Microstructural evolution of transition zone of clad X70 with
duplex stainless steel
Xinjie Di a, b, Zhentang Zhong a , Caiyan Deng a, b*, Dongpo Wang a, b，Xiaojiang Guo c a School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China
b Tianjin Key Laboratory of Advanced Joining Technology, Tianjin, 300072, China
c National Engineering Laboratory Transportation Safety of Oil & Gas Pipeline, Langfang, 065000, China
Corresponding author: Caiyan Deng
E-mail: dengcaiyan@https://www.doczj.com/doc/2f8949705.html,
Tel: +86 022 ********
Fax: +86 022 ********
Address: School of Materials Science and Engineering, Tianjin University, No. 92 Weijin Road, Tianjin, 300072, China
Abstract: The microstructural evolution of the transition zone of clad X70 with duplex stainless steel 2209 by hot-wire tungsten inert gas (TIG) welding was examined with an optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM). The relationship between the dilution rate, the value of Cr eq / Ni eq, the solidification mode, and the microstructural evolution within the transition zone were investigated using a Schaeffler diagram and Fe-Cr-Ni pseudobinary diagram. It was found that the dilution rate ranged from 99% to 14% within the transition zone, causing a variation in the value of Cr eq / Ni eq from 0.36 to
2.55. The transition zone was composed of alpha ferrite + martensite, martensite, martensite + delta ferrite, and austenite + martensite + delta ferrite microstructure regions corresponding to the changes of the solidification mode from A mode to F mode. The microhardness testing results showed that a softened region was obtained in the base metal adjacent to the fusion line, while a hardened region was observed in the clad layer.
Keywords:Cladding; Dilution; Transition zone; Microstructural evolution; Microhardness
1. Introduction
Cladding of duplex stainless steels (DSSs) on low-carbon or low-alloy steels is often used in the oil, petrochemical, nuclear, and marine industries, owing to their perfect combination of high tensile strength, good toughness, adequate weldability, and stress-corrosion cracking resistance [1-3]. However, the joining of dissimilar materials is generally more challenging than that of similar materials because of the differences in the physical, chemical, mechanical, and metallurgical properties of the metal welded, which make the dissimilar joints complicated.
The transition zone is a heterogeneous zone in dissimilar metal welds. Previous investigations indicated that the chemical compositions, microstructures, and physical and mechanical properties all change sharply across the transition zone [4-12]. Therefore, it is essential to study the microstructural evolution of the transition zone for a better understanding of service performance. Reddy et al. [13] deposited austenitic stainless steel on a high-strength low-alloy steel to study the structure and
properties correlations. They discovered that the bond interface was nearly flat and the clad layer exhibited a dendritic austenite structure. Tensile strength and notch-tensile strength of the base metal were higher than those of the weld overlay interface. Eghlimi et al. [14] cladded super-duplex stainless steel on high-strength low-alloy steel to evaluate the microstructure and texture across the welded interface. They pointed out that the major factors affecting the texture were unidirectional solidification, competitive growth, and δ/γ orientation relationship. Lippold et al. [15] carried out the weld overlay cladding of AISI8630 with Ni-base Alloy 625 to research the evolution of microstructures that could promote a susceptibility to hydrogen-assisted cracking with a variation in the dilution rate across the transition zone. They conveyed that hydrogen-assisted cracking was most often associated with the planar growth zone and the formation of martensite owing to the extreme gradients in compositions. Kejelin et al. [16] pointed out that transition zone formation was basically dependent on the chemical composition of the welded metal, dilution of the base metal, and the cooling rate.
The chromium equivalent (Cr eq) represents the influence of Cr and other elements that form ferrite, and the nickel equivalent (Ni eq) represents the influence of Ni and other elements that form austenite [17]. The main factors that influence microstructures and properties are chemical composition and heat input [18], and the effects of the chemical composition on the microstructures are determined by the value of Cr eq/ Ni eq, which varies with the dilution rate for DSSs. The dilution rate is defined in terms of the change of the filler-metal composition by mixing with the base metal [19]. The
dilution rate within the transition zone can be determined by equation 1 [20]:
tz fm
bm fm D C C C C --= (1)
where D represents the dilution, C tz is the concentration of any element within the transition zone, C fm represents the nominal concentration of any element in the filler metal, and C bm is the nominal concentration of any element in the base metal.
