Study On Microstructure And Properties Of HAZ For In-service Welding On High Pressure Natural Gas Pipeline

With the rapid development of the economy, the demand for natural gas isbecoming higher and higher and the total length of the pipeline increases year by rear.The technology of in-service welding is indispensable for repairing the damagedpipeline and modification of gas pipeline. It is very dangerous to welding on thepipeline transporting flammable hazardous materials, such as natural gas and oil. Sothe microstructure and mechanical properties of heat affected zone (HAZ) forin-service welding on high pressure natural gas pipeline was examined using anoptical microscope (OM), scanning electron microscopy (SEM), transmission electronmicroscopy (TEM) and physical simulation technology. The results indicate that, inthe single thermal cycles, the microstructure of coarse grained heat affected zone(CGHAZ) of API5L X65pipeline steel is composed of lath martensite (LM) andacicular ferrite (AF), when the cooling rate is fast. A lot of retained austenite filmsand punctate or short rod M-A components forms between the lath of LM and AF inCGHAZ. So CGHAZ has a high hardness with good impact toughness. Fine grainedheat affected zone (FGHAZ) has a hybrid organization of quasi-polygonal ferrite (QF)and AF. The average grain size of FGHAZ is very small and the content of M-Acomponent is seldom, which make FGHAZ has excellent comprehensive mechanicalproperties. The microstructure of intercritical heat affected zone (ICHAZ) iscomposed of QF with uneven size and lots of M-A component, distributed along thegrain boundary. So that the toughness of ICHAZ is poor, while the hardness is lower.As the cooling rate is reduced, the microstructure type of each sub-region of HAZbasically unchanged, but the hardness is reduced. The impact toughness of CGHAZand ICHAZ improved with the reducing of the cooling rate, and the impact toughnessof ICHAZ is still the worst sub-region.In the secondary thermal cycle, the microstructure of surpass critical coarsegrained heat affected zone (SCCGHAZ) of API5L X65pipeline steel is composed oflots of AF and a few QF, when the cooling rate is fast. A lot of strip or punctate M-Acomponents forms in the microstructure of SCCGHAZ. So SCCGHAZ has a highhardness. The microstructure of intercritical coarse grained heat affected zone(ICCGHAZ) is composed of QF with uneven size and lots of M-A component, distributed along the grain boundary. So the hardness of ICCGHAZ is lowest.Subcritical coarse grained heat affected zone (SCGHAZ) has a hybrid organization ofLM and AF. There are some carbides precipitating in the microstructure of SCGHAZ.The hardness of SCGHAZ is between SCCGHAZ and ICCGHAZ. As the cooling rateis reduced, the microstructure type of each sub-region of HAZ basically unchanged,but the hardness is reduced. When t8/5is greater than4s, the hardness of SCCGHAZis the smallest sub-region.