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Submerged arc welding of stainless steel and the challenge from the laser welding process
Journal of Materials Processing Technology 134 (2003) 174±179
Submerged arc welding of stainless steel and the challenge from the laser welding process N.A. McPhersona,*, K. Chib, T.N. Bakerb a
b
BAE Systems, 1048 Govan Road, Glasgow G51 4XP, Scotland, UK Department of Mechchanical Engineering, University of Strathclyde, Glasgow, Scotland, UK Received 13 August 2001; received in revised form 6 March 2002; accepted 28 June 2002
Abstract The welding of austenitic and duplex stainless steels has been reassessed by questioning traditional requirements of the weld metal and/or the heat affected zone (HAZ). The use of high dilution submerged arc welding of austenitic and duplex stainless steels has been shown to produce acceptable properties, despite the high heat input used in some instances. Corrosion characteristics have been established as being acceptable too. These ®ndings have been further validated by examination of the weld region material using thin foil transmission electron microscopy (TEM). This showed that while some intermetallic phases were present, they did not adversely affect the weld metal properties. In addition, an examination has taken place of Nd:YAG laser-welded austenitic and duplex stainless steels, to establish the potential viability of this route compared to the submerged arc welding process. The material properties, including the relevant corrosion testing, have been found to be acceptable. TEM has again shown that some intermetallic phases are present in the weld metal. It has been suggested that a segregation mechanism is responsible for their presence in this case. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Austenitic stainless steel; Duplex stainless steel; Submerged arc welding; Nd:YAG laser welding; High dilution; Intermetallic phases
1. Introduction The submerged arc welding process was developed in the 1930s and 1940s, and since then it has become wellestablished. It is used wherever its limitations allow, as bene®ts from the high productivity, and high quality components have to be maximised. It has been used on a number of different types of steels, and with the upsurge in the market for chemical carrier ships [1], its use was adopted, wherever possible, for welding the integral 316LN type stainless steel tanks. The concept of single-sided, single pass welding using a square edge preparation with a single wire was formerly `considered' as being detrimental to the corrosion properties of the stainless steel weld metal and the heat affected zone (HAZ). There were, however, no scienti®c data to justify this opinion, merely the view that the high heat input involved would lead to the precipitation of undesirable intermetallic phases and carbides, due to the slow cooling rates. In addition, as the use of duplex stainless steel has increased [2], so too has its range of potential applications. *
Corresponding author.
Most of the `rules' for the duplex stainless steel requirements have been developed from its use in offshore industry sectors. However, in other areas such as shipbuilding, where corrosion resistance is the main criterion, some of the `rules' may not necessarily be applicable. As in the case of the 316LN austenitic stainless steels, the single-sided single pass welding process was considered to be undesirable, as the required high heat input would generate too low a ferrite content in the weld metal and create favourable conditions for the precipitation of intermetallic phases. In a similar vein, the exceptionally high cooling rates created in laser welding would result in too high a ferrite content if autogenous welding was being carried out, when welding duplex stainless steels. For austenitic stainless steels it has been reported [3] that the high cooling rates of laser welding and electron beam welding favour the formation of austenitic microstructures, which are more susceptible to internal cracking. This paper will describe work that was undertaken to question the validity of some of the `rules' and `standards' related to the submerged arc welding of stainless steels. In addition, the results of some autogenous Nd:YAG laser welding will be introduced into the discussion.
0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 4 6 6 - 1
N.A. McPherson et al. / Journal of Materials Processing Technology 134 (2003) 174±179
175
(20 wt.% Cr±14.5 wt.% Ni±3.7 wt.% Mo) and for the 2205 steel, the wire was 2209 (23 wt.% Cr±9.0 wt.% Ni±3.2 wt.% Mo). The welding ¯ux used in both cases was the same, i.e. a 2.3 basicity ¯ux. Each weld was X-rayed and crack tested, and found to be satisfactory. Mechanical testing was carried out using a longitudinal weld metal tensile test to establish the key mechanical properties. Other relevant testing, including corrosion testing, was carried out, and is detailed in Table 4.
