-Laves-phase precipitation and coarsening of tungsten strengthened 9% Cr steel under thermal aging at 923K up to 8000 h was reported in this paper, where the information of the evolution of mean particle size, the number density, the volume fraction of Laves-phase precipitates and the partition coefficients of W and Mo in the matrix over the aging time were determined. The change of hardness over time was also measured. The main results of this investigation were: 1) Laves-phase nucleates and grows rapidly on grain boundaries and lath boundaries within the first 1,500 h of aging time; 2) the coarsening of Laves-phase is much faster than that of M 23 C 6 carbides, meanwhile it demonstrates the two stages characteristics and kinetics of Laves-phase nucleation and growth which were determined experimentally; 3)The precipitation of Laves-phase produces a pronounced matrix depletion of W and Mo atoms; and 4) The precipitated Laves-phase gives rise to weaker precipitation strengthening in comparison with M 23 C 6 carbides, and causes the loss of hardenss and creep strength due to the Corresponding author. Tel.: +86 13554693820; fax: +86 02768772253. E-mail address: wxue2011@whu.edu.cn (Xue Wang) 2 depletion of Mo and W from the solid solution. This paper contributes to the knowledge of kinetics of Laves-phase precipitation and coarsening, as well as understanding the creep damage broadly.
Abstract-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.
This paper mainly discusses the technique of using quenching at 1000°C and tempering at 650°C for 2 hours to replace the heat treatment process of quenching at 1225°C and tempering at 560°C for 1 hour for three times.Due to the old heat treatment process is traditional multifarious waste energy and relatively backward.The experiment results prove that after the new treatment using the W6Mo5Cr4V2 high-speed steel to make cutting tools such as cutters, drill, taps and so on. It aslo achieves better abrasion resistance and ductility. This meeting the objective of improving the wear resistance, toughness, lifespan enhancement and energy conservation.It opens up a new way of simple and energy saving for W6Mo5Cr4V2 high-speed steel heat treatment process.The material of W6Mo5Cr4V2 high-speed steel has certain value to research.
Type IV failure, the key life time limiting factor for high Cr steel strengthened with W steel such as P92, occurs in the fine grain heat affected zone (FGHAZ) of a weld. However, the actual size of FGHAZ in a weld is too small (1-2mm), it is necessary to re-produce sizeable uniform specimen for creep tests (such as smooth and notched bar) in order to investigate its creep properties and micro-structural evolutions. This paper investigated two simulation methods for the re-production of FGHAZ, e.g. weld simulator and heat treatment in furnace. The microstructures, hardness, and creep rupture time under 923K/100MPa were investigated and compared with the actual FGHAZ of a weld joint. The dimensional big enough FGHAZ specimen, re-produced by het treatment in furnace, were also crept at 923K/110MPa and 100MPa and compared with that of the base metal. It was found that 1) both the simulated FGHAZ show the similar microstructures and hardness as that of actual FGHAZ of a weld joint, e.g. small grain size (less than 10m), the disappearance of martensitic lath and the formation of equiaxed grain, and a lowering micro-hardness of 30-40HV , 2) both the simulated FGHAZ demonstrate the similar creep rupture time as that of actual FGHAZ of a weld joint and it is shorter than that of base metal, which is similar to others' finding, 3) in comparison with the base metal, the creep test of FGHAZ reproduced by heat treatment in furnace show a short transitional period and early occurrence of and a prolonged tertiary stage, indicating a deterioration of creep strength. It is concluded that 1) the uniform FGHAZ reproduced via a weld simulator is in the order of 10 to 15 mm long and is not longer enough for creep test; 2) heat treatment in furnace can reproduce the representative FGHAZ required for creep research and the difference of the thermal cycle experienced in a actual welding and heat treatment in furnace is minor and negligible.
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