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Role of inclusions in flux aided backing submerged arc welding
Journal of Materials Processing Technology 240 (2017) 145–153
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Role of inclusions in flux aided backing submerged arc welding Juan Pu a,b , Shengfu Yu a,∗ , Yuanyuan Li b a b
State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China
a r t i c l e
i n f o
Article history: Received 20 July 2016 Received in revised form 19 September 2016 Accepted 20 September 2016 Available online 22 September 2016 Keywords: Inclusions Thermodynamic calculation Flux aided backing submerged arc welding (FAB-SAW) CeO2 -containing alloy powders Microstructure
a b s t r a c t The formation of inclusions and their roles in Ce microalloyed weld metal by flux aided backing submerged arc welding (FAB-SAW) were studied by combining experimental investigation and thermodynamic calculation. The results showed that in Ce microalloyed weld metal, the core of effective inclusions was composed of Ce2 O3 , Al2 O3 , SiO2 , MnO, and TiO compound oxides, while the surface of inclusions was composed of CeS. The first-formed Ce2 O3 particles in molten pool provided more nucleation sites for Al, Si, Mn, Ti oxides and CeS, which promoted an increase on the areal density and a decrease on the mean diameter of inclusions. The small sized inclusions increased the formation of acicular ferrite (AF) and suppressed the formation of grain boundary ferrite (GBF) and ferrite side-plate (FSP). The elongation, tensile strength and impact toughness of weld metals were improved with the increasing amount of AF. When 3.0 wt.% CeO2 was added, the highest proportion of AF was obtained, producing the optimal mechanical properties of weld metal with elongation of 24%, tensile strength of 635 MPa and impact energy of 89 J at −20 ◦ C. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Welding on the large engineering structures such as ocean platforms, large-diameter pipelines and shipbuilding are usually performed in cold and wet environments. With the development of marine construction industry and shipbuilding industrial clusters, improving large heat-input welding technologies is rapidly becoming a critical subject. Flux aided backing submerged arc welding (FAB-SAW) can easily achieve single-pass welding on the steel plates with a thickness up to 25 mm and can satisfy the requirements of the applications above, as reported by Liu (1986). As the diagram shown in Fig. 1, FAB-SAW is a simple and practical method which can be used in cold and wet environments. In this method, the alloy powders are directly added into the weld groove which will alloy with the molten weld metal, while a ceramic backing is installed on the reverse side of steels to provide refractory containment for the alloy powders and restrict the molten weld bath dispersion. By using FAB-SAW method, two surfaces weld forming can be achieved by a one surface welding operation. However, the high FAB-SAW process heat-input of over 60 kJ/cm results in grain coarsening, generating large quantities of both grain
∗ Corresponding author. E-mail addresses: pu [email protected] (J. Pu), [email protected] (S. Yu), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.jmatprotec.2016.09.016 0924-0136/© 2016 Elsevier B.V. All rights reserved.
boundary ferrites (GBFs) and ferrite side-plates (FSPs) and even bainite and martensite-austenite (M-A) constituents in the weld metal. These unwanted weld products resulted in a deterioration of the impact toughness of the weld metal, as described by Zhao et al. (2002). A method to achieve a refined weld metal microstructure with the required toughness in the weld metal is an urgent issue for FAB-SAW. Oxide metallurgy technology is considered to be an effective approach to improve the mechanical properties of the weld metal for high strength low alloy (HSLA) steel. The deliberate creation of oxide inclusions promotes the formation of acicular ferrite (AF) and increases the ratio of AFs to GBFs and FSPs in the weld metal. Recently, researchers have paid significant attention to the role of Ce in the weld metal and heat affect zone (HAZ) in large heat-input welding. Thewlis (2006) studied the effect of cerium sulphide particles on the microstructure evolution of HSLA steel. Subsequently, Thewlis et al. (2008) investigated the development of acicular ferrite in steels containing dispersed cerium sulphide particles when welded by an autogenous laser. They concluded that Ce-oxysulfides can provide effective nucleation sites for the formation of AF in both the steel and weld metal. Yu et al. (2008) studied the microstructure and properties of Ce-containing weld metal which were obtained from multi wire welding. Yan et al. (2015) reported that Ce in weld metal can promote the formation of Ce-oxysulfides and the nucleation of AF in the weld metal of flux copper backing (FCB) submerged arc welding. Pu et al. (2016) studied the transition of Ce
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from CeO2 to Ce-oxysulfides and its effect on AF nucleation during FAB-SAW. Ce-oxysulfide inclusions can be considered as effective nucleation agents which induce the formation of AF during the large heat-input welding. Usually, the size, distribution and composition of these inclusions control the formation of AF. The optimum size of inclusions for inducing AF is associated with their composition. The optimal size of Ti oxides was 2.0 m and that of MnS particles was more than 1.0 m as reported by Hajeri et al. (2006), the optimum size of Ti-Al oxides inclusions range from 0.5 to 1.5 m as reported by Huang et al. (2015), TiOx + MnS composite inclusions should be greater than 0.85 m as investigated by Mu et al. (2014), Al2 O3 + MgO composite inclusions should be in the range from 0.3 to 0.8 m according to the research by Zhang et al. (2015), while Ti oxide inclusions should have a size of 0.6 m as reported by Seo et al. (2013). Thus it can be concluded that the effective inclusion size range is between 0.3 and 2 m. Up to now, the effects of Ce on the formation, size and distribution of inclusions in weld metal have not been systematically investigated. In this research, CeO2 alloy powders were added to a weld groove prior to FAB-SAW and Ce-containing oxide inclusions were produced in the weld process. CeO2 was decomposed and transferred into the molten metal pool under the arc action, and Ce-containing inclusions were produced while the characteristics of inclusions were controlled through the metallurgical reactions occurring in the molten pool. The various compositions of Cecontaining oxide inclusions were thermodynamically calculated and experimentally verified. The number, size and distribution of inclusions were analyzed. The microstructure evolution, tensile and impact properties of weld metal were studied. The relationships among the characteristics of inclusions, microstructure evolution and mechanical properties of weld metals were discussed.