2. Welding of stainless steels In each case the plate material used was either chromium± nickel±molybdenum 316LN austenitic stainless steel or 2205 grade duplex stainless steel. Table 1 shows chemical compositions typical of the plates used throughout this work. A number of welds were produced using a Y preparation, a V preparation, a square edge preparation ][ with one pass from each side, and a square edge preparation, with one pass only. In the case of laser welds, the weld preparation was square edge ][. The submerged arc welds produced with the relevant heat inputs, corrected using an ef®ciency factor of 0.95, have been compiled in Table 2. In each case, a single 3.2 mm diameter electrode wire was used. For the 316LN steel, the consumable wire was 317L
3. Results With the exception of the laser welds, the weld metal chemical analysis of each weld is shown in Table 3.
Table 1 Typical chemical analysis of the plates used
316LN 2205
C (wt.%)
Si (wt.%)
S (wt.%)
P (wt.%)
Mn (wt.%)
Ni (wt.%)
Cr (wt.%)
Mo (wt.%) Cu (wt.%)
Al (wt.%)
N2 (wt.%)
0.02 0.03
0.43 0.45
0.005 0.006
0.028 0.025
1.55 0.86
11.3 5.60
18.1 22.1
2.9 3.2
0.01 0.01
0.150 0.155
0.24 0.22
Table 2 Details of the welds produced by arc weldinga Austenitic 316LN
Duplex 2205
Plate thickness (mm)
Prepration
Passes
Heat input (kJ/mm)
Pass
Plate thickness (mm)
Prepration
Passes
Heat input (kJ/mm)
Pass
16
Y
4
1.36 1.84 2.06 1.80
1/1 1/2 1/3 1/4
15
Y
3
1.51 2.88 3
1/1 1/2 2/1
16
][
2
2.75 3.29
1/1 2/1
15
V
4
1.1 1.35
1/1 1/2
20
][
2
3.37 3.90
1/1 2/1
2.56 2.70
1/3 2/1
10
][
1
3.37
1/1
12
][
1
4.45
1/1
15
][
1
4.45
1/1
15
][
1
5.50
1/1
a
Y: Y shape preparation; V: V shape preparation; ][: square edge preparation.
Table 3 Chemical analysis of the weld metal from each arc weld used in this study Material
Plate thickness (mm)
Prepration
Passes
C (wt.%)
Si (wt.%)
S (wt.%)
P (wt.%)
Mn (wt.%)
Ni (wt.%)
Cr (wt.%)
Mo (wt.%)
Cu (wt.%)
Al (wt.%)
N2 (wt.%)
Austenitic 16 316LN 16 20 10 15 15
Y ][ ][ ][ ][ ][
4 2 2 1 1 1
0.007 0.020 0.018 0.020 0.020 0.030
0.65 0.60 0.48 0.47 0.51 0.50
0.004 0.007 0.004 0.006 0.006 0.005
0.018 0.038 0.020 0.030 0.034 0.023
1.52 1.28 1.38 1.49 1.42 1.42
12.7 11.2 12.7 11.6 11.6 12.2
18.4 18.6 18.6 18.5 18.2 18.1
3.60 3.10 3.02 3.00 2.90 3.20
0.19 0.26 0.13 0.22 0.24 0.22
0.014 0.016 0.020 0.014 0.103 0.014
0.06 0.10 0.13 0.12 0.12 0.12
Duplex 2205
V Y ][
4 3 1
0.030 0.010 0.030
0.61 0.62 0.50
0.012 0.008 0.008
0.031 0.027 0.021
1.32 1.29 1.22
22.2 22.5 21.8
3.10 3.00 3.20
0.24 0.22 0.17
0.012 0.011 0.014
0.14 0.15 0.15
15 15 12
7.00 7.80 6.40
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The dilution effects are evident in the square edge preparation welds compared with the Y or V preparation, where the chemical analysis is closer to the consumable wire analysis. The results in Table 4 show that, irrespective of which welding procedure is used, the mechanical properties and other test requirements have been met. In the case of the 316LN steel, the use of the higher alloyed 317L consumable has counteracted the dilution effects from the parent plates and as a result, acceptable strength levels have been maintained. Duplex 2205 has shown a similar trend, and in the case of the single-sided single pass weld, acceptable mechanical properties have been achieved, despite a slightly leaner weld metal chemistry resulting from the higher dilution of the square edge preparation. In the case of both steels, the high dilution submerged arc welding of stainless steels is a feasible option. In all cases, of the 316LN material, the optical microstructure consisted of lacy/vermicular ferrite [4]. In the duplex 2205 material, the optical microstructure consisted of an acicular austenite± ferrite structure. However, to enhance the ®ndings at this stage, it was essential to ensure that potentially damaging artefacts were not present within the weld metal. To con®rm this, transmission electron microscopy (TEM) of thin foils taken from de®ned positions within the weld was carried out. The 316LN welds were found to contain small quantities of the intermetallic chi (w)-phase. A typical example is shown in Fig. 1(a), and shows the precipitates on the g/d boundaries, which is their predominant location. There were some instances of isolated sigma (s)-phase particles, in some of the welds, but they were relatively small and have not signi®cantly affected the mechanical properties. An example of a s-phase particle is shown in Fig. 1(b). One unusual feature was the observation of precipitation of M7C3 in the 15 mm thick single-sided single pass weld. This is shown in Fig. 1(c). All the precipitates were positively identi®ed by selected area electron diffraction in the TEM. The positions where the samples were taken from are shown in Fig. 2, which is a macroetched sample of the 10 mm thick weld. The duplex stainless steels welds showed no evidence of precipitation in the weld metal in any of the welds. There were some isolated instances of w in the HAZ, but these were relatively infrequent. This work has further con®rmed the earlier view that high dilution submerged arc welding practices for 316LN and duplex 2205 steels can be used effectively for applications in chemical carriers. 4. Laser-welded stainless steels Plate which was 6 and 7.5 mm thick was used to evaluate autogenous Nd:YAG laser welding of stainless steels. The main ®ndings from this work have been presented in Table 5.
