The complex nitride Z-phase, Cr(V,Nb)N, has recently been identified as a major cause for premature breakdown in creep strength of a number of new 9-12%Cr martensitic steels, especially the high Cr variants. A thermodynamic model of the Z-phase has been created based on the Thermo-Calc software. The model predicts the Z-phase to be stable in all of the new 9- 12%Cr martensitic steels, and this has generally been confirmed by experimental observations. Z-phase precipitation seems then to be a kinetic problem, and driving force calculations, using Thermo-Calc with the developed model, have been used to predict steel compositions, which could delay Z-phase precipitation. The model also predicted the existence of a new niobium free Z-phase variant, which has since been discovered in a niobium free 12CrMoV steel.

New Monte Carlo models have recently been developed to predict microstructural evolution in steels and aluminum alloys during heat treatment and high-temperature service. These models can control precipitate type and size distribution, distinguishing between pure lattice and grain boundaries. Consequently, they can forecast the precipitate size distribution within grains and on grain boundaries as a function of time. This paper describes the model validation for ferritic Fe-9Cr P92 steels. The model provides new information over a range of time intervals adding up to the total plant lifetime in an ultra-supercritical plant. This information can be incorporated into continuum damage mechanics models for predicting creep rate and stress rupture life. The paper discusses how this technique is used as a materials development tool to forecast necessary compositional modifications for improving creep properties in ferritic steels.

Materials are developed and improved by adjusting both the alloy chemistry and the processing conditions to achieve desired microstructures and properties. Traditionally, these improvements have been made by a slow and labor-intensive series of experiments. But it is now possible to replace this expensive trial and error process by carrying out only a few ‘key’ experiments in conjunction with thermodynamic calculations. These calculations are powerful tools for alloy design, enabling improvement in the selection of alloy chemistry and the parameters used for fabrication steps such as heat treatments. In order to have the utmost confidence in the results obtained from the calculations, it is essential to have high quality thermodynamic databases. Such databases can be used not only in phase equilibrium calculations but also as the critical input for further kinetic simulations. In the present paper, we present our work on the development of reliable thermodynamic databases for nickel-based superalloys and iron alloys. We first briefly describe the methodology of developing these databases and then discuss some specific examples using the databases. With the aid of these examples, the usefulness of thermodynamic databases in aiding the development of advanced materials is discussed.

The Institute of Materials Science, Welding and Forming (IWS) conducts research activities on ferritic/martensitic 9-12% Cr steels through an interconnected network of projects. These projects focus on mechanical properties of base and weld metals, microstructural characterization of creep and damage kinetics, weldability, microstructure analysis during creep, modeling of precipitation and coarsening kinetics, and deformation behavior under creep loading. The individual projects are briefly described, outlining the conceptual approach towards quantitatively describing the creep behavior of 9-12% Cr steels. The research efforts aim to comprehensively understand and model the creep performance of these advanced steel grades by investigating their microstructural evolution, damage mechanisms, precipitation kinetics, and deformation characteristics under creep conditions. The integrated projects examine both base metals and welded joints, providing insights into material properties, weldability, and microstructure-property relationships critical for their application in high-temperature components.

A novel multi-component, multi-particle, multi-phase precipitation model is used to predict the precipitation kinetics in complex 9-12% Cr steels investigated within the European COST project. These steels are used for tubes, pipes, casings and rotors in USC (ultra super critical) steam power plants for the 21 st century. In the computer simulations, the evolution of the precipitate microstructure is monitored during the entire fabrication heat treatment including casting, austenitizing, several annealing treatments. The main interest lies on the concurrent nucleation, growth, coarsening and dissolution of different types of precipitates.

TenarisDalmine, in collaboration with CSM, developed and characterized ASTM Grade 91 steel tubes and pipes for high-temperature boilers in ultrasupercritical power plants. This paper details the mechanical properties of both untreated and long-term exposed tubes. Extensive creep testing allowed extrapolation of stress values for rupture in 100,000 hours (93 MPa at 600°C) and 1% elongation in 250,000 hours (83 MPa at 600°C), satisfying ECCC and TÜV requirements. Additionally, STEM analysis investigated microstructure and precipitate evolution after long-term exposure, revealing changes in precipitate chemistry, size, and distribution.

