Analysis of the Reasons for the Smaller Size of Inconel 718 Alloy Precision Rod Bolts

(HY-industry technical centre)

  1. Inconel 718 alloy corresponds to the domestic grade GH4169, which is a nickel-base superalloy strengthened by body-centered γ′′ and face-centered cubic γ′ phase precipitation. It has high yield strength, tensile strength and durability at low temperature and below 700°C. Strength, good fatigue resistance, oxidation resistance, corrosion resistance, good processing properties, weldability and long-term structural stability, widely used in high-temperature components of aircraft engines, is a common high-temperature alloy for engine fasteners. In the production of Inconel In the final inspection process of the 718 high-temperature alloy precision rod bolt, the diameter of the bolt polished rod part is smaller than the product design size requirement. After inspection and inspection, the inspection result of the bolt rod part size processing process meets the requirements.

    In order to find the cause of the problem, Before and after the process, the dimensional measurement and comparison were carried out, and the tracking and troubleshooting were finally determined. The diameter of the precision rod was reduced due to the aging heat treatment process. This paper analyzed the causes of the reduction of the size of the precision rod of Inconel 718 superalloy caused by aging heat treatment and its influence law to avoid product scrapping. At the same time, one against the other, on the other Superalloy precise dimensions commonly used portion of the bolt by the influence of aging heat treatment was studied systematically.

  2. For the stress-sustaining Inconel 718 alloy precision rod bolt, the main processing flow is: head forming → solution treatment → machining → rolling thread → aging treatment. The machining process between solution and aging treatment completes the size and rolling of the bolts, and the diameter of the polished rods has been machined to the final dimensions.

    In each process, it was found that the diameter of the polished rod after the aging of the precision rod bolt of the specification φ10 was reduced by 0.009 mm to 0.010 mm before the aging. It is indicated that aging heat treatment is the cause of the smaller size of Inconel 718 alloy, but its internal mechanism needs further analysis. The Inconel 718 alloy structure consists mainly of γ matrix phase, δ phase, carbide phase and γ′′ (Ni3Nb) phase and γ′ (Ni3(Al, Ti, Nb)) phase as the main strengthening phase.

    Studies have shown that solid solution Treatment, δ phase, γ′′ phase and γ′ phase dissolve into the matrix γ phase. The alloy is kept at 720 ° C for 8 h, and the γ′′ phase and γ′ phase are precipitated. The γ′ phase is mainly precipitated at 620 ° C for 8 h.In addition, studies have shown that  Inconel 718 undergoes solid phase transformation after standard heat treatment, and the precipitated γ′′ phase and γ′ phase are embedded in the γ matrix in a coherent manner. It is proved by experiments that after aging, γ The lattice constants of the γ” and γ’ phases are slightly reduced. According to the preliminary analysis, after the aging heat treatment of Inconel 718 alloy, the lattice constant and volume fraction of each phase change microscopically, and thus the dimensional change is macroscopically observed.

    In order to verify the above theoretical analysis results, the following process tests were carried out: the solid solution Inconel 718 alloy was ground to a diameter of 0.0 0.010 10 mm − with a centerless grinding machine, and its ends and the middle were measured with an indicator micrometer (accuracy of 0.001 mm). The diameter of the three parts, and the position mark, after the standard heat treatment aging (aging process: the product is installed in the furnace and vacuumed to (0.1 ~ 0.5) MPa → warmed to 720 ° C for 8 h → 55 ° C / h Cool to 620 ° C for 8 h → air cooling), use the same method to measure the diameter of the same position, found that the size is reduced by 0.009 mm ~ 0.013 mm before aging.The above test shows that after the aging treatment, the diameter of the sample becomes smaller by 0.009 mm to 0.013 mm.

