Influence of design parameters in weld joint performance

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN

0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME

202

INFLUENCE OF DESIGN PARAMETERS IN WELD JOINT

PERFORMANCE

Maqbool Ahmed1, M. Azizuddin

2, B.Balakrishna

3,

1Associate Prof RITS,

2Prof & Head Mech Dept RITS,

3Associate Prof JNTU Kakinada

1. INTRODUCTION

A flange connection used in oil and gas industry failed premature. Investigation was

conducted to analyze the failure causes. Micro/macro structure study , hardness, light and

SEM microscopes analysis of the chemistry near and away from the crack suggested that:

a) The failure is most probably caused by recent practice of reducing the wall thinness of

the nipple by grinding to suit to the flange ends

b) Welding has caused a brittle micro-structure to develop, making it vulnerable to

crack. Also, sulphur pick – up (either as a result of heat induced by welding or as a

result of ingress from the flowing mediam) near the cracking area shows relatively

high concentrations (about twice that of the bulk material about 10 mm away from the

crack line).

These findings did emphasize the importance of design factor in accelerating failure. At the

end, some recommendations have also been introduced to mitigate the occurrence of such

failures in the future.

2. DEFINITION OF THE PROBLEM

Failure of a weld joint of 3/4” flange had resulted in hydrocarbon leak. This 3/4”

tapping had been taken from 24” and routed with 1” analyzer line ( new ) with reducer. The

failure had apparently happened within a time span of about four months.

The circumferential crack (65%) was observed in top side HAZ of weld joint.

INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN

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ISSN 0976 - 6480 (Print) ISSN 0976 - 6499 (Online) Volume 4, Issue 3, April 2013, pp. 202-210 © IAEME: www.iaeme.com/ijaret.asp Journal Impact Factor (2013): 5.8376 (Calculated by GISI) www.jifactor.com

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Figure 1. The cracked-face of the failed flange

Figure 1 shows the appearance of the crack externally. The ¾” flange was directly

welded with the pipe as a butt weld joint.

The new 1” analyzer line found rigidly fixed with L- angle and clamps. Also line

routed 90 deg in the direction of 24” header. As the client has stated to us, no pressure gauge

installed, it was end blinded before EDM used it as tie-in for analyze sample line. The flu gas

within the pipe is sour. The design pressure of 24” pipe is 79.8 bar (G) and design

temperature is 85 Deg C. The flange material is a low temperature carbon steel (A350 LF2,

¾ inch Sch 80. 3.91mm) and the Nipple material sis also a low temperature carbon steel

A333 Gr6.3/4 inch Sch 80. 3.91 mm).

1. Examination: The examinations that were carried out on the failed flange were as follows:

1.1. Macro-examination

1.2. Micro-examination (Metallography, SEM)

1.3. Chemical analysis (spark emission)

1.4. Mechanical Hardness Test

2.1. Macro-examination:

Below, we will look at the results obtained from the examinations mentioned above. Figure

2 shows the profile of the flange + nipple :

Figure 2. Distinguishing the nipple and flange parts in order to define the location of the

crack



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Furthermore, The failed flange and nipple were sectioned vertically. This is shown in Figure

3a. As seen in this figure, the crack is on a portion (about 70%) of the material of the flange.

It is evident from these macro-examination images (Figures 2 and 3a) that the location of the

crack is within the flange area. This is important as this will allow us to concentrate

more on the flange area and investigate more deeply on the cause (es) of the crack in this

area.

Figure 3a. The flange and the nipple parts after being cut into two halves.

As seen From the half on the right, the crack is very evident. The fracture surfaces were also

examined across A-A as shown in Figure 3b.

Figure 3b. Fracture surfaces of the failed flange as sectioned through A-A.

One of these surface was examined by both macro-and micro-examination. Figures 3c

and 3d show the failed surface. As seen in Figure 3c, at least one crack visible with naked eye

being developed on the fracture surface.

A A



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Figure 3c. A transverse crack being developed at the cross-section of the fracture surface

along A-A section from figure 4b. The crack covers about 60% of the length of the cross

section area.

Figure 3d: magnified view of Figure 3c (close-up). Some of the “beach- marks” typicl of

fatigue are shown within the oval

Figure 3e. Beach marks (on the fracture surface) suggesting the likelihood of fatigue

It must be noted that observing bench-marks is one way of suggesting that the failure

has been due to fatigue. In this particular case, there is also another evidence which is the

mode of the crack (see Figures 10a and 10b). Before and after sectioning the flange + nipple.

