| Hydrogen induced delayed fracture under static load is a kind of sudden fracture which occurred at a load below the designed allowable stress, whose consequence is disastrous. Since many accidents happened to the overloaded components of maraging steel, the delayed fracture behaviors of maraging (stainless) steels treated in the under-aged, peak-aged and hover-aged conditions in hydrogen environment--such as H2 and saturated H2S solution-ave been studied by many researchers, and steels with under-aged treatment have been proved to have lowest resistance to hydrogen induced delayed fracture. However, the tolerable ultimate hydrogen content in maraging (stainless) steels is extremely low, so it has not been studied whether the hydrogen induced delayed fracture happened after normal smelting and hot working. In this paper, the influence of aging temperature on the delayed fracture resistance of maraging (stainless) steels was investigated.18Ni350 maraging steels and Cr-Ni-Mo-Ti maraging stainless steels (including the forms of rods and cold-rolled plates of 2.0mm in thickness) were used in this investigation. The materials were processed through double vacuum smelting (vacuum induction melting followed by vacuum arc remelting) which could make the hydrogen content no more than 2.0ppm, and aged at the temperatures (420?540?) which covered the under-aged, peak-aged and over-aged conditions. Notched tensile tests were carried out to compare the fracture strength between the "fast tension" with a displacement rate of 0.15mm/min and the "slow tension" with a displacement rate from 0.0015mm/min to 0.005mm/min, which were used to evaluate the hydrogen induced delayed fracture tendency trend. Follows are the main conclusions:18Ni350 maraging steels aged at the under-aged condition of 420? and 450? had a low strength and a high notch sensitivity, which illustrated high hydrogen induced delayed fracture sensitivity even in the case of<1.0ppm hydrogen content. The strength of 18Ni350 maraging steels aged at 480? is a little lower than that peak-aged at 510?, while the notched tensile strength of the former is more than 100MPa compared with that peak-aged, with a low notch sensitivity at the same time. The strength decreased when over-aged at 540?,but the notch sensitivity did not decrease. No hydrogen induced delayed fracture tendency showed when aged between 480? to 540?, although the same hydrogen content was applied.In notched fast tensile test, the peak strength of Cr-Ni-Mo-Ti maraging stainless steels from the rod material appeared at temperature between 480? to 500?. In the case of<2.0ppm hydrogen content, hydrogen induced delayed fracture was detected in steels under-aged at 420? and 440? and not detected in steels aged between 460? to 520?.The ultimate tensile strength of Cr-Ni-Mo-Ti maraging stainless steels from the cold-rolled plates decreased when aged between 440? to 520?. However, the peak strength of the steels from the hot-rolled plates appeared between 480? to 500? On the other hand, the cold-rolled deformation had the function of enhancing the aging kinetics due to its enhancement of the over-aged tendency. Cold rolling could significantly improve the strength after aging without increasing the notch sensitivity. Especially aging above 460?, the notch sensitivity ratios of cold-rolled materials were even close to that of hot-rolled. Though the strength increment of the cold-rolled plates is about 230-460MPa, the cold-rolled plates aged above 480? did not show the hydrogen induced delayed fracture tendency, which demonstrates cold-rolling was a reliable method to increase the strength of maraging stainless steels. |
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