Creep behavior and degradation of creep properties of advanced 9-12%Cr ferritic steels are phenomena of major practical relevance, often limiting the lives of power plant components and structures designed to operate for long periods under stress at elevated temperatures. Because life expectancy is, in reality, based on the ability of the material to retain its high-temperature creep strength for a period of at least twice the projected design life, methods of creep property assessment based on physical changes in the material that are likely to occur during service exposure rather than simple parametric extrapolation of the short-term data are necessary. This work attempts to highlight the problem areas just in this respect. The proposed approaches are illustrated by recent experimental results on advanced high creep strength 9-12%Cr ferritic- martensitic steels (P91 and P92).

High precision stress relaxation tests (SRT) at temperatures between 550C and 700C were performed on serviced and reheat treated T91, 9%Cr steel. The service exposure was 116,000 hours at steam temperatures to 550C. Constant displacement rate (CDR) tests were also run at 600C on notched specimens for the two conditions. Specimens, heat treated after service, were stronger at the lower test temperatures in terms of both tensile strength and creep strength. This difference was reflected in the CDR results, which also suggested a lower fracture resistance in the heat treated condition. Thus, service exposure appears to have softened the alloy and enhanced its resistance to fracture, with no evidence of embrittling reactions. Based on the analysis of the SRT tests, projections were made of the times to 1% creep and the times to rupture as well as direct comparisons with minimum creep rate data'. When plotted on the basis of a Larson- Miller parameter (C=30), the calculated values compared well with actual long time rupture testing for exposed and re-heat treated specimens, and generally showed higher precision. The longest test time was about eighteen months for the stress rupture data compared with the use of one machine for a few weeks for the SRT data. The latter actually covered a far greater range of creep rates and projected lives. The SRT test is especially consistent at higher parameter values, i.e., higher temperatures and/or lower stresses. This method of accelerated testing is now being applied to a wide range of alloys for fossil power plants for composition and process optimization, design analysis, and life assessment.

The effect of Cr content on the creep strength at 650°C was examined with high Cr heat resistant steels for the USC high-temperature rotor shafts. The amount of Cr was varied from 8.5% to 11.5%, and then, the alloying effect of Cr was investigated on the stability of the precipitates at 650°C. Within the present range of the Cr content, the short-term creep rupture life under the higher applied stress increased with the Cr content in the steels, whereas the long-term creep rupture life under the lower applied stress decreased with the Cr content in the steels. For example, under the applied stress of 98MPa, the 9%Cr steel exhibited the longest creep rupture life among the experimental steels. Also, it was found from the experiment using the extracted residues that the degree of solution strengthening and the sorts of precipitates scarcely changed regardless of the Cr content in the steels. The Laves phase precipitated finely in the lath was enlarged in the 11.5%Cr steel even after a short-term creep. This result indicates that the coarsening of precipitates such as the Laves phase promotes the recovery of the lath in the early stage of creep deformation. It was suggested that 9%Cr is desirable content in the ferritic steel for suppressing the degradation of creep strength in 98MPa at 650°C.

Creep deformation behavior of the T122 type steels with different matrix phases such as α’ (martensite) and α’+δ (martensite and delta-ferrite) at different stress levels has been studied comparing with those of the model steels with the initial microstructures consisting of the various combination of matrices such as ferrite (α), martensite (α’) and austenite (γ), and precipitates such as MX and M 23 C 6 . The heterogeneous creep deformation is found to be pronounced at lower stress level in the steel with a dual phase matrix of α’+δ, resulting in a complex sigmoidal nature in the creep rupture life. The creep deformation process of the steel with the dual phase matrix is similar to that of the model steel with the α phase matrix which exhibits a typical heterogeneous creep deformation and the early transition to the acceleration creep at a very small creep strain. Such a heterogeneous creep deformation is much pronounced along the interfaces between the soft δ ferrite and the hard martensite (α’) phases, and has a viscous nature in creep deformation which was first identified in P91 steel. It is concluded that the homogeneous microstructure is a key for achieving the long-term creep strength in the advanced ferritic steels at elevated temperatures over 600°C.

Effects of Ni content and heat treatment condition on impact toughness and creep strength of precipitation strengthened 15Cr ferritic steels were investigated in order to discuss a possibility of improvement in both mechanical properties. Both creep strength and impact toughness of the developing steels were improved drastically by solid solution treatment with water quenching. However, an addition of Ni reduced the long-term creep strength of the steels, though Ni was effective in improvement in impact toughness. It was found that water quenching suppressed formation of coarse block type particles and precipitate free zones around them, and precipitation of plate type fine particles and thermal stability of them within ferrite phase were promoted by solid solution treatment with water quenching. However, martensite phase with sparsely distributed coarse block type particles were formed in the Ni added steels, and such microstructure reduced the precipitation strengthening effect slightly. On the other hand, increase in impact values of the steel indicated no relation to volume fraction of martensite phase. It was supposed that the impact toughness of ferrite phase itself was improved by solid solution treatment and addition of Ni.