2. Experimental procedures A commercial grade D32 steel plate with a thickness of 20 mm was used as the base metal. AWS 5.17 F7A4-EH14 welding wire was used with a diameter of 4.8 mm. AWS 5.17 F7A0-EH14 flux was dried at 350 ◦ C for 2 h before use. As depicted in Fig. 1, a single V type groove with an angle of 40◦ was machined into the base metal and the root gap of the base metal was maintained between 1 and 3 mm during pre-assembly. The V type weld groove was filled with alloy powders. TG-B1 ceramic backing was installed on the weld groove. Run-on and run-off plates were used at two ends of the base metal. To investigate the effect of CeO2 content on the microstructure and mechanical properties of weld metal, alloy powders with five different concentrations of CeO2 (0, 0.5, 1.5, 3.0 and 5.0 wt.%) were designed. The balance of alloy powders was composed of Mn powder with a constant concentration of 2.0 wt.% and iron powder. FAB-SAW experiments were conducted using a Lincoln IDEALARC® DC-1500 welding machine. The welding parameters were set as follows: a welding current of 1050 A, a welding voltage of 34 V, a wire feed speed of 170 cm/min, a welding speed of 20 cm/min, a wire extension of 35 mm and a welding heat input of 107.1 kJ/cm. Samples used for metallurgical and mechanical test were extracted and machined from each weld sample using an electric discharge machine (EDM), as shown in Fig. 2. The sample for chemical composition analysis was extracted from the center region of weld metal. Two samples for metallurgical testing including one sample for optical microstructure (OM) and another sample for transmission electron microscopy (TEM) were obtained. Two longitudinal samples for tensile test and three samples for Charpy
Fig. 1. The schematic diagram of FAB-SAW.
Fig. 2. The locations of all test samples.
V-notch impact test were prepared. The locations and sizes of samples for mechanical properties test are shown in Fig. 3. The chemical compositions of the weld metals were investigated by using an X-ray fluorescence spectrometer, a CS600 carbon-sulfur analyzer for C and S, a TCH600 oxygen-nitrogen analyzer for O content and an ICP-MS X Series II for Ce content. The microstructure of weld metal was observed by a ZEISS optical microscope. The proportions of AF, GBF and FSP in weld metal were calculated according to ASTM-E562-02. Inclusions morphologies were observed by TEM. Samples were first sliced into discs with a thickness of 0.25 mm with a diamond saw. These discs were mechanically ground and then thinned by a twin-jet electro-polisher using an electrolyte which comprising of 10% perchloric acid and 90% ethyl alcohol at −30 ◦ C and 20 mA. TEM analysis was conducted by using a TECNAI 2 transmission electron microscope. Bright field images, selected area electron diffraction (SAED) patterns and energy dispersive spectrometer (EDS) spectrums of inclusions in the weld metals were obtained. The number, size and distribution of the inclusions were examined using an IBAS-2000 automatic image analyzer. The mechanical properties, including the tensile strength, yield strength, elongation, and Charpy V-notch impact toughness at 0 and −20 ◦ C, were tested using a SANS-CMT 5205 tensile machine and a JB-300B impact testing machine, respectively. The fracture surfaces were observed with a JEOL JSM-6480 scanning electron microscope (SEM).
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Fig. 3. The locations and dimensions of samples: (a) the locations of tensile and impact samples: (b) dimensions of tensile samples and (c) dimensions of impact samples.
Table 1 EDS analysis results of zone A–C (wt.%) of P3 inclusion in Fig. 4(c). Inclusions
Al
Si
Mn
Ti
Ce
O
S
C
Fe
Zone A Zone B Zone C
33.7 9.2 –
9.8 5.8 –
19.3 10.2 5.2
1.3 4.3 –
4.9 31.9 –
29.8 32.1 –
1.2 6.5 –
– – 1.02
– – 93.78
3. Results and discussion 3.1. The inclusions in weld metals 3.1.1. Character analysis on inclusions in weld metals Fig. 4 shows the TEM bright field images and the EDS spectra of typical inclusions in weld metals. For the CeO2 -free sample (Fig. 4a), the inclusion P1 is mainly composed of Al, Si, Mn, Ti, O and S. For samples with 0.5, 1.5, 3.0, 5.0 wt.% CeO2 (corresponding to Fig. 4b–e), inclusions P2-P5 are composed of Al, Si, Mn, Ti, O, S and Ce. It was observed that the typical inclusion chemistry was altered with the addition of Ce. This indicated that Ce was successfully transferred from CeO2 -containing powders to the weld metal, and finally into the inclusions. Fig. 5 depicts the EDS-mapping results of typical inclusions with the addition of CeO2 . The bulk of the inclusions comprised oxides of Al, Si, Mn, Ti, and a small amount of Ce oxides. S and Ce were detected on the surface of inclusions, and this was inferred to be Ce-sulfides. EDS results of the center A, edge B and outer surface C for the inclusion P3 (as shown in Fig. 4c) are listed in Table 1. It can be observed that the center A of inclusion was composed of Al, Si, Mn, O, Ti together with 4.9 wt.% Ce and 1.2 wt.% S, while the edge B of inclusion contained Ce, S, Ti, O, together with 31.9 wt.% Ce and 6.5 wt.% S. Therefore, Ce and S were mainly precipitated at the surface of the inclusion. The outer surface C of inclusion contained
Fe, C and Mn, and was considered to be a ferrite was nucleated by Ce-sulfides present at the surface of the inclusion. The selected area electron diffraction (SAED) patterns for zones A, B and C for inclusion P3, demonstrate that the compound in zone A has a facecentered cubic (fcc) structure with a lattice constant of 0.8 nm, which corresponds to Al2 O3 ·MnO, as shown in Fig. 6A. The substance in zone B has an fcc structure with a lattice constant of 0.6 nm, which is consistent with CeS, as shown in Fig. 6B. While the substance in zone C is ferrite, as shown in Fig. 6C. Consequently, the inclusion was composed of Al2 O3 ·MnO, SiO2 , TiO, Ce2 O3 complex oxides in the center, and predominately CeS at the surface. During the FAB-SAW process, CeO2 powders were first decomposed to Ce atoms or ions under weld arc action. These ions were transferred into the molten pool where the deoxidization and desulphurization reactions took place. Ce had priority over the other alloying elements, such as Al, Ti, Si and Mn, in reacting with oxygen to form Ce2 O3 owing to its stronger deoxidizing ability. The first-formed Ce2 O3 particles provided nucleation sites for Al, Ti, Si, Mn oxides, hence the Ce, Al, Ti, Si, Mn complex oxide inclusions were formed. During the metallurgical reaction process, most of complex oxides inclusions floated into the slag and the oxygen content of molten pool was decreased. Meanwhile, Ce was also a strong sulfide-forming element, and when the temperature of weld pool decreased, a desulphurization reaction was initiated and Ce-sulfides precipitated on the surface of the complex oxides inclusions. During the metallurgical reaction process in FAB-SAW molten pool, chemical and metallurgical reactions among Ce, O and S occurred at temperatures from 1873 K to 2173 K, and followed the laws of thermodynamics obtained by Huang (2002): G = −19.147TlgK = −19.147Tlg
a[Mx Ny ] a[M]
x
· a[N]
y
(1)
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Fig. 4. TEM micrographs of inclusions and EDS analysis results of P1–P5 inclusions: (a) CeO2 -free; (b) 0.5 wt.% CeO2 ; (c) 1.5 wt.% CeO2 ; (d) 3.0 wt.% CeO2 and (e) 5.0 wt.% CeO2 .
where G is the standard free energy, M is a substitute for element Ce, N stands for the nonmetallic elements (O, S), x and y are the stoichiometric coefficients, and a is the activity value of compounds in FAB-SAW molten pool. a[Mx Ny ] is supposed to be equal to 1 in FAB-SAW molten pool.
