Microstructure and Mechanical Properties of S355 Steel Welded Joints

Microstructure and Mechanical Properties of S355 Steel Welded Joints
S355 square steel bar is a kind of low alloy high strength structural steel produced in accordance with European standards. S355 steel square bar has strict requirements on chemical composition, strength, plasticity and low temperature toughness. S355 steel square bar is widely used in electric power, construction, bridges, and pressure vessel welding structures, and is also used in railway vehicle manufacturing. At present, China is implementing the high-speed railway passenger transportation strategy, the train running speed is greatly improved, and the steering frame will bear a greater dynamic load. Therefore, higher requirements are placed on the mechanical properties of the welded components, and the performance of the welded joints directly affects the safety and reliability of high-speed railway passenger cars.
The tensile test results of the front fillet weld cross joint of S355 square steel bar can be seen that the cross joint without gap and without gap and the cross joint with 45° K groove and without gap are stretched at the weld position. This indicates that under the test conditions, the room temperature shear strength of the welded joint of S355 steel is much higher than that of the base metal under static load conditions. The welding head without the groove can meet technical requirements. At the same time, it should be noted that the roots of the welded joints of these two types of joints have cracked. By observing the end faces of the eroded welds, it is found that the roots of the cross joints of the 45° K-shaped groove are not welded completely and become crack source, which eventually cracks at the root of the weld under tensile load. The welded joints are in the form of a bevel and a certain assembly clearance is an effective measure to avoid incomplete penetration. It is recommended that the welded joints be welded at the opening and leaving a gap to avoid incomplete penetration and sufficient strength. As for S355 steel welded joints, the weld structure is mainly a large number of coarse and strip-shaped pro-eutectoid ferrite precipitated along the original austenitic columnar grain boundary, close to the pro-eutectoid ferrite near a small amount of pearlite structure. The crystal is acicular ferrite, and the size is large, and there is a certain amount of granular bainite. The weld zone of the fusion zone and the superheat zone of the heat-affected zone is the weld metal, and the right side is the microstructure of the superheat zone. It can be seen that the grain size of the fusion zone and the superheat zone are different, mainly the block pro-analytic analysis along the grain boundary of the prior austenite. The heat affected zone structure is uniform bulk ferrite and pearlite, and the grains are very fine. The base material is uniform small piece of ferrite and pearlite. The microstructure of the weld depends on the welding material and welding procedure specifications. In engineering practice, the welding heat input should be strictly controlled to obtain more desirable mechanical properties.