The development of 9-12% chromium steels during the last twenty years is reviewed. The significant increases in creep strength that have been achieved by minor alloying additions of V, Nb, W, Mo, N and B are discussed and the mechanisms by which the individual elements contribute to the long-term creep strength are evaluated. The basic strengthening is provided by the martensitic transformation that allows the formation of a sub-grain structure from the martensite laths. The sub-grain boundaries are stabilized by precipitates, mainly M 23 C 6 ; within the sub-grains, fine nitride and carbonitride precipitates interact with dislocations, thereby enhancing the strength. The relative contributions of the martensitic transformation and the various precipitates to the overall creep strength of the steels are assessed. Of particular importance for the long-term creep strength is the stability of the microstructure, especially the time dependent coarsening of the various precipitates and the possible formation of additional phases, such as Laves phase (Fe 2 (W,Mo) and the Z phase (CrNbN). It is shown that microstructural changes that occur during exposure at anticipated service temperatures have a large impact on the strength and these changes must be taken into account in the derivation of long-term design stresses. Finally, the potential for achieving further increases in the creep strength of 9-12% chromium steels is discussed, especially in view of the need for higher chromium contents to ensure adequate steam oxidation resistance.

Effect of thermomechanical and magnetic treatment on creep characteristics of advanced heat resistant ferritic steels for USC power plants has been investigated to explore fundamental guiding principles for improving creep rupture strength at elevated temperatures over 600°C. A model steel with a composition of Fe-0.08C-9Cr-3.3W-3Co-0.2V-0.05Nb-0.05N-0.005B-0.3Si-0.5Mn (in mass%) has been prepared by vacuum induction furnace. Creep tests at 650 °C and microstructural observations were performed on the thermomechanical and magnetic treated specimens after tempering. New thermomechanical treated samples without magnetic field showed some improvement in creep strength comparing with ordinarily normalized and tempered specimens. Further improvement was observed in the specimen that had been exposed to a magnetic field during transformation into the martensite. From the result of microstructural observation, it was found that the finely distributed precipitates such as MX and M 23 C 6 caused this improvement. And it was suggested that the magnetic treatment at martensitic transformation increase the precipitation sites during tempering, resulting in increasing the amount and preventing the growth of the precipitates.

Significant developments have been made in recent years in the description of creep as a damage process. Advanced martensitic steels that expend over half their lives in the tertiary creep stage have been the focus of attention since their use temperatures are being extended to higher temperatures. Data available for assessing the predictability of the damage models are somewhat scarce since long-time exposure testing has generally been for constant temperature and load conditions. In this work, data are reviewed from relatively long-time tests that involved temperature and stress changes. The prediction of several damage models are compared to material behavior. Most of the comparisons are for 9Cr-1Mo-V steel in the temperature range of 538 to 649 C and for times in the range of 10,000 to 80,000 hours.

This paper investigates the cause of premature failures in certain Grade 91 steel components used in UK power plants. The failures were linked to both low material hardness and specific chemical compositions that fell within ASTM specifications but had a low nitrogen-to-aluminum ratio (N:Al). The investigators examined eight material batches, including those involved in failures, new stock, and in-service components with similar properties. Testing confirmed these materials had lower creep resistance compared to standard Grade 91 steel. Microscopic analysis revealed the presence of large aluminum nitride precipitates, which limited the formation of beneficial vanadium nitride precipitates, leading to reduced creep strength. These findings suggest that even within the ASTM specification limits, a low N:Al ratio can negatively impact the performance of Grade 91 steel.

