Effect Of Aging Treatment On Precipitates In Z2CND18.12N Austenitic Stainless Steel

Z2CND18.12N austenitic stainless steel is a commonly used safety end pipe steel for the CPR1000 pile type of the third generation pressurized water reactor nuclear power plant.It has good toughness and corrosion resistance.In the test of the safety end welding witness primary side of a steam generator,there was a case where the impact energy of the stainless steel side heat affected zone failed.Aiming at this problem,a series of tests,including microscopic microstructure SEM,fracture morphology,impact toughness,precipitated phase EBSD and TEM and other related test analysis,were carried out by means of optical microscope,field emission scanning electron microscope,transmission electron microscope,microhardness meter and Charpy V-notch impact test,in order to reveal the causality.The specific research contents of this paper are as follows:（1）Through the test analysis of the Z2CND18.12N delivery state specimen of the safety end parts with unsatisfactory impact energy,it was found that the unsatisfactory impact toughness was caused by the precipitation of abnormal tissues,resulting in brittleness of the material.It was confirmed that the precipitated phase was mainly a chain-like carbide and a bulkσphase.In addition,the material was re-solution treatment and the microstructure and properties of the material before and after solution treatment were copmpared.It was found that after the re-solution treatment,the precipitate disappeared,and only theδferrite and austenite phases existed in the base material,and the impact toughness of the material returned to normal.It was concluded that the precipitation of abnormal tissue is related to the insufficient solid solution treatment of the delivery state.（2）At 800°C,the aging treatment of Z2CND18.12N material in re-solution treatment was carried out for different durations（2h,12h,24h,72h,200h）,and the effects of aging treatment on its precipitation phase and properties were investigated.The two kinds of tissues were mainly generated in aging period,Cr₂₃C₆ in the form of particle chains andσphase in block.The two phases mainly precipitated in theδferrite region.With the aging time increased,the precipitated phase gradually gathered and grew at the grain boundary,and the impact energy of the sample gradually decreased.After 72 hours,the precipitation was gradually stabilized and the impact energy was53J.Finally,after re-solution treatment,the impact toughness of the sample returned to285J.At the same time,the microhardness of the sample gradually increased,after the re-solution treatment,the microhardness value of the sample returned to 142HV.（3）The cause of the safety end component quality accident was confirmed to be the failure of the solution treatment due to insufficient supply solution treatment temperature and insufficient holding time.In essence,it became a higher temperature aging treatment,which accelerated the diffusion rate of alloying elements and the transformation ofδferrite toσphase,and quickly formed carbide andσphase.Eventually,the impact energy of the heat-affected zone of welding witness was unqualified.The experimental results showed that the solution treatment could eliminate the abnormal structure of carbide andσphase,but theδferrite cannot be eliminated.The staining identification ofσphase was an effective method for the identification of abnormal tissues.（4）The following experimental cycle process was carried out:the precipitation phase of the base metal material sample of the safety end part appeared→the performance was unsatisfactory→re-dissolution treatment state→precipitation phase was eliminated→aging treatment at different time（2h to 200h）→precipitation phase gothered and aggregation→re-dissolution treatment→precipitation phase elimination.And the precipitation,evolution and elimination of carbide andσphase in this process were explored.On the basis of this,in order to avoid the problem of material brittleness caused by precipitation phase,it was proposed to controlδferrite content in Z2CND18.12N austenitic stainless steel.