A narrow martensite band that caused a hardened region was formed in the as-welded condition within the transition zone. The development of the hardened region might be expected to induce sensitivity to hydrogen embrittlement [15] and is partially responsible for the premature failure of dissimilar metal welds [20, 21]. A better understanding of the microstructural evolution within the transition zone becomes an important input into the selection of welding procedures to prolong the service life. Although several investigations [5-16] have been conducted on the transition zone, the microstructural composition within the transition zone and the influence of the heat input on the microstructure are not clear. In particular, research studies on microstructural evolution by means of the solidification mode and solid-state transformation resulting from variations in the dilution rate are still limited. In order to understand the mechanism of premature failure and prevent the premature failure, further studies are necessary to provide more documents on the microstructural evolution details within the transition zone. In the present study, an attempt was made to evaluate the changes of the solidification mode and the microstructural evolution with variations in the dilution within the transition zone.
2. Experimental procedures
In the present study, cladding of DSS2209 on pipeline steel X70 (420 mm × 100 mm × 25 mm) was performed by hot-wire tungsten inert gas (TIG) welding. The chemical compositions of the base metal and overlay metal are given in Table 1. Before cladding, the base metal surface was ground and washed with acetone to remove oxide scales and contaminants. The cladding process parameters are summarized in Table 2.
Table 1 Chemical compositions of base metal and overlay metal (wt. %)
Elements C Si Mn P S Cr Mo Ni Cu Nb N Fe X70 0.07 0.24 1.59 0.015 0.003 0.13 0.17 0.16 0.15 0.05 - Bal.
Table 2 Welding process parameters
Parameters Shielding
gas
gas flow
rate
Filler rod
diameter
Hot wire
current
Hot wire
voltage
Heat input
1 0.74kJ/mm
2 Pare argon
(99.99%)
15L/min 1.2mm 50A 3.0V 1.33kJ/mm
3 1.93kJ/mm After the cladding process, some transverse samples were sliced from the clad composite for subsequent analysis. Specimens for microstructural observation were prepared by standard mounting, grinding, and polishing methods. Then the base metal was etched with 4% Nital while the clad layer was etched with a solution of CuCl2 (5 g) + HCl (100 ml) + alcohol (100 ml). The metallographic microstructure of the transition zone was characterized by an optical microscope (OM) and scanning electron microscope (SEM). Moreover, the chemical composition profile within the transition zone was analyzed by an SEM equipped with Energy Dispersive X-ray Spectroscopy (EDS). A PHILIPS CM200 transmission electron microscope (TEM)
was used for analyzing the microstructures’ characteristics of the transition zone in a more detailed way. The thin foils from the transition zone for TEM were produced by twinjet polishing with 3% perchloric acid and 97% ethanol at temperatures ranging from -25 °C to -29 °C.
Microhardness measurements across the transition zone was carried out on the FM-ARS9000 Vickers microhardness testing machine with a 200 g load and a holding time of 10 s.
The cladding was conducted under three different heat inputs of 0.74 kJ/mm, 1.33 kJ/mm, and 1.93 kJ/mm. The thesis mainly investigated the microstructural evolution of the transition zone under optimal welding parameters (1.33 kJ/mm).
