Study On Microstructure And Properties Of Heat Affected Zone In Large Wire Energy Welding Of AH36 Steel

In recent years,the use of thick plate steel is becoming more and more popular in the field of marine manufacturing.In order to improve welding efficiency,reduce construction period and reduce production cost,high linear energy welding technology is widely used;However,large linear energy welding will make the austenite grains in the heat affected zone,especially in the coarse-grained heat affected zone,and coarse grain boundary ferrite,side slab ferrite,granular bainite and other structures that have negative effects on toughness will be formed during thermal cycling cooling,which will affect the service life of steel plates;AH36 ship plate steel is often used to make important structural parts because of its excellent strength and toughness and weldability.In this thesis,the effects of different high-energy welding on the microstructure and properties of AH36 ship plate steel are studied.The main contents and results are as follows:(1)The welding thermal simulation experiment of AH36 steel plate with high linear energy of 550 KJ/cm ～ 750 KJ/cm was carried out;Using pendulum impact testing machine to carry out impact test at 0 ℃ on thermal simulation samples;The microstructure was observed by metallographic microscope and scanning electron microscope.The results show that with the increase of linear energy,the impact energy of HAZ after welding decreases first and then tends to be stable;The microstructure changes from massive ferrite and acicular ferrite to grain boundary ferrite,acicular ferrite and lath bainite.When the linear energy increases to 700KJ/cm,lateral lath ferrite begins to appear in the microstructure.(2)The inclusions in the base metal and the coarse grained heat affected zone(CGHAZ)after typical wire heat input were analyzed by SEM and OTS.The results show that the inclusions in the base metal are much larger than those in the welding heat affected zone in the range of 0.5 ～ 1.5 μ m;After heat input,the small size of Nitride(mainly Ti N)decreased the most,Aluminate-Sulfide inclusions were almost absent,Oxide-Sulfide inclusions increased;The main types of inclusions that can promote acicular ferrite nucleation are(Al,Mg,Ti,Ca,Si)-O or(Al,Mg,Ti)-O,the size is about 3 μ m,the shape is round,and the nucleation mechanism is the inert interface mechanism.(3)In-situ observation of austenite growth and microstructure transformation during welding thermal cycle shows that with the increase of linear energy,the preferred nucleation position of ferrite changes from intragranular to grain boundary,and the temperature range from the beginning of nucleation in intragranular to the end of growth decreases,while the temperature range from the end of nucleation in grain boundary increases;With the increase of linear energy,the average size of austenite also increases,and ferrite precipitates in the range of 85 ～ 200 μ m;Inclusions with a size of about 1.5 μ m can hinder the growth of austenite grain boundaries.(4)Using gas-electric vertical welding(EGW)and submerged arc welding(SAW)to weld 120 mm thick AH36 ship plate steel,the microstructure and properties of welded joints were studied;The results show that the microstructure of the weld surface of gaselectric vertical welding(EGW)is coarse dendrite at the fusion line,and there are grain boundary ferrite and lath bainite at 2mm away from the fusion line,and there is a small amount of pearlite between ferrite;Fine ferrite and pearlite are located 5mm and 7mm away from the fusion line.The hardness of welded joint increases gradually from base metal to weld,and the hardness of heat affected zone and fusion line of EGW is lower than that of SAW;The impact energy of gas-electric vertical welding(EGW)surface and root increases with the increase of fusion line distance,and finally tends to be stable.