بررسي تجربي ويژگي هاي سطح شكست نمونه نامتجانس (با درز جوش افقي) آزمايش ضربه سقوطي فولاد API X65

Experimental Study of Fّracture Surface Characteristics of Inhomogeneous dro‎p Weight Tear Test Specimen Made from API X65 Steel

در اين پژوهش شكست نگاري نمونه نامتجانس (با درز جوش افقي) آزمايش ضربه سقوطي از جنس فولاد API X65 براي اولين بار انجام شده است. مقطع شكست نمونه آزمايش شامل سه ناحيه فلز پايه، متاثر از حرارت و جوش مي باشد. آزمايش طبق استاندارد API 5L انجام شده است. . ﺗﺮك ﺑﺎ ﺷﮑﺴﺖ ﺗﻮرﻗﯽ از ﻧﺎﺣﯿﻪ رﯾﺸﻪ ﺷﯿﺎر ﺷﺮوع ﺷﺪه و در اداﻣﻪ ﺑﺎ ﺗﻐﯿﯿﺮ زاوﯾﻪ 45 درﺟﻪ ﻧﺴﺒﺖ ﺑﻪ ﺻﻔﺤﻪ ي ﺳﻄﺤﯽ، ﺑﻪ ﺷﮑﺴﺖ ﻧﺮم ﺑﺮﺷﯽ ﺗﺒﺪﯾﻞ ﺷﺪه و ﺗﺎ ﻧﺎﺣﯿﻪ ﺷﮑﺴﺖ ﻣﻌﮑﻮس اداﻣﻪ دارد. در ﻋﮑﺲ ﻫﺎي ﺣﺎﺻﻞ از ﻣﯿﮑﺮوﺳﮑﻮپ اﻟﮑﺘﺮوﻧﯽ روﺑﺸﯽ در ﻫﺮ ﺳﻪ ﻧﺎﺣﯿﻪ، ﻋﻼﺋﻢ ﺷﮑﺴﺖ ﻧﺮم ﻣﺸﺎﻫﺪه ﺷﺪ ﮐﻪ ﺷﺎﻣﻞ ﺣﻔﺮه ﻫﺎي ﻣﺨﺮوﻃﯽ ﺑﺎ اﻧﺪازه، ﺟﻬﺖ و ﺷﮑﻞ ﻣﺘﻔﺎوت اﺳﺖ. ﺑﺮرﺳﯽ ﺣﻔﺮه ﻫﺎي ﻣﺨﺮوﻃﯽ از اﺑﺘﺪا ﺗﺎ اﻧﺘﻬﺎي ﺳﻄﺢ ﺷﮑﺴﺖ ﻧﺸﺎن ﻣﯽ دﻫﺪ ﮐﻪ ﺑﺎرﮔﺬاري از ﺣﺎﻟﺖ ﮐﺸﺸﯽ در رﯾﺸﻪ ﺷﯿﺎر ﺑﻪ ﺑﺎرﮔﺬاري ﺑﺮﺷﯽ در اﻧﺘﻬﺎي ﺳﻄﺢ ﺷﮑﺴﺖ ﺗﺒﺪﯾﻞ ﻣﯽ ﺷﻮد؛ ﻫﻤﭽﻨﯿﻦ در ﻧﺎﺣﯿﻪ ﻓﻠﺰ ﭘﺎﯾﻪ ﻋﻼﺋﻢ ﺷﻮرون ﻧﺎﻗﺺ ﺑﺎ ﺗﺮاﮐﻢ ﺑﺎﻻ در ﻣﺮﮐﺰ ﺳﻄﺢ ﺷﮑﺴﺖ ﻣﺸﺎﻫﺪه ﺷﺪ. در ﻧﺎﺣﯿﻪ ﺑﺮﺧﻮرد ﭼﮑﺶ، ﺷﮑﺴﺖ ﻣﻌﮑﻮس ﺗﻮرﻗﯽ ﻣﺸﺎﻫﺪه ﺷﺪ ﮐﻪ از ﻫﺮ دو ﻃﺮف ﺗﻮﺳﻂ ﺳﻄﻮح ﺑﺮﺷﯽ ﺑﺎ زاوﯾﻪ 45 درﺟﻪ ﻣﺤﺼﻮر ﺷﺪه اﺳﺖ. ﺳﻄﺢ ﺷﮑﺴﺖ ﻧﺮم ﻣﺤﺎﺳﺒﻪ ﺷﺪه در ﻣﺤﺪوده ي اﺳﺘﺎﻧﺪارد ﺑﻮده و ﺑﯿﺎﻧﮕﺮ ﭼﻘﺮﻣﮕﯽ ﻣﻄﻠﻮب اﯾﻦ ﻓﻮﻻدﻫﺎ ﺟﻬﺖ اﺳﺘﻔﺎده در ﻟﻮﻟﻪﻫﺎي اﻧﺘﻘﺎل ﮔﺎز اﺳﺖ

In this study, the fractograghy of the inhomogeneous specimen (including seam weld) of API X65 steel was performed for the first time by the drop weight tear test. The fracture surface of the test specimen consisted of three zones of base metal, heat affected zone and weld metal. The test was performed according to the API 5L standard. The crack initiated with a cleavage fracture from the notch root and then transformed to a shear failure with a 45 degree angle change to the surface plate and continued to the inverse fracture zone. SEM images of all three zones showed signs of ductile fracture including dimples of varying size, orientation and shape. Dimples study from beginning to the end of fracture surface showed that loading from the tensile state at the notch root was changed to shear loading at the end of fracture surface. Also, faint chevron marks with high density were observed in the center of fracture surface in the base metal. In the hammer impact zone, an inverse cleavage fracture was observed, which was enclosed on both sides by shear surfaces with an angle of 45 degree. The calculated ductile fracture surface was within the standard range and indicated the confirmed toughness of these steels for the use in gas transportation pipe lines.