Advanced Heat Resistant Steels for Power Generation

The precipitates obtained by EPE technology from the 9Cr-1.7W-0.4Mo-Co ferritic heat-resistant steel subject to isothermal aging were investigated using SEM, TEM and XRD. The particle size distribution and the coarsening kinetics of M23C6 with duration of isothermal aging were also analyzed with or without consideration of Laves phase. The results show that the isolated dislocations were detected in delta ferrite interior, and the precipitates on delta ferrite and martensite boundaries are obviously larger than other locations. Fe2W-Laves phase can only be found as duration of aging time to 2000 h, and is preferential to form adjacent to M23C6 particles. The small M23C6 particles firstly coarsen, but the large M23C6 are relatively stable during short aging. The total coarsening rate of M23C6 precipitates is 9.75 × 10−28m3s−1, and the coarsening of M23C6 depends on the formation of Laves phase.

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Precipitates and Particles Coarsening of 9Cr-1.7W-0.4Mo-Co Ferritic Heat-Resistant Steel after Isothermal Aging.

The changes occurring in the microstructure and mechanical properties of interrupted G115 steel samples during creep at 650 °C and 140 MPa were systematically investigated. M23C6 carbides exhibited a low coarsening rate in the G115 steel upon the addition of boron. Cu-rich phase particles can significantly stabilize the microstructure in the early stages of creep. However, they gradually dissolved after ~ 1000 h. The geometrically necessary dislocation (GND) density decreased significantly during the transient creep stage and was relatively stable after 1045 h. Finally, it decreased rapidly. The mean width of the martensite lath structures increased slightly with an increase in the creep time from 0 h to 302 h, after which it dramatically increased, which is closely related to the evolution of Cu-rich phase particles. On the basis of the obtained experimental results, a microstructural-based yield strength model was developed for establishing the correlation between the microstructure and mechanical properties of G115 steel at different creep stages. The results calculated by the model were in good agreement with the experimental data.

A physical-based yield strength model for the microstructural degradation of G115 steel during long-term creep

FeCoNiCrAl high entropy alloys (HEAs) are regarded as promising coating materials for traditional steel. However, the brittle nature always contributes to the cracking of the equiatomic FeCoNiCrAl HEA coating. In this work, FeCoNiCrAl coatings on C45 carbon steel and 304 stainless steel (304SS) pipes were fabricated using extreme high-speed laser cladding (EHLA) with a cladding speed of 50 m/min. The results showed the high-hardness character of the two coatings on C45 and 304SS substrates. The C45 substrate showed a remarkable effect on the cracking of the coating. The martensitic transformation of the heat-affected zone (HAZ) of the C45 substrate was considered the main factor contributing to cracking of the coating. The finite element method showed that the martensite transformation of C45 expanded the volume and increased the hardness of the HAZ. This restricted the plastic deformation of the substrate and increased the tensile stress and cracking tendency of the coating. As a martensitic transformation did not occur in 304SS, the low-hardness substrate effectively relieved the tensile stress in the coating and avoided cracking.

Cracking mechanism of brittle FeCoNiCrAl HEA coating using extreme high-speed laser cladding

The effect of a prior short-term ageing on mechanical and creep properties of P92 steel

Laves-phase evolution during aging in 9Cr-1.8W-0.5Mo-VNb steel for USC power plants

Laves-phase evolution during aging in fine grained heat-affected zone of a tungsten-strengthened 9% Cr steel weldment

Martensitic filler metals with low transformation temperatures can efficiently mitigate harmful tensile welding residual stress. It is vital to clarify the difference in the influence of martensitic transformation for different shapes of welded joints. This article outlines the influence of martensitic transformation in a butt-welded plate and a butt-welded pipe which were designed to have the same dimensions of the cross-section perpendicular to weld line. A clear difference in the influence of martensitic transformation was found in these two joints. Longitudinal tensile stress in weld zone is efficiently reduced in both joints, whereas longitudinal tensile stress is formed in base metal near the weld zone in the pipe. A notably greater influence on transverse stress is found in the pipe than in the plate.

Influence of martensitic transformation on welding residual stress in plates and pipes

Microstructure, mechanical properties and residual stress of a 2205DSS/Q235 rapidly formed LBW joint

A nonlinear ultrasonic technique has been used to characterize damage evolution in structural steel subjected to tensile loading. The results show that the nonlinear parameter β′ increases monotonously with increasing tensile strain within its tensile strength. Compared with the original specimen, the nonlinear parameter rose by nearly 3.37 times when stretched to its tensile strength. This result indicates that the nonlinear ultrasonic technique is able to characterize the material damage at the early stages, which can become an effective means for improving the integrity and safety of the structural component.

Lei Hu

Papers

Laves-phase evolution during aging in 9Cr-1.8W-0.5Mo-VNb steel for USC power plants

Laves-phase evolution during aging in fine grained heat-affected zone of a tungsten-strengthened 9% Cr steel weldment

Influence of martensitic transformation on welding residual stress in plates and pipes

Microstructure, mechanical properties and residual stress of a 2205DSS/Q235 rapidly formed LBW joint

Application of Nonlinear Ultrasonic Technique to Characterize the Damage Evolution in Structural Steel after Tensile Deformation