Solid solution strengthening deformation cobalt-based superalloy cobalt alloy Haynes 188/UNS R30188/GH5188 (HY-industry technical centre)
introduction
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Due to the use of cobalt-based alloys in a few aero-engines imported from the West, China trial-produced cobalt-based alloys of corresponding brands in the 1880s. At present, the technology has gradually matured and it has begun to export to foreign countries with advantageous price-performance ratios, the most representative of which The one is GH5188/Haynes 188, and the corresponding Shanghai HY Industry Co., Ltd brand is HY 188
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HY 188 Material grades and equivalents: Udimet alloy 188;Werkstoff 2.4683;Alloy 188;Inconel 188; Haynes Alloy 188;UNS R30188;GH 5188
Composition and characteristics of Haynes 188/UNS R30188/GH5188 alloy:
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HY 188/GH5188/Haynes 188 alloy contains 22% Ni to stabilize the austenite structure, which prevents it from transforming to a close-packed hexagonal structure at lower temperatures, and can also reduce deformation resistance and improve hot workability. Adding a large amount of insoluble metal element W (14.5%) for solid solution strengthening is an important feature of GH5188 alloy. The W atoms dissolved in the γ matrix can reduce stacking fault energy and diffusion coefficient, and can increase the modulus of elasticity, thus improving the high temperature creep resistance of the alloy.
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The high tungsten content also promotes the precipitation of M6C carbides. Another feature of the alloy composition is the addition of a large amount of Cr (22%) to form a stable Cr2O3 oxide film, improve the oxidation resistance and thermal corrosion resistance of the alloy, and also produce a moderate solid solution strengthening effect. However, the content of W and Cr is on the upper limit, and there is a tendency to form Co2W type Laves in the alloy.
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HY 188/GH5188/Haynes 188 alloy is developed from L-605 alloy by calculating the number of electron holes, and it should be an alloy with stable structure. However, due to the wide range of alloy composition, if both Cr and W are controlled at the upper limit, the average number of electron holes in the alloy matrix will exceed the critical value for forming the Laves phase, and the Laves phase will be precipitated. As long as the composition range of the existing alloy is carefully designed with the phase calculation method (PHACOMP), and the smelting process is carried out through a strict smelting process, the content of Ni, Cr, W and Si is well matched, and the good effect of no Laves precipitation can be achieved.
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Another important feature of HY 188/GH5188/Haynes 188 alloy composition is the use of rare earth element La for microalloying. Although the La added in the alloy is only 0.03%~0.12%, it has a great influence on the mechanical properties, especially the plasticity, and the effect of improving the plasticity is very obvious. At the same time, it also effectively improves the oxidation resistance. The La added to the alloy forms a La-rich compound, which combines with MC to form La, M, phase, which exists as an inclusion phase in γ austenite, and the remaining La is mainly enriched in grain boundaries. From the results of Auger spectroscopy analysis on the facets along the crystal fracture, it can be seen that with the increase of the peeling time, the concentration of lanthanum gradually decreases until it balances with the concentration in the crystal, and it can be seen that lanthanum atoms are obviously segregated in the grain boundaries. Before electroslag remelting, the degree of lanthanum segregation is greater than that after electroslag remelting, which is related to the burning loss of lanthanum during the electroslag remelting process. The lanthanum atoms segregated at the grain boundary affect the segregation of other elements at the grain boundary, affect the precipitation of carbides, slow down the precipitation rate of carbides, and increase the bonding force of the grain boundaries, so that the plasticity of the alloy is significantly improved.
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Rare earth element La is added to the GH5188 alloy, because its atoms are larger than the Co atomic radius, the matrix causes lattice distortion, reduces the activation energy of Cr*3 diffusion, increases the diffusion coefficient of Cr*3, and promotes the rapid and large-scale formation of Cr2O3. Good protection. The oxide in the surface layer of lanthanum can increase the bonding force between the oxide film and the substrate, and make the oxide film adhere to the surface of the substrate closely, thereby further improving the oxidation resistance.
Hot working and heat treatment of Haynes 188/UNS R30188/GH5188
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GH5188 alloy contains a large amount of solid solution strengthening element W, which deteriorates its hot workability. In the actual forging hot working process, forging cracks and other phenomena often occur. The research results of the thermal simulation process performance of the cobalt-based alloy GH5188 show that when the deformation rate is constant at 20/s, the maximum allowable deformation cmx of the alloy at different deformation temperatures shows that the best thermal deformation temperature range of GH5188 alloy is 1100~1200℃. When the maximum allowable deformation of the alloy reaches its peak and the εmax-T curve appears to be a plateau, the deformation resistance remains at a low level within the range of medium deformation, and the surface of the deformed sample is smooth, without wrinkles and cracks, and the deformed structure at the section is uniform , No abnormal structure and microcracks caused by stress concentration and uneven deformation.
