Mechanical Properties and Fracture Toughness in a RCCM 18MND5 (Type ASME A508 Gr3) Forging for Nuclear Applications Affected by Local Segregations

Abstract The effects of carbon macro-segregation on the mechanical properties of low alloy steel forgings have been extensively studied and reported in several papers in the past. The present paper focuses on a forged ring made with a hollow ingot of the grade RCCM 18 MND5 (similar to ASME A508 Gr3), in the so called “end of solidification zone”. This zone at the top end of the ring and close to mid-thickness, has a limited macro-segregation (%C∼ 0.23) and contains “band type” segregations at a millimetric scale. These segregated bands are inherited from the equiaxed solidification structure of the original ingot in which inter-dendritic segregations are embedded. The non-homogeneity of the structure at the millimetric scale is considered to be the cause of an important scatter of Charpy notched bar impact tests. The segregated bands were shown to be prone to inter-granular cracking because of a relatively high amount of Phosphorus concentrated in the segregation. In the present paper, fracture toughness tests are reported with 1TCT specimens taken in the same zone of the forging containing these local segregations. The Reference Temperatures T0 are determined in this position, and in the conventional ¼ Thickness position. Fractographic examinations are performed in order to verify if the same fracture mechanisms as those observed in Charpy specimens, can also be observed in CT specimens. The scatter of the KJC data in the segregated position is analyzed in terms of the ASTM E1921 homogeneity criteria.