Joachim Konrad

Preparation, phase stability and structure of the C36 Laves phase Nb1-xCo2+x

Abstract The C36 Laves phase Nb1–xCo2+x has been re-investigated in order to study phase stability and structure. Constitutional data have been obtained by investigating homogenized single- and two-phase samples and from diffusion couples. The C36 phase Nb1–xCo2+x crystallizes with hexagonal MgNi2 structure type (Z = 8, space group P63/mmc, a = 4.7414(4) Å and c = 15.458(1) Å at x = 0.265(4)). The homogeneity range extends from 24.6(2) to 25.3(5) at% Nb. The temperature range of the phase field is limited by a eutectoid (C36 Nb1–xCo2+x = Nb2Co7 + C15 NbCo2) and a peritectic (L + C15 NbCo2 = C36 Nb1–xCo2+x) reaction at ∼ 1050 °C and 1264 °C, respectively. In addition, the title phase is involved in the peritectoid reaction Co(Nb) + C36 Nb1–xCo2+x = Nb2Co7 at 1086 °C and in the eutectic reaction L = Co(Nb) + C36 Nb1–xCo2+x at 1239 °C. The C36 and C15 Laves phases of the Nb—Co system are separated by an extremely small two-phase field (<0.5 at%). The crystal structure exhibits pronounced deviations from ideal parameters obtained from a hard sphere model. The Co network displays a type of distortion known from many hexagonal Laves phases. Kagom, layers display an elongation of the Co—Co edges of the basal triangles of Co5 trigonal bipyramids and a contraction of Co—Co edges of the uncapped triangles. The Nb atoms are also displaced from their idealized sites. In the crystal structure of C36 Nb1–xCo2+x excess Co atoms randomly substitute Nb atoms — (Nb1–xCox)Co2. The excess Co atoms occupy preferentially the Nb2 site approximately twice as much as Nb1. These positions differ mainly in the conformation of the corresponding Nb6Nb2 fragments (Nb1—Nb1 eclipsed and Nb2–Nb2 staggered). In addition, Co atoms are displaced from the original Nb positions. The distortion of the Co and the Nb network is a consequence of the bonding situation of the defect-free crystal structure. The preferential site occupation of excess Co atoms is triggered by interactions with atoms beyond the first coordination shell. The C36 phase Nb1–xCo2+x exhibits Pauli-paramagnetic behavior (χP = +1.31 · 10–3 emu mol–1). The temperature dependent part of the electrical resistivity ρ(300 K) – ρ0 is only 17 μΩ cm whereas the residual resistivity is very large with ρ0 = 140 μΩ cm indicating strong structural disorder.

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.

Investigation of Nucleation Mechanisms of Recrystallization in Warm Rolled Fe3Al Base Alloys

It has been shown in literature that the mechanical properties of Fe3Al base alloys are strongly dependent on the heat treatment subsequent to warm rolling. Therefore, the recrystallization behavior of 3 different hot and warm rolled and annealed Fe3Al-based alloys has been investigated. Two of these alloys contain different forms and amounts of second phase particles, while a pure binary alloy was taken as reference. All alloys develop a-(<110>||RD) and g-(<111>||ND) fiber bcc-type rolling and annealing textures, however, the amount of a- and g-fibers vary in dependence of the alloy composition. The current work presents the investigations on the nucleation process during annealing that has been studied by means of high resolution backscatter electron diffraction (EBSD) in the SEM. In particular the occurrence of orientation gradients in the deformed structure and their crystallographic relationship to the formation of new grains was investigated. It was shown, that small particles favor the a-fiber component by hindrance of the growth of new grains. In contrast, large particles lead to particle stimulated nucleation. This weakens the overall texture but does not randomize it since the orientation gradients around particles keep a relationship with the matrix orientation.

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

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