Behaviour of Short Intergranular Stress Corrosion Crack in Type 304 Austenitic Stainless Steel

Austenitic stainless steels with their good weldability, superior corrosion resistance and excellent performances in higher temperatures are an important material for engineering applications in industrial plants. Intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels is a critical failure mechanism where cracking can result from sensitisation of certain grain boundaries after heat treatment (e.g. post-weld stress relief) or fast neutron irradiation in nuclear plant. Sensitisation is a decrease in the local resistance to stress corrosion, to a degree that depends on the grain boundary structure. Developments of predictive models for stress corrosion crack nucleation require more information about the effect of several external parameters (e.g. stress and time) on the likely extent of crack growth. Understanding is also required about how the grain boundary crystallography and the orientations of grain boundary plane and its surrounding grains affect crack propagation.
In this PhD thesis, the effects of time, applied stress and microstructure on populations of short crack nuclei have been investigated in sensitised type 304 austenitic stainless steel, tested under static load in an acidified potassium tetrathionate (K2S4O6) environment. Statistical evaluation, using the Gumbel extreme value distributions enables analysis of the growth rate of the population of short crack nuclei. This methodology has been developed, in order to quantitatively evaluate the influence of grain boundary control on crack development. These investigations showed an increase in the expected crack length with increasing time and grain size. Although the crack length tends to increase with stress, the effect is not strong. The grain boundary controlled microstructures exhibited significantly higher resistance to intergranular crack propagation. Direct observations of intergranular crack initiation and propagation, using digital image correlation (DIC) along with electron back scatter diffraction (EBSD), in various microstructures has been used to study the crack nucleation sites and crack interactions with grain boundaries of different characteristics. The effect of microstructural modification on crack growth kinetics has also been investigated. A significantly longer incubation period for crack initiation and lower crack growth rate were observed in thermo-mechanically treated microstructure.
New methods have been developed to assess the clustering characteristics of grain boundaries of particular properties. The network properties of boundaries classified by EBSD data have been compared with the network of corroded grain boundaries in electro-chemically tested samples. Image analyses (IA) was employed to evaluate the geometrical properties of susceptible boundaries clusters in a range of microstructures produced by sequential thermo-mechanical processing (TMP).
DL-EPR testing method of sensitisation assessment has been augmented by large area image analysis (IA) assessments of optical images to measure the dimensions and connectivity of the attacked grain boundary network. This approach determines the degree of sensitisation of the susceptible grain boundaries in the microstructure, and is used to explain IGSCC behaviour. A new method of degree of sensitisation determination is proposed, based on normalisation by a cluster parameter for the network of susceptible grain boundaries.