failure of weld joints between carbon steel pipe and 304 stainless steel elbows

Engineering Failure Analysis 12 (2005) 181–191

www.elsevier.com/locate/engfailanal

Failure of weld joints between carbon steel pipe and304 stainless steel elbows

Anwar Ul-Hamid *, Hani M. Tawancy, Nureddin M. Abbas

Materials Characterization Laboratory, Research Institute, King Fahd University of Petroleum and Minerals, P.O. Box 1073,

Dhahran 31261, Saudi Arabia

Received 2 July 2004; accepted 3 July 2004

Available online 12 September 2004

Abstract

A number of weld joints between carbon steel (CS) pipe and type 304 stainless steel (SS) elbows constituting a gas

piping system of a petrochemical unit developed cracks after a relatively short period of usage, resulting in leakage. The

gas flowing through the pipe, was hydrogen rich at a temperature of 45 �C and a pressure of 16 kg/cm2. Light optical

metallography and scanning electron microscopy, combined with energy dispersive X-ray spectroscopy, inductively

coupled plasma and microhardness testing were used to determine the most probable cause of failure. Analysis showed

that the cracks originated at the interface between the CS pipe and the SS root weld. A narrow band between the CS

pipe and SS weld exhibited a hardness of Rockwell C 60 suggesting the formation of martensite due to C segregation at

welding temperature and subsequent quenching during cooling. The ferritic region of CS adjacent to the weld was

decarburized and was devoid of pearlite; corroborating C diffusion. The weld region was diluted comprising mainly

of Fe with small amounts of Cr and Ni. Cracking is thought to have initiated at the hardened region. However, the

failure might have been aided by hydrogen rich medium and soft C-depleted ferrite region.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Cracks; Failure analysis; Heat affected zone; Hydrogen damage; Welds

1. Introduction

This paper reports the investigation into the failure of dissimilar weld metal joints in a piping system of a

petrochemical plant. The process involved a reformed gas that passed through a water cooler followed by

compression in a three stage centrifugal synthesis gas compressor. The outlet piping of the suction drum

1350-6307/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2004.07.003

* Corresponding author. Tel.: +966 3 860 2017; fax: +966 3 860 4442.

E-mail address: [email protected] (A. Ul-Hamid).

mailto:[email protected]

182 A. Ul-Hamid et al. / Engineering Failure Analysis 12 (2005) 181–191

constituted weldments of carbon steel (CS) pipe and SS 304 elbow fittings as schematically illustrated in

Fig. 1. A number of cracks appeared at these weld joints after a relatively short period of service. Radio-

graphic tests conducted on-site showed that the cracks were circumferential and developed along the weld

seam adjacent to the CS pipe. The length of these cracks varied between 120 and 600 mm. Some of these

joints leaked following weld repair in the past. One of these joints failed after two and half years of servicefollowing weld repair. The length of the crack formed at this point was 300 mm and it had occurred at the

weld seam. A sample piece comprising failed CS pipe and the weld seam was cut at this point and analyzed

to determine the most probable cause of failure.

2. Materials and processes

The suction drum was made of CS internally clad with SS 304 and did not present any corrosion or leak-age problems. The outlet piping of the suction drum comprised of ASTM A53 Grade B CS straight pipe

(600 mm NB and 8 mm wall thickness) and type 304 SS elbow fittings with similar dimensions. The CS pipe

was joined to SS 304 elbows through butt weld joints by arc welding process using E309 electrodes and

ER309 filler metal. These joints were made at a number of points in the installation. Chemical composition

of the pipe obtained using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) con-

firmed the pipe material to be ASTM A53 Grade B [1], as shown in Table 1. This grade of material contains

0.3 wt% C (max). The composition of the weld deposit was also measured and confirms the use of E/ER 309

type electrodes/filler metal [2] (see Table 1). The pressure and temperature of the reformed gas at the suctionof the compressor was 16 kg/cm2 and 45 �C, respectively. The gas constituents and their respective mole

percentages are given in Table 2.

