Statistical modelling of intergranular brittle fracture in a low alloy steel

The intergranular brittle fracture (IBF) behaviour of a low alloy steel 16M ND5 (A508 Cl. 3) was investigated. A temper embrittlement heat treatment wa s applied to the material to simulate the effect of local brittle zones (gh ost lines) which can be found in the as-received material condition. An inc rease in the Charpy V toughness transition temperature and a significant de crease in the fracture toughness measured on CT-type specimens were observe d in the embrittled material, as compared to the reference material which w as submitted to the same austenitizing and tempering heat treatment, but wh ich was not subjected to the temper embrittlement treatment. Tensile tests un notched specimens were carried out to measure the Weibull stress and sca tter in the results. A statistical model, the Beremin model, originally pro posed for brittle cleavage fracture was applied to IBF. It is shown that th is model is not able to fully account for the results, in particular for th e existence of two slopes in a Weibull plot. Systematic fractographic obser vations showed that the low slope regime in this representation was associa ted with the existence of MnS inclusions initiating brittle fracture, while the larger slope was related to microstructural defects. Initiation of IBF from MnS inclusions can occur when the material is still elastically defor med while the second population of microstructural defects is active in the plastic regime. A modified statistical model based on the Beremin model an d taking into account these specific aspects is proposed in the framework o f the weakest link theory. The parameters of this model are identified from test results on notched specimens. It is shown that this model is able to predict the temperature dependence of fracture toughness and the scatter in the experimental results.