Sensitivity Of In-service Welding Tee X70 Blocking Cold Cracking

Domestic and international natural gas pipeline has entered the era of high-grade, large diameter. Large-scale use of X80line pipe in the Second West-East natural gas pipeline has been built in China, while using the X90-level test pipe section diameter of φ1219mm. The pipe diameter φ1422mm natural gas pipeline has been used in the West-East Gas Pipeline construction of three-wire. In this pipeline maintenance and emergency process,16MnR is unable to meet the construction requirements of the status.The slab X70grade pipeline steel as a closure tee material cold cracking susceptibility was studied in this paper. Analog pipeline maintenance and emergency girth welding arc are transformed into linear welding by an experimental box. Under the conditions of in the flow rate of0.6m/s and0.9m/s、the preheating temperature is room temperature (25℃)、50℃and100℃, Analog-dimensional repair welded joints is detected by stress concentration detector and its’weld heat affected zone of the lateral is measured by residual stress tester; the microstructure and mechanical properties of welded joints are studied; lap joint cold cracking susceptibility is tested and HAZ maximum hardness value is tested by method of cold cracking susceptibility of welded joints, at the same time,X70pipeline steel cold cracking susceptibility is quantitative analyzed by the implant test. Research shows that:Cold cracking sensitivity testing of a lap joint shows:under the different preheating temperature conditions, The rate cold cracking of the welded joints of X70pipeline steel plate thickness of52mm and X80pipeline steel plate thickness of20mm is very low, while the highest hardness value of welded joints is not exceeded350HV10, and the highest hardness values appear in X70pipeline steel side. The stress concentration analysis of welded joint shows that a smaller range of stress concentration occur in the welded joints (0.6m/s、Preheating to50℃) in which there are a mass of martensite The heat affected zone of welded joints transverse residual stress tests found that the heat affected zone transverse residual stresses are compressive stress, and did not exceed the yield strength.The researches of microstructure and properties of welded joints show that there is a transition area between the base metal and the weld heat affected zone of fine-grained area, The microstructure of this area composed of the X70base metal, M-A or pearlite, in the light microscope display white areas. The welded joints CGHAZ is cold cracking sensitive area, coarse microstructure of this region, mostly bainitic ferrite M-A, most of this regions is highly hardened martensite. In analog-dimensional welding experiments to repair block three-way service, cold cracking rate of welded joints is zero, indicating that it has strong anti-cold crack sensitivity ability; However, The hardness of some heat-affected zone of welded joints is very high, The highest hardness value is381HV10(leakage in the welding process), when the preheating temperature reaches100℃, the highest hardness value of the root welding position of the welded joints is also reached349HV10, indicating that the welded joints of hardened is higher, easy to produce cold cracking. Latch test shows The critical fracture stress of X70pipeline steel plate than the yield strength of the base metal, indicating that the base material has a strong resistance to cold cracking sensitivity.