Fig. 1. Precipitation in 316LN single-sided pass weld: (a) w-phase precipitates on the austenite/d-ferrite boundary on 15 mm thick weld towards the root adjacent to the fusion line; (b) s-phase precipitation in cap region in 10 mm thick weld; (c) M7C3 precipitates in centre region of weld near the fusion line in 15 mm thick weld.
The main feature in the laser-welded material was the increased hardness compared to the submerged arc welded material. The optical microstructure of the 316LN material consisted of areas of vermicular ferrite plus areas of what appeared to be a cellular structure. In some instances the structure appeared to be totally austenitic. In some instances there was evidence of grain boundary liquidation at the
N.A. McPherson et al. / Journal of Materials Processing Technology 134 (2003) 174±179
177
Table 4 Mechanical properties of the weld metal Material
Plate thick (mm)
Preparation
Passes
Yield strength (N/mm2)
UTS (N/mm2)
e (%)
20
Austenitic 16 316LN 16 20 10 15 15
Y ][ ][ ][ ][ ][
4 2 2 1 1 1
374 298 482 318 320 308
618 558 718 585 586 584
43 37 33 46 40 38
Duplex 2205
V Y ][
4 3 1
533 526 564
766 722 769
30 28 30
a
15 15 12
Impact energy (J)
113 182 67 135a 143
20
60
106 190 54 123a 131 127
79 168 42 114a 117 99
45
16 20 11
99
Hardness (VPN) 100 69 146 30 88a 85 54
HAZ WM HAZ WM (cap) (cap) (root) (root)
Corrosion test (A262)
185 173 196 193 180 192
187 169 200 171 174 189
188 189 207 173 178 199
192 181 201 175 176 198
Pass Pass Pass Pass Pass Pass G48
242 247 264
233 242 259
256 243 268
270 256 267
Pass Pass Pass
Sub-size charpy specimen used results recalculated to be equivalent to other tests on standard charpy specimens.
Table 5 Properties of laser-welded material Material
316LN 2205 a
Plate thickness (mm)
7.6 5.8
UTS (N/mm2)
a
658 793a
Maximum hardness
Corrosion test
Ferrite (wt.%)
Weld
HAZ
A262
G48
220 308
N/A 285
Pass N/A
N/A Pass
8.5 68
UTS is based on a cross weld tensile test which fractured in the parent plate.
centreline region of the plate. This has then been followed by micro®ssuring along any centreline segregation that is present. The optical microstructure of the duplex 2205 steel consisted of large ferrite grains growing in from the HAZ to the weld centreline, where some smaller more regular
grains were present. The higher ferrite content of the duplex 2205 would contribute to the higher hardness of that weld. There was no evidence to indicate the presence of cracking in the microstructure in either the duplex or the 316LN weld metal.
Fig. 2. Macroetch of 10 mm thick 316LN single-sided single pass weld, showing the positions of the transmission electron micrographs shown in Fig. 1.
