Cleavage Micromechanisms Study in a Ship Plate Steel through Four-Point Double-Notch Bend and Charpy Tests

Abstract

In this work instrumented impact Charpy tests were performed to analyze the ductile-brittle transition of Grade A ship plate steels, which are composed of ferrite matrix with pearlite bands in the rolling direction (RD) and pearlite lands in the transverse direction (TD). Observations on fracture surfaces of Charpy specimens showed regions of cleavage from room to down temperatures, therefore blunt four-point double–notch bend specimens (4PBT), were also tested at 25°C, 0°C, –60°C and –196°C in RD and TD, to study cleavage nucleation and propagation mechanisms. In the RD the following four different cleavage microfeatures nucleated microcracks in the notch region of 4PBT in all test temperatures: lamellar-pearlite microstructure, pearlite-boundary, ferrite-grains boundary and grains ferrite-inclusions. However, in the TD at –60°C, close to the lower shelf, only lamellar-pearlite and pearlite-boundary microfeatures were found nucleating microcracks. Nevertheless, the higher density of microcracks was found in the RD. The biggest microcracks were developed in TD in pearlite lands of critical size, in all test temperature, which are the link for the development of microcracks of critical size, which can propagate and fracture steel plates. In RD most microcracks nucleated in a critical region of about 1mm from notch root for all test temperatures, however for the TD the critical region decreases with test temperature, from about 450 µm (+25°C), 370 µm (0°C) and 225µm (–60°C). The analysis also showed the effect of microstructure orientation on the number and size of microcracks, and microcracks arrest.