Effect Of Adding Magnesium On Temper Embrittlement In 2.25Cr1Mo Steel With Different Phosphorus Content

2.25Cr-1Mo steel is widely used in the energy and petrochemical industries,at present,it has been selected as the main structural material of the evaporator of my country's fourthgeneration advanced fast neutron reactor(CFR600).It usually requires to be operated at the intermediate temperature(450-600°C)combined with a high stress for 40 years.However,after long-term service at this temperature,the temper embrittlement caused by impurity elements such as P segregation at the grain boundary is one of its main failure modes.In order to improve the temper embrittlement,this project is based on the mechanism of impurity elements such as P segregation,introduces molten steel Mg treatment method in 2.25Cr1 Mo steel and makes use of the potential advantages of Mg grain boundary segregation and carbides refinement.This project is designed to add Mg to 2.25Cr1 Mo steel doped with different P content,systematically analyzes the evolution of impact toughness during the long-term aging and the embrittlement law of step cooling test,combined with the grain boundaries segregation behavior of P,Mg,S and grain boundary carbides characteristics,to reveal the interaction between Mg and P elements.The specific research content and results are as follows:(1)The effect of Mg treatment on the impact toughness and microstructure evolution of2.25Cr1 Mo steel doped with different P content was studied at 580 ? aging up to 5000 h.The results show that the impact toughness of steel is closely related to the P content in steel.As the P content increases from 0.002 % to 0.05 %,the impact toughness is greatly reduced,which is related to the strong weakening of the grain boundary strength by P segregation.Whether the addition of Mg can improve impact toughness of steel during the long-term aging is related to the content of P in the steel.when the P content is 0.056 %,the addition of 0.006 % Mg can improve the impact toughness of 2.25Cr1 Mo steel to a certain extent;while when the P content is 0.002 %,the addition of Mg reduces its impact toughness.The analysis proves that after adding Mg under the high P condition(0.056%),the improvement of the impact toughness of2.25Cr1 Mo steel during the long-term aging is related to the segregation of Mg at grain boundaries,promotion of ferrite formation and refinement of grain boundary precipitation.After long-term aging,there will be obvious Mg segregation at the grain boundaries of the Mg-treated steel,and its segregation will reduce the grain boundary segregation amount of P to a certain extent,thereby reducing the serious harm caused by P segregation.(2)The effect of Mg treatment on temper embrittlement tendency of 2.25Cr1 Mo steel doped with different P content was studied after step cooling test.The results show that when the P content in steel increases from 0002 % to 0.05 %,the concentration of P segregation at the grain boundary increases from 0.87 at.% to 2.35 at.% after step cooling test,and the temper embrittlement sensitivity coefficient of steel increases from-51.4 ? raising to 174°C,the temper embrittlement of steel is greatly increased.When the P content is 0.056 %,adding0.006 %Mg,the temper embrittlement sensitivity coefficient of steel drops to 147.3 ?,and the temper embrittlement tendency of steel is reduced to a certain extent.When the P content is0.002 %,adding 0.005 %Mg,the temper embrittlement sensitivity coefficient of steel increases to 15.3 ?,compared with no Mg addition,and the embrittlement tendency of the steel increases.Auger electron spectroscopy(AES)results shows that after Mg addition under high P conditions(0.056%),Mg,P and S will co-segregate at the grain boundaries,and the co-segregation phenomenon will reduce the segregation amount of P and S,thereby reducing the temper embrittlement tendency of steel.After Mg addition under low P conditions(0.002 %),the amount of P segregation at the grain boundary is lower,and the segregation of Mg increases the concentration of S segregation at the grain boundary to a certain extent,therefore,the temper embrittlement tendency of steel increases.