Intergranular fracture stress and phosphorus grain boundary segregation of a Mn-Ni-Mo steel

Abstract

*EDF, Direction des Etudes et Recherches, Dpt. Etude des Mate´riaux, Ecuelles, BP1,77283 Moreˆt sur Loing, France**Ecole des Mines de Paris, Centre des Mate´riaux, UMR CNRS 7633, BP87,91003 Evry Ce´dex, France(Received February 3, 1999)(Accepted February 18, 1999)IntroductionNuclear Reactor Pressure Vessel (RPV) steel A508 class 3 which is a low alloyed steel is not usuallysensitive to reversible temper embrittlement when properly heat treated. However heterogeneous zonesmay be present in particular near the inner side of the vessel. These zones result from the segregationof the alloying elements (C, Mn, Ni, Mo) and impurities (S, P) taking place during solidification of thematerial. They are called segregated zones (or ghost lines). They can reach 2 mm thick along the radiusand 30 mm long through the circumferential direction. Their susceptibility to reversible temperembrittlement is mainly due to grain boundary phosphorus segregation triggering brittle intergranularfracture when the material is tested at low temperature. In this material like in other steels the influenceof some other alloying elements (Mo, Mn...)isclearly significant (1) and should also be taken intoaccount. But phosphorus effect has proved to be predominant.The effect of ghost lines on A508 class 3 toughness properties has been pointed out (2) but has neverbeen quantitatively studied. The aim of the present study is therefore to find out a quantitativerelationship between grain boundary phosphorus segregation and critical intergranular fracture stress. Asynthetic steel with a chemical composition representative of an average segregated zone was preparedfor the present study. A number of heat treatments were applied to reach different embrittlementconditions. Then brittle fracture properties were obtained by performing cryogenic fracture tests onnotched tensile specimens while the corresponding grain boundary phosphorus levels were measured byAuger electron spectroscopy. Systematic fractographic observations were carried out. Moreover anattempt to determine the influence of temperature on the critical intergranular fracture stress was made.Materials and Heat TreatmentsThe intrinsic mechanical characteristics of segregated zones cannot be easily determined because oftheir very small dimensions (;2 mm width). A material with a chemical composition representative ofan average segregated zone was therefore prepared. Another plate was also elaborated with the nominalcomposition of A508 class 3 (3). A comparison between both materials can be made on Table 1 whereit is noted that the synthetic steel is enriched with most of the alloying elements. In particular