178
N.A. McPherson et al. / Journal of Materials Processing Technology 134 (2003) 174±179
weld. Fig. 3(c) is a typical area in the 2205 weld showing relatively high dislocation densities in the ferrite, and angular intragranular austenite particles. In the work carried out by Chen et al. [6] using CO2 laser-welded 2205, no precipitates were identi®ed, but the austenite in that structure was heavily dislocated and contained evidence of twinning. 5. Discussion
Fig. 3. TEM structures in laser-welded stainless steels: (a) vermicular ferrite structure in 316LN weld (dark field image); (b) chromium nitride particle in the centre of a high dislocation density femite region in 316LN weld; (c) austenite needles growing in the ferrite grain boundary of duplex stainless steel weld metal.
TEM of the 316LN weld showed the presence of areas of ferrite, as shown in Fig. 3(a). In addition, the dislocation density in all the samples examined was higher than that seen in the submerged arc welded samples. Initial work [5] did not show any boundary precipitation, but subsequently precipitates were observed which have been identi®ed as w phase and Cr2N in both the 316LN and the 2205 welds. The particle shown in Fig. 3(b) is a Cr2N particle in the 316LN
It has been shown that high dilution single-sided welding of 316LN steel is a feasible option either as a submerged arc weld or as a laser weld. In each case the properties are acceptable, and the corrosion requirements have been met. However, the situation regarding autogenous laser welding has still to be fully resolved. The presence of the w and Cr2N precipitates is unexpected, and can only have occurred due to some high temperature segregation effect, as neither particle would have time to precipitate via a conventional mechanism due to the very high cooling rates of the weld metal [7]. It is clear that some further work will have to be done in this area to gain a fuller understanding of the weld metal material. A similar discussion can be made related to the 2205 duplex steel. There appears to be signi®cant scope to utilise high dilution submerged arc welding of that grade. In addition to this work, other work [8] has shown that high dilution double-sided submerged arc welding of 20 mm thick duplex stainless steel can produce an acceptable quality, although a slight chamfer on the weld preparation is required. The potentially higher ferrite contents of the laser-welded material, although showing acceptable corrosion properties in this work and other investigations, has to be fully considered. The use of ®ller wire to reduce the ferrite content is a potential route, but this has the effect of reducing the welding speed to that of conventional processes. The effect of hydrogen in higher ferrite welds has to be considered, as it has the possibility of stress corrosion cracking. One of the major bene®ts that has to be borne in mind is the reduced distortion that is a feature of laser-welded plate. The double-sided submerged arc welding process creates a situation where distortion can be countered [9]. The singlesided welding process will tend to exacerbate distortion. The application of autogenous laser welding on an industrial scale will in the future be limited to specialised applications. This is due to the demanding requirements for material ®t up prior to welding. It is becoming more evident [10] that the use of a hybrid laser system involving the metal inert gas (MIG) process and a laser (CO2 or Nd:YAG) will tolerate less critical material ®t up. This will involve only a marginal reduction in the bene®t to distortion of autogenous laser welding. 6. Conclusions The high dilution submerged arc welding of stainless steels has been shown to be a viable process for even further
N.A. McPherson et al. / Journal of Materials Processing Technology 134 (2003) 174±179
expansion in the cases of 316LN and duplex 2205. The challenge from laser welding will exist in the future, due to its bene®cial effects for distortion. A number of areas require further work within the laser welding process. Acknowledgements The authors wish to thank BAE Systems for permission to publish this paper. References [1] N.A. McPherson, D. Doig, Stainless Steel World (1999) 29±33. [2] J. Charles, in: Proceedings of the Duplex Stainless Steels 1994 Conference, Glasgow, 1994, pp. 3±48.
179
[3] S. Fukumoto, W. Kurz, ISIJ Int. 37 (1997) 677±684. [4] H. Inoue, T. Koseki, S. Ohkita, M. Fuji, Sci. Technol. Weld. Join. 5 (2000) 385±396. [5] N.A. McPherson, T.N. Baker, C. Hu, J.D. Russell, in: Proceedings of the Stainless Steel 1999 Conference, Vol. 3, Sardinia, 1999, pp. 361± 368. [6] C.P. Chen, N.J. Ho, H.L. Huang, in: Proceedings of the Taiwan International Welding Conference 1998 on Technology Advancements and New Industrial Applications in Welding, Taipei, 1998, pp. 291±298. [7] W. Kurz, R. Trivedi, Trends in welding research, in: Proceedings of the Fourth International Conference, Gatlinburg, 1995, pp. 115± 120. [8] N.A. McPherson, C. Baxter, BAE Systems and Avesta Sheffield, 1999, Unpublished work. [9] N.A. McPherson, T.N. Baker, Y. Li, J. Hoffmann, Sci. Technol. Weld. Join. 5 (2000) 35±39. [10] P. Seyffarth, B. Anders, J. Hoffmann, in: Proceedings of the JOM-8 International Conference on the Joining of Materials, Helsingor, Denmark, 1997, pp. 120±125.
