Abstract: The welding and heat treatment processes for the X70 low-temperature service pipe fitting are studied by means of analyzing the chemical composition, metallographic microstructure and mechanical properties of the steel plate as material thereof.As a result, it is identified that homogenous and fined primer welding state metallographic microstructure will be obtained with the multi-layer and multi-pass, and 20 kJ/cm low welding wire energy SAW welding process;and that rather good comprehensive mechanical performance of the weld seam and the HAZ of the pipe fitting will also be obtained via the after-weld quenching and tempering heat treatment process featuring quenching at(950±10)℃ ×1.5 h.+tempering at(630±10)℃ ×2.5 h.