Failure Analysis

Laboratory Report

Name: Neville Lawless

Lecturer: Dr. Neal Murphy

Demonstrator: Paul Bright

Student No: 06523587

Date Taken: 3/11/09

Due date: 17/11/09

Introduction

Component being investigated: Scaffolding Hook.Mild steel with carbon content of 0.15% or less.Conforms to AISI-SAE 1015

The Hook being investigated is welded onto a scaffolding pole. Its purpose is to secure the scaffolding alongside the wall to the building which works are being carried out on.The reason for the investigation is that many of these have failed in service and are seen as unfit for their purpose in the current state.A Hook of German manufacture which did not fail in service is being used as a comparison.The differing compositions are laid out in the table 1 below. These were found from an optical emission spectrometer.

Manufacturing technique:

It is known that the Hooks are produced by cold-forming the steel bars and then welded onto the Scaffolding.

Nature of failure:

The failure occurs in the bend of the hook. On inspecting similar specimens it is seen that a microscopic flaw or crack is present in this bend.

As the fracture occurs in the bend, it can be assumed that the failure is of a mixed mode fracture type. This is the result of a combined tensile and shears force being present here.

Figure 1Actual failed hook

Failure Investigation Carried out

Chemical analysis:

An optical emission spectrometer was used to analyse the Hook being investigated and also the similar German Hook being used for comparison. The elemental amounts are given by weight percent in Table 1 below. The balance was Iron. (Fe)

As said already the component was manufactured from Mild steel with a carbon content of 0.15% or less.The analysis that was provided suggested that the main differences that were of a notable nature, are the 0.038% Aluminium content in the failed Hook and the much higher silicon presence in the German steel. This shows that it was a cold-heading or forging grade of steel as it has been completely de-oxidised.

Failed Hook German Hook

C 0.150 0.130

Si 0.150 0.410

Mn 0.730 1.380

P 0.011 0.020

S. 0.028 0.004

Cr 0.040 0.100

Ni 0.100 0.080

Mo 0.020 0.010

Cu 0.200 0.280

Al 0.003 0.038

Ti 0.002 0.002

V 0.000 0.020

Table 1: Elemental Amounts

Hardness testing

In order to investigate whether or not a physical change occurred in the bar during its manufacture, a Vickers hardness test was carried out. This can be seen in figure 1 belowThis was done on the outside of the bar and also its core.Surface hardness: 260HV5KgCore hardness: 215HV5KgThe obvious cause of this is Work hardening being incurred during the cold rolling process. Work Hardening is the strengthening of a metal by plastic deformation. This strengthening is brought about by the movement of dislocations within the crystal structure of the material and the entanglement of these dislocations.

Figure 2 Vickers Hardness test

Bend Testing

3 bend tests were carried out on different samples with different heat treatments being applied in each case. The aim of these was to recreate the fractures that occurred and narrow down the possible causes. The type of test can be seen in figure 2

Figure 3: Bend test carried out on bar (Journal: Industrial Lubrication and Tribology Volume: 57)

1. Test 1 was a simple bending test being carried out on the bar with no heat treatment being applied.The test could not re-straighten the bar to its original shape, however no fracture was seen to have occurredIt is felt that because the bar could not be straightened, this is a further indication that work hardening has taken place.

Figure 4 Bar after bending with no treatment

.

2. Test 2 was carried out after a heat treatment of heating to 300 ○ C and a cooling to ambient temperature was performed. The sample fractured in a brittle manner during the test with

only a small force being applied. It was felt by the team that this is a good indicator that welding causes significant embrittle-ment to the hook. Further inspection also showed that the fracture surface was similar to the

original fractures.

Figure 5 Fracture after heat treatment

3. Test 3 reproduced heat treatment 2 and a further treatment of 650 ○ C for 1 hour and then cooling to ambient. When the bending test was then carried out the hook was able to be bent back to its original shape as can be seen in figure 5.This treatment normalizes the steel. Normalizing is a type of heat treatment applicable to ferrous metals only. It differs from annealing in that the metal is heated to a higher temperature and then removed from the furnace for air cooling. The purpose of normalizing is to remove the internal stresses induced by heat treating, welding, casting, forging, forming, or machining.

Figure 6: Hook bent back to normal shape after heat treatment

Recommendations:

On investigation of the Hook it was found that aluminium in steel acts as a stabiliser to Nitrogen. With the decreased levels of aluminium present, the nitrogen is free to move in the steel. The bending process is thought to have caused a build up or entanglement of dislocations in the bend of the hook During the welding process, the nitrogen moves and becomes trapped in the bend within these dislocations and accumulates here.

It is known that Nitrogen affects the toughness of the heat-affected zone (HAZ) of welded steel. This loss in toughness is often referred to as HAZ embrittlement. When a load is applied, as in the case of the scaffolding being inspected, fracture occurs much easier.

It is felt that the German steel in the hooks did not fail because it didn’t become brittle like our test pieces did. This was due to the Aluminium in steel stabilising the nitrogen.

To prevent this failure re-occurring, it was felt by our team that the heat treatment used in test 3 should be utilised. This should be only used if the current mild steel is been used and performed after welding has taking place and the material cold.A better alternative would be to use forging grade of steel conforming to BS EN 10263-2:2001. These standards are for steels which are used for cold-deformation. Using this would not require any heat treatment after welding. This is the most sensible solution depending on cost and customer requirements.

References

http://steel.keytometals.com/default.aspx?ID=CheckArticle&NM=202. Retrieved 18/11/09

Lecture notes. Materials of science 1. Alun Carr