STUDY ON FRACTURE PROCESS OF NITRIDED CASES OF STEEL 30CrMo WITH ACOUSTIC EMISSION TECHNIQUE

ABSTRACT In this paper, initiation and propagation of cracks in nitrided cases of two different structures on 300rMo steel are studied on notched three-point bending specimens with the acoustic emission (AE) technique. It has been found that the formation of the first macrocrack in specimens with an τ′ -Fe 2-3 N case occurs at lower loads and cracks propagate much faster than in specimens with a γ′ -Fe 4 N case. For both structures, the number of AE counts per unit length of crack is nearly the same. Study on AE amplitude distribution shows that the fracture process proceeds first with initiation of microcracks and then linkage with the main crack or between themselves occurs. Optical microscopic and SEM observations support the findings of the AE amplitude study.