3. Experimental procedure

The cracked region at the weld joint was sectioned and mounted in cross-section using standard metal-

lographic techniques. The sample was ground with 600 grit size SiC paper and polished using 1 lm dia-mond paste. It was then coated with a thin layer of carbon using a carbon evaporator. This was

necessary to make the surface electrically conducting to avoid charge build-up during scanning electron

microscopy (SEM) analysis. Fractured surfaces were analyzed in an as-received condition. Both light micro-

scopy and SEM were used to conduct a microstructural study of the fracture surface, base metal, weld

SS 304 Elbow

CS to SS Butt Weld(Cracked region)

Carbon Steel 24" NB Pipe

Fig. 1. Schematic illustration of the weld joints between the CS pipe and SS 304 elbow where cracking occurred.

Table 2

Constituents (mole%) of the reformed gas in the outlet piping

Gas composition

Constituent Mole%

CH4 3.53

CO 14.5

CO2 7.48

H2 73.71

N2 0.21

H2O 0.57

Table 1

Chemical composition (wt%) of ASTM A53 Grade B steel along with that measured by ICS-AES

Chemical composition, wt%

C Fe Cr Ni Mn Mo Si Cu P S V

ASTM A53 grade B [1] 0.30 max Bal. 0.40 0.40 1.2 0.15 – 0.40 0.05 0.06 0.08

CS pipe (measured) – 99.3 0.05 0.04 0.55 0.003 0.02 – – – –

ER309 [2] 0.12 max Bal. 23–25 12–14 1.0–2.5 – 0.25–0.6 – 0.03 0.03 –

Typical E309 weld deposit 0.07 Bal. 22.0 12.0 1.40 – 0.60 – 0.018 0.008 –

SS weld (measured) – 67.8 19.7 11.2 1.66 0.003 0.18 0.04 – – –

A. Ul-Hamid et al. / Engineering Failure Analysis 12 (2005) 181–191 183

metal and the heat affected zone. A JEOL SEM JSM-5800LV having a probe resolution of 3.5 nm and cou-

pled with an EDS detector was used to examine the samples. The Si(Li) X-ray detector had an atmospheric

thin window capable of detecting elements down to Be. The accelerating voltage was maintained at 20 kV

during analysis and imaging was performed using secondary electrons. SEM/EDS analysis was used to

determine the elemental constitution of microstructural features. Bulk composition of CS pipe and weld

metal was verified using ICP-AES. Vickers microhardness testing was performed at the weld region and

the base metal.

4. Results

4.1. Visual inspection

Visual examination of the failed sample indicated a separation of the weld bead from the CS pipe. Cir-

cumferential cracking was evident at areas adjacent to the weld region. There was no evidence of branchingin the primary crack. Corrosion deposits were not observed at the pipe surface.

4.2. Light optical microscopy

Light optical microscopy at low magnification revealed primary circumferential crack in the CS pipe

close to the weld region as shown in Fig. 2(a). Separation between the weld and the CS pipe is evident

at one point in the sample as shown in the low magnification macrograph of Fig. 2(b).

General fracture surface of the CS pipe obtained by separation is shown in the optical macrograph ofFig. 3(a). The fracture appeared flat and brittle and had its origin at the weld region. The marked region

Fig. 2. Light optical micrographs showing: (a) the crack formed at the CS pipe circumference; (b) separation between the CS pipe and

the weld metal at the fusion zone.


in Fig. 3(a) encompasses several pinholes and is shown at a higher magnification in Fig. 3(b). These pin-

holes are material defects and were detected in significant proportions within the pipe. Presence of such de-

fects can aid in initiation and/or propagation of cracks particularly at the weld region.

The fracture at the inner surface of the pipe was continuous through the pipe circumference. On the

other hand, the outer surface of the pipe exhibited localized areas that were still intact after failure. This

observation indicates that the cracking originated at the inner surface of the pipe near the root weld. It also

follows that the primary crack was circumferential while the leakage occurred when this crack branchedand traversed through the wall thickness of the pipe.

