How to comprehensively assess the degree of embrittlement by combining multiple test results
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It is necessary to combine the results of three core tests – impact testing, metallographic examination, and non-destructive testing – in order to cross-verify and comprehensively assess the degree of brittleness in petroleum refining equipment. The specific methods are as follows: Quantitative grading through impact testing – Using the measured impact absorption energy and the temperature at which toughness changes to brittleness as key indicators, and comparing them with the standard values for new materials; if the impact energy decreases by more than 50% or the temperature at which toughness changes to brittleness rises significantly, it indicates that the material has reached a high level of brittleness. Microscopic evidence from metallographic testing, combined with an analysis of the tissue structure based on the metallographic results: if only slight carbide precipitation is observed and there are no grain boundary cracks, this indicates mild embrittlement ； The presence of large amounts of Weibull tissue and grain boundary network precipitates indicates moderate to severe embrittlement. Defect grading in non-destructive testing: Based on the results of non-destructive testing, the absence of any microcracks indicates mild brittleness; the presence of only a small number of non-propagating microcracks suggests moderate brittleness, while the occurrence of through-going macroscopic cracks indicates severe brittleness. In multi-result cross-validation, a classification is determined directly when the matching levels of the three types of test results are consistent. If discrepancies occur, the degree of mechanical property degradation observed in impact tests serves as the primary criterion; this is combined with the depth of structural damage as determined by metallographic analysis and the extent of defect propagation detected by non-destructive methods, to ultimately determine the accurate grade of brittleness.
The original poster’s sharing is very practical; it explains the idea of cross-verifying the results of the three types of tests quite clearly. I would like to add a few points from my own understanding: data consistency is crucial—for example, impact tests may show severe brittleness, but metallographic analysis reveals only slight precipitation; in such cases, it is necessary to first consider whether there are differences in the sampling locations, or whether the impact sample happened to hit a local defect, and one should not simply take an average value. The weights can be adjusted dynamically – for equipment that has been in service for a long time, even if the microcracks detected by non-destructive testing do not expand, it is advisable to maintain a high level of vigilance, as fatigue or stress corrosion could trigger their growth; critical size assessment can be carried out using fracture mechanics. Pay attention to high-temperature hydrogen exposure conditions – if the equipment is in contact with hydrogen for an extended period, slight carbonite precipitation in the metal structure may indicate only initial hydrogen erosion; in such cases, a decrease in impact strength often occurs later on. It is recommended to conduct additional tests for hydrogen content or hydrogen-induced cracking to assist in making judgments. Furthermore, different devices have varying design margins; it is recommended to refer to the equipment’s original design documents or relevant industry standards (such as API 579, GB/T 30579) for making a final determination. If possible, it is advisable to consult experts in material failure analysis, as safety is of paramount importance.