Nieq on pitting corrosion resistance and mechanical properties of UNS S32304 duplex stainless steel welded joints

Nieq on pitting corrosion resistance and mechanical properties of UNS S32304 duplex stainless steel welded joints

Corrosion Science 70 (2013) 252–259 Contents lists available at SciVerse ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/c...

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Corrosion Science 70 (2013) 252–259

Contents lists available at SciVerse ScienceDirect

Corrosion Science journal homepage: www.elsevier.com/locate/corsci

Influence of Creq/Nieq on pitting corrosion resistance and mechanical properties of UNS S32304 duplex stainless steel welded joints Yiming Jiang a, Hua Tan a,d, Zhiyu Wang b, Jufeng Hong a, Laizhu Jiang b, Jin Li a,c,⇑ a

Department of Materials Science, Fudan University, Shanghai 200433, PR China Research and Development Center, Baosteel Co. Ltd., Shanghai 201900, PR China c Institute of Metal Research, Chinese Academy of Science, Shenyang 110016, PR China d Nuclear Power Plant Service Company, Shanghai 200233, PR China b

a r t i c l e

i n f o

Article history: Received 16 August 2012 Accepted 22 January 2013 Available online 31 January 2013 Keywords: A. Stainless steel C. Welding C. Pitting corrosion

a b s t r a c t Pitting corrosion resistance and mechanical properties of 2304 duplex stainless steel with different Creq/Nieq values after plasma-arc welding and welding thermal simulation were systematically studied. The results showed that the lower the Creq/Nieq value in the experimental range, the better the microstructure after welding or welding thermal cycle. High pitting resistance equivalent number in the chemical composition brought in low weight loss rate and high critical pitting temperature for base metal. Furthermore, as the Creq/Nieq value decreased, the degradation of pitting corrosion resistance after welding thermal cycle reduced. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The success of modern duplex stainless steels (DSSs) is due to their weldability as well as their excellent mechanical properties and corrosion resistance. Compared with the earlier generation of duplex steels, weldability has been improved appreciably, by introduction of nitrogen as an alloying element and vacuum and argon oxygen decarburization (VOD and AOD) processes. The properties of DSS are dependent on the ferrite (a)-austenite (c) phase ratio which in the base metal is designed to be approximately 1:1. Moreover, the precipitation of secondary phases including intermetallic phase (r, v) and nonmetallic compounds (Cr23C6, Cr2N) has shown a detrimental influence on the properties especially the pitting corrosion resistance and toughness [1–6]. In the heat-affected zone (HAZ) and fusion zone (FZ), the microstructure strongly depends on thermal cycle and chemical composition. For DSS, among the parameters of welding thermal cycle, heat input is the most important one which determines the cooling rate of welding process directly. The lower the heat input, the faster the cooling rate. Low heat input brings on an extremely unbalance microstructure with excess of ferrite phase and also results in plenty of chromium nitrides precipitating in the interior of the ferrite grains or the interface of austenite and ferrite grains. Generally, a relatively high heat input is beneficial to the microstructure of welded joints with more austenite reformation during cooling ⇑ Corresponding author at: Department of Materials Science, Fudan University, Shanghai 200433, PR China. Tel./fax: +86 21 6564 3648. E-mail address: [email protected] (J. Li). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.01.037

stage and without chromium nitrides. However, intermetallic phases are easy to form when the cooling rate is too slow, especially for the high alloyed super duplex stainless steel. Therefore, it must be borne in mind that an upper limit to heat input is set by the prevention of intermetallic phase precipitation, and this risk increase with increased alloy element level [7–13]. As for the influence of chemical composition on welding of duplex stainless steel, a great number of investigations have carried out on the effect of alloying elements, especially the most effective nitrogen [14–23]. Ogawa and Koseki [24] had pointed out that both nitrogen and nickel could increase austenite content of the weld, but N increase the pitting corrosion resistance while Ni degrades the pitting corrosion resistance. Muthupandi and his coworkers [25,26] had investigated the influence of N and Ni on microstructure and mechanical properties of 2205 weld metals and the results that the addition of Ni and N could significantly improve the microstructure, phase balance and impact toughness, but it seems to have no appreciable influence on the hardness of the weld zone were obtained. Liou et al. [27,28] have carried out meaningful researches to prove the beneficial influence of nitrogen on microstructure and corrosion resistance such as pitting corrosion and stress corrosion cracking in simulated heat-affected zones of DSS. All these studies are focused on the influence of single alloying element. However, a more important parameter – the ratio of chromium equivalents (Creq) and nickel equivalents (Nieq), which present the ability of stabilizing the ferrite and austenite structure from the angle of alloying elements, was less investigated. DSS contains ferrite stabilizing elements like Cr, Mo, Si and W as well as austenite stabilizing elements like Ni, Mn, C, N and Cu. Long and DeLong

