A Study On The Relationship Between The Heat Input And Microstructure In The Heat-affected Zone Of EH36 And EH420 Shipbuilding Steels

The marine trade has increasingly developed,and large-scale ships are widely constructed in order to lower operating costs as well as reduce the environmental burden.High heat input welding is commonly used to improve the welding efficiency of thick plates,thereby decreasing the duration of shipbuilding.Unfortunately,use of this effective method deteriorates the microstructures in the heat-affected zone(HAZ),markedly reducing its properties in terms of strength and toughness.Oxide inclusions in steel welds can affect formation of acicular ferrite(AF),which improves the toughness of the HAZ.The microstructure varied at different heat input,which means that it would be greatly meaningful to investigate the variation of microstructure.Because this work could make contributions to an optimal welding parameter.In this present study,EH36 and EH420 steels with Ti-Mg-Ca treatment and subjected to different heat input(50,100,150 and 200 kJ/cm)were systematically investigated by means of scanning electron microscopy(SEM),energy dispersive spectrometry(EDS),electron backscattered diffraction(EBSD),differential scanning calorimetry(DSC),and confocal scanning laser microscopy(CSLM).The results both in EH36 and EH420 shipbuilding steels show that the density of typical MnS inclusions will gradually decrease,even disappear,with the increase of heat input;More importantly,S-Mn-Ca-O-Ti-Si-Al-Mg and S-Mn-Ca-O-Ti-Al-Mg induced by the Ti-Mg-Ca treatment,whose average sizes between 2?5 ?m and 1?2 ?m respectively,could effectively promote the nucleation of acicular ferrites at all heat inputs.For EH36,there is 43.32%of bainite ferrites at 50 kJ/cm.When the heat input increases to 100 kJ/cm,although 8.99%of grain boundary ferrites precipitate,the content of acicular ferrites reaches the highest(58.34%).As the heat input continually increases,the content of acicular ferrites decrease,and only 16.85%at 200 kJ/cm.When the grade of steels raises from EH36 to EH420,for one thing,the heat input corresponding to the highest content of acicular ferrites changes to 150 kJ/cm.For another,there is still 41.33%of acicular ferrites at 200 kJ/cm.In addition,it was found that ferrite laths prefer to nucleate on the surface of inclusions instead of grain boundaries under the heat input of 50 kJ/cm in EH36,while FSPs are easier to form when the heat input increases.However,ferrites plates precipitated easier than acicular ferrites under all heat input in EH420.