segregation; electroslag remelting; superalloy; microstructure; homogeneity

Abstract: The dendrite structure and precipitation phase differences of electroslag remelting ingot of Inconel 718, Incone 706 and Inconel 706M were studied. The results show that among the three kinds of superalloys, the difference between the dendrite spacing in the center of 718 ingots and the edge is the smallest, and the difference between the segregation rate of the main segregation elements Nb and Ti in the center of 718 ingots and the edge region is the smallest, and the macroscopic microstructure uniformity of 718 ingots is the best. The dendrite spacing difference between the center and the edge of 706M ingots is the largest, and the difference between the Nb element segregation rate in the center of the ingots and the edge is 0.91%. The microstructure uniformity of the ingots is the worst. 718 ingot dendrites are prone to enrich positive segregation elements Nb and Mo, with the highest contents reaching 6.82% and 3.01%, respectively. The highes Nb content between dendrites of 706 and 706M ingots is less than 4%, and the highest Ti element content is more thar 2%%. Meanwhile. the Nb segregation rate in the microstructure of 706 and 706M ingots is still very high. The phase of Laves in 706 ingot structure is the most, and the phase content of Laves in the center reach 3.9%, and the edge conten decrease by more than 2%, with uneven distribution. 706M Laves phase is the least in the cast ingot structure, and tha content in the center and edge is less than 2%. The content distribution is relatively uniform. Compared with 718 and 706, the average length of carbonitrides in the central region of 706M ingots varied the most from the edge, and there are fewer acicular phases in the microstructurc.

Superalloy is a material that has excellent strength, creep resistance and other properties at high temperatures (working temperature above 1000 ° C). When materials need to have excellent load resistance under static, fatigue and creep conditions, nickel-based superalloys have become the materials of choice. The as-cast microstructures of nickel-based superalloys mainly include matrix γ, strengthening phase γ / and γ // phase intergranular Laves phase, carbonitrides and a small amount of needle-like TCP phases. The as-cast structure often has a significant impact on the subsequent heat treatment process and properties of nickel-based superalloys. For example, the increase in the laves phase will often extend the heat treatment time, increase the difficulty of heat treatment, and affect the mechanical properties of the alloy. A large number of acicular phases such as δ will cause the alloy matrix to weaken, reduce the strength, and provide channels for crack initiation and propagation. Less and easily cause notch sensitivity of the alloy. Research shows that alloy elements affect the type and number of new phases in high-temperature alloys, such as the quantified phases γ / and γ // phases and acicular phases are often directly related to the content of elements such as Al, Ti, and Nb; increase the alloy The ratio of Al / Ti and the content of Al + Ti will reduce the precipitation of acicular δ phase and precipitated phase γ // phase; when the Ti content in the alloy exceeds 1%, a certain amount of acicular phase will precipitate; Al content Above 1.5%, the σ phase begins to precipitate: the Nb content reaches more than 5%, there will be a large number of δ phases in the structure, the element composition is the thermodynamic factor of the bridge phase in the superalloy, and the cooling rate determines the growth kinetics of the precipitate phase. Studies have shown that the Laves phase in superalloys is formed by the segregation of easily segregated elements such as Nb and Ti in the final residual liquid phase. Changes in the cooling rate will change the solute partition coefficient and affect the segregation of alloying elements; with the increase of the cooling rate, the average diameter, average length and average width of the carbides are significantly reduced, and the Laves phase is more likely to form a eutectic (selected in this paper Three typical nickel-based superalloys, Inconel 718 (referred to as “718”), Inconel 706 (referred to as “706”), and Inconel 706M (referred to as “706M”), were compared as the research objects. The segregation of components at different positions and the phase precipitation between products. The effects of elemental affinity and cooling rate on the microstructure of superalloys were studied, thereby revealing the uniformity of the three types of electroslag locks and providing theoretical guidance for subsequent component design and heat treatment processes

1 Experimental Method

The Inconel718 Inconel706 and Inconel706M high-temperature alloy ingots used in the experiment were obtained from the Institute of Iron and Steel Research, Luzhou Production Base, and obtained electrode ingots through vacuum induction melting (VIM). The electroslag remelting process was used for refining. The remelting current is 3.1 kA, the voltage is 22.2 V, and the size of the electroslag ingot after remelting is φ (220-230) mm × 170 mm. Table 1 shows the composition of the electroslag ingot. After the remelting, the samples of the center (marked as C) and edges (marked as E) are taken at the riser of the slag ingot in the radial direction, which are marked as: 718C, 718E, 706C, 706E, 706MC, 706ME, and slag. The ingot sample was mechanically polished.The 706M used 5 mL H2SO4 + 150 mLH-C1 + 20g CuSO4.5H2O + 80 mLH2O mixed solution as an etchant for chemical etching, and 718 and 706 used a 10% oxalic acid solution for electrolytic corrosion under an optical microscope. Microstructure analysis, Proimaging software was used to measure the distance between dendrite arms, quantitative metallographic analysis of Laves phase; field emission scanning electron microscopy was used to characterize the microstructure, and the size of the precipitated phase was analyzed statistically; phase scanning component EDS spectroscopy was used for phase composition analysis ; Using the emission electron probe microanalyzer (FE-EPMA), 20 groups of point components were taken at each position to analyze the composition segregation at the center and edges of the ingots of the three types of superalloys.

