Fracture of High-Strength Bolt: Cause Analysis

Abstract: During the straightening process in the bolt manufacturing process, the bolt fracture occurs.

Through the macroscopic examination, chemical composition analysis, mechanical property test, metallographic structure analysis and fracture analysis of the broken bolt, the causes of the bolt fracture were analyzed.

The results show that the internal casting defects of the bolt are not eliminated due to improper hot forging process during the manufacturing process of the bolt, resulting in the reduction of the bolt bearing capacity and cracking in the straightening process.

Manufacture a batch of high-strength hexagonal bolts with specification of M42 mm, material of 42CrMoA and performance grade of 10.9.

The processing technology of bolts is as follows: annealing of raw materials → centerless turning → sawing → chamfering of flat end face → phosphating lubrication → shrinking rod → hot forging → chamfering of hexagon head → heat treatment (tempering) → straightening → rolling thread, and the tempering process is treated by mesh belt furnace.

During the straightening process, two pieces broke at about 1/2 of the screw axis (see Fig. 1).

The straightening of the remaining bolts in this batch was stopped immediately after 2 bolts broke during the straightening process.

In order to find out the cause of bolt fracture and avoid the recurrence of similar events, the author conducted relevant inspection and analysis on the broken bolts.

1. Physical and chemical testing

1.1 Macro inspection

1.1.1 Macro analysis of fracture

Both bolts were broken at about 1/2 of the screw to straighten the bending part, as shown in Fig. 1 (a).

The fracture surface is characterized by brittle fracture as a whole. The fracture surface presents radial stripes from the center to the periphery.

The outer layer of the fracture surface is a smooth and flat brittle fracture.

No macroscopic plastic deformation and slag inclusion are found on the surface of the fracture surface.

The arrow position in Fig. 2 shows cracks in the straightening process.

It shows that the crack starts from the center and expands to the periphery, finally leading to bolt fracture.

1.1.2 Macroscopic test

A transverse sample shall be taken 20 mm below the fracture surface of the broken bolt for low magnification inspection.

A large number of shrinkage cavities are found in the bolt center.

The inspection results are as follows: general looseness is Grade 1, central looseness is Grade 2, and general speckled segregation is < Grade 1.

See Fig. 3. No other macro defects such as cracks are found.

Fracture of High-Strength Bolt: Cause Analysis 1
(a)General drawing of broken bolt;
Fracture of High-Strength Bolt: Cause Analysis 2
(b)The breakpoint of bolts

Fig.1 Broken bolt

Fracture of High-Strength Bolt: Cause Analysis 3
Fig.2 Macromorphology of fracture
Fracture of High-Strength Bolt: Cause Analysis 4
Fig.3 Macro inspection of fracture of bolt

1.2 Chemical composition analysis

Take samples at about 20 mm near the fracture position of the bolt for chemical composition analysis.

The QSN750 direct reading spectrometer produced by OBLF of Germany is used.

The chemical composition of the material is tested to meet the requirements of GB/T 3077-1999 Alloy Structural Steel for the chemical composition of bolts of this material by spectral analysis. See Table 1.

The ONH-836 oxygen, nitrogen and hydrogen analyzer of American Liko Company is used to determine the oxygen, nitrogen and hydrogen content of the sample taken from the broken bolt. The results are: 0.0011% O, 0.0090% N, 0.0001% H.

The contents of O, N and H are low.

Table 1 Chemical composition of broken bolt (w,%)

Element

C

Si

Mn

P

S

Cr

Mo

Ni

Cu

Detection value

0.42

0.34

0.69

0.010

0.004

1.10

0.20

0.04

0.02

0.43

0.33

0.70

0.010

0.006

1.07

0.20

0.04

0.02

0.39

0.33

0.67

0.010

0.004

1.06

0.19

0.04

0.02

Standard value

0.38-0.45

0.17-0.37

0.50-0.80

0.025

0.025

0.90~1.20

0.13~0.25

<0.30

<0.25

1.3 Mechanical properties

Take one of the bolts in the same batch for tensile test.

The diameter of the tensile sample is 10 mm, which is not the actual bolt.

The HUT605A microcomputer controlled electro-hydraulic servo universal testing machine of Wance Group is used for mechanical performance test.

See Table 2 for the test results.

The hardness test is carried out on the metallographic sample of the broken bolt, and the test results are shown in Table 3.

