Study On Welding Softening Behavior Of Submarine Pipeline Steel

In addition to being able to transport oil and natural gas,pipeline transportation can also be an important transportation tool for many primary energy sources.However,based on the recent global energy analysis and outlook,the reduction of land resources and the increasing demand for energy by humans have led to relying on land transport is far from meeting human needs for energy.So people are turning to maritime transport.Compared with other transport tools of oil and gas storage tanks,submarine pipelines have become the “lifeline”of marine transportation tools because of its continuous,efficient and fast characteristics.Submarine pipelines face with more srict quality requirements when they performance in harsh marine environments than land transportation,such as high pressures,surges,and vortex surges.Therefore,spend more time and energy to develope and research in high quality and performance submarine pipeline steel have great significance.Firstly this paper by studying the actual welded joints,it was found that the two-phase microstructure steel softened in the heat affected zone of the weld,it appeared in the fine grain zone,which caused the heat affected zone to become the weakest part of the welded joint.However,in actual production,submarine pipeline steel based on strain design must meet over-strengthen than Base metal,and the softening phenomenon is completely inconsistent with the performance requirements of actual submarine pipeline steel,thus restricting the vigorous development of submarine pipeline steel.Therefore mainly analyzes the softening causes of submarine pipeline steel during the welding thermal cycle and finds improvement measures.Subsequently,the experimental method of welding thermal simulation was used to study the influence of different base metal components on the microstructure and properties of submarine pipeline steel.It is found that the experimental steels also have a softening zone in the fine-grained zone of the heat affected zone.The effect of changing the Nb content on the softening zone is limited,and the softening can be significantly improved by increasing the V content,the mainly reason is that the addition of V can play the role of precipitation strengthening;on the other hand,due to the low solid solubility of V,and the greater influence of V on the hardenability,it can greatly improve the hardenability and increase the low temperature phase.It promote the deformed of bainite,and the ferrite content is decreased,thereby increasing the microhardness.Finally,the effects of different heat inputs(10 KJ/cm,25 KJ/cm,35 KJ/cm)on the softening zone were studied by means of welding thermal simulation experiments.The results show that the welds under the three heat input conditions are softened in the fine-grained zone,which reduces the heat input,the softening phenomenon can be greatly improved.This is mainly because the cooling time is reduced,the cooling rate is increased so the residence time in the high temperature region is shortened,and the crystal grains are not grown.Therefore,the grain size is reduced under the low heat input condition,the fine grain strengthening effect is remarkable,and the hardness is increased from 240 HV up to 260 HV.