Microstructure Stability And Creep Behavior Of Modified 9Cr-1Mo Welded Joint

Modified 9Cr-1Mo martensite heat-resistant steels have been employed as the preferred material of steam turbine rotor of ultra-supercritical power plants because of their excellent high temperature creep performance,corrosion resistance,oxidation resistance.The narrow gap welding technology has been widely used for the large size components of rotor.The safety and reliability of the welded joint depend largely on its creep behavior and microstructure stability.In this paper,microstructure stability and creep behavior of modified 9Cr-1Mo welded joint was characterized in detail.The coarsening mechanism of second phase in martensite steels during aging and creep was studied systematically.The creep damage models of modified 9Cr-1Mo welded joint under different conditions were analyzed.The relationship between creep rupture and microstructure evolution was revealed.The results may help to understand better the microstructure stability and creep behavior of9Cr-1Mo welded joint and provide a theoretical reference for the operation of 9Cr-1Mo steels welded joint.Firstly,the microstructure of 9Cr-1Mo welded joint was characterized systematically.It is found that welded joint contains coarse grain heat affect zone（CGHAZ）,fine grain heat affected zone（FGHAZ）,over tempered heat affect zone（OTHAZ）and weld metal（WM）.Banded structure composed of small grains with size of 2-5μm is found along the equiaxial grain in the middle of WM.Dissolution of high temperature ferrite occours in CGHAZ of 10Cr1Mo due to high weld thermal cycle temperature.W,Mo and Cr elements in high temperature ferrite can diffuse to martensitic matrix,which can be a supplement of coarse second phase.Prior austenitic grain boundaries（PAGBs）are mostly the large angle boundaries（15°-60°）while the small angle boundaries（2°-15°）are found in subboundaries.Secondly,mechanical properties and microstructure evolution of 9Cr-1Mo welded joint after aging under different conditions were studied.The results reveal that hardness of WM after aging rises obviously but hardness of 10Cr1Mo and 9Cr1Mo B has no obvious change.The toughness of WM drops fistly and then rises slightly,finally keeps stable with the extension of aging time.Second phase in 10Cr1Mo and WM continuously coarsen and form chain structure along the large angles boundaries.Coarsened second phase were also observed in PAGBs of 9Cr1Mo B.At the same time,a lot of small second phase precipitates along martensite subboundaries because of B addition in 9Cr1Mo B.Local element enrichment and interface energy increasing because of the inhibition of grain boundary and second phase on dislocation movement are considered as the main reason of second phase coarsening.A phenomenon of Fe element insteads of Cr element occurs in M₂₃C₆ phase coarsening process.Laves phase nucleates preferentially around M₂₃C₆phase and along the large angle grain boundaries,and grows up through the way of grain boundary diffusion.Finally,the microstructure evolution,creep behavior and creep fracture morphology were studied and the the relationship between creep behavior and microstructure evolution was analyzed.Creep stress causes directional movement of dislocation,which contributes to the coarsening of second phase.Creep rupture with cup and cone-shaped fracture occurred in 10Cr1Mo OTHAZ due to the dislocation glide at 500℃and 538℃with high stress.Under the condition of 566℃/230 MPa free dislocation movement is hindered by high angle boundaries.The initiation of creep crack occurs firstly in banded structure of equiaxial grain zone.And a wave-shaped fracture is revealed.Creep deformation in weaken plastic region due to dislocation glide is always considered as the main creep damage mechanism under high stress and low temperature.Under high temperature and low stress,coarsened second phase and creep voids occurs along large angle PAGBs due to the inhibition of free dislocation movement.The creep damage always generates in the area with small size grains.