From Eq. (1), the relationship between a[O] and a[S] during the formation process of Ce-containing compounds in equilibrium was calculated and shown in Table 2. The precipitation graph on transformation between Ce-sulfides and Ce-oxysulfides from the results in Table 2 can be plotted as shown in Fig. 7.
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Fig. 5. EDS-mapping results of inclusions which induced the formation of AF: (a) 0.5 wt.% CeO2 ; (b) 1.5 wt.% CeO2 ; (c) 3.0 wt.% CeO2 and (d) 5.0 wt.% CeO2 .
149
150
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Fig. 6. Selected-area diffraction patterns corresponding to zone A–C showed in Fig. 4(c): (A) center of the inclusion; (B) edge of the inclusion and (C) outer surface of the inclusion.
Fig. 7. Precipitation diagram of lga[Ce] − lga[O] − lga[S] . Table 2 Relationship between a[O] and a[S] in equilibrium of different Ce-containing inclusions. Chemical reaction equations
Relationship between a[O] and a[S]
2CeO2 = Ce2 O3 + [O] Ce2 O3 + [S] = Ce2 O2 S + [O] 2CeO2 + [S] = Ce2 O2 S + 2 [O] Ce2 O2 S + [S] = 2CeS + 2 [O]
a[O] = 0.13 a[S] /a[O] = 12.59 a[S] /a2[O] = 97.72
Ce2 O2 S + 2 [S] = Ce2 S3 + 2 [O] Ce2 S3 = 2CeS + [S]
a[S] /a[O] = 263 a[S] = 0.51
Table 3 Chemical composition of all FAB-SAW weld metals. CeO2 (wt.%)
0 0.5 1.5 3.0 5.0
a[S] /a2[O] = 13489
Chemical elements (wt.%) C
Si
Mn
P
S
Ti
Al
Ce
Oa
0.14 0.14 0.13 0.12 0.09
0.29 0.28 0.27 0.26 0.19
1.56 1.61 1.53 1.45 1.23
0.023 0.023 0.020 0.021 0.024
0.010 0.011 0.007 0.003 0.009
0.0004 0.0003 0.0002 0.0003 0.0002
0.018 0.027 0.016 0.023 0.016
0 0.037 0.058 0.079 0.092
136 139 141 257 382
a:ppm.
Meanwhile, the relations between the activity of O, S and the concentration of solute elements in FAB-SAW molten pool can be expressed by: lgfi =
ai = fi [%i]
j ei
and can be calculated by the following equation obtained by Wei (1980): j
j
ei = (2557/T − 0.365) · ei (1873) [%j]
(2)
(4)
j
ei (1873) is the interaction parameter of elements at 1873 K.
(3)
where j is the solute element (Al, Si, Mn, etc.), i is the solution elements (Ce, O and S), [%j] and [%i] are the concentration of j and j i in FAB-SAW weld metal, ei is the interaction parameter of j on i
The results werea[O] = 8.4738 × 10−5 , a[S] = 0.003, a[S] /a[O] = 63, anda[S] /a2[O] = 742300, calculated using Eqs. (2)–(4) with the concentration of elements in weld metal from Table 3. When these results are combined with Fig. 7, it may be observed that Ce2 O3 primarily nucleates in the liquid metal, and then CeS precipitates
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Table 4 The size, number and distribution of inclusions in weld metals. CeO2 (wt.%)
Mean diameter Dmax /m
Areal density A /mm−2
Areal percentage (AA /%)
Mean area (A/m2 )
0 0.5 1.5 3.0 5.0
1.17 0.86 0.82 0.72 1.02
3494 4525 5108 6071 3762
0.465 0.301 0.255 0.237 0.344
0.692 0.517 0.467 0.385 0.586
Table 5 The proportion of AF, FSP and GBF in FAB-SAW weld metals. CeO2 (wt.%)
GBF
FSP (area percentage, %)
AF
0 0.5 1.5 3.0 5.0
40.7 21.2 19.1 14.6 46.3
27.8 13.5 10.5 6.8 8.1
31.5 65.3 70.4 78.6 45.6
AF-acicular ferrite; GBF-grain boundary ferrite; FSP-ferrite side-plate.
