Coline Beal

Investigation of creep damage in advanced martensitic chromium steel weldments using synchrotron X-ray micro-tomography and EBSD

Abstract In recent years, a design concept for the stabilisation of the microstructure by addition of boron and nitrogen was developed. This so called martensitic boron–nitrogen strengthened steel (MARBN) combines boron strengthening by solid solution with precipitation strengthening by finely dispersed nitrides. Welded joints of MARBN steels showed no formation of a uniform fine grained region in the heat affected zone (HAZ) which is in general highly susceptible to Type IV cracking. In this work, the crossweld creep strength of a newly developed MARBN steel was analysed and the evolution of damage was investigated using synchrotron microtomography supported by electron microscopy. Three-dimensional (3D) reconstructions of the tested samples together with electron backscatter diffraction investigations revealed an intense void formation in a restricted area along small grains at prior austenite grain boundaries in the HAZ as the main reason for premature creep failures in the HAZ of welded joints.

Investigation on the Microstructural Evolution in a Medium-Mn steel (X10Mn5) after Intercritical Annealing*

Abstract TRansformation Induced Plasticity (TRIP)-assisted Medium-Mn steels, with Mn-contents in the range of 4–10 ma.-%, have recently gained a lot of interest due to their promising mechanical properties. This steel group contains ≥ 30 % of retained austenite, which is stabilized by Carbon and mainly Manganese during intercritical annealing. The present work investigated the influence of annealing temperature and cooling rate on the microstructural evolution by means of dilatometry. Two thermodynamical models for the prediction of optimal annealing temperature and maximum retained austenite content have also been thoroughly evaluated. For further characterization, scanning electron microscopy, EBSD, micro-hardness testing and X-ray diffraction were carried out. The investigations manifested a pronounced influence of both annealing temperature and cooling rate, on the phase fractions of ferrite, austenite and martensite, which must be taken into account by design of batch annealing route for Medium-Mn TRIP steels in order to obtain superior combination of strength and ductility.

Creep and damage investigation of advanced martensitic chromium steel weldments for high temperature applications in thermal power plants

Abstract With the aim to increase base material creep strength and overcome the type IV cracking problem, a new design concept was developed. This so called martensitic boron–nitrogen strengthened steel (MARBN) combines boron strengthening through solid solution with precipitation strengthening by finely dispersed nitrides. In this work, uniaxial creep tests of the MARBN base material and welded joints have been carried out. The creep strength of the welded joints was analysed, and the evolution of creep damage was investigated. The creep tests of MARBN revealed increased strength of the base material of about +20% compared to the best commercially available 9Cr steel grade. At higher stress levels, the creep strength of crosswelds is between that of the MARBN base material and the conventional 9Cr base materials. Nevertheless, long term creep tests revealed a drop in creep strength of the MARBN welded joints. The underlying phenomena of crossweld creep behaviour are discussed in detail.

Combination of Microstructural Investigation and Simulation during the Heat Treatment of a Creep Resistant 11% Cr-Steel

This work deals with the microstructural evolution of creep resistant martensitic/ferritic 11% Cr-steel during thermomechanical treatment from an experimental as well as modeling point of view. The creep resistance of this material group is highly dependent on the precipitate status. The initial precipitate status is controlled by the chemical composition of the alloy and the heat treatment after casting or hot rolling. It is therefore of utmost interest to understand and model the precipitate kinetics during this process. Once the microstructural evolution has been modeled successfully, only minimum effort is required to computationally test variants in the composition or heat treatment in order to optimize the process. In this work, the material was hot rolled, austenitized and subsequently annealed. All heat treatments have been performed during dilatometry tests. In order to investigate the microstructural evolution during the process, specimens were extracted at definite stages of the treatment. The specimens were then investigated applying various microscopical techniques in order to quantify the microstructural features (grain size, martensite lath width and precipitate data). The experimental data were then compared to thermodynamic simulations (MatCalc). General data such as nucleation sites for precipitates were taken from literature, grain size and martensite lath widths from the experimental data. Simulations include equilibrium calculations and precipitate kinetic simulations. In general, the simulations showed good agreement with the experimental findings, with minor room for improvements. The work thus lays a solid ground for future improvements of the heat treatment process.

Influence of Thermal History on the Hot Ductility of Ti-Nb Microalloyed Steels

The hot ductility of Ti-Nb microalloyed steel has been investigated to evaluate the sensitivity to surface crack formation during the continuous casting process. Tensile samples were subjected to different thermal treatments and were tested at deformation temperatures ranging from 650°C to 1000°C using a strain rate of 10-3s-1. It has been found, that the investigated steel evinced poor ductility over almost the whole testing temperature range characterized by marked grain boundary cracking, irrespective of which thermal cycle has been utilized or whether the samples have been melted or only reheated. Microstructural examinations and supplementary thermo-kinetic computer simulations revealed distinct Ti-Nb precipitation throughout the microstructure being responsible for the deteriorated materials hot ductility.

