It has been reported for as-quenched AISI 4340 s~eel that high temperature austenitizing treatments at 1200°C, instead of conventional heattreatment at 870°C, result in a twofold increase in fracture toughness,. ~Ic'• but a decrease in Charpy impact energy. This paper seeks to find an explanation for this discrepancy in Charpy and fracture toughness data in terms of the difference between Kic and impact tests. It is shown that the observed behavior is independent of shear lip energy and strain rate effects, but can be rationalized in terms of the differing response of the structure produced by each austenitizing treatment to the influence of ' notch root radius on toughness. The microstructural factors which affect this behavior are discussed. Based on these and other observations, it is considered that the use of h;i.gh temperature austenitizing b~ questioned as a practical heat-treatment procedure for ultra-high strength, low alloy steels. Finally, it is suggested that evaluation of material toughness should not be based solely on Kic or Charpy impact energy values alone; both sharp crack fracture toughness and rounded notch impact energy tests are required.

No abstract

Critical fracture stress and stress modified fracture strain models are utilized to describe the variation of lower and upper shelf fracture toughness with temperature and strain rate for two alloy steels used in the manufacture of nuclear pressure vessels, namely SA533B-1 (HSST Plate 02) and SA302B (Surveillance correlation heat). Both steels have been well characterized with regard to static and dynamic fracture toughness over a wide range of temperatures (-190 to 200~ although valid JIc measurements at upper shelf temperatures are still somewhat scarce. The present work utilizes simple models for the relevant fracture micromechanisms and local failure criteria to predict these variations in toughness from uniaxial tensile properties. Procedures are discussed for modelling the influence of neutron fluence on toughness in irradiated steel, and predictions are derived for the effect of increasing fluence on the variation of lower shelf fracture toughness with temperature in SA533B-1.

By applying a slip-line fields solution for general yielding, which includes the tup indenter for Charpy V-notch and precracked Charpy tests, estimates of high loading-rate yield stress values can be obtained. These values of yield stress are of particular interest for loading-rate sensitive materials such as low strength steels. Normal tensile tests are not performed at strain-rates indicative of impact tests; dynamic fracture toughness tests require the knowledge of dynamic yield stresses for the determination of validity limits for the fracture toughness measurements. This paper reviews the calculation of yield stresses from static and dynamic three-point bend tests and presents data for a nuclear pressure vessel steel which shows good agreement with high rate and conventional tensile test data.

Neutron irradiation can promote significant changes in the microstructure and associated mechanical properties of low alloy steels. In particular, irradiation can induce the formation of non-equilibrium phases and segregation, which may lead to a degradation in toughness. In this study, the microstructural changes caused by neutron irradiation have been characterized in A508 Grade (Gr) 4N-type steels (~3.5% Ni) using a variety of state-of-the-art analytical techniques including 3D-Atom Probe Field-Ion Microscopy and Small Angle Neutron Scattering, along with post-irradiation annealing studies combining Positron Annihilation Lineshape Analysis and hardness measurements. Important differences between conventional and "superclean" A508 Gr 4N steel have been identified in this investigation. The data indicate that Ni is not the controlling factor in the irradiation damage behavior of these materials; rather, the Mn content of the steel is a dominant factor in the irradiation-induced microstructural development of soluterelated hardening features.