In this study, 25Cr2Ni2Mo1V filler metal was deposited to weld low pressure steam turbine shafts, which are operated in fossil power plants. A comparison experiment was conducted on the weld metals (WMs) before and after varied various aging duration from 200 hours up to 5000 hours at 350 ℃. Microstructure was characterized by means of scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD) techniques. In addition, mechanical properties of corresponding specimens were evaluated, e.g. Vickers microhardness, Charpy V impact toughness and tensile strength. It is shown that the tensile strength remained stable while impact energy value decreased with increasing aging duration. Based on the experiment above, it was concluded that the variation of mechanical properties can be attributed to the redissolution of carbides and reduction of bainite lath substructure.

A modified version of Alloy 706, designated FENIX-700, was developed using the CALPHAD method to improve high-temperature stability above 700°C. The new alloy features reduced Nb and increased Al content, relying on γ' (Ni 3 Al) strengthening while eliminating γ'' (Ni 3 Nb), δ, and η phases. This modification improved both creep temperature capability (from 650°C to 700°C) and segregation properties. Successful manufacturing trials included a 760 mm² forging shaft using triple melt processing and a 1050 mm ESR ingot, demonstrating industrial viability. The study also explores compatible Ni-base welding materials for joining FENIX-700 to 12% Cr ferritic steel in 700°C class steam turbine applications.

The global transition toward high-efficiency steam power plants demands increasingly advanced steel rotor forgings capable of operating at temperatures of 600°C and above. The European Cost program has been instrumental in developing creep-resistant 10%-chromium steels for these critical applications, with Steel Cost E emerging as a prominent material now widely utilized in steam turbine shafts and experiencing significant market growth. Saarschmiede has pioneered a robust, fail-safe manufacturing procedure for Cost E rotors, establishing a comprehensive database of mechanical properties and long-term performance data that enhances turbine design reliability. The company has expanded its manufacturing capabilities to include Cost F rotor forgings for high-pressure and intermediate-pressure turbines, with component weights reaching up to 44 tonnes. Investigating methods to further increase application temperatures, researchers within the Cost program discovered the potential benefits of boron additions to 10%-chromium steels. Leveraging this insight, Saarschmiede has produced full-size trial rotors to develop and refine production procedures, with these prototype components currently undergoing extensive testing to validate their performance and potential for advanced high-temperature applications.

Steels with 9-12% chromium content are widely used in steam turbines operating above 550°C due to their improved creep properties. Saarschmiede has extensive experience manufacturing high chromium steels, such as the X12CrMoWVNbN10-11-1 steel designed through the European COST program for application up to 610°C (COST Rotor E). From this steel, Saarschmiede produces high-pressure rotor shafts and gas turbine discs. To meet ever-increasing steam temperatures, a modified steel type with elevated boron content was developed, and pilot rotors have been manufactured. For ingot manufacturing of high chromium steels, Saarschmiede utilizes the Electro-Slag-Remelting process, allowing ingots up to 165 tons. Optimized forging and heat treatment procedures ensure reproducible forging properties. All products undergo rigorous destructive and non-destructive testing.

Three Ni-base wrought alloys with different hardening mechanisms (INCONEL 706, Waspaloy and INCONEL 617) were investigated as candidates for steam turbine rotor applications at temperatures up to 700 °C in respect to their microstructure and microstructural stability. The Nb containing alloy Inconel 706 had a complex microstructure with γ', γ" and η phases which are stable in long term service up to 620 °C. At higher temperatures significant particle coarsening and phase transformation were observed. Waspaloy is hardened by γ' particles and after ageing at 700 °C and higher, it tended to a stable microstructure. Inconel 617 is a solid solution hardened material additionally hardened by homogeneously distributed fine M 23 C 6 carbides. After long term ageing at temperatures of 650 °C to 750 °C the carbides tended to form carbide films along the grain boundaries and at 700 °C to 750 °C γ' precipitated as homogeneously distributed particles with low coarsening during long term service. In order to optimize the candidate alloys Inconel 706 and Waspaloy were modified to the new alloys DT 706 and DT 750. The aspects of modification and first experimental results are reported.