Treatment method of local deformation in welding

- Dec 30, 2020-

Treatment method of local deformation in welding

    For the deformed workpiece, if the deformation is not large, it can be corrected by fire. If the deformation is large, use the method of jacking while baking to correct.

   1 Reasons for welding cracks in steel structures and preventive measures

    1.1 Thermal cracking

    Thermal cracks refer to cracks generated at high temperatures, also known as high-temperature cracks or crystalline cracks. They usually occur inside the weld, and sometimes may also appear in the heat-affected zone. The manifestations are: longitudinal cracks, transverse cracks, root cracks, arc craters Cracks and heat-affected zone cracks. The reason is that there is segregation in the welding pool during the crystallization process. The low melting point eutectic and impurities exist in the form of liquid interlayer during the crystallization process to form segregation. The strength after solidification is also low. When the welding stress is large enough , It will pull apart the liquid interlayer or the solid metal that has just solidified to form cracks. In addition, if there are low-melting eutectics and impurities on the grain boundaries of the base material, they will be pulled apart when the welding tensile stress is large enough. In short, the occurrence of thermal cracks is the result of the combined effect of metallurgical and mechanical factors. The preventive measures are as follows:

    Limit the content of easily segregated elements and harmful impurities in base metal and welding materials (including welding rod, welding wire, flux and shielding gas). In particular, control the content of sulfur and phosphorus and reduce the carbon content. Generally, the content of sulfur in steel used for welding The content of phosphorus should not be greater than 0.045%, and the content of phosphorus should not be greater than 0.055%; In addition, the further the carbon content of the steel, the worse the welding performance. Generally, when the carbon content in the weld is controlled below 0.10%, the thermal cracking sensitivity can be greatly reduced; Adjust the chemical composition of the weld metal, improve the weld structure, refine the weld grains to increase its plasticity, reduce or disperse the degree of segregation, and control the harmful effects of low melting point co-products; use alkaline electrodes or fluxes to reduce welding The impurity in the seam is captured to improve the degree of segregation during crystallization; the shape factor of the weld is appropriately increased, and the multi-layer and multi-pass welding method is adopted to avoid centerline segregation and prevent centerline cracks; use reasonable welding sequence and direction, adopt The smaller welding line can exceed, the overall preheating and hammering method, and the arc crater is filled when the arc is closed.

    1.2 Cold cracks

    Cold cracking generally means that the temperature of the welding seam drops to the martensite transformation temperature range (below 300-200℃) during the cooling process. It can appear immediately after welding, or it can happen a long time after welding. It is also called delayed cracking. There are three basic conditions for its formation: the formation of a hardened structure in the welded joint; the existence and concentration of diffusible hydrogen; there is a large welding tensile stress. The preventive measures mainly include:

    Choose reasonable welding specifications and line energy to improve the weld and heat-affected zone structure, such as preheating before welding, controlling the interlayer temperature, slow cooling or post-heating after welding, etc. to accelerate the escape of hydrogen molecules; use alkaline electrodes or Flux to reduce the diffusion oxygen content in the weld; welding rods and flux should be dried in strict accordance with the specified requirements before use (low hydrogen electrode 300℃~350℃ heat preservation 1h; acid electrode 100℃~l50℃ heat preservation 1h; flux 200℃~250℃ heat preservation for 2h), carefully clean the groove and welding wire, remove oil stains, moisture and rust spots and other dirt to reduce the source of hydrogen; timely heat treatment after welding. One is to perform annealing treatment to eliminate internal stress and make The quenched structure is tempered to improve its toughness; the second is to carry out hydrogen elimination treatment to fully escape hydrogen from the welded joint; improve the quality of the steel and reduce the layered inclusions in the steel; adopt various technological measures that can reduce the welding stress.

   2 Related issues in welding inspection of steel structures

   2.1 The difference and connection of weld grade, inspection grade and assessment grade

    Welds that require internal quality inspection are classified into first and second grades according to their quality grades. They are called first-grade welds and second-grade welds. This is the weld grade.

    The inspection level refers to the accuracy achieved by the inspection, that is, the *degree of the inspection result obtained by the combination of the inspection instrument and the inspection method. Ultrasonic flaw detection adopts GB/T11345-1989 standard and is divided into three levels according to the detection level from low to high. Radiographic flaw detection adopts GB/T3323-1987 standard and is divided into A, AB, B according to the detection level from low to high. There are three levels, which respectively stipulate the detection method, detection surface, detection range and allowable defect equivalent (dB value) of manual ultrasonic flaw detection and the sensitivity (the relationship between the transillumination thickness and the image quality indicator) for radiographic flaw detection.

    The assessment level refers to the internal quality level of the weld determined by the inspector after the defect is detected by measuring the defect according to the standard. Specifically, ultrasonic flaw detection refers to the defect grading according to the standard GB/T11345-1989 Table 6 after measuring the wave height between the length measuring line and the reject line (zone Ⅱ) defect; radiographic testing refers to measuring the defect indication on the film Length and size, according to the standard GB/T3323-1987 Table 6, Table 7, Table 9, Table 10 and comprehensive rating (see the standard 16.1 ~ 16.4), this article must be mastered by every flaw detector.

   2.2 Exceeding standard defect treatment and re-exploration and expansion

    GB50205 "Code for Acceptance of Construction Quality of Steel Structure Engineering" only specifies the testing method, testing ratio and qualification level, and does not have clear requirements for the treatment of defects. With reference to the requirements of JG181 "Technical Specification for Welding of Building Steel Structures" and welding inspection standards for other industries, the detected defects can be treated as follows:

    The detected impermissible defects must be repaired. After repairing, the welded grid shall be considered as qualified after the same inspection method is passed.

    For welds that require random inspection, after discovering impermissible defects, the inspection shall be carried out in an area of 10% and not less than 200mm (when the length is allowed) of the entire weld length at both ends of the inspected area.

    a. If no over-standard defects are found in the expanded inspection area, the weld grid shall be considered. b. If an over-standard defect is found in the expanded inspection area, the weld shall be inspected completely.

    For the welds that require spot inspection for on-site installation, if any defects are found, the inspection shall be expanded according to the following principles;

    a. Increase the inspection of two welds welded by the same welder of this type. If no excess defects are found in the two expanded welds, the batch of welded seams shall be considered. b. If excess defects are found in these two expanded welds, each weld containing excess defects shall be randomly inspected according to the above principles. c. If no excess defects are found in the welds that are randomly inspected again, the batch of welded seams shall be considered. d. If the welds of the re-sampling inspection are still found to have over-standard defects, the welds of this type welded by the welder shall be inspected completely. At the same time, it can be negotiated to increase the proportion of remaining weld inspections.