- Email: [email protected]
Metallographic analysis of embedded crack in electron beam welded austenitic stainless steel chemical storage tank
Engineering Failure Analysis 8 (2001) 157±166
www.elsevier.com/locate/engfailanal
Metallographic analysis of embedded crack in electron beam welded austenitic stainless steel chemical storage tank Abhay K. Jha*, S. Arumugham Material Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Trivandrum 695 022, India Received 6 December 1999; accepted 4 January 2000
Abstract An AISI 304L austenitic stainless steel tank used for chemical storage showed cracks during the post weld quali®cation programme. A crack of 75 mm length embedded within the weld pool was subjected to detailed metallographic analysis. The results revealed that the cracking was due to a shift in the solidi®cation mode from primary ferrite to primary austenite. The residual stress introduced during rolling and forming of material as well as additional contractional strain during welding under ®xtured condition, are additional factors which caused cracking. 7 2001 Elsevier Science Ltd. All rights reserved. Keywords: Cracks; Residual stress; Chemical-plant failures; Tank failures; Welded fabrications
1. Introduction AISI 304L austenitic stainless steel is used for a variety of components for chemical plants which demand good formability in addition to high corrosion resistance. One such component is a cylindrical shape with torospherical end domes. The tank is fabricated out of cold rolled AISI 304L austenitic stainless steel sheet of 6 mm thickness. A schematic diagram is shown in Fig. 1. The tanks are fabricated by forming and electron beam (EB) welding. Fabrication does not involve annealing or any stressrelieving treatment before welding. During recent fabrication of tanks, cracks were noticed in a few tanks during post weld radiographic quali®cation. Such cracks were found to be embedded as there was no indication of cracks on the surface. A cut piece of the weldment with an embedded crack was subjected to detailed metallographic examination, as described in this paper. * Corresponding author. Tel.: +91-471-563-748; fax: +91-471-415-348. E-mail address: [email protected] (A.K. Jha). 1350-6307/01/$ - see front matter 7 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 6 3 0 7 ( 0 0 ) 0 0 0 0 3 - 0
158
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
Fig. 1. Schematic diagram of chemical storage tank.
2. Material The material from the dome portion as well as the cylindrical shell was analysed for chemical composition, which con®rmed the material to be AISI 304L except for the carbon content, which is more than the speci®ed limit of 0.03% (Table 1). The material has ultimate tensile strength (UTS) = 550 MPa, 0.2% proof stress = 245 MPa and elongation = 55% (on 50 mm G.L.).
Table 1 Chemical composition of the tank material (wt%) Element
C
Mn
Si
S
P
Ni
Cr
Fe
Speci®cation Tank
0.030max 0.045
2.0max 1.80
1.0max 0.41
0.03max 0.019
0.045max 0.023
8±12 10.6
18±20 18.4
Bal Bal
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
159
3. Experiments A cut piece measuring 200 mm 100 mm was taken across the weld of a tank which showed the presence of cracks during post weld radiographic observation. The cut piece was again radiographically examined to con®rm the presence of a crack extending up to a length of 75 mm. A strip 25 mm 75 mm was taken from the cracked piece for detailed investigation which included radiography, dye penetrant test, stereo microscopy, optical microscopy and scanning electron microscopy. Visual observation as well as dye penetrant tests did not reveal any crack on the surface. One cut piece from the cracked region and another away from the cracked area were taken for metallographic observation. Both the cut pieces were polished along lateral as well as transverse directions using conventional metallographic techniques. The cut piece of the cracked area polished in a transverse direction (i.e., through thickness) showed the presence of an embedded crack, 2.5 mm deep, below 0.75 mm from the outer surface (Fig. 2). The crack plane lies perpendicular to the weld direction and is in the plane of the thickness. The hardware in the presence of such a large crack will not be quali®ed for service. The polished pieces were electro-etched with 10% oxalic acid in methanol as electrolyte to reveal the microstructure. The microstructure of the cracked piece revealed a typical welded structure, interface of weld pool and parent phase (Fig. 3). Dendritic coring, typical of a rapidly frozen austenite weld pool was seen. Within the weld pool, the presence of an interface between the previously solidi®ed pool and the subsequently solidi®ed pool was seen (Fig. 3(a)). Locations 1 and 2 (Fig. 3(a)) along the centerline of weld pool at higher magni®cation are shown in Fig. 3(b) and (c). This shows the presence of primary austenite in and around the crack. Within the weld pool, at the center, coring with equiaxed grains of primary austenite was seen surrounding the crack (Fig. 3). This interface did not penetrate through the thickness as evidenced by the lowest contour of the interface (Fig. 4). Little acicular or Widmanstatten austenite was seen near the interface at the grain boundary region (Fig. 4). The microstructure of the parent metal revealed the typical structure of stainless steel without any thickening of grain boundaries at the weld/parent interface (Fig. 5). The microstructure of the cut piece taken away from the cracked region revealed a typical welded structure (Fig. 6(a)) with the absence of any interfaces within the weld
Fig. 2. Presence of subsurface crack, 8.
