| In this paper,at the room temperature and -50℃the properties and fracture mechanisms of MAG and TIG welding joint of a 980MPa high strength steel are studied by impact test, tensile test and scanning electron microscopy (SEM)observations,the weak link of welding joints are determined,effects of weak link on the whole impact toughness of joints are obtained.By a series of experiments,the following results are obtained:(1)3.11% Ni content of MAG welded metal's yield strength can not reach at the base metal.Which yield strength of 802MPa, but base metal yield strength of 949Mpa at the room temperature.At -50℃,weld metal and base metal yield strength are 858MPa and 990MPa respectively, but the tensile strength of weld metal approximately reaches that of the base material.The impact toughness of MAG weld metal for 100-130J at room temperature,but the impact toughness was measured to be only 24.5J to 51.5J at -50℃.While at the room temperature,the measured values of the impact toughness of the base metal,coarse grain heat affected zone and fine-grain heat affected zone are 175J,138J and 193J respectively;-50℃their impact properties are 177J,183J and 155J respectively. It is much lower than that of base metal and HAZ. The main reason is that the mircostructural is composed of upper bainite with M-A island and some acicular ferrite;at the same time,which it includes a large number of inclusions.At -50℃,the fracture surface morphologies shows a quasi-cleavage mode and poor performance.(2) 6.48% Ni content of TIG weld metal's tensile properties is slightly lower than that of the base metal.Which yield strength of 901MPa,but base metal yield strength of 949Mpa at the room temperature.At -50℃,weld metal and base metal yield strength are 974MPa and 990MPa respectively, but the tensile strength of weld metal approximately reaches that of the base material.The impact toughness of TIG weld metal for 163-196J at room temperature, the impact toughness was measured to be 179J to 142J at-50℃.It is suggesting that the impact toughness of TIG weld metal is close to base materials,and higher than that of the weakest link (900℃heat zone)of base materials HAZ in the corresponding performance.That is to say TIG weld metal is qualified to weld,the reason of improving performance is 6.48% Ni content,high Ni content increases its hardenability, so lath bainite microstructure is obtained and only very fine carbide particles precipitate inside the bainite lath.In addition the 6.48% Ni content makes more residual austenite in the weld joints which increases the crack resistance.Even in -50℃remains a ductile fracture morphology.(3)Effects of the weld metal fraction on the toughness of welding joint:the properties of original weld metal of MAG weld metal is very bad,fracture mainly presents quasi-cleavage,the impact toughness values are greatly reduced with increasing the fractions of the weld metal in the ligaments.When the weld metal with inferior toughness even in a fraction as high as 20% causes remarkable deterioration about 45% on the impact toughness of the welding joint. The weld metal with comparable toughness gives little effects to the property of the welding joint.TIG weld metal has a good performance,the impact toughness values are little effected in the fractions of the weld metal in the ligaments. But own to the base material and the coarse grain of thermal simulated 1320℃heated zone has better performance than that of TIG weld metal,so the proportion of weld metal lower than 60% was still facilited the impact toughness.Based on the MAG weld joint gain the local performance parameters of weld metal by double hole micro-shear tests.
|
PDF Full Text Request