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41 I MARS-AVRIL 2013 I SOUDAGE ET TECHNIQUES CONNEXES

ÉTUDES ET RECHERCHE

D. LARGE, F. SCANDELLA, A. ROBINEAU1, J. PEULTIER, A. FANICA2, F. DUPOIRON3, B. PETIT4, A. THULIN, R. PETTERSSON5, L. CARLIER, B. TILLARD6, F. WEISANG-HOINARD7, J. B. BEUQUE8, H. DUMONTET9

Recent price fl uctuations of raw materials such as chromium and nickel have led to the development of stainless steel grades containing less nickel, which are known as lean duplex stainless steels. A research project has been initiated involving stainless steel manufacturers and end-users in order to study the weldability and properties of such grades. Investigations have been carried out on three lean duplex grades: UNS S32101, UNS S32202 and UNS S32304. Test pieces have been welded using several arc welding processes: Gas Tungsten Arc Welding (GTAW), Gas Metal Arc Welding (GMAW) using solid and cored wires, plasma, Submerged Arc Welding (SAW) and Shielded Metal Arc Welding (SMAW) and two plate thicknesses, 5 and 12 mm. These welded test pieces were then used for characterization work including metallographic examinations (in order to check weld quality and to measure the ferrite content), mechanical testing (hardness measurements, tensile and impact tests) and pitting corrosion on welded joints. The infl uence of gas shielding upon ferrite content and pitting corrosion behaviour was also studied. Pitting corrosion of welded joints has been evaluated in terms of critical pitting temperature (CPT) using a modifi ed ASTM (ASTM International, 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959, United States) G150 [1] test method. CPT and pits location, as well as mechanical testing results have been related to ferrite content and chemical analysis of both base metal and fusion zone.

1. INTRODUCTION

Recently, the price of nickel has shown considerable variability and an increase seems to be inevitable in the long term. So, it is predictable that the cost of austenitic stainless steels will grow too. It is well-known that it is possible to reduce the cost of steels by replacing nickel with manganese and nitrogen, which are the two basic elements of low cost auste-nitic steels (AISI 200 series). However these steels are less resistant to stress corrosion cracking or pitting corrosion, particularly in media with high content of chlorides. An alternative is to use duplex stainless steels with low nickel content (called lean duplex stainless steels), which are more resistant to these types of corrosion and which have a lower raw material cost. Besides this corrosion resistance, these steels offer better mechanical properties than AISI 304L or AISI 316L (with roughly twice the yield strength) and make it possible to carry out a substan-tial economy on the cost of raw materials by reduc-tion of thickness. However, welding of these steels requires well controlled procedures. In particular, as for traditional Cr-Ni-Mo duplex stainless steels, the corrosion resistance seems to be dependent on the ferrite/austenite ratio in weld metal and HAZ (Heat Affected Zone). Consequently, it is necessary to have a good understand and knowledge of the infl uence of welding parameters on the mechanical properties and corrosion resistance of welded joints and repairs by welding.

A research project has been initiated thanks to a partnership between the Institut de Soudure and stainless steel manufacturers and end-users (Industeel Arcelor Mittal, Aperam, Outokompu SAS, Total Petrochemical France IPEC, Roquette Frères, Barriquand and EDF Ceidre). The objective of this project was fi rstly to defi ne optimal welding proce-dure specifi cations of lean duplex stainless steels in terms of welding operation, and secondly, to charac-terize mechanical and corrosion resistance proper-ties of different types of welded joints (Gas Tungs-ten Arc Welding, Gas Metal Arc Welding, Flux Cored Arc Welding, Plasma, Submerged Arc Welding and Shielded Metal Arc Welding) for comparison with base metal and austenitic welded joints.

