The fine-grained heat-affected zone (FGHAZ) specimens for P92 steel weld were produced by welding thermal simulator, then the simulated specimens were aged at 923 K for 0∼5000 h. The Laves phase parameters of FGHAZ were measured using SEM-back-scattered electron, and a two-dimensional (2D) precipitate/matrix model was used to calculate the stress-strain concentration at Laves phase/matrix interface by ANSYS. The results indicate that the maximum precipitated quantity of Laves phase in both FGHAZ and base metal (BM) is around 1.0%; however, the time to the saturated value is about 500 h shorter in the FGHAZ than in the BM, meanwhile. In fact, the differences in precipitation and coarsening of Laves phase between the FGHAZ and BM are closely correlated to the grain size. Furthermore, the matrix of FGHAZ is softened due to its polygonal sub-grains with low dislocation densities replacing the martensitic lath structure. The concentration of stress-strain at the Laves phase/matrix interface is larger in the FGHAZ than in BM, resulting from its softened matrix and coarser Laves phase, which contributes to the formation of creep voids on grain boundaries. Among these two factors, the softening of matrix has more significant influence on the formation of creep voids over the coarsening of Laves phase.

The precipitation and coarsening of Laves-phase in the fine grained heat-affected zone (FGHAZ) of a 9% Cr steel P92 welded joint during thermal aging at 923 K were investigated and compared to the base metal (BM), in order to clarify their effects on the Type IV fracture. Laves-phase precipitated mostly on the prior austenite grain boundaries of the FGHAZ. In comparison with BM, FGHAZ contained more grain boundary areas and can provide more nucleation sites for Laves-phase, resulting in an accelerated precipitation and rapidly reaching to the around 1.0% of saturated volume fraction. The coarsening of Laves-phase precipitates in FGHAZ was also much faster than that in BM, enhanced by the contribution of grain boundary diffusion resulted from its finer prior austenite grains. The FGHAZ had denser and smaller Laves-phase precipitates during the precipitation period in comparison with BM, obviously improved the creep strength by precipitation hardening. However, this effect in FGHAZ reduced sharply during coarsening owing to its coarsening rate greater than that of BM. In addition to the initial coarse polygonal subgrains with low dislocation density in FGHAZ produced by the weld thermal cycle and subsequent tempering in post-weld heat treatment (PWHT), coarse Laves-phase precipitates on grain boundaries formed in the long-term thermal aging, contributing to the formation of the creep cavities, can also play a key role in Type IV fracture of welded joint in 9% Cr steels.

•The characteristics of precipitation and coarsening of Laves-phase were determined.•The matrix depletion of W and Mo due to Laves-phase precipitation was quantified.•The effect of precipitated Laves-phase on the hardness was evaluated.Long term precipitation and coarsening of Laves-phase in tungsten strengthened 9% Cr steel under thermal aging at 923 K was investigated and reported in this paper. It experimentally measured the evolution of mean particle size, the number density, the volume fraction of Laves-phase precipitates, the partition coefficients of W and Mo in the matrix, as well as the change of hardness. Its main conclusions were: 1) Laves-phase nucleates and grows rapidly on grain boundaries and lath boundaries within the first 1500 h of aging time; 2) The two stages characteristics and kinetics of Laves-phase nucleation and growth which were determined experimentally; 3) The coarsening of Laves-phase is much faster than that of M23C6 carbides; 4) The precipitation of Laves-phase produces a pronounced matrix depletion of W and Mo atoms; and 5) The precipitated Laves-phase gives rise to weaker precipitation strengthening in comparison with M23C6 carbides, and causes the loss of hardness due to the depletion of Mo and W from the solid solution. This paper contributes to the knowledge of kinetics of Laves-phase precipitation and coarsening, providing the essential information for comparative investigation of creep damage mechanisms. This paper also contributes to the understanding the creep damage broadly.

The methods for reproducing fine grained heat affected zone of high Cr steel for creep test were investigated. This research was motivated by the need for reproducing uniform FGHAZ to investigate the creep damage process and untangle the coupling between thermal aging and cavitation (nucleation, growth, and coalesce), and the influence of states of stress. The two methods for producing FGHAZ were weld simulator and heat treatment in furnace. The microstructures, hardness, and creep rupture time under 923 K/100 MPa were investigated and compared with the actual FGHAZ of a weld joint. The FGHAZ specimens, re-produced by heat treatment in furnace, were also crept and compared with that of the base metal at 923 K/110 MPa and 100 MPa. It was found that (1) both the re-produced FGHAZ showed the similar microstructures and hardness as that of actual FGHAZ of a welded joint, (2) both the re-produced FGHAZ demonstrated the similar creep rupture time and it was shorter than that of a weld joint, and (3) in comparison with the base metal, the creep test of FGHAZ, reproduced by heat treatment in furnace, shows a short transitional period and early occurrence of a tertiary stage. It is concluded that (1) the uniform FGHAZ reproduced via a weld simulator is in the order of 10–15 mm long and is not big enough for creep test and (2) heat treatment in furnace can reproduce the representative FGHAZ required for creep research.

The creep rupture behavior and microstructure changes of W strengthened P92 steel weld joints have been investigated at 873 K, 898 K and 923 K. The joints were prepared by submerged arc welding (SAW). The results showed that low ductility type IV fracture took place more easily at higher temperature and lower stress. There would be a critical Larson–Miller parameter (LMP) of 35.5 and a critical applied stress of 120 MPa for type IV fracture. The critical stress was independent of creep temperature. Type IV cracks occurred in the fine grained heat affected zone (FGHAZ), corresponding to the maximum heating temperature just above Ac3, which showed the fine equiaxed microstructure without lath structure and the lowest hardness due to the instability of microstructure. An increased number density of Laves phases precipitated on grain boundaries in FGHAZ compared with other zones of weldment during creep, while the coarsening of M23C6 carbides was not very significant in W strengthened P92 steel. The fracture location in FGHAZ exhibited the most severe creep damage among the various zones of weldment and many cavities formed at the grain boundaries during creep. It was considered that the coarse Laves phase at the grain boundaries acted as the preferential cavity nucleation sites. We believe that the degradation of lath substructure and fast formation of Laves phase may be the main metallurgical factors for the type IV cracking.

•Near eutectic phase segregation was observed in Sn-15Bi solder joint.•Near eutectic phase layer segregates near the anode side.•Near eutectic phase segregation relies on the up-hill diffusion of Bi atom.•The rapid diffusion of Bi atom relies on the electron flow and temperature.The electro-migration test of Sn-15Bi solder joint was conducted at 150 °C with a current density of 160 A/cm2. The near eutectic phase layer thickening as the extending of electro-migration time was observed on the anode side interface of Sn-15Bi solder joint. The Bi fraction of cathode side reduced as the increase of electro-migration time. The Bi atom of near eutectic phase layer was from solid solution Bi atom in β-Sn phase. The diffusion of Bi atom was up-hill diffusion under the effect of electron flow.