For long-distance natural gas pipeline, using high grade pipeline steel is the main way to save costs. Canada’s pipeline industry practice proved that: compared with the X60, adoption of X70 pipeline wall thickness can be reduced 14%;Compared with X70, adoption of X80 pipeline wall thickness can be further reduced by 12.5%. Welding is one of the key links in steel pipe manufacturing and pipeline construction, due to uneven welding thermal cycle, the welding heat affected zone (HAZ) is a weak part of the pipeline. Cooling rate is the main parameter to determine HAZ organization performance, it is related to the welding heat input, the thickness of workpiece and the surrounding radiation condition. Therefore, making the reasonable welding process, control welding cooling speed can get excellent organization performance of the HAZ, avoid HAZ cracks, so as to ensure the safety operation of pipeline.

With the increase of cooling rate, the microstructure of X80 pipeline steel HAZ is given priority to with polygonal ferrite, to give priority to with B or grain of lath martensite, the hardness will increase. When the welding heat input is too big or preheating temperature is too high and the cooling speed is lower than 2 ℃ / s, X80 pipeline steel HAZ organization is given priority to with or B polygon ferrite grains, but due to the existence of pearlite and MA island group block distribution, leading to the coarse crystal impact performance poorer. When the welding heat input is too small or preheating temperature is too low and the cooling speed is greater than 30 ℃ / s, X80 pipeline steel HAZ organization is given priority to with BF or lamellar martensite, resulting in the decrease of the coarse crystal impact performance. Making reasonable welding process, controlling the cooling rate within the scope of 2 ~ 30 ℃ / s, X80 pipeline steel HAZ organization is given priority to B, MA island group is scattered, the coarse grain zone has proper hardness and excellent impact performance.