Effect Of Tempering Times On The Microstructure And Mechanical Properties Of P91 Weld Joint

The P91 steel,a modified 9Cr-1Mo steel by restricting the content of carbon and adding vanadium,niobium and nitrogen,is a ferritic heat-resistant steel.Its domestic brand is 10Cr9Mo1 VNb and the microstructure in delivery state is martensite.The P91 seamless pipe,one of the most promising materials,has been widely applied to reheater and superheater tubes in subcritical and supercritical power generation systems around the world for its excellent high-temperature properties.Actually,during the on-site installation,it is required for non-destructive testing and hardness testing after heat treatment of the weld joint according to the requirements of the welding process.Furthermore,it is necessary for supplementary heating or repairing and the corresponding tempering heat treatment in unqualified places,and then test the hardness to determine whether the quality of weld joints qualified.Therefore,there is a objective reality of the multiple heat treatment at the welded joint,however,the effect of multiple tempering on the microstructure and mechanical properties of the P91 pipe joint has not been well known.Based on the research status and the practical problems appeared in engineering application of P91 seamless pipe,several times of high-temperature tempering were carried out on the P91 pipe joint in this paper respectively.On the one hand,the relationship between microhardness and mechanical properties under different heat treatment conditions of the local regions was analyzed by Vickers microhardness test,tensile testing at ambient temperature and at elevated temperature,which can providesome reference for the quality inspection of welding area.On the other hand,the influence of tempering times on the microstructure and mechanical properties of the P91 weld joint,including that at ambient temperature and elevated temperature,were investigated on the basis of the evolution of its microstructure by using OM,XRD diffraction analysis,TEM,SAED and above-mentioned tensile tests in this work,which may also provide some reference for the actual installation of P91 seamless pipe.The results are as follows:(1)Under the premise of welding and post-weld heat treatment processes identified for P91 in the experiment,the relationship between microhardness and mechanical property under ambient temperature is Rm(25 ?)? 3.626HV0.2,the correlation coefficient of the reference sample reaches maximum,That is 3.69,in addition,the relationship of that under 650 ? is Rm(650 ?)? 1.406HV0.2,and its maximum correlation coefficient reaches 1.48 when re-tempering once.(2)Under the experimental conditions,with the increase of tempering times,the microstructure of the P91 weld transforms coarse lath martensite into tempered sorbite reserved the orientation of lath martensite,while that of the heat-affected zone(HAZ)is first homogenized and then coarsened.As for the main phase under vary conditions,it remains a mixed phase of body-centered cubic ?-Fe phase and Fe-Cr phase.When tempered once,the microstructure of HAZ is more uniform,the MX(M= V/Nb,X=C/N)type carbonitrides with fine grain are precipitated,which have a pinning effect,and dislocation jogs,dislocation tangles and metastable dislocation nets are also found here.With a further increase of tempering times,the microstructure of the HAZ is gradually replaced by cell wall structure with less dislocation.Additionally,the initially dispersed precipitates,gathering and growing on the grain boundaries based on the Ostwald ripening theory,are gradually formed a face-centered cubic carbide of Cr23C6,whose lattice constant is 1.0638 nm.(3)Under the experimental conditions,the microhardness HV0.2 of the HAZ re-tempered once to four times respectively is larger than that of the reference sample,and the trend is first increased and then decreased,moreover,the maximum value is161.02 when tempered three times,which is increased by 3.35 % than the reference sample.(4)When tempered once,the tensile strength at ambient temperature Rm(25 ?)and that at elevated temperature Rm(650 ?)of the experimental P91 heat-affected zone,under the action of precipitation strengthening,dislocation strengthening and interface strengthening,have both reached the maximum values,that is,are 580.00 MPa and232.66 MPa and improved by 0.87 % and 11.02 % separately.The experimental results show that there is a significant influence of the tempering times after welding on the microstructure and properties of P91 seamless pipe.Under the experimental conditions,the HAZ microstructure of P91 pipe tempered 1~2 times is more uniform,and the mechanical properties at ambient temperature and elevated temperature are superior owing to the comprehensive action of solid solution strengthening,precipitation strengthening,dislocation strengthening and interfacial strengthening.