Intergranular and intragranular phosphorus segregation in Russian pressure vessel steels due to neutron irradiation

doi:10.1016/S0022-3115(00)00007-6

Journal of Nuclear Materials

Volume 279, Issues 2–3, June 2000, Pages 259-272

https://doi.org/10.1016/S0022-3115(00)00007-6 Get rights and content

Abstract

Russian reactor pressure vessel steels have been studied in three conditions: initial, irradiated and annealed. It has been established that irradiation induces both intergranular as well as intragranular phosphorus segregation. Fractographic studies demonstrated that brittle intergranular and ductile intergranular fracture surfaces of Charpy specimens appear as a result of intergranular and intragranular segregation, respectively. Transmission electron microscope (TEM) studies have revealed radiation-induced precipitates on interface boundaries to which intragranular phosphorus segregation occurs. Auger electron spectroscopy (AES) has been applied to detect phosphorus enrichment of fracture surfaces in the regions of brittle and ductile intergranular fractures.

Introduction

It is well known that radiation embrittlement (RE) of reactor pressure vessel steels (RPVS) induces the shift observed in transition curves obtained using Charpy tests defined as an increase in DBTT [1], [2].

In addition to hardening, a significant contribution to RE can be brought about by intragranular segregation of impurities (primarily, phosphorus), occurring at interface boundaries of precipitates (including those arising under irradiation for, e.g., copper-enriched or copper-vacancy clusters) [3]. Certain contributions to RE in RPVS can be brought about by intergranular segregation – for, e.g., phosphorus [3].

Many studies devoted to grain-boundary phosphorus segregation in steels (the so-called phenomenon of `reversible temper brittleness') are available [4], [5]. In contrast, the number of studies, where intragranular phosphorus segregation was considered, is much less. However, in these few studies, the existence of phosphorus segregation at interface boundaries of the precipitate/matrix type together with an increase in DBTT induced by intragranular phosphorus segregation in steels was demonstrated using direct experimental methods. In particular, in [6] such segregation was observed in RPVS A533, A508 and in some model steels. It should be noted that intragranular segregation in these alloys was due to thermal aging at various temperatures. In [7] three different mechanisms of DBTT shift, i.e., hardening, formation of grain-boundary and intragranular phosphorus segregation, contributing to embrittlement occurring in steels subjected to thermal aging, were experimentally separated. In [6], [7] it was shown that in the RPVS and model steels studied, intragranular phosphorus segregation proceeds mainly on M₆C type carbides, Laves phases and non-metallic inclusions. It was shown in the same studies that if any of the above types of precipitates are located along grain boundaries, then phosphorus segregation at their interfaces can lead to the appearance of ductile intergranular fractures on the surfaces of Charpy specimens tested at temperatures in the upper shelf (US).

The phenomena related to intragranular segregation were found in investigations of RE in some model binary alloys namely Fe–P and Fe–Sn in [8]. In this paper, the influence of concentration of the second element in the series of binary alloys, Fe–Ta, Fe–W, Fe–Nb, Fe–Ti, Fe–Cu, Fe–P, Fe–Sn, on the value of their RE resulting from irradiation to neutron fluences (3–5)×10¹⁹ cm⁻² (E > 0.5 MeV) at 50°C was investigated. Moreover, it was demonstrated that for all binary alloys (including the alloys Fe–Cu), but excluding Fe–P and Fe–Sn, the concentration dependencies of the yield stress after irradiation have the same shape as the concentration dependencies of the DBTT. It should be noted that for binary alloys Fe–P and Fe–Sn this relation does not exist. It should be emphasized as well that in Charpy specimens for alloys Fe–P and Fe–Sn the regions with intergranular fractures were not observed. As the authors [8] believe, this fact proves the dominant role of intragranular segregation in RE proceeding in these binary alloys.

In [6], [7] the methods of FEGSTEM and APFIM were used to study intragranular phosphorus segregation at interface boundaries for Russian and American grade steels irradiated in conditions characteristic of an operating PWR.

