| There are significant differences in chemical composition,microstructure and mechanical property among low alloy steel,austenitic stainless steel and stainless steel or nickel base alloy weld metals in the safe-end weld joint of reactor pressure vessel(RPV)in pressurized water reactors(PWRs).The composition diffusion and welding heat effect between different materials in the welding process can lead to the formation of microstructures different from base metals in the weld joint.Also,it is easy to introduce welding defects in the welding process of dissimilar metals.These factors make the safe-end weld joint become the weak part in the primary water system.The safe-end weld joint is subjected to high temperature,high pressure,coolant and mechanical load during long-term operation,which possibly cause corrosion,creep,fatigue,stress corrosion cracking(SCC),environmentally assisted fatigue(EAF)or other damages.The austenitic alloys and its welding metals in the safe-end weld joint of Generation ? and Generation ? PWRs were studied in this paper.The EAF tests of austenitic stainless steel,austenitic stainless steel weld metal and nickel-base alloy weld metal were carried out,focusing on the influence of material factors,mechanical factors and environmental factors on EAF performance.The EAF tests aim to accumulate EAF life data of austenitic alloy and its weld metals and further to reveal EAF damage mechanism and differences between austenitic alloy and its weld metals,so as to provide guidance for life assessment and safety design of domestic nuclear power plants.The effect of grain boundary engineering(GBE)on EAF behavior of 316LN stainless steel in high-temperature pressurized water was studied.The as-received(AR)316LN stainless steel is in solution annealed state.Two types of GBE specimens with different fraction of low-? CSL boundaries labeled as GBE and Non-GBE respectively were obtained by a GBE treatment from the AR 316LN material.The average grain size of the AR,GBE and Non-GBE specimen is 64.8?m,21.1?m and 22.1?m respectively,and the corresponding fraction of low-? CSL boundary is 25.7%?82.6%and 41.7%respectively.The fatigue lives of the AR,GBE and Non-GBE specimens decreased significantly in high-temperature pressurized water,regardless of GBE treatment or grain size.The EAF lives of the GBE specimens are generally 20%higher than those of the AR specimens.The EAF lives of the GBE and Non-GBE specimens are comparable.The longer EAF life for the GBE specimen compared to the AR specimen is attributed to grain refinement rather than increased fraction of low-? CSL boundaries obtained from GBE treatment.Microstructural small cracks(MSCs)initiates predominantly in a manner of transgranular fracture with some intergranular feature,including crack initiation along the random high angle boundaries(RHABs)and crack initiation along the ?3 grain boundaries,and propagates in a transgranular manner for the AR,GBE and Non-GBE specimens in high-temperature pressurized water.The fraction of MSCs initiation along the?3 boundary increases with increasing the fraction of the ?3 boundary.The ?3 boundaries are not superior to the RHABs in preventing fatigue crack intergranular initiation.The growth of MSCs is sensitive to microstructure.Grain refinement can prevent fatigue crack from initiation along the grain boundaries and decrease the propagation rate of MSCs.The EAF behavior of 308L weld metal in the safe-end weld joint of Generation? PWRs in high-temperature pressurized water was studied.The EAF life was comparable for 308L weld metal in high-temperature water with 0.005 ppm DO and 0.1 ppm DO,respectively.The EAF life of 308L weld metal decreased slightly with decreasing the strain rate from 0.04%/s to 0.004%/s.The EAF effects were more slightly pronounced for 308L weld metal at a low strain amplitude of 0.3%or 0.4%than that at a high strain amplitude of 0.6%or 0.8%.The columnar crystals and associated dendrite boundary orientation of 308L weld metal affected the EAF crack initiation and propagation process.The long crack was prone to transgranular initiation and propagation when dendrite boundary orientation was not well perpendicular to cyclic loading direction.The cracks were prone to intergranular initiation and propagation along the dendrite boundary perpendicular to cyclic loading direction.The typical cracking along ?/? phase boundary morphology was not found on the fracture surface,which indicated that the EAF main crack mainly initiated and propagated across the ?-ferrites,and the effect of strain rate on the EAF behaviors of 308L weld metal were not significant,which indicate that the ?-ferrite in 308L weld metal mainly plays the role of inhibiting crack growth in EAF cracking process.Differences of EAF behaviors among 316LN base metal(BM),316LN heat-affected zone(HAZ)and 308L weld metal(WM)in the safe-end weld joint of Generation ? PWRs in high-temperature pressurized water were studied.The fatigue lives of 316LN HAZ specimen in room-temperature air are relatively longer than those of 316LN BM specimen.Of all three kinds of specimens,the 308L WM specimen shows the worst resistance to fatigue in air.The fatigue lives of 3 16LN BM,316LN HAZ and 308L WM specimens were remarkably reduced in high-temperature water in comparison with those in room-temperature air.The 316LN HAZ shows the highest EAF effect and the 308L WM has the lowest EAF effect.The poor EAF performance of 316LN HAZ specimen is mainly because of the higher residual strain and poor corrosion resistance in comparison with 316LN BM specimen.The effect of?-ferrite in 308L WM played an important role in preventing fatigue crack growth in the EAF process in high-temperature water,where the higher Cr content in 308L WM and Cr-rich oxide film formed at ?-ferrites have higher corrosion resistance and further reduce the EAF damage.The EAF behavior of Alloy 52/52M nickel-base alloy weld metal in the safe-end weld joint of Generation ? PWRs in high-temperature pressurized water was studied.The results indicate that the grain boundaries of Alloy 52/52M weld metal are mainly RHABs and low angle boundaries.The welding residual strain mainly distributes in 52Mw and at the interface of 52b and 52Mw,especially near the RHABs in these regions.The welding columnar grains of 52b and 52Mw are highly elongated of 1-2 mm long and 100-400?m wide,with the orientations parallel to the cyclic loading axis for the 52b and at a 45°-75° angle to the cyclic loading axis for the 52Mw.The DDC cracks mainly locate in 52Mw and at the interface of 52b and 52Mw.The larger welding residual strain in 52Mw and near the interface promotes the formation of DDC defects at the straight RHABs in 52Mw and at the interface.Two types of DDC defects can be identified,namely,open DDC and closed DDC defects.The closed DDC is defined as the not fully open DDC crack,whose grain boundary interfaces on both sides could have a good combination.The fatigue life of Alloy 52/52M decreased in high-temperature pressurized water in comparison with that in air,the EAF effects were significant under the condition of lower strain amplitudes.The EAF lives of Alloy 52/52M weld metal in high-temperature water were comparable with decreasing the strain rate from 0.4%/s to 0.0004%/s.The EAF cracking behaviors of Alloy 52/52M depended on columnar grain orientation.Little or no cracking was found in the 52b with columnar grain orientation parallel to the cyclic loading axis,but significant cracking in the direction parallel to columnar grains was observed in the 52Mw,whose columnar grains are oriented at a 45°-75° angle to the cyclic loading axis.The grain boundaries of Alloy 52/52M weld metal are intrinsically resistant to EAF crack initiation and propagation even at a lower strain rate of 0.0004%/s except for the formation of open DDC defects at RHABs. |
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