Effect Of Martensite-Austenite On Charpy Impact Toughness Of Simulated Inter-critically Reheated Coarse Grained Heat-affected Zone

Inter-critically reheated coarse grained heat-affected zone(ICCGHAZ)had lower toughness in all heat-affected zone(HAZ).The deterioration of toughness was associated with the volume fraction?size?morphology?the interfacial energies of martensite-austenite(M-A)constituent.However,the mechanism of various factors and quantitative analysis were not clearly reported and still needed to be further considered.So the microstructure and mechanical properties of ICCGHAZ of X70 pipeline steel?X70 high deformed pipeline steel?E550 offshore platform steel was examined using an optical microscope(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and physical simulation technology.The results indicated that when the second peak temperature were 750 and 800?,M-A constituent in ICCGHAZ of X70 steel was located along the prior austenite boundaries with the obvious ‘necklace' structure;when the second peak temperature was 850?,M-A constituent was dispersely located along the prior austenite boundaries with the unobvious ‘necklace' structure.The obvious ‘necklace' structure was main factors on the deterioration of toughness under different peak temperature.When the second peak temperature was 750?,M-A constituent in ICCGHAZ of E550 steel was located along the prior austenite boundaries with blocky shape and displayed closely;With the increase of second peak temperature,M-A constituent increased and interconnected with the shape of ring,the martensite lath coarsened and plenty of carbide precipitated.The recovery of martensite?the size and fraction of M-A were main factors on the improvement of toughness of E550 steel under different peak temperature.When the second peak temperature was 850?and the first heat input was 10kJ/cm,with the increase of second heat input,the volume fraction and size of M-A with blocky shape along prior austenite grain boundaries in X70 steel increased.When the second heat input was 10kJ/cm,with the increase of first heat input,the volume fraction and size of M-A were sensitive to prior microstructure and increased largely.With the increase in volume fraction and size of M-A,Charpy impact toughness gradually deteriorated.The volume fraction and size of M-A were main factors on the deterioration of toughness under different heat input.To analyze the effect of M-A constituent on the initiation and propagation of crack comprehensively,the blocky M-A constituent along austenite grain boundaries was more detrimental than slender M-A constituent in the interior of grain.