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Microalloying effects on microstructure and mechanical properties of 18Cr–2Mo ferritic stainless steel heavy plates
Materials and Design 58 (2014) 518–526
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microalloying effects on microstructure and mechanical properties of 18Cr–2Mo ferritic stainless steel heavy plates Jian Han a,⇑, Huijun Li a, Haigang Xu b a b
School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia Baoshan Iron & Steel Co., Ltd., Shanghai 200431, China
a r t i c l e
i n f o
Article history: Received 24 November 2013 Accepted 6 February 2014 Available online 18 February 2014 Keywords: Ferritic stainless steel Heavy plate Microalloying Microstructure Mechanical property
a b s t r a c t The microstructure, including grain size and precipitation, tensile strength and Charpy impact toughness of (Nb + V) 18Cr–2Mo ferritic stainless steel heavy plates with/without Ti were investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and standard tensile strength and Charpy impact toughness testing. It was found that for 18Cr–2Mo heavy plate, a good combination of Nb–V stabilized method without Ti induces refinement of grain sizes due to the precipitation of amounts of fine Nb carbonitrides and V nitrides. Meanwhile, the mechanical testing results indicate that optimal transformation of grain size, precipitation that Nb–V composition system brings to 18Cr–2Mo heavy plate is beneficial to improvement of strength and impact toughness. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Ferritic stainless steel (FSS), which is body centered cubic (BCC) structure, is essentially Fe–Cr or Fe–Cr–Mo alloy [1]. FSS has various advantages in comparison with austenitic stainless steel (ASS): lower cost, higher thermal conductivity, smaller linear expansion and better resistance to chloride stress-corrosion cracking, atmospheric corrosion and oxidation. Because of these merits, FSS is very attractive in numbers of application fields [2–4]. The Cr and Mo balance in 18Cr–2Mo FSS, together with stabilizing additions, such as Nb, V, Ti, provides higher corrosion resistance than other ferritic grades [5] and is even comparable to austenitic AISI 316 [6]. However, one limitation of 18Cr–2Mo FSS is its relatively high ductile to brittle transition temperature (DBTT) [7], which is always above room temperature and particularly as thickness is beyond 5 mm [8]. In addition, it is impossible to reduce the thickness of FSS products due to its comparatively low strength. For instance, for 18Cr–2Mo FSS, which is named S44400 according to ASTM: A240/A240M-13c, the bottomline of its yield strength is only 275 MPa. At present, 18Cr–2Mo FSS heavy plates (above 4 mm) have been applied in limited industrial fields, such as water treatment and brewing. Therefore, the challenge of 18Cr–2Mo FSS heavy plate is how to improve toughness and utilize the other attributes of this steel group. ⇑ Corresponding author. Tel.: +61 405427056; fax: +61 242213238. E-mail address: [email protected] (J. Han). http://dx.doi.org/10.1016/j.matdes.2014.02.009 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved.
Some factors affecting the toughness properties have already been identified for FSS. Wright [9] proposed that the BCC crystallography limits the number of available slip systems, lowers the deformation compatibility and increases the probability of initiation and propagation of brittle fracture. Xiao [10] explained the brittleness of FSS from the point of crack cores that form at the boundaries of deformation bands, which influences DBTT of high Cr FSS [8]. Van Zwieten and Bulloch [11] suggested that for Fe–Cr stainless steel, C and N dramatically reduce the impact toughness properties due to carbides and nitrides form at the grain boundaries, while O has only a small detrimental effect since O in BCC metals promotes the occurrence of intergranular fracture. The presence of second phases, viz. carbides, nitrides and oxides, can negatively influence the toughness properties of FSS. In high C and/or N alloys, the toughness is decreased after a high temperature annealing treatment, which increases the lattice friction stress and the flow stress as a result of the precipitation of carbides and nitrides on dislocations. Ohashi et al. [9] believed that coarse grains tend to promote crack initiation, and thus the grain size contributes mainly to resistance to initiation of brittle fracture and only slightly to crack propagation. The detrimental effect of interstitials may be controlled by the addition of stabilizing elements, such as Nb, V, Ti and Zr. These elements form strong carbides and nitrides [7,12,13], which are more stable than Cr carbides and nitrides. Semchyshen et al. [7] found Nb and Ti are both effective in retarding the increment in transition temperature. V has a strong tendency to form carbides and ni-
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J. Han et al. / Materials and Design 58 (2014) 518–526 Table 1 Chemical compositions (wt.%) of the studied 18Cr–2Mo steels. Steel no.
C
Si
Mn
Cr
Mo
Nb
V
Ti
N
Fe
FSS-1# FSS-2#
0.0047 0.0046
0.093 0.080
0.090 0.081
17.83 17.95
1.81 1.78
0.15 0.15
0.14 0.11
– 0.05
0.0105 0.0101
Bal. Bal.
Fig. 1. Calculated equilibrium molar fractions of precipitates of FSS-1#: (a) Y axis from 0–1 mol; (b) Y axis from 0–2 103 mole.
Fig. 2. Calculated equilibrium molar fractions of precipitates of FSS-2#: (a) Y axis from 0–1 mol; (b) Y axis from 0–6 103 mole.
Table 2 Tensile properties of the studied 18Cr–2Mo steels. Steel no.
