Dynamic Strain Aging in New Generation Cr-Mo-V Steel for Reactor Pressure Vessel Applications

Abstract

A new generation nuclear reactor pressure vessel steel (CrMoV type) having compositional similarities with thick section 3Cr-Mo class of low alloy steels and adapted for nuclear applications was investigated for various manifestations of dynamic strain aging (DSA) using uniaxial tests. The steel investigated herein has undergone quenched and tempered treatment such that a tempered bainite microstructure with Cr-rich carbides was formed. The scope of the uniaxial experiments included tensile tests over a temperature range of 298 K to 873 K (25°C to 600°C) at two strain rates (10 À3 and 10 À4 s À1 ), as well as suitably designed transient strain rate change tests. The flow behavior displayed serrated flow, negative strain rate sensitivity, plateau behavior of yield, negative temperature (T), and strain rate _ e ð Þ dependence of flow stress over the temperature range of 523 K to 673 K (250°C to 400°C) and strain rate range of 5 9 10 À3 s À1 to 3 9 10 À6 s À1 , respectively. While these trends attested to the presence of DSA, a lack of work hardening and near negligible impairment of ductility point to the fact that manifestations of embrittling features of DSA were significantly enervated in the new generation pressure vessel steel. In order to provide a mechanistic understanding of these unique combinations of manifestations of DSA in the steel, a new approach for evaluation of responsible solutes from strain rate change tests was adopted. From these experiments and calculation of activation energy by application of vacancy-based models, the solutes responsible for DSA were identified as carbon/nitrogen. The lack of embrittling features of DSA in the steel was rationalized as being due to the beneficial effects arising from the presence of dynamic recovery effects, presence of alloy carbides in the tempered bainitic structure, and formation of solute clusters, all of which hinder the possibilities for strong aging of dislocations.

Studies of Bainitic Steel for Rail Applications Based on Carbide-Free, Low-Alloy Steel

“…The effect of force oscillation during low-cycle fatigue is visible across all applied deformation amplitudes. This material behaviour could be a manifestation of the dynamic strain ageing (DSA) occurrence in steel, [83] which may occur at room temperature in low-alloy, low-carbon steel following cold working, i.e. static strain ageing (SSA), or during plastic deformation, as is the case with the tested steel.…”

Descriptors of temporal signatures of serrations in a low alloy steel

“…This insight could be useful in understanding the impact of serrations on tensile and fracture properties in new generation low alloy steels that have been compositionally designed to suppress yield point phenomena. Previous studies by the authors in one such non-aging advanced low alloy steel [20,21] for reactor pressure vessel applications showed reduced influence of dynamic strain aging on tensile properties. In comparison with a martensitic stainless steel, which showed intense serrations and lowered fracture toughness [22], this advanced low alloy steel did not show systematic temperature-strain rate domain of appearance of serrations and lowering of ductility [20,21].…”

“…Previous studies by the authors in one such non-aging advanced low alloy steel [20,21] for reactor pressure vessel applications showed reduced influence of dynamic strain aging on tensile properties. In comparison with a martensitic stainless steel, which showed intense serrations and lowered fracture toughness [22], this advanced low alloy steel did not show systematic temperature-strain rate domain of appearance of serrations and lowering of ductility [20,21]. Extending the influence of temporal metrics of serration obtained from an inhomogenous flow curve to the observations of lowered fracture properties due PLC phenomenon, the γ-parameter values promoting lower ductility and a rising ξ parameter could also reduce crack growth resistance by resulting in rapid fluctuations in crack tip plastic zone size [23].…”

Deformation behavior of AA2017–SiCp in warm and hot deformation regions

“…Moreover, the work hardening exponent and the occurrence of instability owing to DSA were studied in this research. Gupta et al [4] investigated the plastic deformation in a new generation of pressure vessel steels utilizing tensile testing in temperatures between 25°C and 600°C under low strain rates. It was found that the embrittling via DSA has been reduced owing to the concurrent occurrence of dynamic recovery as well as the influence of Mn/Ni atoms on mobility of interstitial atoms.…”