There are many crack shapes in stainless steel heat exchange tubes, which are the result of complex interactions between material properties, process defects, and environmental factors. The formation mechanism and crack morphology characteristics are inherently related.
From the intrinsic characteristics of the material, 304 stainless steel is prone to pitting corrosion in chloride containing environments, while 316L can significantly enhance its corrosion resistance due to its 2-3% molybdenum content. If 304 material is misused in coastal or chemical environments, the elbow will rupture due to local corrosion perforation.
Defects in cold processing technology are equally critical. If there is uneven wall thickness or excessive ellipticity during the elbow forming process, it can lead to abnormal stress distribution. A case study showed that when the wall thickness deviation of cold drawn pipes exceeds 15% due to mold wear, the risk of rupture increases threefold.
The intergranular corrosion in the heat affected zone of the welding process is particularly hidden. When the weld stays in the sensitization temperature range of 450-850 ℃ for a long time, chromium carbide precipitation will form a chromium poor zone. The 304 elbow of a certain chemical plant, due to uncontrolled interlayer temperature during multi-layer welding, cracked along the grain boundary after 6 months of use.
Longitudinal cracks are often closely related to material composition and welding quality. When the carbon element content is too high, it can cause the pipe to become harder, and cracking may occur after welding. Improper selection of welding materials may cause creep of the weld under the combined action of high temperature and load, leading to cracking. If the pollution of the welding interface is not removed or the welding speed is improper during the welding process, it may lead to weld cracking. Work hardening is also an important factor. 304 stainless steel water pipes undergo hardening during cold working, inducing martensitic structure, which is brittle and prone to cracking. Under the combined action of specific corrosive media (such as chloride ions) and stress (such as welding residual stress), stress corrosion cracking may occur in materials. These cracks are usually in the form of “rock sugar” and distributed along grain boundaries, with strong concealment.
Lateral cracks often occur at the base metal location rather than at the weld seam, and their formation is closely related to the enrichment of copper elements along grain boundaries. In a certain case, a large number of transverse short cracks perpendicular to the weld seam appeared on the base metal of 316L stainless steel welded pipe after hydrostatic testing. Scanning electron microscopy observation showed that the fracture surface had a sugar like transgranular feature, and energy spectrum analysis confirmed that copper elements were enriched along the grain boundaries at the crack site. The mechanism of crack formation involves the synergistic effect of grain boundary weakening and local stress concentration, and thin-walled pipes (with a thickness less than 1.0mm) are more prone to such defects. Grain refinement increases the number of grain boundaries, increases the energy of grain boundaries, and thus reduces the toughness of stainless steel. Under stress, grain boundaries are prone to cracking.
The network cracks are mainly caused by the combined effect of welding thermal stress and low melting point eutectic materials. Austenitic stainless steel has a low heat transfer coefficient and a high thermal expansion coefficient, which can cause significant stress and deformation during the welding process. When welding crystals, low melting point residues or co crystals are prone to concentrate on the crystals, causing thermal cracks under welding stress. This type of crack is distributed in a network pattern and is directly related to the segregation of impurity elements such as sulfur and phosphorus in the material. A study has shown that when the sulfur content in stainless steel exceeds 0.015%, the tendency for hot cracking significantly increases. Preventive measures include strictly limiting the S and P content in the base material and welding material, using welding materials containing an appropriate amount of ferrite, and reducing heat input through low current rapid welding.
Circular cracks are commonly found at the connection between tube plates. Macroscopic observations show no significant thinning or plastic deformation of the fracture surface, with corrosion products on the surface and bright new fracture characteristics in some areas. Metallographic analysis shows that the cracks are slender and dendritic in shape, with sharp tips. The propagation mode is mainly transgranular, presenting typical stress corrosion cracking characteristics. The formation of such cracks requires the simultaneous presence of three conditions: sensitive materials, specific corrosive media (such as chloride ions), and tensile stress. In a certain case, a circular crack appeared on the heat exchange tube after working in a chloride ion environment for 240 hours, while the 316L sample remained intact under the same conditions.
Solving cracks requires a multi pronged approach: in harsh environments, 316L or duplex steel materials should be selected, and after cold processing, solution treatment at 1050-1100 ℃ should be carried out to eliminate internal stress. During welding, ultra-low carbon welding materials should be used and the interlayer temperature should be controlled below 150 ℃.