Hot ductility and microstructure in casted 2205 duplex stainless steels

Hot ductility and microstructure in casted 2205 duplex stainless steels

Materials Science and Engineering A 515 (2009) 108–112 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepag...

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Materials Science and Engineering A 515 (2009) 108–112

Contents lists available at ScienceDirect

Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea

Hot ductility and microstructure in casted 2205 duplex stainless steels G.W. Fan a,b , J. Liu a,c , P.D. Han d,∗ , G.J. Qiao a a

School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, 710049, China Taiyuan Iron and Steel (Group) Company Ltd., Taiyuan, 030003, China c College of Materials Science and Engineering, Taiyuan Science and Technology University, Taiyuan, 030024, China d College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan, 030024, China b

a r t i c l e

i n f o

Article history: Received 25 September 2008 Received in revised form 14 February 2009 Accepted 16 February 2009 Keywords: Duplex stainless steel Hot ductility Microstructure

a b s t r a c t The effects of the warm parameters (the strain rate and forming temperature) on the deformation behavior and microstructure in 2205 duplex stainless steels (DSS) was studied. The hot deformation behavior of 2205 DSS was investigated through hot tensile and hot compression testing on a Gleeble-1500 thermalmechanical simulator within the temperature range of 1023–1623 K, in order to determine their reduction of area, resistance to deformation, deformability, and microstructure. The experimental results show that the hot deformation curves of the test steel has a high temperature brittlement region at temperatures higher than 1523 K, and the optimum hot ductility region is in the temperature range of 1423–1523 K. The microstructure of the steel obtained after hot deformation is much finer compared with the as-cast microstructure of the steel. It was found that dynamic recovery only occurs within the ␥-phase during hot deformation, but the ␦ phase undergoes dynamic recrystallization. Small secondary ␥ phase islands are precipitated on the ␦ matrix, as demonstrated by TEM analysis, which can markedly promote hot ductility behavior. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.

1. Introduction Duplex stainless steels (DSS) are a special class of steels that are produced with both ferrite (␣) and austenite (␥) within the grain structure. Because of the duplex microstructure, DSS have higher strength than austenitic stainless steels, higher toughness than ferritic stainless steels, good weldability, and high resistance to stress corrosion cracking. It is well known that the good properties of duplex stainless steels rely on a two-phase microstructure comprising approximately equal amounts of ␥ austenite and ␣ ferrite. Due to their high performance, DSS are widely used in the chemical, food, pharmacy, and marine industries, as well as many other fields [1–3]. DSS can be processed by different routes, i.e., casting, forging, extrusion or rolling. These forming operations are usually performed at high temperatures. However, the difference in the mechanical responses of austenite and ferrite under hot working conditions, can lead to the formation of edge cracks or an inappropriated surface finish. It is thus difficult to establish the hot working regime. So far, many reports have focused on the heat treatment process, mechanical properties, and fracture mechanism as well as the fatigue life of 2205 DSS [4–8], but the flow behaviors and

∗ Corresponding author. Fax: +86 351 6010311. E-mail address: [email protected] (P.D. Han).

microstructural changes during hot deformation require further systematic investigation. Despite the large amount of effort invested in studying the behaviors of 2205 DSS, the effects of the hot forming processing parameters, including the strain rate, forming temperature and degree of deformation, on the microstructures of hot deformed 2205 DSS must be investigated further to study the workability and establish the optimum hot forming processing parameters. The objective of this study is to examine the hot deformation behavior, dynamic recovery and recrystallization of 2205 DSS by hot deformation testing with a Gleeble-1500 thermal mechanical simulator.

