Study On Process,microstructure And Mechanical Properties Of Transient Liquid Phase Bonded Joints Of CB2 Ferritic Heat Resistant Steels

CB2 ferritic heat-resistant steel(G12Cr9Mo1Co1NiVNbNB)has been widely used in the ultra-supercritical power plants due to their high creep strength at elevated temperature and oxidation resistance.The joining of the CB2 is often presenting in the process of industrial production.The difference exists in the microstructure of each region of the joint prepared by traditional joining technology,which causes the failure of the joint in the process of plastic deformation.The joint prepared by the transient liquid phase(TLP)bonding has more uniform microstructure and better mechanical property.In this paper,Ni-Cr-B and BNi-2 amorphous foils were used as interlayers,TLP diffusion bonding technique was used to join the CB2 heat resistant steel.The bonding of CB2 heat resistant steel was carried out in the vacuum furnace with various temperature and holding time.Then,the technology of post weld heat treatment(PBHT)was used to improve the uniformity of the microstructure and toughness of the joint.At the same time,scanning electron microscope(SEM)observation,tensile testing,microhardness testing and X-ray diffraction(XRD)were employed to investigate the realationship of the structure and performance of the joint.The results show that the high quality and high efficiency of the joining of CB2 heat resistant steel be achieved by the method of TLP diffusion bonding and the appropriate PBHT process.The athermally solidified zone(ASZ)presents in the center of the joint when the bonding pemperature is below 1150? and the holding time is lower than 600 s with the Ni-Cr-B interlayer.The ASZ is disappearing with the increasing temperature and holding time.The maximum tensile strength reach to 924 MPa for the joint bonded at 1150 °C for 1800 s with elongation of only 10.9%,and the fracture take palce at the joint.PBHT result in significant improvement in the homogenization of the joint and ductility of the joint as elongation of 24.9% is achieved with a decreased strength of 744 MPa,the fracture takes place at base materials.The ASZ only presents in the center of the joint bonded at 1050 °C for 60 s.After PBHT the precipitated phase almost disappear while the size and amount of BN precipitates rise.The joint bonded at 1150 °C for 1800 s has the best tensile strength 934 MPa with the elongation is 5.3%,and the fracture present in the joint.After PBHT,the uniformity of the microstructure is improved obviously,the tensile fracture occurr in the base metal with the tensile strength was 720 MPa and the elongation increase to 18.3%.