Sandip Ghosh Chowdhury

Turbine blade failure in a thermal power plant

The failure of a LP (low pressure) turbine blade of a 220 MW thermal power plant is presented. The blade was made of martensitic stainless steel and the structure was tempered martensite. There was no evidence of degradation of blade material. The fracture took place at the aerofoil region, 113-mm from the root. Throughout the blade surface Si rich phases were detected. Several pits/grooves were found on the edges of the blades and chloride was detected in these pits. These were responsible for the crevice type corrosion. The probable carriers of Cl− were Ca and K, which were found on the blade. The failure mode was intergranular type. Possibly the ultimate failure was due to corrosion-fatigue.

Failure of a super heater tube

The failure of two adjacent platen super heater tubes of a thermal power plant has been analysed. One tube fractured with a fish mouth opening and another thinned down considerably. A significant amount of oxide layer was observed in the inner side of the failed tube. In the other tube the inner oxide layer was very small and the tube was thinned down due to fireside corrosion. The unexposed tube from the same lot revealed a ferritic bainitic microstructure. The failed tube also showed a ferrite bainite microstructure but most of the bainitic carbide had transformed to globular form. Considerable carbide precipitation both in grain boundary (GB) and inside grains were observed. A large number of cavities and micro-cracks were observed along the grain boundaries. A similar microstructure was observed on the same tube 10 cm away from the failed region. The micrograph at the fracture location showed a similar microstructure but the grains are very much elongated. The formation of globular oxide on the other tube was compared to that of the failed tube. Due to breakage of oxide scale of the failed tube, the metal surface of the failed tube was exposed to steam at a higher temperature and this accelerated the oxide growth. This led to creep damage of the material. Ultimately, failure occurred due to rapid overheating of the tube material.

Premature fatigue failure of a spring due to quench cracks

The premature fatigue failure of a compression spring during service has been analysed. Optical and scanning electron microscopy, hardness and tensile testing and X-ray diffraction techniques were employed. Quench cracks, presumably due to improper quenching, were observed across the cross-section of the spring. It is established theoretically that the diameter of the spring rod was smaller than the critical diameter for this composition of steel. X-ray diffraction showed the presence of a small amount of retained austenite (average grain size of 2 μm).

Twinning-induced sluggish evolution of texture during recrystallization in AISI 316L stainless steel after cold rolling

This paper deals with the evolution of texture in AISI 316L austenitic stainless steel during annealing after 95 pct cold rolling. After 95 pct cold rolling, the texture is mainly of the brass type {110}〈112〉, along with a scatter toward the S orientation {123}〈634〉 and Goss orientation {011}〈100〉. Weak evidence of Cu component is observed at this high deformation level. During annealing, recovery is observed before any detectable recrystallization. Recrystallization proceeds through nucleation of subgrain by twinning within the deformed matrix and, later, preferential growth of those to consume the deformed matrix. After recrystallization, the overall texture intensity was weak; however, there are some discernible texture components. There was no existence of the brass component at this stage. Major components are centered on Goss orientation and Cu component {112}〈111〉 as well as the BR component {236}〈385〉. Also, a few orientations come up after recrystallization (i.e., {142}〈2−11〉 and {012}〈221〉). With increase in annealing temperature, the textural evolution shows emergence of weak texture with another new component, {197}〈211〉. The evolution of texture was correlated with the deformation texture through twin chain reaction.

Texture evolution during recrystallization in a boron-doped Ni76Al24 alloy

The evolution of recrystallization texture in the intermetallic compound Ni76Al24(B) has been investigated using the conventional pole figure as well as orientation distribution function (ODF) methods. The initial L12 structure which transforms to DO22 on cold rolling reverts back to the L12 during annealing. The annealing process can be divided into three stages: recovery, reordering and recrystallization. The moderately strong deformation texture resulting from cold rolling becomes very weak during the recovery and reordering processes by an ‘in-situ’ orientation change due to reordering that precedes recrystallization. The texture remains weak throughout recrystallization and grain growth stages. TEM investigations show evidence of twin like features during the reverse DO22→L12 transformation. This is expected to lead to fragmentation of the matrix grains resulting in a weak texture. The prominent recrystallization texture components are {025} and {011} , of which the latter component survives best with continued annealing.

