Fractographic Analysis of Gaseous Hydrogen Induced Cracking in 18Ni Maraging Steel

Fracture surfaces, produced by sustained load gaseous hydrogen assisted cracking of 18Ni maraging steel, were examined by scanning electron microscopy. Cracking developed along boundaries associated with the maraging steel microstructure. The specific crack path depended on test temperature and correlated with the influence of this variable on crack growth rates. At low temperatures, crack-growth proceeded predominantly along prior austenite grain boundaries. Increasing temperature produced a continuously increasing amount of transgranular quasicleavage associated with lath martensite boundaries. The proportions of quasicleavage fracture correlated with temperature induced reductions in the crack rate over those predicted from low temperature Arrhenius behavior. Both reduced hydrogen pressure and yield strength decreased the temperature for the onset of the transition to transgranular fracture but had no influence on the crack path. The lower strength steel fracture morphology contained an increased proportion of features typical of ductile rupture. The crack path through the microstructure was independent of the applied stress intensity factor, and hence crack growth rate, from very low values to those beyond the Stage I and Stage II transition for all temperature conditions. Comparison between fractographic observations and known sites for hydrogen segregation suggested that microstructural features play a significant role in the mechanism for gaseous hydrogen embrittlement.