Effect Of Ce On Microstructure And Impact Properties In CGHAZ Of Low Carbon Nb-Ti Under High Heat Input Welding

The high heat input and high efficiency green welding manufacturing of modern large-scale important structural equipment is the current cutting-edge technology.How to restrain the low temperature embrittlement of coarse grain heat affected zone(CGHAZ)is an important metallurgical and material problem.For this reason,in this paper,the common low-carbon Nb-Ti steel and low-carbon Nb-Ti-CE steel with rare earth Ce were studied from the following four different aspects.The precipitation thermodynamics of low carbon Nb-Ti-Ce steel is affected by Ce,O,S,Al and N.The main precipitation phases include Ce₂O₂S,Al₂O₃,MnS,TiN and Nb(C,N),among which Al₂O₃ is the most stable and Ce₂O₂S is the second.Rare earth inclusions are mainly affected by O and S.when o content is insufficient,they decompose into Ce₂S₃,and when s content is low,Ce₂O₃ precipitates out.When 0.0077%Ce is added to the low carbon Nb-Ti steel,the inclusions increase and refine,and the strip shape gradually changes into the ball shape,and the composite inclusions composed of Al₂O₃·MnS become Al₂O₃,MnS,TiN,Ce₂O₂S and Ce₂S₃.The microstructure of CGHAZ welded by low carbon NB Ti steel and low carbon Nb-Ti-Ce steel in rolling state and simulated 100 kJ/cm heat input is composed of acicular ferrite(AF),massive ferrite(PF)and granular bainite(GB).After the addition of Ce,the heterogeneous nucleated intragranular acicular ferrite(IGAF)and intragranular massive ferrite(IGPF)in the structure change,resulting in large angle grain boundary(MTA?15)°)With the increase of the proportion of grains or substructures,the effective grains are refined,and the low temperature impact toughness of CGHAZ is improved accordingly.When the welding heat input of low carbon Nb-Ti-Ce steel increases from 25 kJ/cm to 150 kJ/cm,the starting temperature Ar3 of austenite to ferrite transformation of CGHAZ steel increases gradually,the intragranular acicular ferrite(IGAF)decreases and the intragranular massive ferrite(IGPF)increases.Moreover,the large angle grain boundary(MTA?15)decreases due to the decrease of transformation undercooling°)With the increase of the average grain size(MED)of grains or substructures,the cleavage fracture trend increases and the low temperature impact toughness decreases.Therefore,the suitable welding heat input of low carbon Nb-Ti-Ce steel should not exceed100 kJ/cm.The results show that the-40?KV2 impact energy of 100 kJ/cm high heat input double wire saw actual joint heat affected zone(1 mm outside the fusion line)of the test steel is increased from 38 J to 120 J,the impact joint morphology is changed from brittle fracture to ductile fracture,and the low temperature toughness of the actual welded joint is obviously improved,which provides a reference for industrial application.