There is no obvious difference in the hardness of bolt surface and center, and the mechanical property test results meet the requirements of Mechanical Properties of Fasteners – Bolts, Screws and Studs (GB/T 3098.1-2010).

Table2 Test result of mechanical properties

propertiesTensile strength Rm/MPaYield strength Rel/MPaElongation A/%Reduction of area Z/%
Detection value 106997014.653.5
Standard value ≥1040≥940≥9≥48

Table 3 Test result of hardness

Part

Surface/HBW

Core/HBW

Detection value

333、329、337

321、329、329

Standard value

316~375

1.4 Microstructure analysis

Take metallographic samples at the surface and core near the fracture of the broken bolt, and conduct metallographic inspection with OLYMPUS-GX51 metallographic microscope.

There are many holes in the center under polishing, as shown in Fig. 3 (a), and no obvious abnormalities are found on the surface.

The microstructure of the bolt’s surface and core is tempered sorbite.

The number of holes in the microstructure near the surface is relatively small, as shown in Fig. 3 (b).

The number of holes in the microstructure at the core is relatively large, as shown in Fig.3 (c). No obvious decarburization is found on the bolt surface, as shown in Fig. 3 (d).

Fracture of High-Strength Bolt: Cause Analysis 5
  • (a)polish state;
  • (b)near sunface;
  • (c)core;
  • (d)surface

Fig.4 Microstructure of bolt fractuire

1.5 Fracture morphology analysis

After ultrasonic cleaning, IT300 scanning electron microscope was used to analyze the fracture morphology.

SEM morphology observation on the radial area of the fracture surface shows that the fracture surface shows obvious cleavage fracture characteristics, and there are secondary cracks and a small number of holes at local locations, as shown in Fig. 4.

Fracture of High-Strength Bolt: Cause Analysis 6

Fig.5 SEM morphologyy of fracture

2. Comprehensive analysis

The chemical composition of the broken bolt meets the standard requirements.

The fracture shows transverse cracking. Macro analysis of the fracture surface shows that the fracture surface as a whole shows brittle fracture characteristics.

The fracture surface is divided into two parts. The crack initiation source area in the center of the fracture surface and the radial expansion area from the center to the periphery.

There is no plastic deformation at the fracture edge, showing the characteristics of brittle fracture.

Macroscopically low magnification test found that there was a serious problem of center looseness on the low magnification test surface (center looseness level 2).

In the process of casting, the loose system gradually solidifies the molten steel from the surface to the center, and the columnar crystal area grows toward the center in the form of dendrites.

The first crystallized dendrites are relatively pure and have a high melting point.

Segregated elements, gases, non-metallic inclusions and a small amount of non solidified molten steel are enriched between dendrites.

As the temperature decreases, the solidified part shrinks.

When the non solidified molten steel between dendrites is insufficient to supplement the gap, shrinkage cavities are formed Loose defects are called general looseness.

When the porosity occurs in the central equiaxed area, it is called central porosity.

The metallographic test results show that there are tiny holes, which are consistent with the low magnification test results.

Fracture analysis shows cleavage fracture morphology, and there are secondary cracks and a small number of holes on the fracture surface.

The mechanical property test results meet the requirements of relevant standards.

To sum up, the fracture shows transverse cracking, and the microstructure and mechanical property indexes meet the standard requirements.

In addition, the mesh belt furnace is used in the quenching and tempering process, and there is no untimely tempering, indicating that the bolt cracking is not formed in the quenching and tempering process.

The crack cracked from the center to the outside, and the hydrogen content was insufficient to cause hydrogen embrittlement, and no hydrogen embrittlement feature (chicken claw pattern) was found in the SEM photos, indicating that the bolt cracking was not caused by hydrogen induced delayed cracking.

The low magnification inspection found that there was obvious central looseness in the bolt center, its position was consistent with the crack source position of the fracture surface, and the fracture system was transversely cracked, so there must be axial tensile stress when cracking.

Therefore, combined with the bolt manufacturing process, it can be determined that the bolt had central looseness because of the raw materials.

In the hot forging process, the casting defects were not eliminated, and under the continuous axial tensile stress in the rod shrinking process.

The holes in the bolt initiate cracks and gradually expand outward, and then brittle cracks occur in the bolt during the straightening process, resulting in the failure of the bolt.

3. Conclusion

The root cause of the bolt fracture is that in the hot forging process, the casting defects (central looseness) in the bolt blank are not eliminated, resulting in the reduction of the bearing capacity of the bolt.

In the correction process, the cracks are caused under the tensile force.

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