Study And Prevention Of Multi-pass Welding Cracks In Steam Cooler

As a heat exchanger,steam cooler can raise utilization rate of coal-fired,thus improving the thermal economics of the units.Steam cooler is belong to the third grade pressure vessel with high danger coefficient.It would result in incalculable damage and very serious consequences in case of leakage and explosion.Therefore,it is significant to improve the welding technique of pressure vessel and strengthen the welding quality control.Cracks initiate at the weld toe of the 17th welded joint,which propagate at the head side and the length is about 50cm.In this thesis,combining with electron microscope,optical microscopy,X-ray diffraction observations and hardness test,numerical simulation of welding process,the reasons of crack are analyzed in the several aspects of material,process and stress.The welding procedure is optimized to ensure higher welding quality,and the manufacture of steam cooler has been further improved to provide quality assurance of equipment safe operation.The constituents of the steel for head(13MnNiMoR)and for forge pipe(20MnMoNb)are both qualified to the standard requirement.The carbon content of head and forge pipe are 0.54 and 0.61,respectively.The metallurgical structure of head material is Ferrite and tempered Sorbite with grain size of 5.5-6;the metallurgical structure of forge pipe material is Ferrite and Pearlite with grain size of 7.5-8.The metallurgical structure of weld metal is proeutectoid Ferrite and acicular Ferrite.Some welding seam structure is converted to Ferrite and granular Bainite influenced by next weld reheat effect.The structure of overheated zone in head side HAZ is grain coarsening lath Martensite and Bainite with grain size of 4.5-5.In hardness testing,the hardness of head material is about 210 HV10;the hardness are 242?245 HV10,387?425 HV10,433 HV10 and 249 HV10 for the 17th weld metal,the area of fusion line,overheated zone and normalized zone,respectively.Influenced by reheat,the hardness of the 15th welded joint had largest decline to 221?227 HV10;242?272 HV10,309 HV10 and 249?250 HV10 for weld metal,fusion line,overheated zone and fine grain zone,respectively.Cracks initiate at the weld toe of the 17th welded joint.The main crack on the surface consist of multiple internal cracks generation nearby the weld toe,expending along the direction of weld joint.In crack initiation,crack initiating through Martensite,and the spherical metal inclusions are observed in crack initiation.Shear lips are found when cracks propagate to the head surface,the length is 1 mm.The facture morphology in initiation and expansion regions both are quasi-cleavage fracture and dimple fracture,which conform to the characteristics of cold crack fracture.A large number of metallic spherical inclusions on Fe substrate are found in the head material and the fracture surface of the crack,which are widely distributed and have a strong bonding strength with the base metal.Several unfused defects were found in fusion zone.The transition from the 17th weld to the base metal is not smooth,distribution of surface residual stress near weld toe shows that the circumferential tensile stress is 491 MPa.the axial tensile stress is 306.3 MPa and the axial tensile stress is 76 MPa.Coarse lath Martensite structure in HAZ resulting in a great decrease of crack resistance of the welded joint.Spherical metal inclusions diminishing its mechanical properties.High residual stress provides power for crack initiation.These factors all led to this cold crack.In view of the causes of cracks,preventive measures are put forward,including adjusting process parameters,strictly implementing welding procedures,ensuring the correct execution of heat treatment,improving groove shape and strict quality control of raw materials.