1、 Welding Procedure Qualification Concept

It includes the process of preparation before welding, welding, test and result evaluation. Welding procedure qualification is also an important process in production practice, which has premise, purpose, result and limited scope. Therefore, the welding procedure qualification shall be carried out according to the proposed welding procedure plan to prepare before welding, weld test pieces, inspect test pieces, and determine whether the welding joints of the test pieces have the required service performance. Finally, the welding process factors, welding data and test results accumulated in the whole process shall be sorted into conclusive and recommended data to form a "welding procedure qualification report".

 

2、 Significance of welding procedure qualification

Welding procedure qualification is an important link to ensure the welding quality of boilers, pressure vessels and pressure pipes. Welding procedure qualification is an indispensable and important part of the technical preparation work before the welding of boilers, pressure vessels and pressure pipes. It is a necessary item for the national quality and technical supervision organization to carry out project inspection. It is a necessary way to ensure that the welding process is correct and reasonable. It is an important guarantee to ensure the quality of weldments. The performance of welded joints must meet the product technical conditions and the corresponding standards,

 

Therefore, the correctness and rationality of the welding process must be verified through the corresponding experiment, namely, welding procedure qualification. The welding procedure qualification can also improve the welding production efficiency as much as possible and reduce the production cost as much as possible under the premise of ensuring the quality of welding joints, so as to obtain the greatest economic benefits.

 

3、 Purpose of welding procedure qualification

(1) It is a technical document that should be followed in the production process of boiler, pressure vessel, pressure pipeline and equipment manufacturing, installation, maintenance, etc., as well as welder training and teaching.

(2) It is a key link or important measure for welding quality management.

(3) It is an important sign reflecting the welding ability and technical level of a unit.

(4) It is a mandatory project stipulated by relevant industrial and national regulations.

 

4、 Applicable scope of welding procedure qualification

(1) The welding procedure qualification is applicable to the welding of the fabrication, installation and maintenance of steel equipment such as boilers, pipes, pressure vessels and load-bearing steel structures, as well as the training and technical examination of welders. The welding procedure qualification shall be conducted before the implementation of these works to determine the correctness of the proposed welding procedure.

(2) Welding procedure qualification is applicable to welding methods such as shielded metal arc welding, argon tungsten arc welding, gas metal arc welding, flux cored wire arc welding, gas welding and submerged arc welding.

(3) Enterprises engaged in manufacturing, installation or maintenance.

(4) The welding procedure qualification is targeted. The technical requirements of various products are different. If the product is a pressure vessel, the test results of the welding procedure qualification shall meet the requirements of the technical conditions and standards of the pressure vessel; If the product is a load-bearing steel structure, its process qualification test results should meet the standard requirements of the technical conditions of the load-bearing steel structure. The welding process qualification work is to meet the technical conditions of the product as the primary requirement of the qualification standard of the welding process qualification test.

 

5、 Characteristics of welding procedure qualification

(1) Welding procedure qualification is to solve the welding procedure problem of any steel under specific conditions, rather than selecting the best process parameters. It has a certain range for most people to accept.

(2) Welding procedure qualification is to solve the performance problems under specific process conditions, but it can not solve the overall quality problems involved in eliminating stress, reducing deformation and preventing welding defects.

(3) The welding procedure qualification shall be based on the welding performance of raw materials, and the production shall be guided through reliable technical condition test of welding procedure qualification, so as to avoid the disadvantage of using actual products as test pieces.

(4) Human factors shall be excluded from the welding procedure qualification test, and welding procedure qualification and welder skill qualification shall not be confused. The person in charge of the welding procedure qualification shall be able to distinguish whether the defect is caused by the welding procedure or the welder's skill. If it is a skill problem, it shall be solved through the welder training.

(5) The tests required by the existing welding procedure qualification procedures are mainly the mechanical tests of welded joints at room temperature. That is to say, it has passed the appearance inspection, non-destructive testing and mechanical test at room temperature, and it is generally considered that it has passed the welding process test. This result is not completely reliable for the new steel grades of high temperature and high pressure pipelines in the power industry, and the high temperature endurance test, creep test, stress corrosion test and other tests of joints should also be considered.

