Kaikki aineistot
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Slurry erosion is a severe problem and a major concern for slurry handling equipment, as it leads to considerable expense caused by failures, downtime and material replacement costs. Slurry erosion is dependent on several parameters such as slurry properties, service conditions, and material properties. Hence, much high-quality research has been aimed at obtaining a fundamental understanding of this complex failure mode and developing new test methodologies and erosion resistant materials to minimize erosion rates. This is a review of the literature covering research into the effects of the main parameters influencing the slurry erosion of different types of steels, focusing on those which have been developed for pipeline applications. The types of bench-scale erosion test rigs, the mechanisms involved, and the behavior of different microstructures under slurry erosion conditions are discussed.
Grain refinement has been widely used to enhance the hardness and toughness properties of metallic materials. However, the effect of prior austenite grain refinement on the final martensitic microstructure and wear performance of steels is not yet fully understood. In this study, induction hardening treatment with heating rates in the range 50–500 °C/s to the peak temperatures of 900 and 1000 °C followed by water quenching has been employed to produce through-hardened microstructures in a new medium-carbon, low-alloy steel intended as a slurry transportation pipeline material. The results revealed that in the range of achieved prior austenite grain size i.e. 2-15 μm, during different heating paths, the final martensitic microstructures experienced only a slight difference in the size of blocks and level of hardness. The mean hardness, hardness homogeneity, and grain structure uniformity were highest with a heating rate of 50 °C/s, especially for those samples which were re-austenitized at the peak temperature of 900 °C. A pin-mill type of high-speed slurry-pot wear tester was used to evaluate the slurry erosion behavior of the steel. It was found that prior austenite grain size in the above-mentioned range had no significant effect on the final microstructure and hardness value, however, the slight difference in martensite block size did notably influence the work hardening behavior and consequently the wear mechanism of the samples during the tests.
Abstract Double Loop Electrochemical Potentiokinetic Reactivation testing has been employed to experimentally determine the degree of sensitization (DOS) of an austenitic stainless steel subjected to isothermal heat treatment for various times in the temperature range 700–820°C. For the different heat treatment conditions, the chromium concentration profiles across grain boundaries were calculated using the diffusion module in Thermo-Calc® based on the assumptions that sensitization is caused by grain boundary M23C6 precipitates and that local multicomponent equilibrium and flux balance exist at the carbide - matrix interface. Comparison of the experimental DOS values and the details of the chromium concentration profiles was used to establish a quantitative depletion factor that predicts sensitization at short annealing times.
Abstract The effects of forced cooling, meaning forced cooling rate and forced cooling finish temperature, on the tensile and impact toughness properties of simulated weld coarse-grained heat-affected zones have been studied for a commercial grade martensitic steel with a yield strength of 960 MPa. The simulations were done by using a Gleeble 3800 to give forced cooling finish temperatures of 500, 400, 300, 200, and 100 °C and forced cooling rates of 50 and 15 °C/s. For the steel studied, strength significantly increased with no significant negative effects on impact toughness when the steel was cooled rapidly to 200 or 100 °C at 15 °C/s. The results indicate that it may be possible to improve welding productivity and mechanical properties of the steel by using forced cooling down to 100 °C to reduce waiting time between weld passes.
Abstract A new method to calculate the plastic anisotropy r-values of austenitic and ferritic stainless steels has been developed. The mean orientation of individual grains is obtained from SEM-EBSD data and r-values for individual grains are calculated by weighting all slip systems according to their Schmid factors. Calculated and measured r-values are in good agreement for austenitic stainless steels. However, in ferritic stainless steels, which are highly anisotropic, good agreement requires the introduction of a Schmid factor threshold below which slip systems are inactive. The present method can be used to estimate local differences in r-values in ferritic stainless steels showing the local variations in texture responsible for ridging.
Abstract A new method to quantify the ridging phenomenon in ferritic stainless steels has been developed based on the evaluation of surface profiles after the tensile elongation of 100 mm wide sheet specimens. The ridging components of the surface profiles are extracted by a tailored spline filtering procedure. A ridging index is proposed to quantify the severity of the surface defect based on surface profile height and spacing parameters. The procedure is independent of the type of profilometer used as long as unfiltered raw profiles can be recorded. The reproducibility of the measurement method and its correlation with the visual assessment of strained specimens is discussed.
Abstract A computational model based on the Johnson-Mehl-Avrami-Kolmogorov equation for simulating the onset and kinetics of austenite to bainite and martensite transformation has been fitted to experimental continuous cooling data for two different steels. We investigated how deformation below recrystallization temperature affected the transformation onset and kinetics in comparison to the same steel in the undeformed state. The fitted model can be used to simulate phase transformations occurring when the steel is cooled along any cooling path. The model can be fully coupled to heat transfer and conduction simulations in order to optimize cooling practice, for example in industrial thermomechanical processing of steel. The fitted model can also be used to predict the hardness of the steel after cooling.