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Y. Jiang et al. / Corrosion Science 70 (2013) 252–259 Table 1 Chemical composition of a series of 2304 duplex stainless steels. Materials

2304-1 2304-2 2304-3 2304-4

Concentration of alloying elements (wt.%) Cr

Ni

Mo

N

Mn

C

Si

S

P

Cu

23.42 23.54 23.01 23.66

4.03 4.60 5.35 4.87

0.31 0.31 0.30 0.71

0.13 0.13 0.12 0.16

1.56 1.67 1.64 1.49

0.021 0.011 0.015 0.030

0.40 0.41 0.36 0.37

0.004 0.005 0.003 0.005

0.010 0.010 0.009 0.007

0.29 0.31 0.30 0.31

Creq

Nieq

Creq/Nieq

PREN

23.73 23.85 23.31 24.37

7.44 7.66 8.35 9.20

3.19 3.11 2.79 2.65

27.04 27.16 26.40 29.20

Creq = %Cr + %Mo + 0.7%Nb [14]. Nieq = %Ni + 35%C + 20%N + 0.25%Cu [14]. PREN = %Cr + 3.3%Mo + 20%N [4,5].

[29,30] suggested the effect of the elements on Creq and Nieq with the following equations:

Creq ¼ wt:%Cr þ wt:%Mo þ 1:5 wt:%Si þ 0:5 wt:%Nb

ð1Þ

Nieq ¼ wt:%Ni þ 0:5 wt:%Mn þ 30ðwt:%N þ wt:%CÞ

ð2Þ

In this paper the influence of Creq/Nieq on corrosion resistance and mechanical properties of the 2304 welded joint including the practical plasma-arc welded (PAW) joint and simulated high temperature HAZ has been systematically studied. A series of 2304 duplex stainless steel base metal with different Creq/Nieq values has been chosen as the studying object and the chemical composition of 2304 duplex stainless steel for the application was optimized through this study. 2. Experimental procedures 2.1. Materials Four kinds of 2304 duplex stainless steels with different Creq/ Nieq value from 2.65 to 3.19, also with different pitting resistance equivalent number (PREN = %Cr + 3.3%Mo + 20%N), were investigated in this paper, and their composition and other important information were shown in Table 1. Here the Creq and Nieq were calculated used the universe formulas according to WRC 1992 Constitution Diagram [17], which is more reasonable than the former formulas obtained by Long and DeLong in 1973 [29,30].

Creq ¼ %Cr þ %Mo þ 0:7%Nb

ð3Þ

Nieq ¼ %Ni þ 35%C þ 20%N þ 0:25%Cu

ð4Þ

PREN is an experienced formula widely used to evaluate the pitting corrosion resistance of austenitic stainless steels and duplex stainless steels from the angle of chemical composition. A lot of alloying elements have the influence on the pitting corrosion resistance including beneficial effect and harmful effect. For example, Cr, Mo, N, Cu, etc. have the beneficial effect while Mn, S, P, etc. have the harmful effect. There is no universe formula for calculation of PREN. As we known that PREN = %Cr + 3.3%Mo + x%N only considering the beneficial effect of the major three element Cr, Mo and N, while the nitrogen factor x is in range of 16–30. Generally, x is chosen among 16, 20 and 30. A middle value 20 is the most widely employed to calculate the PREN value during the study for duplex stainless steels [4,5]. They were melted in a 50 kg vacuum furnace and then cast as a single square ingot. After removing the oxide skin, the ingot was forged into square bloom at the temperature ranging from 900 °C to 1200 °C and divided into several blooms with a dimension of 150 mm  100 mm  42 mm. The blooms were reheated at 1200 °C for 1 h and hot-rolled, using a laboratory hot-rolling mill, into 12 mm thick plates. After hot-rolling, DSS 2304 was solution-annealed at high temperature for 12 min and quenched in water. Due to the different chemical compositions, the annealing temperatures for 2304-1, 2304-2, 2304-3, 2304-4 were 1020 °C, 1040 °C, 1060 °C, 1100 °C respectively.