on the surface of the Ce2 O3 -containing inclusions. Consequently, a typical inclusion was composed of Ce2 O3 , Al2 O3 ·MnO, SiO2 , TiO in the center, and CeS at the surface. CeS induced AF nucleation because of the low lattice mismatch. 3.1.2. Number, size and distribution of inclusions in weld metals Table 4 lists the number, size and distribution of inclusions in FAB-SAW weld metals. For CeO2 -free sample, the areal density of inclusions in weld metal has a minimum value of ∼3494 mm−2 , the mean diameter of inclusions has a maximum value of ∼1.17 m, and the proportion of inclusions with size of >2.0 m is more than 40%. After adding CeO2 , the areal density of inclusions in weld metal increased, while the mean diameter of inclusions decreased, and the fraction of inclusions with size of >2.0 m significantly reduced. When the content of CeO2 was 3.0 wt.%, the areal density of inclusions in weld metals achieved a maximum value and the fraction of inclusions with size of <2.0 m was over 95%. This can be attributed to the refinement of inclusions in weld metals induced by Ce. Following the deoxygenation progress, high density Ce2 O3 were formed (see Fig. 7) and precipitated in molten pool. The precipitated Ce2 O3 can provide a large quantity of nucleation sites for Al, Ti, Si, Mn oxides, resulting in refinement of the inclusions in weld metal. However, when the content of CeO2 in alloy powders was increased to 5.0 wt.%, the mean diameter of inclusions and the fraction of inclusions with size of >2 m increased. Excessive content of CeO2 in alloy powders led to the increase of inclusions density in molten pool. When the density of inclusions exceeded a critical value, the collisions between inclusions were intensified under the drastic agitation induced by the weld arc, resulting in the growth of inclusion size. In conclusion, an appropriate of CeO2 content in weld groove can increase the density and control the size of inclusions. However, too much CeO2 will lead to a larger size and a smaller areal density for inclusions in the process of FAB-SAW. 3.2. Microstructure of weld metals Fig. 8 shows the optical microstructure of FAB-SAW weld metals with different CeO2 content. Table 5 presents the corresponding results for the proportion of GBF, FSP and AF in weld metals derived using quantitative statistical analysis. As shown in Fig. 8a, the proportion of GBF (with an area fraction of 40.7%) and FSP (27.8%) in the CeO2 -free weld metal was higher than the weld metals containing CeO2 . In the samples with differing CeO2 contents, Fig. 8b–d, the
Size distribution of inclusions (area percentage,%) <1 m
1–2 m
2–3 m
3–4 m
>4 m
31.20 51.23 53.15 62.31 45.23
23.76 40.49 42.23 35.08 40.18
36.66 7.19 3.61 2.06 11.30
8.17 1.02 0.96 0.52 3.17
0.21 0.07 0.05 0.03 0.12
proportion of GBF and FSP decreased gradually while the proportion of AF increased with the increasing content of CeO2 . The addition of Ce into weld metal promotes the formation of complex inclusions, composed of a body mainly comprising Ce2 O3 , Al2 O3 ·MnO, SiO2 , TiO in the center and CeS at the surface. Ce can also increase both the areal density of the effective inclusions and the fraction of the inclusions with size of <2.0 m, and consequently increase the proportion of AF in the microstructure. When the content of CeO2 was 3.0 wt.%, the areal density of the inclusions achieved a maximum and the fraction of the inclusions with size of <2.0 m was more than 95% (see Table 4), thus, the number of effective inclusions peaked, and the proportion of AF in the weld also peaked. As a result, the proportion of GBF and FSP was minimized. When 5.0 wt.% CeO2 was added into the weld groove, the proportion of GBF and FSP increased while the proportion of AF decreased as shown in Fig. 8e. This can be attributed to the decreasing areal density of the effective inclusions and the increasing number of inclusions with size of >2.0 m (see Table 4). In conclusion, during FAB-SAW, Ce can promote the formation of AF and conversely inhibit the formation of GBF and FSP by controlling the characteristics, size and distribution of inclusions. Tang (2011) has suggested that Ce could segregate on grain boundaries, producing a decrease of surface tension and grain boundary energy, a reduction of the driving force for grain growth, and consequently suppressing the growth of austenite grains. Thus the presence of Ce results in grains refinement in weld metal. However, further investigations need to be completed in order to confirm this point of view. 3.3. Mechanical properties of weld metals The tensile properties and impact toughness of FAB-SAW weld metals are shown in Fig. 9. For the CeO2 -free sample, the elongation of weld metal was 15% and the impact energy at −20 ◦ C was 30 J, which will not satisfy the application requirement of FABSAW. When CeO2 was added into the weld metal, the elongation, yield strength, tensile strength and the impact toughness at 0 ◦ C and −20 ◦ C of Ce microalloyed weld metal were increased significantly. The weld metal welded with the alloy powders containing 3.0 wt.% CeO2 possessed the optimal mechanical properties with the elongation of ∼24%, the tensile strength of ∼635 MPa and the impact energy value of ∼89 J at −20 ◦ C. However, when excessive amount of Ce (5.0 wt.% CeO2 ) was added, the mechanical properties of weld metal were significantly decreased due to a degradation of the microstructure (Table 5). With the addition of CeO2 , Ce can refine the inclusions of weld metal and promote the formation of AF while inhibiting the formation of GBF and FSP. It is well known that AF has high angle boundaries and fine grain size, which leads to a high resistance to crack propagation. Hence, the mechanical properties of weld metals were improved by increasing the amount of AF. When the content of CeO2 was 3.0 wt.%, the mechanical properties of weld metals achieved the optimum value due to the presence of the highest proportion of AF in the weld metal. Fig. 10 shows the fracture morphologies of fibrous region for FAB-SAW weld metals tested at −20 ◦ C. The fracture surface of
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Fig. 8. Microstructure of FAB-SAW weld metals: (a) CeO2 -free; (b) 0.5 wt.% CeO2 ; (c) 1.5 wt.% CeO2 ; (d) 3.0 wt.% CeO2 and (e) 5.0 wt.% CeO2 .
Fig. 9. The mechanical properties of FAB-SAW weld metals with different CeO2 -containing alloy powders in the weld groove: (a) the tensile properties and (b) the lowtemperature impact toughness.
Fig. 10. SEM images on ductile fracture surfaces of impact specimens tested at −20 ◦ C: (a) CeO2 -free; (b) 0.5 wt.% CeO2 ; (c) 3.0 wt.% CeO2 and (d) 5.0 wt.% CeO2 .
CeO2 -free sample contained some large dimples and cleavage region, as shown in Fig. 10a. The CeO2 -free weld metal had lower impact toughness. After adding CeO2 into the weld groove, the dimples grew deeper in the fracture with a smaller size as shown in Fig. 10b–d. When the content of CeO2 was 3.0 wt.%, a large number of tiny dimples were observed while the weld metal exhibited good toughness. Zhou and Chew (2003) reported that there was an inverse relationship between the impact toughness and the square
root of average dimple size. The formation of dimples in the fracture could be ascribed to the higher fraction of AF in weld metals.
4. Conclusions To improve the microstructure and mechanical properties of the FAB-SAW weld metal on D32 steel, alloy powders with containing differing quantities of CeO2 powder were added to the weld
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groove to induce the formation of Ce-oxysulfide inclusions. The composition, size, number and distribution of the inclusions, and their effects on the microstructure, tensile properties and impact toughness of the weld metal were systematically studied. The main conclusions include: (1) Ce-oxysulfide inclusions were produced in the FAB-SAW metal, and were composed of Al, Ce, Si, Mn, Ti oxides in the core and CeS at the surface according to TEM and thermodynamic analysis. (2) The number, size and distribution of the inclusions were determined by the amount of CeO2 powders. The increased Ceoxysulfides inclusions promoted a proportional increase of AF. The tensile strength, yield strength, elongation and low temperature impact energy of weld metals were increased due to the large amounts of AF in the microstructure. (3) When 3.0 wt.% CeO2 was added, the areal density of inclusions and the fraction of inclusions with size of <2.0 m were reach to the peak value, meanwhile, the optimal mechanical properties of weld metal were obtained with elongation of 24%, tensile strength of 635 MPa and impact energy of 89 J at −20 ◦ C. Conflicts of interest The authors declare no conflict of interest. Shengfu Yu, as a funding sponsor for this paper, designed the study and made a decision to publish the results. Acknowledgments This work was supported by the National Natural Science Foundation of China (NO. U1260103) and Baoshan Iron & Steel Co., Ltd. The authors would like to express our gratitude to the technical stuff in the Provincial Key Laboratory of Advanced Welding Technology, Jiangsu University of Sci. & Technology for various assistances.