2. Experimental method The test steel employed for our experiments is DSS 2205 duplex stainless steel, commercially produced by Taiyuan Iron and Steel Company Limited; the chemical composition is listed in Table 1. The microstructures of 2205 DSS before hot deformation are shown in Fig. 1. It can be clearly seen in Fig. 1 that acicular ␥-austenite was distributed in the ␦-ferrite matrix as cast. High temperature tensile and compression tests were carried out over the temperature range of 1023–1623 K on a Gleeble-1500 thermal mechanical simulator. The tests were conducted at strain rates of 0.1, 1 and 10 s−1 . Cylindrical compression specimens of 10 mm in diameter and 15 mm in height were machined from hot isostatic pressing bars, and tensile specimens were machined to ␸10 mm × 100 mm. Two different

0921-5093/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.02.022

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Table 1 Chemical composition of the steel used (mass fraction in %). Element

C

Si

Mn

P

S

Cr

Ni

Mo

N

Content

0.017

0.72

1.04

0.031

0.001

21.64

5.41

3.02

0.18

heating methods were used: method A involved heating directly to between 1023 and 1623 K, and method B involved first heating to 1523 K and then cooling to the testing temperature at about 10 K/s. The hot working temperatures ranged between 1023 and 1623 K, with average steps of 50 K. In order to investigate the microstructural evolution during deformation, the specimens were quenched from the testing temperature in water immediately after deformation to a certain amount of strain. This work was carried out to investigate the hot ductility, hot strength at different temperatures and strain rates. The microstructure after high temperature deformation was investigated using optical microscopy along the longitudinal surface of the gauge length.

3. Results and discussion 3.1. Mechanical properties during hot deformation In order to understand the hot ductility behavior of 2205 stainless steels, Gleeble hot ductility data was taken with the hot tension and compression testing methods. Two different heating methods were examined through the hot tension testing method. The curves of the reduction in the area versus the testing temperature for tensile testing are shown in Fig. 2. The hot ductility responses of the two methods were similar. The reduction of area values using heating method B were much greater than those using method A at a strain rate of 10 s−1 in the temperature range of 1123–1523 K (Fig. 2). The relation of the deformability and resistance to deformation with testing temperature during compression testing is shown in Fig. 3. At steady-state conditions, the plastic deformation process is greatly influenced by the operating temperature and compression speed. For 2205 DSS using method B, the influence of temperature on the resistance to deformation and the deformability during compression testing is shown in Fig. 3. These results show that the hot ductility curve of the test steel comprises a high temperature brittlement region at temperatures higher than 1523 K and a high temperature ductility region at the temperatures from 1423 to 1523 K.

3.2. Analysis of microstructure Hot compression testing was performed using heating method B. The microstructures were strongly influenced by the strain rate and temperature after hot compression. Figs. 4–6 are optical micrographs of the microstructures after deformation at strain rates of 0.1, 1 and 10 s−1 in the temperature range of 1323–1473 K for 0 s. Similar microstructural characteristics appear at 1323–1473 K. Figs. 4–6 show the evolution of the ␦-phase volume fraction with the deformation temperature; the amount of ␦ phase increases between 1323 and 1523 K with a maximum at 1523 K. Above 1323 K, the amount of ␥ phase decreases considerably with deformation temperature, which is attributed to the ␥ → ␦ phase transformation. Figs. 4–6 show that, above 1323 K, the original grains are heavily elongated along the deformation direction. Even the small grains form an extended array in a line perpendicular to the deformation axis. Increasing the strain rate causes a great increase in the flow stress. A large number of dislocations in the ferrite phase grains are activated and move rapidly, leading to severe changes

Fig. 2. The variation in the reduction of area as a function of temperature for 2205 DSS.

Fig. 1. Optical micrograph showing the microstructure of 2205 DSS as cast (the dark areas are ferrite, and the light ones are austenite).

Fig. 3. The variation in the resistance to deformation as a function of temperature for 2205 DSS.

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Fig. 4. Microstructures of cast 2205 DSS after hot compression with a constant degree of deformation = 50% and strain rate = 0.1 s−1 , but different temperatures: (a) T = 1323 K, (b) T = 1373 K, (c) T = 1423 K, (d) T = 1473 K.

