Single-Specimen Test Measurement of Ji and J-Δa with a Pulsed D-C Potential-Drop Technique

Abstract

Fracture mechanics testing [KIc, J-integral, and crack-tip opening displacement (CTOD)] of a QT-steel (AISI 4137 H) has been carried out in order to compare the testing methods. The tests were performed at room temperature, and the material toughness was varied by applying annealing temperatures from 300 to 650°C. The J-initiation values were successfully determined with a specially developed d-c potential-drop (PD) method. The technique is based upon pulsed current and reference electrodes. Only the potential-drop difference caused by a current difference is measured; this eliminates the thermo-electric effect and offset voltage associated with PD equipment. On the basis of the PD measurements both the J-initiation value and the R-curve (J-Δa) could be determined from a single-specimen test. The R-curves could be fitted to a third-order polynomials, but even if the initiation values were nearly constant for replicate specimens, the slope of the R-curve would vary significantly. The mechanisms of crack initiation and growth are different, and the observed results indicate that the crack growth properties, as characterized with the R-curve, of the present steel quality vary from one specimen to the next. The J-initiation values determined with the multispecimen technique recommended in ASTM E 813 showed considerably more scatter in comparison with the present PD technique. The results suggest that the J-initiation value should be determined on each specimen and not calculated with extrapolation techniques.

Effect of microstructure and grain size on the fracture toughness of a micro-alloyed steel

“…3a). Such observations are in agreement with the findings reported by Thaulow et al and Wang et al [26,27] for similar steels. The microstructure in HTA conditions showed proeutectoid ferrite in the form of grain boundary allotriomorph (shown in arrows) along with some amount of side plate morphology, area marked B, which is a characteristic feature of upper bainitic microstructure as shown in Fig.…”

“…At 4% formation, as observed in increasing austenitising condition, the impact toughness reduces to one fourth of the base material toughness. Thaulow et al [26,28] attributed this drop in the toughness to the formation of ferrite side plate/upper bainitic microstructure in micro-alloyed C-Mn steels. As the martensitic phase appears at higher austenitising conditions (HTO1125 and HTO1275), the impact toughness reduces to a level of 15 joules.…”

High-strength steel development for pipelines: A brazilian perspective

“…[4] It should be remembered, however, that the excellent combination of strength and toughness exhibited by modern bainitic-ferritic microalloyed steels produced by controlled rolling can be significantly degraded by the thermal cycles imposed during the fabrication of the final pipeline product and its on-site assembly for service. [8] It has been shown [9,10] that low fracture toughness typically associated with the HAZ is often provoked by certain specific microstructural aspects. [5,6] In order to counteract such effects, especially the potential loss of toughness of the HAZ, steel producers must take into account the effect of such postrolling processing from the outset, when designing their steel's composition.…”

The Influence of Arc Welding Process on the Mechanical and Microstructural Characteristics of Low Carbon Steel in a Corrosive Environment

“…There are quite limited publications in respect to some of the related previous studies of welded mild steel structures. [ 4–9 ] For instance, Eroglu and Aksoy investigated the effect of initial grain size on the microstructure and toughness of intercritical heat‐affected zone of low carbon steel and established a correlation linking the heat input, initial grain size, microstructure, hardness, and toughness of the intercritical HAZ. It was evident from the microstructural and toughness analysis that the fine initial grain size was effective on the evolution of ductile phases and on the higher toughness.…”