Charles Stallybrass

Dual-scale phase-field simulation of grain growth upon reheating of a microalloyed line pipe steel

Abstract The austenite grain growth of a microalloyed steel was investigated via annealing experiments and phase-field simulations using the phase-field code Micress. The technique described in a previous work was enhanced and applied to an Nb, Ti microalloyed linepipe steel for the case of isothermal heat treatment between 1 050 and 1 200 °C. The input parameters for the phase-field simulations were deduced from physical models based on the results of isothermal holding experiments. A further improvement was the use of the software package MatCalc to simulate at a lower scale the coarsening of the pinning particles. The results of these simulations showed good agreement with the experimental results.

Formation of primary TiN precipitates during solidification of microalloyed steels – Scheil versus DICTRA simulations

Abstract Modern high-strength low-alloy steels commonly contain microalloying additions of titanium, niobium or vanadium in different combinations in order to obtain the desired microstructure and mechanical properties. Titanium has a strong tendency to form TiN in the range of the solidus temperature. This has been reported to have a negative effect on the impact toughness of the material. Thermodynamic calculations showed that the titanium and nitrogen content and the titanium to nitrogen ratio determine if the formation of TiN takes place during solidification or in the solid state. These calculations where complemented by simulations of solidification using the Scheil – Gulliver model and DICTRA. The results were compared with microstructure investigations of plate and slab material with titanium contents between 0.003 wt.% and 0.015 wt.% using light-optical microscopy and electron probe microanalysis. While the formation of TiN particles cannot be ruled out even at the lowest Ti levels, the temperature of formation and the volume fraction varied significantly depending on the Ti content. With respect to the first results of this preliminary study, i. e. the comparison of equilibrium, Scheil and DICTRA calculations, it can be assumed that the Scheil model is the most appropriate one at present.

Strengthening mechanisms for Fe–Al-based alloys with increased creep resistance at high temperatures

Lack of strength, in particular creep resistance, has been considered as the principal shortcoming of Fe-Al-based alloys for long which prevented their use for structural applications at high temperatures. Only recently it has been realized that various strengthening mechanisms are available by which Fe-Al-based alloys can be sufficiently strengthened for applications at high temperatures. From the recent studies at the Max-Planck-Institut fur Eisenforschung GmbH (MPIE) examples of applying different strengthening mechanisms are presented by which Fe-Al-based alloys with considerable creep resistance have been obtained.

Modern Approach to the Microstructure Characterization of Large Diameter Linepipes

Over the past decades, the complexity of requirements regarding the properties of large-diameter linepipes has increased steadily. This is driven by factors such as increasing operating pressures or more hostile environmental conditions. Steel producers all over the world have responded to these demands by continuous development along the entire processing route from steelmaking to thermomechanical rolling and pipe production. Understanding the influence of the microstructure on pipe properties is a key element to extend the use of linepipe steels to more challenging conditions. For this reason, the techniques that are used for microstructure characterization are constantly refined.The microstructure of modern microalloyed linepipe steels that are produced by thermomechanical rolling in combination with accelerated cooling depends strongly on the processing parameters during production. The grain size of the base metal is typically below 10 μm and may contain fractions of ferrite, bainite and M/A-constituents. Because of their size, these microstructure constituents are often not readily accessible to a quantitative analysis by classical light-optical microscopy. This was also found to be true within the heat-affected zone (HAZ) of large-diameter pipes. High-resolution scanning electron microscopy in combination with electron backscatter diffraction was found to offer a wide range of possibilities to characterize the microstructure quantitatively with regard to the effective grain size, the volume fraction of constituents and their variation over the wall thickness. The effects of variations in processing parameters in laboratory-scale trials on the microstructure and properties are illustrated. Based on these investigations, it was possible to refine the alloy design and processing parameters in order to improve the low-temperature toughness of the base metal of high strength plate material and the HAZ of longitudinal weld seams.© 2014 ASME

Development of Modern High Strength Heavy Plates for Linepipe Applications

High strength linepipe steels have to fulfil increasing property demands in modern pipeline applications. The transport of large gas volumes at high pressures from remote areas to the market is achieved in the most economical way by large diameter pipelines. For the last 30 years, high strength heavy plates for pipes and pipe bends were developed and produced at Salzgitter Mannesmann Grobblech. These products were steadily improved for example in terms of toughness and fracture behaviour at low temperatures. This is a strong focus of materials development around the world.Modern high-strength heavy plates used in the production of UOE pipes are generally produced by thermomechanical rolling followed by accelerated cooling (TMCP). The combination of high strength and high toughness of these steels is a result of the bainitic microstructure realised by TMCP and are strongly influenced by the rolling and cooling conditions. This paper gives an overview of the development of high strength plates for line pipe application at Salzgitter Mannesmann Grobblech. From comparably thin-walled X80 plates with no or medium DWTT requirements to recent requirements for approx. 28 mm thick X80 plates with requirements of 75/85% shear area fraction at −30°C and more than 250 J Charpy energy at −40°C the development work and the result of the last five years are described and presented.Classical light-optical characterisation of the microstructure of these steels is at its limits because the size of the observed features is too small to allow reliable quantitative results. Therefore Salzgitter Mannesmann Grobblech and Salzgitter Mannesmann Forschung (SZMF) developed alternative methods with the aim of a quantification of microstructure features and a correlation of those with the mechanical properties and processing conditions.In several investigations, the information is related to the mechanical properties of the plate material. It was found that a variation of the processing conditions has a direct influence on parameters that are accessible through the EBSD method and correlates with mechanical properties. The detailed correlations vary depending on steel grade and TMCP strategy. The results have to be carefully interpreted and help understanding the connection between processing and properties. Consequently this can be used as valuable input for the definition of the processing window for heavy plate production with optimized properties.Copyright

High Strength Heavy Plate Optimised for Application in Remote Areas and Low-Temperature Service

The last decades have seen a steady increase in the demand for high-strength linepipe steels. These offer the most economical option to transport large gas volumes at high pressures from remote areas to the market. Since the beginning of the 1980’s, high strength heavy plates, pipes and pipe bends were developed and produced at Salzgitter Mannesmann Grobblech GmbH and EUROPIPE. Since these days, these products were steadily improved for example in terms of toughness and weldability. As gas resources in increasingly hostile environments are developed, the requirements with regard to deformability and low-temperature toughness have gained growing significance. This is a strong focus of materials development around the world. Modern high-strength heavy plates used in the production of UOE pipes are generally produced by thermomechanical rolling followed by accelerated cooling (TMCP). If accelerated cooling starts above the ferrite-austenite transformation temperature, this processing route results in a microstructure that consists predominantly of bainite. The combination of high strength and high toughness of these steels are a result of the microstructure realised by TMCP and are strongly influenced by the rolling and cooling conditions. Classical light-optical characterisation of the microstructure of these steels is at its limits because the size of the observed features is too small to allow reliable quantitative results. Therefore alternative methods have to be used to obtain a better understanding of the influence of processing conditions on the microstructure. The mechanical properties of high strength plates produced at Salzgitter Mannesmann Grobblech (MGB) and of material rolled using a laboratory rolling mill at the Salzgitter Mannesmann Forschung (SZMF) was characterised with special emphasis on low-temperature toughness. The microstructure was investigated using the electron backscatter diffraction (EBSD) method. With this method, it is possible to gain quantitative information related to features of the microstructure and relate these to the mechanical properties of the plate material. It was found that a variation of the processing conditions has a direct influence on parameters that are accessible through the EBSD method and correlates with mechanical properties. These results can be used as valuable input for the definition of the processing window for heavy plate production depending on the required plate properties.Copyright