Browsing by Author "Sherry, Andrew H."
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Item Analysis of intergranular carbide precipitate in haz of martensitic stainless steel(Taylor’s University, Malaysia, 2017) Ahmed, Ismaila Idowu; Adebisi, Jeleel Adekunle; Yahaya, Taiwo; Abdulkareem, Sulaiman; Sherry, Andrew H.Analysis of intragranular carbide precipitate in the Heat Affected Zone (HAZ) of Martensitic Stainless Steel (MSS) weldment was carried out. Low carbon grade martensitic stainless steel weldment subjected to four point bend test in simulated sweet crude environment was analysed with Transmission Electron Microscope (TEM). The optical microscopy of the failed sample revealed the presence of intergranular cracks on both sides of the weldment in the HAZ. Electron transparent sample for TEM was prepared from the HAZ of the weldment using extraction replica technique. The examination of TEM specimen in imaging mode revealed the presence of precipitates on grain boundaries. The compositional analysis of the precipitates was carried out with Energy Dispersive X-ray (EDX). The result of EDX analysis showed the presence of chromium and molybdenum, this suggests, the precipitates were carbides of the form M23C6. The study therefore upholds sensitisation as the mechanism behind the intergranular cracks observed in the HAZ of the MSS weldment.Item Assessment of Crack Propagation Mode in Martensitic Stainless Steel HAZ with Electron Back Scatter Diffraction: Effects of Environmental Variables(Universitatea Politehnica Timisoara (UPT), Romania, 2016-05) Ahmed, Ismaila Idowu; Adebisi, Jeleel Adekunle; Yahaya, Taiwo; Aremu, Ishaq Na'Allah; Sherry, Andrew H.In this study, an assessment of crack propagation mode in martensitic stainless steels heat affected zone was carried out with electron back scatter diffraction (EBSD) with focus on the effect of environmental variables. Girth welded samples were subjected to four point bend tests in sour and sweet environments. Scanning electron microscope and electron back scatter diffraction technique were both used to study the crack propagation mode and interaction of crack tip with microstructures. Study showed that sample tested in sour environment failed with transgranular cracking and samples tested in sweet environment failed with intergranular stress corrosion cracking mode. The proposed mechanism for the failure was sulphide stress corrosion cracking and intergranular precipitation of carbide by sensitisation process respectively. The correlation between crack path and grains misorientation was studied with aid of EBSD technique.Item Deformation path effects on the internal stress development in cold worked austenitic steel deformed in tension(Elsevier, 2014-07-05) Ahmed, Ismaila Idowu; Grant, Benedict; Sherry, Andrew H.; Quinta da Fonseca, JoaoThe effects of cold work level and strain paths on the flow stress of austenitic stainless steels, including Bauschinger effect and associated internal stresses were investigated with both mechanical testing and neutron diffraction techniques. The main objective was to assess the effects of cold rolling: to 5%, 10%, 20% and 40% reduction and uniaxial straining on the evolution of the internal strains during the re-straining to 5% tensile strain in-situ, which is relevant for stress corrosion cracking (SCC) studies. The results of mechanical testing showed that the yield strength of material increased when it was reloaded in the forward direction and decreased well below the flow stress when the loading direction was reversed, showing a strong Bauschinger effect. The magnitude of Bauschinger effect is independent on whether tensile or compressive prestraining comes first but rather on the amount of prestrain. The assessment of the effect of prestraining methods showed that the magnitude of yield asymmetry was higher in the material prestrained by uniaxial deformation than those prestrained by cold rolling. Neutron diffraction test results showed that the elastic lattice strain difference between the maximum and minimum strain values increased consistently with the applied stress during the re-straining to 5% tensile strain in-situ along the 3 orthogonal directions of the rolled plate. It also emerged that, following the in-situ loading of cold rolled materials to 5% tensile strain, the largest strain difference occurred in the material prestrained to 20% reduction. In cold rolled samples, the peak width increased with cold work levels and during re-straining to 5% along rolling, transverse to rolling and normal directions which simulated reversed condition. In contrast to the cold rolled samples, there was neither increase nor decrease in the peak width of samples prestrained by uniaxial deformation on re-straining in reverse direction. This was rationalised in terms of the development of