Papers by Keyword: Microstructural Stability

Abstract: Applications of advanced heat resistant ferritic steels in boiler repairs require detailed information on the creep behaviour of welds made of various combinations of steels. The paper deals with the results of hardness and microstructure characterization of a dissimilar circumferential weld of 14MoV6-3 and P91 tubes after about 10 years of service exposure in a boiler operated at 580 °C and steam pressure of 10.3 MPa. The P91 tube (f38x4 mm) was welded to the 14MoV6-3 tube (f38x6.3 mm) using the GTAW (141) technology. Bőhler-FOX IN 9-IG (3Cr0.5Mo0.3V) wires were applied as a filler material. Microhardness evaluation after long-term service exposure revealed two carburized zones, values in these zones did not exceed 350 HV0.5. The slowdown of recrystallization in partially decarburized areas of the 14MoV6-3 and the WM suppressed the formation of soft ferritic bands along fusion lines. This phenomenon is related to the high thermodynamic stability of V(C,N) particles in vanadium-bearing low alloy heat resistant ferritic steels at temperatures below 600 °C.

Abstract: Co-28Cr-6Mo alloy is widely used in aerospace industry and biomaterial engineering where high demands are placed on material strength, fatigue endurance as well as corrosion resistance. As complex shapes and weight savings are required for such components, additive manufacturing is the ideal production way. For example, Selective Laser Melting (SLM) of metallic powders is capable of achieving complex components with fine details. However, Co-28Cr-6Mo alloy prepared by SLM shows a characteristic microstructure which causes its instability at elevated temperatures that can occur during operation. To eliminate such operational problem, the component must be appropriately heat-treated. This paper demonstrates the instability of Co-28Cr-6Mo alloy prepared by SLM and proposes the heat treatment yielding its stabilization.

Abstract: The role played by transition elements and rare earths, added in small quantities (fractions of unity in weight percent) was proved by several authors to be beneficial for the age-hardening response and for the microstructural stability at high temperature of Al-based alloys. The paper illustrates the results of an experimental investigation on the effects of erbium (Er) additions on the microstructural features, microstructural stability and mechanical properties of an Al-7Si-0.4Mg alloy of wide industrial use. The effects are multifold: in the cast microstructure, a silicon (Si) eutectic modification was observed with the formation of several Er-containing intermetallic phases. After solution treatment and aging, an increase in peak hardness and an enhanced microstructural stability were obtained. In this preliminary study, the beneficial effects of Er additions were confirmed.

Abstract: GH984G is a new Ni-Fe-based alloy which has been designed for use as superheater, reheater, and header materials for boilers in 700°C advanced ultra-supercritical (A-USC) coal-fired power plants. In this paper, the tensile properties and microstructure evolutions of GH984G were investigated during long-term thermal exposure at 700 and 750°C up to 15000h. The results show that the major precipitates were MC, M₂₃C₆ and γ ́, and no detrimental TCP or GCP phase appeared during long-term thermal exposure. The morphology of γ ́ was spherical and had no obvious change, but the size increases. After thermal exposure at 700°C for 15000h, the 700°C yield strength slightly increased, but the yield strength decreased after thermal exposure at 750°C for 15000h. The variation of strength is attributed to the coarsening of γ ́ precipitation. The deformation mechanism is the moving dislocations shear γ ́ precipitates and the stacking faults form in γ ́ precipitates at room temperature and 700°C. At 750°C, the deformation mechanism is characterized by the formation of dislocation loops and tangles.

Abstract: Tungsten is a promising armour material for plasma facing components of nuclear fusion reactors (NFR) because of its low sputtering rate and favourable thermo-mechanical properties (high melting point and good thermal conductivity). This paper reports some results of an experimental campaign carried out for investigating the microstructural characteristics and the mechanical properties of tungsten (99.97% purity; 5% porosity) for fusion applications. Tungsten has been heat treated at 500 °C and 800 °C with increasing soaking time. The samples in as-supplied condition and after each step of the heat treatments have been examined by optical microscopy and TEM observations, X-ray diffraction (XRD) and micro-hardness tests. The original material has a dislocation density of 1.5 x 10¹⁰ cm^-2 and a mean grain size of 65 μm. Grain size is not affected by the heat treatment at 500 °C which induces only a weak decrease of dislocation density leading to a little smaller hardness. The microstructure can be considered substantially stable even if a weak recovery of dislocations takes place. On the contrary, grain growth is observed after heating at 800 °C: 10 hours of treatment nearly doubles the average grain size.