YS (MPa)
UTS (MPa)
Elongation (%)
FSS-1# FSS-2#
337.5 302.5
470.0 442.5
32.0 33.0
trides, and meanwhile, the solid solubility of its carbides and nitrides are very small in ferrite, which will disperse V precipitates at specific temperature [14]. Paton [15] pointed that some benefit is obtained in impact toughness by the addition of V. In this study, Ti was added to traditional (Nb + V)-stabilized 18Cr–2Mo FSS heavy plates to investigate the role of microalloy additions on microstructure and mechanical properties. V is added to the studied steels instead of part Nb in consideration of its effect of solution and dispersion strengthening, which is beneficial for the development of FSS heavy plate.
Fig. 3. Charpy impact values of FSS-1# and FSS-2# from 40 to 60 °C.
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Fig. 4. Overall fracture appearance of FSS-1# specimens impacted at: (a) 60 °C; (c) 0 °C; (e) 40 °C and SEM fractographs of FSS-1# specimens impacted at: (b) 60 °C; (d) 0 °C; (f) 40 °C.
2. Experimental procedures 2.1. Materials and processing The 18Cr–2Mo FSSs used in this study were melted in a 50 kg vacuum induction furnace. The casting ingots with a diameter of 120 mm were hot forged into the size of 30 mm 250 mm 200 mm (thickness width length), hot rolled into 6 mm plate, and annealed at 950 °C for 30 min, followed by cooling in the air. The chemical compositions of the 18Cr–2Mo steels measured are listed in Table 1. The stabilization ratios of FSS-1# and FSS-2# are 19.1 and 21.2, respectively, which are calculated by interstitial elements divided by stabilizing elements, shown in Eq. (1). The results mean both steels can satisfy the requirements of intergranular corrosion resistance.
Stabilization ratio ¼ ðNb þ V þ TiÞ=ðC þ NÞ
ð1Þ
2.2. Phase calculation The equilibrium phase diagrams were calculated using ThermoCalc software based on TCFE6 database, and the recommended temperature range is from 500 to 1600 °C. The details of precipitation were illustrated by changing the scale of Y axis from the range of 0–1 mol to 0–2 103/6 103 mole.
impact toughness of 18Cr–2Mo FSSs was investigated in the temperature range between 40 and 60 °C, using sub-size (5 mm 10 mm 55 mm) specimens. All Charpy specimens were prepared transverse to the rolling direction of the plate. The impact testing is based on the section of Charpy Impact Testing in ASTM: A370-12a. 2.4. Microstructure analysis As-annealed specimens for microstructural analysis were cut, prepared according to the standard metallographic procedures, and then etched in 5 vol.% ferric chloride (FeCl3) and 5 vol.% hydrochloric acid (HCl) solution. The longitudinal section of specimen was observed using optical microscopy (OM) of Zeiss Axioplan 2. The grain sizes were measured by computer program using planimetric method. The precipitates of the specimens were observed with a FEI Quanta 600 FEG scanning electron microscopy (SEM). Thin foils were prepared for the observation of fine precipitates with transmission electron microscopy (TEM) of JEOL JEM 2100F. Qualitative microanalysis of precipitates was determined by energy dispersive spectroscopy (EDS). Further detailed analysis of precipitates in steel FSS-1# was achieved by complete extraction of non-metallic particles in a solution of 90 vol.% formaldehyde and 10 vol.% hydrochloric acid (HCl) and examined by a Philips X’Pert PRO MPD X-ray diffractometer. 3. Results and discussion
2.3. Tensile strength and impact toughness testing 3.1. Equilibrium diagram Tensile tests were performed at room temperature with strain rate of 5 103 s1. The yield strength (YS, 0.2% proof stress), ultimate tensile strength (UTS) and elongation were measured. The Charpy-V
Figs. 1 and 2 show the equilibrium precipitation of the studied 18Cr–2Mo steels by Thermo-Calc calculation. It is well known that
J. Han et al. / Materials and Design 58 (2014) 518–526
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Fig. 5. Overall fracture appearance of FSS-2# specimens impacted at: (a) 60 °C; (c) 40 °C and SEM fractographs of FSS-2# specimens impacted at: (b) 60 °C; (d) 40 °C.
Nb, V and Ti are all strong forming elements of carbides and nitrides, however, the precipitation behavior of second phase is complicated and mainly dependent on the liquid/solid condition, the solid solubility, the effect of other elements [16]. It is already clear that: (1) TiN forms above the liquidus, which is extremely stable and only dissolve at very high temperature. Thus, for FSS-2#, FCC_A1#1 is confirmed to be TiN; (2) below the liquidus, the reported precipitation order according to the temperature is NbN, VN, TiC, NbC, VC [17], but for different composition system and producing process, the precipitation situation will be changed. Fig. 1a and b display that for the composition of FSS-1#, Zphase, accepted as Cr(V, Nb)N [18], precipitates at 1130 °C, and then the temperatures for the precipitates of Nb/V to appear are 1100 °C and 1070 °C, respectively. And for FSS-2#, due to the fact that TiN appears in the liquid, the other precipitates are prone to forming around TiN. Then, the corresponding temperature for Nb/V/Ti precipitation is 1440 °C, which is influenced by TiN and illustrated in Fig. 2a and b.
