Appendix No 1

8/10/2019 Appendix No 1

1/7


2/7

Report by: 5 Camber Grove

Metspect Sarel Cilliers Circle

Pinetown

4041

2

BACKGROUND

We were told that the bolt failed on a building site in Mocambique. At the time of failure it

had been mounted in a concrete plinth which had been allowed to cure. To align it with the

mating girder it was necessary to straighten the bolt using a hand-held pipe as a lever. It wasduring this operation that the bolt snapped.

EXPERIMENTAL

The failed bolt was examined visually and with a stereo microscope. Sections were then

removed for:

a) The fracture face of the failed bolt was examined optically and on a SEM to

determine the failure mode.

b) Longitudinal and transverse sections were removed from both the failed and

the new bolt. These were mounted, polished, etched in 2% nital and

examined on an optical microscope to determine the microstructure. These

samples were then used for Vickers hardness testing

c) Tensile test pieces were machined from both the failed and the new bolt.

Because of the size of the failed sample it was only not possible to obtain a

full 50mm gauge length sample

d) Spectroscopic chemical analysis was done on the sectional remains from the

tensile test pieces.

RESULTS

Visual Examination

Photo 2 Photo 3

Initiation

point


3/7



Pinetown

4041

3

Photo 2 shows the failed bolt and the threaded portion of the new one. It can be seen that

failure occurred on the last, or very near the last, thread. Due to bending moments this

would have been the highest stress section during the straightening operation.

Photo 3 shows the fracture face with the initiation point marked. The fracture is flat faced

with no significant macro deformation, this is indicative of brittle fracture on a macro level.

Microscopic Examination

Photo 4 Photo 5

Photo 4 and 5 show the microstructure of the failed bolt at 200x magnification. Photo 4 is a

transverse section and photo 5 is a longitudinal direction. The samples have been etched in

2% nital to reveal a microstructure of tempered martensite with some grain boundary

bainite.

There is some banding evident in the longitudinal direction. Banding occurs when there is

non-uniform chemical composition across the section. This can cause variations in hardness

between bands.

Photo 6 Photo 7

Photos 6 and 7 show the microstructure of the new bolt at x200 magnification. Photo 6 is a

transverse section and photo 7 is a longitudinal direction. The samples have been etched in


4/7



Pinetown

4041

4

2% nital to reveal a microstructure, which again was tempered martensite with some grain

boundary bainite. Again there is some banding evident in the longitudinal direction.

Scanning Electron Microscope Examination

The fracture face of the broken bolt was examined on a scanning electron microscope (SEM)

to determine the failure mode. Scale bars and magnification are shown on the photos.

Photo 8 Photo 9

Photo 8 was taken at 600x and photo 9 is the same location at 1200x magnification. The

photographs are typical of cleavage failure. Decohesion or cracking can also be seen.

Cleavage is a mechanism of brittle transgranular fracture, resulting in cleaving of the crystals

along their crystallographic planes.

Photo 10 Photo 11

Dimple


5/7



Pinetown

4041

5

Photo 9 shows more clearly the river patterns associated with brittle cleavage failure. More

than 90% of the fracture face showed a cleavage or quasi-cleavage mode of failure. There

were also small areas of dimple rupture, synonymous with a more ductile failure mode, as

shown in photo 11.

Chemical Analysis and Mechanical Properties

Elements Failed Bolt New Bolta709M40 (En19)

bPD970-709M40

Carbon 0.38 0.40 0.35-0.45 0.36-0.44

Manganese 0.82 0.87 0.50-0.80 0.70-1.00

Sulphur


6/7



Pinetown

4041

6

Sample

Diameter

mm

Area

mm2

Gauge

Length

mm

0.2%

Proof

Load

kN

Max

Load

kN

Extension

mm

0.2%

Proof

Stress

MPa

Elongation

%

RA

%

UTS

MPa

Failed Bolt 10.21 81.87a36.26 66.44 78.49 7.07 812 19.5 54.1 959

New Bolt 9.92 77.29 51.14 62.80 78.19 6.85 813 13.4 51.3 1012

Specification665

min

13

min

850-

1000

Table 2: Tensile test data.

Note that because of the size of the available sample it was not possible to get a standard

tensile test piece on the failed bolt. The reduced gauge length could be expected to affect

the % elongation result, causing it to be slightly high.

Distance from surface(mm) 0.25 0.5 1.0 2.0 3.0 4.0 5.0

Failed Bolt 322 304 310 317 317 322 303

New Bolt 347 331 325 321 322 336 307

Table 3: Vickers hardness readings (HV)

Table 3 is a plot of Vickers hardnessreadings, using a 5kg load, against the distance from

the surface of the bolt. It shows that there is variation in hardness across the section, due to

banding. In fact when separate readings were taken in the light and dark bands variation

between 288HV and 331Hv were found on the failed bolt. Similar readings on the new bolt

varied between 307HV and 345 HV

DISCUSSION

Hydrogen Embrittlement

Hydrogen Embrittlement (HE) is a form of embrittlement caused by the absorption of

hydrogen ions (H+) into the structure of susceptible steels. The presence of hydrogen in

concentrations of only a few parts per million (ppm) is sufficient to cause disastrous brittle

failure.

The susceptibility of a steel to hydrogen embrittlement is a function of its strength, not itschemical composition. Usually only steels with an Ultimate Tensile Strength (UTS) greater

than 1000MPa are susceptible.

The mode of failure with hydrogen embrittlement can be cleavage, quasi-cleavage, micro-

void coalescence or intergranular, depending on the crack tip stress, temperature, the

hydrogen concentration and its effects on its plasticity (reference: Professor M.N. James,

University of Plymouth, England)


7/7



Pinetown7

Hydrogen embrittlement is known to occur during the acid pickling process commonly used

to clean components prior to hot-dip galvanizing.

ASTM A143 (Standard Practice for Safeguarding against Embrittlement of Hot-Dip

Galvanized Structural Steel Products)

Section 3.2"Hydrogen embrittlement may also occur due to the possibility of atomic hydrogen being

absorbed by the steel. The susceptibility to hydrogen embrittlement is influenced by the type of steel;

it's previous heat treatment, and degree of previous cold work. In the case of galvanized steel, the acidpickling reaction prior to galvanizing presents a potential source of hydrogen. In practice hydrogenembrittlement of galvanized steel is usually of concern only if the steel exceeds approximately 150 ksi(1000 MPa) in ultimate tensile strength, approximately 310 HV

CONCLUSIONS

Although the bolts were found to be nominally to specification, the failed bolt failed in a

brittle manner, both macroscopically (minimal distortion) and on a microstructural level

(cleavage). The bolts were found to have a banded structure resulting in areas having ahardness of up to 347HV, equivalent to a UTS of 1100 MPa. There was evidence of

decohesion present, (photos 8 & 9), which is common with hydrogen embrittlement.

From these results our conclusion is that this bolt failed as a result of hydrogen

embrittlement.

RECOMMENDATIONS

1) Those bolts presently mounted in concrete could be tested by affixing a

mounting plate and tightening the bolts up to the correct torque and leaving

them for 48hours. Hydrogen embrittlement is often a delayed failure but, in

our experience, if they are going to fail they will usually do so within 48 hours.

2) All future bolts might be mechanically cleaned, rather than acid pickled, prior

to hot-dip galvanizing. Alternatively a hydrogen relieving heat treatment

process might be instituted after galvanising.

Signed:

_____________________ ______________________ __12/06/2014_____

Alan Blundell Dr. Clinton Bemont (Pr Eng) Date

Appendix No 1

Documents

Transcript of Appendix No 1