Degradation of the welded joint microstructure of a 40-year-old pressure vessel made OF 16Mo3 steel

Abstract

The article concerns a pressure vessel operated for forty years at elevated temperature in in an ammonia production installation. During the planned control tests, unacceptable defects were detected in the circumferential welded joint of the vessel, using the ultrasonic method. In the repair process, a fragment containing a crack located in the heat-affected zone was cut out of the welded joint. The cut fragment of the joint was subjected to tests, the results of which are discussed in this article. On the basis of chemical composition, hardness, macroscopic and microscopic tests, it was determined which factors had a negative impact on the formation of the crack. The most important of them are: an increase in the hardness of the base material and grain growth in the overheating zone, as well as the formation of numerous precipitates and their segregation at grain boundaries. The reheating of the joint during its subsequent repairs should be considered a negative factor.