Weld cracks causes and precautions in steel structure

Hot welded cracks

The hot welded cracks are also known as high-temperature crack or solidification cracking, usually produced in Weld, sometimes also occur in the heat-affected zone, the manifestations are: longitudinal cracks, transverse cracks, roots crack crater cracks and heat-affected zone cracking. Its cause is due to the weld puddle during the crystallization process, the existence of segregation, low melting point eutectic and impurities in the crystallization process, thereby forming segregation exists in the form of liquid interlayer, after solidification, the strength lower, when welding stress is sufficiently large will interlayer liquid or solid metal solidified near just opened the formation of cracks. In addition, if the grain boundary of the base material, there is a low melting point eutectic and impurities, when welding tensile stress is large enough, will be opened. In short, the heat cracks are the result of the role of metallurgical factors and mechanical factors. For its causes, its prevention measures are as follows:
(1) limits the base material and welding material (including the welding rod, wire, flux and shielding gas) easy segregation elements and the content of harmful impurities, in particular, should be controlled, sulfur and phosphorus content and lower carbon content, are generally used for welding of steel the sulfur content should not be greater than 0.04 5%, the phosphorus content should not be greater than 0.055%; Further more carbon content of steel from poorer welding performance Usually weld the carbon content is controlled less than 0.10%, hot cracks The sensitivity can be greatly reduced.
(2) to adjust the chemical composition of the weld metal, improve weld refine weld grain products to increase its plasticity, reduce or disperse the degree of segregation, control the harmful effects of the low melting point eutectic.
(3) using the alkaline electrode or flux to reduce the impurities in the weld containing camera, to improve the degree of segregation in the crystalline.
(4) an appropriate increase in the shape factor of the weld, multi-layer and multi-pass welding method to avoid centerline segregation prevents centerline cracks.
(5) a reasonable welding sequence and direction can be super small welding line, overall preheat and hammering method, crater fill the crater and process measures.
Cold welded cracks
The cold welded cracks usually weld temperature during the cooling process down to the martensitic transformation temperature range (300 - 200 ° C or less) generated can appear immediately after the welding, can also occur in a long time until after the welding, it is also referred to as delayed cracking. Formed three basic conditions: the welded joints form a hardened organizations; presence and concentration of diffusible hydrogen; welding tensile stress. Precautions:
(1) reasonable welding specifications and lines can improve the state of the weld metal and heat affected zone, such as preheating before welding control layer asked temperature, slow cooling after welding or after the heat to accelerate the escape of hydrogen molecules.
(2) using the alkaline electrode or flux to reduce the diffusion of the oxygen content in the weld.
(3) electrode and flux before use should be strictly in accordance with the requirements specified drying (low hydrogen electrodes 300 ℃ ~ 3 50 ℃ insulation lh; acid electrode l 00 ℃ ~ l50 ℃ insulation lh; flux 200 ° C to 250.C insulation 2h), carefully cleaning the groove and wire too in addition to oil, moisture, dirt and rust, in order to reduce the hydrogen source.
(4) post-weld heat treatment in a timely manner. First, the annealing process to eliminate the internal stress, so that the tempering of the quenched structure to improve its toughness;: hydrogen elimination processing, so that the hydrogen to escape fully from the welded joint.
(5) to improve the quality of steel, to reduce the layered inclusions in the steel. (6) to take a variety of process measures can reduce welding stresses.