Fig. 3. (a) Welded microstructure with interface, 125. (b) and (c) Locations 1 and 2 of (a) magni®ed at 500.
160 A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
161
pool as well as the coring as seen in the microstructure of the cracked region. Location 1 (Fig. 6(a)) is shown at higher magni®cation in Fig. 6(b). Another strip 25 mm 75 mm containing a crack and minimum remaining ligament within the weld pool was opened out in two halves to view the cracked surface morphology under the Scanning Electron Microscope (SEM). The fracture and opened surfaces consist of two areas, one darker area representing the fracture surface due to the preexisting crack and another brighter area representing the fresh opened fracture surface. The darker area was seen at higher magni®cation under the SEM. The center of the dark area revealed a very ®ne aligned equiaxed fracture surface morphology typical of solidi®ed cast material (Fig. 7). Alignment of equiaxed grains was clearly seen along the direction of the solidi®cation front. Fig. 8 shows equiaxed grains at the center along with the larger elongated grains aligned toward the interface of weld pool and parent metal. Fig. 9 shows typical features of cast material fracture surface. 4. Discussion Electron beam welding was carried out on the formed dome and cylindrical shell involving severe deformation of the cold rolled sheet which introduced residual stresses into the components. The presence of deformation twins as seen in the microstructure justi®es that the components have undergone severe deformation. Components with residual stresses from cold deformation during rolling and forming have been welded together. The crack exists up to a length of 75 mm along the welding direction.
Fig. 4. Bottom contour of another interface within the weld pool, 500.
Fig. 5. Crack morphology, deformation twins and absence of grain boundary thickening, 500.
162 A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
163
The EB welding practice adopted for fabrication of this tank allowed for an over run to a length in the range of 75±100 mm. The presence of an interface within the weld pool and the crack at the center up to a length of 75 mm con®rms that the cracking occured within the over run area. The crack is found exactly at the center. Primary austenite is found at the center and the crack is seen within this phase. There is no appreciable chemical variation across the weld pool. The absence of solidi®cation segregation as observed in the present case con®rms that the undercooling at the dendritic tip is of the order of the solidi®cation interval. Austenitic stainless steel solidi®es as fully primary austenite during rapid resolidi®cation. This rapid resolidi®cation condition prevails at the center of the weld particularly in a high-energy-density weld. The solidi®cation behaviour and microstructure of high-energy-density
Fig. 6. (a) Welded microstructure away from cracked region, 125. (b) Location 1 of (a) magni®ed at 500.
164
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
Fig. 7. Equiaxed grains aligned along the solidi®cation direction.
welds like EB welds are dierent from that of conventional welds such as Submerged Arc Welding (SAW) and Gas Tungsten Arc Welding (GTAW). This dierence has been attributed to the rapid solidi®cation velocity and cooling rates of high-energy-density welds. The most important eect of rapid solidi®cation is a shift in the solidi®cation transition from primary austenite to primary ferrite (high
Fig. 8. Weld central pool (equiaxed grains) and larger grains aligned toward the weld/parent interface.
