1.5~6) mm.

(2) Individual arc welding with electrodes: The thickness of the weld metal is assessed to be 8 mm, and its applicable thickness range is (6 to 12) mm. The above-mentioned Ds/Ws welding seam welding process can also be used in combination after the argon arc welding and electrode arc welding welding processes are qualified separately. For the "qualification" of gas welding welding methods, the maximum thickness of the weldment applicable is the same as the thickness of the "qualification" test piece.

4. Type of test piece

(1) The process for "evaluating" the qualification of plate specimens is applicable to tubular specimens, and vice versa. But there are various welding positions to consider. For example: a flat plate or an upright plate can replace the horizontal fixed pipe, and a vertical plate can replace the vertical pipe.

(2) "Evaluation" of butt joint test pieces, applicable to corner joint test pieces.

(3) The "evaluation" of full penetration test pieces is applicable to non-full penetration test pieces.

(4) The welding process used to qualify plate fillet weld specimens is applicable to fillet welds between tubes and plates or tubes and tubes, and vice versa.

5. Welding materials

(1) Welding materials such as welding rods, welding wires, and fluxes melt as the welding process proceeds and are melted into the weld metal in the form of filler metal. They are the main components of the weld metal. Selecting and changing them has a negative impact on the welding joint. The properties of weld metals have a great impact, but their wide variety makes "assessment" very difficult. In order to reduce the number of assessments and conduct "assessment" reasonably, the selection of welding materials is based on the same principles as the selection of steel, divided by class level (the regulations are available in tables) to facilitate the "assessment" work.

(2) For foreign welding rods, welding wires and fluxes, the relevant information can be consulted before use or verified by testing, and they can only be used after confirming that they meet the requirements. Its chemical composition and mechanical properties are similar to those in the domestic welding consumables list. It can be classified into the corresponding class level and treated the same as domestic welding consumables. Welding rods, welding wires and fluxes that are not listed in the welding consumables table can be classified into the corresponding category and can be used if their chemical composition, mechanical properties and process characteristics are similar to those in the table. Those who cannot be classified should be "assessed" separately.

(3) Each category of welding rods and wires should be evaluated separately. For the same category but different levels, the higher level assessment can be applied to the lower level; among the welding electrodes of the same level, those that have been assessed by acidic electrodes are exempted from the alkaline electrode assessment.

(4) The filler metal is changed from solid core welding wire to flux cored welding wire, or vice versa.

(5) Change the type of flammable gas or protective gas and cancel the back protective gas.

(6) The selection of materials for welding dissimilar steels should comply with the principles specified in DL/T752.

(7) For foreign materials, especially welding materials for high-alloy steel, the basic properties of the materials should be fully understood. Some important indicators directly related to product performance should be verified through tests before use.

6. Pipe specimen diameter

There are no strict regulations on the "evaluation" of pipe diameters in general regulations. In the power industry, due to the wide variety of pipe specifications, the following regulations are made in consideration of the large differences in technology:

(1) When the outer diameter of the "evaluation" test piece pipe is Do≤60mm and the argon arc welding method is used, the outer diameter of the weldment pipe applicable to the process is not specified.

(2) For the "evaluation" of other pipe diameters, the range applicable to the outer diameter of the weldment pipe is: the lower limit is 0.5D0, and the upper limit is not specified.

7. Welding position of test piece

Based on the characteristics of the industry, the electric power industry has made special regulations on the welding positions and scope of application for "assessment". (See the table above in the regulations) In the following situations, the following regulations should also be followed:

(1) In the vertical welding position, when the root bead is changed from upward welding to downward welding or vice versa, it should be re-evaluated.

(2) For gas welding and tungsten arc welding of pipes with a diameter of ≤60mm, except for special requirements on welding process parameters, generally only horizontal pipes are "qualified", which can be applied to all welding positions of the weldment.

(3) When the pipe is automatically welded in all positions, tubular test pieces must be used for "evaluation" and cannot be replaced by plate-shaped test pieces.

8. Preheating and interlayer temperature

When the preheating temperature of the evaluation specimen exceeds the proposed parameters, the evaluation should be re-evaluated:

(1) The preheating temperature of the evaluation specimen decreases by more than 50°C;

(2) For weldments with impact toughness requirements, the interlaminar temperature increases by more than 50°C.

9. Post-weld heat treatment

(1) Inspection is required in the middle and the test piece cannot be welded in one go. Post-heat treatment is required.

(2) There should be a certain interval between the completion of post-weld heat treatment and the welding operation, and the interval between post-weld heat treatment should be strictly in accordance with the heat treatment specifications of various types of steel and comply with the provisions of DL/T 819 and DL/T 868. : For example, P91 martensitic steel requires that after the welding work is completed, all austenite will be transformed into martensite after the weld is cooled to 100°C and then the temperature will be raised for post-weld heat treatment.

10. Welding specification parameters and operating techniques

When there are changes in the welding specification parameters and operating techniques, the process instructions should be re-evaluated or changed according to the parameter type.

