Abstract
Q690E high strength low alloy (HSLA) steel plays an important role in offshore structures. In addition, underwater local cavity welding (ULCW) technique was widely used to repair important offshore constructions. However, the high cooling rate of ULCW joints results in bad welding quality compared with underwater dry welding (UDW) joints. Q690E high strength low alloy steels were welded by multi-pass UDW and ULCW techniques, to study the microstructural evolution and mechanical properties of underwater welded joints. The microstructure and fracture morphology of welded joints were observed by scanning electron microscope and optical microscope. The elemental distribution in the microstructure was determined with an Electron Probe Microanalyzer. The results indicated that the microstructure of both two welded joints was similar. However, martensite and martensite-austenite components were significantly different with different underwater welding methods such that the micro-hardness of the HAZ and FZ in the ULCW specimen was higher than that of the corresponding regions in UDW joint. The yield strength and ultimate tensile strength of the ULCW specimen are 109 MPa lower and 77 MPa lower, respectively, than those of the UDW joint. The impact toughness of the UDW joint was superior to those of the ULCW joint.
Highlights
Underwater welding has become the main method employed for the repair and maintenance of offshore oil and gas pipelines, mining platforms, watercraft, seashore components, piers as well as harbor devices and systems
The peak temperature varies with position in the and, the sub-zones of the are characterized by different position in the heat affected zone (HAZ) and, the sub-zones of the HAZ are characterized by different position in the and, the sub-zones of be the are characterized by different different microstructures
During the rapid cooling process of the underwater local cavity welding (ULCW) joint, the austenite transforms to M and B, and the base material (BM) microstructure is retained in the other regions (Figure 5g)
Summary
Underwater welding has become the main method employed for the repair and maintenance of offshore oil and gas pipelines, mining platforms, watercraft, seashore components, piers as well as harbor devices and systems. Simulated the local dry underwater welding process of E550 steel They concluded that rapid cooling rate can improve the impact toughness and tensile strength of weld metal in local dry underwater welding. Zhai et al [8] simulated the local dry underwater tungsten inert gas welding with a flux-cored wire and analyzed high-speed photographs of the metal transfer process. They concluded that the metal transfer mode varies from the no-contact mode/slag column mode to the continuous liquid transfer mode, with an increase in the wire-electrode gap. The microstructure of the BM consisted mainly of tempered martensite (TM) and a small amount of bainite (B)
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