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
165
temperature) to higher chromium to nickel equivalent (Creq/Nieq) ratios. Susceptibility to cracking is more for a weld that solidi®es as primary austenite than as primary ferrite. Similar centerline cracking of an EB weld due to a change in solidi®cation mode from primary ferrite to primary austenite has also been reported by other investigators [1]. Near the interface of the resolidi®cation front a few Widmanstatten austenite phases are seen which have a lesser resistance to cracking than ferrite. Absence of grain boundary thickening at the interface between weld and parent material as observed in the present case rules out any possibility of sensitisation of material. The cracked surface morphology was seen under the Scanning Electron Microscope (SEM). Equiaxed grains are seen at the center which were aligned along the direction of solidi®cation whereas comparatively larger elongated grains were aligned toward the interface of weld/parent metal. The presence of equiaxed grains in the vicinity of the crack indicates the possibility of cracking at high temperature at which the critical shear stress is less. If sucient strain/stress develops before the low melting liquidus is reached, then solidi®cation cracks can easily form and propagate. The component has undergone severe deformation and forming. In the absence of any intermediate annealing or stress relieving treatment, the residual stress introduced into the component during fabrication and constrained welding condition during ®xturing for ovality correction have played a major role in cracking. The longitudinal restraint is high at the weld stop position [2]. The crack caused by such restraint lies normal to the weld bead surface. One mechanism of cracking due to the in¯uence of stress from shrinkage is that cracking occurs in the ®nal stage of solidi®cation because the partially solidi®ed dendritic structure is unable to tolerate the contraction strain or external restraint. As cracks are seen in the overlap region (i.e., the area in the preceding weld remelted by the succeeding weld), the possibility of low melting liquid running as a frontier in the moving weld pool and accumulation in the over run region cannot be ruled out. Contractional strain during welding has played an important role and primary austenite as discussed earlier makes the region susceptible to cracking.
Fig. 9. Typical cast fracture feature.
166
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
Detailed experimentation on the in¯uence of solidi®cation velocity on the relative amount of primary and secondary phases that solidify from the melt is described elsewhere [3]. Based on this study it is found that solidi®cation as primary austenite can be avoided by reducing the interface velocity to less than 40 mm/s by adjusting the heat input and speed of welding. Another attempt [4] to avoid cracking in such high-energy-density welds is to have chemical composition control by reducing the S, P and Si content. 5. Conclusions The cracking has been attributed mainly to the shift in the solidi®cation mode from primary ferrite to primary austenite and residual stress introduced during rolling and forming of material as well as additional contractional strain during welding under ®xtured condition. Acknowledgements The authors are thankful to Shri A. Natarajan, Deputy Head, Material Characterisation Division, Dr. T.S. Lakshmanan, Head, Material Characterisation Division, Dr. K.V. Nagarajan, Group Director, Materials & Metallurgy Group and Shri K.S. Sastri, Deputy Director (PCM), VSSC for their valuable technical guidance during the work. The authors wish to thank Shri Madhavan Nair, Director, VSSC to permit us to publish this paper. References [1] ASM Specialty Hand Book. In: Davis JR, editor. Stainless steel. Material Park, OH 44073: ASM International, The Material Information Society, 1994. p. 372. [2] Gooch TG, et al. In: Proc. Conf. `Weldability of Materials' Detroit, MT, October. Materials Park, OH 44073: ASM Welding Metallurgy Committee, ASM International, 1990. p. 31. [3] Elmer JW, et al. In: Proc. Conf. `Weldability of Materials' Detroit, MT, October. Materials Park, OH 44073: ASM Welding Metallurgy Committee, ASM International, 1990. p. 143. [4] Metals hand book. 9th ed., vol. 12. Metals Park, OH 44073: ASM International, 1987. p. 138.
