Article title: Microstructure evolution and mechanical performance of NV E690 steel repaired by underwater laser directed energy deposit technique 5
First author: Chen Mingzhi
Communication author: Sun Guifang
Powder-feeding laser direct metal deposition (DMD) is a type of directional energy deposition (DED). This study has made a breakthrough in applying DMD technology to underwater environment restoration of NV E690 steel plates. The influence of underwater environment and environmental pressure (0.01 MPa - 0.35 MPa) on the microstructure evolution, phase transformation and mechanical properties of repair materials was systematically studied. The precipitation kinetics of (Ti, V)N particles were analyzed using underwater pressurized nitriding theory. The research results show that the quenching effect of water refines the grain size, improves the dislocation density and martensite content of the slats. The microstructure evolution and mechanical properties of underwater repair samples have no obvious relationship with the underwater environmental pressure. The research has built a bridge between marine engineering and directional energy deposition technology, providing a new alternative for underwater restoration technology.
01 Research background
NV E690 steel for marine engineering has excellent comprehensive mechanical properties. However, marine engineering equipment often suffers surface damage, resulting in unpredictable damage to structural parts. Therefore, carrying out underwater in-situ repair of damaged parts can greatly improve the service life of offshore equipment. Commonly used underwater in-situ repair technologies include underwater arc welding (UAW) and underwater laser welding (ULBW). UAM is prone to cause large heat-affected zones and poor repair quality. The wire feeding ULBW has poor repair flexibility due to the fixed wire feeding direction. This study is based on the self-built powder-feeding underwater local dry direct metal deposition (UDMD) technology, which efficiently, highly flexible, high quality and cost-effective underwater in-situ repair of NV E690 steel. The influence of underwater environment and different environmental pressures (0.01 MPa-0.35 MPa) on tissue evolution and mechanical properties was systematically studied.
02 Innovation points
1. Based on UDMD technology, the repair experiment of NV E690 steel under different environmental pressures was realized
2. The mechanism of influence of water environment on microstructure and mechanical properties in UDMD process
3. The underwater pressurized nitriding theory was analyzed (Ti, V)N Precipitation kinetics of particles
0303 Research content
1. Effect of underwater environment on microstructure evolution
Compared with onshore DMD repair samples, the quenching effect of water increases the dislocation density of UDMD samples, and the high cooling rate inhibits the transformation of slat martensite to tempered martensite , reducing the intrinsic heat treatment effect (IHT) of the deposited layer. In addition, precipitation of (Ti, V)N nanoparticles was found in the deposition layer at a pressure of 0.35 MPa.
Figure 1. Microstructure morphology
2. Underwater pressurized nitriding and precipitation kinetic analysis of (Ti, V)N particles
Expand the traditional pressurized nitriding theory to the underwater environment and calculate the theoretical nitrogen content of the liquid molten pool. Compared with the onshore DMD experiment, the theoretical nitrogen content in the UDMD melt pool was increased by 2.01 times under the ambient pressure of 0.35 MPa, which led to the formation of (Ti, V)N nanoparticles formed by the mutual dissolution of TiN and VN in the deposited layer. Theoretical calculations show that VN nucleates on the first precipitated TiN particles, and finally forms (Ti, V)N particles. Furthermore, (Ti, V)N particles exhibit high thermal stability due to their extremely high dissolution temperature and extremely low coarse rate.
Figure 2. Solubility of nitrogen in liquid phase melt pool and calculation results of TiN and VN precipitation kinetics
Figure 3. Precipitation process and strengthening mechanism of (Ti, V)N particles
3. Effect of underwater environment on mechanical properties
2 33. Compared with onshore DMD repair samples, the quenching effect of water leads to higher hardness of UDMD repair samples.All samples were tough tensile fractured at room temperature. In addition, when the underwater ambient pressure P ≤ 25MPa, the UDMD sample was tensile fractured in the repair area. There was no obvious relationship between the tensile performance of the sample and the low-temperature impact performance and the underwater ambient pressure. When P = 0.35 MPa, the precipitated thermally stable (Ti, V) N particles under pressurized nitriding strengthened the repair area, and the UDMD repair sample was tensile and broken on the base material. Figure 4. Tensile properties of onshore repair samples and underwater repair samples
04 enlightenment
This study expanded the traditional onshore DMD technology to the underwater environment. Based on the powder-feeding underwater laser direct metal deposition technology (UDMD), an underwater in-situ repair experiment for NV E690 steel was carried out. The influence of underwater environment and environmental pressure (0.01 MPa - 0.35 MPa) on the microstructure evolution, phase transformation and mechanical properties of materials were systematically studied. In addition, this study proposes for the first time to apply the traditional pressurized nitriding theory to the field of underwater in situ restoration, which provides a theoretical basis for subsequent in-situ manufacturing of high nitrogen deposited layers in large water depths.
05 Author introduction
Chen Mingzhi, Southeast University School of Mechanical Engineering doctoral degree. The main research directions are microstructure evolution, mechanical properties, process detection and corrosion properties in laser direct energy deposition and underwater laser direct energy deposition.
Sun Guifang, professor/doctoral supervisor of the School of Mechanical Engineering, Southeast University, has devoted himself to research on laser green remanufacturing theory and technology for nearly 20 years. With the support of GF basic scientific research projects, JKW fund , ZF fund, National Natural Science Foundation , Jiangsu Provincial Natural Science Foundation , etc., we have focused on breaking through the strengthening and lifting mechanism of laser surface remanufacturing of failed components, underwater laser remanufacturing technology, extreme environmental metallurgy theory, stress/defect online monitoring methods and regulation mechanism, forming a theoretical and technical system for in-situ high-quality and efficient repair of failed components. 72 SCI search papers have been published in industry journals (including more than 60 first authors or corresponding authors), and 10 EI search papers have been published. The paper was published in well-known journals in the fields of machinery, materials, metallurgy, instruments, and other related fields such as Acta Mater., Addit. Manuf., Scripta Mater., Mater. Sci. Eng. A, Mater. Des., Appl. Surf. Sci., Metall. Mater. Trans. A, Surf. Coat. Technol., J. Manuf. Process., Measurement and other related fields. national invention patents are authorized, and invention patents are reviewed.
06Article information
Mingzhi Chen, Kun Yang, Zhandong Wang, Shibin Wang, Erke Wu, Yi Lu, Zhonghua Ni, Jinzhong Lu, Guifang Sun, Microstructure evolution and mechanical performance of NV E690 steel repaired by underwater laser directed energy deposit technique, Adv. Powder Mater. 2023, 2, 100095. https://doi.org/10.1016/j.apmate.2022.100095