Abstract We present a computational method for calculating the phase transformation start for arbitrary cooling paths and for different steel compositions after thermomechanical treatments. We apply the method to quantitatively estimate how much austenite deformation and how many different alloying elements affect the transformation start at different temperatures. The calculations are done for recrystallized steel as well as strain hardened steel, and the results are compared. The method is parameterized using continuous cooling transformation (CCT) data as an input, and it can be easily adapted for different thermomechanical treatments when corresponding CCT data is available. The analysis can also be used to obtain estimates for the range of values for parameters in more detailed microstructure models.
Abstract In high- and ultrahigh-strength steel welding, interpass cooling time is an important factor affecting productivity and welding costs. Usually, welding heat input is restricted to meet the relatively short recommended cooling times between 800 and 500 °C (t8/5), which are prescribed by the need to meet weld strength and toughness properties. This, in turn, leads to the need for multipass welding with the interpass waiting times needed for the weld to cool to a sufficiently low interpass temperature. Welding productivity is affected by both the number of passes and the interpass waiting time. With a view to minimizing the total number of passes needed for a given preparation, it is beneficial for the interpass temperature to be as low as possible as this permits higher heat input for a given t8/5. On the other hand, low interpass temperature requires longer interpass waiting times. Therefore, this research concerns the potential of introducing copper heat sinks adjacent to the weld to reduce the time it takes for the weld to cool down to the interpass temperature. It is demonstrated that, in the case of a butt weld in a 6 mm thick base plate MAG welded with a weld energy of 1 kJ/mm and an interpass temperature of 100 °C, copper heat sinks almoust halve the interpass waiting time. This can have a marked effect on the overall productivity when welding highand ultrahigh-strength steels and increase their attractiveness for steel construction.
Abstract Ridging means the appearance of surface profile undulations, i.e. peaks and valleys, as a result of plastic strain. The reasons for the different ridging behaviour of industrially produced, stabilized ferritic stainless steel sheets (EN 1.4509) have been investigated after straining in the rolling and transverse directions with low and high resistance towards ridging. The evolution of macro-texture has been measured by X-ray diffraction (XRD) both before and after ridging tests in the rolling and transverse directions. The macro texture results showed that straining along the rolling direction strengthened the α fibre whereas the γ fibre was strengthened by grain rotations after straining along transverse direction. Electron backscatter diffraction (EBSD) imaging was used to establish the micro-textural variations over the thickness of the sheets among the high and low ridging materials. Mean orientations of individual grains determined from the EBSD data were utilized to calculate plastic strain ratio r-values by considering all slip systems weighted according to their Schmid factors. The calculated r-values were used to predict the ridging surface profile after straining along the rolling and transverse directions. The results demonstrated the influence of local variations in micro-texture on the severity of ridging.
Abstract The effects of forced cooling, i.e., forced cooling rate and forced cooling finish temperature, on the tensile and impact toughness properties of simulated weld coarse-grained heat-affected zones has been explored in the case of a low-carbon thermomechanically processed steel with a yield strength of 700 MPa. The forced cooling finish temperatures that were studied were 400, 300, 200, and 100 °C and the forced cooling rates were 50 and 15 °C/s. Coarse-grained heat-affected zones were simulated using a Gleeble 3800 thermomechanical simulator. For the steel concerned, strength and impact toughness improved significantly when the steel was cooled rapidly to 200 or 100 °C. The results indicate that it may be possible to substantially improve welding productivity by using forced cooling to reduce interpass times.
Abstract Due to the volume change accompanying the fcc to bcc or bct crystal structures in steels, it is a common practice to determine phase transformation temperatures using dilatometry. The martensite start temperature (Ms) is often of particular interest. Experimentally, it is found that the start of the martensite transformation is not indicated by a sharp change in the slope of the dilatation curve as is predicted by the Koistinen–Marburger equation. Rather, there is a gradual change in the slope such that the martensite start temperature is ill-defined. The current work shows that this gradual change in slope can be related to chemical inhomogeneity in the steel caused by interdendritic microsegregation. It is shown that combining the Koistinen–Marburger equation with measured concentration profiles allows experimental dilatation curves to be well predicted.
Novel high-hardness medium carbon martensitic laboratory steel has been produced and tested for wear resistance. Different finish rolling temperatures (FRT) and quenching finish temperatures (QFT) were utilized. Composition was selected based on earlier experiments and carbon content was set to 0.35 % to obtain surface hardness of approximately 600 HB. FRT was varied to investigate the effect of prior austenite deformation on the mechanical properties. Direct quenching was implemented in the laboratory rolling trials. Plates were either quenched to room temperature or quenching was finished at 250 °C. The interrupted quenching was tested in order to improve the toughness of the steel via autotempering and possible austenite retention without drastic loss of hardness. The steel samples were tested for hardness and impact toughness. Material characterization included SEM and optical microscopy for microstructural inspection. Direct quenched steel samples exceeded the desired 600 HB surface hardness, but interrupted quenching to 250 °C resulted in lower hardness values. In contrast, the impact toughness was improved with latter quenching method. Impact-abrasion wear testing was conducted for the experimental steels to understand the effect of rolling and quenching parameters on wear resistance. Impeller-tumbler tests were carried out at Tampere Wear Center using natural granite as the abrasive. The results indicate that surface hardness is the main controlling factor of wear, and samples with the highest surface hardness showed the lowest mass loss.