| The control of the microstructure and mechanical property of welding lineis very important to reliability of weld structure, while the microstructure andmechanical property of welding line is determined by thermal cycle of weldingline during welding process, which is determined by the size of weld structureand welding process parameters. It is significant to set up relations amongwelding line microstructure, mechanical property, the size of weld structure andwelding process parameters. It is also the purpose of this thesis to set up therelationships mentioned above for X70 pipeline steel.Based on numerical simulation of weld thermal cycle, considering thecharacters of submerge arc welding (SAW), using gauss heat source model, bygiving material equivalent thermal conductivity coefficients at hightemperature, a numerical model of SAW has been set up to simulate weldthermal cycle parameters.After studied the effect of the blow force of arc and heat transition inwelding pool and heating radius of arc on temperature field and thermal cycleparameters, the result that all factors mentioned above have great effect ontemperature field but have little effect on weld thermal cycle parameters ofwelding line has been get. By comparing the numerical result and practical dataof cooling time from 800 ℃ to 500 ℃(t8/5) of welding line, 10% difference hasbeen found. It is shows that the numerical model is reliable.By numerical simulating the weld thermal cycle parameters of X70 pileline steel with the numerical model set up above, relationships among t8/5 ofHAZ and the size of weld structure and weld line energy has been set up. Bythermal simulating the microstructure and mechanical properties of weldingline with different t8/5 with Gleeble-3500, relationships of microstructure,mechanical properties of HAZ and t8/5 has been set up. Based on therelationships set up above, relationships between content of pro-eutectoidferrite, HV and the size of weld structure, weld line energy has been set up.
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