Fig. 5. Microstructure of cast 2205 DSS after hot deformation with a constant degree of deformation = 50% and strain rate = 1 s−1 , but different temperatures: (a) T = 1323 K, (b) T = 1373 K, (c) T = 1423 K, (d) T = 1473 K.

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Fig. 6. Microstructure of cast 2205 DSS after hot deformation with the same degree of deformation = 50% and strain rate = 10 s−1 , but different temperatures: (a) T = 1323 K, (b) T = 1373 K, (c) T = 1423 K, (d) T = 1473 K.

in grain shape. No precipitation or intermetallic phases appear on the micrograph, and the microstructure consists only of ferrite and austenite. Dynamic recrystallization occurs in the ␦ phase. The degree of recrystallization becomes larger with increasing temperature. Careful examination shows that no recrystallization occurs in the ␥ phase; only dynamic recovery takes place within the ␥

phase. In contrast to the microstructures deformed at low strain rates, more obvious plastic deformation characteristics appear at strain rates higher than 10 s−1 . As for the influence of the heat treatment temperature and the time the specimen remained a secondary phase on the structure, Fig. 7 shows the microstructures of cast 2205 DSS after hot defor-

Fig. 7. Microstructure of cast 2205 DSS after hot deformation at 1223 K for 120 s, with strain rate = 1 s−1 and ε = 50%: (a) optical image, (b) transmission electron micrograph.

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Table 2 Chemical composition of ␥, ␥ and ␦ phase determined by EDAX analysis. Phase

␥ ␥ ␦

Element (wt%) Si

Cr

Ni

Mo

Fe

0.88 0.82 0.84

21.81 22.38 23.12

6.86 5.07 4.48

2.30 2.74 3.34

68.16 68.46 68.02

Totals may exceed 100.00 due to rounding.

mation at 1223 K for 120 s, with a strain rate = 1 s−1 and ε = 50%. Fig. 7(a) and (b) are an optical image and a transmission electron micrograph, respectively, which clearly show the presence of a secondary gamma phase inside the ␦ ferrite grain. It is known that the nucleation of a secondary gamma phase occurs through the ␦ → ␥ transformation [9]. This transformation accompanies the formation of the Ni-rich gamma phase within ␦ ferrite. In addition, during the formation of the gamma phase, Ni is absorbed and Cr is rejected to the adjacent ferrite region. The EDAX analysis technique was used to identify the phases constituting each microstructural region (shown in Table 2). The microstructure obtained after 120 s essentially consists of bulk ␥ austenite phase, islands of secondary ␥ phase and ␦ ferrite. The results indicate that the secondary gamma phase, i.e., the recrystallization of ferrite, can markedly promote hot ductility behavior and reduce the risk of cracking.

ably when the temperature was sufficiently high, and the ␥ phase changed from acicular to a globular. The deformation parameters, such as the strain rate and temperature, determined the behaviors of hot ductility and dynamic recrystallization. Increasing the temperature and decreasing the strain rate facilitated the dynamic recrystallization of the ␦ ferrite phase. Only dynamic recovery occurred within the ␥ phase, but the ␦ ferrite phase underwent dynamic recrystallization, Considering workability and microstructural control, the optimum hot deformation conditions were determined to be in the temperature range of 1423–1523 K. Small secondary gamma phase islands precipitated on the ␦ matrix as shown by TEM, which can markedly promote hot ductility behavior. Acknowledgments The authors would like to gratefully acknowledge the support of the National Natural Science Foundation of China (No. 50874079) and the National Natural Science Foundation of Shanxi Province (No. 2008012008-2). References [1] [2] [3] [4]

4. Conclusion It was found that the microstructure and deformation behavior are very sensitive to the hot ductility. When the deformation temperature is comparatively low, the volume fraction of austenite phase increases, however, the microstructure changed consider-

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