Study on persistent fracture behavior of GH907 alloy forgings

By comparing the composition and structure of the qualified and unqualified samples of the notch durability of GH907 forgings, the behavior of the persistent fracture of GH907 forgings was studied. The results show that the permanent fracture mode of GH907 forgings is mainly intergranular cracking, and the cracks mainly propagate along the grain boundaries without or insufficient precipitation, and terminate at the grain boundaries with sufficient precipitation of ε phase. There is a certain epsilon phase in the sample, there is no precipitation or insufficient precipitation at the grain boundary, and the notch endurance life of the forging is relatively low, which cannot meet the requirements of the technical condition of 60h.

Introduction

GH907 alloy is a new type of low-expansion high-temperature alloy developed in the 1970s. It is based on iron-nickel-cobalt and is comprehensively strengthened with niobium, titanium, silicon and trace boron. It has high strength below 650℃ , Low thermal expansion coefficient, good cold and hot fatigue performance, and almost constant elastic modulus. It is the material of choice for high-performance engine parts [1]. It is widely used abroad as the outer ring and sealing ring of the engine turbine to accurately control the gap between the turbine blades and the outer ring, improve engine performance and increase combustion efficiency.
In recent years, GH907 alloy with low expansion coefficient has been widely used in China as important casing components of aeroengines. For example, the rear casing of the high-pressure compressor, the bearing ring and the heat insulation ring are all ring forgings. Due to the small demand for forgings and few manufacturers, the forging factory does not have enough control over some details of forging production. The relationship between performance and element composition and the corresponding relationship with the process have not been fully grasped. The long-lasting performance of forgings is often in the factory for re-inspection. Unqualified phenomenon occurred.
At present, research institutes and institutes have conducted in-depth research on the structure and properties of the alloy [2], and some thermodynamic laws have been formed, which has a guiding role in the production of alloy forgings. This paper studies the corresponding relationship between the persistent fracture behavior of GH907 alloy and the structure, provides a basis for analyzing the reasons for the unqualified persistent properties of forgings, and provides some control for solving the engineering production of forgings.

Test method

Samples were taken from two forgings with different batches and the same forging process. The chemical composition of the forging blanks is shown in Table.1:
Table.1 Chemical composition

The size of the casing forgings is Ф574×Ф506×77, and 3 samples are taken from each forging, and samples are taken from the chord direction. The sample heat treatment system is 980℃±15℃, 1h air cooling or fast cooling +775℃±15℃, 12h furnace cooling at a rate of 55℃/h to 620℃±15℃, 8h air cooling.
The 6 specimens were respectively subjected to endurance life tests on the GWT-100 high temperature endurance testing machine according to the technical requirements of forging reinspection, and the fractured sample structure and fracture were observed under the SUPER55 scanning electron microscope and GX51 optical microscope.

Test results and discussion

The relationship between the gap durability and chemical composition

Table.2 Lasting results of gap

From the test results (see Table.2), the endurance life of the three samples of one furnace of raw materials are all qualified, and the endurance of the three samples of the other furnace of raw materials are all unqualified. It can be seen from Table 1 that the elements with large differences in composition are Si and Cr. The data show [3]: Si, Al, Ti, Nb elements all have an effect on the notch persistence of GH907 alloy, and Si has the greatest influence, and the notch persistence life increases greatly with the increase of Si.
The Si content in the raw material of one of the forgings was below the middle limit, and the endurance life did not meet the technical standard requirements. Therefore, in production, the Si content is generally required to be controlled at the upper limit.

Metallographic structure observation

The structure of the samples with qualified and unqualified endurance life is shown in Figure.1. It can be seen that the ε phase precipitated in the unqualified samples is obviously less, and there is almost no precipitation on the grain boundaries. There are many acicular epsilon phases in the qualified samples and they precipitate feathers along the grain boundaries.
Select one of the fractured samples, take a sample from the fractured part of the sample to observe the cross-sectional structure, and the grain size is 3.5-4, as shown in Figure.2. The fracture surface partially cracked along the smooth grain boundary. A secondary small crack can be seen below the fracture. It can be seen from magnification that the small crack extends along the grain boundary without ε phase precipitation, and finally ends at the grain boundary where ε phase precipitates sufficiently, as shown in the figure.3.
The ε phase is an important phase in the alloy, which has an important influence on the mechanical properties. The precipitation temperature is 700～920℃, and the precipitation peak temperature is about 800℃. The massive precipitation of ε phase is beneficial to eliminate the notch sensitivity of the alloy and greatly improve Its longevity [4]. As the Si content increases, the ε phase in the alloy increases significantly.
The notch permanent fracture of GH907 alloy is directly related to the absence of epsilon phase precipitation in the grain boundaries of the alloy, and the epsilon phase precipitation is related to the Si content under the same forging process conditions.
The data indicate [5]: The precipitation of ε phase is also related to the heat treatment process and the forging process.

Fig.1 Sample organization with qualified and unqualified endurance life

Fig. 2 Secondary cracks near the fracture surface of No.7 permanent specimen

Fig. 3 Morphology of ε phase at small cracks

Fracture observation

The characteristics of the persistent fracture of the notch are as follows: the fracture starts from the notch of the sample, and obvious intergranular fracture characteristics and secondary cracks can be seen in the initiation area of the crack, as shown in Figure 4. It can be seen from the figure that the fracture in the red line mark is not an intergranular feature, indicating that the alloy structure is not uniform, which may be related to the forging process. The magnified observation of the fracture zone along the grain shows that the grain boundaries of the secondary cracks are smoother, which is consistent with the metallographic observation results.

Fig. 4 Intergranular fracture characteristics and secondary cracks

Conclusion

• 1) The permanent fracture mode of GH907 forgings is mainly intergranular cracking. The crack propagates along the grain boundary without ε phase precipitation, and finally ends at the grain boundary where ε phase precipitation is sufficient.
• 2) The permanent fracture behavior of GH907 forgings is directly related to the absence of ε phase precipitation in the grain boundaries of the alloy, and the ε phase precipitation is related to the Si content under the same forging process conditions.

Author: Liu Jing, Zhang Yindong, Zhang Lixin, white Hao

Source: Network Arrangement – China GH907 alloy forgings Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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References:

• [1] GH907 alloy material research report
• [2] GH907 high pressure rear casing forging notch endurance performance analysis report
• [3] The effect of alloying elements on low-expansion superalloys
• [4] GH907 ring structure and durability analysis results
• [5] Review report of GH907 alloy development summary