2.2. Welding and thermal simulation Welding was performed using autogenous PAW without filler metal, and the corresponding parameters were listed in Table 2. Welding thermal cycle simulation was carried on DSS 2304 base metal through the Gleebe 3800 thermal–mechanical simulator. Fig. 1 shows the relationship between the temperature and the elapsed time registered by thermocouple during welding simulations. The rising rate was 350 °C/s; peak temperature was 1350 °C; holding time at peak temperature was 3 s; heat input was 1.5 kJ/mm. 2.3. Characterization To observe microstructure, each specimen was electrochemically etched by 30%KOH solution at 2 V for 15 s. The ferrite volume fraction of the base metal, the welded joint, and the simulated high temperature HAZ was measured by Helmut Fischer MP3 Feritscope. The microstructure and the morphologies after pitting corrosion were observed by both optical microscope (OM) and scanning electron microscope (SEM). The mechanical properties such as yield strength, tensile strength, and elongation of the PAW joints were measured while the impact toughness of the simulated HAZ specimens was charac-

Table 2 Welding conditions applied for PAW of duplex stainless steel DSS 2304. Welding current (A) Welding voltage (V) Orifice gas nozzle diameter (mm) Welding speed (cm/min) Nozzle height (mm) Plasma gas flow: Ar (L/min) Shielding gas flow: Ar (L/min)

165 50 3.2 27 5–6 15 15

Fig. 1. Simulated welding thermal cycle curve of high temperature HAZ by thermomechanical simulator Gleebe 3800.

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Fig. 2. Equilibrium diagrams of 2304 duplex stainless steels with different chemical compositions obtained by Thermo-Cal software.

terized. The impact test was carried out for Charpy V-notched specimens of 10  10  55 mm dimension at 40 °C, which was a common test temperature specified for offshore application. Pitting corrosion resistance of PAW joint was evaluated by immersing in 6%FeCl3 + 0.05 M HCl solution according to GB/T 17897-1999. After 24 h immersion, the weight loss rate was measured and the morphology was observed by OM and SEM. While the pitting corrosion resistance of the simulated HAZ specimen was evaluated through the potentiostatic critical pitting temperature (CPT) in 1.0 M NaCl solution according to the ASTM G150 standard. Since pitting of stainless steels depends strongly on the surface finish, the surface of all testing specimens were ground with successive grade silicon carbide sand paper up to 1000 grit, degreased with ethanol, rinsed with distiller water and dried in air. The test was carried out through the electrochemical station PAR-STAT 2273 with a three-electrode cell containing a Pt foil auxiliary electrode and a saturated calomel electrode (SCE) as reference; all potentials quoted in this paper refer to this reference electrode. The specimen acting as working electrode was mounted in epoxy resin. Prior to each CPT measurement, the working electrode was ground with successive grade silicon carbide sand paper up to 1000 grit, degreased with ethanol, rinsed with distiller water and dried in air. The CPT test was repeated at three times for the same specimen and the average value of the results was adopted.

temperature Tf. As the Creq/Nieq value decreased from 3.19 to 2.65 for four kinds of 2304 DSS, both Tb and Tf increased, manifesting that the ability of stabilizing the austenite phase was becoming stronger as it is well known that ferrite phase is more stable at high temperature. 3.1. Microstructure and properties of the PAW joints In Fig. 3, typical macro cross-section of the weld bead produced by PAW was shown. A relative wide fusion zone was formed with great number of large ferrite columnar grains while the austenite distributed along the ferrite grain boundaries. After enlarging, the microstructures of the fusion zone and the heat-affected zone for 2304-1 (Creq/Nieq: 3.19 the largest) and 2304-4 (Creq/Nieq: 2.65

3. Results and discussion Thermo-Cal software was employed to obtain the equilibrium diagram of these 2304 duplex stainless steels and the results were shown in Fig. 2. Two important temperatures should be paid attention to: one is the temperature at which the volume of ferrite phase is equal to that of austenite phase, called the balance temperature Tb, and the other is the temperature at which all the austenite transforms to ferrite at high temperature, called the ferritizing

Fig. 3. Macro plasma-arc welded joints cross section image of 2304 duplex stainless steel.

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Fig. 4. Optical plasma-arc welded joints microstructure of 2304 duplex stainless steel including fusion zone and heat-affected zone.

Table 3 Mechanical properties of 2304 base metal and PAW joints. Materials

2304-1 2304-2 2304-3 2304-4

Ferrite proportion (%) BM

WZ

50.9 51.9 48.7 46.6

70.0 68.1 61.3 57.0

Yield strength 0.2% (MPa)

Tensile strength (MPa)

Elongation (%)

462 450 443 475

645 660 653 700

40.0 38.0 36.5 38.0

Fig. 5. Weight loss rate of 2304 base metal and plasma-arc welded joints after immersion in corrosive 6%FeCl3 + 0.05 M HCl mixed solution for 24 h at 35 °C.

the smallest) welded joints were presented and compared in Fig. 4. In the fusion zone, there were more and coarser austenite grains existed in 2304-4 compared with 2304-1. The large ferrite grains

Fig. 6. SEM image of pit morphology after immersion in corrosive 6%FeCl3 + 0.05 M HCl mixed solution for 24 h at 35 °C.

have been divided into several small parts by Widmanstattenshaped austenite apparently for 2304-4. The HAZ is a gradually

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Fig. 7. Microstructure of 2304 high temperature heat-affected zone with different chemical composition.

