| Corrosion fatigue (CF) is one of the most important failure modes of primary coolant pipe used in nuclear power plant (NPP), which attributed to the thermal cyclic stress caused by a variety of start-up and shutdown as well as transients when severed in high temperature water environment. The materials used in pressurized water reactor (PWR) NPP were the Z3CN20.09M duplex stainless steel (DSS) and 316LN stainless steel (SS). While due to the ferrite phase existed in the former, the precipitation of σ phase and the thermal aging were often occurred during the welding or heat treatment and the service process respectively. Moreover, the uneven grain size of 316LN SS was often aroused during the complex forging process. All these microstructure evolutions could have obvious effect on the mechanical, corrosion and CF properties of primary coolant pipe. In this study, the mechanisms of fatigue crack initiation and propagation were elucidated by investigating the composition and mechanical properties of oxide films formed in simulated service environment. The effects of grain size of 316LN SS, as well as the precipitation of σ phase and thermal aging of Z3CN20.09M DSS on the CF properties were investigated. And the related mechanisms of the effects of microstructure evolution on CF behavior were analyzed. The main results are as following:(1) The hot-working property of 316LN SS was investigated by Gleeble-1500 thermomechanical simulator. The effects of the forging and solution temperatures on the microstructure and mechanical property were studied. The result indicated that the best forging technology of 316LN SS was forged at 1100℃ and followed by solution treatment at 1150℃ for two hours, which can get excellent microstructure and mechanical property. After the above treatment, the grain size, elongation, tensile and yield strengths were 80μm,68%,601.5 and 266.2MPa respectively, which can meet the criterion formulated by Westinghouse Corporation.(2) The fatigue crack initiation behaviors of the two SSs in high temperature water environment were investigated by CF cracking test system. And the mechanisms of promoting the crack initiation by high temperature water for the two SSs were analyzed concentrating on the effect of the composition of oxide film. The experimental results indicated that the cracks were begin to be initiated at persistent slip bands (PSBs) during the first 20%of fatigue life of the 316LN SS tested in 320℃ water. While they were initiated successively at the PSBs, phase boundaries and pitting corrosion zones of the Z3CN20.09M DSS when tested in 290℃ water, and the cracks were found mainly to be initiated at the PSBs. Comparing to the high temperature air environment, the fatigue crack was initiated more and earlier when the specimen tested in high temperature water.(3) The effect of high temperature water on crack propagation was investigated, and the mutual promoted effect of oxidation by high temperature water and crack propagation at crack tip was analyzed. The acceleration of high temperature water on crack propagation can be attributed to the deteriorative mechanical property of oxide film at crack tip. Moreover, the ferrite phase can postpone the fatigue crack initiation and propagation of the Z3CN20.09M DSS. This can be attributed to it can hinder the extending of PSBs, which finally retarding crack initiation at this zone. On the other hand, the crack propagation was also postponed by better strength and corrosion resistance of ferrite phase at crack tip.(4) The effects of grain size on the fatigue life and crack initiation mechanism of 316LN SS in 320 ℃ water environment were investigated. The results indicated that the specimens with fine grains (30μm) own the longest fatigue lives than those with intermediate (80μm) and coarse grains (210μm). The fatigue stress amplitudes of the specimens increased with the grain refinement, although the fatigue lives of the specimens with intermediate and coarse grains were near. Fatigue cracks were often initiated at the PSBs and crystal orientation interfaces on the surfaces of the specimens with intermediate and coarse grains, while it was not easy to be initiated on the fine grain specimens. Moreover, the dislocation structures in the interior of specimens were consisted of numerous dislocation walls and cells for the specimens with fine grain as well as planar slip bands for the specimens with intermediate and coarse grains respectively.(5) The effect of precipitation of σ phase on the corrosion, mechanical and CF properties of the Z3CN20.09M DSS was investigated, and the combined effect of the deteriorative corrosion and mechanical properties on fatigue crack initiation mechanism was analyzed. The results indicated that the content of a phase increased, and the corrosion, mechanical and fatigue properties deteriorated with increasing the aging time at 750℃. Different to the as-received specimens, the fatigue cracks were often initiated at corrosion pitting point zone on the surface and were also initiated around the fractured ferrite in the interior of aged specimens. It can be say that the deteriorative corrosion and mechanical properties induced by the precipitation of σ phase was the main reason for the decreased fatigue life of the Z3CN20.09M DSS.(6) The effects of long time thermal aging at 475℃ on the corrosion, mechanical and CF properties as well as related mechanisms of the Z3CN20.09M duplex DSS were investigated. The results indicated that the corrosion and mechanical properties deteriorated with increasing the thermal aging time at the first 1000h, while they all slightly recovered when aging for 2000h. This means that the possible self-healing behavior was occurred during thermal aging process. Similarly, the fatigue life was also deceased with increasing the thermal aging time, while the difference was minor between the specimens aged for 1000 and 2000h. |
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