Submerged arc welding of stainless steel and the challenge from the laser welding process N.A. McPhersona,*, K. Chib, T.N. Bakerb a
b
BAE Systems, 1048 Govan Road, Glasgow G51 4XP, Scotland, UK Department of Mechchanical Engineering, University of Strathclyde, Glasgow, Scotland, UK Received 13 August 2001; received in revised form 6 March 2002; accepted 28 June 2002
Abstract The welding of austenitic and duplex stainless steels has been reassessed by questioning traditional requirements of the weld metal and/or the heat affected zone (HAZ). The use of high dilution submerged arc welding of austenitic and duplex stainless steels has been shown to produce acceptable properties, despite the high heat input used in some instances. Corrosion characteristics have been established as being acceptable too. These ®ndings have been further validated by examination of the weld region material using thin foil transmission electron microscopy (TEM). This showed that while some intermetallic phases were present, they did not adversely affect the weld metal properties. In addition, an examination has taken place of Nd:YAG laser-welded austenitic and duplex stainless steels, to establish the potential viability of this route compared to the submerged arc welding process. The material properties, including the relevant corrosion testing, have been found to be acceptable. TEM has again shown that some intermetallic phases are present in the weld metal. It has been suggested that a segregation mechanism is responsible for their presence in this case. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Austenitic stainless steel; Duplex stainless steel; Submerged arc welding; Nd:YAG laser welding; High dilution; Intermetallic phases
1. Introduction The submerged arc welding process was developed in the 1930s and 1940s, and since then it has become wellestablished. It is used wherever its limitations allow, as bene®ts from the high productivity, and high quality components have to be maximised. It has been used on a number of different types of steels, and with the upsurge in the market for chemical carrier ships [1], its use was adopted, wherever possible, for welding the integral 316LN type stainless steel tanks. The concept of single-sided, single pass welding using a square edge preparation with a single wire was formerly `considered' as being detrimental to the corrosion properties of the stainless steel weld metal and the heat affected zone (HAZ). There were, however, no scienti®c data to justify this opinion, merely the view that the high heat input involved would lead to the precipitation of undesirable intermetallic phases and carbides, due to the slow cooling rates. In addition, as the use of duplex stainless steel has increased [2], so too has its range of potential applications. *
Corresponding author.
Most of the `rules' for the duplex stainless steel requirements have been developed from its use in offshore industry sectors. However, in other areas such as shipbuilding, where corrosion resistance is the main criterion, some of the `rules' may not necessarily be applicable. As in the case of the 316LN austenitic stainless steels, the single-sided single pass welding process was considered to be undesirable, as the required high heat input would generate too low a ferrite content in the weld metal and create favourable conditions for the precipitation of intermetallic phases. In a similar vein, the exceptionally high cooling rates created in laser welding would result in too high a ferrite content if autogenous welding was being carried out, when welding duplex stainless steels. For austenitic stainless steels it has been reported [3] that the high cooling rates of laser welding and electron beam welding favour the formation of austenitic microstructures, which are more susceptible to internal cracking. This paper will describe work that was undertaken to question the validity of some of the `rules' and `standards' related to the submerged arc welding of stainless steels. In addition, the results of some autogenous Nd:YAG laser welding will be introduced into the discussion.
0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 4 6 6 - 1
N.A. McPherson et al. / Journal of Materials Processing Technology 134 (2003) 174±179
175
(20 wt.% Cr±14.5 wt.% Ni±3.7 wt.% Mo) and for the 2205 steel, the wire was 2209 (23 wt.% Cr±9.0 wt.% Ni±3.2 wt.% Mo). The welding ¯ux used in both cases was the same, i.e. a 2.3 basicity ¯ux. Each weld was X-rayed and crack tested, and found to be satisfactory. Mechanical testing was carried out using a longitudinal weld metal tensile test to establish the key mechanical properties. Other relevant testing, including corrosion testing, was carried out, and is detailed in Table 4.
