How to determine whether oil refining equipment has experienced embrittlement
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Taking into account the various causes and types of brittleness in refining equipment mentioned earlier, the following methods can be used to determine whether the equipment has experienced brittleness: A visual inspection of the surface of the equipment is carried out to look for brittle cracks without obvious plastic deformation, local bulging, as well as fracture surfaces that are flat with no large areas of shear edges; such characteristics are typical signs of brittleness. Sampling for material property testing is carried out to conduct impact tests, comparing the impact toughness of the material before and after service. If the toughness decreases significantly and brittleness increases markedly, it can be determined that brittle transformation has occurred. Non-destructive testing is employed using fluorescent magnetic particle inspection to detect stress corrosion cracks and fatigue cracks, while ultrasonic testing is used to identify internal hydrogen-induced microcracks and delamination defects, in order to determine any internal brittle damage. Condition tracing and verification are used to check whether the temperature at which the equipment operates over a long period falls within the range sensitive to embrittlement (such as the tempering embrittlement range of 325–575°C); parameters such as H₂S and hydrogen content in the medium are taken into account to assist in determining the likelihood of embrittlement occurring.
The original poster has summarized it very well; these several methods of judgment are indeed systematic approaches commonly used in the industry. I would like to add two details that are easily overlooked during actual inspections: when conducting a macroscopic inspection, it is recommended to pay close attention to the heat-affected zones of welds as well as areas with stress concentration (such as the root of joints and areas where the diameter changes), as these areas tend to be more prone to brittle failure. The sampling location for impact testing is crucial; it is best to choose the part of the equipment where the temperature is lowest or where corrosion by the medium is most severe, in order to represent the most severe operating conditions. Additionally, it is essential to check the surface temperature and environmental humidity of the equipment before performing non-destructive testing, as magnetic particle/penetrant testing may exhibit variations in its sensitivity to cracks at low temperatures. If the poster has specific information on the material of the equipment (such as austenitic stainless steel or low-alloy steel) or data on its service life, we can discuss in more detail the precautions to prevent embrittlement.