Short Process Recovery of Scrap 690 Alloy by Carbon Thermal Reduction AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY23040020

Chinese Library Classification Number:TG146

Citation Information:

Nickel is an important raw material for fabrication of high-temperature alloys and corrosion-resistant alloys due to its high melting point，corrosion resistance，oxidation resistance，ductility and other excellent properties. As one kind of key strategic resources，nickel has been widely used in aerospace，defense，nuclear energy and other important fields. However，China is a typical nickel-deficit country，and the dependence on foreign nickel ore reaches as high as 95%. Recycling of nickel-contained scrap can effectively alleviate the shortage of nickel resources，and is of great significance to ensure the safe and sustainable supply of nickel resources in China. Compared with other types of nickel-contained scraps，nickel-based high-temperature alloy scrap exhibits higher recovery value because of its advanced preparation process and containing various kinds of strategic metal elements. Currently，different methods including pyrometallurgical，hydrometallurgical，and pyro-hydrometallurgical processing are applied for recycling scrap nickel-based high-temperature alloys. However，these methods have disadvantages such as multi-step processing and relatively low utilization rates. What's more，the achieved products by the above methods usually are metal elements or corresponded salts. To achieve same-level utilization of scrap nickel-based alloys，a direct re-melting method through vacuum induction melting is used to recycle scrap 690 alloy. However，re-melted 690 alloy shows poor performance，especially deterioration of ductility，which is resulted from a large number of Cr₂O₃ inclusions distrusted in re-melted 690 alloy. Consequently，Cr₂O₃ inclusions cannot be removed effectively through the direct re-melting method. Therefore，carbon thermal reduction was proposed to solve this problem in this study. Scrap 690 alloy was obtained through aging treatment at 850 ℃ for 360 h by considering that oxidation as well as microstructure coarsening were the main factors for serving failure of 690 alloys. The above scrap 690 alloy were re-melted using carbon thermal reduction at 1550 ℃，and the carbon reduced 690 alloy was obtained. Fe content of the alloy was analyzed using inductively coupled plasma optical emission spectrometer（ICP-OES），while titration was used to determine Cr content. Oxygen-nitrogen-hydrogen analyzer was utilized to analyze O and N content，and carbon-sulfur analyzer was used to determine C and S content. The microstructure and fracture morphology of three types of 690 alloys were observed using optical microscopy （OM），scanning electron microscopy （SEM） equipped with energy dispersive spectroscopy （EDS），and X-ray diffraction（XRD）. Microhardness was performed using Vickers microhardness tester under conditions of Load retention for 10 s at test force of 1960 N. The tensile properties of three types of 690 alloys were tested by electronic universal testing machine at room temperature and 650 ℃. SEM images indicated that a large amount of Cr₂₃C₆ precipitates were uniformly distributed in carbon reduced 690 alloy，while there were almost no inclusions of Cr₂O₃. Moreover，the oxygen content of carbon reduced 690 alloy decreased to 88×10^-6. Therefore，it could be reasonably included that the inclusions of Cr₂O₃ were successfully transformed into precipitates of Cr₂₃C₆ through the carbon thermal reduction method. As a result，the mechanical properties of carbon reduced 690 alloy significantly improved. The hardness of carbon reduced 690 alloy was HV 278. The tensile strength of carbon reduced 690 alloy at room temperature and 650 ℃ was 672 and 395 MPa，respectively，which was 10.5% and 44.2% higher than those of standard 690 alloy. More importantly，the elongation at room temperature and 650 ℃ of carbon reduced 690 alloy was increased 18.8% and 96.0%，respectively，compared with direct re-melted 690 alloy. The difference in mechanical properties between the two types of remelted 690 alloys was mainly related to the type and distribution of second-phase particles. The inclusions of Cr₂O₃ were brittle particles，which were prone to stress concentration at the interface between inclusions and matrix. Their distribution would not get changed accordingly during plastic deformation，where voids and cracks were easily formed，leading to an obvious decrease in the tensile ductility of direct re-melted 690 alloy. For carbon reduced 690 alloy，the unfavorable inclusions of Cr₂O₃ were transformed into beneficial precipitates of Cr₂₃C₆ benefited from the carbon thermal reduction method. During the tensile test，Cr₂₃C₆ precipitates could slow down the aggregation and growth of micropores，hinder crack generation，and improve the ductility of alloy. Furthermore，a large amount of uniformly distributed Cr₂₃C₆ precipitates contributed to the higher strength of carbon reduced 690 alloy due to dispersion strengthening effect. Also，these precipitates of Cr₂₃C₆ contributed to the fine grain microstructure，where grain boundary strengthening was achieved. These results indicated that recycling of scrap 690 alloy using carbon thermal reduction could effectively remove the oxide inclusions，and improve both the strength and ductility of re-melted 690 alloy.