3.2. Tensile strength Table 2 lists the tensile strength properties of FSS-1# and FSS2# specimens at room temperature. It is demonstrated that the YS value of Nb–V stabilized steel is higher, and meanwhile, the elongations of two steels are similar. Nb and V addition will improve strength of the studied steels, which are induced by solution and dispersion strengthening of Nb and V, together with refinement of grain size, etc. More details will be discussed in subsequent sections.
3.3. Impact toughness Fig. 3 shows the impact toughness values of FSS-1# and FSS-2# from 40 to 60 °C. For each FSS, the drop of impact values occurs predominantly when temperature is reduced to a low degree. Comparatively, FSS-1# owns better toughness since its critical temperature is 0 °C, but for FSS-2#, 20 °C, at which their toughness has been decreased to an unstable or a low level. The fracture surfaces of FSS-1# and FSS-2# were analyzed by SEM, shown in Figs. 4 and 5. The specimens impacted between 40 and 60 °C were observed. When impacted at 60 °C, the fractograph of FSS-1# (Fig. 4b) presents a fracture with plastic deformed dimples, which shows a characteristic of ductile fracture [19]. Some holes can be seen, which are larger and deeper in contrast to that of FSS-2# (Fig. 5b), and indicate more severe deformation [20]. If the impacting temperature is turned to 40 °C, the fracture morphology of FSS-1# is river-like, which consists of many flat facets and shear cracks (Fig. 4f). But for FSS-2# tested at 40 °C, the fracture surfaces become smoother (Fig. 5d), which implies lower impact toughness of sample compared with that of FSS-1#. For FSS-1#, when measured at 0 °C, the fracture morphology is mixed one, which consists of many small flat facets surrounded by ductile dimples, and indicates that its impact energy may be changeable. But for FSS-2# tested at the critical temperature, the mixed fracture does not appear at 20 or 40 °C. 3.4. Microstructure It is well known from Fig. 6 that the microstructure of 18Cr– 2Mo FSSs is single ferrite phase. The average grain sizes of FSS-
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Fig. 6. OM images of steels: (a) FSS-1#; (b) FSS-2#.
1# and FSS-2# are approximating to 155 and 186 lm, respectively. The distribution of grain sizes is given in Fig. 7. For the grain that from 40 to 180 lm, FSS-1# owns 20% more than that of FSS-2#, reaching 75%. And for the grain that from 180 to >300 lm, FSS2# owns 37% grains of this size. Even more than 13% grains are bigger than 300 lm in FSS-2#. By comparison, Nb–V stabilized steel obtained finer grains than that of Nb–V–Ti stabilized one, which means the addition of Nb and V exerted the grain refining effect, but if Ti is added to Nb–V steel, the refining effect degrades. Due to the limitation of annealing temperature (above the sigma solvus) and rolling reduction (from 30 mm to 6 mm, 80%), the grain size is hard to be greatly fined by producing process in this paper. But besides some thermomechanical processing, viz. controlled rolling, heat treatment, etc., precipitation of Nb and V also influence the grain size [21]. The grain refinement of Nb–V steel is possibly related with the amount of fine precipitates, such as Nb and V carbonitrides, which form at the grain boundaries, shown in Fig. 8, and contributes not only to toughness improvement of FSS-1#, but also to strength increment due to dispersion strengthening [22]. From the results of Thermo-Calc calculation (Fig. 1c and d), the formation of TiN happens in the molten steel, and brings the effect of refining the solidification structure [23]. However, Du et al. [24] did not find the relationship between TiN particle size and grain size by statistical analysis of some previous
work, which means that the large TiN is not effective in preventing the growth of ferrite grain size. For other fine precipitates, Andersen and Grong [25,26] set up the analytical modeling of grain growth in metals in the presence of precipitation, which determined the grain boundary pinning effect together with particle coarsening. Usually, it is accepted that refinement of grain size will be an advantage to strength and toughness. For strength, the fine grain generates the pinning effect, which will increase the tensile strength undoubtedly. And for toughness, Petch explained the reason why fine grain size will be beneficial to the increasing of toughness [7], and pointed that DBTT occurs when
ry ky d1=2 ¼ k2y þ r0 ky d1=2 > C lc
ð2Þ
where ry is flow or fracture stress, ky is Hall–Pecth slope, d is grain size, r0 is lattice friction stress, C is a constant related to stress state and average ratio of normal to shear stress on the slip plane, l is shear modulus, c is effective surface energy of an implied crack. The Eq. (2) illustrates that the growth of grain size will increase the value of DBTT. Meanwhile, the Eq. (3) [15] relating grain size to transition temperature has been verified for several metals. The relationship can be stated as 1=2
T c ¼ A þ Bd
ð3Þ
where Tc is DBTT, A and B are constants, and d is the mean linear intercept in mm. Paton [15] gave an example of Eq. (3) for the ferritic stainless steel that he studied: Tc = 80–11.5d1/2.
Fig. 7. The distribution of grain sizes.
Fig. 8. SEM image of precipitation at the grain boundaries of FSS-1#.
J. Han et al. / Materials and Design 58 (2014) 518–526
Fig. 9. (a) SEM image of FSS-1# and (b) EDS analysis of precipitate.
Fig. 10. (a) SEM image of FSS-2# and (b) EDS analysis of precipitate.
