Directional coarsening of the γ' phase (rafting) in Ni-based single crystal superalloys during creep at 1273 K was simulated by the phase-field method. The inelastic strain introduced in the γ phase was assumed to be composed of plastic strain (ε p ) and creep strain (ε c ). The simulations were performed with various sets of values of material parameters and the magnitude of external tensile stress. We let a feed-forward neural network learn the simulation data in order to enable fast and exhaustive prediction of the time to rafting, t raft . From the analysis based on the trained neural network, it has been shown that t raft becomes longer with increasing magnitude of γ/γ' lattice misfit, with decreasing creep coefficient, and with increasing yield stress of the γ phase (σγ ys ). The sensitivity of t raft to σ γ ys is high when the ratio of ε p to the total inelastic strain (ε p + ε c ) is high.

The paper deals with microstructural evolution in the AISI 316LN + 0.1 wt.% Nb steel during long-term creep exposure at 600 and 625°C. The following minor phases formed: Z-phase (NbCrN), M 23 C 6 , M6X (Cr3Ni2SiX type), η-Laves (Fe2Mo type) and σ-phase. M6X gradually replaced M 23 C 6 carbides. Primary Z-phase particles were present in the matrix after solution annealing, while secondary Z-phase particles formed during creep. Precipitation of Z-phase was more intensive at 625°C. The dimensional stability of Z-phase particles was excellent and these particles had a positive effect on the minimum creep rate. However, niobium also accelerated the formation and coarsening of σ-phase, η-Laves and M6X. Coarse particles, especially of σ-phase, facilitated the development of creep damage, which resulted in poor long-term creep ductility.

Structural changes in P92-type steel after creep at temperature of 600°C under a stress of 140 MPa were investigated. The steel was solution treated at 1050°C and tempered at 780°C. The structure in the grip portion of the creep specimen changed scarcely after creep exposure for 6876 h. In contrast, the structural changes in the gage and neck sections were characterized by transformation of the tempered martensite lath structure into relatively coarse subgrain structure. The formation of a well-defined subgrain structure in the gage and neck sections was accompanied by the coarsening of M 23 C 6 carbides and precipitations of Laves phase during creep. Mechanisms of grain boundary pinning by precipitates are 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.

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.

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.

This study explores methods to enhance the creep strength of 12%Cr martensitic/ferritic steels. The approach focuses on utilizing various precipitates to hinder microstructure coarsening and dislocation movement. A combination of Laves phase (slow precipitation) and MX carbonitrides (dislocation pinning) is used for sustained strengthening. Different MX-forming elements (V, Ta, Ti) are investigated to identify the optimal combination for high quantities of finely distributed strengthening particles. Additionally, cobalt and copper are employed to promote a fully martensitic microstructure and potentially slow down diffusion or provide nucleation sites for Laves phase precipitation. Long-term creep tests confirm the effectiveness of Laves phase precipitation, particularly with tungsten present. Tantalum's influence on both MX precipitation and the Laves phase is also observed. Combining multiple MX-forming elements (V/Ta, V/Ti, Ta/Ti) further improves creep strength, supported by predictions of high MX carbonitride formation from Thermo-Calc calculations. Partially replacing cobalt with copper (1%) also demonstrates positive effects on creep properties.