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

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1 Engineering Failure Analysis 12 (2005) Failure of weld joints between carbon steel pipe and 304 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/cm 2. 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. Ó 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 * Corresponding author. Tel.: ; fax: address: anwar@kfupm.edu.sa (A. Ul-Hamid) /$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi: /j.engfailanal

2 182 A. Ul-Hamid et al. / Engineering Failure Analysis 12 (2005) 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. Radiographic 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 service following 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 leakage 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) confirmed 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 suction of the compressor was 16 kg/cm 2 and 45 C, respectively. The gas constituents and their respective mole percentages are given in Table Experimental procedure The cracked region at the weld joint was sectioned and mounted in cross-section using standard metallographic techniques. The sample was ground with 600 grit size SiC paper and polished using 1 lm diamond 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 microscopy and SEM were used to conduct a microstructural study of the fracture surface, base metal, weld SS 304 Elbow Carbon Steel 24" NB Pipe CS to SS Butt Weld (Cracked region) Fig. 1. Schematic illustration of the weld joints between the CS pipe and SS 304 elbow where cracking occurred.

3 A. Ul-Hamid et al. / Engineering Failure Analysis 12 (2005) 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 CS pipe (measured) ER309 [2] 0.12 max Bal Typical E309 weld deposit 0.07 Bal SS weld (measured) Table 2 Constituents (mole%) of the reformed gas in the outlet piping Gas composition Constituent Mole% CH CO 14.5 CO H N H 2 O 0.57 metal and the heat affected zone. A JEOL SEM JSM-5800LV having a probe resolution of 3.5 nm and coupled 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. Circumferential cracking was evident at areas adjacent to the weld region. There was no evidence of branching in the primary crack. Corrosion deposits were not observed at the pipe surface 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 of Fig. 3(a). The fracture appeared flat and brittle and had its origin at the weld region. The marked region

4 184 A. Ul-Hamid et al. / Engineering Failure Analysis 12 (2005) 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 pinholes are material defects and were detected in significant proportions within the pipe. Presence of such defects 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 branched and traversed through the wall thickness of the pipe Scanning electron microscopy 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 intergranular separation and coarse dimples within the grains. Typical composition of weld metal for E309 type rods contains 22 wt% Cr and 12 wt% Ni (see Table 1). The SEM/EDS analysis of the weld regions in Figs. 4(a)

5 A. Ul-Hamid et al. / Engineering Failure Analysis 12 (2005) 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 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 fracture surface showed the presence of Cr and Ni, which is thought to be from the weld fusion zone. This indicates that analyzed region above was located close to the weld region 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 macrograph in Fig. 6(a), originated from the inner surface and extended towards the outer surface of the pipe.

6 186 A. Ul-Hamid et al. / Engineering Failure Analysis 12 (2005) 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 itõs difficult to judge the exact location of its origin on a micro-level with respect to the weld joint. However, 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 ÔAÕ in Fig. Fig. 6(b). Small cracks parallel to the rolling direction were also observed. These cracks were rolling defects and were not produced during service. 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 optical 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 this area 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

7 A. Ul-Hamid et al. / Engineering Failure Analysis 12 (2005) 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 orientated perpendicular to the direction of maximum tensile stress to which the pipe was subjected Microhardness testing The results of Vickers microhardness tests carried out at different regions of the weld cross-section sample 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

8 188 A. Ul-Hamid et al. / Engineering Failure Analysis 12 (2005) 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 during the welding process where the carbon in the pipe segregated towards the SS weld. This resulted in a carbon 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 martensitic zone.

9 A. Ul-Hamid et al. / Engineering Failure Analysis 12 (2005) 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 excessive 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 a metallurgical 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

10 190 A. Ul-Hamid et al. / Engineering Failure Analysis 12 (2005) 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 temperature which is comparable to the service temperature encountered in the piping system under consideration 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 propagation 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 development 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 presence 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 small diameter electrodes at a low current and relatively higher voltage in order to prevent dilution. The welding

11 A. Ul-Hamid et al. / Engineering Failure Analysis 12 (2005) 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 reduced. 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 American Society for Testing of Metals; p. 1. [2] Welding and brazing. In: Metals handbook, vol. 6, 8th ed. Metals Park (OH): American Society of Metals; p [3] Welding and brazing. In: Metals handbook, vol. 6, 8th ed. Metals Park (OH): American Society of Metals; p [4] Failure analysis and prevention. In: Metals handbook, vol. 10, 8th ed. Metals Park (OH): American Society of Metals; p [5] Failure analysis and prevention. In: Metals handbook, vol. 10, 8th ed. Metals Park (OH): American Society of Metals; p. 230.