The working temperature of the combustion chamber of aerospace engines can reach over 1100 ℃, requiring the combustion chamber material to have good high-temperature strength, oxidation resistance, and tissue thermal stability. GH 3230 alloy is used in key parts such as engine combustion chamber flanges. This alloy has the following breakthroughs and improvements compared to traditional combustion chamber alloys: replacing the single strengthening of W element with W and Mo element composite strengthening, which improves the solid solution strengthening effect; The high-temperature endurance and high-temperature oxidation resistance of the alloy were improved through microalloying of La element; A large amount of M6C carbides in the alloy have good thermal stability in the Ni matrix, which not only plays a high-temperature strengthening role but also improves the thermal stability of the alloy structure.
Due to the high degree of alloying of GH 3230, the total amount of W + Mo + Cr reaches 36%. During the processing, the deformation resistance is high, and the processing and forming are difficult. Improper production technology is prone to ring cracking. The carbon content in the alloy is relatively high, which is twice that of other solid solution strengthened high-temperature alloys in the same category. It forms a large amount of primary carbides M6C with W, Cr, and Mo. Uniformly distributed M6C carbides cannot only strengthen the alloy to a certain extent but also effectively bind grain boundaries, constrain grain growth, and improve the thermal stability of the alloy structure. However, in the actual production process, this type of carbide has a clear tendency of segregation. In the steel ingot formed by smelting, the carbide tends to concentrate between the as-cast dendrites and exists as a carbide band structure in the bar after being drawn and opened (Figure 1). Due to improper forging process or insufficient deformation, it is easy to cause the carbide banded structure to be inherited into the finished ring, leading to a weakened strengthening effect of the alloy matrix, uneven grain structure, etc., which in turn affects the mechanical properties and thermal stability of the ring.
This article mainly introduces the forming process of GH 3230 ring parts. It conducts forging process research to eliminate the banded structure of carbides in the ring parts, achieve uniform distribution of carbides, and improve the microstructure and mechanical properties of the finished ring parts in order to meet the material requirements of the combustion chamber flange of aerospace engines.
1. Process plan
1.1 Dimensions of raw materials and finished circular parts
The raw material of GH 3230 alloy is melted using a dual process of vacuum induction and electroslag remelting and poured into a quasi 460mm electroslag ingot. After billet opening and forging, it is formed into a quasi 200mm bar. The actual measured chemical composition is shown in Table 1.
Table.1 Measured Chemical Composition of GH 3230 Alloy (Mass Fraction, %)
The microstructure of GH 3230 bar is shown in Figure 1. From the figure, it can be clearly observed that the carbide banded structure in the alloy bar is obvious, and the structure is very uneven. A large number of M6C carbides are distributed in a banded manner along the flow direction of the bar. The grain size of the bar changes with the difference in carbide distribution; that is, the grain size is smaller in the positions where carbides gather and larger in the positions where carbides are sparse, which is related to the pinning effect of carbides on grain size.
Figure.1 Ribbon Structure of Carbide in GH 3230 Alloy Bar
The size of the rolled circular part is: outer diameter Ф 770mm × internal diameter Ф 650mm × Height 55mm.
1.2 Process flow and testing equipment
The processing process of circular parts generally includes: cutting, upsetting, punching, expanding, rolling and final rolling forming. When forming conventional high-temperature alloy products, the upsetting process is generally carried out by direct upsetting. Due to its particularity – the presence of a large number of carbides and strip structures, GH 3230 alloy is formed using two different forging processes for comparative analysis. In the process of upsetting the cake, two processes, unidirectional upsetting and multi-directional forging, are used to make the cake into blanks. Finally, the ring rolling process is followed to roll and form. The specific process routes for the two methods are:
- (1) Blanking – unidirectional upsetting – punching – expanding – pre rolling – final rolling forming;
- (2) Blanking multi-directional forging punching expanding pre-rolling final rolling forming.
Both unidirectional upsetting and multi-directional forging belong to a type of free forging, and their process characteristics are:
(1) Unidirectional upsetting
Unidirectional upsetting refers to the direct upsetting of a bar into a cake during compression deformation in the z-direction. Its characteristics are low fire frequency, simple operation, time-saving and material saving, but there may be problems such as insufficient deformation and uneven organization.