intragranular and intergranular stresses in the two cases and implications to SCC susceptibility were discussed.Item Effects of martensite development on lattice strain evolution during the in situ deformation of austenitic stainless steels at cryogenic temperatures(SAGE, 2013-03-15) Ahmed, Ismaila Idowu; Quinta da Fonseca, J; Sherry, Andrew H.The effects of martensite development on lattice strain evolution during the in situ compression of austenitic stainless steels, Type 304L, at ambient and cryogenic temperatures were investigated. Engin-X neutron diffractometer was used to study the change in the lattice strain during the uniaxial compression to 10% strain in situ. The report found no evidence of martensite during the compression test at ambient temperature. Similarly, no martensite was observed at cryogenic temperatures up to the applied load of 200MPa in the elastic region. Martensite was only observed during the plastic deformation at cryogenic temperatures up to the applied strain of 10%. The presence of martensite in the austenitic stainless steels microstructure reinforces the austenite matrix. This was evident from the decrease in the lattice strain as the deformation temperature decreases.Item The Influence of Composition and Plastic Strain on the Electrochemistry of Stainless Steels Doped with Platinum Group Metals(Materials Performance Centre, University of Manchester, 2009-03) Govender, Kuveshni; Necib, O.; Ahmed, Ismaila Idowu; Lyon, Stuart B.; Sherry, Andrew H.Corrosion-related degradation of stainless steel components of Naval Nuclear Power Plant (NNPP) can have significant impact on the cost of nuclear ownership through inspection requirements, the reliability and, in some cases, the availability of plant. This project aims to establish an improved physical understanding of the mechanisms of Stress Corrosion Cracking (SCC) of 304 stainless steel of the type used for a range of NNPP components. In order to accomplish this aim, the research is studying the influence of platinum group metal (PGM) alloying additions (i.e. < 1 wt.%, additions palladium and/or ruthenium) on the SCC of 304 stainless steel as these have predictable effects on the electrochemical processes of corrosion. The approach includes mechanical and electrochemical testing together with pre- and post-test metallurgical examination (over an appropriate range of length scales), and finally predictive model development. This Report presents experimental data relating to the influence of PGM additions and plastic-strain (cold work) on the electrochemistry of 304L stainless steel in a range of environments including simulated PWR water. The electrochemical data reveal ruthenium to be more effective than palladium in enhancing the corrosion resistance of 304 stainless steel in PWR water chemistry and this was traced to removal of palladium from the alloy matrix by precipitation of a PdMn Intermetallic 2nd phase precipitate (reported previously). For all alloy variants, increasing temperature shifted the Open Circuit Potential (OCP) to more noble values and increased the anodic current density; observations consistent with enhanced to both metal dissolution and oxygen diffusion rates at higher temperatures. Increasing the oxygen concentration increased the OCP but decreased the passive current density; observations rationalised in terms of the efficiency of the cathodic reaction in which PGM additions enhance the exchange current density for oxygen reduction leading to faster passivation kinetics. In hydrogenated water, the PGM modified alloys displayed a preferred lower OCP and a higher anodic current density than the standard alloy and this is because PGM additions increase the exchange current density for hydrogen oxidation. In deaerated pressurised water at 260oC, the ruthenium-doped alloy displayed an improved corrosion resistance compared with standard 304 stainless steel whilst the palladium-doped alloy displayed a higher dissolution current density. Studies in acidified aerated potassium tetrathionate revealed an abrupt transition of the OCP to more positive potentials the effect being more rapid for the ruthenium-doped alloy. Analysis of the anodic polarisation curves suggests that PGM additions enhance the corrosion resistance, the effect being most pronounced for ruthenium. Regarding the effect of plastic strain (cold work), the influence of strain path on the evolution of internal strain in cold-worked austenitic stainless steel was investigated using neutron diffraction and mechanical testing. As expected, hardness and yield strength increase with the level of cold work, however the magnitude of the increase is dependent upon the strain path due to the mixed isotropic/kinematic hardening response. Neutron diffraction demonstrated that residual strain in cold-worked material increases