Submerged arc welding process and performance of Q345B seamless steel pipe

Q345B seamless steel pipe belongs to hot rolled and normalized steel, and its main structure is ferrite + pearlite. This kind of steel has high hardness, strength, low plasticity and toughness, and has a certain tendency to harden. It is prone to cold cracks when welding large thick plates and excessive cooling speed; when welding with large process parameters, Due to the excessive heat input, it is easy to cause thermal cracks, pores or thermal stress cracks in the heat-affected zone. In this paper, the large-scale I-beam of Q345B seamless steel pipe is used as the welding object, and the submerged arc welding process performance of this material and the mechanical properties of the weld are studied.

The welding equipment is MZ-1250 submerged arc automatic welding machine. The chemical composition (mass fraction, %) of Q345B seamless steel pipe for welding is: 0.18C, 0.32Si, 1.32Mn, 0.005S, 0.021P; its σs=360MPa, σb=500MPa, δ=23%, AKU=193J. Use Φ4.0mm H08MnA welding wire, and the flux is HJ431. The weldment structure is a large I-beam, and the total length after assembly is 5000mm. Before welding, remove oil, rust and other dirt within 20mm of the groove surface of the weldment and its front and back sides, until the metallic luster is exposed. Spot welding is performed with electrode arc welding, the spot welding distance is 150mm, and the length of each spot weld is 20mm. After spot welding, it is polished to reduce the source of cracks.
When using submerged arc welding to weld Q345B seamless steel pipes, in addition to the correct selection of flux, welding wire and electrode, it is also necessary to clean up the surface of the workpiece before welding, such as oil rust and moisture, and strictly control the drying temperature and time of the flux and electrode. Short-arc welding and other measures are adopted during operation, which can prevent the generation of pores to a large extent, improve the welding quality and reduce the repair rate. Using a reasonable welding process can prevent welding cracks to a large extent. Q345B seamless steel pipe welded joints have higher strength and lower plastic toughness. There is Widmanstatten structure in the welded joint of Q345B seamless steel pipe, which is prone to embrittlement, which is a weak link in the joint, and small heat input welding should be used during production.

ASTM A500-Standard Specification for Cold-Formed Welded and Seamless Carbon Steel Structural Tubing in Rounds and Shapes.According to ASTM A500, the weld seam location for square, rectangle and other specially shaped tubing shall not be located within the corner radius, unless specified by the customer or manufacturer.Under ASTM A500, the corner radius for square and rectangular tubing shall not exceed three times the corner radius.

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