Fracture analysis of 304 stainless steel probe rod

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1. Case background: After being in service inside an oil pipeline for 1 month, this 304 stainless steel probe rod fractured in the area near the weld at its connection to the flange. The probe rod has a diameter of Φ16×3 mm, the outer diameter of the oil transport pipeline is Φ406 mm, the flow velocity is approximately 3 m/s, and the operating temperature is 300°C. 2. Brief description of the analysis method: The fracture surface was examined morphologically. Through this observation and analysis, it was found that the fracture pattern of the probe rod resembled that of a typical fatigue fracture, with the source of fatigue located on the outer side of the tube wall ； There are numerous cleavage facets at the port, as well as many fine, parallel fatigue striations; the mode of fracture is cleavage fracture. An analysis of the microstructure in the area near the fracture surface revealed that both the base material and the region near the weld consist of austenite, δ-ferrite distributed in bands along the direction of deformation, and carbides distributed along the grain boundaries. It can be concluded that the probe rod was not subjected to effective heat treatment prior to welding, and the presence of high-temperature δ-ferrite and grain boundary carbides reduces its mechanical properties. Figure 2. Macroscopic fracture morphology (microregion). Figure 3. Typical SEM image of weakened grain boundaries. 3. Results and discussion: The fracture site of the probe rod was located at the root of the weld, in a stress concentration zone; the fracture mode was single-source fatigue fracture under high stress. The heat treatment process of the raw materials prior to welding was defective; no effective heat treatment was carried out. The presence of high-temperature δ-ferrite and grain boundary carbides led to a significant decline in its mechanical properties as well as a reduced fatigue resistance, **which shortened the service life of the product. 4. Recommendation: As indicated by the above analysis, the heat treatment process applied to the base material of the probe rod was inappropriate; therefore, the client should conduct on-site inspections of the raw materials supplied by the supplier, discard those that do not meet the standards, or apply an appropriate heat treatment process prior to welding in order to eliminate high-temperature δ-ferrite and grain boundary carbides.