(1) Changes in flame properties during gas welding;

(2) During automatic welding, change the distance between the conductive tip and the workpiece;

(3) The variation range of welding speed is 10% larger than the rated value;

(4) Change from single-sided welding to double-sided welding;

(5) Change from manual welding to automatic welding;

(6) Multi-pass welding is changed to single-pass welding, etc.

How to determine the items for welding procedure qualification can be determined based on the above requirements or other special conditions.

4. Welded test pieces and test piece inspection

1 Welding test pieces must be carried out under effective supervision and in strict accordance with the requirements and regulations of the process qualification plan.

2 During the welding process, each step should be carefully recorded by a dedicated person, and a parameter recorder that can save the recorded data should be equipped. The record should be properly preserved for review and approval.

3 The inspection items must be complete and carried out in accordance with relevant regulations.

The main inspection items are:

(1) Weld appearance inspection: The reinforcement of the weld metal should not be lower than the base metal, the depth and length of the undercut should not exceed the standard, and there should be no cracks, lack of fusion, slag inclusions, arc craters and pores on the surface of the weld.

(2) Non-destructive inspection of welds: Radiographic inspection of tubular specimens shall be carried out in accordance with the provisions of DL/T821, and the quality of welds shall not be lower than Class II standards. Non-destructive testing is not related to the mechanical properties of welded joints, but it is necessary to understand the status of welding defects in the "evaluation". At the same time, it is also considered that they should be avoided when cutting test pieces. For this reason, it is included in the inspection items. It should. The main purpose of the fracture inspection is to check the macroscopic welding defects in the weld metal section. It falls within the scope of measuring the welder's operating skills and cannot be directly used to measure the mechanical properties, so it is cancelled.

(3) Tensile test (size specimen)

① The reinforcement of the sample is removed mechanically and is flush with the base material.

② Thickness of the test piece: When the thickness is less than 30mm, a full-thickness test piece can be used. When the thickness is greater than 30mm, it can be processed into two or more pieces of test pieces.

③The tensile strength of each sample shall not be lower than the lower limit of the base material.

④The tensile strength of the dissimilar steel sample shall not be lower than the lower limit of the base metal on the lower side.

⑤ Carry out tensile test on two or more specimens, and the average value of each group of specimens shall not exceed the lower limit of the specified value of the base material.

(4) Bending test

①Bending specimens can be divided into transverse face bending (back), longitudinal face bending (back), and transverse side bending.

②When T is less than 10, T=t; when T is greater than t, t=10. Width of specimen: 40, 20, 10 (unit: mm).

③ The reinforcement of the sample shall be removed mechanically to maintain the original surface of the base metal. The undercut and weld root notch are not allowed to be removed.

④ If there are defects on the transverse side bending surface, the more serious one should be measured as the tensile surface.

⑤The three main factors that affect the bending test are: the ratio of the width to thickness of the specimen, the bending angle and the diameter of the bending axis. The bending test method and related provisions of SD340-89 regulations do not correspond to the elongation of the material itself. Therefore, the elongation of the bent outer surface of the sample has exceeded the lower limit of elongation for some steel materials, so it is not reasonable. .

In order to make the bending test more reasonable for plasticity determination, the new regulations make the following provisions: The bending test method shall be carried out according to GB/T232 metal bending test method.

The bending test conditions are as follows: sample thickness ≤10, bending axis diameter (D) 4t. The distance between the supports (Lmm) is 6t+3, and the bending angle is 180 degrees.

For steel materials with a lower limit of elongation less than 20% specified in standards and technical conditions, if the bending test fails and the actual measured elongation is <20%, it is allowed to increase the diameter of the bending axis for testing. After bending to the specified angle, each piece On the tensile surface of the specimen, there shall be no cracking defects with a length exceeding 3 mm in any direction in the weld and heat-affected zone, except for cracks on edges and corners, but cracks caused by slag inclusion defects shall be included.

(5) Impact test: As long as the pressure-bearing and load-bearing components have the conditions to be used as impact specimens, the impact test should be carried out. Therefore, it should be done when the following conditions are met:

① When the thickness of the weldment is not enough for sampling (5? 0? 5mm), it is not necessary to do it.

②When the thickness of the weldment is ≥16mm, an impact test is required, 10?0?5mm.

③Evaluation qualification standard: The average value of the three samples shall not be lower than the lower limit specified in the relevant technical documents, and one of them shall not be lower than 70% of the specified value.

(6) Metallographic inspection: For tube-sheet corner joints, the same cut shall not have two inspection surfaces.

(7) Hardness test: The hardness of the weld and heat-affected zone should not be less than 90% of the hardness value, not exceed the Brinell hardness of the base metal plus 100HB, and not exceed the following regulations:

When the total alloy content is less than 3%, the hardness is less than or equal to 270HB;

When the total alloy content is equal to 3 to 10, the hardness is less than or equal to 300HB;

When the total alloy content is greater than 10, the hardness is less than or equal to 350HB;

P91 steel 220~240 is the best.

(8) The preparation, cutting and evaluation of the above samples shall be carried out in accordance with relevant standards.

(9) After the inspection, a formal report must be issued by a person with corresponding qualifications.

(10) Inspection procedures and requirements must comply with regulations.

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