www.elsevier.com/locate/engfailanal
Metallographic analysis of embedded crack in electron beam welded austenitic stainless steel chemical storage tank Abhay K. Jha*, S. Arumugham Material Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Trivandrum 695 022, India Received 6 December 1999; accepted 4 January 2000
Abstract An AISI 304L austenitic stainless steel tank used for chemical storage showed cracks during the post weld quali®cation programme. A crack of 75 mm length embedded within the weld pool was subjected to detailed metallographic analysis. The results revealed that the cracking was due to a shift in the solidi®cation mode from primary ferrite to primary austenite. The residual stress introduced during rolling and forming of material as well as additional contractional strain during welding under ®xtured condition, are additional factors which caused cracking. 7 2001 Elsevier Science Ltd. All rights reserved. Keywords: Cracks; Residual stress; Chemical-plant failures; Tank failures; Welded fabrications
1. Introduction AISI 304L austenitic stainless steel is used for a variety of components for chemical plants which demand good formability in addition to high corrosion resistance. One such component is a cylindrical shape with torospherical end domes. The tank is fabricated out of cold rolled AISI 304L austenitic stainless steel sheet of 6 mm thickness. A schematic diagram is shown in Fig. 1. The tanks are fabricated by forming and electron beam (EB) welding. Fabrication does not involve annealing or any stressrelieving treatment before welding. During recent fabrication of tanks, cracks were noticed in a few tanks during post weld radiographic quali®cation. Such cracks were found to be embedded as there was no indication of cracks on the surface. A cut piece of the weldment with an embedded crack was subjected to detailed metallographic examination, as described in this paper. * Corresponding author. Tel.: +91-471-563-748; fax: +91-471-415-348. E-mail address: [email protected] (A.K. Jha). 1350-6307/01/$ - see front matter 7 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 6 3 0 7 ( 0 0 ) 0 0 0 0 3 - 0
158
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
Fig. 1. Schematic diagram of chemical storage tank.
2. Material The material from the dome portion as well as the cylindrical shell was analysed for chemical composition, which con®rmed the material to be AISI 304L except for the carbon content, which is more than the speci®ed limit of 0.03% (Table 1). The material has ultimate tensile strength (UTS) = 550 MPa, 0.2% proof stress = 245 MPa and elongation = 55% (on 50 mm G.L.).
Table 1 Chemical composition of the tank material (wt%) Element
C
Mn
Si
S
P
Ni
Cr
Fe
Speci®cation Tank
0.030max 0.045
2.0max 1.80
1.0max 0.41
0.03max 0.019
0.045max 0.023
8±12 10.6
18±20 18.4
Bal Bal
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
159
3. Experiments A cut piece measuring 200 mm 100 mm was taken across the weld of a tank which showed the presence of cracks during post weld radiographic observation. The cut piece was again radiographically examined to con®rm the presence of a crack extending up to a length of 75 mm. A strip 25 mm 75 mm was taken from the cracked piece for detailed investigation which included radiography, dye penetrant test, stereo microscopy, optical microscopy and scanning electron microscopy. Visual observation as well as dye penetrant tests did not reveal any crack on the surface. One cut piece from the cracked region and another away from the cracked area were taken for metallographic observation. Both the cut pieces were polished along lateral as well as transverse directions using conventional metallographic techniques. The cut piece of the cracked area polished in a transverse direction (i.e., through thickness) showed the presence of an embedded crack, 2.5 mm deep, below 0.75 mm from the outer surface (Fig. 2). The crack plane lies perpendicular to the weld direction and is in the plane of the thickness. The hardware in the presence of such a large crack will not be quali®ed for service. The polished pieces were electro-etched with 10% oxalic acid in methanol as electrolyte to reveal the microstructure. The microstructure of the cracked piece revealed a typical welded structure, interface of weld pool and parent phase (Fig. 3). Dendritic coring, typical of a rapidly frozen austenite weld pool was seen. Within the weld pool, the presence of an interface between the previously solidi®ed pool and the subsequently solidi®ed pool was seen (Fig. 3(a)). Locations 1 and 2 (Fig. 3(a)) along the centerline of weld pool at higher magni®cation are shown in Fig. 3(b) and (c). This shows the presence of primary austenite in and around the crack. Within the weld pool, at the center, coring with equiaxed grains of primary austenite was seen surrounding the crack (Fig. 3). This interface did not penetrate through the thickness as evidenced by the lowest contour of the interface (Fig. 4). Little acicular or Widmanstatten austenite was seen near the interface at the grain boundary region (Fig. 4). The microstructure of the parent metal revealed the typical structure of stainless steel without any thickening of grain boundaries at the weld/parent interface (Fig. 5). The microstructure of the cut piece taken away from the cracked region revealed a typical welded structure (Fig. 6(a)) with the absence of any interfaces within the weld
Fig. 2. Presence of subsurface crack, 8.
