In view of developing novel heat-resisting steels for applications in conventional power plants with service temperatures of 650 °C, a series of martensitic/ferritic model steels with 12 wt.%Cr were studied to achieve an increased creep resistance through additional alloying with various elements for controlled precipitation of M23C6 carbides, MX carbonitrides and intermetallic Laves phase. The alloy design relied on thermodynamic simulation calculations using Thermo-Calc and DICTRA. The mechanical testing concentrated on creep at 650 °C for up to 8000 h. The alloy optimization resulted in creep rupture strengths above those of the martensitic/ferritic P92 steel. The work was part of a cooperative project within the German MARCKO program.

Design of martensitic/ferritic heat-resistant steels for application at 650 °C with supporting thermodynamic modelling

Evolution of microstructure and deformation resistance in creep of tempered martensitic 9-12%Cr-2%W-5%Co steels

Laves phases with their comparably simple crystal structure are very common intermetallic phases and can be formed from element combinations all over the periodic table resulting in a huge number of known examples. Even though this type of phases is known for almost 100 years, and although a lot of information on stability, structure, and properties has accumulated especially during the last about 20 years, systematic evaluation and rationalization of this information in particular as a function of the involved elements is often lacking. It is one of the two main goals of this review to summarize the knowledge for some selected respective topics with a certain focus on non-stoichiometric, i.e., non-ideal Laves phases. The second, central goal of the review is to give a systematic overview about the role of Laves phases in all kinds of materials for functional and structural applications. There is a surprisingly broad range of successful utilization of Laves phases in functional applications comprising Laves phases as hydrogen storage material (Hydraloy), as magneto-mechanical sensors and actuators (Terfenol), or for wear- and corrosion-resistant coatings in corrosive atmospheres and at high temperatures (Tribaloy), to name but a few. Regarding structural applications, there is a renewed interest in using Laves phases for creep-strengthening of high-temperature steels and new respective alloy design concepts were developed and successfully tested. Apart from steels, Laves phases also occur in various other kinds of structural materials sometimes effectively improving properties, but often also acting in a detrimental way.

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Laves phases: a review of their functional and structural applications and an improved fundamental understanding of stability and properties

Simulation of the kinetics of precipitation reactions in ferritic steels

The role of Laves phase on microstructure evolution and creep strength of novel 9%Cr heat resistant steels

In the recent two decades advanced martensitic/ferritic 9-12 % Cr steels are recognized to be the most potential materials for 650°C Ultra Super Critical (USC) Power Plants. The critical issues are the improvement of long-term creep strength and corrosion resistance. The aim of the present research is to design new super heat-resistant 12 % Cr martensitic/ferritic steels using basic principles and concepts of physical metallurgy, to test and optimize model alloys and to investigate and clarify their behavior under long-term creep conditions with emphasis on microstructural stability and corrosion resistance. Fine distributions of stable precipitates, which block the movement of subgrain boundaries (M 23 C 6 carbides, Laves phase) and dislocations (MX carbonitrides) and delay coarsening of microstructure, are the key to high creep strength of this type of steels. Therefore, different carbide, nitride and Laves phase forming elements (Cr, W, Nb, V, Ta, Ti) have been used to provide precipitation hardening. Furthermore, the aim is to produce a sequence of precipitates with different kinetics, i.e. with precipitation of a new phase during coarsening of the prior one. Co has been used for obtaining 100% martensite initially and for slowing down diffusion processes and particle coarsening. The partial replacement of Co by Cu is also investigated to reduce costs. The first results of mechanical tests of the studied model alloys have shown positive effects of the addition of W and Ta as Laves phase and MX forming elements, respectively, as well as of the addition of B. Alloying with Co has also shown beneficial effects on the creep strength.

Heat-resistant 650°C steels - Design and testing of model alloys

https://www.jstage.jst.go.jp/article/isijinternational1989/42/12/42_12_1505/_pdf

Martensitic/Ferritic Super Heat-resistant 650°C Steels - Design and Testing of Model Alloys

The demands for reduction of monetary and environmental costs of electricity generation have resulted in the development of a new class of coal-fired plants—Ultra Super Critical (USC) Power Plants. The higher steam parameters (operating temperature and pressure) of the new plants require new materials with improved long-term high-temperature properties. This has resulted in intensive world-wide activities in research and development of new steels. The new martensitic/ferritic super heat-resistant 9–12% Cr steels (always mass%) have been recognized as an alternative for widely used high-strength austenitic stainless steels because of their favorable physical properties (thermal expansion and heat conductance) and lower price. The critical issues are the improvement of long-term creep strength, as well as oxidation and corrosion resistance. As is well known, the increased temperature accelerates all degradation processes in steels, such as migration of atoms and dislocations, recovery and recrystallization, grain and subgrain coarsening, precipitation coarsening, corrosion etc. Therefore, under service conditions (elevated temperature and pressure) pronounced microstructural changes occur, leading to the degradation of long-term properties. Hence, improvement of microstructural stability is the key for decelerating degradation processes and obtaining excellent long-term creep strength. For this it is necessary to slow down the recovery of microstructure, provide enough solid solution and precipitation strengthening and slow down precipitation coarsening. A new German project (within the frame of the MARCKO program) has been initiated to develop a new super heat-resistant 12% Cr ferritic steel for application at 650°C and 300 bar in conventional power plants using basic principles and concepts of physical metallurgy, to test and optimise model alloys and to investigate and clarify their behaviour under long-term creep conditions with emphasis on microstructural stability and corrosion resistance. The aim is to investigate the effect of different types of precipitates (M23C6, MX, Laves phase), which block the movement of subgrain boundaries and dislocations and delay coarsening of microstructure, as well as the effect of alloying elements on creep behavior. In particular, the Laves phase is studied, which precipitates during service and which is to strengthen the alloys when M23C6 precipitate particles, besides finely distributed other carbides and nitrides, become less effecISIJ International, Vol. 42 (2002), No. 12, pp. 1505–1514

J. Vilk

Papers

Heat-resistant 650°C steels - Design and testing of model alloys

Martensitic/Ferritic Super Heat-resistant 650°C Steels - Design and Testing of Model Alloys

Martensitic/ferritic super heat-resistant 650 °C steels - Thermodynamics and kinetics of precipitation reactions

Design of model alloys for martensitic/ferritic super heat-resistant 650 °C steels