Table 4 Volume fraction of ferrite phase in duplex stainless steel 2304 simulated HTHAZ. Materials

Direction

BM

HTHAZ No. 1

No. 2

No. 3

Aver.

2304-1

Surface Cross

52.6 51.0

69.9 67.1

71.1 63.8

71.8 64.5

70.9 65.2

2304-2

Surface Cross

51.9 51.5

70.0 65.0

67.7 60.8

68.9 63.6

69.2 63.1

2304-3

Surface Cross

48.7 48.0

64.9 53.5

63.1 54.4

63.4 54.5

63.8 54.1

2304-4

Surface Cross

47.2 44.6

56.4 48.4

55.1 51.4

54.8 52.0

55.4 50.6

Fig. 9. Critical pitting temperature measured curves of 2304 high temperature heat-affected zone according to ASTM G 150. The test condition is listed as follow: applied potential was 0.75 V (SCE), increasing rate of solution temperature was 1 °C/min, starting temperature was 2 °C.

Table 5 CPT values of 2304 base metal and simulated HAZ.

Fig. 8. Critical pitting temperature measured curves of 2304 base metal according to ASTM G 150. The test condition is listed as follow: applied potential was 0.75 V (SCE), increasing rate of solution temperature was 1 °C/min, starting temperature was 2 °C.

Materials

2304-1

2304-2

2304-3

2304-4

PREN Creq/Nieq CPT of BM (°C) CPT of HAZ (°C) CPT variation (°C)

27.04 3.19 26.5 11.5 15.0

27.16 3.11 29.0 13.0 16.0

26.40 2.79 32.0 18.0 14.0

29.20 2.65 33.5 23.0 10.5

transiting zone from fusion zone to base metal which was affected by the thermal cycle. For duplex stainless steel, the high temperature HAZ should be emphasized on, because the most typical

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257

Fig. 10. Typical morphologies after CPT measurement for the simulated high temperature heat-affected zone specimens.

less resistant than the austenite phase, which has also been proved by a great number of former studies [31–34]. 3.2. Microstructure and properties of the simulated HAZ

Fig. 11. Impact energy of 2304 high temperature heat-affected zone.

problems of the welded joints were associated with it. The same to the fusion zone, more and coarser austenite grains was displayed in HAZ of 2304-4. Moreover, it was observed that a portion of primary austenite grains of base metal were preserved in 2304-4, while the primary austenite phase in 2304-1 has transformed to ferrite phase totally in 2304-1 during heating stage and all the austenite phase in the high temperature HAZ was reformed during the cooling stage of the welding process. As shown in Fig. 1, the lower Creq/Nieq, the higher Tf value. The temperature range of single ferrite phase was very narrow for 2304-4 and therefore only a part of original austenite grains dissolved into ferrite matrix during the heating stage. On the contrary, the temperature of austenite reforming was higher during the cooling stage, therefore, more and coarser austenite grains precipitated. The ferrite phase volume fraction mechanical properties of the PAW joints were listed in Table 3. As Creq/Nieq value decreased, ferrite phase in weld zone was decreased gradually. However, the mechanical properties such as strength and elongation changed without apparent law, because the strength was not only by the ratio of two phases. PAW joint of 2304-4 has the highest strength with yield strength value equal to 475 MPa. The weight loss rate of base metal and PAW joints for the four kinds of 2304 DSS after immersing in corrosive 6%FeCl3 + 0.05 M HCl solution were displayed in Fig. 5. Due to the highest PREN value (29.20) of 2304-4 base metal had the smallest weight loss rate only 0.469 g m2 h1, representing the best pitting corrosion resistance. Other three kinds of 2304 base metal exhibit similar pitting corrosion resistance. From 2304-1 to 2304-4, the weight loss rate of the welded joint was decreased as the Creq/Nieq value decreased gradually. 2304-4 welded joints presented the best pitting corrosion resistance too. Pit morphology of 2304-1 PAW joints after immersion was shown in Fig. 6 as the typical. It was easily found that the coarse ferrite grains have been corroded and the Widmanstatten-shaped austenite grains retained, demonstrating that the ferrite phase was