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Role of inclusions in flux aided backing submerged arc welding Juan Pu a,b , Shengfu Yu a,∗ , Yuanyuan Li b a b
State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China
a r t i c l e
i n f o
Article history: Received 20 July 2016 Received in revised form 19 September 2016 Accepted 20 September 2016 Available online 22 September 2016 Keywords: Inclusions Thermodynamic calculation Flux aided backing submerged arc welding (FAB-SAW) CeO2 -containing alloy powders Microstructure
a b s t r a c t The formation of inclusions and their roles in Ce microalloyed weld metal by flux aided backing submerged arc welding (FAB-SAW) were studied by combining experimental investigation and thermodynamic calculation. The results showed that in Ce microalloyed weld metal, the core of effective inclusions was composed of Ce2 O3 , Al2 O3 , SiO2 , MnO, and TiO compound oxides, while the surface of inclusions was composed of CeS. The first-formed Ce2 O3 particles in molten pool provided more nucleation sites for Al, Si, Mn, Ti oxides and CeS, which promoted an increase on the areal density and a decrease on the mean diameter of inclusions. The small sized inclusions increased the formation of acicular ferrite (AF) and suppressed the formation of grain boundary ferrite (GBF) and ferrite side-plate (FSP). The elongation, tensile strength and impact toughness of weld metals were improved with the increasing amount of AF. When 3.0 wt.% CeO2 was added, the highest proportion of AF was obtained, producing the optimal mechanical properties of weld metal with elongation of 24%, tensile strength of 635 MPa and impact energy of 89 J at −20 ◦ C. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Welding on the large engineering structures such as ocean platforms, large-diameter pipelines and shipbuilding are usually performed in cold and wet environments. With the development of marine construction industry and shipbuilding industrial clusters, improving large heat-input welding technologies is rapidly becoming a critical subject. Flux aided backing submerged arc welding (FAB-SAW) can easily achieve single-pass welding on the steel plates with a thickness up to 25 mm and can satisfy the requirements of the applications above, as reported by Liu (1986). As the diagram shown in Fig. 1, FAB-SAW is a simple and practical method which can be used in cold and wet environments. In this method, the alloy powders are directly added into the weld groove which will alloy with the molten weld metal, while a ceramic backing is installed on the reverse side of steels to provide refractory containment for the alloy powders and restrict the molten weld bath dispersion. By using FAB-SAW method, two surfaces weld forming can be achieved by a one surface welding operation. However, the high FAB-SAW process heat-input of over 60 kJ/cm results in grain coarsening, generating large quantities of both grain
∗ Corresponding author. E-mail addresses: pu [email protected] (J. Pu), [email protected] (S. Yu), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.jmatprotec.2016.09.016 0924-0136/© 2016 Elsevier B.V. All rights reserved.
boundary ferrites (GBFs) and ferrite side-plates (FSPs) and even bainite and martensite-austenite (M-A) constituents in the weld metal. These unwanted weld products resulted in a deterioration of the impact toughness of the weld metal, as described by Zhao et al. (2002). A method to achieve a refined weld metal microstructure with the required toughness in the weld metal is an urgent issue for FAB-SAW. Oxide metallurgy technology is considered to be an effective approach to improve the mechanical properties of the weld metal for high strength low alloy (HSLA) steel. The deliberate creation of oxide inclusions promotes the formation of acicular ferrite (AF) and increases the ratio of AFs to GBFs and FSPs in the weld metal. Recently, researchers have paid significant attention to the role of Ce in the weld metal and heat affect zone (HAZ) in large heat-input welding. Thewlis (2006) studied the effect of cerium sulphide particles on the microstructure evolution of HSLA steel. Subsequently, Thewlis et al. (2008) investigated the development of acicular ferrite in steels containing dispersed cerium sulphide particles when welded by an autogenous laser. They concluded that Ce-oxysulfides can provide effective nucleation sites for the formation of AF in both the steel and weld metal. Yu et al. (2008) studied the microstructure and properties of Ce-containing weld metal which were obtained from multi wire welding. Yan et al. (2015) reported that Ce in weld metal can promote the formation of Ce-oxysulfides and the nucleation of AF in the weld metal of flux copper backing (FCB) submerged arc welding. Pu et al. (2016) studied the transition of Ce
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from CeO2 to Ce-oxysulfides and its effect on AF nucleation during FAB-SAW. Ce-oxysulfide inclusions can be considered as effective nucleation agents which induce the formation of AF during the large heat-input welding. Usually, the size, distribution and composition of these inclusions control the formation of AF. The optimum size of inclusions for inducing AF is associated with their composition. The optimal size of Ti oxides was 2.0 m and that of MnS particles was more than 1.0 m as reported by Hajeri et al. (2006), the optimum size of Ti-Al oxides inclusions range from 0.5 to 1.5 m as reported by Huang et al. (2015), TiOx + MnS composite inclusions should be greater than 0.85 m as investigated by Mu et al. (2014), Al2 O3 + MgO composite inclusions should be in the range from 0.3 to 0.8 m according to the research by Zhang et al. (2015), while Ti oxide inclusions should have a size of 0.6 m as reported by Seo et al. (2013). Thus it can be concluded that the effective inclusion size range is between 0.3 and 2 m. Up to now, the effects of Ce on the formation, size and distribution of inclusions in weld metal have not been systematically investigated. In this research, CeO2 alloy powders were added to a weld groove prior to FAB-SAW and Ce-containing oxide inclusions were produced in the weld process. CeO2 was decomposed and transferred into the molten metal pool under the arc action, and Ce-containing inclusions were produced while the characteristics of inclusions were controlled through the metallurgical reactions occurring in the molten pool. The various compositions of Cecontaining oxide inclusions were thermodynamically calculated and experimentally verified. The number, size and distribution of inclusions were analyzed. The microstructure evolution, tensile and impact properties of weld metal were studied. The relationships among the characteristics of inclusions, microstructure evolution and mechanical properties of weld metals were discussed.