2. EXPERIMENTAL PROCEDURE

2.1 MATERIAL AND WELDING PROCESSES

Investigations have been carried out on three lean duplex grades: UNS S32101, UNS S32202 and UNS S32304, and two austenitic stainless steels AISI 304L and AISI 316L for comparison. Chemical com-position of each grade is given in Table 1 and Pitting Resistance Equivalent values (PRE-N) are determi-ned according to Equation 1 below.PRE-N = %Cr + 3.3 x %Mo + 16 x % N (1)

WELDING OF LEAN DUPLEX STAINLESS STEEL GRADES: MICROSTRUCTURE, CORROSION RESISTANCE AND MECHANICAL PROPERTIES

1. Institut de soudure, 4 boulevard Henri Becquerel, 57970 Yutz2. Industeel CRMC, 56 rue Clémenceau, BP 19, 71202 Le Creusot3. Total Petrochemical France IPEC, 6 allée Irène Joliot-Curie, Bat. H, 69800 Saint-Priest4. Aperam, BP 15, Isbergues5. Outokumpu Avesta Research Centre, Koppardalvsägen 65, PO Box 74, Avesta – Sweden6. EDF Ceidre, 2 rue Ampère, 93206 Saint-Denis Cedex 017. Outokumpu SAS, 100 rue Petit, 75019 Paris8. Roquette Frères, 62136 Lestrem9. Barriquand, 9-13 rue Saint-Claude, 42334 Roanne Cedex

1302_0130_P_040_000_ETUDES_2.indd 411302_0130_P_040_000_ETUDES_2.indd 41 19/03/13 19:4419/03/13 19:44

CONJONCTURE ÉTUDES ET RECHERCHE

42 SOUDAGE ET TECHNIQUES CONNEXES I MARS-AVRIL 2013 I

Test pieces have been welded using several arc wel-ding processes: GTAW, GMAW using solid and cored wires, plasma, SAW and SMAW (see Table 2).Depending on the process, two plate thicknesses have been used, 5 and 12 mm (see Table 2).For AISI 304L grade, the fi ller metal used was of type EN ISO 19 9 L and for AISI 316L grade, EN ISO 19 12 3 L.For all the lean duplex grades, the same fi ller was used in order to facilitate comparisons. This was EN ISO 22 9 3 NL, which is usually matched to UNS S32205 base material rather than lean duplex grades and which contains appreciably more molybdenum.All welded joints were subject to radiographic exa-mination according to EN ISO 5817 B in order to detect any welding defects and allow sampling of sound zones for testing.

2.2 TESTING PROGRAM PERFORMED ON LEAN DUPLEX AND AUSTENITIC STAINLESS STEELS

In order to characterize mechanical, metallurgical and corrosion resistance properties of welded joints, dif-ferent tests and examinations have been performed.Metallographic examinations (micro and macro), Vickers microhardness tests and ferrite content measurements by image analysis have been perfor-med on each type of welded joints.For ferrite content measurements, the etchant was diluted hydrochloric acid (33% Vol.) and sodium disulfite (10 g/L). The etching time was 25 s. This etching gives a very satisfying contrast, ferrite is

coloured from blue to brown and austenite still appears white. Ferrite content measurements were made on weld metal and HAZ by image analysis on the basis of 30 pictures which were located as shown in Figure 1 in both the weld metal and heat affected zone. Magnification is adapted to the dimension of the studied zone (X500 or X1000).Each picture is converted in order to obtain a two-coloured image (black and white). A software deve-loped by Institut de Soudure was used to determine the proportion of pixels in black areas in comparison with the proportion of pixels in white areas. This method has been validated by all partners after a comparison with other types of ferrite measure-

Table 2 – Welding Program

of Lean Duplex and Austenitic Stainless

Steels grade (heat inputs are given without thermal

effi ciency factor). Filler Metal has

been Used except for Plasma.

SMAW : Shield Metal Arc Welding,

SAW : Submerged Arc Welding,

GTAW : Gas Tungsten Arc

Welding, GMAW : Gas Metal

Arc Welding

Figure 1: Location of 30 fi elds on weld metal and HAZ for ferrite content measurements – UNS S32202 (12 mm) GTA Welded joint.

Table 1 – Chemical Composition and PRE-N (Pitting Resistance Equivalent) of Lean Duplex and Austenitic Stainless Steel Base Materials investigated