Fig. 11. (a) TEM image of FSS-1# and (b and c) EDS analysis of precipitates.
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Fig. 12. (a) TEM image of FSS-2# and (b and c) EDS analysis of precipitates.
Fig. 13. TiN particles appearing at the initiation of cleavage fracture.
3.5. Precipitation Figs. 9 and 10 display the SEM images and EDS analysis of the precipitates of FSS-1# and FSS-2#. For FSS-1#, the formation of Z phase, Cr(V,Nb)N, is unstable in short annealing time, hence, the typical MX precipitates appearing are Nb/V carbonitride according to Thermo-Calc calculation. Usually, in the high Cr steels with Nb and V, MX precipitates can be divided into two groups, Nb(C,N) and VN [18]. Amounts of fine precipitates distributing in the grain are probably Nb(C,N) and VN particles, which are shown in Fig. 9a. Nb(C,N) is confirmed by EDS (Fig. 9b), however, VN precipitate (in the upper left circle of Fig. 9a) is hard to distinguish by means of SEM since the VN particle is the smallest particle found in high Cr steel [18]. From Fig. 11a and c, it is observed that some fine particles of MX adhering to Nb(C,N) in FSS-1# are VN, and the size of this complex precipitate is less than 200 nm. In the meantime, Fig. 11a and b show the precipitate of NbC. It is reported the average diameters of the spherical Nb(C,N) and the plate-like V-nitride adhering to Nb(C,N) are around 500 nm and 180 nm in 9 wt.% Cr
ferritic steels [27], and meanwhile, the size of Nb and V precipitates together with C and N will be changed with the concentration ratio of Nb/V [28]. For FSS-2#, in the presence of Ti, which is the strong nitride forming element, some cuboidal TiN precipitate of the average size of about 3 lm is found distributing in the matrix (Fig. 10a and b). As the amount of Ti in steel is more than critical value as-certained by the ideal chemical matching of TiN, and thus, the interstitial atom nitrogen has fully been stabilized by Ti theoretically. Then, NbC and TiC have the similar solid solubility. In low carbon steels with Nb and Ti additions, NbC is more stable than TiC at higher temperatures in ferrite throughout the ferrite stability range [29]. From Fig. 12a–c, it is observed that some fine particles of MX besides the coarse particle TiN in FSS-2# are rich in Nb, which are (Nb, Ti)C and NbC. VN is seldom found since N is almost occupied by Ti. At present, it is accepted that the coarse precipitates, such as TiN, result in a loss of toughness, since they will be associated with the initiation of brittle fracture [24,30,31], and meanwhile, the precipitation of TiN will also influence the quantity of fine Nb and V
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(3) Ti addition will degrade its toughness properties due to the large TiN particles, which usually initiate the cleavage fracture process. Hence, Ti is not a suitable element for stabilizing 18Cr–2Mo FSS heavy plate, to which high impact toughness is necessary.
Acknowledgements This work is financially supported by the R&D Project of Baoshan Iron & Steel Co., Ltd. (Grant No. D09ECX0205). The authors are grateful to Zhu Zhixiong and Ma Yan for their help in discussion and experiment. References
Fig. 14. XRD result of extracted precipitates of FSS-1#.
carbonitrides, which is related with the modification of grain size. Fig. 13a reveals the found precipitate TiN locating at the initiation of cleavage fracture, which testifies the negative influence of large particle and can be regarded as inclusions in some steels. The EDS result of the selected particle is given in Fig. 13b. Three critical steps are involved in a TiN particle initiated cleavage fracture process, i.e. microcrack nucleates at a TiN particle; the microcrack penetrates the matrix across the particle–matrix interface and grows into a grain-sized crack; the grain-sized crack continues to propagate across the grain boundaries and becomes unstable, resulting in cleavage fracture [32]. This point together with grain size can be taken into account regarding the toughness comparison of two studied steels. Since Nb–V–Ti system is testified not to be suitable for composition design of 18Cr–2Mo FSS heavy plate, the paper focuses on investigating the exact precipitation of FSS-1#, and X-ray diffraction (XRD) was used to precisely distinguish the precipitates. In Fig. 14, XRD result of FSS-1# is given, which confirms the results of SEM and TEM. It is clear that the precipitates extracted mainly consist of NbC and VN.81. NbN, NbCrN, Fe–Cr formed in a small amount during the whole process according to strength of the peaks. NbCrN is one type of Z phase, which is always in the steel without V. The lack of VC is possibly related with its unstability and low precipitation temperature, above which carbon is always occupied by Nb totally. The dynamic producing process modified the Thermo-Calc prediction of precipitation in some degree. 4. Conclusions The effects of microalloying on the grain size and precipitation of 18Cr–2Mo FSSs stabilized by (Nb + V) and (Nb + V + Ti) were investigated. The relationship between Ti addition and the properties was discussed. The following conclusions can be drawn: (1) For (Nb + V) stabilized 18Cr–2Mo FSS, fine grain size, which is related with fine precipitation of the particles at the grain boundaries, is beneficial to the tensile strength and impact toughness of 18Cr–2Mo FSS heavy plate. (2) For (Nb + V) steel, Nb(C,N) and VN (precisely NbC, NbN and VN.81) are main precipitates, however for (Nb + V + Ti) steel, primary precipitates are TiN and (Nb, Ti)C. The element of Ti can greatly modify the precipitation behavior of (Nb + V) composition system.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microalloying effects on microstructure and mechanical properties of 18Cr–2Mo ferritic stainless steel heavy plates Jian Han a,⇑, Huijun Li a, Haigang Xu b a b