3. Results and analysis
3.1 Morphologies of the transition zone
Fig. 1a shows the OM image of the transition zone, which demonstrates that the interface of the base metal and clad layer was nearly flat. Fig. 1b shows a micrograph of the transition zone obtained by SEM. The transition zone can be divided into two regions (named the “A” and “B” region s) by the fusion line. In Fig. 1b, line 1 represents a dilution rate of 99%, line 2 is the fusion line, and lines 3 and 4 represent the boundaries of different growth modes. The width of the “A” region is approximately 5–7 μm.The “B” region is composed of the zones of planar and cellular growth. The widths of the planar and cellular zones are approximately 6–9 and 9–15 μm,respectively. Owing to the higher thermal conductivity of the X70 substrate compared with that of DSS2209, large amounts of heat transfer and a steep
temperature gradient existed near the fusion line, causing a lower constitutional supercooling. Hence the zone adjacent to the fusion line was planar. Nevertheless, as solidification proceeded, the base metal became warm, resulting in a gentle temperature gradient, and the constitutional supercooling increased, leading to the domination of cellular structure growth [14]. In situations where the base metal and overlay metal were both ferritic at temperatures near the melting point, the solidification of the weld metal could always grow epitaxially from the unmelted or partially melted base metal that served as the nuclei. Thus, the cellular grains grew directly from the planar grains without the formation of a border which is called Type II grain boundaries in the overlay, as shown in Fig. 1b.
Fig. 1 Morphologies of the transition zone: (a) OM image of the interface, and (b) SEM
micrograph of the different regions
3.2 Composition profile within the transition zone
SEM-EDS was conducted to characterize the composition gradients within the transition zone. Fig. 2a and b show the chemical composition profile of the transition zone of a heat input of 1.33 kJ/mm. The mass fraction of the major alloy elements in points 1 to 13 with an interval of approximately 2.5 μm in Fig. 2a are shown in Table 3. The dilution rate of point 1 is approximately 99% as evaluated by the Fe element according to equation 1. Point 3 is in the fusion line with a dilution rate of 90%. Points 6 and 13 are on the boundaries of planar and cellular with a dilution rate of 46% and 14%, respectively. Fig. 2b, which is an example of SEM-EDS line scans across the transition zone, shows that a bond of steep composition gradient was found within the transition zone. In the transition zone, Fe and Cr content varied markedly, while that of Ni increased slower. The main reasons for these gradients were the dilution of clad metal with the base metal and the fast cooling rate of the molten pool. Generally, the transition zone composition was dependent on the individual compositions of the base metal and overlay metal as well as the dilution rate [19]. The transition zone covered a dilution rate ranging from approximately 99% to 14%.
In addition, the dilution rate was related to the welding parameters. Fig. 2c and d show the composition profile of a heat input of 0.74 kJ/mm, and Fig. 2e and f show the composition profile of a heat input of 1.93 kJ/mm. Although there was also a bond of steep composition gradient across the transition zone, the composition gradient of the heat input of 1.93 kJ/mm was gentler, while that of 0.74 kJ/mm was steeper compared with that of the heat input of 1.33 kJ/mm.
The microstructures within the transition zone are dependent on the dilution and the
cooling rate. The width of the transition zone of the lower heat input was narrower than that of the higher heat input (Fig. 2a, c, e). The elements segregated more seriously under the lower heat input so that each microstructure region became narrower, including the martensite region, while the higher heat input was the opposite. However, the phase balance of ferrite and austenite of the clad metal was upset when the heat input was lower or higher.
Fig. 2 EDS composition analysis of transition zone of different heat inputs: (a), (b) 1.33 kJ/mm;
(c), (d) 0.74 kJ/mm; (e), (f) 1.93 kJ/mm.
