Abstract
The effect of Mg treatment on the microstructure and toughness of the heat-affected zone (HAZ) of shipbuilding steel after high-heat-input welding was investigated via laboratory and industrial testing. The welding process and Charpy impact tests were also carried out to evaluate the HAZ toughness of steel plates. First, typical inclusion characteristics were characterised with an ASPEX PSEM Explorer. Then, confocal laser scanning microscopy (CLSM) was used to observe the diameters of austenite grains under different holding times. The results showed that when the addition of microalloy elements were in the order of Al–Mg–Ti, this had an effect on dispersing inclusions, the largest proportion of which were micro-inclusions that had a particle size range of 1.0–1.5 μm. This accounted for 25.4% of the total inclusions, which was the highest amount. The micro inclusion particle size that was mainly distributed in the range of 0.5–3.5 μm accounted for 82.8% of all the micro-inclusions. The inclusion structure induced intragranular acicular ferrite (IAF) in austenite as follows: MgO and Al2O3 formed the core and Ti2O3 adhered to the Al–Mg complex inclusions to produce smaller particle sizes and dispersions of Al, Mg, and Ti complex inclusions. The 40-mm-thick plate obtained in the industrial test after welding had an average impact absorbed energy 2 mm from the weld joint in the heat-affected zone of 198.9 J at −20 °C, while the welding heat input was 150 kJ/cm, compared with the parent material’s low-temperature performance, which exceeded 88%.
Highlights
Oxide metallurgy [1] is an effective way to improve the toughness of heat-affected zone (HAZ) welding by utilizing non-metallic inclusions to nucleate an intragranular acicular ferrite (IAF) structure, thereby refining the microstructure and increasing the strength of the HAZ [2,3,4]
When the alloy addition order was Al–Mg–Ti for high-heat-input welding, shipbuilding steel obtained dispersed, small, and abundant particles that effectively induced interlocking IAFs in austenite, for which Al2O3 and MgO served as the core, TixOy attached to the complex inclusions, Figure
When the alloy addition order was Al–Mg–Ti for high-heat-input welding, shipbuilding steel obtained dispersed, small, and abundant particles that effectively induced interlocking IAFs in austenite, for which Al2O3 and MgO served as the core, TixOy attached to the complex inclusions, When the alloy addition order was Al–Mg–Ti for high-heat-input welding, shipbuilding steel obtained dispersed, small, and abundant particles that effectively induced interlocking IAFs in austenite, for which Al2 O3 and MgO served as the core, TixOy attached to the complex inclusions, and carbonitrides of Nb precipitated at the complex inclusions, which have a small two-dimensional disregistry with α-Fe, and induce IAF effectively, refining the HAZ microstructure, and inclusions with a size of 0.5–2.5 μm accounted for 71.3% of the total inclusions
Summary
Oxide metallurgy [1] is an effective way to improve the toughness of HAZ welding by utilizing non-metallic inclusions to nucleate an intragranular acicular ferrite (IAF) structure, thereby refining the microstructure and increasing the strength of the HAZ [2,3,4]. Shipbuilding steel was treated with Mg and produced inclusions that had an effect on the formation of acicular ferrite in the microstructure. These inclusions were small, highly dispersed particles that acted as heterogeneous nucleation sites for new phases and facilitated the formation of acicular ferrite to divide the austenite, while the refined microstructure effectively improved the toughness of the steel
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