with the level of cold work. The elastic anisotropy and the lattice strain difference increased consistently with the applied stress during in-situ re-straining tests. In considering all the cold work material reloaded in-situ to 5 % in tension along the 3 principal directions, the largest strain was found to occur in the material with the pre-strain level of 20 % reduction. Electrochemical data indicate that the sample orientation (and, consequently the alloy texture) has a greater effect than the absolute level of cold work. Thus, the L-T plane displayed more noble electrochemical behaviour than the L-S plane, and the T-S plane was found to be least noble. Generally, material cold rolled to 20% reduction showed the least noble electrochemical behaviour of all the cold rolled materials on all three orthogonal planes.Item Investigation of surface residual stress profile on martensitic stainless steel weldment with X-ray diffraction(Elsevier, 2016-01-19) Ahmed, Ismaila Idowu; Adebisi, Jeleel Adekunle; Abdulkareem, Sulaiman; Sherry, Andrew H.The development of residual stresses during fabrication is inevitable and often neglected with dire consequences during the service life of the fabricated components. In this work, the surface residual stress profile following the martensitic stainless steel (MSS) pipe welding was investigated with X-ray diffraction technique. The results revealed the presence of residual stresses equilibrated across the weldment zones. Tensile residual stress observed in weld metal was balanced by compressive residual stresses in the parent material on the opposing sides of weld metal.Item Metallographic Characterisation of Cold Worked Type 304 Austenitic Stainless Steels(Materials Performance Centre, University of Manchester, 2007-09) Ahmed, Ismaila Idowu; Scenini, Fabio; Sherry, Andrew H.The influence of cold work on the Stress Corrosion Cracking (SCC) susceptibility of Austenitic Stainless Steels (ASS) is an ongoing topic of research. While some workers have suggested that low levels of cold work are deleterious to SCC, others argue that high levels may be favourable to SCC under certain conditions. The objective of this report is to quantify a number of key parameters associated with the influence of cold work on SCC susceptibility. The microstructures of ASS subjected to different deformation modes and different levels of cold work have been examined using a range of techniques: optical metallography was carried out to determine the grain size and elongation ratio; the X-ray diffraction technique was used to quantify the amount of strain- induced martensite and residual stress; and lastly, deformation texture was examined using Electron Back Scatter Diffraction (EBSD) technique. In addition, Vickers hardness was used to quantify the material hardness in each condition of cold work. The outcome of the characterisation showed that the Vickers hardness and the volume fraction of strain induced martensite increases with both increasing levels of cold work and weight percent of carbon in the material composition. The stability of delta ferrite was also affected by the proportion of ferrite-stabilising elements (e.g. Si) in the composition. Bi-rolling (i.e. cross- rolling) significantly increased the hardness value and the amount of strain induced martensite compared with samples in which cold work was generated either by uniaxial rolling and pulling (i.e. tensile cycling). Cross-rolling also led to a more uniform distribution of delta-ferrite within the microstructure than either unidirectional rolling or pulling.Item Microstructural Correlation of Hardness Profile in Martensitic Stainless Steel Weldment(Springer, 2016-02-10) Ahmed, Ismaila Idowu; Adebisi, Jeleel Adekunle; Yahaya, Taiwo; Abdulkareem, Sulaiman; Sherry, Andrew H.The correlation between microstructures and hardness profiles in low carbon martensitic stainless steel girth weldment were investigated. Optical microscopy and a scanning electron microscope equipped with electron back scatter diffraction system were used for assessment of microstructural phases, grain sizes, and grain misorientation across the weldment. The hardness value fluctuated between the peak and lowest values in the heat-affected zone of the weldment. The hardness profile observed is consistent with microstructural evolution across the weldment. The low hardness value observed in the weld metal was attributed to the large proportion of ferrite introduced by the superduplex filler metal used for the fabrication and grain coarsening caused by prolonged cooling of the weld metal. The electron backscatter diffraction data showed that higher grain sizes occurred in the weld metal and this is consistent with the highest degree of misorientation and recrystallisation observed in the weld metal than parent and HAZ region of weldment.