Issue 21

R. Valentini et alii, Frattura ed Integrità Strutturale, 21 (2012) 30-36 ; DOI: 10.3221/IGF-ESIS.21.04 35 (a) (b) Figure 8 : (a) Fracture surface for Maraging 250 specimen peak-aged at 535°C and (b) X1000 magnification for test in air. (a) (b) Figure 9 : (a) Fracture surface for Maraging 250 specimen peak-aged at 535°C and (b) X1000 magnification for test in 3.5% NaCl aqueous solution with galvanic coupling. D ISCUSSION xperimental evidence for AISI 4340 confirms severe re-embrittlement when the steel is subject to hydrogen uptake. As expected, only test in air resulted in a ductile fracture whereas the others environments, chosen for their high conductivity for HE[5-9], lead to brittle fractures in approximately one fifth of time. Test results for Maraging 250 proved that any of the grades suffered from the embrittling effect of the corrosive environments, neither in the most severe conditions, that is coupling to AISI 316 and paint-stripper. Comparing Figure 3 (a) to 3 (b) it is clear as Maraging peak-aged at 535°C exhibits a greater percent elongation than the steel in its original condition (peak-aged at 480°C). Difference is due both to the lower yield strength and the possible higher content of reversion austenite. The lack of uniformity for SSRT performance of Maraging peak-aged at 535°C may be attributed to non homogeneous heat treatment conditions for the specimens treated in Laboratory. Regarding SEM analysis, it's worth noting the difference between the fracture in air for AISI 4340 (Fig. 5) and that related to the specimen loaded in after paint-stripper application (Fig. 6). In the first case, the material displays a ductile aspect with an external ring associated to the first phase of necking and a more fibrous core better shown in Figure 5 (b). For the second case, Fig. 6 (b) shows intergranular fractures due to hydrogen uptake inside the growing crack[10]. As previously stated, Maraging fractures follow the classical cup-cone model for ductile necking. From pictures taken at higher magnification (Figures from 7 to 9) it can be noticed as the presence of precipitates had influenced the modality of rupture. These formations, probably Titanium nitrides, formed E