2. Experimental procedures A commercial grade D32 steel plate with a thickness of 20 mm was used as the base metal. AWS 5.17 F7A4-EH14 welding wire was used with a diameter of 4.8 mm. AWS 5.17 F7A0-EH14 flux was dried at 350 ◦ C for 2 h before use. As depicted in Fig. 1, a single V type groove with an angle of 40◦ was machined into the base metal and the root gap of the base metal was maintained between 1 and 3 mm during pre-assembly. The V type weld groove was filled with alloy powders. TG-B1 ceramic backing was installed on the weld groove. Run-on and run-off plates were used at two ends of the base metal. To investigate the effect of CeO2 content on the microstructure and mechanical properties of weld metal, alloy powders with five different concentrations of CeO2 (0, 0.5, 1.5, 3.0 and 5.0 wt.%) were designed. The balance of alloy powders was composed of Mn powder with a constant concentration of 2.0 wt.% and iron powder. FAB-SAW experiments were conducted using a Lincoln IDEALARC® DC-1500 welding machine. The welding parameters were set as follows: a welding current of 1050 A, a welding voltage of 34 V, a wire feed speed of 170 cm/min, a welding speed of 20 cm/min, a wire extension of 35 mm and a welding heat input of 107.1 kJ/cm. Samples used for metallurgical and mechanical test were extracted and machined from each weld sample using an electric discharge machine (EDM), as shown in Fig. 2. The sample for chemical composition analysis was extracted from the center region of weld metal. Two samples for metallurgical testing including one sample for optical microstructure (OM) and another sample for transmission electron microscopy (TEM) were obtained. Two longitudinal samples for tensile test and three samples for Charpy
Fig. 1. The schematic diagram of FAB-SAW.
Fig. 2. The locations of all test samples.
V-notch impact test were prepared. The locations and sizes of samples for mechanical properties test are shown in Fig. 3. The chemical compositions of the weld metals were investigated by using an X-ray fluorescence spectrometer, a CS600 carbon-sulfur analyzer for C and S, a TCH600 oxygen-nitrogen analyzer for O content and an ICP-MS X Series II for Ce content. The microstructure of weld metal was observed by a ZEISS optical microscope. The proportions of AF, GBF and FSP in weld metal were calculated according to ASTM-E562-02. Inclusions morphologies were observed by TEM. Samples were first sliced into discs with a thickness of 0.25 mm with a diamond saw. These discs were mechanically ground and then thinned by a twin-jet electro-polisher using an electrolyte which comprising of 10% perchloric acid and 90% ethyl alcohol at −30 ◦ C and 20 mA. TEM analysis was conducted by using a TECNAI 2 transmission electron microscope. Bright field images, selected area electron diffraction (SAED) patterns and energy dispersive spectrometer (EDS) spectrums of inclusions in the weld metals were obtained. The number, size and distribution of the inclusions were examined using an IBAS-2000 automatic image analyzer. The mechanical properties, including the tensile strength, yield strength, elongation, and Charpy V-notch impact toughness at 0 and −20 ◦ C, were tested using a SANS-CMT 5205 tensile machine and a JB-300B impact testing machine, respectively. The fracture surfaces were observed with a JEOL JSM-6480 scanning electron microscope (SEM).
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Fig. 3. The locations and dimensions of samples: (a) the locations of tensile and impact samples: (b) dimensions of tensile samples and (c) dimensions of impact samples.
Table 1 EDS analysis results of zone A–C (wt.%) of P3 inclusion in Fig. 4(c). Inclusions
Al
Si
Mn
Ti
Ce
O
S
C
Fe
Zone A Zone B Zone C
33.7 9.2 –
9.8 5.8 –
19.3 10.2 5.2
1.3 4.3 –
4.9 31.9 –
29.8 32.1 –
1.2 6.5 –
– – 1.02
– – 93.78
3. Results and discussion 3.1. The inclusions in weld metals 3.1.1. Character analysis on inclusions in weld metals Fig. 4 shows the TEM bright field images and the EDS spectra of typical inclusions in weld metals. For the CeO2 -free sample (Fig. 4a), the inclusion P1 is mainly composed of Al, Si, Mn, Ti, O and S. For samples with 0.5, 1.5, 3.0, 5.0 wt.% CeO2 (corresponding to Fig. 4b–e), inclusions P2-P5 are composed of Al, Si, Mn, Ti, O, S and Ce. It was observed that the typical inclusion chemistry was altered with the addition of Ce. This indicated that Ce was successfully transferred from CeO2 -containing powders to the weld metal, and finally into the inclusions. Fig. 5 depicts the EDS-mapping results of typical inclusions with the addition of CeO2 . The bulk of the inclusions comprised oxides of Al, Si, Mn, Ti, and a small amount of Ce oxides. S and Ce were detected on the surface of inclusions, and this was inferred to be Ce-sulfides. EDS results of the center A, edge B and outer surface C for the inclusion P3 (as shown in Fig. 4c) are listed in Table 1. It can be observed that the center A of inclusion was composed of Al, Si, Mn, O, Ti together with 4.9 wt.% Ce and 1.2 wt.% S, while the edge B of inclusion contained Ce, S, Ti, O, together with 31.9 wt.% Ce and 6.5 wt.% S. Therefore, Ce and S were mainly precipitated at the surface of the inclusion. The outer surface C of inclusion contained
Fe, C and Mn, and was considered to be a ferrite was nucleated by Ce-sulfides present at the surface of the inclusion. The selected area electron diffraction (SAED) patterns for zones A, B and C for inclusion P3, demonstrate that the compound in zone A has a facecentered cubic (fcc) structure with a lattice constant of 0.8 nm, which corresponds to Al2 O3 ·MnO, as shown in Fig. 6A. The substance in zone B has an fcc structure with a lattice constant of 0.6 nm, which is consistent with CeS, as shown in Fig. 6B. While the substance in zone C is ferrite, as shown in Fig. 6C. Consequently, the inclusion was composed of Al2 O3 ·MnO, SiO2 , TiO, Ce2 O3 complex oxides in the center, and predominately CeS at the surface. During the FAB-SAW process, CeO2 powders were first decomposed to Ce atoms or ions under weld arc action. These ions were transferred into the molten pool where the deoxidization and desulphurization reactions took place. Ce had priority over the other alloying elements, such as Al, Ti, Si and Mn, in reacting with oxygen to form Ce2 O3 owing to its stronger deoxidizing ability. The first-formed Ce2 O3 particles provided nucleation sites for Al, Ti, Si, Mn oxides, hence the Ce, Al, Ti, Si, Mn complex oxide inclusions were formed. During the metallurgical reaction process, most of complex oxides inclusions floated into the slag and the oxygen content of molten pool was decreased. Meanwhile, Ce was also a strong sulfide-forming element, and when the temperature of weld pool decreased, a desulphurization reaction was initiated and Ce-sulfides precipitated on the surface of the complex oxides inclusions. During the metallurgical reaction process in FAB-SAW molten pool, chemical and metallurgical reactions among Ce, O and S occurred at temperatures from 1873 K to 2173 K, and followed the laws of thermodynamics obtained by Huang (2002): G = −19.147TlgK = −19.147Tlg
a[Mx Ny ] a[M]
x
· a[N]
y
(1)
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Fig. 4. TEM micrographs of inclusions and EDS analysis results of P1–P5 inclusions: (a) CeO2 -free; (b) 0.5 wt.% CeO2 ; (c) 1.5 wt.% CeO2 ; (d) 3.0 wt.% CeO2 and (e) 5.0 wt.% CeO2 .
where G is the standard free energy, M is a substitute for element Ce, N stands for the nonmetallic elements (O, S), x and y are the stoichiometric coefficients, and a is the activity value of compounds in FAB-SAW molten pool. a[Mx Ny ] is supposed to be equal to 1 in FAB-SAW molten pool.
