S960 and S700 are two types of high strength low alloy steels (minimum yield strengths at 960 MPa and 700 MPa, respectively) developed recently by Tata Steel. These steels are typically used in heavy lifting equipment. This research examines the feasibility and characteristics of single pass autogenous laser welding (ALW), multi-pass ultra-narrow gap laser welding (NGLW) of 8 mm thick S960 and 13 mm thick S700 high strength low alloy (HSLA) steels and compared the characteristics of the welds with those of gas metal arc welding (GMAW). The work aims to understand the development of welding induced residual stresses, microstructures, microhardness, tensile properties, bending properties and Charpy impact toughness at different temperatures as produced by different welding techniques (ALW, NGLW and GMAW).Design of experiments and statistical modelling were used to predict and optimise laser welding parameters of S960 and S700 HSLA steels. The contour method was used to measure the 2D distribution of residual stresses of the welded specimens. X-ray diffraction was carried out to measure the surface residual stresses of the welded specimens. The main novel contributions include:(1) Development of welding procedures for ultra-NGLW of HSLA steels. The ultra-NGLW process was successfully applied to the welding of 8 mm thick S960 and 13 mm thick S700 HSLA steels with a very narrow groove (1.2-1.4 mm wide) using a moderate laser power (2-3 kW).(2) Resolving the melt sagging problem for single pass autogenous laser welding of thick section materials. Horizontal (2G) welding position was applied to successfully resolve the melt sagging problem when single pass flat (1G) position ALW was applied to welding a 13 mm thick S700 steel plate. Computational fluid dynamic (CFD) modelling was carried out to understand the dynamic force interactions in the weld pool and the factors affecting the formation of the weld bead profile.(3) Understanding the effects of heat input on the microstructures evolution and mechanical properties of the welded high strength steel joints. The much lower heat input for ALW of 8 mm thick S960 steel and ultra-NGLW of both 8 mm thick S960 and 13 mm thick S700 steels results in the generation of hard martensite in the narrow fusion zone (FZ) and heat affected zone (HAZ), which strengthened the welded joints but deteriorated the toughness of the welded joints. The strengthened narrow FZ and HAZ for both the ALW and ultra-NGLW of 8 mm thick S960 steels demonstrated almost the same tensile strength and elongation as the base material. A relatively high heat input for the ALW of 13 mm thick S700 steel results in the generation of bainite in the FZ, which has almost the same microstructure and hardness as the base material.(4) Understanding the effect of solid-state phase transformation on the residual stresses of the welded specimens. It was demonstrated that the solid-state phase transformation from austenite to ferrite, bainite and martensite changes the magnitude of residual stress in the fusion zone for the welded S700 steel plates. In addition, it also changes the yield strength of the FZ, which also has a significant effect on the welding residual stress. In summary, this work has resulted in a significantly enhanced understanding of the way in which the choice of welding process affects the properties of welded joints in high strength steels. Laser welding was found to offer strengthened welded joints. However, the laser welded joints presented low impact toughness. If the toughness of the laser welded joints can be improved, laser welding will be a promising technique for joining high strength steels.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:684830 |
| Date | January 2016 |
| Creators | Guo, Wei |
| Contributors | Li, Lin ; Francis, John |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/laser-welding-of-high-strength-steels(4158f5b9-4d58-4f99-b2e9-dc7fd316410f).html |
Page generated in 0.0132 seconds