Volume 56, Issue 6

This work demonstrates the efficiency of microstructural tuning by external loading during the martensitic phase transformation of a 12Cr-9Ni-2Mo-0.7Al-0.35Ti maraging steel. Additional prior austenite grain size refinement is achieved by thermal cycling yielding industrially relevant grain sizes, for which not the full set of all 24 characteristic Kurdjumov-Sachs orientation relation variants of martensite is obtained within each grain. For the last thermal cycle three different cases are investigated: i) one specimen is annealed at a higher temperature before the last cooling, ii) one specimen is loaded with a tensile stress of 120 MPa and iii) one specimen with an equivalent 120 MPa Mises stress in torsion during the transformation. This study is one of the first combining the effects of a small grain size, a high dislocation density and external loadings. Orientation imaging microscopy data is presented, revealing the effect of loading type during transformation on the final texture and interface/boundary character.

The effects of different sample preparation techniques on a set of laboratory hot-rolled and thermally cycled ferritic-austenitic stainless steels containing 19 – 20 wt.% Cr have been studied. The differences in chemical composition led to different volume fractions of ferrite and austenite and austenite stabilities, which were characterized using composition-based estimates of M d30 and M s temperatures. The sample preparation methods were combined mechanical and chemical polishing, electrolytical polishing and ion beam cross section polishing. The ferrite-austenite phase ratio and tendency for phase transformation varied with the chemical composition of the steel. Electropolishing is shown to be the only sample preparation method not to cause martensite formation in samples containing metastable austenite.

The aim of this work was to investigate the deformation mechanism in a highly alloyed austenitic Mn-steel, because these steels can deform by twinning and/or slip. Deformation by localized twinning or slip leads to formation of deformation structures on the surface, which are either bundles of deformation twins or slip bands. Such deformation structures around a standard Vickers hardness indent were investigated by means of scanning electron microscopy and electron backscatter diffraction to study the active deformation mechanism. Energy dispersive x-ray spectroscopy was conducted in order to study influences of segregation on the deformation. The main result of the investigations is that the deformation structures are formed by dislocation slip in the (101)-orientation and that an influence of segregations in the alloy composition on the deformation mechanism was not observed.

A first stage turbine vane of a heavy-duty gas turbine engine, exhibiting an outward bulge of approximately 0.6 mm and a visible crack in the top coating, was delivered by the client for a detailed destructive failure investigation. The subject component was an experimental part used in a test field engine. The suction side wall of the casting was cut off to reveal a gaping crack on the inside of the aft cavity that was also visible by borescopic inspection. The cooling insert of the aft cavity was clean and did not have any blocked cooling holes. The cooling pattern on the inside wall of the aft cavity was unremarkable. No evidence whatever of impact damage was found on the suction side surface of the subject vane in the cracked region. The hypothesis as to the most probable root cause of the cracking that is still standing is a combination of low wall thickness, non-optimal process parameters for bond coat pre-heat, and potentially high thermal/mechanical loading during engine operation. It appears this is not a wide-spread phenomenon but rather an isolated occurrence.