Fig. 3. (a) Welded microstructure with interface, 125. (b) and (c) Locations 1 and 2 of (a) magni®ed at 500.
160 A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
161
pool as well as the coring as seen in the microstructure of the cracked region. Location 1 (Fig. 6(a)) is shown at higher magni®cation in Fig. 6(b). Another strip 25 mm 75 mm containing a crack and minimum remaining ligament within the weld pool was opened out in two halves to view the cracked surface morphology under the Scanning Electron Microscope (SEM). The fracture and opened surfaces consist of two areas, one darker area representing the fracture surface due to the preexisting crack and another brighter area representing the fresh opened fracture surface. The darker area was seen at higher magni®cation under the SEM. The center of the dark area revealed a very ®ne aligned equiaxed fracture surface morphology typical of solidi®ed cast material (Fig. 7). Alignment of equiaxed grains was clearly seen along the direction of the solidi®cation front. Fig. 8 shows equiaxed grains at the center along with the larger elongated grains aligned toward the interface of weld pool and parent metal. Fig. 9 shows typical features of cast material fracture surface. 4. Discussion Electron beam welding was carried out on the formed dome and cylindrical shell involving severe deformation of the cold rolled sheet which introduced residual stresses into the components. The presence of deformation twins as seen in the microstructure justi®es that the components have undergone severe deformation. Components with residual stresses from cold deformation during rolling and forming have been welded together. The crack exists up to a length of 75 mm along the welding direction.
Fig. 4. Bottom contour of another interface within the weld pool, 500.
Fig. 5. Crack morphology, deformation twins and absence of grain boundary thickening, 500.
162 A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
163
The EB welding practice adopted for fabrication of this tank allowed for an over run to a length in the range of 75±100 mm. The presence of an interface within the weld pool and the crack at the center up to a length of 75 mm con®rms that the cracking occured within the over run area. The crack is found exactly at the center. Primary austenite is found at the center and the crack is seen within this phase. There is no appreciable chemical variation across the weld pool. The absence of solidi®cation segregation as observed in the present case con®rms that the undercooling at the dendritic tip is of the order of the solidi®cation interval. Austenitic stainless steel solidi®es as fully primary austenite during rapid resolidi®cation. This rapid resolidi®cation condition prevails at the center of the weld particularly in a high-energy-density weld. The solidi®cation behaviour and microstructure of high-energy-density
Fig. 6. (a) Welded microstructure away from cracked region, 125. (b) Location 1 of (a) magni®ed at 500.
164
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
Fig. 7. Equiaxed grains aligned along the solidi®cation direction.
welds like EB welds are dierent from that of conventional welds such as Submerged Arc Welding (SAW) and Gas Tungsten Arc Welding (GTAW). This dierence has been attributed to the rapid solidi®cation velocity and cooling rates of high-energy-density welds. The most important eect of rapid solidi®cation is a shift in the solidi®cation transition from primary austenite to primary ferrite (high
Fig. 8. Weld central pool (equiaxed grains) and larger grains aligned toward the weld/parent interface.
