Study On The Fracture Toughness And Fracture Mechanism Of 9Cr/CrMoV Dissimilar Steels Welded Joint

The steam turbine rotor is the key part in the power plant and it is becoming the new technology in the manufacturing industry because of its low cost and high performance.9Cr steel is the new material used in the steam turbine rotor which has good creep and fatigue property in high temperature and pressure.In order to satisfy the material requirement in different zones in the steam turbine,9Cr and CrMoV steels are welded together.Thus,safety evaluation is very important for welded steam turbine rotor.Fracture is one of the failure types for the steam turbine rotor when it starts or stops.Thus,it is significant to study on the fracture toughness and its mechanism both for the reliability assessment and the optical structure design for the welded joint.We investigated on the fracture toughness and its mechanism from the analysis of microstructure,mechanical property,fracture mode as well as the simulation of the crack propagation.The research results not only supply basic data for engineers but also help researchers understand the fracture mechanism of welded joint better.In this paper,the microstructure of the welded joint was firstly analyzed systemically through optical microscope,scanning electronic microscope and micro-hardness.The microstructure differs a lot in various regions in the 9Cr/CrMoV welded joint and obvious boundary exists between different regions.The main microstructure of CrMoV-BM is granular bainite and lath marten-site is the main microstructure in9Cr-BM.Different microstructure results in different mechanical property.The CrMoV-HAZ and 9Cr-HAZ contains two regions:coarse grain zone and fine grain zone.More carbides precipitated in the HAZ during the welding process.Element diffusion occurred beside the 9Cr-HAZ resulting in the carbon enriched zone and carbon depleted zone.The width of the carbon enriched zone is about 37?m.The micro-hardness varies a lot along the welded joint.The micro-hardness of 9Cr is much larger than that of CrMoV because of the different microstructure.The micro-hardness of HAZ is higher than that of BM because of the precipitated carbides in HAZ.What's more,there is a soft zone between BM and HAZ resulting from‘over temper'in the welding procedure,which is harmful for the mechanical property of the welded joint.Secondly,the fracture toughness of the various region in the9Cr/CrMoV welded joint was measured through three points bending experiment,applying multi-specimens method.The J_IC of CrMoV-BM,CrMoV-HAZ and WM is 321.7 k J/m²,266.1 k J/m²,176.2 k J/m²,respectively.And the K_Q of 9Cr-BM and 9Cr-HAZ is 124.9 MPam^1/2,115.3 MPam^1/2,respectively.The crack propagation mechanism was investigated based on the fracture morphology analysis.Comparing the fracture morphology of CrMoV-BM and CrMoV-HAZ,larger void in the fracture morphology consumes more energy during the crack propagation process,which will result in better fracture toughness.We observed the shape and distribution of the second phase particles.As for the 9Cr-BM,the second phase particles distributed around the lath martensite boundary which will easily become the crack initiation point.In order to explain the data scatter in the J-R curve of WM,the fracture toughness of equaxed crystal zone and columnar crystal zone was tested.Moreover,the fracture mode has been analyzed for both equaxed crystal zone and columnar crystal zone.Finally,the FEA model was established using ABAQUS to simulate the crack propagation behavior and crack propagation resistance.The GTN model as well as the Brittle Cracking model was used to simulate the ductile crack and brittle crack.We calibrated the GTN parameters for CrMoV-BM,CrMoV-HAZ and WM and those parameters have been validated by experiment results.The interface crack between CrMoV-BM and CrMoV-HAZ contains main-crack and sub-crack.The main-crack propagates along the interface.The interface crack between 9Cr-HAZ and WM will propagate to the direction of WM and the crack beside the interface(0.5 mm or 1 mm)will even cross the interface to the WM.The crack propagation resistance was influenced by the initial crack length.Longer initial crack length has smaller resistance because of the higher stress triaxiality and equivalent plastic strain right ahead of the crack tip.Through experiment and simulation investigation,we find that different fracture toughness was obtained for various regions in the welded joint because of the discrepancies of microstructure.The fracture toughness of CrMoV is much better than that of 9Cr.The fracture mode of CrMoV is ductile fracture mechanism while 9Cr is brittle fracture mechanism.The much difference of fracture toughness for varies regions in the welded joint result from the matrix microstructure and the distribution of carbides.The matrix of bainite has lower hardness and higher fracture toughness comparing with the matrix of martensite.Larger grain size has lower fracture toughness when the matrix is the same in different zones.Moreover,we find that the interface crack will propagate toward to the lower strength material because of the mismatch mechanical property.Thus,in order to assess the fracture property of the whole welded joint,we need to test the fracture toughness for all the regions.Refining welding parameters and applying reasonable material can improve the property of week zone.Thus,the mechanical property of the whole welded joint can be improved.