School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia Baoshan Iron & Steel Co., Ltd., Shanghai 200431, China
a r t i c l e
i n f o
Article history: Received 24 November 2013 Accepted 6 February 2014 Available online 18 February 2014 Keywords: Ferritic stainless steel Heavy plate Microalloying Microstructure Mechanical property
a b s t r a c t The microstructure, including grain size and precipitation, tensile strength and Charpy impact toughness of (Nb + V) 18Cr–2Mo ferritic stainless steel heavy plates with/without Ti were investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and standard tensile strength and Charpy impact toughness testing. It was found that for 18Cr–2Mo heavy plate, a good combination of Nb–V stabilized method without Ti induces refinement of grain sizes due to the precipitation of amounts of fine Nb carbonitrides and V nitrides. Meanwhile, the mechanical testing results indicate that optimal transformation of grain size, precipitation that Nb–V composition system brings to 18Cr–2Mo heavy plate is beneficial to improvement of strength and impact toughness. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Ferritic stainless steel (FSS), which is body centered cubic (BCC) structure, is essentially Fe–Cr or Fe–Cr–Mo alloy [1]. FSS has various advantages in comparison with austenitic stainless steel (ASS): lower cost, higher thermal conductivity, smaller linear expansion and better resistance to chloride stress-corrosion cracking, atmospheric corrosion and oxidation. Because of these merits, FSS is very attractive in numbers of application fields [2–4]. The Cr and Mo balance in 18Cr–2Mo FSS, together with stabilizing additions, such as Nb, V, Ti, provides higher corrosion resistance than other ferritic grades [5] and is even comparable to austenitic AISI 316 [6]. However, one limitation of 18Cr–2Mo FSS is its relatively high ductile to brittle transition temperature (DBTT) [7], which is always above room temperature and particularly as thickness is beyond 5 mm [8]. In addition, it is impossible to reduce the thickness of FSS products due to its comparatively low strength. For instance, for 18Cr–2Mo FSS, which is named S44400 according to ASTM: A240/A240M-13c, the bottomline of its yield strength is only 275 MPa. At present, 18Cr–2Mo FSS heavy plates (above 4 mm) have been applied in limited industrial fields, such as water treatment and brewing. Therefore, the challenge of 18Cr–2Mo FSS heavy plate is how to improve toughness and utilize the other attributes of this steel group. ⇑ Corresponding author. Tel.: +61 405427056; fax: +61 242213238. E-mail address: [email protected] (J. Han). http://dx.doi.org/10.1016/j.matdes.2014.02.009 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved.
Some factors affecting the toughness properties have already been identified for FSS. Wright [9] proposed that the BCC crystallography limits the number of available slip systems, lowers the deformation compatibility and increases the probability of initiation and propagation of brittle fracture. Xiao [10] explained the brittleness of FSS from the point of crack cores that form at the boundaries of deformation bands, which influences DBTT of high Cr FSS [8]. Van Zwieten and Bulloch [11] suggested that for Fe–Cr stainless steel, C and N dramatically reduce the impact toughness properties due to carbides and nitrides form at the grain boundaries, while O has only a small detrimental effect since O in BCC metals promotes the occurrence of intergranular fracture. The presence of second phases, viz. carbides, nitrides and oxides, can negatively influence the toughness properties of FSS. In high C and/or N alloys, the toughness is decreased after a high temperature annealing treatment, which increases the lattice friction stress and the flow stress as a result of the precipitation of carbides and nitrides on dislocations. Ohashi et al. [9] believed that coarse grains tend to promote crack initiation, and thus the grain size contributes mainly to resistance to initiation of brittle fracture and only slightly to crack propagation. The detrimental effect of interstitials may be controlled by the addition of stabilizing elements, such as Nb, V, Ti and Zr. These elements form strong carbides and nitrides [7,12,13], which are more stable than Cr carbides and nitrides. Semchyshen et al. [7] found Nb and Ti are both effective in retarding the increment in transition temperature. V has a strong tendency to form carbides and ni-
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J. Han et al. / Materials and Design 58 (2014) 518–526 Table 1 Chemical compositions (wt.%) of the studied 18Cr–2Mo steels. Steel no.
C
Si
Mn
Cr
Mo
Nb
V
Ti
N
Fe
FSS-1# FSS-2#
0.0047 0.0046
0.093 0.080
0.090 0.081
17.83 17.95
1.81 1.78
0.15 0.15
0.14 0.11
– 0.05
0.0105 0.0101
Bal. Bal.
Fig. 1. Calculated equilibrium molar fractions of precipitates of FSS-1#: (a) Y axis from 0–1 mol; (b) Y axis from 0–2 103 mole.
Fig. 2. Calculated equilibrium molar fractions of precipitates of FSS-2#: (a) Y axis from 0–1 mol; (b) Y axis from 0–6 103 mole.
Table 2 Tensile properties of the studied 18Cr–2Mo steels. Steel no.