20.6 0.020 18.10 0.35 0.082 8.10 1.4 0.5 0.0040 0.025

24.8 0.020 17.00 2.20 0.036 10.60 1.5 0.4 0.0060 0.032

Hot Rolled Coil 5 mm 25.5 0.023 21.32 0.20 0.220 1.62 5.0 0.6 0.0010 0.021

Hot Rolled Plate 12 mm 25.3 0.023 21.30 0.19 0.210 1.57 5.0 0.7 0.0010 0.022

5 mm 27.5 0.017 22.98 0.33 0.214 2.53 1.3 0.4 0.0003 0.023

12 mm 27.4 0.020 22.83 0.35 0.215 2.51 1.2 0.4 0.0005 0.023

Hot Rolled Coil 5 mm 26.0 0.027 22.77 0.31 0.140 4.20 1.2 0.5 0.0005 0.024

Hot Rolled Plate 12 mm 25.4 0.019 22.82 0.18 0.128 4.18 1.3 0.5 0.0004 0.026

Hot Rolled Coil

Hot Rolled Plate

%Si %SThickness

UNS S32304

UNS S32202

UNS S32101

5 mm

%C %CrProcess %P

UNS S30403

UNS S31603

%Mo %N %Ni %MnGrade PREN

SMAW SAW Plasma GTAWGTAW

Low EnergyGTAW

High Energy

GMAW with Solid Wires Low Energy

GMAW with Solid Wires High Energy

GMAW with Cored Wires

UNS S30403

7.5 kJ/cm 100% Ar

UNS S31603

7.5 kJ/cm 100% Ar

5 mm8 kJ/cm No gas

protection

8.9 kJ/cm 100% Ar

9 kJ/cm Ar + 2.5% N2

12 mm19.3 kJ/cm

No gas protection

8.8 kJ/cm Ar + 2.5% N2

13.8 kJ/cm Ar + 2.5% N2

8.1 kJ/cm Ar + 30% He + 2.5% CO2

10.5 kJ/cm Ar + 30% He + 2.5% CO2

11.6 kJ/cm Ar + 18% CO2

5 mm8 kJ/cm No gas

protection

9.4 kJ/cm 100% Ar

9 kJ/cm Ar + 2.5% N2

12 mm19.3 kJ/cm

No gas protection

8.8 kJ/cm Ar + 2.5% N2

13.8 kJ/cm Ar + 2.5% N2

8.1 kJ/cm Ar + 30% He + 2.5% CO2

11.4 kJ/cm Ar + 30% He + 2.5% CO2

11.3 kJ/cm Ar + 18% CO2

5 mm8 kJ/cm No gas

protection

8.9 kJ/cm 100% Ar

9 kJ/cm Ar + 2.5% N2

12 mm19.3 kJ/cm

No gas protection

14 kJ/cm Ar + 2.5% N2

8.1 kJ/cm Ar + 30% He + 2.5% CO2

10.5 kJ/cm Ar + 30% He + 2.5% CO2

11.6 kJ/cm Ar + 18% CO2

x kJ/cm Average energy

Material

5 mm

Welding Process

Welding process performed

UNS S32101

UNS S32202

UNS S32304


WELDING OF LEAN DUPLEX STAINLESS STEEL GRADES


ment. A comparison of results obtained using point counting (ASTM E562) and computer-assisted image analysis has been performed by Institut de Soudure and Industeel CRMC (all current specifi cations use the point counting method as reference procedure). Ferrite contents measured using computer-assisted analysis were found to be about 10% higher than those obtained using point counting. Comparison of results obtained for the same sample by several laboratories using different computer-assisted image analysis software has been performed. Results do not show signifi cant differences between laborato-ries (maximum difference of 5% in weld metal and 3% in HAZ).Tensile tests have been performed only on lean duplex stainless steels according to NF EN 895 standard [2] in order to determine UTS (Ultimate Tensile Strength) of welded joints and to identify the rupture zone. Welds have been levelled off before tests.

Impact tests have been performed only on 12 mm thickness lean duplex grades at – 10°C, both on HAZ (VHT 0/b, see explanation in impact strength results) and weld metal (VWT 0/b, see explanation in impact strength results) according to NF EN 875 standard [3].Pitting corrosion tests have been carried out accor-ding to a modifi ed ASTM G150 standard. CPT (Cri-tical Pitting Temperatures) have been determined in argon-deaeratedNaCl solution (58 g/L, 1 mol/L or 35,400 ppm Cl-). Before testing, samples were pickled in a bath containing 2% hydrofluoric acid (HF) and 20% nitric acid (HNO3) for a maximum time of 2 hours and a maximum temperature of 40°C. Then, samples were passivated during 30 minutes at room temperature in a bath containing 20% nitric acid (HNO3). The weld face and back of the weld were exposed but cut sides and edges were hidden by polyester putty in order to avoid prefe-rential pits on the edges. A conventional three