 

6、 Procedure for welding procedure qualification

Prepare and issue the welding procedure qualification specification - prepare the welding procedure qualification scheme - weld test pieces and inspection test pieces - prepare the welding procedure qualification report - prepare the welding operation instruction (or welding procedure card) according to the welding procedure qualification report.

The main function of the assignment is to assign assessment tasks. Therefore, its main contents should be: assessment purpose, assessment indicators, assessment items, and the qualifications of the departments and personnel undertaking the assessment tasks.

（1） Determination of evaluation index

Various technical indexes shall be determined according to regulations and theoretical basic knowledge (weldability) of steel. According to the provisions of Welding Procedure Qualification Specification DL/T869, it is required that the chemical composition and mechanical properties (strength, plasticity, toughness and other indicators) of the weld metal should be equal to or not lower than the lower limit of the corresponding specified value of the base metal.

（2） Determination of evaluation items

According to the requirements of the actual work of the project, the relevant coverage of the project shall be done according to the scope of application of the procedure, and the assessment items shall be determined.

 

The items of welding procedure qualification shall be determined from the following aspects:

1. Steel

(1) Classification of steel

(2) Basic regulations of steel grade in "assessment"

(3) Classification of dissimilar steels

The meaning of dissimilar steel welding joint is that: the steel combination of dissimilar steel welding joint is basically divided into two categories: one is the same type of metal structure with different chemical compositions, for example, the welded joints of low carbon steel and low alloy steel are all of pearlite structure type, and the difference in physical properties is small, only the chemical compositions are different; The other is the welded joint of low alloy pearlitic steel and high alloy martensitic steel or austenitic stainless steel.

The main characteristics of dissimilar steel welded joints are that the welded joints formed by them are uneven in chemical composition, metallographic structure, mechanical properties and distribution of welding residual stress, and the welding process needs to take necessary technological measures to solve these problems.

1) Type A dissimilar steel joint: one side of the welded joint is austenitic steel, and the other side is steel with other structures. The specific types are: A+M, A+B, A+P, etc.

2) Type M dissimilar steel joint: one side of the welded joint is martensitic steel, and the other side is steel with other structures. The specific types are: M+B, M+P, etc.

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

 

2. Thickness of evaluation test piece

(1) Butt weld applies to weldment thickness

1) The thickness of the evaluation test piece is 1.5 ≤ δ< When 8 (mm), the range applicable to the thickness of weldment is: the lower limit is 1.5mm, and the upper limit is 2 δ， And no more than 12mm.

2) The thickness of the evaluation test piece is 8 ≤ δ When ≤ 40 (mm), the range applicable to the thickness of weldment is specified as: lower limit 0.75 δ， Upper limit 1.5 δ。 When the thickness of the evaluation test piece is greater than 40mm, the upper limit is unlimited.

(2) Fillet weld applies to the thickness of weldment: the thickness of qualified fillet joint δ， The range applicable to the thickness of weldment is the same as that of butt joint, but the thickness of test piece shall be calculated according to the following provisions:

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

2) The thickness of tube plate fillet weld specimen is the thickness of tube wall.

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

In addition, double side welding of submerged arc welding, small diameter thick wall, etc. shall be carefully checked and implemented according to the regulations.

 

3. Welding method

Various welding methods shall be "qualified" separately and shall not be substituted for each other. "Qualification" in the form of combination of more than one welding method is adopted, in which each welding method can be "evaluated" separately or combined. The weld metal thickness of each welding method shall be within the scope of application of respective "qualification".

 

For example, welding procedure qualification (other conditions) shall be conducted by argon arc welding root layer (3mm thick), welding rod arc welding filling and covering process (8mm thick in total). This is a combination of two welding methods. The qualified welding process is applicable to:

 

(1) Separate argon arc welding: the weld metal thickness is 3 mm, and the applicable thickness range is (1.5~6) mm.