Abstract: The effects of Re on solidus/liquidus temperatures, solidification segregation characteristics of alloying elements, γ′ morphology and volume fraction as well as microstructural stability at 1150 °C have been investigated in two experimental Ni₃Al-based single crystal superalloys with and without Re (1.0 wt%) addition. The results indicated that Re addition increased the solidus/liquidus temperatures. The segregation characteristics of Ni₃Al-based single crystal superalloys were similar with Ni-based single crystal superalloys, and the solidification segregation ratios of W and Mo increased slightly due to Re addition. Meanwhile, Re addition resulted in more negative lattice misfit, more cuboidal γ′ phase and higher γ′ volume fraction. The kinetics of γ′ coarsening and rafting at 1150 °C was lowered substantially compared with the addition of Re.

Abstract: This paper deals with microstructural stability of P23/P91 dissimilar welds during long-term creep exposure at 500°C. Detailed TEM investigations on minor phase evolution were performed on specimens after long-term creep exposure. Microstructural changes in the decarburized layer of the P23 steel were studied using EBSD and TEM. Experimental data on minor phase evolution were compared to the results of thermodynamic (Thermocalc) and kinetic (Dictra) simulations. Evolution of minor phases in the P23/P91 welds during creep at 500°C was in a good agreement with results of microstructural modelling. Partial decarburization of the P23 occurred at the expense of dissolving M₇C₃ phase. Microstructure of the partly decarburized zone in the P23 steel remained bainitic in all specimens investigated. In spite of the partial decarburization in the P23 steel during longterm creep exposure at 500°C, creep failures occurred in the ICHAZ of the P23 steel.

Abstract: The paper presents results of microstructural investigations of Mg-5Al-3Ca-0.7Sr-0.2Mn (ACJM53) magnesium alloys in the as-cast condition and after heat treatment at 450°C for 4.5 hours. Two kinds of transformation were observed in ACJM53 alloy after heat treatment: (Mg,Al)2Ca (C36) → Al2Ca (C15) and Al3Mg13(Sr,Ca) → Mg17(Sr,Ca)2 transformations without the change in morphology of output compounds. Morphology of Mg2Ca (C14) have been changed from fine lamellar to globular and precipitation process of Al2Ca (C15) phase inside the grains of solid solution was found.

Abstract: A low cost Ni-Fe-based wrought superalloy for 700 advance ultra-supercritical coal-fired power plants was developed. The stability of microstructure and mechanical properties of this alloy during long-term thermal exposure was investigated by SEM,TEM and tensile tests. The experimental results showed that the major precipitates in the alloy were spherical γ, MC and discrete M₂₃C₆ distributing along grain boundary after the long-term exposure at 700 and 750 and no harmful phases, such as σ phase and η phase, were found. However, after exposure at 800 up to 3000 h, small amount of lath-like η phase precipitated at grain boundary by consuming the surrounding γ. The η phase exhibited a fixed orientation relationship with the γ matrix. During thermal exposure γ coarsened with increasing the exposure time and exposure temperature. In addition, all major phases and their stability temperature ranges were calculated by JMatPro and these results were confirmed by the experimental results. The 700 tensile tests revealed that the alloy after exposure at 700 and 750 for 3000 h exhibited excellent ductility and strength. Therefore, the GH984G alloy possessed excellent stability of microstructure and mechanical properties between 700 and 750 up to 3000 h, and it is a promising material for 700 advance ultra-supercritical coal-fired power plants.

Abstract: Influences of aging on the creep rupture properties of super-clean 9%CrMoV steel and 1%CrMoV steel, the heat resistant steels for steam turbine rotors of thermal power plants, are investigated. Using the as-received and the aging-treated materials of the two steels, creep rupture tests are carried out at 566°C. Creep rupture lives, creep fracture modes as well as the microstructural changes of the specimens are examined. It is made clear that the creep strength and the microstructural stability of super-clean 9%CrMoV steel are superior to those of 1%CrMoV steel in long-term services.