How to implement the quality control of steel pipe welds in production

The quality inspection of steel pipe welding not only includes the inspection of welded components but also the inspection of its welding process. The following is a detailed description of the three aspects of pre-welding inspection, welding inspection, and post-welding inspection.


First, pre-welding inspection
The preparation work before steel pipe welding is mainly controlled by the following aspects.
1. Personnel: Steel pipe welding operators should have a certain level of operating skills and hold a valid qualification certificate before they can take up their posts. The scope of their welding operations should be within the coverage of their valid qualification certificates.
2. Steel pipe welding construction machinery: The welding equipment should be good, the welding power supply should be stable, and the indicating instruments such as ammeters and voltmeters should be qualified and within the validity period. Other auxiliary equipment such as grounding wires, tungsten electrodes, welding guns, etc. should not be damaged.
3. Materials: Including parent materials and welding materials. The parent material is the metal material that needs to be welded and connected. Check whether the information in the parent material and the material list is consistent (specifications, grades, heat treatment status, etc.) and whether the size of the parent material groove meets the requirements of the processing drawings. The inner and outer surfaces of the groove should be polished within 20mm to expose the metallic luster before welding. Welding materials include welding wire, welding rod, flux, shielding gas, broken wire, etc. The brand, specification, and quantity of the welding material should be checked. The welding rod and flux must be dried before use. During the carrying process, they should be carried in a sustainable insulation bucket or other auxiliary insulation equipment. The dried welding rod and flux generally need to be dried again after more than 4 hours.
4. Steel pipe welding method: Steel pipe welding methods are divided into fusion welding, brazing, and pressure welding. Among them, commonly used are arc welding, plasma arc welding, tungsten inert gas shielded welding, metal inert gas shielded welding, submerged arc welding, vacuum diffusion brazing, resistance welding, flash butt welding, laser welding, electron beam welding, etc. Before welding, check whether the steel pipe welding method to be adopted meets the requirements of the steel pipe welding process assessment regulations.
5. Steel pipe welding environment: Different steel pipe welding methods and metal materials have different requirements for steel pipe welding environments. Cr-Mo heat-resistant steel itself has a strong tendency to harden. When the cooling speed is too fast during and after welding, cold cracks are easily generated. Therefore, this type of Cr-Mo steel should be preheated before welding and heated and insulated on both sides of the steel pipe welding joint. At the same time, it is greatly affected by the temperature and humidity in the air. Generally, welding is prohibited before effective measures are taken to control the following situations: relative humidity of air ≥ 90%, weldment temperature below -10℃, ambient temperature below 5℃, rainy and snowy weather, and wind speed ≥ 8m/s during arc welding, and wind speed ≥ 2m/s during gas shielded welding.
6. Steel pipe welding measurement: Steel pipe welding process inspection In order to ensure that the steel pipe welding can be correctly welded according to the steel pipe welding parameters in the steel pipe welding process instruction, it is often necessary to increase the quality inspection procedures of the steel pipe welding process. The quality inspection of the steel pipe welding process is usually carried out by full-time or part-time quality inspectors. Starting from the preparation work of steel pipe welding, the staffing, welding equipment, welding materials, welding environment, welding methods, welding parameters, and other aspects are measured and monitored.
7. Applicability: Before welding, the applicability of the welding process specification and the weldment should be checked; the applicability of the working conditions (environment) to welding should be checked; the applicability of the assembly, fixtures, and positioning.


Second, inspection during steel pipe welding
1. During the steel pipe welding process, the welder's self-inspection should be strengthened. The welder himself is usually the first to find the problem in the process, especially in multi-layer and multi-pass welding. The welder observes carefully. If the steel pipe welding defects are found, they can be directly removed. If the same defects with high density and high frequency are found, they should be reported in time.
2. During the steel pipe welding process, the use and storage of welding materials should be strengthened, as well as the inspection of the welding parameter range (current, voltage, welding speed, interlayer temperature, shielding gas flow), especially when there are impact toughness and hardness requirements, to ensure that the welding parameters are within the welding process specification.
3. During the steel pipe welding process, the welding equipment and its indicating instruments should be sensitive and reliable.
4. During the steel pipe welding process, the cleaning and shape of the weld, the number of layers of deposited metal, the welding sequence of welded parts, the control of steel pipe welding deformation, the use and storage of welding materials, and the size inspection should also be checked.


Third, post-welding quality inspection
After welding, it should be checked whether the welded joint meets the acceptance criteria, which can generally be divided into the following aspects.
1. Appearance inspection: Use a 5x magnifying glass to inspect the weld surface. There should be no defects such as cracks, pores, slag inclusions, incomplete penetration, and incomplete welding; stainless steel and low-temperature steel are not allowed to have undercuts, and the undercut size of carbon steel and low-alloy steel welds should meet the standard requirements, and the misalignment should be recorded. Use a welding inspection ruler to measure the weld width, excess height, weld toe, weld leg size, etc. During welding production, the straightness, roundness, concentricity, and other deformed dimensions of the weld should also be checked.
2. Nondestructive testing: PT and MT are generally used for near-surface nondestructive testing of welds, and UT and RT are used for internal nondestructive testing of welds. TOFD ultrasonic testing, phased array testing, leak detection, digital radiography, etc. can be used in other situations. Each testing method is applicable to different conditions, has different defect detection rates, and has different defect sensitivities.
3. Destructive testing: When conditions permit, destructive physical and chemical tests can be performed on welds to effectively and accurately test whether the quality of welded joints meets the design and use requirements.
4. Tightness test: Commonly used tightness test inspection methods include liquid filling leak test, air tightness test, ammonia test, kerosene leak test, helium test, and vacuum box test.
(1) Liquid filling leak test is mainly used to check non-pressure containers, pipelines, and equipment.
(2) The principle of the air tightness test is: that in a closed container, use compressed air that is much lower than the working pressure of the container to apply soapy water on the outside of the weld. When compressed air is introduced, bubbles will appear in the soapy water due to the pressure difference between the inside and outside of the container.
5. Strength test inspection: Strength test inspection is divided into hydraulic strength test and air pressure strength test. The hydraulic strength test is often carried out with water as the medium, and there are certain requirements for the test pressure. Usually, the test pressure is 1.25 to 1.5 times the design pressure.