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Dissimilar metal welds (DMWs) between ferritic steel grades are found extensively in the construction of thermal power plants. The potential combinations and approaches for joining dissimilar ferritic steels are nearly limitless. For DMWs, the difference in alloy composition (specifically chromium and carbide-forming elements) provides the main driving force for carbon diffusion during welding, post-weld heat treatment and long-term service at elevated temperatures. Since the high temperature creep strength of local, carbon-denuded zones can be dramatically reduced from that of the parent or filler material, the service performance of ferritic DMWs can be severely reduced. This article reviews experimental observations on microstructural evolution in dissimilar ferritic welds, activities to describe the observed phenomena by modelling and simulation and discusses the performance of these welds at high temperature. Lastly, a well-engineered approach to the design of ferritic DMWs is discussed in the context of thermal power plants which are subject to damage by creep.

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.

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 z20% 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.

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Two modified X120Mn12 Hadfield steels, differing in the amount of the alloying elements Al and N, are analyzed with respect to AlN precipitation and its effects on ductility. Charpy impact tests are performed, demonstrating the loss of ductility in the one grade containing a high density of AlN precipitates. The characterization of the precipitates is carried out by high-resolution scanning electron microscopy (HRSEM). Depending on chemical composition, primary and secondary AlN precipitates are detected on prior austenite grain boundaries and within the bulk volume. The experimental observations are confirmed by thermokinetic simulations, using the software package MatCalc (

Synchrotron X-ray microtomography(SR-μCT) scans have been carried out on sample coupons extracted from the fracture specimens of a 10.86% Cr heat resistant steel exposed to crep deformation at 873K over stresses of 120, 150, and 180 MPa. The 3D cavitation characteristics in terms of void volume fraction, numbwer density and size distribution as a function of the applied stress has been determined by quantitative analysis of the reconstructed tomograohy slice datasets. The relationship between heterogenous spatial distribution of creep voids and variation in rupture life has been exploited in terms of microstructural sites during the onset of creep embrittlement.