Microstructure and properties of UNS S32750 super-duplex stainless steels processed by hot isostatic pressing
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摘要: 采用等离子旋转电极雾化-热等静压工艺制备了UNS S32750超级双相不锈钢。利用光学显微镜、扫描电子显微镜、电子背散射衍射、万能试验机等手段研究了热等静压UNS S32750超级双相不锈钢固溶处理前后的显微组织和力学性能。结果表明：采用等离子旋转电极雾化制备的UNS S32750超级双相不锈钢粉末在150 MPa 压力下，经1200 ℃×3 h热等静压烧结后实现了致密化，相对密度为99.7%。随炉缓冷过程中，烧结件中析出的σ相导致材料韧性显著下降。经1035 ℃×1 h固溶处理后水淬，σ相完全溶解，材料韧性显著提高，显微组织为α和γ两相组织，体积比为65:35，抗拉强度791 MPa，屈服强度586 MPa，断后伸长率38%，冲击吸收功236 J。Abstract: The UNS S32750 super-duplex stainless steels were prepared by plasma rotating electrode processing and hot isostatic pressing in this study. The microstructure and mechanical properties of the steels before and after the solution treatment were investigated by optical microscope (OM), scanning electron microscope (SEM), electron back scattering diffraction (EBSD), and electronic material testing machine. The results indicate that the UNS S32750 super-duplex stainless steel powders prepared by plasma rotary electrode processing can be densified by hot isostatic pressing sintering at 1200 ℃ for 3 h at 150 MPa, and the relative density of the sintered parts is 99.7%. The σ phases precipitate in the sintered parts during the furnace slow cooling process, significantly undermining the impact toughness. In contrast, the σ phases are dissolved completely after the solution treatment at 1035 ℃ for 1 h followed by the water quenching, leading to a notable increase in the impact toughness. A duplex structure of the α and γ phases is identified, which significantly affects the properties of the specimens. In detail, the volume ratio between the α and γ phases is 65:35, the tensile strength and yield strength of the specimens are 791 MPa and 586 MPa, respectively, the elongation is approximately 38%, and the impact absorbing energy is 236 J.
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图 1 等离子旋转电极雾化粉末形貌
Figure 1. Morphology of the plasma rotating electrode atomized powder
图 2 JMatPro模拟的UNS S32750双相不锈钢相图
Figure 2. Phase diagram of the UNS S32750 duplex stainless steels simulated by JMatPro software
图 3 不同固溶处理试样金相组织：（a）S1；（b）S2；（c）S3；（d）S4；（e）S5
Figure 3. Microstructure of the specimens after the different solution treatments: (a) S1; (b) S2; (c) S3; (d) S4; (e) S5
图 4 S1试样电子背散射衍射组织
Figure 4. Electron back scattering diffraction images of the S1 sample
图 5 不同固溶处理工艺试样拉伸断口形貌：（a）S1；（b）S3
Figure 5. Tensile fracture morphology of the specimens after the different solution treatments: (a) S1; (b) S3
图 6 不同固溶处理工艺试样冲击断口形貌：（a）S1；（b）S3
Figure 6. Impact fracture morphology of the specimens after the different solution treatments: (a) S1; (b) S3
表 1 UNS S32750双相不锈钢粉末化学成分（质量分数）
Table 1. Chemical composition of the UNS S32750 duplex stainless steel powders
% Cr Ni Mo N Si Mn S P C O Fe 25.7100 6.0800 3.5600 0.2000 0.4500 0.7200 0.0040 0.0300 0.0280 0.0057 余量 
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表 2 UNS S32750双相不锈钢热处理工艺
Table 2. Heat treatment of the UNS S32750 super-duplex stainless steels
试样编号 加工工艺 S1 热等静压（1200 ℃ × 3 h、150 MPa）+ 炉冷 S2 热等静压（1200 ℃ × 3 h、150 MPa）+ 炉冷 + 固溶处理（1000 ℃ × 1 h）+ 水冷 S3 热等静压（1200 ℃ × 3 h、150 MPa）+ 炉冷 + 固溶处理（1035 ℃ × 1 h）+ 水冷 S4 热等静压（1200 ℃ × 3 h、150 MPa）+ 炉冷 + 固溶处理（1070 ℃ × 1 h）+ 水冷 S5 热等静压（1200 ℃ × 3 h、150 MPa）+ 炉冷 + 固溶处理（1100 ℃ × 1 h）+ 水冷
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表 3 不同固溶处理试样中α和γ两相体积比
Table 3. Volume ratio of α and γ phases in the specimens after the different solution treatments
试样编号 α与γ体积比 S1 52:44 S3 65:35 S4 65:35 S5 71:29
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