2 Experimental Results And Discussion

2.1 Comparison Of Equilibrium Phase Diagrams

Equilibrium phase diagrams of three superalloys were calculated using Thermo-calc 2018 software, as shown in Figure 1. In the equilibrium state of 718 alloy, there are a large number of δ, σ and other phases and a small amount of γ / phase precipitation. The carbide type is M23C6, and the δ phase precipitation of the 706 alloy significantly decreases, and the γ / precipitation amount increases. There is precipitation of δ phase, and the precipitation amount of σ phase is significantly reduced.The precipitation amount of γ / is also high, and the carbide type is MC.

2.2 Comparison Of Dendrite Structure And Segregation Rate

Fig. 2 is a photograph of the dendrite structure at the center and edges of the electroslag ingot. In the electroslag ingot, the primary and secondary arms are perpendicular to each other as γ columnar dendrites. Comparing the dendrite arm spacing (DAS) in Figure 2, it can be seen that the center and edge DAS value of the 718 alloy ingot is 15 μm, the center and edge DAS value of the 706 alloy ingot is 16 μm, and the center and edge DAS of the 706M alloy ingot The value difference reached 33 μm. It can be seen that the 706M ingot is more sensitive to the cooling rate and the macrostructure is more uneven.

Table 2 and Table 3 list the segregation element content and segregation rate in the center and edge regions of the superalloy, respectively, where the segregation rate (SR) is the ratio of the interdendritic element content to the dendritic stem element content. It can be seen that the closer the SR is to 1, the lower the degree of segregation. SR greater than 1 indicates positive segregation, otherwise it is negative segregation. From Table 2 and Table 3, in the three alloys, Nb, Mo, Ti and Si are all positive segregation, while Fe and Cr are negative segregation. On the one hand, the cooling rate of the central area of the ingot is smaller than that of the edge. The element segregation rate of the three kinds of high-temperature alloy ingots in the central area is more deviated from that of the edges, and the segregation in the central area of the ingot is more serious. A comprehensive comparison of the main segregation elements Nb and Ti segregation rates at the center and edges of each alloy ingot shows that the differences between the center and edge Nb and Ti segregation rates of 718 alloy ingots are 0.25% and 0.61%, respectively. The smallest, which indicates that the uniformity of the macrostructure of the 718 ingot is the best; and the difference between the Nb element segregation rate between the center and edges of the 706M alloy ingot reaches 0.91%. The ingot has the worst macrostructure uniformity. On the other hand, the 718 ingots are easily enriched with interstitial elements Nb and Mo, the highest content of which is 6.82% and 3.01%, respectively. The content is almost negligible, but the content of Si and Ti elements increases between dendrites, especially the highest content of Ti element is more than 2%. Although the contents of Nb and Mo in the components of 706 and 706M alloys are significantly reduced, due to the significant increase in the content of Si in the composition, more segregation between dendrites can promote the segregation of other elements that are liable to segregation, which is beneficial to the segregation of elements such as Nb. Concentrated at the intergranular position, the overall content of Nb, Mo and other elements in the interdendritic enrichment of 706 and 706M ingots is significantly reduced, and the segregation rate of Nb in the structure of 706 and 706M ingots is still large.