2. Welding of stainless steels In each case the plate material used was either chromium± nickel±molybdenum 316LN austenitic stainless steel or 2205 grade duplex stainless steel. Table 1 shows chemical compositions typical of the plates used throughout this work. A number of welds were produced using a Y preparation, a V preparation, a square edge preparation ][ with one pass from each side, and a square edge preparation, with one pass only. In the case of laser welds, the weld preparation was square edge ][. The submerged arc welds produced with the relevant heat inputs, corrected using an ef®ciency factor of 0.95, have been compiled in Table 2. In each case, a single 3.2 mm diameter electrode wire was used. For the 316LN steel, the consumable wire was 317L
3. Results With the exception of the laser welds, the weld metal chemical analysis of each weld is shown in Table 3.
Table 1 Typical chemical analysis of the plates used
316LN 2205
C (wt.%)
Si (wt.%)
S (wt.%)
P (wt.%)
Mn (wt.%)
Ni (wt.%)
Cr (wt.%)
Mo (wt.%) Cu (wt.%)
Al (wt.%)
N2 (wt.%)
0.02 0.03
0.43 0.45
0.005 0.006
0.028 0.025
1.55 0.86
11.3 5.60
18.1 22.1
2.9 3.2
0.01 0.01
0.150 0.155
0.24 0.22
Table 2 Details of the welds produced by arc weldinga Austenitic 316LN
Duplex 2205
Plate thickness (mm)
Prepration
Passes
Heat input (kJ/mm)
Pass
Plate thickness (mm)
Prepration
Passes
Heat input (kJ/mm)
Pass
16
Y
4
1.36 1.84 2.06 1.80
1/1 1/2 1/3 1/4
15
Y
3
1.51 2.88 3
1/1 1/2 2/1
16
][
2
2.75 3.29
1/1 2/1
15
V
4
1.1 1.35
1/1 1/2
20
][
2
3.37 3.90
1/1 2/1
2.56 2.70
1/3 2/1
10
][
1
3.37
1/1
12
][
1
4.45
1/1
15
][
1
4.45
1/1
15
][
1
5.50
1/1
a
Y: Y shape preparation; V: V shape preparation; ][: square edge preparation.
Table 3 Chemical analysis of the weld metal from each arc weld used in this study Material
Plate thickness (mm)
Prepration
Passes
C (wt.%)
Si (wt.%)
S (wt.%)
P (wt.%)
Mn (wt.%)
Ni (wt.%)
Cr (wt.%)
Mo (wt.%)
Cu (wt.%)
Al (wt.%)
N2 (wt.%)
Austenitic 16 316LN 16 20 10 15 15
Y ][ ][ ][ ][ ][
4 2 2 1 1 1
0.007 0.020 0.018 0.020 0.020 0.030
0.65 0.60 0.48 0.47 0.51 0.50
0.004 0.007 0.004 0.006 0.006 0.005
0.018 0.038 0.020 0.030 0.034 0.023
1.52 1.28 1.38 1.49 1.42 1.42
12.7 11.2 12.7 11.6 11.6 12.2
18.4 18.6 18.6 18.5 18.2 18.1
3.60 3.10 3.02 3.00 2.90 3.20
0.19 0.26 0.13 0.22 0.24 0.22
0.014 0.016 0.020 0.014 0.103 0.014
0.06 0.10 0.13 0.12 0.12 0.12
Duplex 2205
V Y ][
4 3 1
0.030 0.010 0.030
0.61 0.62 0.50
0.012 0.008 0.008
0.031 0.027 0.021
1.32 1.29 1.22
22.2 22.5 21.8
3.10 3.00 3.20
0.24 0.22 0.17
0.012 0.011 0.014
0.14 0.15 0.15
15 15 12
7.00 7.80 6.40
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The dilution effects are evident in the square edge preparation welds compared with the Y or V preparation, where the chemical analysis is closer to the consumable wire analysis. The results in Table 4 show that, irrespective of which welding procedure is used, the mechanical properties and other test requirements have been met. In the case of the 316LN steel, the use of the higher alloyed 317L consumable has counteracted the dilution effects from the parent plates and as a result, acceptable strength levels have been maintained. Duplex 2205 has shown a similar trend, and in the case of the single-sided single pass weld, acceptable mechanical properties have been achieved, despite a slightly leaner weld metal chemistry resulting from the higher dilution of the square edge preparation. In the case of both steels, the high dilution submerged arc welding of stainless steels is a feasible option. In all cases, of the 316LN material, the optical microstructure consisted of lacy/vermicular ferrite [4]. In the duplex 2205 material, the optical microstructure consisted of an acicular austenite± ferrite structure. However, to enhance the ®ndings at this stage, it was essential to ensure that potentially damaging artefacts were not present within the weld metal. To con®rm this, transmission electron microscopy (TEM) of thin foils taken from de®ned positions within the weld was carried out. The 316LN welds were found to contain small quantities of the intermetallic chi (w)-phase. A typical example is shown in Fig. 1(a), and shows the precipitates on the g/d boundaries, which is their predominant location. There were some instances of isolated sigma (s)-phase particles, in some of the welds, but they were relatively small and have not signi®cantly affected the mechanical properties. An example of a s-phase particle is shown in Fig. 1(b). One unusual feature was the observation of precipitation of M7C3 in the 15 mm thick single-sided single pass weld. This is shown in Fig. 1(c). All the precipitates were positively identi®ed by selected area electron diffraction in the TEM. The positions where the samples were taken from are shown in Fig. 2, which is a macroetched sample of the 10 mm thick weld. The duplex stainless steels welds showed no evidence of precipitation in the weld metal in any of the welds. There were some isolated instances of w in the HAZ, but these were relatively infrequent. This work has further con®rmed the earlier view that high dilution submerged arc welding practices for 316LN and duplex 2205 steels can be used effectively for applications in chemical carriers. 4. Laser-welded stainless steels Plate which was 6 and 7.5 mm thick was used to evaluate autogenous Nd:YAG laser welding of stainless steels. The main ®ndings from this work have been presented in Table 5.