(2) Separate welding by shielded metal arc welding: the weld metal thickness is assessed to be 8mm, and the applicable thickness range is (6~12) mm. The above welding procedures of Ds/Ws welds can also be used in combination after argon arc welding and shielded metal arc welding are qualified. The "qualification" of the gas welding method is applicable to the maximum thickness of the weldment which is the same as the thickness of the "qualification" test piece.

 

4. Type of test piece

(1) The process of "qualification" of plate test pieces is applicable to tubular test pieces, and vice versa. However, various welding positions shall be considered. Example: The horizontal fixed pipe can be replaced by the horizontal vertical pipe, and the vertical pipe can be replaced by the vertical pipe.

(2) The "qualification" of butt joint test pieces is applicable to corner joint test pieces.

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

(4) The welding process qualified by the plate fillet weld test piece is applicable to the fillet weld between tube and plate or tube and tube, and vice versa.

 

5. Welding materials

(1) Welding materials such as electrodes, welding wires and fluxes are melted with the welding process, 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 great impact on the weld metal properties of the welded joint. However, they have a wide variety, which brings great difficulties to the "assessment". In order to reduce the quantity of assessment and carry out "assessment" reasonably, the selection of welding materials is the same as the selection principle of steel materials, which are classified by class and level (the procedures are available in the table) to facilitate the "assessment" work.

(2) For foreign welding rods, wires and fluxes, relevant data can be inquired or verified by tests before application, and they can only be used after confirming that they meet the requirements. Its chemical composition and mechanical properties are similar to those of domestic welding materials. It can be classified into corresponding classes and treated as domestic welding materials. Electrodes, wires and fluxes not listed in the welding materials table, such as chemical composition, mechanical properties and process characteristics, are similar to those in the table, which can be classified into corresponding categories and applied. Those that cannot be included shall be "assessed" separately.

(3) All types of welding rods and wires shall be evaluated separately. For those of the same category but different levels, the higher level assessment can be applied to the lower level; Among the welding rods of the same grade, those who have been assessed as acid welding rods can be exempted from the assessment of basic welding rods.

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

(5) Change the type of combustible gas or shielding gas, and cancel the back shielding gas.

(6) Materials for dissimilar steel welding shall be selected according to the principles specified in DL/T752.

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

 

6. Diameter of pipe test piece

There is no strict regulation on the "assessment" of pipe diameter in the general regulations. Due to the wide range of pipe specifications in the power industry, the following regulations are made in consideration of the large differences in technology:

(1) When the outer diameter Do of the "qualification" test piece pipe is ≤ 60mm and the argon arc welding method is adopted, the process is applicable to the outer diameter of the welded piece pipe.

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

 

7. Welding position of test piece

According to the characteristics of the industry, the electric power industry has made special provisions on the welding position and scope of application of the "assessment". (See the table above in the procedure) In the following cases, the following provisions should also be followed:

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

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

(3) When the pipe is automatically welded in all positions, the tubular test piece must be used for "evaluation", and the tabular test piece cannot be used for "evaluation" instead.

 

8. Preheating and interpass temperature

When the preheating temperature of the evaluation test piece exceeds the proposed parameters, it shall be re evaluated:

(1) The preheating temperature of the evaluation test piece decreases by more than 50 ℃;

(2) For weldments with impact toughness requirements, the interpass temperature shall be increased by more than 50 ℃.

 

9. Post weld heat treatment

(1) It is required to conduct inspection in the middle and post heat treatment is required instead of welding the test piece at one time.

(2) The interval between PWHT and PWHT shall be in strict accordance with the heat treatment specifications of various steels and DL/T 819 and DL/T 868 For example, P91 martensitic steel requires that after the welding work is completed, after the weld is cooled to 100 ℃, the austenite is completely transformed into martensite, and then the temperature is raised for post weld heat treatment.

 

10. Welding specification parameters and operation technology

When the welding specification parameters and operation technology change, the process instruction shall be re evaluated or changed according to the parameter type.

(1) Change of flame property 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 assessed 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.

The items of welding procedure qualification can be determined according to the above requirements or other special conditions.

