Hydrogen embrittlement for austenitic alloys: behaviour of microstructure and segregation of sulphur and phosphorus impurities

The mechanical properties of the austenitic alloys generally depend on thei r chemical composition and microstructure, but they can be significantly mo dified by the presence of hydrogen. In this paper, the effect of hydrogen o n two nickel (800 and 600) alloys has been investigated for different compo sition and microstructure. The alloy 800 has been studied in three structural states : A (as received condition) and Al, A2, representing two different heat treatments. The allo y 600 has been investigated in as received condition (B) and after heat tre atment (B1). The state A of alloy 800 was represented by the austenitic microstructure w ith the presence of M23C6 carbides in the matrix, nevertheless the grain bo undaries remained clean. Further, the carbides and carbonitrides of Ti, as well as the carbosulfides of titanium (Ti4C2S2) are detected in the intragr anular space. The heat treatment designated Al was represented by the annea ling treatment for la h at 700 degrees C, whereas the heat treatment design ated A2 consisted of the solution treatment for 20 min at 1000 degrees C fo llowed by annealing for 50 h at 700 degrees C. Both treatments A I and A2 h ave been employed to provoke the continuous precipitation of the carbides M 23C6 at the grain boundaries. The microstructure of the alloy 600 in the as delivered condition (B) was f ormed by the austenite with the precipitation of the carbides M23C6 on the austenitic grain boundaries. The same carbides were present in the matrix b ut were of the acicular morfology. The next heat treatment BI which consist ed of the solution treatment for 20 min at 1050 degrees C produced the net austenitic microstructure with all the carbides diluted in the solid soluti on. The effect of the microstructure on the mechanical behaviour of both alloys at the presence and the absence of hydrogen has been studied by means of t he tensile test at ambient temperature. The hydrogen charging was realized before the mechanical testing by the potentiostatic cathodic polarization o f the specimens, by the electrolysis of the water injected into the salt ba th at the temperature of 300 degrees C for the time intervals from 5 to 48 h. To study the microsegregation behaviour of both alloy and impurity (phospho rus and sulphur) elements on the grain boundaries the free surface segregat ion has been monitored on the samples of alloys in the broad range of annea ling temperatures. The procedure consisted of the stepwise annealing of the samples at increasing temperatures and each anneal was followed by the mea surement of the Auger electron spectra at the temperature lower than 300 de grees C and subsequent ion etching of the segregated layer. The most intensive hydrogen embrittlement has been observed in case of solu tion treated and sensitized alloy 600 with significant precipitation of car bides on the grain boundaries, the fractures being of the fully intergranul ar brittle nature. The solution treated state of the same alloy demonstrate d less embrittlement and less intergranularity of fractures. On the contrar y, the alloy 800 has been resistant to the hydrogen embrittlement, both in the solution treated and sensitized conditions. The observed differences ha ve been explained by the different picture of phosphorus free surface segre gation as measured by the Auger spectrometry.