As seen from Figure 4, there is a reduction of 0.60 mm in the cross section. This trimming

action will actually reduce the effective cross section to carry the load. This will result in

higher stresses being developed

For a given stress, then, this reduction in size would mean an increase of the local

stresses by about 110 %. A possible consequence of developing such stresses is that they

may reach past the yield point of the material, causing plastic deformation by encouraging the

formation of internal micro-cracks. If the material, micro-structurally, has also become brittle

due to developing of brittle phases, this can enhance the likelihood of crack initiation/

propagation especially at structure imperfections.



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Figure 4. (Left) the dimension of the flange before removing (right) after cold cutting

Among the characteristics of the fluid, it had also been mentioned that it contains

“impurities” such as dust. The dust particles will cause erosion-corrosion as it is apparent

from Figure 5.

Figure 5. Some signs of erosion-corrosion

It suggests that hard impurities that are accompanying the fluid can also have an

impact on accelerating the failure of the piece. These impurities can hit the surface and

through this physical contact, the effective cross section that may reduce

Erosion-corrosion is also further enhanced by the impact of improper design as imposed by

inappropriate trimming: the difference between the cross sections thus generated is capable of

increasing the detrimental of the dust micro-particles that are entrained with gas. Figure 6

shows how to change in cross section due to design can cause internal deterioration in an

equipment.



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Figure 6. Schematic presentation of how change in cross section can induce impingement

As it is seen from Figure 6, when the cross section is reduced, because of relative

change in the pressure of the fluid (as a function of fluid velocity), erosion-corrosion can be

induced.

Another important matter from Figure 6 is that due to erosion induced as such

getting a uniformly eroded surface is not likely. As it appears, the inside of the piece will be

selectively ploughed

This will create a topography on the surface that will reflect light in different angles. This

is seen in Figure 5 as bright and dark areas. However, it is not possible to estimate the

relative contribution of this factor (erosion-corrosion) to the general set of internal factors

facilitating corrosion.

2.2. Micro-examination:

Scanning electron microscope (SEM) of the crack is shown in Figure 7.



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Figure 7 shows that in addition to the main crack, some micro-cracks are being developed

and start propagating. Another finding that is important from a micro-structural point of view

was the change observed from a mixed pearlitic-ferritic microstructure to a fully ferritic

microstructure near the weld zone.

There are several processes that can lead to intergranular fracture.

1. Micro-void nucleation and coalescence at inclusions or second phase particles

located along grain boundaries

2. Grain boundary crack and cavity formation associated with elevated temperature

stress rupture condition.

3. De-cohesion between contiguous grain due to presence of impurity at grain

boundaries and in the presence of hydrogen in liquid metals.

4. Stress corrosion cracking associated with chemical dissolution along grain

boundaries.

5. Cyclic loading when the material has insufficient number of independent slip

systems to accommodate plastic deformation between contiguous grain leading to

grain boundaries rupture

Chemical analysis of sulphur near and away from crack along with other findings in this

investigation may suggest that a combination of the mechanisms above could have been

responsible for observing this crack. However, based on the facts that:

a) the part has undergone IG,

b) the fluid is a sour gas where there is a relatively high concentration of sulphur near the

crack compared with that of the bulk material, (the impact to be mentioned in section

3.3)

c) the trimming of the effective load surface that can stimulate conditions of sudden

change in the velocity of the fluid inside, developing (internal) cyclic loading,

(explained in section 3.1)

Mechanisms 2, 3 and 5 could be the main mechanisms contributing to the failure of the part.

2.3. Chemical analysis (spark emision)

The main cracked area, as shown below in Figure 10, was also studied for relative

concentration of sulphur near the cracked area and some 10 mm away from it.The main

reason for selecting sulphur was that the gas was of sour nature, having a relatively high

concentration of sulphur in it. In addition, during welding, a molten pool maintains a

concentration gradient for the alloying elements. The alloying elements will be attracted into

this molten pool and afterwelding the cooling process starts, the alloying elements that now

have been precipitated at or near the weld line, start to change the mechanical properties of

the material at that venue.

The results of the spectroscopy have been superimposed on the figure that shows the

crack, altogether shown in Figure 12. The dark column represents the sulphur values from

within the bulk of the crack whereas the light column shows the relative values near the

crack.



