Fracture Causes of GH4698 Superalloy Bolts

Bolt is an important connecting piece in the aeroengine, which plays an important role in the stable operation of the engine.

The fixing bolt of the aeroengine turbine disk should not only withstand the high working temperature, but also bear the high load brought by the high speed.

A bolt on the low-pressure turbine disk of an aeroengine broke.

The bolt material is GH4698 superalloy.

Its production process is: cutting and blanking → heat treatment (primary solution+secondary solution+aging) → machining → thread rolling → fluorescence inspection.

When assembling the engine, the bolt is used to insert the through hole of the turbine disk and the retainer ring, and the washer (made of 1Cr18Ni9Ti steel) and lock nut (made of GH4033 superalloy) are used at the corresponding position on the other side of the turbine disk for fixing.

The working temperature is 560~580 ℃, and the tightening torque of the nut is 7.84~9.8N · m.

The macro morphology of the turbine disk and its bolt assembly is shown in Figure 1.

Fracture Causes of GH4698 Superalloy Bolts 1
Fig. 1 Macro appearance of turbine disk bolt assembly

In order to find out the cause of the bolt fracture, the researchers conducted the physical and chemical inspection and analysis on the bolt.

1. Physical and chemical inspection

1.1 Macro observation

Fracture Causes of GH4698 Superalloy Bolts 2
Fig. 2 Macro morphology before and after bolt fracture

The fracture location of GH4698 superalloy bolt is bolt interlayer, which is basically located at the fitting place of washer and nut.

The macro morphology of the bolt before and after fracture is shown in Fig. 2.

Dismantle the broken bolt assembly and observe it under the stereo microscope.

It can be seen that the entire section is relatively flat and basically presents the intergranular fracture morphology. The macro morphology of the bolt fracture is shown in Fig. 3.

Fracture Causes of GH4698 Superalloy Bolts 3
Fig. 3 Macro morphology of bolt fracture

It can be seen from Fig. 3 that the crack source area (Area A) is distributed within the circumference of the bottom of the bolt, and the crack extends to the inside of the bolt;

The expansion area (Zone B) accounts for about 60% of the total section area, and the section is rough;

The central area (Zone C) is a transient fault area, which is fibrous.

Fracture Causes of GH4698 Superalloy Bolts 4
Fig. 4 Macroscopic Appearance of Circumferential Surface of Bolt Retaining Ring Fitting Section

From the side of the bolt, no obvious necking deformation or microcrack is found near the fracture surface of the bolt.

The fracture surfaces on both sides of the fracture can be completely matched.

The circumferential surface morphology of the fitting section of the bolt and retainer ring on the cracked side and the non cracked side are obviously different.

The circumferential surface on the cracked side is bright metal color, while the metal color on the circumferential surface on the non cracked side has been basically worn (see Fig. 4).

Macroscopic observation of the gasket shows that there are obvious differences in indentation depth and indentation morphology between the left and right sides (see Fig. 5).

Fracture Causes of GH4698 Superalloy Bolts 5
Fig. 5 Macro morphology of washer

1.2 Fracture analysis

Fracture Causes of GH4698 Superalloy Bolts 6
Fig. 6 Micro morphology of bolt fracture

The fracture morphology of the bolt was observed by scanning electron microscope (SEM).

No obvious inclusion defects are found near the crack source area [see Fig. 6a)], the fracture surface of the crack source area is of cleavage like morphology, fatigue bands are visible locally [see Fig. 6b)], obvious intergranular secondary cracks are visible in the propagation area [see Fig. 6c)], and the transient fracture area is of cleavage like morphology [see Fig. 6d)].

1.3 Metallographic inspection

Fracture Causes of GH4698 Superalloy Bolts 7
Fig. 7 Microstructure of Longitudinal Section of Bolt Fracture

The bolt fracture shall be sampled along the longitudinal section, and the microstructure in polished and corroded state shall be observed with an optical microscope.

No inclusion defects are found near the section, and obvious intergranular secondary cracks are visible, with grain size of 0-1 (see Fig. 7).

Fracture Causes of GH4698 Superalloy Bolts 8
Fig. 8 Microstructure of Fractured and Unfractured Bolt Threads

Take samples from the broken bolt to observe the micro morphology of the thread.

