Constant extension-rate tensile tests are performed to investigate the effects of strain rate and environmental hydrogen concentration on the tensile properties of various aged T-250 specimens. The 426 *C (800 ~ underaged specimens are very sensitive to strain rate; the 482 ~ (900 oF) peak-aged specimens exhibit a reduced ductility under low strain rates; and the 593 *C (II00 oF) overaged specimens are insensitive to strain rate when tested in air. The excellent resistance to embrittlement of the overaged specimens in gaseous hydrogen could be associated with the extensive formation of reverted austenite and the incoherent Ni3Ti precipitates. The tensile-fractured surfaces of such specimens reveal a ductile dimple fracture. However, the peak-aged specimens are susceptible to gaseous hydrogen embrittlement, and the embrittled region shows a primary fracture mode of quasi-cleavage. The least resistant to hydrogen embrittlement of the underaged specimens is characterized by a more brittle fracture appearance, that is, intergranular fracture, under a low strain rate or in the gaseous hydrogen environment.
The tensile properties of laser-welded T-250 maraging steel are measured, with attention paid to the influence of strain rate and gaseous hydrogen on the fracture behavior of welded specimens. Post-weld heat treatments are performed on laser-welded specimens to obtain underaged (WU), peak-aged (WP), and overaged (WO) specimens. Hydrogen embrittlement (HE) affects the tensile fracture behavior of the welded specimens; HE changes not only the fracture mode but also the fracture location. Without the influence of hydrogen, the fracture location is at the softest region, the weld metal (WM), and the fracture appearance reveals a ductile dimple fracture. For welds sensitive to HE, the fracture is initiated at the heat-affected zone (HAZ) with coarse grain size, and the associated fracture surface exhibits intergranular and quasi-cleavage fractures that are brittle in nature. In addition, the HAZ with coarse grain size is more prone to HE, as compared to other regions in the welded specimens. The WU specimens are susceptible to HE in air under a low strain rate, while the WP specimens are only susceptible to gaseous hydrogen embrittlement (GHE). However, the WO specimens are immune to GHE and insensitive to strain rate.
The effects of environmental hydrogen content on fatigue crack growth rates (FCGRs) in T-250 maraging steel plates and laser welds were investigated. The influence of ageing treatments on fatigue characteristics of the alloy was also studied. Experimental results revealed that the accelerated FCGRs in the presence of hydrogen were always associated with changes in fracture modes that appear in compact-tension specimens. Even for overaged specimens with excellent resistance to gaseous hydrogen embrittlement, such an acceleration of crack growth in hydrogen could not be avoided. The crack path of underaged specimens in hydrogen was found mainly along prior austenite boundaries for steel plates and along coarse columnar boundaries for welds. In gaseous hydrogen, peak-aged welds exhibited intergranular and quasi-cleavage mixed fracture modes, compared to mainly quasi-clevage for similar aged steel plates. Hence, the enhancement of crack growth in hydrogen was more pronounced for the welds. Overaged welds showed hi her FCGRs than the same aged steel plates only in hydrogen and for AK values greater than 20MPa&.
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