Papers by Author: T. Manzoor

Abstract: Fatigue crack growth near-threshold stress intensity factor is affected by the microstructure of the material. A large portion of microstructural influence is due to the change in grain size of the material. Grain size in the dual phase steel was varied and found that the near-threshold stress intensity factor (rK_th) increased as the grain size increased. Influence of load ratio nearthreshold fatigue crack propagation was also studied. It was observed that the near-threshold stress intensity range, rK_th for fatigue growth decreased with increasing load ratio.

Abstract: A low alloy steel containing 0.2%C was heat treated with three cycles of heat treatments with the aim to acquire different morphologies of martensite in dual phase microstructure. Microscopic examination revealed that the morphologies consisting of grain boundary growth, scattered laths and bulk form of martensite were obtained. These morphologies have their distinct patterns of distribution in the matrix (ferrite). In tensile properties observations the dual phase steel with bulk morphology of martensite showed minimum of ductility but high tensile strength as compared to other two morphologies. This may be due to poor alignments of bulk martensite particles along tensile axes during deformation. Tempering was employed with various holding times at 550°C to induce ductility in the heat treated material. The tempering progressively increased the ductility by increasing holding time. However, tempering response to strengths and ductilities was different to all three morphologies of martensite.

Abstract: Wedge open loaded (WOL) specimens of age hardened Zeron 100 duplex stainless steel were tested in 3.5 wt % NaCl solution with cathodic polarizes applied at-900mV/SCE to investigate stress corrosion cracking mechanism in duplex stainless steel. The interaction between microstructure and mechanism of stress corrosion cracking was studied. Fracture mechanism was studied by using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The material was found cracked by ferrite cleavage, austenite tearing and austenite dissolution by environment. The ferrite cleavage took place along {100} planes and {112} twin habit planes. The austenite grains appear to act as crack bridging & crack arrester and failed by tearing & stress corrosion cracking.

Abstract: The phenomenon of ultrafine-grain refinement of ferrite during transformational grain refinement (TGR) followed by static re-crystallization of vanadium micro-alloyed steels was studied. A substantial grain refinement (2.8m) was attained during TGR process by rolling at 900°C. Cold rolling with 70% of reduction introduced strain, utilized for re-crystallization during annealing at different temperatures. Electron Backscattered Diffraction (EBSD) technique was employed to quantify the low angle grain boundaries (LAGB) and high angle grain boundaries (HAGB) spacings and results were correlated with hardness drops during annealing process. At higher annealing times and temperatures the vanadium precipitates restricted the process of grain growth probably due to effective dispersion strengthenening. The abnormal grain growth during annealing, predicted previously for niobium steels, found absent in the present vanadium microalloyed steels.