electrodes potentiostatic method was used with a calomel-saturated electrode (reference electrode) immersed into a Luggin capillary, and a platinum disc as counter electrode. After stabilization of open circuit potential (OCP) for 1 hour, an anodic poten-tial scanning was performed at 0.5 mV/s from OCP to + 300 mV/SCE (Saturated Calomel Electrode). This potential was chosen instead of the 700 mV recommended by the ASTM G150 to decrease the aggressiveness of the test and increase the CPT so that is possible to evaluate the effect of a detrimental microstructure. Then, at a constant potential of + 300 mV/SCE, a variable tempera-ture scan between 0.5°C/min and 1°C/min (this last value was the maximum scan rate performed by our control device) was applied until current density reach 100 µA/cm², this temperature being defi ned as the CPT. After this, the temperature was increased by 2°C and maintained for 15 minutes. For each type of welded joint, at least 3 tests were per-formed. Error bars corresponding to standard devia-tion are reported.Finally, chemical analysis of weld metal has been performed on lean duplex stainless steels.

3. RESULTS

3.1 FERRITE CONTENT AND METALLOGRAPHIC ANALYSIS

An example of a cross-section examination is shown in Figure 2. This fi gure shows the base metal, heat affected zone and weld metal for UNS S32202 lean duplex stainless steel welded with GTAW with a high heat input. For this welded joint, ferrite content measured by image analysis was 67% in HAZ and 35% in weld metal.Table 3 gives ferrite content measured for all type of lean duplex stainless steel welded joints by image analysis.

Figure 2:Microstructure of a GTA welded joint in UNS S32202 lean duplex stainless steel with a high heat input.

Table 3 – Ferrite Content Measured by Image Analysis on cross-sections for all type of Lean Duplex Stainless Steel Welded Joint

GradeThicknes

s (mm)Process

Number of weld passes

%Ferrite HAZ

%Ferrite Weld Metal

UNS S32101 5 GTAW with N2 3 65 45

UNS S32202 5 GTAW with N2 3 69 58

UNS S32304 5 GTAW with N2 3 68 48

UNS S32101 5 SMAW 2 60 48

UNS S32202 5 SMAW 2 67 44

UNS S32304 5 SMAW 2 61 42

UNS S32101 5 Plasma (without N2) 1 61 68



UNS S32101 12 SAW 2 63 55

UNS S32202 12 SAW 2 61 58

UNS S32304 12 SAW 2 61 57

UNS S32101 12 GMAW High Energy 5 69 44



UNS S32101 12 GMAW Low Energy 7 69 57



UNS S32101 12 GMAW with Cored Wire 4 64 51



UNS S32101 12 GTAW High Energy 8 69 36



UNS S32101 12 GTAW Low Energy 12 64 28

UNS S32202 12 GTAW Low Energy 12 72 26




As can be seen in Table 3, the ferrite content is high, around 70%, for the weld metal in plasma welding and in the heat affected zone for GMA Welding with low energy, especially for the UNS S32304 grade. Concerning plasma welding, the high ferrite content in weld metal is due to the absence of fi ller metal and nitrogen in the shielding gas and the as-solidi-fi ed condition without weld reheating.In the case of GMA Welding with low energy, the high ferrite content in HAZ is due to the low heat input which gives a rapid thermal cycle in the HAZ, and less time for forming austenite from the ferrite during cooling.

3.2 PITTING CORROSION RESISTANCE

3.2.1 Results on base metalAustenitic and lean duplex stainless steel base materials have been tested according to modified ASTM G150 in order to determine their Critical Pitting Temperature (CPT). Results presented in Figure 3 show CPT for 5 mm and 12 mm thickness grades. Examples of pits observed in base metal for each grade are presented in Figure 4.It is possible to establish a ranking of grades according to their pitting corrosion resistance in terms of CPT:

AISI 304L < AISI 316L ≤ UNS S32101 < UNS S32202 ≤ UNS S32304

This ranking can be correlated with PRE-N determi-ned in Table 1. First of all, AISI 304L grade which has the lowest PRE-N presents logically a low CPT. The UNS S32101 lean duplex has PRE-N (5 mm and 12 mm thickness) slightly above that of AISI 316L and shows a similar CPT as it was described in pre-vious study [4]. UNS S32202 has the highest PRE-N value of all the steels investigated, but the CPT of the 12 mm material is similar to that of UNS S32304 while the CPT for 5 mm UNS S32202 is signifi cantly lower.