Fig. 1. Precipitation in 316LN single-sided pass weld: (a) w-phase precipitates on the austenite/d-ferrite boundary on 15 mm thick weld towards the root adjacent to the fusion line; (b) s-phase precipitation in cap region in 10 mm thick weld; (c) M7C3 precipitates in centre region of weld near the fusion line in 15 mm thick weld.
The main feature in the laser-welded material was the increased hardness compared to the submerged arc welded material. The optical microstructure of the 316LN material consisted of areas of vermicular ferrite plus areas of what appeared to be a cellular structure. In some instances the structure appeared to be totally austenitic. In some instances there was evidence of grain boundary liquidation at the
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Table 4 Mechanical properties of the weld metal Material
Plate thick (mm)
Preparation
Passes
Yield strength (N/mm2)
UTS (N/mm2)
e (%)
20
Austenitic 16 316LN 16 20 10 15 15
Y ][ ][ ][ ][ ][
4 2 2 1 1 1
374 298 482 318 320 308
618 558 718 585 586 584
43 37 33 46 40 38
Duplex 2205
V Y ][
4 3 1
533 526 564
766 722 769
30 28 30
a
15 15 12
Impact energy (J)
113 182 67 135a 143
20
60
106 190 54 123a 131 127
79 168 42 114a 117 99
45
16 20 11
99
Hardness (VPN) 100 69 146 30 88a 85 54
HAZ WM HAZ WM (cap) (cap) (root) (root)
Corrosion test (A262)
185 173 196 193 180 192
187 169 200 171 174 189
188 189 207 173 178 199
192 181 201 175 176 198
Pass Pass Pass Pass Pass Pass G48
242 247 264
233 242 259
256 243 268
270 256 267
Pass Pass Pass
Sub-size charpy specimen used results recalculated to be equivalent to other tests on standard charpy specimens.
Table 5 Properties of laser-welded material Material
316LN 2205 a
Plate thickness (mm)
7.6 5.8
UTS (N/mm2)
a
658 793a
Maximum hardness
Corrosion test
Ferrite (wt.%)
Weld
HAZ
A262
G48
220 308
N/A 285
Pass N/A
N/A Pass
8.5 68
UTS is based on a cross weld tensile test which fractured in the parent plate.
centreline region of the plate. This has then been followed by micro®ssuring along any centreline segregation that is present. The optical microstructure of the duplex 2205 steel consisted of large ferrite grains growing in from the HAZ to the weld centreline, where some smaller more regular
grains were present. The higher ferrite content of the duplex 2205 would contribute to the higher hardness of that weld. There was no evidence to indicate the presence of cracking in the microstructure in either the duplex or the 316LN weld metal.
Fig. 2. Macroetch of 10 mm thick 316LN single-sided single pass weld, showing the positions of the transmission electron micrographs shown in Fig. 1.
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weld. Fig. 3(c) is a typical area in the 2205 weld showing relatively high dislocation densities in the ferrite, and angular intragranular austenite particles. In the work carried out by Chen et al. [6] using CO2 laser-welded 2205, no precipitates were identi®ed, but the austenite in that structure was heavily dislocated and contained evidence of twinning. 5. Discussion
Fig. 3. TEM structures in laser-welded stainless steels: (a) vermicular ferrite structure in 316LN weld (dark field image); (b) chromium nitride particle in the centre of a high dislocation density femite region in 316LN weld; (c) austenite needles growing in the ferrite grain boundary of duplex stainless steel weld metal.
