The crystallography of interfaces in a duplex stainless steel having an equiaxed microstructure produced through the ferrite to austenite diffusive phase transformation has been studied. The five-parameter interface character distribution revealed a high anisotropy in habit planes for the austenite-ferrite and austenite-austenite interfaces for different lattice misorientations. The austenite and ferrite habit planes largely terminated on (1 1 1) and (1 1 0) planes, respectively, for the austenite-ferrite interfaces associated with Kurdjumov-Sachs (K-S) and Nishiyama-Wasserman (N-W) orientation relationships. This was mostly attributed to the crystallographic preference associated with the phase transformation. For the austenite-ferrite interfaceswith orientation relationships which are neither K-S nor N-W, both austenite and ferrite habit planes had (1 1 1) orientations. sigma 3 twin boundaries comprised the majority of austenite-austenite interfaces, mostly showing a pure twist character and terminating on (1 1 1) planes due to the minimum energy configuration. The second highest populated austenite-austenite boundary was sigma 9, which tended to have grain boundary planes in the tilt zone due to the geometrical constraints. Furthermore, the intervariant crystallographic plane distribution associated with the K-S orientation relationship displayed a general tendency for the austenite habit planes to terminate with the (1 1 1) orientation, mainly due to the crystallographic preference associated with the phase transformation.

Characterizing complex surface topologies is necessary to understand stress concentrations created by rough surfaces, particularly those made via laser power-bed additive manufacturing (AM). Synchrotron-based X-ray microtomography (\(\upmu \hbox {XCT}\)) of AM surfaces was shown to provide high resolution detail of surface features and near-surface porosity. Using the CT reconstructions to instantiate a micromechanical model indicated that surface notches and near-surface porosity both act as stress concentrators, while adhered powder carried little to no load. Differences in powder size distribution had no direct effect on the relevant surface features, nor on stress concentrations. Conventional measurements of surface roughness, which are highly influenced by adhered powder, are therefore unlikely to contain the information relevant to damage accumulation and crack initiation.

The effect of the ferrite to austenite phase transformation route on the microstructure and interface plane character distributions was studied in a duplex stainless steel. Two markedly different austenite morphologies (i.e., equiaxed and Widmanstätten) were produced through diffusional (slow cooling) and semi-shear (air-cooling) transformations, respectively. Both austenite morphologies had textures similar to the as-received condition, which was attributed to a “texture memory” effect. The air-cooled microstructure displayed a significantly higher content of Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) interfaces (39%) compared with the slow-cooled one (16%), due to the change in the austenite nucleation and growth mechanism during the phase transformation. A five-parameter analysis of different interfaces revealed that for K-S/N-W orientation relationships, ferrite and austenite terminated on (110) and (111) planes, respectively, regardless of the transformation route. The population of these planes, however, increased as the transformation rate increased. A higher fraction of Σ3 boundaries was observed in the equiaxed austenite morphology compared with its Widmanstätten counterpart, which was mainly attributed to the different kinetics and the growth mode of austenite plates during the phase transformation. Σ9 boundaries were mostly formed where two Σ3 boundaries met and were largely of tilt character because of geometric constraints. The intervariant boundary plane distributions of both austenite microstructures displayed more frequent {111} orientations than other planes for a majority of the boundaries. This trend was markedly stronger for Widmanstätten austenite.

A new approach is presented for calculating the parent orientation from sets of variants of orientations produced by phase transformation. The parent austenite orientation is determined using the orientations of bainite variants that transformed from a single parent austenite grain. In this approach, the five known orientation relationships are used to back transform each observed bainite variant to all their potential face-centered-cubic (f.c.c.) parent orientations. A set of potential f.c.c. orientations has one representative from each bainite variant, and each set is assembled on the basis of minimum mutual misorientation. The set of back-transformed orientations with the minimum summation of mutual misorientation angle (SMMA) is selected as the most probable parent (austenite) orientation. The availability of multiple sets permits a confidence index to be calculated from the best and next best fits to a parent orientation. The results show good agreement between the measured parent austenite orientation and the calculated parent orientation having minimum SMMA.

Radiant tubes of an ethane furnace at a petrochemical plant fabricated from an austenitic heat resistant steel casting (HP grade) failed along longitudinal direction after a fraction of anticipated service life. To study the cause of failure, microstructures of as-received and used tubes were investigated by optical and scanning electron microscopy and the microchemical composition of tubes and precipitated carbide were determined by energy dispersive X-ray (EDS). Also, the morphology of deposited coke particles was determined by SEM/EDS. Finally to measure the extent of carbon penetration, hardness testing was performed on the inner and outer surface of tube. The experimental results show that the improper coking and decoking cycles remove the protective oxide layer (Cr2O3) that forms on the exposed surfaces and that, with this layer removed, the coke could easily deposit on inner, non-protected surface. The carbon diffusion into the metal was accelerated with deposited coke and caused microstructural degradation and drastically reduced the ductility of material at high temperatures.