It can be seen that the integrity of the unbroken part of the thread is good, and no obvious processing defects are found [see Fig. 8a)].

By analyzing the microstructure of normal unbroken bolt [Fig. 8b)], it can be seen that the grain size of unbroken bolt is obviously smaller than that of the broken bolt, and its grain size is grade 3-4.

1.4 Chemical composition analysis

The broken bolt was sampled, and the chemical composition was analyzed by ICP-OES (Inductively Coupled Plasma Emission Spectrometer). The results met the technical requirements.

2. Analysis and discussion

Under normal circumstances, when assembling bolts, tightening and locking nuts will produce axial tensile stress on the bolts, while there is obvious indentation difference on both sides of the circumference of the bolt retainer ring fitting section.

The different indentation morphologies on both sides of the washer showed that the fractured bolt produced deflection tensile stress after assembly.

When the bolt is deflected, the additional bending stress increases with the increase of deflection angle, and the overall bearing capacity and fatigue life of the bolt will decrease significantly.

Generally, the finer the grain size of the material, the higher its strength, plasticity and impact toughness.

At high temperatures, the creep strength and rupture strength of coarse-grained materials are higher than those of fine-grained materials.

Generally, at 750~800 ℃, the grain boundary strengthening effect of GH4698 superalloy is enhanced, while coarse grains will shorten the grain boundary length, weaken the grain boundary strengthening effect, and intergranular fracture will occur under stress.

Although the chemical composition of GH4698 superalloy bolts meets the technical requirements, compared with normal bolts, their grains are relatively coarse.

At room temperature, the grain boundary can prevent crack growth, and the coarse grain will reduce the fatigue resistance of the material, so the coarse grain will also cause the bolt to fracture.

During the test run of the engine, under the combined action of axial stress, bending stress and engine vibration, the notch sensitivity of the bolt at the thread interlayer will increase, and a crack source will be generated.

The coarse grain of the material will also make the crack rapidly grow along the grain in the expanding area, and finally a transient fracture will occur at the center.

3. Prevention and improvement measures

The detailed assembly process can avoid the deviation of bolts due to improper assembly to a certain extent.

As there is no requirement for the grain size of bolts in the current process, the grain size of each batch of bolts varies greatly.

The original heat treatment process (1120 ℃ × 8h, air cooling+1000 ℃ × 4h, air cooling+775 ℃ × 16h, air cooling) adjusted to (1110 ℃ × 2h, air cooling+1000 ℃ × 4h, air cooling+775 ℃ × 16h, air cooling), the mechanical properties and grain size test results of bolts at different temperatures are shown in Table 1.

It can be seen from Table 1 that the mechanical properties of bolts at room temperature (23 ℃) are improved due to grain refinement, and the mechanical properties of bolts at high temperature (750 ℃) are decreased, which is consistent with the above analysis results.

At the same time, the bolt was installed and tested, and no crack or fracture occurred on the bolt.

Table 1 Test Results of Mechanical Properties and Grain Size of Bolts at Different Temperatures

Heat  treatment

After adjustment

Before adjustment

Requirement

Hardness/HBW

334

315

285~341

Grain size/grade

3.5

1~2

2~6

Room temperature tensile properties

Tensile strength/MPa

1307

1233

≥1130

Yield strength/MPa

857

787

≥705

Elongation after fracture/%

30

22.5

≥17

Reduction of area/%

40

23

≥19

Tensile property at 750 ℃

Tensile strength/MPa

908

958

≥740

Elongation after fracture/%

8.5

8.5

≥5

Reduction of area/%

11

15

≥8

Endurance life (750 ℃, 412MPa)/h

77.97

84.38

≥50

4. Conclusion

(1) The bolts are subject to fatigue fracture, which is mainly characterized by intergranular fracture, and the main reason for fracture is the deflection stress generated during assembly.

(2) The coarse grain of the broken bolt reduces the plasticity and impact toughness of the material, increases the sensitivity of the notch, and promotes the rapid intergranular growth of the crack.

(3) Detailed assembly requirements can avoid bolt deflection;

The mechanical properties of the material were improved by adjusting the heat treatment process and refining the grains.

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