Dirk Engelberg

Observations of intergranular stress corrosion cracking in a grain-mapped polycrystal.

Nondestructive three-dimensional mapping of grain shape, crystallographic orientation, and grain boundary geometry by diffraction contrast tomography (DCT) provides opportunities for the study of the interaction between intergranular stress corrosion cracking and microstructure. A stress corrosion crack was grown through a volume of sensitized austenitic stainless steel mapped with DCT and observed in situ by synchrotron tomography. Several sensitization-resistant crack-bridging boundaries were identified, and although they have special geometric properties, they are not the twin variant boundaries usually maximized during grain boundary engineering.

X-ray microtomographic observation of intergranular stress corrosion cracking in sensitised austenitic stainless steel

Abstract Intergranular stress corrosion cracking in a sensitised type 302 stainless steel wire has been observed in situ using high resolution X-ray microtomography. Tomography enables the development and failure of crack bridging ligaments to be studied in detail in three dimensions. Direct comparison of these features has been made with scanning electron microscopy fractography. The crack bridges failed in a ductile manner, with a morphology that is consistent with non-sensitised low energy grain boundaries.

In situ observation of intergranular crack nucleation in a grain boundary controlled austenitic stainless steel

Grain boundary engineering has been proposed to increase the lifetime performance of sensitized austenitic stainless steel in aggressive environments. Increased microstructure resistance is typically associated with higher fractions of twin (Σ3) grain boundaries, but there is uncertainty about the properties and role of other boundaries. To develop predictive models for stress corrosion crack nucleation, more information is required about how grain boundary crystallography and the orientations of the grain boundary plane and its surrounding grains affect crack development. Digital image correlation combined with electron backscatter diffraction has been used to characterize the microstructure and to observe, in situ, the nucleation and propagation of short stress corrosion cracks in thermo‐mechanically processed type 304 stainless steel. The crack path and its growth rate have been determined and are found to be influenced by the microstructure.

The influence of low-strain thermo-mechanical processing on grain boundary network characteristics in type 304 austenitic stainless steel

Grain boundary engineering of austenitic stainless steel, through the introduction of plastic strain and thermal annealing, can be used to develop microstructures with improved resistance to inter‐granular degradation. The influence of low‐strain thermo‐mechanical processing on grain boundary network development, with systematic variations of annealing treatments, has been investigated. Three stages of the microstructure development during grain boundary engineering in low‐strain processing conditions are identified, and correlated with changes in grain boundary character and deviation distributions. Low‐energy connected length segments at triple junctions, which have been proposed to be responsible for crack bridging during inter‐granular stress corrosion cracking, can be influenced by the choice of the annealing treatment parameters. The development of individual grain boundary length segments of different character showed consistent trends with increasing grain size. Crack length predictions are consistent with the beneficial effect of designing microstructures with high fractions of twin grain boundaries and smaller grain size.

An experimental investigation into strain and stress partitioning of duplex stainless steel using digital image correlation, X-ray diffraction and scanning Kelvin probe force microscopy

The evolution of microstructure strain partitioning during quasi in situ tensile loading of grade 2205 duplex stainless steel has been investigated. Digital image correlation revealed the development of tensile strain initially in austenite and at interphase boundaries, and further extending into the ferrite with increasing load. Higher resolution digital image correlation observations indicated strain hardening of austenite, followed by deformation of the ferrite. Digital image correlation analysis of a 20% cold-rolled microstructure revealed tensile strain development at interphases, with discrete tensile and compressive strain pockets observed within the austenite. X-ray diffraction measurements indicated the presence of tensile stresses primarily developing in the ferrite, with full-width at half maximum values indicating plastic strain accumulation primarily in the austenite. The effect of tensile loading on Volta potential differences, obtained via scanning Kelvin probe force microscopy, highlighted the development of discrete anodic and cathodic sites with the introduction of strain. A Volta potential roughness parameter (ΨRa) is introduced describing Volta potential changes as a function of strain. This observation supports the concept of an enhanced propensity of local electrochemical activity with increasing applied strain in duplex stainless steel.