From Eq. (1), the relationship between a[O] and a[S] during the formation process of Ce-containing compounds in equilibrium was calculated and shown in Table 2. The precipitation graph on transformation between Ce-sulfides and Ce-oxysulfides from the results in Table 2 can be plotted as shown in Fig. 7.
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Fig. 5. EDS-mapping results of inclusions which induced the formation of AF: (a) 0.5 wt.% CeO2 ; (b) 1.5 wt.% CeO2 ; (c) 3.0 wt.% CeO2 and (d) 5.0 wt.% CeO2 .
149
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Fig. 6. Selected-area diffraction patterns corresponding to zone A–C showed in Fig. 4(c): (A) center of the inclusion; (B) edge of the inclusion and (C) outer surface of the inclusion.
Fig. 7. Precipitation diagram of lga[Ce] − lga[O] − lga[S] . Table 2 Relationship between a[O] and a[S] in equilibrium of different Ce-containing inclusions. Chemical reaction equations
Relationship between a[O] and a[S]
2CeO2 = Ce2 O3 + [O] Ce2 O3 + [S] = Ce2 O2 S + [O] 2CeO2 + [S] = Ce2 O2 S + 2 [O] Ce2 O2 S + [S] = 2CeS + 2 [O]
a[O] = 0.13 a[S] /a[O] = 12.59 a[S] /a2[O] = 97.72
Ce2 O2 S + 2 [S] = Ce2 S3 + 2 [O] Ce2 S3 = 2CeS + [S]
a[S] /a[O] = 263 a[S] = 0.51
Table 3 Chemical composition of all FAB-SAW weld metals. CeO2 (wt.%)
0 0.5 1.5 3.0 5.0
a[S] /a2[O] = 13489
Chemical elements (wt.%) C
Si
Mn
P
S
Ti
Al
Ce
Oa
0.14 0.14 0.13 0.12 0.09
0.29 0.28 0.27 0.26 0.19
1.56 1.61 1.53 1.45 1.23
0.023 0.023 0.020 0.021 0.024
0.010 0.011 0.007 0.003 0.009
0.0004 0.0003 0.0002 0.0003 0.0002
0.018 0.027 0.016 0.023 0.016
0 0.037 0.058 0.079 0.092
136 139 141 257 382
a:ppm.
Meanwhile, the relations between the activity of O, S and the concentration of solute elements in FAB-SAW molten pool can be expressed by: lgfi =
ai = fi [%i]
j ei
and can be calculated by the following equation obtained by Wei (1980): j
j
ei = (2557/T − 0.365) · ei (1873) [%j]
(2)
(4)
j
ei (1873) is the interaction parameter of elements at 1873 K.
(3)
where j is the solute element (Al, Si, Mn, etc.), i is the solution elements (Ce, O and S), [%j] and [%i] are the concentration of j and j i in FAB-SAW weld metal, ei is the interaction parameter of j on i
The results werea[O] = 8.4738 × 10−5 , a[S] = 0.003, a[S] /a[O] = 63, anda[S] /a2[O] = 742300, calculated using Eqs. (2)–(4) with the concentration of elements in weld metal from Table 3. When these results are combined with Fig. 7, it may be observed that Ce2 O3 primarily nucleates in the liquid metal, and then CeS precipitates
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Table 4 The size, number and distribution of inclusions in weld metals. CeO2 (wt.%)
Mean diameter Dmax /m
Areal density A /mm−2
Areal percentage (AA /%)
Mean area (A/m2 )
0 0.5 1.5 3.0 5.0
1.17 0.86 0.82 0.72 1.02
3494 4525 5108 6071 3762
0.465 0.301 0.255 0.237 0.344
0.692 0.517 0.467 0.385 0.586
Table 5 The proportion of AF, FSP and GBF in FAB-SAW weld metals. CeO2 (wt.%)
GBF
FSP (area percentage, %)
AF
0 0.5 1.5 3.0 5.0
40.7 21.2 19.1 14.6 46.3
27.8 13.5 10.5 6.8 8.1
31.5 65.3 70.4 78.6 45.6
AF-acicular ferrite; GBF-grain boundary ferrite; FSP-ferrite side-plate.
on the surface of the Ce2 O3 -containing inclusions. Consequently, a typical inclusion was composed of Ce2 O3 , Al2 O3 ·MnO, SiO2 , TiO in the center, and CeS at the surface. CeS induced AF nucleation because of the low lattice mismatch. 3.1.2. Number, size and distribution of inclusions in weld metals Table 4 lists the number, size and distribution of inclusions in FAB-SAW weld metals. For CeO2 -free sample, the areal density of inclusions in weld metal has a minimum value of ∼3494 mm−2 , the mean diameter of inclusions has a maximum value of ∼1.17 m, and the proportion of inclusions with size of >2.0 m is more than 40%. After adding CeO2 , the areal density of inclusions in weld metal increased, while the mean diameter of inclusions decreased, and the fraction of inclusions with size of >2.0 m significantly reduced. When the content of CeO2 was 3.0 wt.%, the areal density of inclusions in weld metals achieved a maximum value and the fraction of inclusions with size of <2.0 m was over 95%. This can be attributed to the refinement of inclusions in weld metals induced by Ce. Following the deoxygenation progress, high density Ce2 O3 were formed (see Fig. 7) and precipitated in molten pool. The precipitated Ce2 O3 can provide a large quantity of nucleation sites for Al, Ti, Si, Mn oxides, resulting in refinement of the inclusions in weld metal. However, when the content of CeO2 in alloy powders was increased to 5.0 wt.%, the mean diameter of inclusions and the fraction of inclusions with size of >2 m increased. Excessive content of CeO2 in alloy powders led to the increase of inclusions density in molten pool. When the density of inclusions exceeded a critical value, the collisions between inclusions were intensified under the drastic agitation induced by the weld arc, resulting in the growth of inclusion size. In conclusion, an appropriate of CeO2 content in weld groove can increase the density and control the size of inclusions. However, too much CeO2 will lead to a larger size and a smaller areal density for inclusions in the process of FAB-SAW. 3.2. Microstructure of weld metals Fig. 8 shows the optical microstructure of FAB-SAW weld metals with different CeO2 content. Table 5 presents the corresponding results for the proportion of GBF, FSP and AF in weld metals derived using quantitative statistical analysis. As shown in Fig. 8a, the proportion of GBF (with an area fraction of 40.7%) and FSP (27.8%) in the CeO2 -free weld metal was higher than the weld metals containing CeO2 . In the samples with differing CeO2 contents, Fig. 8b–d, the
Size distribution of inclusions (area percentage,%) <1 m
1–2 m
2–3 m
3–4 m
>4 m
31.20 51.23 53.15 62.31 45.23
23.76 40.49 42.23 35.08 40.18
36.66 7.19 3.61 2.06 11.30
8.17 1.02 0.96 0.52 3.17
0.21 0.07 0.05 0.03 0.12
