Abstract: This paper analyzes the reason that the size of Inconel718 Alloy Precision Bolts polished rods decreases due to the solid-state phase transformation during aging heat treatment. At the same time, the effect of aging heat treatment on the size of Inconel 718 and other four kinds of superalloys is studied. The results show that the size of the precision rod of Inconel 718 superalloy decreases due to the solid-state phase transformation of the aging heat treatment. The size of Inconel 718, Rene41, Waspaloy and A286 alloys all decrease after aging, and the reduction is proportional to the size before aging. . A certain size allowance is reserved before aging, which can effectively solve the problem that the size of the polished rod of the permanent stress superalloy precision rod bolt is too small.
Inconel 718 alloy corresponds to the domestic brand GH4169. It is a nickel-based superalloy with body-centered tetragonal γ″ and face-centered cubic γ′ phase precipitation strengthened, with high yield strength, tensile strength and lasting strength at low temperature and below 700℃ , At the same time, it has good fatigue resistance, oxidation resistance and corrosion resistance, as well as good processing performance, welding performance and long-term microstructure stability. In the final inspection process of Alloy Precision Bolts, it was found that the diameter of the polished shank of the bolt was smaller than the product design size requirements. The inspection records of the inspection process showed that the inspection results of the dimensional processing process of the bolt shank met the requirements. In order to find the cause of the problem, before and after the processing of each process The size measurement and comparison were carried out, and it was finally determined after the follow-up investigation that the aging heat treatment process caused the diameter of the precision rod to become smaller. In this paper, the reasons for the size reduction of the Inconel 718 superalloy precision rod caused by the aging heat treatment and its influence rule were analyzed to avoid product scrap. , a systematic study was carried out on the influence of aging heat treatment on the dimensions of other three commonly used superalloy precision bolts.
2. Analysis of the reasons for the smaller diameter of the Inconel718 Alloy Precision Bolts
For the stress durable Inconel718 Alloy Precision Bolts, the main processing flow is: head forming → solution treatment → machining → rolling thread → aging treatment. The machining process between solution and aging completes the dimensional finishing of the bolt and the rolled thread, and the polished rod section diameter is machined to final size. During the investigation of each process, it was found that the diameter of the polished rod of the precision rod bolt with a specification of φ10 after aging was reduced by 0.009 mm~0.010 mm compared with that before aging. It shows that aging heat treatment is the reason for the size reduction of Inconel 718 alloy, but its internal mechanism needs further analysis. The microstructure of Inconel 718 alloy is mainly composed of γ matrix phase, δ phase, carbide phase and γ″(Ni3Nb) phase and γ′(Ni3(Al,Ti,Nb)) phase as the main strengthening phase. After treatment, δ phase, γ″ phase and γ′ phase were dissolved into the matrix γ phase. The alloy was kept at 720 °C for 8 h, γ″ phase and γ′ phase were precipitated, and at 620 °C for 8 h, γ′ phase was mainly precipitated. , Some studies have shown that after standard heat treatment of Inconel 718, a solid-state phase transition occurs, and the precipitated γ″ phase and γ′ phase are embedded in the γ matrix in a coherent manner. And it is proved by experiments that the lattice constants of γ, γ″ and γ′ phases decrease slightly after aging.
Preliminary analysis shows that after the aging heat treatment of Inconel 718 alloy, the lattice constant and volume fraction of each phase have changed on the microscopic level, so the size of the alloy has changed on the macroscopic level. In order to verify the above theoretical analysis results, the following process tests were carried out: the Inconel 718 alloy in the solid solution state was finely ground with a centerless grinder to a diameter of 0 0.010 10 mm − , and the two ends and the middle were measured with an indicating micrometer (with an accuracy of 0.001 mm). The diameters of the three parts are marked, and the positions are marked. After standard heat treatment and aging (the aging process is: put the product in a furnace and vacuumize to (0.1~0.5) MPa → heat up to 720 °C for 8 h → heat at 55 °C/ h Cool to 620°C for 8 h → air cooling.), measure the diameter of the same position by the same method, and find that the size is 0.009 mm~0.013 mm smaller than that 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 subjected to solution treatment again, and the size of the same part was measured by the same method. It was found that the diameter size at this time was basically the same as the size before aging, indicating that after the solution treatment, the diameter size ratio Before solution treatment (aging state), it becomes larger by 0.009 mm~0.013 mm. This further proves that the dimensional change of the alloy is due to solid-state phase transformation.
3. Experimental verification and influence rule 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, the laboratory of Shanghai HY Industry Co., Ltd selected 3 raw materials with diameter specifications (φ6.1, φ8.1, φ10.1) for testing. FM69, HT0593EY. Using a centerless grinder, they were finely ground into cylindrical samples with diameters ( 0 0.010 φ6− , 0 0.010 φ8− , 0 0.010 φ10− ) (as shown in Figure 1), and 10 samples were prepared for each specification. Each sample was measured at both ends and three parts in the middle, and each part was measured vertically twice to obtain the average value, to obtain 3 sets of data for each sample, 10 samples for each diameter specification, a total of 30 sets of data. Use an indicating micrometer to measure, accurate to 0.001 mm, measure the dimensions D and D’ before and after aging, and then calculate the corresponding dimensional change (D-D’), a total of 30 sets of data, and take the average value for data analysis. OriginPro7.5 software was used for analysis, and a straight line was fitted to obtain the dimensional relationship function before and after aging. The size reduction of Inconel 718 alloy after aging is proportional to the size before aging. The functional relationship is obtained by linear fitting with OriginPro7.5 software: D-D′ = 0.00125D − 0.00133, D and D′ are the diameters before and after aging, respectively.
4. The effect of aging on the size of Rene41, Waspaloy and A286 alloys
Based on the principle of drawing inferences from others, the other three high-temperature alloys Rene41, Waspaloy and A286 alloys involved in precision rod bolts are predicted to be the same as Inconel 718 alloys, and their sizes will become smaller after aging, but due to the difference in composition, the change rules are as follows: differentiated. The size reduction of Rene41, Waspaloy and A286 alloys after aging is proportional to the size before aging. The functional relationship is obtained by linear fitting of OriginPro7.5 software: D-D′ = 0.001D − 0.003 (Rene′41), D-D′ = 0.000761D + 0.00239 (Waspaloy) and D-D′ = 0.001259D − 0.00073 (A286), D and D′ are the diameters before and after aging, respectively.
According to the above test and analysis results, the superalloy precision rod bolts are divided into fatigue type and stress endurance type, both of which have high requirements on the dimensional accuracy of the polished rod part. For fatigue type precision shank bolts, in order to avoid the problem of reducing the size after aging, the bolt shank and rolling threads can be finished after the aging process, which is not affected by aging; and for stress lasting precision shank bolts, generally in Before the aging process to the final size of the rod, according to the influence of the aging heat treatment on the size of different grades of superalloy, a certain size allowance can be reserved for the final diameter size to avoid the size reduction after aging. After multiple batches of production verification, after a certain size allowance is reserved before aging, the final product size meets the product design requirements, effectively solving the problem of the small diameter of the polished rod.
The solid-state phase transformation of the aging heat treatment leads to a reduction in the diameter of the polished rod of Inconel718 Alloy Precision Bolts;
After aging, the size of Rene41, Waspaloy, A286 and Inconel 718 alloys are reduced, and the reduction is proportional to the size before aging;
A certain size allowance is reserved before aging, which can effectively solve the problem that the diameter of the polished rod of the stress-type superalloy precision rod bolt is too small.
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