Failure analysis of the brittle fracture of a thick-walled 20 steel pipe in an ammonia synthesis unit

In this paper, the analysis of a brittle fracture explosion accident of a thick-walled 20 steel pipe, which connects the outlet of an ammonia separator and the inlet of a cold exchanger in an ammonia synthesis unit of a chemical fertilizer plant, is presented in detail. The causes and mechanism of the material degradation are investigated by analyzing chemical compositions (especially the elements of nitrogen, hydrogen), testing mechanical properties, observing microstructures and defects, and strain aging sensitivity, and analyzing annealing behavior of the pipe. Furthermore, the mechanism of crack initiation and propagation are analyzed carefully through macro- and micro-crack observations by means of Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The research results not only support the conclusions given by the accident survey group, but also clarify the uncertainty in the report of the survey at that time. The further research makes it clear that the decrease of material toughness due to strain aging embrittlement (SAB), and the crack initiation and propagation is caused by SAB and its lowering effect on the threshold value of hydrogen-induced cracking (HIC). The research work described in the paper has a significant meaning to recognize or eliminate SAB occurred in quite ductile plain carbon steels very commonly used in industry, such as 20 steel in China.