Microstructure and Properties of X70 Pipeline Steel Joints Welded by Welding Process without Preheating

X70 pipeline steel was welded using two welding processes with preheating and without preheating before welding, respectively. The microstructure, tensile property and impact property of welded joints were studied by optical microscopy, tensile testing and impact testing. The results show that the microstructure of the joint with preheating before welding consisted of small acicular ferrites and polygonal ferrites. The grains of the joint without preheating grew up significantly, coarse granular bainites and a small amount of martensites could be found in its microstructure. The joint with preheating had the better tensile strength and impact toughness, compared to that without preheating. But the properties of the joint without preheating still met requirements. Considering the engineering economic costs, the process without preheating can be used in the welding of X70 steel.