types of Welding crack, causes and the solution to prevent
The most common welded serious flaw. Welding of the metal include two categories: one is welding material properties caused by deterioration, so that the original weldment lost the unique properties of materials such as stainless steel after welding losing its corrosion resistance; the other is In the welded joints in or near the base material defects such as cracks and pores. Crack the safe use of welding parts, is a very dangerous process defects. Welding cracks occur not only in the welding process, some still have a certain incubation period, while others are produced in the re-heating after welding process. Welding crack in accordance with its location, size, cause and mechanism of the different categories can have different methods. According to the conditions of crack formation can be divided into hot cracking, cold cracking, reheat cracking and lamellar tearing other four categories.
Thermal cracks produced in many high temperature near the solidus, there is along the grain boundary (see Interface) distribution characteristics; but sometimes it can also lower than the solidus temperature, along the "polygon of the boundary" formed. Thermal cracks are usually generated in the weld metal more, but are also likely to form near the weld fusion line was welded metal (base material) above. The characteristics of their formation process, can be divided into the following three cases.
Generated in the weld metal solidification cracking the end of the crystallization process, "brittle temperature" range, this time there is a thin intergranular liquid layer, which metal plastic is very low, the cooling resulting from the uneven contraction of the tensile deformation of more than allowable value, the liquid layer along the grain boundary cracking. Elimination of solidification cracking of the main metallurgical measures by adjusting the composition, grain refinement, and strictly control the formation of low melting point eutectic impurities, in order to improve the material in the brittle temperature range of plasticity; In addition, from the design and workmanship to minimize The tensile deformation within the temperature range.
Liquefied cracks mainly occurred in the vicinity of the weld fusion line in base material, sometimes produced in the first multi-layer welding welding welding tract. Formation is due to the role of heat in the welding, the weld metal fusion line is generated outside the local melting along the grain boundary and in the subsequent cooling caused by the contraction of liquid along the grain boundary layer of cracking. Cracks caused by this situation is twofold: First, materials with low melting point grain boundary has more substance; the other is due to rapid heating, the decomposition of some metal compounds, but no time to spread, causing some local grain boundary of alloy elements even to the enrichment of the eutectic composition. Principles of these cracks to prevent strict control impurities, a reasonable selection of welding materials, to minimize the effect of welding heat.
Multilateral crack is below the solidus temperature of the formation. Its characteristics are along the "polygon boundaries" distribution, with a crystalline grain boundary was no significant relationship; easy to produce in single-phase austenitic metals. This phenomenon is explained as a result of overheating and uneven heat welding the crystallization conditions for the formation of a large number of vacancies within the crystal and the dislocation, at a certain temperature, stress, arranged under the sub-grain boundaries (polygon boundary), when This boundary coincides with the harmful impurities rich region, often the formation of microcracks. Elimination of such defects can be improved by adding the polygon is activation energy of alloying elements, such as the addition of Ni-Cr alloy W, Mo, Ta, etc.; the other is to reduce overheating when soldering and welding stress.
The main cause cold cracking can be divided according to quenching cracks, hydrogen induced delayed cracking and deformation crack.
Quenching cracks in steel martensite transition point () near (seen Austenite Transformation Diagrams) or less in the 200 cracks, mainly in the medium and high carbon steel, low alloy high strength steel and titanium, etc. produce parts primarily in the weld metal and heat affected zone within. Intergranular cracks or wear to the crystal. The formation of cold cracking of the main factors: ① high hydrogen content of metals; ② brittle organization or organizations sensitive to hydrogen embrittlement; ③ welding restraint stress (or strain).
Hydrogen induced delayed cracking in the welding process of hydrogen dissolved in the weld metal heat-affected zone diffusion, segregation, especially in easy crack initiation of triaxial tensile stress concentration area concentration, causing hydrogen embrittlement, which reduces the metal in the crack initiation position (or crack front) of the critical stress, when the local stress exceeds here when this critical stress to cause cracking. The crack formation time delay significantly, and their concentration hydrogen diffusion because it takes time (incubation period). The conditions to produce such cracks exist hydrogen and hydrogen-sensitive tissues, while there are large restraint stress. Therefore, it often produces severe stress concentration at the root of welds and seam edge, and the overheated zone. Prevention measures include: ① lower weld hydrogen content, such as using low hydrogen electrodes, welding materials, strict drying; ② after a reasonable warm-up and heat; ③ low carbon equivalent of raw materials used; ④ reduce the binding stress, avoid stress concentration (see the metal hydride).
Deformation crack this crack formation is not necessarily because of the high hydrogen content, multi-layer welding or fillet weld in the strain concentration case, because the tensile strain exceeds the plastic deformation of metal produced.
Reheat cracks in some high-strength low-alloy steel, pearlitic heat-resistant steel, austenitic stainless steel and nickel-based alloys after welding high temperature heating process again. The main reason is generally believed that when the welding heat again to 500 to 700, in the heat affected zone of the hot zone, due to the special carbide precipitation strengthening due to the secondary crystallization, some weakening of the grain boundary precipitation of trace elements, and to Additional welding deformation during stress relaxation concentrated in the grain boundary, resulting in intergranular cracking. Thus, the crack has the characteristics of intergranular cracking, and both have serious stress concentration in the heat affected zone of coarse-grained region. In order to prevent such cracks, the first in the design and then to select materials with low sensitivity to hot cracking, followed on from the process to minimize the internal stress near the weld zone and stress concentration problems.
Lamellar tearing arising primarily from the angle plate welding, which is characterized by parallel to the plate surface, along the rolling direction the development ladder. Such cracks are often not limited to heat-affected zone, but also can occur far from the surface of the base metal. The main cause is due to non-metallic inclusions in the metal layer in the distribution of the plastic sheet along the thickness direction than along the rolling direction, while the thick fillet welding in the thickness direction when the welding has caused great stress , so the cause lamellar tearing. Sulfide sheet generally considered the most damaging, and layered silicate and alumina inclusions over-intensive also have an impact. Prevent such defects, primarily in the metallurgical process in the strict control of the number and distribution of inclusions in the state. In addition, the improved joint design and welding technology, but also to a certain extent.