(2) Multidirectional forging
The principle of the multi-directional forging process is shown in Figure 2, that is, in the process of upsetting the cake, first upsetting along the z-direction, then 90 ° flipping the y-direction upsetting, and then 90 ° flipping the x-direction upsetting. Through multiple axial changes, the cake is finally upset. Compared with traditional unidirectional upsetting, the biggest characteristic of multi-directional forging is that during the deformation process, the material is continuously compressed and stretched with the axial changes of external loading, and the deformation is sufficient. Through repeated deformation, the effect of refining grains and improving performance is achieved. At present, multi-directional forging technology has been applied to various materials such as titanium alloy, aluminum alloy, and magnesium alloy. Still, it is less applied in the field of high-temperature alloys.
Figure.2 Schematic diagram of multi-directional forging
Use an electric furnace to heat the circular workpiece blank, and use an 8MN fast forging machine to upset, punch, and expand the hole; the Ф1600mm ring rolling mill is used for rolling forming.
2. Test results
2.1 Finished circular parts
The finished circular part was formed using both methods and the GH 3230 circular part, after multi-directional forging and rolling is shown in Figure 3. The formed circular parts have a standardized shape, no distortion or bulge, good surface quality, and no obvious defects such as cracks.
Figure.3 GH 3230 Alloy Finished Ring Parts
2.2 Alloy Structure
The microstructure of GH 3230 alloy is mainly composed of γ matrix is composed of a large amount of M6C type carbides and a trace amount of M23C6 type carbides. Among them, M6C is a primary carbide distributed in granular form within the grain and at grain boundaries. At the same time, M23C6 precipitates in small amounts at grain boundaries in the form of small particles or chains.
For the rolled microstructure and 1230 ℃ of circular parts formed by two different forging processes and rolling × After 1 hour of AC solid solution heat treatment, the microstructure was observed, and the results are shown in Figure 4.
Figure.4 Metallographic Structure of GH 3230 Alloy Ring Parts Formed by Different Processes
From Figure 4, it can be seen that the rolled microstructure of the circular part formed by the unidirectional upsetting process is still observable due to insufficient deformation, and the dendrite cast microstructure left over from the original bar can still be observed. At 1230 ℃ × After 1 hour of AC solid solution treatment, the banded structure of carbides is still relatively obvious. Although the grains undergo recrystallization and growth, the grain size is uneven, ranging from 25 to 55 μm. The non-uniformity of grain structure is precisely due to the uneven distribution of carbides, which leads to inconsistent pinning of carbides at grain boundaries during grain growth. The circular part formed by the multi-directional forging process has sufficient deformation, and the rolled structure of the finished ring part is uniform. At 1230 ℃ × After 1 hour of AC solid solution treatment, the distribution of carbides is uniform and the banded structure disappears. The grain size is uniform after recrystallization and growth, with an average grain size of about 40 μm.
The uniformity of the structure of the circular part formed by the multi-directional forging process is significantly better than that of the circular part formed by the unidirectional upsetting process.
2.3 Mechanical properties
The mechanical properties of GH 3230 alloy circular parts formed by two processes, unidirectional upsetting and multi-directional forging, were tested, and the results are shown in Tables 2 and 3.
Table.2 Tensile Properties of GH 3230 Alloy Ring Parts
|Test temperature/°C||Forging process||Tensile Property|
|Room temperature||Unidirectional upsetting||901||381||43.5||45|
From Tables 2 and 3, it can be seen that the tensile strength and plasticity level of the ring parts produced by unidirectional upsetting and multi-directional forging processes at room temperature and 1000 ℃ stretching are equivalent. Still, there is a significant difference in their endurance performance. The ring parts formed by a multi-directional forging process at 1000 ℃/60 MPa have better consistency and stability in endurance performance than unidirectional upsetting.
Table.3 High temperature endurance performance of GH 3230 alloy ring components
|Project||Forging process||Stress rupture property|
|1000 °C/60 MPa||Unidirectional upsetting||130||114||87|
GH 3230 alloy ring parts were formed using two processes: unidirectional upsetting and multi-directional forging. The GH 3230 annular part formed by a multi-directional forging process has a more uniform grain structure, uniform distribution of carbides, disappearance of banded structure, and more stable mechanical properties.
Author: Zhang Yonglu