The demand for higher efficiency and reduced emissions in coal-fired power boilers will result in the use of higher steam temperatures and pressures. A significant materials effort is required to reach a target steam condition of 760°C/35MPa. These new Ultrasupercritical (USC) units will require the use of nickel-based superalloys. Long-term creep strength will be a determining factor in achieving the highest possible steam conditions. To this end, the creep strength of commercially available (Haynes 230), modified/controlled chemistry (CCA617/Maгco 617), and new (INCONEL 740) alloys, including weldments, are being investigated at Oak Ridge National Laboratory (ORNL). Creep tests at ORNL show that the CCA617 provides a significant improvement in strength over the standard alloy 617 at 650°C to possibly 750°C. The strength of alloy 230 is well characterized, thus the testing on 230 has focused on specific specimen configurations for evaluating the high temperature behavior of weldments. Creep testing on INCONEL alloy 740 has shown good strengths (higher than 230 or CCA617) that may meet the target steam conditions. Microstructural analysis by electron microscopy on aged and tested material is being used to further understand the structure-properties relationship in these materials and determine long-term stability of the microstructures.

This study examines the deformation behavior of P92 steel (ferritic, 9% Cr) at high temperatures (600°C to 900°C) using isothermal hot tensile tests. Particular focus is placed on the stress-strain behavior around its alpha-gamma transition temperature (825°C). Additionally, fire accident simulation heating tests were conducted to assess the integrity of P92 beyond 650°C (relevant for short-term creep) and compare it to stainless steel 1.4404 (potential building material). Finally, microstructural analysis was performed on tested samples, revealing that the martensitic structure with characteristic laths was retained at temperatures up to 750°C.

This paper explores the low cycle fatigue (LCF) and creep-fatigue properties of a hot-forged, normalized, and tempered 9Cr-1Mo ferritic steel. This steel offers good performance in high-temperature applications (up to 873K) in power plants and reactors. The steel was forged into 70 mm diameter rods and then heat-treated with normalizing (1313K for 1 hour, air cooling) and tempering (1033K for 1 hour, air cooling). LCF tests were conducted at 300-873K with varying strain amplitudes and strain rates to understand the influence of both factors. Additionally, some specimens were aged at different temperatures for 10,000 hours before testing. Finally, creep-fatigue interaction tests were performed at 823K and 873K using tensile hold times ranging from 1 to 30 minutes.

Tempered martensitic 9-12%Cr steels bearing tungsten, such as P92 and P122 showing higher creep rupture strength than the conventional steel P91, have been developed for thick section components in ultra-supercritical (USC) boilers. However, their creep strength is not sufficient for applying at the steam condition of 650°C/35MPa or above, which is a recent target condition in order to increase plant efficiency. The research and development project in NIMS on advanced high-Cr steels which can be applied at the steam condition of 650°C/35MPa as boiler components with large diameter and thick section has been carried out since 1997. In this project, it has been revealed that the addition of boron more than 0.01 mass% to the 0.08C-9Cr- 3W-3Co-V,Nb-<0.00ЗN steel remarkably improves creep strength. The boron enriched in M 23 C 6 carbides near prior-austenite grain boundaries suppresses coarsening of these carbides during creep deformation, leading to excellent microstructural stability and creep strength. Further improvement of creep strength is achieved by the addition of appropriate amount of nitrogen which enhances precipitation of fine MX. Excess addition of nitrogen to the high-B containing steel reduces creep rupture lives and ductility. The highest creep strength is obtained in the 0.08C-9Cr-3W-3Co-0.2V-0.05Nb-0.0139B-0.0079N (mass%) steel, resulting in excellent creep strength in comparison with that of P92 and P122. This steel shows good creep ductility even in the long term. It is, therefore, concluded that this high-B bearing 9Cr-3W-3Co-V,Nb steel with the addition of nitrogen in the order of 0.008 mass% is the promising candidate which shows superior creep strength without impairing creep ductility for thick section components in the 650°C-USC plant.

The effect of a reduced-temperature austenization treatment on the microstructure and strength of two ferritic-martensitic steels was studied. Prototypic 9% and 12% Cr steels, modified 9Cr-1Mo (ASME T/P91) and Type 422 stainless (12Cr-1Mo-W-V), respectively, were austenized at the standard 1050°C and an off-normal 925°C, both followed by tempering at 760°C. The reduced austenization temperature was intended to simulate potential inadequate austenization during field construction of large structures. The microstructure, tensile behavior, and creep strength were characterized for both steels treated at each condition. While little change in microstructure was observed for the modified 9Cr-1Mo steel, the creep strength was reduced at higher temperatures and in long duration tests. The microstructure of the Type 422 stainless in the off-normal condition consisted of polygonized ferrite instead of tempered martensite. In this case the creep strength was reduced for short duration tests (less than ~1000 hr), but not for long duration tests. Slight reductions in tensile strength were observed at room temperature and elevated temperatures of 450,550, and 650°C.