YS (MPa)
UTS (MPa)
Elongation (%)
FSS-1# FSS-2#
337.5 302.5
470.0 442.5
32.0 33.0
trides, and meanwhile, the solid solubility of its carbides and nitrides are very small in ferrite, which will disperse V precipitates at specific temperature [14]. Paton [15] pointed that some benefit is obtained in impact toughness by the addition of V. In this study, Ti was added to traditional (Nb + V)-stabilized 18Cr–2Mo FSS heavy plates to investigate the role of microalloy additions on microstructure and mechanical properties. V is added to the studied steels instead of part Nb in consideration of its effect of solution and dispersion strengthening, which is beneficial for the development of FSS heavy plate.
Fig. 3. Charpy impact values of FSS-1# and FSS-2# from 40 to 60 °C.
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Fig. 4. Overall fracture appearance of FSS-1# specimens impacted at: (a) 60 °C; (c) 0 °C; (e) 40 °C and SEM fractographs of FSS-1# specimens impacted at: (b) 60 °C; (d) 0 °C; (f) 40 °C.
2. Experimental procedures 2.1. Materials and processing The 18Cr–2Mo FSSs used in this study were melted in a 50 kg vacuum induction furnace. The casting ingots with a diameter of 120 mm were hot forged into the size of 30 mm 250 mm 200 mm (thickness width length), hot rolled into 6 mm plate, and annealed at 950 °C for 30 min, followed by cooling in the air. The chemical compositions of the 18Cr–2Mo steels measured are listed in Table 1. The stabilization ratios of FSS-1# and FSS-2# are 19.1 and 21.2, respectively, which are calculated by interstitial elements divided by stabilizing elements, shown in Eq. (1). The results mean both steels can satisfy the requirements of intergranular corrosion resistance.
Stabilization ratio ¼ ðNb þ V þ TiÞ=ðC þ NÞ
ð1Þ
2.2. Phase calculation The equilibrium phase diagrams were calculated using ThermoCalc software based on TCFE6 database, and the recommended temperature range is from 500 to 1600 °C. The details of precipitation were illustrated by changing the scale of Y axis from the range of 0–1 mol to 0–2 103/6 103 mole.
impact toughness of 18Cr–2Mo FSSs was investigated in the temperature range between 40 and 60 °C, using sub-size (5 mm 10 mm 55 mm) specimens. All Charpy specimens were prepared transverse to the rolling direction of the plate. The impact testing is based on the section of Charpy Impact Testing in ASTM: A370-12a. 2.4. Microstructure analysis As-annealed specimens for microstructural analysis were cut, prepared according to the standard metallographic procedures, and then etched in 5 vol.% ferric chloride (FeCl3) and 5 vol.% hydrochloric acid (HCl) solution. The longitudinal section of specimen was observed using optical microscopy (OM) of Zeiss Axioplan 2. The grain sizes were measured by computer program using planimetric method. The precipitates of the specimens were observed with a FEI Quanta 600 FEG scanning electron microscopy (SEM). Thin foils were prepared for the observation of fine precipitates with transmission electron microscopy (TEM) of JEOL JEM 2100F. Qualitative microanalysis of precipitates was determined by energy dispersive spectroscopy (EDS). Further detailed analysis of precipitates in steel FSS-1# was achieved by complete extraction of non-metallic particles in a solution of 90 vol.% formaldehyde and 10 vol.% hydrochloric acid (HCl) and examined by a Philips X’Pert PRO MPD X-ray diffractometer. 3. Results and discussion
2.3. Tensile strength and impact toughness testing 3.1. Equilibrium diagram Tensile tests were performed at room temperature with strain rate of 5 103 s1. The yield strength (YS, 0.2% proof stress), ultimate tensile strength (UTS) and elongation were measured. The Charpy-V
Figs. 1 and 2 show the equilibrium precipitation of the studied 18Cr–2Mo steels by Thermo-Calc calculation. It is well known that
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Fig. 5. Overall fracture appearance of FSS-2# specimens impacted at: (a) 60 °C; (c) 40 °C and SEM fractographs of FSS-2# specimens impacted at: (b) 60 °C; (d) 40 °C.
Nb, V and Ti are all strong forming elements of carbides and nitrides, however, the precipitation behavior of second phase is complicated and mainly dependent on the liquid/solid condition, the solid solubility, the effect of other elements [16]. It is already clear that: (1) TiN forms above the liquidus, which is extremely stable and only dissolve at very high temperature. Thus, for FSS-2#, FCC_A1#1 is confirmed to be TiN; (2) below the liquidus, the reported precipitation order according to the temperature is NbN, VN, TiC, NbC, VC [17], but for different composition system and producing process, the precipitation situation will be changed. Fig. 1a and b display that for the composition of FSS-1#, Zphase, accepted as Cr(V, Nb)N [18], precipitates at 1130 °C, and then the temperatures for the precipitates of Nb/V to appear are 1100 °C and 1070 °C, respectively. And for FSS-2#, due to the fact that TiN appears in the liquid, the other precipitates are prone to forming around TiN. Then, the corresponding temperature for Nb/V/Ti precipitation is 1440 °C, which is influenced by TiN and illustrated in Fig. 2a and b.
