Abstract
Acoustic emission (AE) is used to monitor cleavage microcracking activity in weld thermal simulated HAZ microstructures of a 420 MPa rolled plate. Fracture mechanics testing at different temperatures is carried out for three different simulated HAZ microstructures: ICCGHAZ Δt8/5=15 s, CGHAZ Δt8/5=5 s, and ICCGHAZ Δt8/5=5 s. Two parameters are extracted from the AE measurements: the rate of microcrack nucleation and the distribution of arrested cleavage microcrack sizes. The latter is obtained based on a first-order relationship between microcrack sizes and AE signal amplitude, earlier established by the authors. The results are discussed in terms of the effects of temperature and microstructure. It is shown that on average the arrested microcrack sizes are smaller in the microstructures with faster cooling rate, i.e. with smaller prior austenite grain size. It is also demonstrated that the effect of temperature on the microcrack nucleation rate depends on the microstructure. Further, it is shown that a rapid increase in fracture toughness with temperature is usually associated with a significant reduction in the microcrack nucleation rate. The results are interesting both in terms of understanding the temperature effects on fracture toughness and also as input to development of micromechanical models for cleavage fracture.