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The variation of tensile properties and grain sizes, on adding silicon, up to 1.03 wt% in five grades of low carbon structural steel was studied. The reduced sensitivity of the lower yield stress, [σy], at sub-zero test temperatures, otherwise called "softening", of these steels with reduced grain sizes, brought about by the silicon additions was also investigated. Optical and transmission electron microscopes were used to characterize the ferrite-pearlite structures, decarburized layers, random interlamellar spacings in the pearlite constituent, slip line, twin and dislocation structures. The precipitates observed were studied qualitatively with the transmission electron microscope, and microanalysis was carried out by using the dispersed energy of the X-rays (EDAX). Dilatometric studies were done to establish the critical temperatures of the steel grades. Uniaxial tensile tests were carried out at between 77 and 350 K, with strain rates of 1.7x10⁻⁴, 3.3x10⁻⁴s and 0.33 s⁻¹. The data from these tests were analyzed in terms of the contribution silicon and nitrogen make to the tensile properties, and were used to determine the thermal activation parameters, hence "softening". It was found that silicon additions beyond 0.31 wt% in these steels inhibit grain growth, due to the effect of silicon on the grain nucleation kinetics. In conformity to previous reports, the Hall-Patch slope, Ky was reduced to a limit, on the initial additions of silicon, but further to this observation, it was found that low silicon steels, below 14 μm possess higher strength than high silicon steels, within 0.31 to 0.78 wt% Si. In contrast with previous reports, in which SiN precipitates were identified in similar. steel grades, under aged conditions, low temperature (∞) Si₃N₄ precipitates were observed in both the annealed and the aged samples of the steels in the present study. It was also found that reduced grain sizes increase the "softening" tendency of these steels. These results we correlated with those from limited reports found in the literature, regarding the effect of silicon (above 0.7 wt%) on the impact behaviour of these grades of steel. From the correlation, it is suggested that a new theory should be sought to explain how the initial additions of silicon, with reduced grain sizes, improve the impact behaviour. Hypotheses are advanced linking the improvement of impact behaviour with the "softening" phenomenon. It is also suggested that a higher silicon to manganese ratio, with silicon not exceeding 1 wt%, may improve not only the strength and the impact behaviour, but also-the cost indices of these grades of steel.

The role of composition, heat treatment and microstructural variables on the hardness, flow stress and fracture toughness has been investigated for a range of C-Mn steels. The study is divided into two parts. In Part 1, the influence of various ferrite-pearlite microstructures on the hardness and flow stress has been examined with respect to a Hall-Petch analysis. In contrast with previous reports it was found that a Hall-Petch equation can be applied satisfactorily in describing these properties provided care is taken in obtaining the appropriate mean slip distance for a given microstructure. The mean random interlamellar spacing was found to best quantify the microstructure when account is taken of the ferrite volume fraction, ferrite grain size, prior-austenite grain size and calculated cementite thickness. These parameters have been combined in a simple law of mixtures model to evaluate the mean slip distance in ferrite for a range of pearlite volume fractions between 20 and 100%. By substituting the mean slip distance for the effective ferrite grain size in a Hall-Petch equation both a positive friction stress and a very good correlation was obtained with measured hardness and flow stress data. In Part 2, the effect of changes in microstructure on the toughness of high carbon pearlitic steels was studied using standard Charpy, instrumented-impact and plane strain fracture toughness tests. Controversy in the literature regarding the influence of pearlite colony and prior-austenite grain boundaries in obstructing cleavage crack propagation has been resolved by close examination of the microstructure and fracture surface. The pearlite nodule size was found to be directly related to the cleavage facet size when proeutectoid ferrite is considered in hypo-eutectoid steels. Refining the pearlite nodule size by low austenitising temperatures and accelerated cooling gives improved toughness. Although the pearlite nodule size was shown to primarily determine the ductile-brittle transition temperature, it is suggested that the pearlite spacing may be of more importance as regards fracture toughness.

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The overall aim of the present investigation is to adopt a fundamental approach towards the identification, measurement and modelling of the effect of pre- and post-processing upon shot-peening residual stresses. At present this is carried out in an ad hoc fashion with little understanding of the fundamental properties which are being affected by these treatments and of how these basic properties influence product performance and determine (the peening) process parameters. Indeed, current concern of reliability and efficiency of machinery alone justifies the widespread interest in the present investigation. The specific objectives arising from the above overall aim can be summarised as follows: (i) to identify, measure and model the effect of pre-processing upon the locked-in residual stress field and surface morphology of a prospective peening component, (ii) to identify, measure and synthesize the effect of shot-peening treatment upon the resulting stress field and surface profile of the treated component, (iii) to identify, measure and model the effect of post-processing upon the aforementioned parameters, and (iv) to examine the effect of residual stresses and incomplete-coverage upon ultimate product performance. Consideration of the surface profile will be accounted for in the analysis. The outcome of these investigations should provide the foundation necessary for process control and optimisation of working surfaces of components currently employed in aerospace, automotive and power generation industries. Accordingly a number of investigations will be carried out, in parallel, to highlight these effects. In view of the large number of variables involved in these processes, we focus our attention to the following: for pre-processing, controlled turning was selected and for post-processing, controlled grinding was selected. Residual stress measurements were performed by the centre hole air-abrasive off-centre rotating nozzle method, and whenever possible, the results were verified by other methods such as X-ray diffraction and etching. An experimental rig was designed, built and commissioned to allow for locating different sizes and shapes of cylindrical and flat components accurately near the 'drilling head'. The surface topography was measured, using a motorised stage and a profilometer of the stylus type. This arrangement enabled the three dimensional determination of treated surfaces. The materials investigated were typical of aircraft alloys: steel 817M40 and aluminium 7075, which were provided by a major landing-gear manufacturer (Dowty Rotol) . In order to examine the effect of residual stresses and surface profile upon ultimate product performance, fatigue tests using electro-hydraulic servo-controlled test equipment were used to evaluate the effect of the treatment upon the fatigue life and relaxation of residual stresses. Both complete and incomplete coverage were considered in the analysis. The results of the work show that pre-processing can induce unfavourable tensile residual stresses, which are detrimental to the fatigue life of the component. They also show that peening residual stresses play an important role ~n negating these tensile residual stresses. Effects of surface profile due to the pre-treatment also indicated a major influence on the success of the treatment, and upon the selection of the appropriate peening parameters. As for post-processing, it was shown that the residual stresses varied linearily with depth removed and remained significant even after 50 percent of the arc height had been removed from the component.

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No description available.