proportion of GBF and FSP decreased gradually while the proportion of AF increased with the increasing content of CeO2 . The addition of Ce into weld metal promotes the formation of complex inclusions, composed of a body mainly comprising Ce2 O3 , Al2 O3 ·MnO, SiO2 , TiO in the center and CeS at the surface. Ce can also increase both the areal density of the effective inclusions and the fraction of the inclusions with size of <2.0 m, and consequently increase the proportion of AF in the microstructure. When the content of CeO2 was 3.0 wt.%, the areal density of the inclusions achieved a maximum and the fraction of the inclusions with size of <2.0 m was more than 95% (see Table 4), thus, the number of effective inclusions peaked, and the proportion of AF in the weld also peaked. As a result, the proportion of GBF and FSP was minimized. When 5.0 wt.% CeO2 was added into the weld groove, the proportion of GBF and FSP increased while the proportion of AF decreased as shown in Fig. 8e. This can be attributed to the decreasing areal density of the effective inclusions and the increasing number of inclusions with size of >2.0 m (see Table 4). In conclusion, during FAB-SAW, Ce can promote the formation of AF and conversely inhibit the formation of GBF and FSP by controlling the characteristics, size and distribution of inclusions. Tang (2011) has suggested that Ce could segregate on grain boundaries, producing a decrease of surface tension and grain boundary energy, a reduction of the driving force for grain growth, and consequently suppressing the growth of austenite grains. Thus the presence of Ce results in grains refinement in weld metal. However, further investigations need to be completed in order to confirm this point of view. 3.3. Mechanical properties of weld metals The tensile properties and impact toughness of FAB-SAW weld metals are shown in Fig. 9. For the CeO2 -free sample, the elongation of weld metal was 15% and the impact energy at −20 ◦ C was 30 J, which will not satisfy the application requirement of FABSAW. When CeO2 was added into the weld metal, the elongation, yield strength, tensile strength and the impact toughness at 0 ◦ C and −20 ◦ C of Ce microalloyed weld metal were increased significantly. The weld metal welded with the alloy powders containing 3.0 wt.% CeO2 possessed the optimal mechanical properties with the elongation of ∼24%, the tensile strength of ∼635 MPa and the impact energy value of ∼89 J at −20 ◦ C. However, when excessive amount of Ce (5.0 wt.% CeO2 ) was added, the mechanical properties of weld metal were significantly decreased due to a degradation of the microstructure (Table 5). With the addition of CeO2 , Ce can refine the inclusions of weld metal and promote the formation of AF while inhibiting the formation of GBF and FSP. It is well known that AF has high angle boundaries and fine grain size, which leads to a high resistance to crack propagation. Hence, the mechanical properties of weld metals were improved by increasing the amount of AF. When the content of CeO2 was 3.0 wt.%, the mechanical properties of weld metals achieved the optimum value due to the presence of the highest proportion of AF in the weld metal. Fig. 10 shows the fracture morphologies of fibrous region for FAB-SAW weld metals tested at −20 ◦ C. The fracture surface of
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Fig. 8. Microstructure of FAB-SAW weld metals: (a) CeO2 -free; (b) 0.5 wt.% CeO2 ; (c) 1.5 wt.% CeO2 ; (d) 3.0 wt.% CeO2 and (e) 5.0 wt.% CeO2 .
Fig. 9. The mechanical properties of FAB-SAW weld metals with different CeO2 -containing alloy powders in the weld groove: (a) the tensile properties and (b) the lowtemperature impact toughness.
Fig. 10. SEM images on ductile fracture surfaces of impact specimens tested at −20 ◦ C: (a) CeO2 -free; (b) 0.5 wt.% CeO2 ; (c) 3.0 wt.% CeO2 and (d) 5.0 wt.% CeO2 .
CeO2 -free sample contained some large dimples and cleavage region, as shown in Fig. 10a. The CeO2 -free weld metal had lower impact toughness. After adding CeO2 into the weld groove, the dimples grew deeper in the fracture with a smaller size as shown in Fig. 10b–d. When the content of CeO2 was 3.0 wt.%, a large number of tiny dimples were observed while the weld metal exhibited good toughness. Zhou and Chew (2003) reported that there was an inverse relationship between the impact toughness and the square
root of average dimple size. The formation of dimples in the fracture could be ascribed to the higher fraction of AF in weld metals.
4. Conclusions To improve the microstructure and mechanical properties of the FAB-SAW weld metal on D32 steel, alloy powders with containing differing quantities of CeO2 powder were added to the weld
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groove to induce the formation of Ce-oxysulfide inclusions. The composition, size, number and distribution of the inclusions, and their effects on the microstructure, tensile properties and impact toughness of the weld metal were systematically studied. The main conclusions include: (1) Ce-oxysulfide inclusions were produced in the FAB-SAW metal, and were composed of Al, Ce, Si, Mn, Ti oxides in the core and CeS at the surface according to TEM and thermodynamic analysis. (2) The number, size and distribution of the inclusions were determined by the amount of CeO2 powders. The increased Ceoxysulfides inclusions promoted a proportional increase of AF. The tensile strength, yield strength, elongation and low temperature impact energy of weld metals were increased due to the large amounts of AF in the microstructure. (3) When 3.0 wt.% CeO2 was added, the areal density of inclusions and the fraction of inclusions with size of <2.0 m were reach to the peak value, meanwhile, the optimal mechanical properties of weld metal were obtained with elongation of 24%, tensile strength of 635 MPa and impact energy of 89 J at −20 ◦ C. Conflicts of interest The authors declare no conflict of interest. Shengfu Yu, as a funding sponsor for this paper, designed the study and made a decision to publish the results. Acknowledgments This work was supported by the National Natural Science Foundation of China (NO. U1260103) and Baoshan Iron & Steel Co., Ltd. The authors would like to express our gratitude to the technical stuff in the Provincial Key Laboratory of Advanced Welding Technology, Jiangsu University of Sci. & Technology for various assistances.
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