1 The causes of local deformation in welding and preventive measures

1.1 Cause

The rigidity of the workpiece is small or uneven, shrinkage after welding, inconsistency in the inconsistency; uneven arrangement of the welds in the workpiece itself, resulting in uneven shrinkage, large contraction of the weld seam, and large deformation; improper handling by the processing personnel, not symmetrical Layer, section, intermittent welding, welding current, speed, direction are inconsistent, resulting in inconsistent deformation of the workpiece; welding bite too large, causing welding stress concentration and excessive deformation; welding placed uneven, stress concentration caused by deformation.

1.2 Preventive measures

When design, try hard to make the parts rigid and weld uniform layout, set the welds symmetrically to reduce the cross and dense weld; make a reasonable welding sequence to reduce deformation. For example, first welding the main weld after the minor weld, first welding the symmetric part of the weld after the asymmetric weld, first welding the large shrinkage of the weld after the weld shrinkage of the small weld, after welding the butt weld Welded fillet welds; for large workpieces with large welds, segmented, layered, intermittent welding, and control of current, velocity, and direction are consistent; when manually welding longer welds, intermittent welding should be performed in sections. , From the middle of the workpiece to the two back welding, welding personnel should be symmetrically distributed to avoid deformation due to heat concentration; large parts such as the shape of the asymmetry, the small parts after welding should be corrected after the deformation, in order to reduce the assembly welding The overall deformation; workpiece welding should often flip, so that the deformation cancel each other; for welding easy to produce angular deformation of the parts, should be pre-distortion treatment before welding, such as v-shaped steel plate butt welding, the interface should be appropriate before welding The pad height, so that can be flattened after welding; external welding reinforcement to increase the rigidity of the workpiece to limit the welding deformation, the position of the reinforcement should be set on the opposite side of the contraction stress.

1.3 Processing Methods

If the deformed workpiece is not deformed, it can be corrected by fire. If the deformation is large, use the edge of the edge with the method of correction.

2 causes of welding cracks in steel structures and preventive measures

2.1 Hot cracks

Hot cracks refer to cracks produced at high temperatures, also known as high-temperature cracks or crystal cracks, and are usually generated within the weld seam, and sometimes in the heat-affected zone, in the form of longitudinal cracks, transverse cracks, root cracks, and arc pits. Cracks and heat affected zone cracks. The reason for this is that there is segregation in the crystallization process of the weld pool. Low-melting eutectic and impurities exist in the form of liquid intermetallic layers during the crystallization process to form segregation, and the strength after solidification is also low, when the welding stress is large enough. The liquid metal layer or solid metal that has just been solidified will be pulled apart to form cracks. In addition, if the low-melting point eutectic and impurities are also present on the grain boundary of the base metal, they will be pulled away when the welding tensile stress is large enough. In short, the occurrence of hot cracks is the result of the joint action of metallurgical and mechanical factors. For its causes, the preventive measures are as follows:

Limit the content of separable elements and harmful impurities in the 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. The sulfur content in the steel generally used for welding Should not be greater than 0.045%, phosphorus content should not be greater than 0.055%; In addition, the carbon content of the steel is farther away, the worse the welding performance, the general content of carbon 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 microstructure of the weld, refine the weld bead, increase its plasticity, reduce or disperse the segregation, and control the harmful effects of the low melting point co-product; use a basic electrode or flux to reduce the weld The impurities in the seam are included, improving the degree of segregation during crystallization; appropriately increasing the shape factor of the weld, using multi-layer multi-pass welding methods to avoid centerline segregation, preventing centerline cracks; using a reasonable welding sequence and direction, using Smaller welding line can be super, overall preheating and hammering, arc filling and other technological measures.

2.2 Cold crack

Cold cracking generally refers to the fact that the temperature of the weld during the cooling process falls below the martensitic transformation temperature range (below 300-200°C) and may occur immediately after welding or it may take a long time after welding. It is also called delay crack. There are three basic conditions for its formation: the formation of hardened microstructures by welded joints; the presence and concentration of diffusible hydrogen; and there is a large welding tensile stress. The main preventive measures are:

Select reasonable welding specifications and line energy to improve the microstructure of welds and heat-affected zones, such as preheating before welding, temperature control, post-weld cooling, or post-heating to accelerate the evolution of hydrogen molecules; use alkaline electrodes or Flux, to reduce the diffusion of oxygen content in the weld; electrode and flux should be used in strict accordance with the requirements before the use of drying (low hydrogen electrode 300 °C ~ 350 °C insulation 1h; acid electrode 100 °C ~ l50 °C insulation 1h; flux 200 °C ~ 250 °C heat 2h), carefully clean the groove and wire, remove oil, water, rust and other dirt, in order to reduce the source of hydrogen; heat treatment in time after welding. First, annealing, to eliminate internal stress, so that Quenching the tempering of the structure to improve its toughness; Second, the hydrogen removal process allows hydrogen to escape from the welded joint; improve the quality of the steel and reduce the layered inclusions in the steel; and adopt various technological measures that can reduce the welding stress.

3 Problems Related to Welding Inspection of Steel Structures

3.1 Weld seam grade, inspection grade, assessment grade difference and contact

Welds that require internal quality inspections are classified into primary and secondary grades according to quality grades, which are called first grade and second grade welds. This is the weld grade.

The inspection level refers to the accuracy of the inspection test, that is, the accuracy of the test results obtained by the combination of the inspection instrument and the detection method. Ultrasonic inspection using GB/T11345-1989 standard according to the detection level from low to high is divided into A, B, C three levels, ray detection using GB/T3323-1987 standard according to the detection level from low to high is divided into A, AB, B At three levels, they specify the detection method for manual ultrasonic flaw detection, the detection surface, the detection range and the allowable defect equivalent (dB value), and the sensitivity to be achieved by radiographic inspection (the relationship between the transillumination thickness and the image quality meter).

The rating level refers to the internal quality level of the weld determined by the flaw detection personnel after the defect is detected and then measured according to the standard. Specifically, the ultrasonic flaw detection refers to the classification of defects according to the standard GB/T11345-1989 Table 6 after measuring the height of the wave height between the measurement line and the discard line (zone II); radiographic inspection 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 is that each flaw detection personnel must be proficient.

3.2 Exceeding Standard Defects Handling and Reconnaissance, Extended Exploration

GB 50205 "Code for Acceptance of Construction Quality of Steel Structures" only stipulates inspection methods, test proportions and qualification levels, and there is no explicit requirement for the handling of defects. According to the requirements of JG 181 "Technical Specification for Welding of Building Steel Structures" and other industry welding&