3.2.2 Results on welded jointsAustenitic and lean duplex stainless steel welded joints have been tested according to modifi ed ASTM G150 in order to determine their Critical Pitting Temperature (CPT). Results presented in Figure 5 show CPT for 5 mm and 12 mm thick materials with all welding processes. The location of pits are also given in Figure 5 and the side where pits appear (weld face or back of the weld) is specifi ed.The results show that all the welds exhibit a cri-tical pitting temperature which is higher than that of GTA welded AISI 304L and in many cases also higher than GTA welded AISI 316L. The autogenous Plasma welds showed superior corrosion resistance compared to the welded AISI 316L for all three lean duplex grades, with very similar CPT for UNS S32101 and UNS S32202 but a slightly lower CPT for UNS S32304. Similar good results, at least on a par with the AISI 316L welds, were seen for the SMAW welds in 5 mm material, and for the SAW welds in 12 mm material, although in the latter

Figure 3:CPT of lean duplex and austenitic base metals.

(a)

(d)

(b)

(e)

(c)

Figures 4:Pits observed on base metals after modifi ed ASTM G150 tests – UNS S30403 (a), UNS S31603 (b), UNS S32101 (c), UNS S32202 (d), UNS S32304 (e), magnifi cation X16.

0

10

20

30

40

50

60

AISI 304L AISI 316L UNSS32101

5mm

UNSS3210112mm

UNSS322025mm

UNSS3220212mm

UNSS32304

5mm

UNSS3230412mm

CPT (°C)




case the values for UNS S32304 were borderline. The pitting attack occurred either in the base mate-rial or in the heat affected zone, as would be expec-ted since the filler was the more highly alloyed 22 9 3 NL. The results for GMAW and GTAW welds were more surprising. In numerous cases for all three lean duplex grades pitting was observed to occur in the weld metal. This is unexpected because the fi ller material is more highly alloyed than the base materials. Analysis of the weld metal, shown in Table 4, indicates that the weld metal PRE-N is in the range 30-37 while that of the base material is 25-27. The only plausible explanation is that the

pickling process was not suffi cient to remove the weld oxides, so that residues remained on the weld metal and initiated pitting. The normal recommen-dation is to use a combination of mechanical clea-ning and pickling in order to obtain the best results. Nevertheless the CPT data is encouraging in that it shows that even inadequately cleaned welds can readily attain a corrosion resistance on a par with AISI 304L welds. The SMAW, SAW and Plasma results show that results similar to AISI 316L welds can also be achieved in some cases. There is thus considerable room for improvement by adopting good weld cleaning routines for the GMAW and GTAW welds.

3.3 IMPACT STRENGTH RESULTS

Impact test results measured at – 10°C on weld metal and HAZ for 12 mm thickness lean duplex stainless steels are given in Figures 6 and 10. V is the type of notch. W and H mean Weld metal and HAZ, respectively. T means notch through thickness. 0 is the distance between notch centre and the reference line (fusion line). So, VHT 0/b means that V-notch is positioned on the fusion line.

3.3.1 Results in weld metalFigure 6 presents impact strengths and ferrite contents in weld metal for all 12 mm lean duplex grades welded joints. It is possible to establish a ranking of weld types according to their weld metal impact strengths:

SAW < GMAW with cored wire < GMAW with solid wire < GTAW

This figure shows that high impact strengths are associated with low ferrite content below 40 % (below red line). This observation concerns all GTA Welded grades (with low and high energy). Indeed, in these processes, Ar + N2 gas protection obtained in weld metal (see Table 4) encouraged austenite formation and thus induces low ferrite content in weld metal.Figure 6 shows that lowest impact strengths concern the three Submerged Arc Welded (SAW) lean duplex stainless steels (UNS S32101, UNS S32202 and UNS S32304). This result is confirmed by the fracture surfaces presented in Figure 7. It can be seen that SAW joints in UNS S32304 have an impact strength which averaged 65 J, i.e. higher than the 28 J threshold [5,6] and its fracture surface presented a semi-brittle behaviour. The impact strength for UNS S32101 and UNS S32202 was lower, close to 28 J, and the fracture surfaces for these two grades appeared brittle.All the other welded grades present a ductile beha-viour according to chosen test conditions as can be seen for example in Figures 8 (GMAW High energy of UNS S32101, GTAW Low energy of UNS S32202, GMAW Low energy of UNS S32304).The Ni content in weld metal presented in Table 4 can be correlated with the impact strength beha-viour. In Figure 9, the impact strengths obtained in weld metal have been plotted as a function of nickel content in the lean duplex weld metal. This fi gure shows that impact strength values associated with brittle fracture surfaces observed on SA welded UNS S32101, UNS S32202 and UNS S32304 lean duplex grades (experimental points circled in blue) are all related to low nickel content in weld metal (between 5% and 6,5%).From these result it can be concluded that nickel content higher than 7% seems suffi cient to obtain a ductile behaviour in weld metal with impact strengths higher than 80 J at – 10°C.

Figure 5:CPT and pits location of welded joints realised on lean duplex and austenitic grades.




3.3.2 Results in heat affected zoneIn HAZ, results show that all types of welded lean duplex grades present impact strengths higher than the 28 J threshold (see Figure 10). However, there was some variability in the results because part of the fracture surface could lie within the weld metal. This is illustrated for UNS S32101 welded by SA and shown in Figure 11. The left hand side of the frac-ture surface is brittle and probably associated with weld metal while the right hand side is in the heat affected zone.

3.4 TENSILE BEHAVIOR

Tensile test results performed at ambient tempera-ture according to NF EN 895 standard are given in Table 5.Table 5 confirms that UTS values (from 717 MPa for 12 mm grades and 749 MPa for 5 mm grades) for lean duplex grade welded joints are higher than UTS values for austenitic grade welded joints (624 and 633 MPa respectively for AISI 304L and AISI 316L).Fractures in weld metal only occurred in the auste-nitic grades and some 5 mm lean duplex welded

Grade Welding ProcessFerrite content

PRE-N %C %Mn %Si %Cr %Ni %Mo %S %P %Cu %N

SMAW 5mm 48 34.0 0.033 1.30 0.8 23.2 7.3 2.40 0.005 0.020 0.10 0.18SAW 12mm 55 31.2 0.022 2.50 0.8 22.1 5.1 1.80 <0.003 0.020 0.17 0.20Plasma 5mm 68 24.8 0.030 5.00 0.7 21.2 1.7 0.17 <0.003 0.022 0.23 0.19

GMAW with cored wire 12mm 51 33.9 0.029 1.80 0.6 23.1 7.6 2.40 0.008 0.026 0.29 0.18GTAW 5mm 45 31.1 0.021 3.20 0.5 22.1 4.9 1.60 <0.003 0.018 0.13 0.23

GTAW Low Energy 12mm 28 35.8 0.013 2.00 0.4 22.7 8.3 2.80 <0.003 0.020 0.09 0.24GTAW High Energy 12mm 36 34.9 0.008 2.10 0.5 22.6 7.7 2.60 0.005 0.020 0.11 0.23GMAW Low Energy 12mm 57 34.0 0.032 2.00 0.4 22.7 8.0 2.70 <0.003 0.020 0.10 0.15GMAW High Energy 12mm 44 33.7 0.027 2.10 0.5 22.7 7.4 2.50 0.005 0.021 0.11 0.17



GTAW Low Energy 12mm 26 37.0 0.014 1.70 0.4 22.9 8.8 3.10 <0.003 0.020 0.08 0.24GTAW High Energy 12mm 35 34.4 0.018 1.50 0.4 22.8 7.4 2.50 <0.003 0.019 0.10 0.21GMAW Low Energy 12mm 57 34.2 0.033 1.50 0.4 23.0 8.1 2.70 <0.003 0.020 0.09 0.14GMAW High Energy 12mm 46 34.5 0.033 1.50 0.4 22.9 8.1 2.70 <0.003 0.021 0.09 0.17



GTAW High Energy 12mm 36 34.4 0.009 1.60 0.4 22.9 8.0 2.50 0.004 0.020 0.14 0.20GMAW Low Energy 12mm 51 33.3 0.032 1.60 0.4 23.0 8.2 2.50 <0.003 0.021 0.14 0.13GMAW High Energy 12mm 40 33.5 0.029 1.50 0.4 23.0 7.7 2.40 <0.003 0.021 0.15 0.16

UNS S32101

UNS S32202

UNS S32304

Table 4 – Chemical Composition and Ferrite Content of Weld Metal of Lean Duplex Stainless Steel Welded Joints (chemical analysis performed by OES, Optical Emission Spectrometry)

Figure 6:Impact strengths and ferrite contents in weld metal of 12 mm thickness lean duplex grades welded joints.