3.2. Tensile strength Table 2 lists the tensile strength properties of FSS-1# and FSS2# specimens at room temperature. It is demonstrated that the YS value of Nb–V stabilized steel is higher, and meanwhile, the elongations of two steels are similar. Nb and V addition will improve strength of the studied steels, which are induced by solution and dispersion strengthening of Nb and V, together with refinement of grain size, etc. More details will be discussed in subsequent sections.
3.3. Impact toughness Fig. 3 shows the impact toughness values of FSS-1# and FSS-2# from 40 to 60 °C. For each FSS, the drop of impact values occurs predominantly when temperature is reduced to a low degree. Comparatively, FSS-1# owns better toughness since its critical temperature is 0 °C, but for FSS-2#, 20 °C, at which their toughness has been decreased to an unstable or a low level. The fracture surfaces of FSS-1# and FSS-2# were analyzed by SEM, shown in Figs. 4 and 5. The specimens impacted between 40 and 60 °C were observed. When impacted at 60 °C, the fractograph of FSS-1# (Fig. 4b) presents a fracture with plastic deformed dimples, which shows a characteristic of ductile fracture [19]. Some holes can be seen, which are larger and deeper in contrast to that of FSS-2# (Fig. 5b), and indicate more severe deformation [20]. If the impacting temperature is turned to 40 °C, the fracture morphology of FSS-1# is river-like, which consists of many flat facets and shear cracks (Fig. 4f). But for FSS-2# tested at 40 °C, the fracture surfaces become smoother (Fig. 5d), which implies lower impact toughness of sample compared with that of FSS-1#. For FSS-1#, when measured at 0 °C, the fracture morphology is mixed one, which consists of many small flat facets surrounded by ductile dimples, and indicates that its impact energy may be changeable. But for FSS-2# tested at the critical temperature, the mixed fracture does not appear at 20 or 40 °C. 3.4. Microstructure It is well known from Fig. 6 that the microstructure of 18Cr– 2Mo FSSs is single ferrite phase. The average grain sizes of FSS-
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Fig. 6. OM images of steels: (a) FSS-1#; (b) FSS-2#.
1# and FSS-2# are approximating to 155 and 186 lm, respectively. The distribution of grain sizes is given in Fig. 7. For the grain that from 40 to 180 lm, FSS-1# owns 20% more than that of FSS-2#, reaching 75%. And for the grain that from 180 to >300 lm, FSS2# owns 37% grains of this size. Even more than 13% grains are bigger than 300 lm in FSS-2#. By comparison, Nb–V stabilized steel obtained finer grains than that of Nb–V–Ti stabilized one, which means the addition of Nb and V exerted the grain refining effect, but if Ti is added to Nb–V steel, the refining effect degrades. Due to the limitation of annealing temperature (above the sigma solvus) and rolling reduction (from 30 mm to 6 mm, 80%), the grain size is hard to be greatly fined by producing process in this paper. But besides some thermomechanical processing, viz. controlled rolling, heat treatment, etc., precipitation of Nb and V also influence the grain size [21]. The grain refinement of Nb–V steel is possibly related with the amount of fine precipitates, such as Nb and V carbonitrides, which form at the grain boundaries, shown in Fig. 8, and contributes not only to toughness improvement of FSS-1#, but also to strength increment due to dispersion strengthening [22]. From the results of Thermo-Calc calculation (Fig. 1c and d), the formation of TiN happens in the molten steel, and brings the effect of refining the solidification structure [23]. However, Du et al. [24] did not find the relationship between TiN particle size and grain size by statistical analysis of some previous
work, which means that the large TiN is not effective in preventing the growth of ferrite grain size. For other fine precipitates, Andersen and Grong [25,26] set up the analytical modeling of grain growth in metals in the presence of precipitation, which determined the grain boundary pinning effect together with particle coarsening. Usually, it is accepted that refinement of grain size will be an advantage to strength and toughness. For strength, the fine grain generates the pinning effect, which will increase the tensile strength undoubtedly. And for toughness, Petch explained the reason why fine grain size will be beneficial to the increasing of toughness [7], and pointed that DBTT occurs when
ry ky d1=2 ¼ k2y þ r0 ky d1=2 > C lc
ð2Þ
where ry is flow or fracture stress, ky is Hall–Pecth slope, d is grain size, r0 is lattice friction stress, C is a constant related to stress state and average ratio of normal to shear stress on the slip plane, l is shear modulus, c is effective surface energy of an implied crack. The Eq. (2) illustrates that the growth of grain size will increase the value of DBTT. Meanwhile, the Eq. (3) [15] relating grain size to transition temperature has been verified for several metals. The relationship can be stated as 1=2
T c ¼ A þ Bd
ð3Þ
where Tc is DBTT, A and B are constants, and d is the mean linear intercept in mm. Paton [15] gave an example of Eq. (3) for the ferritic stainless steel that he studied: Tc = 80–11.5d1/2.
Fig. 7. The distribution of grain sizes.
Fig. 8. SEM image of precipitation at the grain boundaries of FSS-1#.
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Fig. 9. (a) SEM image of FSS-1# and (b) EDS analysis of precipitate.
Fig. 10. (a) SEM image of FSS-2# and (b) EDS analysis of precipitate.
Fig. 11. (a) TEM image of FSS-1# and (b and c) EDS analysis of precipitates.
