Kiyoshi Bessyo

A property distribution map to explain HAZ CTOD toughness: Study of microcrack initiation properties of a steel HAZ having microscopic strength inhomogeneities (2nd Report)

Summary To visualise the fracture initiation characteristics in the multipass weld heat affected zone of YS = 460 N/mm2 class 30 mm thick TMCP high‐strength steel, the yield strength distribution map and CTOD toughness distribution map are drafted by computer on the basis of a series of HAZ simulation results. A comparison between these computationally estimated property distribution maps and the actual HAZ CTOD test results reveals the following: Fracture of the HAZ CTOD specimens is initiated from the peripheral part of a local hard zone (LHZ) which does not always correspond to the most brittle point on the crack front. The critical CTOD value derived from full‐thickness specimens is adequately represented by the local critical CTOD level at the fracture initiation point. Fracture initiation is due not only to local brittleness, but also to strength inhomogeneity in the HAZ.

Morphology and fracture properties of martensitic islands in a multipass weld HAZ: Study of microcrack initiation properties of a steel HAZ having microscopic strength inhomogeneities (1st Report)

Summary This paper describes an investigation of the relationship between the microstructure and mechanical properties of the HAZ of YS = 460 N/mm2 class 30 mm thick TMCP high‐strength steel as produced in thermal simulations. The loss of HAZ toughness is found to be closely related to the amount and pre‐existing form of the martensitic islands in the matrix. Blocky type martensitic islands surrounded by ferrite grains do not impair the toughness, whereas lath type martensitic islands formed in a large matrix packet of the same crystal orientation depress the toughness depending on the amount of martensite. The latter structure is formed in the unaltered coarse‐grained HAZ (UACGHAZ), the intercritically reheated coarse‐grained HAZ (ICCGHAZ), and the subcritically reheated coarse‐grained HAZ (SCCGHAZ). Among the latter, the ICCGHAZ structure has relatively short lath type martensitic islands with a softer lath type matrix. This structure readily generates microcracks. The dependence of the strength and tou...