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In addition, under the same temperature and deformation, different deformation rates have certain effects on the hot workability and recrystallization structure of the alloy. The alloy has a uniform deformation structure and recrystallized structure within the optimal thermal deformation temperature range and medium reduction rate at a deformation rate of 10-1/8~10/s, no micro-cracks inside, and a smooth outer surface No cracks. Therefore, in the actual production such as forging and other hot working, the initial forging and final forging temperature should be well controlled. Under the normal forging rate and deformation, the alloy will have better forgeability.
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In the industrial production of GH5188 alloy, the forging heating temperature is 1180℃, and the final forging temperature is not less than 980℃. 1. The ring parts are heated by oil furnace at 1150±20℃, and the ring is heated by electric furnace at 1160~1180℃. More than 20% forging workers and 1180℃ solution heat treatment can obtain satisfactory structure and performance
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The heat treatment system of GH5188 alloy is solid solution treatment, 1170~1190℃ for hot-rolled sheet, air cooling; for cold-rolled strip and plate, 1165~1230℃, rapid air cooling; for bar and forging, 1180℃±10℃ , Fast air cooling. In addition to γ-Austenite, the solid solution structure has M.C primary carbides and a small amount of MC and La-rich La, M, phases combined with lanthanum-rich compounds, and a small amount of MB2 and MC phases. After long-term aging at high temperature, M6C decomposes into M23C6. Some GH5188 alloys may also precipitate Laves phase, which will be solid-solved at 1180℃ or re-dissolved in γ matrix after long-term aging at 870~980℃.
Oxidation resistance of Haynes 188/UNS R30188/GH5188 alloy
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As an index required by technical conditions, the technical conditions of superalloys are very important in actual production. Both suppliers and buyers should strictly implement them. Generally, high-temperature alloys do not include oxidation resistance in the technical conditions. However, the technical conditions of GH5188 alloy require oxidation infiltration. Layer depth is used as an inspection index, which is a special requirement of GH5188 alloy. Research work points out that metallurgical quality has a significant impact on the oxidation resistance of GH5188 alloy. If the structure of GH5188 alloy is uniform before oxidation, the microsegregation is small, and fine M and C particles are dispersed and distributed in the crystals. When the processes of diffusion and depletion occur , The microscopic composition of each area will not be too different, and the oxidation will expand slowly and uniformly, such as GH5188 strip; if the structure of the alloy before oxidation is not uniform, the intragranular and grain boundary carbide particles are extremely large and form serious microsegregation At this time, the microscopic composition is very different. When processes such as diffusion and oxidation occur, the metal elements such as Cr in the high-concentration area (large carbide concentration area) first diffuse outward, forming poor Cr and Al at the boundary of the bulk carbide In the “depleted zone” of other metal elements, the oxidation infiltration quickly spreads along these areas first, forming an uneven oxide layer.
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Therefore, the oxidation resistance of GH5188 alloy is closely related to the metallurgical quality of the alloy. The size and distribution of carbides and micro-segregation have a greater impact on the oxidation resistance than the grain size, and oxidation tends to proceed along the carbide boundary; when the grains are fine, La, My, MC, M6C, and M23C6 are small in size and dispersed. , GH5188 alloy has the best oxidation resistance. Because the metallurgical quality of the alloy is very sensitive to the oxidation resistance of the alloy, the AMS technical conditions stipulate that every heat and batch of GH5188 alloy must be tested for oxidation resistance. AMS5608D specifically stipulates as:
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The test surface of the sample (excluding the clamping part) shall not be less than 9.7c㎡. The test surface shall be polished by hand with 120# or finer silicon carbide sandpaper, and the grease shall be removed. During the test, the sample can be inserted into the emotional ceramic tile or suspended on the emotional ceramic rod. The sample must not be placed in the crucible.
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The sample should undergo 4 cycles, each cycle includes heating to (1095~1150)±14℃, keeping it for 25±1h, and air cooling to 150℃ or lower. Heating should be carried out in a furnace that can provide natural ventilation, air flow, and uniform exposure of the sample surface.
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Check the polished cross-section with an instrument with a magnification of not less than 500 times. Each side must not have damaged metal larger than an average of 0.04mm (metal converted oxide scale plus any continuous intergranular oxide).
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If a damaged metal area with a diameter greater than 1.5mm appears locally on the sample, and it is not caused by contact with the ceramic fulcrum, the sample should be considered invalid and the test should be re-tested. This state will still appear after the retest. The product should be rejected.