    However, the sample after the aging heat treatment was again subjected to solution treatment, and the size of the same portion was measured by the same method. It was found that the diameter size at this time was substantially the same as that before the aging, indicating the diameter-to-size ratio after solution treatment. Before the solution treatment (aging state), it becomes larger by 0.009 mm to 0.013 mm. This further proves that the dimensional change of the alloy is due to solid phase transformation.

  3.  Test verification and influence law of the reason for the size reduction of Inconel 718 alloy:In order to study the effect of aging heat treatment on the size of Inconel 718 alloy.

    According to the product size characteristics, three raw materials with diameter specifications (φ6.1, φ8.1, φ10.1) were selected for testing. The technical conditions of the raw materials were AMS5708K, and the furnace numbers were L1543, FM69 and HT0593EY. Grinding them into cylindrical specimens of diameter ( 0 0.010 φ6− , 0 0.010 φ8− , 0 0.010 φ10− ) using a centerless grinding machine (as shown in Figure 1), 10 specimens per specification, each The two samples were measured at the two ends and the middle three parts. Each part was measured twice vertically and averaged. Three sets of data were obtained for each sample. There were 30 sets of data for each sample of 10 diameters. Measurements were made with an indicator micrometer, accurate to 0.001 mm, and the dimensions D and D’ before and after aging were measured, and then the corresponding dimensional change (D-D’) was calculated. A total of 30 sets of data were obtained and averaged for data analysis. The analysis was performed using OriginPro7.5 software and fitted into a straight line to obtain the dimensional relationship function before and after aging.

    The effect of aging on the size of Inconel 718 alloy is shown in Fig. 2. It can be seen from the figure that the size reduction of the alloy after aging is proportional to the size before aging, and the function relationship is obtained by linear fitting with OriginPro7.5 software. :DD’ = 0.00125D − 0.00133, D and D’ are the diameters before and after aging, respectively.

  4. Ageing effect on the size of Rene’ 41, Waspaloy and A286 alloys: For the other three kinds of superalloys Rene’41, Waspaloy and A286 alloys involved in precision rod bolts, it is predicted to be the same as Inconel 718 alloy, and the size is also after aging. It will become smaller, but due to the difference in composition, the law of change is different. The effect of aging on the size of Rene’41, Waspaloy and A286 alloys is shown in Fig. 3, Fig. 4 and Fig. 5 respectively. It can be seen from the figure that the size reduction of the alloy after aging is proportional to the size before aging. The OriginPro7.5 software straight-line fit yields the functional relationships: DD’ = 0.001D − 0.003 (Rene’41), DD’ = 0.000761D + 0.00239 (Waspaloy), and DD’ = 0.001259D − 0.00073 (A286), D and D’ are the diameters before and after aging, respectively.

  5. Preventive measures: According to the above test and analysis results, the dimensional changes of the four alloys after the aging of the common dimensional change range are summarized in Table 1 for different materials and specifications of the products. High-temperature alloy precision rod bolts are classified into fatigue type and stress-sustained type, and both require high dimensional accuracy of the polished rod part.

    For fatigue type precision bolts, in order to avoid the problem of small size after aging, the bolts and rolling threads can be finished after the aging process, which is not affected by aging; for the stress-resisting precision rod bolts, generally Before the aging, the final size of the rod can be processed according to the influence rule of the aging heat treatment of different grades of high-temperature alloy. The final diameter size is reserved according to Table 1 to avoid a small size after aging. After multi-batch production verification, after a certain amount of allowance is reserved according to Table 1 before aging, the final product size satisfies the product design requirements, effectively solving the problem that the diameter of the polished rod is small.

  6. Conclusion :

    The solid phase transformation of the aging heat treatment results in a smaller diameter of the Inconel 718 alloy precision rod bolt rod;

     After aging, the sizes of Rene’41, Waspaloy, A286 and Inconel 718 alloys are all reduced, and the reduction is proportional to the size before aging;

    Reserve a certain amount of size allowance before aging, which can effectively solve the problem that the diameter of the polished rod of the stress type high-temperature alloy precision rod bolt is small.

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