Table 3 The mass fraction of major alloy elements of representative points (wt. %)
Points Fe Cr Ni
X70 97.42 0.13 0.16
1 97.13 0.55 0.21
2 96.78 1.05 0.29
3 94.2
4 2.88 0.83
4 89.16 6.64 1.86
5 82.49 11.73 3.06
6 79.26 14.16 3.73
7 77.59 14.64 4.85
8 75.67 15.96 5.09
9 73.90 17.16 5.42
10 73.04 17.80 5.58
11 71.61 18.41 6.13
12 69.71 19.46 6.39
13 68.47 21.03 6.57
3.3 Microstructural evolution within the transition zone
Despite its narrow width, there will be a complicated microstructural evolution within the transition zone owing to the deep gradient of alloy elements [5, 11]. Although the Schaeffler diagram (Fig. 3) was developed to predict the weld metal constitution of stainless steels, it can also be applied to dissimilar metal welds with carbon steels and stainless steels [7, 14, 19]. By plotting Cr eq and Ni eq on the Schaeffler diagram and connecting X70 pipeline steel and DSS2209 composition points by a tie line, the entire deposited metal constitution then lay along the tie line, as shown in Fig. 3. Moreover, the tie line can be used to determine the effect of the base metal dilution and provide insight into the microstructure that can form in the transition zone [7, 17, 19, 22]. According to Fig. 3, depending on the amount of dilution, the chemical composition of the transition zone normally cut across the composition range of ferrite + martensite (dilution > 97%), martensite (97% > dilution > 59%), ferrite + martensite (59% > dilution > 39%) and austenite + martensite + ferrite (39% >
dilution > 14%) on the Schaeffler diagram.
Fig. 3 Schaeffler diagram for predicting microstructural evolution within the transition zone [17] The TEM observation of the transition zone is shown in Fig. 4. Several mixed microstructure regions were observed within the transition zone: alpha ferrite (α)+ martensite (M), M, M + delta ferrite (δ), and M + austenite (γ) +δ, as illustrated in Fig.
4. Thus, the TEM results are consistent with the prediction of the Schaeffler diagram. In Fig. 4a and c, the microstructures are both composed of M and ferrite although they are δ and α, respectively, owing to different dilutions. The lattice parameter of the ferrite in Fig. 4a is a = b = c = 0.2931 nm, while that of Fig. 4c is a = b = c = 0.3659 nm. More alloy elements forming ferrite dissolved in δ, which caused the lattice distortion of δ. As can be seen in Fig. 4b, the lath martensite exhibited their special appearance, unlike those in homogeneous materials that divided the original austenitic into lath regions and had the same orientation [23]. The width of the typical dislocation martensite lath was 0.08 μm to 0.58μm. Fig. 4d indicated that martensite lath nucleated between the grain of delta ferrite and austenite because of the higher
free energy of the grain boundary. In addition, there were no precipitates existing in the transition zone under the selected welding parameters.
Fig. 4 TEM micrographs of mixed microstructures of different regions within the transition
zone
3.4 Microhardness measurements
Fig. 5 shows the variations of Vickers microhardness across the transition zone. Apparently, a dramatic fluctuation in the microhardness can be seen across the transition zone. A softened region with a microhardness value of 236 HV0.2was obtained in the base metal adjacent to the fusion line, while a hardened region with a microhardness value of 407HV0.2was observed in the clad layer. In addition, the microhardness of the base metal (276 HV0.2) was a little higher than that of the clad layer (259 HV0.2).
Fig. 5 Variations of Vickers Microhardness across the transition zone
4. Discussion
The cladding that developed during the fusion welding technique of dissimilar alloys will achieve a chemical composition intermediate between the two alloys [13, 19]. The chemical composition will, in turn, have a significant influence on the solidification mode and microstructural evolution within the transition zone as well as its mechanical properties [9, 24].
