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Evaluation of the nature of internal defects, causes of defects and preventive measures ⑤

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The echo height is large, the wave amplitude is wide, and there will be multiple peaks. When the ultrasonic flaw detector probe is translated, the reflected wave continuously changes in the amplitude of the reflected wave. Crack is one of the most dangerous defects. In addition to reducing the strength of welded joints, the end of the crack is a notch of sharp pin. After the weldment is loaded, it causes stress concentration and becomes the origin of structural fracture. There are three types of cracks: hot cracks, cold cracks and reheat cracks.



The reasons for hot cracks are: the cooling rate of the molten pool during welding is very fast, causing segregation; the uneven heating of the weld produces tensile stress.



Preventive measures: limit the content of segregation-prone elements and harmful impurities in the base metal and welding materials, mainly limit the sulfur content and increase the manganese content; increase the basicity of the electrode or flux to reduce the impurity content and improve the degree of segregation; improve the welding structure form, Use a reasonable welding sequence to improve the degree of freedom when the weld shrinks.



Causes of cold cracks: The hardenability of the material to be welded is large, and it is easy to crack when it is subjected to human welding tension during the cooling process; the cooling speed is very fast during welding. Hydrogen molecules enter into the fine pores of the metal in a gaseous state, and cause a lot of pressure, which makes the local metal generate a lot of pressure to form cold cracks; welding stress and tensile stress occur at the same time as hydrogen concentration and quenching embrittlement. It is easy to form cold cracks.



Preventive measures: preheating before welding and slow cooling after welding, so that the decomposition of austenite in the heat affected zone can be carried out within a sufficient temperature range to avoid the generation of hardened structures and reduce welding stress; Annealing, dehydrogenation treatment, eliminate the stress generated during welding, and make hydrogen diffuse to the outside world in time; choose low-hydrogen electrode and alkaline flux or austenitic stainless steel electrode welding wire, etc., the welding material is dried according to the regulations, and strictly cleaned Groove; strengthen the protection during welding and the cleaning of the surface of the welded part to avoid the intrusion of hydrogen; select a reasonable welding specification and adopt a reasonable welding sequence to improve the stress state of the weldment.