Fig. 7 shows the microstructure of simulated HAZ of four kinds of 2304 DSS after the same welding thermal cycle in Fig. 1 through Gleebe 3800 thermal–mechanical simulator. A part of primary austenite phase in 2304-3 and 2304-4 existed in high temperature HAZ apparently, especially in 2304-4, resulting from the lower Creq/Nieq value. The ferrite phase volume fraction of base metal and simulated HAZ on the surface and cross section for four kinds of 2304 DSS were summarized in Table 4. As the Creq/Nieq value decreased, the ferrite phase content decreased both in base metal and HAZ. The ratio of ferrite phase and austenite phase in 2304-4 HAZ was near 1:1 and presented a more balanced microstructure. Typical CPT curves of base metal and simulated HAZ were shown in Figs. 8 and 9. The detailed CPT values were summarized in Table 5. The same to the weight loss rate of welded joints, 2304-4 exhibited the best pitting corrosion resistance for both base metal and HAZ with the highest CPT values. Generally, the higher the PREN value, the better the pitting corrosion resistance for duplex stainless steels or austenitic stainless steels. In this paper, the results were not always consistent with this 2304-3 with the lowest PREN value (26.40) possessed the second high CPT value among the investigated four materials. There were two possible causes. One was the negative effect of harmful chemical elements such as P, S was not considered during the calculation of PREN value. Apparently the P, S in 2304-3 was lower than those in 2304-1 and 2304-2. The other was the pitting corrosion resistance of multi-phase alloys was dependent on the weakest phase. It was demonstrated that pitting corrosion resistance of duplex stainless steels had been determined by PREN value of the weaker phase not that of the whole alloy, since the Cr and Mo enriched in ferrite phase, while Ni and N were concentrated in austenite phase. Solution-annealing heat treatment was a key process that not only adjusted the two phase ratio but also modified the distribution of major alloying elements between two phases. Annealing temperature should be differed from each other for the investigated four 2304 duplex stainless steels due to the different chemical composition, and the equilibrium diagrams obtained by Thermo-Cal software had proved this. Due to different annealing temperature, The PREN of weakest phase in 2304-3 may be higher than that of 2304-1 and 2304-2, although PREN of 2304-3 was a little lower than that of 2304-1 and 2304-2. Compared with the base metal, all the HAZ specimens showed impaired pitting corrosion resistance with a drop of CPT value more than 10 °C, especially for 2304-1 and 2304-2. The decrease of pitting corrosion after welding process or welding thermal cycle for duplex stainless steel is resulted from two aspects of the microstructure evolution during welding, one is the lower chromium and molybdenum content of ferrite phase in heat-affected zone or fusion zone compared with parent metal; the other is the precipitation of Cr2N

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in the ferrite phase [7,15,34–36]. It was worthy to pay attention to the variation of CPT and the Creq/Nieq value of the four 2304 DSS. The lower the Creq/Nieq value, the less the degradation of pitting corrosion resistance after welding. Among the investigated four 2304 DSS, the drop of pitting corrosion resistance after the same welding thermal cycle was the least for 2304-4, which indicated that 2304-4 possessed the best weldability properties from the angle of the corrosion resistance. The typical pit morphologies after CPT test were displayed in Fig. 10. Ferrite phase was less resistant for the simulated HAZ specimens and pitting occurred at ferrite grains, which agreed with the previous investigations [15,17]. Besides the pitting corrosion resistance, the impact toughness was another important property significantly affected by welding process for duplex stainless steel. Fig. 11 shows the impact energy of the 2304 duplex stainless steel simulated HAZ. Different from the pitting corrosion resistance, the impact energy of 2304-3 was the highest among four kinds of duplex stainless steel, the other three kinds of HAZ showed similar impact energy. After systematical consideration on pitting corrosion resistance, impact energy, microstructure phase balance, 2304-4 was chosen as the optimum chemical composition for 2304 duplex stainless steels. 4. Conclusions The major conclusions were drawn as follows: 1. Creq/Nieq has a decisive influence on the microstructure of welded joint and simulated high temperature HAZ. The lower the Creq/Nieq value, the more the austenite phase formed after welding or thermal cycle and the microstructure was more balanced. 2. The drop of pitting corrosion resistance after welding or welding thermal cycle was closely related to the Creq/Nieqvalue. The drop increased as the Creq/Nieq value increased and the composition with lower Creq/Nieq value showed better weldability from the angle of pitting corrosion resistance. 3. With systematical consideration of the mechanical properties, corrosion resistance, microstructure phase balance, 2304-4 was chosen as the optimum chemical composition for application of 2304 duplex stainless steels at last.

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