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
165
temperature) to higher chromium to nickel equivalent (Creq/Nieq) ratios. Susceptibility to cracking is more for a weld that solidi®es as primary austenite than as primary ferrite. Similar centerline cracking of an EB weld due to a change in solidi®cation mode from primary ferrite to primary austenite has also been reported by other investigators [1]. Near the interface of the resolidi®cation front a few Widmanstatten austenite phases are seen which have a lesser resistance to cracking than ferrite. Absence of grain boundary thickening at the interface between weld and parent material as observed in the present case rules out any possibility of sensitisation of material. The cracked surface morphology was seen under the Scanning Electron Microscope (SEM). Equiaxed grains are seen at the center which were aligned along the direction of solidi®cation whereas comparatively larger elongated grains were aligned toward the interface of weld/parent metal. The presence of equiaxed grains in the vicinity of the crack indicates the possibility of cracking at high temperature at which the critical shear stress is less. If sucient strain/stress develops before the low melting liquidus is reached, then solidi®cation cracks can easily form and propagate. The component has undergone severe deformation and forming. In the absence of any intermediate annealing or stress relieving treatment, the residual stress introduced into the component during fabrication and constrained welding condition during ®xturing for ovality correction have played a major role in cracking. The longitudinal restraint is high at the weld stop position [2]. The crack caused by such restraint lies normal to the weld bead surface. One mechanism of cracking due to the in¯uence of stress from shrinkage is that cracking occurs in the ®nal stage of solidi®cation because the partially solidi®ed dendritic structure is unable to tolerate the contraction strain or external restraint. As cracks are seen in the overlap region (i.e., the area in the preceding weld remelted by the succeeding weld), the possibility of low melting liquid running as a frontier in the moving weld pool and accumulation in the over run region cannot be ruled out. Contractional strain during welding has played an important role and primary austenite as discussed earlier makes the region susceptible to cracking.
Fig. 9. Typical cast fracture feature.
166
A.K. Jha, S. Arumugham / Engineering Failure Analysis 8 (2001) 157±166
Detailed experimentation on the in¯uence of solidi®cation velocity on the relative amount of primary and secondary phases that solidify from the melt is described elsewhere [3]. Based on this study it is found that solidi®cation as primary austenite can be avoided by reducing the interface velocity to less than 40 mm/s by adjusting the heat input and speed of welding. Another attempt [4] to avoid cracking in such high-energy-density welds is to have chemical composition control by reducing the S, P and Si content. 5. Conclusions The cracking has been attributed mainly to the shift in the solidi®cation mode from primary ferrite to primary austenite and residual stress introduced during rolling and forming of material as well as additional contractional strain during welding under ®xtured condition. Acknowledgements The authors are thankful to Shri A. Natarajan, Deputy Head, Material Characterisation Division, Dr. T.S. Lakshmanan, Head, Material Characterisation Division, Dr. K.V. Nagarajan, Group Director, Materials & Metallurgy Group and Shri K.S. Sastri, Deputy Director (PCM), VSSC for their valuable technical guidance during the work. The authors wish to thank Shri Madhavan Nair, Director, VSSC to permit us to publish this paper. References [1] ASM Specialty Hand Book. In: Davis JR, editor. Stainless steel. Material Park, OH 44073: ASM International, The Material Information Society, 1994. p. 372. [2] Gooch TG, et al. In: Proc. Conf. `Weldability of Materials' Detroit, MT, October. Materials Park, OH 44073: ASM Welding Metallurgy Committee, ASM International, 1990. p. 31. [3] Elmer JW, et al. In: Proc. Conf. `Weldability of Materials' Detroit, MT, October. Materials Park, OH 44073: ASM Welding Metallurgy Committee, ASM International, 1990. p. 143. [4] Metals hand book. 9th ed., vol. 12. Metals Park, OH 44073: ASM International, 1987. p. 138.