0

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30

40

50

60

70

0

50

100

150

200

250

300

SA

W

GM

AW

Cor

ed W

ire

GT

AW

Low

Ene

rgy

GT

AW

Hig

h E

nerg

y

GM

AW

Low

Ene

rgy

GM

AW

Hig

h E

nerg

y

SA

W

GM

AW

Cor

ed W

ire

GT

AW

Low

Ene

rgy

GT

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Hig

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nerg

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GM

AW

Low

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rgy

GM

AW

Hig

h E

nerg

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SA

W

GM

AW

Cor

ed W

ire

GT

AW

Hig

h E

nerg

y

GM

AW

Low

Ene

rgy

GM

AW

Hig

h E

nerg

y

Ferrite co

nten

t in W

eld M

etal

Imp

act

stre

ng

th in

Wel

d M

etal

-V

WT

0/b

(J)




Figure 7:Fracture surfaces after impact tests on SAW joints in 12 mm thickness lean duplex grades, UNS S32101 (a), UNS S32202 (b), UNS S32304 (c).

Figure 8:Example of ductile fracture surfaces after impact tests on welded joints for 12 mm thickness lean duplex grades – GMAW High energy of UNS S32101 (a), GTAW Low energy UNS of S32202 (b), GMAW Low energy of UNS S32304 (c).

Figure 9:Impact strengths at – 10°C as a function of Ni content in weld metal of 12 mm thickness lean duplex grades welded joints.

(a)

(a)

(b)

(b)

(c)

(c)

Figure 10:Impact strengths and ferrite contents in HAZ of 12 mm thickness lean duplex grades welded joints.

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70

80

90

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rgy

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nerg

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Low

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ed W

ire

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rgy

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h E

nerg

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rgy

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ed W

ire

GT

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h E

nerg

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Ene

rgy

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AW

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h E

nerg

y

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nten

t in H

AZ

Imp

act

stre

ng

th in

HA

Z -

VH

T 0

/b (

J)

0

50

100

150

200

250

300

5 5,5 6 6,5 7 7,5 8 8,5 9

Imp

act

str

eng

th i

n W

eld

Me

tal

-V

WT

0/b

(J)

% Ni in Weld Metal

28 J

Submerged Arc Welding

UNS S32304

UNS S32202and

UNS S32101

Gas Tungsten Arc Welding




joints (see Figures 12 and 13). All the 5 mm thick specimens had UTS higher than the corresponding 12 mm specimens.For 5 mm and 12 mm welded grades with fracture in the base metal, all UTS are higher than base materials.It can be concluded that all welded lean duplex stainless steels have good tensile properties higher than base materials and austenitic welded grades.

4. CONCLUSIONS

Three lean duplex grades (UNS S32101, UNS S32202 and UNS S32304) have been welded by using seve-ral arc welding processes with the same 22 9 3 NL fi ller material. A test program has been conducted to evaluate the infl uence of welding process and para-meters and the resulting ferrite content on mechani-cal and corrosion properties.• Pitting corrosion resistance for the welded lean duplex grades according to a modifi ed ASTM G150 test method is higher than AISI 304L GTA welded joints. For SMA welds in 5 mm material and SA welds in 12 mm material the CPT was on a par with or better than that of an AISI 316L GTA welded joint.• Autogenous plasma welds presents high ferrite contents in weld metal and HAZ of about 70%. Pit-ting occurs in these two zones but the CPT remains at a high level. The GTA and GMA welds showed pit-ting in the overalloyed weld metal, probably an indi-cation that weld cleaning had not been adequate. Even so, the pitting resistance was superior to that of welded AISI 304L. • High weld metal impact strengths were seen for GTA welds (with low and high heat input) in all three lean duplex grades and were associated with low ferrite content below 40%. The good austenite refor-mation is due to Ar + N2 gas protection with absence of oxygen and high nitrogen content obtained in weld metal. Lower impact strength values observed

Figure 11:Fracture surface of SA welded UNS S32101 lean duplex grade (impact test in HAZ – VHT 0/b).