2.3 Comparison Of Intergranular Precipitation

Figure 3 shows the Laves phase precipitated between the crystals. The Laves in the center of the ingot are mostly lumpy, and the edges are mostly eutectic. The morphology of the Laves phase is often related to the Nb content in the final residual liquid phase of solidification, and the Nb content in the residual liquid phase has a direct relationship with the cooling rate. According to Miao Zhujun’s calculations, when the cooling rate is lower than 5 ℃ / min, A large amount of Laves phase will be precipitated, and a co-product Laves phase will be generated when the cooling rate is close to 20-30 ℃ / min. The cooling rate is slower near the center of the ingot, and the Laves phase is mostly distributed in a block shape. Table 4 shows the Laves phase composition analyzed by SEM-EDS. The main elements of the Laves phase are Ni Nb, Mo, Cr, Fe, and Ti. Compared with the 718 ingot, the Ni and Mo content in the 706 ingot Laves phase is significantly reduced, and The Fc content increased, and the Ti.Fe content in the Laves phase of the 706M ingot continued to increase. This was due to the decrease in the alloy element Ni.Mo content and the increase in the Ti.Fe content in the original components of the 706 and 706M electroslag chains. The Laves phase quantitative metallographic analysis results are shown in Figure 4.The Laves phase content in the figure represents the area percentage of Laves. In comparison, the Laves phase content in the 706 ingot is the highest, the distribution is uneven, and the Laves phase content in the center of the ingot. It reaches 3.9%, and the edge content is reduced by more than 2% .Laves phase content in the 706M ingot is the smallest and uniformly distributed.The content at the center and edge positions is less than 2%, and the difference is less than 0.1%. The Laves phase change law in the 718 ingot structure Between 706 and 706M. As the Si content in the 706 ingot increased significantly, it promoted the segregation of Nb to form the Laves phase, while 706M had a lower Nb, Mo content, and the Si content was reduced to 0.18%. Therefore, the 706M special mirror has fewer small and edge Laves phases, and the content distribution is more uniform. Figure 5 is a block-shaped or strip-shaped irregular composite carbonitride.The large-size block-shaped carbonitrides have high stability and often increase the difficulty of subsequent heat treatment.The carbonitride composition is shown in Table 5. These composite carbonitrides often have TIN as the core (dark gray phase in Figure 5) and Nb-enriched carbides (light gray phase in Figure 5). Studies have shown that TIN can be used as the nucleation core of NbC. This double-layered carbonitride is prone to appear in Nb and Ti superalloy cast locks.Figure 6 is a quantitative metallographic analysis of three kinds of superalloy carbonitrides.Due to the difference in cooling rate, the average carbonitrides in the center of the casting chain are average. The length is significantly larger than the edges. The length of the carbonitride in the center of the 706, 706M ingot is larger, which can be explained from the following aspects. The distance between the branches in the center of the 706 and 706M ingot is larger. The literature [12] shows that the distance between the dendrites increases and the space for carbide formation increases. This results in an increase in carbide size; the difference between the average length of the center of the 706M mirror and the edge carbonitride is 5 μm. This can be analyzed from the difference in the uniformity of the macrostructure of the 706M ingot. As a result, the length difference of carbonitrides increases. The carbonitride composition is shown in Table 5.Compared with 718 and 706, the Nb content in 706M carbonitrides is significantly increased. According to the aforementioned 706M cast chain, the dendrite is easy, and the Laves phase formed by segregation elements is less. The calculation results show that the γ / content in the 706M ingot structure increases. The amount of δ, σ and other phases is significantly reduced. Changes in the content of these phases will reduce the consumption of Nb elements, which may cause the Nh content in 706M carbonitrides to rise. ,

Needle-like phases often exist around the laves phase of superalloys. According to the literature, a small amount of acicular phase precipitation is beneficial to the high-temperature creep resistance of high-temperature alloys, but too much precipitation will often reduce the performance of superalloys and increase brittleness. 3 The needle-like phases in the cast chain structure of these high-temperature alloys are shown in Figures 3 (d) .3 (c) and 3 (0). Compared with 706M and 706, there are more acicular phases in the 718 ingot. Guo et al. Believe that the amount of acicular phase precipitation is related to the ratio of Ti and AI to the content of Nb. A large number of acicular phases will precipitate. Table 6 shows the contents of elemental Nb and the values ​​of Ti and Al in the three types of superalloys. The Nb content of 718 reaches 5.29%, and the ratio of Ti and Al is low, which leads to the precipitation of a large number of needle-like phases in the as-cast microstructure of 718, which prolongs the subsequent heat treatment and affects the heat-resistant bag performance of the casting. However, the lowest Nb content in 706 not only has a low Nb content but also a low Ti / Al ratio, so the precipitation of acicular phases is significantly reduced, which is consistent with the calculation results of the phase diagram in Figure 1. On the premise of reducing easily segregating elements such as Nb and controlling the uniformity of the superalloy, the content of acicular phase forming elements such as Al.Ti should be strictly controlled to provide a better basis for the subsequent heat treatment and forging process.

segregation

3 Conclusions

(1) Among the three alloy ingots, the difference between the intergranular connection and the edge of the central area of the Inconel718 ingot is the smallest, about 15 μm, and the difference between the main segregation elements Nb and Ti at the center and the edges is the smallest of 0.25% and 0.61%, the best uniformity of the macrostructure: Inconel 706M ingot center has the largest difference between the dendrite distance and the edge, up to 33μm, the difference between the Nb element segregation rate and the edge of the ingot center reaches 091%, and the cast lock has the worst uniformity The macrostructure uniformity of the nconel706 ingot is somewhere in between.

(2) Inconel 718 ingot dendrite is easily enriched with positive segregation elements Nb and Mo, with the highest contents of 6.82% and 3.01%, respectively. Inconel 706 and Inconel 706M ingot dendrites have the highest Nb element [high content below 4 %, Mo content is almost negligible, Ti element contains up to 2%, and the segregation rate of Nb in Inconel 706 and Inconel 706M ingot structure is still very high.

(3) Inconel 706 ingot structure has the most Laves phase, the content of Laves phase in the central position reaches 3.9%, the content of the edge decreases by more than 2%, and the distribution is uneven; the Inconel 706M ingot structure has the least Laves phase, and the center and edge content are low At 2%, the content difference is less than [0.1%, and the distribution is relatively uniform. The content and distribution of Laves phase such as Inconel 718 cast are in between. Compared with Inconel 718 and Inconel 706, the average length of carbonitriding in the central area of Inconel 706M ingot has the largest difference with the sand edge, about 5 μm, and there are fewer needle-like phases in the structure.

 

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