Specimens of duplex X 2 Cr Ni Mo N 25-7-4 stainless steel were characterised at the DCN to assess the feasibility of using such solid parts in the fabrication of equipment operating in seawater.

Two cylindrical specimens (A and B) were manufactured, with a central hole. The main difference between the two specimens was in the heat treatment during manufacture. On specimen A this treatment was carried out on the solid boulder and on specimen B after the pre-machining of the central hole. Core cooling of specimen A was thus slower than for specimen B. The impact toughness characteristics were thus low at the core of specimen A because of the partial decomposition of the ferrite zones into sigma phase.

To gain a better understanding of the influence of the structure associated with slow cooling on other material characteristics, stress corrosion tests in synthetic seawater and toughness tests were carried out on samples taken from the low impact strength areas of specimen A and from the healthy specimen B. In addition, fatigue and crack propagation tests were carried out on specimen B.

The results obtained showed that, as expected, the low impact strength areas possessed lower tearing energy levels J1c than on the healthy specimen with a correct quenching state, where the extreme ductility of the material did not enable valid J1c results to be obtained, as there was generalised plasticity at the bottom of the crack. However, the measured values of KJc in the areas embrittled by the s phase precipitation were not very low and could prove adequate compared with the size of the critical defect for certain applications. This opinion was confirmed by the fact that no sensitivity to corrosion was identified within the limits of validity of the WOL test used, regardless of the thermal state.