4.3. Scanning electron microscopy

4.3.1. Weld deposit

A small portion of the weld metal taken from the sample was examined using a SEM. The morphology

exhibited by the weld at different regions is shown in Figs. 4(a) and (b). The weld fracture shows intergran-

ular separation and coarse dimples within the grains. Typical composition of weld metal for E309 type rodscontains 22 wt% Cr and 12 wt% Ni (see Table 1). The SEM/EDS analysis of the weld regions in Figs. 4(a)

Fig. 3. Optical macrographs of CS pipe showing: (a) general fracture surface revealed after separation, and (b) the presence of pinholes

within the material.


and (b) showed lower concentrations of Cr (14.9 wt% max) and Ni (5.9 wt% max). This indicates that the

weld was diluted during welding. However, proper welding rod was selected for the process since E309 type

rods are used to join dissimilar metals such as SSs and CSs [3]. Type E309 filler metal contains 5–10%

ferrite.

4.3.2. Carbon steel pipe

The morphology of fracture surface located close to the inner surface of the CS pipe is shown at a low

magnification in the secondary electron SEM micrograph of Fig. 5(a). The fracture surface appeared flat.

Another localized region at a higher magnification exhibited dimpled structure formed due to microvoid

coalescence as shown in Fig. 5(b). In addition to a predominant Fe content, SEM/EDS analysis of the frac-

ture surface showed the presence of Cr and Ni, which is thought to be from the weld fusion zone. This indi-

cates that analyzed region above was located close to the weld region.

4.3.3. Microstructure of CS–SS weld interface

Microstructural examination of the damaged cross-section was conducted through the weld region in

order to determine the nature of cracking. The crack, shown by the low magnification light optical macro-

graph in Fig. 6(a), originated from the inner surface and extended towards the outer surface of the pipe.

Fig. 4. (a) and (b) Scanning electron SEM images showing the surface morphology of the weld deposit. The insets show the chemical

composition of imaged regions.


The crack did not exhibit branching and mainly traversed along the weld line. The same region at a slightly

higher magnification is shown in an SEM micrograph in Fig. 6(b). The crack had opened up after failure

and it�s difficult to judge the exact location of its origin on a micro-level with respect to the weld joint. How-

ever, it is clear that its origin lies within the weld zone as also indicated by one of the possible directions it

could have propagated by a coarse crack opening marked as �A� in Fig. Fig. 6(b). Small cracks parallel to

the rolling direction were also observed. These cracks were rolling defects and were not produced duringservice. An example of these cracks, obtained from regions away from the weld zone, is shown in the

SEM micrograph of Fig. 6(c). The primary crack propagated in a transgranular manner as evident in

Fig. 6(a).

The microstructure of the CS pipe constituted bright ferrite and dark pearlite grains as shown in the opti-

cal micrograph of Fig. 7(a). The figure also shows the interface between the steel pipe and the weld. It was

observed that the CS grains adjacent to the interface were devoid of pearlite. This can also be seen in the

SEM micrograph of Fig. 7(b), where pearlite appears bright and the CS–weld interface is apparent as a

whitish boundary. The chemical compositions obtained by SEM/EDS analysis of different regions of thisarea are shown in Fig. 7(b). It can be observed that the CS–weld interface is predominantly composed

of Fe with a small amount of Cr while the regions immediately surrounding the fusion zone has some

Ni in addition to above. Micro-cracks were observed within the pearlite-denuded ferrite region adjacent

Fig. 5. Secondary electron SEM images of the fracture at the inner surface of the CS pipe showing: (a) flat morphology and

(b) dimpled structure at a high magnification.


to the CS–weld interface as shown by the secondary electron SEM image of Fig. 8. These cracks were ori-

entated perpendicular to the direction of maximum tensile stress to which the pipe was subjected.

4.4. Microhardness testing

The results of Vickers microhardness tests carried out at different regions of the weld cross-section sam-ple are summarized in Table 3. The hardness measured at the interface of the CS and the weld was very high

and corresponds to Rockwell C 60. Other microhardness values correlate with the microstructure observed

in the weld cross-section. As expected, the lowest hardness was exhibited by the pearlite-denuded zone due

to a lack of carbon in the region.