The Effects of Intercritical Annealing Temperature and Initial Microstructure on the Stability of Retained Austenite in a 0.1C-6Mn Steel

The effects of the intercritical annealing temperature and initial microstructure on the stability of retained austenite were investigated for a 0.1C-6Mn (wt-%) steel. Medium-Mn transformation-induced plasticity (TRIP) steels exhibit a strong dependence of their mechanical properties on the variation of intercritical annealing temperature. This behavior is strongly linked to the amount and stability of the retained austenite. Thus, interrupted tensile tests were used to examine the effect of annealing temperature on the stabilization of the retained austenite. Detailed microstructural investigations were employed to elaborate the effects of its chemical and mechanical stabilization. Furthermore, the final microstructure was varied by applying the batch annealing step to an initial non-deformed and deformed microstructure respectively. Retained austenite stability along with resulting mechanical properties of the investigated medium-Mn TRIP steel was significantly influenced as the amount and morphology of the respective phases altered as a consequence of both initial microstructure and applied intercritical annealing temperature.

Advanced Microstructures for Increased Creep Rupture Strength of MARBN Steels

Over the past three decades a lot of effort was made to optimize the chemical compositionof 9% Cr martensitic steels, aiming to increase the operating temperature up to 923K and thus im-proving the efficiency of thermal power plants. Under these service conditions (high temperature andstress exposure), the creep strength of such steels is closely related to the long term stability of theirmicrostructure. The time to rupture can also be understood as an equivalent to the time of microstruc-ture deterioration. Optimization of the initial microstructure and understanding of the microstructureevolution during creep exposure are therefore decisive to improve the creep behavior of 9% Cr steels.Selected chemical compositions of MarBN steels (Martensitic 9% Cr steels strengthened by Car-bides, Nitrides and Boron) were subjected to different heat treatments to produce an optimized mi-crostructure to improve the creep rupture time. The initial microstructure before creep exposure wasinvestigated using optical microscopy, SEM and EBSD. Short term creep rupture tests at 923K and150MPa were performed, followed by systematic microstructure investigations.Comparative EBSD investigations confirm an optimized microstructure for creep exposure, pro-duced by an appropriate heat treatment. From comparative creep test results, it can be concluded thatadvanced microstructures increase the time to rupture of the selected MarBN steels by more than 10percent, without reduction of the ductility.

Considerations for Sound Parameter Studies in Electron Beam Welding of Thick Walled Components

Joining new materials, material combinations or geometries by means of electron beam welding requires some preliminary parameter studies to assess the proper welding range. Determining parameters for thick walled applications is usually done by performing bead on plate welds. Due to resource efficiency, several welds are placed in a single block. After welding, the block is cut perpendicularly to the welding direction to judge the geometry of the molten zone and the influence of the used parameter combination on the joint quality. In different bead on plate welding experiments for several steels, the cross-sections revealed cracks, located in the centerline of the weld seam. In further experiments it turned out that these failures were not fully repeatable and therefore could not be attributed to the welding parameters or on the material solely. The experimental setup as a whole is crucial. Therefore, experiments were planned and performed to investigate the issues of these centerline cracks in EBW bead on plate welding studies. This contribution documents the performed failure case analysis. It shows that these cracks where not caused by miscalculated welding parameters or due to the metallurgic sensitivity of the material. Cracks appearance occurs however by reasons of a combination of shrinkage strain and dendritic solidification. Furthermore this document provides some recommendations to avoid this defect and beware of misleading interpretation of welding trials.

Dissimilar Electron Beam Welds of Nickel Base Alloy A625 with a 9% Cr-Steel for High Temperature Applications

Welding of thick walled components with an electron beam has great potential due to the minimal heat input, high reproducibility and cost-efficiency. In the present work electron beam welding was used to weld 50mm thick plates of cast Ni-base alloy A625 to ferritic/martensitic 9% Cr steel plates. The welds were creep exposed at 625°C with stress levels ranging between 156 - 100MPa. Microstructure analysis of the weld-seam and the heat affected zone was carried out using metallography and scanning electron microscopy employing the EBSD technique to determine the location of the creep rupture. Creep fracture is located in the heat affected zone of the 9% Cr steel. Electron beam welded samples were compared to shield metal arc welded samples regarding welding and creep resistance. The performance and related microstructure properties of the electron beam welded specimen are more than competitive to conventional metal-arc-welding procedures.

Application of Thermo-Calc TCFE7 to High-Alloyed Mottled Cast Iron

The thermodynamic modeling of alloy systems consisting of stable and metastable phases e.g. high-alloyed mottled cast iron can be problematic. Thermodynamic databases are rather well-developed for low, medium and high alloyed steels (e.g. HSS) but the application of those databases is not yet very common for high-alloyed (mottled) cast irons. The Thermo-Calc software together with the TCFE7 database is used to calculate isopleth and property diagrams, using the CALPHAD method. Additionally Scheil-Gulliver calculations are performed to simulate the effects of microsegregation during solidification. The results from the thermodynamic calculations are compared with measurements on own samples and with literature values. Those measurements include quantitative light-optical analysis, SEM with BSE detector, EDX measurements for the distribution of the alloying elements as well as XRD and DSC measurements. The investigations show the possibilities which are offered by thermodynamic calculations for high-alloyed mottled cast iron as well as the limitations and the compromises which have to be taken into account when calculating stable and metastable phases existing next to each other.