After nearly a decade of development, China's steel structure has made a qualitative leap. While the steel structure is developing rapidly, high-strength steel, thick plates and various profiles are rapidly increasing with the demand and development of the project; at the same time, high-strength steel, thick plate welding and high-heat input welding technology have also been widely used. These are the cornerstones for China's steel structure to enter the world's advanced ranks.
From the perspective of high-strength steel used in steel structures in the past decade, in addition to the traditional low-alloy high-strength structural steel Q345, higher-strength Q390, Q420, Q460 (hereinafter collectively referred to as high-strength steel) are also used in different degrees in the Beijing Olympics project. The "Bird's Nest", the Water Cube, the Shanghai World Financial Center, the new CCTV site, and the Shanghai Center, Shenzhen Ping An Building and Shenzhen Jingji 100 Center Plaza, which are under construction or completed.
The main factors affecting the performance of welded joints in the welding process of high-strength steel, in addition to the influence of steel properties themselves, mainly depend on the welding method, welding filling materials and welding process.
From the application of high-strength steel welding methods, there are mainly solid wire welding gas welding (GMAW), monofilament or multi-wire submerged arc welding (SAW), electroslag welding (ESW) and electrode arc welding (SMAW) during assembly. Flux cored wire gas shielded welding (FCAW) is generally not recommended for high strength steel welding because of its relatively small penetration depth. Compared with ordinary Q345, high-strength steel has higher carbon equivalent because of its higher strength, resulting in worse thermal processing (cutting, correction) and weldability, and greater tendency to harden the heat-affected zone. .
In practical applications, ensuring the performance of welded joints, cracking of assembled spot welds, generation of hot cracks in hot heat input welding, and degradation of heat affected zone performance are the main problems of high strength steel welding. Therefore, in determining the welding process of different welding methods, Consider the following factors.
(1) The weldability of the material itself includes carbon content and added trace elements; carbon equivalent and crack sensitivity index; thickness of the material to be welded.
(2) The welding method does not require post-heat treatment; the HAZ toughness is good at large heat input welding.
(3) Diffusion hydrogen content of the filler metal deposited metal; the amount of trace metal added.
(4) Heat input quantity of welding process and ambient temperature; shape of groove (limited to semi-melting condition, stress concentration factor);
Weld bead (single bead or multi-pass bead); preheating method (heating speed, preheating width); joint restraint.
In short, in the welding technology, the performance of the welded joint should be guaranteed to meet the design requirements. In the same strong design, in addition to the joint strength can reach the "grade" of the base metal, the maximum strength should be controlled as much as possible to reduce the brittle fracture of the joint. Risk; secondly prevent the occurrence of hot and cold cracks, especially cold cracks. Due to the high strength of the material, more trace elements are added during smelting, so when the carbon content is high, the welding parameters and welding auxiliary measures must be carefully selected.
For more information, please refer to the attachment or read the 18th issue of Metalworking (Hot Processing).
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