High-resolution, in-situ, tomographic Observation of Stress Corrosion Cracking

This paper presents high resolution X-ray tomographic observations of intergranular stress corrosion crack nucleation and growth. In-situ experiments have been performed on beam line ID19 at the European Synchrotron Radiation Facility (ESRF). High-resolution tomography provides non-destructive, three-dimensional information of the shape and depth of damage. To the authors? knowledge, these are the first such observations of stress corrosion, and demonstrate the potential for high resolution, synchrotron, X-ray tomography as a tool for observing pitting, intergranular corrosion and intergranular cracking.The application of high-resolution synchrotron X-ray tomography to two in-situ experiments is described. Localised corrosion and intergranular cracking in sensitised 5083 aluminium alloy has been studied. These results show the progressive development, transition, and coalescence of two forms of damage within the bulk of the sample. In-situ observations of intergranular cracks in sensitised 302 stainless steel have also been obtained. These provide evidence for crack bridging ligaments, caused by the high resistance of special grain boundaries. Further applications of high resolution X-ray tomography are described, such as in-situ studies of pitting and the transition from pitting to cracking in aluminium alloys and stainless steels, including the effects of near-surface residual stress.

Five-parameter grain boundary analysis of a grain boundary-engineered austenitic stainless steel

Two different grain boundary engineering processing routes for type 304 austenitic stainless steel have been compared. The processing routes involve the application of a small level of strain (5%) through either cold rolling or uni‐axial tensile straining followed by high‐temperature annealing. Electron backscatter diffraction and orientation mapping have been used to measure the proportions of Σ3n boundary types (in coincidence site lattice notation) and degree of random boundary break‐up, in order to gain a measure of the success of the two types of grain boundary engineering treatments. The distribution of grain boundary plane crystallography has also been measured and analyzed in detail using the five‐parameter stereological method. There were significant differences between the grain boundary population profiles depending on the type of deformation applied.

Assessing the risk of under-deposit chloride-induced stress corrosion cracking in austenitic stainless steel nuclear waste containers

Abstract This paper provides a short summary of investigations that have focused on the occurrence of the phenomenon that is commonly referred to as atmospheric chloride-induced stress corrosion cracking (AISCC) in types 304L and 316L austenitic stainless steels; the materials from which storage containers for the UK’s intermediate level nuclear waste are currently fabricated. A brief overview of the relationship between localised corrosion phenomena (pitting or crevice corrosion) and stress corrosion cracking from observations derived under conditions of immersion in chloride-containing environments is provided. The impact of environmental variables such as the relative humidity, temperature and chloride deposition density on the development of AISCC and potential difficulties in extrapolating data derived from laboratory investigations to an in-store situation are also discussed. Proposals for the development of laboratory protocols that may more accurately reflect behaviour in an actual storage situation, including the possibility of employing accelerated testing methods, are also briefly addressed.

Low-Temperature Environmentally Assisted Cracking of Grade 2205 Duplex Stainless Steel Beneath a MgCl2:FeCl3 Salt Droplet

The corrosion and environmentally assisted cracking susceptibility of grade 2205 duplex stainless steel beneath a FeCl3:MgCl2-containing salt deposit has been investigated. Long-term exposure to atmospheric environment at 50°C and 30% relative humidity resulted in different forms of corrosion and the formation of cracks depending on the location under the salt-laden droplet. Selective dissolution with closely-spaced microcracks of the ferrite suggested hydrogen embrittlement toward the center of the droplet, with chloride-induced stress corrosion cracking and selective dissolution of the austenite observed toward the rim of the deposit. Cracks in the ferrite had cleavage-like appearances, typically forming within existing cracks, whereas the austenite had branched cracks, initiating from crevices and pits. These observations are discussed in light of expected electrochemical potential variations beneath the droplet.

Probing propensity of grade 2205 duplex stainless steel towards atmospheric chloride- induced stress corrosion cracking

Abstract The propensity of grade 2205 duplex stainless steel towards atmospheric chloride-induced stress corrosion cracking at 50°C has been investigated. Electron backscatter diffraction has been used to characterise as received and 750°C heat-treated microstructures. Screening tests in chloride-containing aqueous environments were employed to investigate the corrosion behaviour of both microstructures. These tests indicated significantly increased corrosion rates when exposed to HCl or FeCl3-containing environments. Stress corrosion cracking tests with atmospheric exposures for up to 12 months showed selective dissolution of the ferrite, accompanied by stress corrosion microcracks in the austenite. This work demonstrates that grade 2205 duplex stainless steel microstructure may be rendered susceptible to stress corrosion cracking under atmospheric exposure conditions at 50°C.