Failure analysis of high temperature studs

Studs in the interceptor valve of a 110 MW unit failed after a service life of 148,700 h. The studs were operated under a steam pressure of 35 kg/cm2 and a temperature of 535°C. The studs were fractured at one end of the threaded end. Various techniques were employed to analyse the failure of the studs. It has been concluded that the failure of the studs was due to reverse temper embrittlement. The failure was delayed due to the presence of Mo and V. To reduce the tendency to this kind of failure, the following steps were recommended: (a) reduce the phosphorus content in the steel to a low level or (b) reduce the grain size to about 10 μm.

Rolling texture in the intermetallic compound Ni76Al24(B)

The development of cold rolling texture in the intermetallic compound Ni3Al(B) has been investigated as a function of rolling reduction, using the conventional pole figure as well as the orientation distribution functions (ODF) method. The textures at low degrees of deformation have been found to be similar to the rolling texture of pure Ni cold rolled to similar levels. With increasing degree of deformation, the texture in Ni3Al(B) changes to alloy type at reductions greater than 45%. This texture is characterized by the presence of a remarkably strong rotated Goss component over and above the Brass (Bs) component. Transmission electron microscopy of thin foils of the material cold rolled by 65% and more shows evidence of twinning. X-ray diffraction analysis indicates a structural change in the material from L12 to DO22 with the progress of cold work. An attempt has been made to explain the texture development in terms of this structural transformation.

Dislocation densities and prevailing slip-system types determined by X-ray line profile analysis in a textured AZ31 magnesium alloy deformed at different temperatures

Tension experiments were carried out at room temperature, 473 K and 673 K on AZ31-type extruded magnesium alloy samples. The tensile deformation has almost no effect on the typical extrusion texture at any of the investigated temperatures. X-ray diffraction patterns provided by a high-angular-resolution diffractometer were analyzed for the dislocation density and slip activity after deformation to fracture. The diffraction peaks were sorted into two groups corresponding either to the major or to the random texture components in the specimen. The two groups of reflections were evaluated simultaneously as if the two texture components were two different phases. The dislocation densities in the major texture components are found to be always larger than those in the randomly oriented grain populations. The overwhelming fraction of dislocations prevailing in the samples is found to be of type, with a smaller fraction of -type dislocations. The fraction of -type dislocations is always obtained to be zero within experimental error.

Evolution of grain-boundary character distribution during iterative processing of an austenitic stainless steel

The present article deals with the analysis of grain-boundary character distribution (GBCD) and microstructural characteristics after iterative processing of austenitic stainless steel, AISI 316L. The steel was subjected to iterative cold reduction and subsequent annealings. After an initial decrease in the fraction of Σ3 boundaries, the number of these increases in subsequent steps. The results relate the importance of iterative processing and the mechanism of obtaining a higher fraction of Σ3 boundaries.

Stress, Texture and Phase Transformation in Titanium Thin Films

{111} fiber textured face centered cubic (fcc) titanium has been found to coexist with the {0002} fiber textured hexagonal close packed (hcp) titanium in polycrystalline titanium (Ti) thin films (thickness: 144 nm to 720 nm) deposited on Si (100) substrate by magnetron sputtering. X-ray diffraction investigation confirms that relative phase fraction of such metastable fcc Ti phase decreases with increasing film thickness indicating thickness dependent fcc-hcp phase transformation of titanium. Texture development in hcp Ti phase was due to film microstructure (thickness effect) rather than the phase trans-formation. Diffraction stress analysis (by d-sin2 method) indicates that fcc to hcp phase transformation is also accompanied by the reduction of compressive stress in the hcp Ti phase with increasing film thickness. Strain energy calculations for both phases of titanium indicate that fcc Ti is a more stable phase compared to hcp Ti at relatively low film thickness (144 nm to 432 nm). It has been concluded that film stress favours fcc to hcp phase transformation towards the higher film thickness. Reverse transformation (hcp to fcc) occurs towards the lower film thickness.