Thus, the accumulated experimental data at the present time on investigation of RPVS show that in addition to hardening, irradiation can result in grain boundary and intragranular segregation of impurities (primarily, phosphorus).

It would be reasonable to suppose that the fractography of Charpy specimens tested at different and comparable temperatures before and after irradiation can provide direct information on the mechanisms responsible for RE in RPVS. As it is well known, in the majority of cases, for steels with bcc crystal lattice (in particular, for RPVS) schedules of final heat treatment are applied in order to prevent the occurrence of temper brittleness. For this reason, the fracture surfaces of unirradiated RPVS specimens represent various combinations of ductile cleavage and quasi-cleavage fracture [9], [10]. In the simplest case, if RE in RPVS is caused only by hardening, then irradiation does not induce the appearance of new types of fracture modes. In the real case, when RE in RPVS is caused by the simultaneous action of several mechanisms, including the formation of grain boundary and intragranular segregation of impurities under irradiation, the fracture surfaces may also contain regions with brittle and ductile intergranular fractures [3], [10]. It is unlikely that a comprehensive evaluation of the mechanisms responsible for RE in RPVS can be achieved without structural investigation of the same materials after recovery annealing and re-irradiation.

Therefore, in the present study a series of fractographic, structural and Auger electron spectroscopy (AES) investigations of RPVS in the initial, irradiated, annealed and re-irradiated states have been performed. The objective of the study was to provide a comparative analysis of all the investigations to achieve detailed information on the conditions of grain boundary and intragranular phosphorus segregation and their role in RE occurring in RPVS.

Section snippets

Materials and experimental

The following steels have been investigated:

•
15Kh2MFA – base metal (WWER-440). After forging, heat treatment: quenching 1000°C/10 h; cooling in oil; tempering 700°C/16 h, cooling in air.
•
15Kh2NMFAA – base metal (WWER-1000). After forging, heat treatment: austenisation 920°C/h, cooling in water; annealing at 650°C, cooling in air; annealing at 620°C/25 h, annealing at 650°C/20 h, final cooling in a furnace to room temperature.
•
25Kh3NM – base metal (prototype reactor). The material was subjected to

Results

Table 2 details typical results of fractographic studies applied to the RPVS in the initial state, irradiated in various conditions, or annealed and re-irradiated. The results permit the following conclusions to be drawn concerning grain boundary and intragranular phosphorus segregation and their evolution under irradiation and recovery annealing.

(1) Irradiation of the steels in conditions characteristic of RPV operation, results in significant changes in Charpy specimen fractures as compared

Discussion

The above experimental data allow the following conclusions to be made on the behaviour of Russian RPVS resulting from irradiation and recovery annealing.

As described earlier, irradiation induces the formation of radiation defects (dislocation loops), copper-enriched (round-shaped) precipitates and also increases the density of ultrafine dispersed vanadium carbides (disc-shaped precipitates), which are present in the steels in the initial state. Furthermore, irradiation induces phosphorus

Conclusions

1.
The principal feature of the fractography of irradiated (or in some cases long-term aged) Charpy specimens of RPVS is the transition from purely transcrystalline to mixed mode fracture, characterised by the presence of brittle and/or ductile intergranular fracture.
2.
The presence of grain boundary phosphorus segregation in the fracture surfaces of RPVS specimens with intergranular fracture both following irradiation and long-term aging (∼60 000 h) at operating temperatures of 270–290°C has been

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Intergranular and intragranular phosphorus segregation in Russian pressure vessel steels due to neutron irradiation

Abstract

Introduction

Section snippets

Materials and experimental

Results

Discussion

Conclusions

J. Nucl. Mater.

Acta Metall.

Theor. Appl. Fracture Mech.

J. Nucl. Mater.

Int. J. Press. Vessel Piping

J. Nucl. Mater.

Acta Metall.

Acta Metall.

Philos. Mag.

Vopr. Material.

J. Nucl. Mater.