TEM of the 316LN weld showed the presence of areas of ferrite, as shown in Fig. 3(a). In addition, the dislocation density in all the samples examined was higher than that seen in the submerged arc welded samples. Initial work [5] did not show any boundary precipitation, but subsequently precipitates were observed which have been identi®ed as w phase and Cr2N in both the 316LN and the 2205 welds. The particle shown in Fig. 3(b) is a Cr2N particle in the 316LN
It has been shown that high dilution single-sided welding of 316LN steel is a feasible option either as a submerged arc weld or as a laser weld. In each case the properties are acceptable, and the corrosion requirements have been met. However, the situation regarding autogenous laser welding has still to be fully resolved. The presence of the w and Cr2N precipitates is unexpected, and can only have occurred due to some high temperature segregation effect, as neither particle would have time to precipitate via a conventional mechanism due to the very high cooling rates of the weld metal [7]. It is clear that some further work will have to be done in this area to gain a fuller understanding of the weld metal material. A similar discussion can be made related to the 2205 duplex steel. There appears to be signi®cant scope to utilise high dilution submerged arc welding of that grade. In addition to this work, other work [8] has shown that high dilution double-sided submerged arc welding of 20 mm thick duplex stainless steel can produce an acceptable quality, although a slight chamfer on the weld preparation is required. The potentially higher ferrite contents of the laser-welded material, although showing acceptable corrosion properties in this work and other investigations, has to be fully considered. The use of ®ller wire to reduce the ferrite content is a potential route, but this has the effect of reducing the welding speed to that of conventional processes. The effect of hydrogen in higher ferrite welds has to be considered, as it has the possibility of stress corrosion cracking. One of the major bene®ts that has to be borne in mind is the reduced distortion that is a feature of laser-welded plate. The double-sided submerged arc welding process creates a situation where distortion can be countered [9]. The singlesided welding process will tend to exacerbate distortion. The application of autogenous laser welding on an industrial scale will in the future be limited to specialised applications. This is due to the demanding requirements for material ®t up prior to welding. It is becoming more evident [10] that the use of a hybrid laser system involving the metal inert gas (MIG) process and a laser (CO2 or Nd:YAG) will tolerate less critical material ®t up. This will involve only a marginal reduction in the bene®t to distortion of autogenous laser welding. 6. Conclusions The high dilution submerged arc welding of stainless steels has been shown to be a viable process for even further
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expansion in the cases of 316LN and duplex 2205. The challenge from laser welding will exist in the future, due to its bene®cial effects for distortion. A number of areas require further work within the laser welding process. Acknowledgements The authors wish to thank BAE Systems for permission to publish this paper. References [1] N.A. McPherson, D. Doig, Stainless Steel World (1999) 29±33. [2] J. Charles, in: Proceedings of the Duplex Stainless Steels 1994 Conference, Glasgow, 1994, pp. 3±48.
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[3] S. Fukumoto, W. Kurz, ISIJ Int. 37 (1997) 677±684. [4] H. Inoue, T. Koseki, S. Ohkita, M. Fuji, Sci. Technol. Weld. Join. 5 (2000) 385±396. [5] N.A. McPherson, T.N. Baker, C. Hu, J.D. Russell, in: Proceedings of the Stainless Steel 1999 Conference, Vol. 3, Sardinia, 1999, pp. 361± 368. [6] C.P. Chen, N.J. Ho, H.L. Huang, in: Proceedings of the Taiwan International Welding Conference 1998 on Technology Advancements and New Industrial Applications in Welding, Taipei, 1998, pp. 291±298. [7] W. Kurz, R. Trivedi, Trends in welding research, in: Proceedings of the Fourth International Conference, Gatlinburg, 1995, pp. 115± 120. [8] N.A. McPherson, C. Baxter, BAE Systems and Avesta Sheffield, 1999, Unpublished work. [9] N.A. McPherson, T.N. Baker, Y. Li, J. Hoffmann, Sci. Technol. Weld. Join. 5 (2000) 35±39. [10] P. Seyffarth, B. Anders, J. Hoffmann, in: Proceedings of the JOM-8 International Conference on the Joining of Materials, Helsingor, Denmark, 1997, pp. 120±125.