INCOLOY825/L415 Bimetal Composite Pipe Straight Seam Welding Process And Its Microstructure And Properties

To meet the demand of environment,technique and safe security,the integrate gas pipe in Turkmenistan used the Argon Tungsten-Arc Welding(GTAW)and Union Melt Welding(SAW)technique to weld the INCOLOY825/L415 bimetal composite sheet which spanned 22 ± 3mm.The Optical Microscope(OM),Scanning Electron Microscope(SEM),Electron Back Scatter Diffraction(EBSD)were taken to indicate the metallographic structure and the profile of sectional area of the welded joint respectively.Energy Disperse Spectroscopy(EDS)characterized the component elements.The mechanical properties of welded joint were tested with Microhardness Tester,Universal Tensile Machine and Brittleness Compact Tester.The results were shown as follow:The properties of welded joint of INCOLOY825/L415 bimetal composite sheet were differentiated owing to disparate in welding process.The average tensile strength of matrix was 582.5MPa under the 760A electric current;With 160A electric current treatment,the weld zone of the coating was 273.5Hv for hardness,716.7MPa for average tensile strength,31.93%for average ductility;The result of brittleness compaction showed that weld joint hadasuperior compact energy while the matrix got the inferior compact energy.The teared ridge extended along the carbon steel which revealed the cleavage fracture.There were equal-axis ductile voids in Nickle alloy which considerable correspond to the ductile fracture.The sample with 160A treatment was 1.08875mm in CTOD test which is much higher than that of 180A treatment(0.3037mm).Beside the CTOD value of 180A treated sample was highly discretized and Pop-in.The welded joint of matrix and coating were studied respectively with SEM and EDS.The results followed:L415 matrix with 780A treatment transformed into Widmannstatten structure which was hard and brittle within heat affected zone.Nb and Mo enriched phase were distributed dispersively in the welded bead of INCOLOY825 coating.The liquation cracks founded in the fusion zone was originated from austenite in matrix,then expanded to welded joint by both intercrystalline fracture and transgranular fracture.The solidification cracker extended along the weld bead with NbC deposition.,while Ti(C,N)blocked at the sectional area only.As the crack occurred to the coating with 180A current,the study on weld joint with EBSD showed that:most weld joint was low angel boundary.The austenite which had high stacking fault energy preferred to lessen the energy as thestrain increased which resulted from welding.Dislocation glide was the very way to lower the stacking fault energy which was<111>crystallographic preferred orientation.The new-formed low-angel boundary alleviated the strain and energy in grains.Crackers expanded along the straight-forward boundary which was separated by the low-angel boundary.The crack stopped when came across the high-angel boundary because of the angular deviation.The weld technique for matrix and coating was finally ascertained with study on the structure of weld joint:SAW with 760A which was used to weld the matrix transition area.GTAW with 160A was adequate to meet industrial demand.