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Fig. 12. (a) TEM image of FSS-2# and (b and c) EDS analysis of precipitates.
Fig. 13. TiN particles appearing at the initiation of cleavage fracture.
3.5. Precipitation Figs. 9 and 10 display the SEM images and EDS analysis of the precipitates of FSS-1# and FSS-2#. For FSS-1#, the formation of Z phase, Cr(V,Nb)N, is unstable in short annealing time, hence, the typical MX precipitates appearing are Nb/V carbonitride according to Thermo-Calc calculation. Usually, in the high Cr steels with Nb and V, MX precipitates can be divided into two groups, Nb(C,N) and VN [18]. Amounts of fine precipitates distributing in the grain are probably Nb(C,N) and VN particles, which are shown in Fig. 9a. Nb(C,N) is confirmed by EDS (Fig. 9b), however, VN precipitate (in the upper left circle of Fig. 9a) is hard to distinguish by means of SEM since the VN particle is the smallest particle found in high Cr steel [18]. From Fig. 11a and c, it is observed that some fine particles of MX adhering to Nb(C,N) in FSS-1# are VN, and the size of this complex precipitate is less than 200 nm. In the meantime, Fig. 11a and b show the precipitate of NbC. It is reported the average diameters of the spherical Nb(C,N) and the plate-like V-nitride adhering to Nb(C,N) are around 500 nm and 180 nm in 9 wt.% Cr
ferritic steels [27], and meanwhile, the size of Nb and V precipitates together with C and N will be changed with the concentration ratio of Nb/V [28]. For FSS-2#, in the presence of Ti, which is the strong nitride forming element, some cuboidal TiN precipitate of the average size of about 3 lm is found distributing in the matrix (Fig. 10a and b). As the amount of Ti in steel is more than critical value as-certained by the ideal chemical matching of TiN, and thus, the interstitial atom nitrogen has fully been stabilized by Ti theoretically. Then, NbC and TiC have the similar solid solubility. In low carbon steels with Nb and Ti additions, NbC is more stable than TiC at higher temperatures in ferrite throughout the ferrite stability range [29]. From Fig. 12a–c, it is observed that some fine particles of MX besides the coarse particle TiN in FSS-2# are rich in Nb, which are (Nb, Ti)C and NbC. VN is seldom found since N is almost occupied by Ti. At present, it is accepted that the coarse precipitates, such as TiN, result in a loss of toughness, since they will be associated with the initiation of brittle fracture [24,30,31], and meanwhile, the precipitation of TiN will also influence the quantity of fine Nb and V
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(3) Ti addition will degrade its toughness properties due to the large TiN particles, which usually initiate the cleavage fracture process. Hence, Ti is not a suitable element for stabilizing 18Cr–2Mo FSS heavy plate, to which high impact toughness is necessary.
Acknowledgements This work is financially supported by the R&D Project of Baoshan Iron & Steel Co., Ltd. (Grant No. D09ECX0205). The authors are grateful to Zhu Zhixiong and Ma Yan for their help in discussion and experiment. References
Fig. 14. XRD result of extracted precipitates of FSS-1#.
carbonitrides, which is related with the modification of grain size. Fig. 13a reveals the found precipitate TiN locating at the initiation of cleavage fracture, which testifies the negative influence of large particle and can be regarded as inclusions in some steels. The EDS result of the selected particle is given in Fig. 13b. Three critical steps are involved in a TiN particle initiated cleavage fracture process, i.e. microcrack nucleates at a TiN particle; the microcrack penetrates the matrix across the particle–matrix interface and grows into a grain-sized crack; the grain-sized crack continues to propagate across the grain boundaries and becomes unstable, resulting in cleavage fracture [32]. This point together with grain size can be taken into account regarding the toughness comparison of two studied steels. Since Nb–V–Ti system is testified not to be suitable for composition design of 18Cr–2Mo FSS heavy plate, the paper focuses on investigating the exact precipitation of FSS-1#, and X-ray diffraction (XRD) was used to precisely distinguish the precipitates. In Fig. 14, XRD result of FSS-1# is given, which confirms the results of SEM and TEM. It is clear that the precipitates extracted mainly consist of NbC and VN.81. NbN, NbCrN, Fe–Cr formed in a small amount during the whole process according to strength of the peaks. NbCrN is one type of Z phase, which is always in the steel without V. The lack of VC is possibly related with its unstability and low precipitation temperature, above which carbon is always occupied by Nb totally. The dynamic producing process modified the Thermo-Calc prediction of precipitation in some degree. 4. Conclusions The effects of microalloying on the grain size and precipitation of 18Cr–2Mo FSSs stabilized by (Nb + V) and (Nb + V + Ti) were investigated. The relationship between Ti addition and the properties was discussed. The following conclusions can be drawn: (1) For (Nb + V) stabilized 18Cr–2Mo FSS, fine grain size, which is related with fine precipitation of the particles at the grain boundaries, is beneficial to the tensile strength and impact toughness of 18Cr–2Mo FSS heavy plate. (2) For (Nb + V) steel, Nb(C,N) and VN (precisely NbC, NbN and VN.81) are main precipitates, however for (Nb + V + Ti) steel, primary precipitates are TiN and (Nb, Ti)C. The element of Ti can greatly modify the precipitation behavior of (Nb + V) composition system.
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