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Figure 10. The change of relative concentration of sulphure (wt%) near the crack and about

10 mm away from it within the bulk of the steel.

As being evident from Figure10, sulphur is a much higher concentration near the crack,

suggesting that either it has been accumulated as a result of welding or as a results of sulphur

ingress from the sour flu gas flowing inside. At this stage, however, it is impossible to

distinguish between these possible two sources of sulphur but the end result is that the

microstructure becomes more vulnerable to cracking.

2.4 Hardness Test A section of the failed flange was cut as shown in Figure 11 and the hardness of both

sides of the cut section was tested. Figure 12 shows the change of hardness on both sides

recorded as HRBW (Rockwell Hardness B Scale Tungsten-Carbide ball Indenter)..

Figure 11 . A cross section (AA) of the flange showing the difference in thickness for both

the original pipe width on the base metal and the reduced wall thickness near heat affected

zone (HAZ). Typically the wall thickness in the welded edge has been reduced by 66.90%.

Figure12. Change of hardness over the outer and inner surfaces (as from Figure 9)

Figure 12 shows that the hardness values (especially near the heat affected zone-

HAZ) are far different from those of the parent material, suggesting that the material is too

brittle and susceptible to develop cracks. This matter becomes of importance when we

consider all other pieces of evidence (micro-/ macro- structure) that suggest that one of the

main causes of the failure can be linked with the welding.

(AA)



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3. CONCLUSIONS AND RECOMMENDATIONS

The failure seems to be a combination of the following factors:

1. Wrong welding edge preparation; the trimming has resulted in a thickness too low for

the joint between the nipple and the flange to stand the required mechanical loads.

The reduced area has already resulted in creating varying cross sections where the

varying local pressures resulting from varying velocities of the fluid gas also

contribute more to the vulnerability of the material to failure

2. Due to factors such as welding or possible ingress from the flu gas inside the flange,

sulphur has been accumulated at the cracking area and most possible around the grain

boundaries, giving rise to an intergranular crack,

3. Due to the heat induced during welding, the microstructure of the weld zone and HAZ

has been transferred into a brittle zone.

4. The gas may contain impurities in the form of very tiny dust particles. These particles

, entrained by the flow of the gas, are inducing erosion-corrosion resulting in making

the internal wall even more vulnerable.

5. The external factor of fatigue has been accelerated during the last four months of

service adding already existing internal factors contributing to failure.

The following can be recommended to prevent similar cases to happen:

a) Avoid any modification in the dimensions of the parts to induce inappropriate

levels of stress as well as the likelihood of getting situations encouraging

cavitation,

b) Selection of a flange of same internal diameter as that of the nipple to avoid the

turbulence in gas flow

c) Use low sulphur welding method with more care not to cause too much

segregation of potentially corrosive alloying elements (such as sulphur) near grain

boundaries,

d) Reduce the level of impurities (micro-dusts) of the gas to avoid internal erosion-

corrosion,

e) Observing and controlling the fatigue as induced by excess vibrations.

4. REFERENCES

1- ASME/ANSI B 16.5:Pipe flanges and flanged fitting ( 1996 ) page 1-2.

2- ASME Pressure Vessel and Boiler Code. Section II, Part A, Ferrous materials specifications,

Materials: Specifications for carbon steel forging for piping applications. (1999) page 180.

3- API specifications 5L; Specifications for line pipe, 42nd ed, ( 2000 ) page 8.

4- Failure analysis of high pressure Butt Weld. F Ahmed,F Hassan and L.Ali. Pak J.Engg &

Appl science Vol 3 July 2008 ( P 26-32 )

5- Equivalent to assess hardenability of steel and prediction of HAZ Hardness Distribution.

Kasuya, T and Hashiba Y.; Nippon Steel Technical Report No 95 January 2007.

6- Failure Examination - Faulty Design, Weld Defect-Fracture of a Cross on a Church Steeple.

Naumann, Friedrich K; Spies, Ferdinand, PRAKT METALLOGR., 12(5), May 1975, pp.

268-271.

7- Failure analysis of a Cross country line pipe using 'CTOD' concept - A case study;

Sova Bhattacharya, Kannan C,Mohapatra B, Makhija R&D Centre, Indian Oil Corporation

Limited, Faridabad, India

Influence of design parameters in weld joint performance

Technology

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