| LIU Rui,YU Jia-cheng,YANG Qi-fan,WU Zhi-jie,XI Ming-zhe,GAO Shi-you.Effect of Laser Power on Mechanical Property and Strengthening Mechanism of DH36 Marine Steel[J],52(3):408-417 |
| Effect of Laser Power on Mechanical Property and Strengthening Mechanism of DH36 Marine Steel |
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| DOI：10.16490/j.cnki.issn.1001-3660.2023.03.039 |
| KeyWord:laser power Rosenthal solution mechanical properties precipitation strengthening mechanism |
| Author | Institution |
| LIU Rui |
School of Mechanical Engineering, Yanshan University, Hebei Qinhuangdao , China |
| YU Jia-cheng |
School of Mechanical Engineering, Yanshan University, Hebei Qinhuangdao , China |
| YANG Qi-fan |
School of Mechanical Engineering, Yanshan University, Hebei Qinhuangdao , China |
| WU Zhi-jie |
School of Mechanical Engineering, Yanshan University, Hebei Qinhuangdao , China |
| XI Ming-zhe |
School of Mechanical Engineering, Yanshan University, Hebei Qinhuangdao , China |
| GAO Shi-you |
School of Mechanical Engineering, Yanshan University, Hebei Qinhuangdao , China |
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| Abstract: |
| The laser remelting experiment was carried out in the in-house system, which consists of a 3 300 W fiber laser and a CNC four-axis working substrate. The DH36 marine steel plate is 200 mm long, 120 mm wide and 12 mm thick. An argon purged chamber with oxygen content less than 8×10‒6 was used to prevent the molten pool from oxidation. The laser remelting processing parameters were as follows:laser power 1 000, 1 500, 2 000 and 2 500 W, spot size 3 mm, laser scanning velocity 10 mm/s, overlapping ratio 50%. After grinding and polishing, the cubics cut by wire-electrode were etched with 4% nitric acid alcohol. The microstructure of yz section of DH36 marine steel was observed under ZEISS AX10 metallographic microscope (OM). The effect of different laser remelting power on the mechanical properties of DH36 Marine steel was studied by Zwick Z010 universal mechanical testing machine which the tensile speed was 1 mm/min. QPIX automatic vickers hardness tester was used to test the microhardness with the load of 200 g and the retention time of 20 s. JEM-2010 transmission electron microscope was used to observe the microstructure and size of precipitates in the xy plane after laser remelting process, and the element distribution was analyzed by mapping. The temperature gradient、cooling rate and solidification speed of molten pool during laser remelting process was simulated by Rosenthal solution. When P=1 000 W, the microstructure of the molten pool is mainly of AF and the overlapping zone consists of QF. It crosses the AF forming region on the CCT curve when the laser power is 1 000 W. The core of AF is Mn and a small amount of Si and Ti in the alloy, which grows fast in the shape of plate. Nanoscale cementites appear intragranular which the average length is 120-140 nm and the width is 20 nm. From the point of view of phase transformation, the carbon content of austenite is much higher than that of ferrite. Because of the lack of carbon content of DH36 steel, the pre-eutectoid reaction inevitably occurs when the molten pool goes through the cooling process. At a certain temperature gradient, however, the diffusion of C is inhibited, which makes it free from the ferrite boundary. At the same time, owing to the large cooling rate, the cementites don’t have enough time to connect and grow into lamellae and disperse in the grain as particles. Meanwhile, a certain amount of Ti and Nb precipitates in the crystal to form (Nb,Ti)C which reduces the nucleation barrier and promotes AF nucleation. From the energy point of view, the high temperature gradient in the molten pool provides sufficient driving force to satisfy the interface energy difference resulting from the formation of nanoscale cementites. When P increases to 2 500 W, spherical precipitates are produced in the grain and at the grain boundary. The average grain size in remelted layer is about 2 μm. The intracrystular precipitates pins for the dislocations, while the dislocations are wound into cells distributed around the grain boundary precipitates. The maximum size of precipitate is about 0.8 μm. There is a high content of Fe in the intragranular precipitates, while the grain boundary precipitates seldom contains of Fe and are mainly composed of Mn, Ti and Al elements. The diffusion driving force of Fe is much higher than that of Mn, Ti and Al. Therefore, it forms compounds intragranular as solid solution rather than moves to the grain boundary. When P=1 500 W, it has the maximum yield strength and extension strength, which is 495.7 MPa and 615.5 MPa, increased 7.5% and 3.2% compared with the DH36 steel. When P=1 000 W, AF appears at the bottom of remelting layer, and the microhardness is significantly increased to 448HV, which is 70.49% higher than that of DH36 steel. The microhardness of the remelting layer decreases gradually along the z direction which is the remelting zone > heat affected zone > DH36 steel. |
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