 

7、 Welded test piece and test piece inspection

(1) The welding test pieces must be made under effective supervision in strict accordance with the requirements and provisions of the process qualification plan.

(2) During the welding process, each step shall be carefully recorded by a specially assigned person, and a parameter recorder capable of saving the recorded data shall be provided for recording. The records shall be properly kept for review.

(3) The inspection items must be complete and carried out according to relevant regulations.

 

The main inspection items include:

Weld appearance inspection: the reinforcement of weld metal shall not be lower than the base metal, the depth and length of undercut shall not exceed the standard, and the weld surface shall be free of cracks, incomplete fusion, slag inclusion, arc crater and air hole. 

Non destructive inspection of welds: radiographic inspection of tubular test pieces shall be carried out according to DL/T821, and the weld quality shall not be lower than Grade II standard. Non destructive testing is not related to the mechanical properties of welded joints, but it is necessary to understand the welding defects in the "assessment". At the same time, it is also necessary to avoid cutting test pieces, so it should be included in the inspection items. The main purpose of fracture inspection is to check the macro welding defects of the weld metal section, which belongs to the scope of welder operation skill measurement and cannot be directly used to measure the mechanical properties, so it is canceled.

 

Tensile test (size specimen)

① The surplus height of the sample shall be removed mechanically and flush with the base metal.

② Thickness of test piece: full thickness test pieces can be used when the thickness is less than 30mm, and two or more test pieces can be processed when the thickness is more than 30mm.

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

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

⑤ Two or more samples shall be subject to tensile test, and the average value of each group of samples shall not exceed the lower limit of the specified value of the base metal.

 

Bending test

① The bending specimen can be divided into transverse bending (back), longitudinal bending (back) and lateral bending.

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

③ The surplus height of the sample shall be removed mechanically to maintain the original surface of the base metal. Undercut and root notch are not allowed to be removed.

④ If there are defects on the lateral bending surface, the more serious one shall be measured as the tensile surface.

⑤ The three main factors affecting the bending test are: the ratio of width to thickness of the specimen, the bending angle and the diameter of the bending shaft. The bending test method and relevant provisions in SD340-89 specification do not correspond to the elongation of the material itself. Therefore, the elongation of the external surface of the sample bending has exceeded the lower limit of elongation for some steels, so it is not reasonable.

 

In order to make the bending test more reasonable for the plastic measurement, the new specification has made the following provisions: the bending test method is carried out according to GB/T232 Metal Bending Test Method.

The bending test conditions are specified as: sample thickness ≤ 10, bending shaft diameter (D) 4t. Support spacing (Lmm) 6t+3, bending angle 180 degrees.

For the steel whose lower limit of elongation specified in the standards and technical conditions is less than 20%, if the bending test fails and the measured elongation is less than 20%, it is allowed to increase the diameter of the bending shaft for the test. After bending to the specified angle, there shall be no cracks with a length of more than 3mm on the tensile surface of each sample in any direction in the weld and heat affected zone, except for cracks on corners, but cracks caused by slag inclusion defects shall be included.

 

Impact test: impact test shall be carried out for pressure bearing and load bearing components as long as they are qualified for impact test. Therefore, impact test shall be carried out when the following conditions are met:

 

① If the thickness of weldment is less than 5 × 10 × 15mm, it can be omitted.

② When the thickness of the weldment is ≥ 16mm, impact test shall be carried out? 0 5mm.

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

Metallographic inspection: there shall be no two inspection surfaces in the same notch of tube sheet corner joint.

Hardness test: the hardness of the weld and heat affected zone shall not be less than 90% of the hardness value, not more than the Brinell hardness of the base metal plus 100HB, and not more than the following provisions:

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 3~10, the hardness is less than or equal to 300HB; When the total alloy content is more than 10, the hardness of 350 HB P91 steel 220~240 is the best.

 

(4) The above samples shall be prepared, cut and evaluated according to relevant standards.

(5) After the inspection, a formal report must be issued by a qualified person.

(6) Inspection procedure