5. Discussion

The CS pipe material cracked from the internal surface at the root weld region which was in contact with

the hydrogen rich gas. The crack propagated primarily circumferentially and traversed through the pipe

Fig. 6. (a) Optical and (b) SEM micrographs of the weld cross-section illustrating the crack formed at the weld zone, and (c) SEM

image of rolling defects within the CS pipe.


wall thickness resulting in leakage. The crack originated at the interface between the carbon and SS weld

and its propagation within the CS pipe was transgranular. The carbon–stainless steel fusion zone was

decarburized resulting in a low hardness pearlite-denuded ferrite region. The decarburization occurred dur-

ing the welding process where the carbon in the pipe segregated towards the SS weld. This resulted in acarbon rich segregated layer at the CS–SS interface as shown in Fig. 7(a). Although the microstructure

of this thin layer was not resolvable by light and scanning electron microscopy, hardness of Rockwell C

60 indicated presence of martensite at this region. The fracture is thought to have initiated within this mar-

tensitic zone.

Fig. 7. (a) Optical micrograph and (b) SEM image of the weld cross-section illustrating the general microstructure of CS pipe, interface

layer and SS weld along with elemental compositions of various regions.


The presence of martensite indicates that sufficient heat was generated during welding to fuse an exces-

sive amount of CS that resulted in dilution of weld deposit locally, as indicated by the EDS results shown

earlier. As a consequence, a thin band of low alloy steel was produced between the CS and the SS weld

which possessed sufficient hardenability to form martensite during cooling from welding temperature.

The layer was thin enough to have undergone quenching as it was cooled while in contact with a large mass

of carbon steel pipe.

The presence of a thin layer of brittle martensite at the interface between CS pipe and SS weld sets up ametallurgical notch which is undesirable and can lead to cracking [4]. The results suggest that a small region

of high hardness produced at the CS–weld interface could have been adequate to initiate cracking. Large

Fig. 8. Secondary electron SEM image of a crack in pearlite-denuded ferrite region formed close to the CS–SS weld interface.

Table 3

Vickers microhardness values obtained from different regions of CS–weld interface

Region analyzed Microhardness, VHN

Stainless steel weld 321

CS–SS weld interface 700

Pearlite-denuded zone near CS–SS interface 137

Ferrite and pearlite near denuded zone 148

CS pipe away from weld 183


amounts of hydrogen in the medium that was transferred through the pipe can result in hydrogen damage

at martensitic region during service [5]. The cracks formed in this manner will initiate at the inner surface of

the pipe as observed in this study. Maximum sensitivity to hydrogen cracking occurs at or near room tem-

perature which is comparable to the service temperature encountered in the piping system under consider-ation here. The lack of multiple branching of the crack is also characteristic of hydrogen induced damage.

The decarburized area adjacent to the carbon steel pipe can also result in crack initiation and/or propaga-

tion under tensile stress, as corroborated by micro-cracks observed in the region.

6. Conclusions

Experimental results indicate that the joint between the CS pipe and the SS weld failed due to the devel-opment of a localized region of high hardness (e.g. martensite) at its interface during cooling from welding

temperature. The hydrogen rich gas transported through the pipe initiated cracking at this region. The pres-

ence of a decarburized layer adjacent to the CS pipe aided in crack propagation under stress.

7. Recommendations

It was recommended that the heat input during welding should be reduced through the use of smalldiameter electrodes at a low current and relatively higher voltage in order to prevent dilution. The welding


parameters should be controlled such that the penetration into the CS pipe is kept to a minimum. It was

also suggested that any risk of hydrogen contamination (e.g. through electrodes, moisture) should be re-

duced. As a long term measure, the CS pipe should be replaced by SS in order to produce sound welds.

Acknowledgement

The authors acknowledge the support of the Research Institute of King Fahd University of Petroleum

and Minerals.

References

[1] ASTM Standard A53-90b. Annual book of ASTM standards. vol. 01.01. American Society for Testing of Metals; 1993. p. 1.

[2] Welding and brazing. In: Metals handbook, vol. 6, 8th ed. Metals Park (OH): American Society of Metals; 1971. p. 252.

[3] Welding and brazing. In: Metals handbook, vol. 6, 8th ed. Metals Park (OH): American Society of Metals; 1971. p. 274.

[4] Failure analysis and prevention. In: Metals handbook, vol. 10, 8th ed. Metals Park (OH): American Society of Metals; 1971. p. 343.

[5] Failure analysis and prevention. In: Metals handbook, vol. 10, 8th ed. Metals Park (OH): American Society of Metals; 1971. p. 230.

failure of weld joints between carbon steel pipe and 304 stainless steel elbows

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