600

640

680

720

760

800

Pla

sma

5mm

GT

AW

5m

m

SM

AW

5m

m

SA

W 1

2mm

GM

AW

with

cor

ed w

ire 1

2mm

GT

AW

Low

Ene

rgy

12m

m

GT

AW

Hig

h E

nerg

y 12

mm

GM

AW

Low

Ene

rgy

12m

m

GM

AW

Hig

h E

nerg

y 12

mm

Pla

sma

5mm

GT

AW

5m

m

SM

AW

5m

m

SA

W 1

2mm

GM

AW

with

cor

ed w

ire 1

2mm

GT

AW

Low

Ene

rgy

12m

m

GT

AW

Hig

h E

nerg

y 12

mm

GM

AW

Low

Ene

rgy

12m

m

GM

AW

Hig

h E

nerg

y 12

mm

Pla

sma

5mm

GT

AW

5m

m

SM

AW

5m

m

SA

W 1

2mm

GM

AW

with

cor

ed w

ire 1

2mm

GT

AW

Hig

h E

nerg

y 12

mm

GM

AW

Low

Ene

rgy

12m

m

GM

AW

Hig

h E

nerg

y 12

mm

AIS

I 304

L G

TA

W 5

mm

AIS

I 316

L G

TA

W 5

mm

UT

S (

MP

a)

Fracture in base metalFracture in weld metal

UNS S32101 UNS S32202 UNS S32304

Figure 12:Ultimate Tensile Strengths of lean duplex and austenitic grades welded joints.

Table 5 – Ultimate Tensile Strength Results at Ambient Temperature and Location of Fracture Zone of Lean Duplex and Austenitic Stainless Steel Welded Joints (NF EN 895 Standard)




on SA welded lean duplex grades were related to low nickel content in weld metal (between 5% and 6.5%).• A nickel content higher than 7% seems suffi cient to obtain a ductile behaviour in weld metal with impact strengths higher than 80 J at – 10°C.

•All welded lean duplex stainless steels have good tensile properties, higher than base material and austenitic welded grades although some 5 mm welded joints showed fracture in weld metal which indicates that the weld process could be further optimised.

ACKNOWLEDGEMENTS

The author would like to acknowledge all partners for fruitful discussions, for providing raw materials and financing this experimental study (A. Fanica / Industeel Arecelor Mittal, B. Petit / Aperam, A. Thu-lin / Outokumpu SAS, F. Dupoiron / Total Petroche-mical France IPEC, J. B. Beuque / Roquette Frères, H. Dumontet / Barriquand and L. Carlier and B. Til-lard / EDF Ceidre)

REFERENCES

[1] ASTM G 150-99, “Standard Test Method for Electrochemical

Critical Pitting Temperature Testing of Stainless Steels”,West

Conshohocken, PA: ASTM.

[2] NF EN 895, “Essais destructifs des soudures sur matériaux

métalliques – Essai de traction transversale”, AFNOR (Tour

Europe, 92049 Paris-La Défense Cedex), Paris, 1995.

[3] NF EN 875, “Essais destructifs des soudures sur matériaux

métalliques - Essai de fl exion par choc” AFNOR, Paris, 1995.

[4] P. Johansson, M. Liljas, “A new lean duplex stainless steel

for construction purposes”, 4th European Stainless Steel,

Science and Market Congress, Session C5, Part 2, Paris,

2002, p. 155.

[5] G. Sanz, “Risque de rupture fragile. Essai de mise au point

d’une méthode quantitative de choix des qualités d’aciers vis-

à-vis du risque de rupture fragile”, AFNOR-IRSID, 1951.

[6] B. Marandet, G. Sanz, “Étude par la mécanique de la rupture

de la ténacité d’aciers à résistance moyenne fournis en forte

épaisseur”, Revue de Métallurgie, pp. 359-383, avril 1976.

Figure 13:Metallographic examinations of fracture in weld metal of austenitic and 5 mm lean duplex grades (NF EN 895 Standard).

Vous rencontrez des problèmes ou avez des questions concernant la corrosion ?

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Institut de Soudure - Direction de la Recherche

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