In general, for stainless steel, to simplify a multi-component system into the Fe-Cr-Ni ternary system, Cr eq and Ni eq were introduced and the solidification mode can be divided into the following four types [25, 26]:
A mode: L → L + γ → γ Cr eq / Ni eq < 1.25
AF mode: L → L + γ → L + δ + γ → δ + γ 1.25 < Cr eq / Ni eq < 1.48 FA mode: L → L + δ → L + δ + γ → δ + γ 1.48 < Cr eq / Ni eq < 1.95 F mode: L → L + δ → δ → δ + γ Cr eq / Ni eq > 1.95
Fig.6 illustrates the solidification modes according to chemical composition. The values of Cr eq and Ni eq can be estimated using the following equations [25]:
eq %35%20%0.25%Ni Ni C N Cu =+?+?+? (2)
eq %%0.7%Cr Cr Mo Nb =++ (3)
Fig. 6 Relative position of four solidification modes of duplex stainless steels in pseudobinary diagram. Dashed line indicates solidification path of the 14% dilution rate region [27]
The dilution rate varied sharply within the transition zone. On average, the dilution rate in the “A” region was in the range of 99% to 90%, and the dilution rate in the “B” region ranged from 90% to 14%. The variation of the dilution will affect the value of Cr eq / Ni eq . According to the Fe-Cr-Ni pseudobinary phase diagram (Fig. 6) and the Schaeffler diagram (Fig. 3), the solidification mode and microstructural evolution are dependent on the value of Cr eq / Ni eq within the transition zone. In view of this, it is useful to know the concentration of alloy elements within the transition zone that significantly influences the value of Cr eq / Ni eq including C, N, Cr, Ni, Mo, Mn, Cu, Si, and Nb. However, not all of the alloy elements can be obtained by EDS because of their low concentration and light element character. These elements can be calculated by the dilution that can be obtained by the concentration of the Fe element within the transition zone [20]. As long as the variation in dilution of the transition zone is
known, together with the nominal concentrations of alloy elements of the overlay metal and base metal, the C, N, Cu, Si, and Nb can be obtained by back-calculation. Of course, the hypothesis of this method is that C, N, Cu, Si, and Nb will be mixed in the liquid state to the same level within the transition zone as the Fe element. The variation in dilution evaluated by the Fe element within the transition zone is determined by equation 1. Thus, the mass fraction of low concentration and light elements can be calculated by equation 4 [19], once the dilution within the transition zone is known.
tz bm fm (1D)C C DC =+- (4)
Therefore, Cr eq and Ni eq will be obtained with the concentration of relevant elements. The dilution rate, the Cr eq / Ni eq value, the solidification mode, the microstructural evolution, and the distance from the fusion line (the clad layer side is positive, while the base metal is negative) within the transition zone are shown in Table 4 in detail.
Table 4 Dilution rate, Cr eq / Ni eq value, solidification mode, and microstructural evolution within
the transition zone
Distance (μm)Dilution (%) Cr eq/Ni eq Solidification mode Microstructures
-5.0 99 0.36 - α+M
-2.5 98 0.51 - α+M
0 90 0.99 A M
2.5 75 1.67 FA M
5.0 55 2.27 F M + δ
7.5 46 2.45 F M + δ
10.0 41 2.15 F M + δ
12.5 35 2.28 F γ + M + δ
15.0 30 2.36 F γ + M + δ
17.5 27 2.39 F γ + M + δ
20.0 23 2.32 F γ + M + δ
22.5 25 17
14
2.20
2.55
F
F
γ + M + δ
γ + M + δ
Table 4 shows that martensite existed in all four regions. As previous studies indicated [8, 20, 21, 28], the formation of intermediate compositions with high hardenability such as Cr and Ni (shown in Fig. 2b), and the rapid cooling rate caused the formation of martensite. Moreover, the lath martensite regions appeared “jumbled” in an original austenitic grain because the M s points varied with distance from the fusion line, as illustrated in Fig. 4b. In addition, the martensite layer was greatly dependent on the local hardenability (as determined by local composition) and local cooling rate [20]. According to Fig. 3, martensite can form only at a dilution above 14%, and single-phase martensite can form only at a dilution between 59% and 97% located approximately 3 μm from the fusion line in the clad layer.
T he “A” region (Fig. 1b), is composed of α and M, as shown in Fig. 4a and Table 4. This region had a high dilution rate, so the solidification mode and microstructural evolution were mainly affected by the composition of the base metal. For the
single-phase martensite region, where the alloy elements increased sharply, it covered a dilution approximately from 90% to 75% causing a solidification mode ranging from A to FA. In regard to the A solidification mode, austenite was the primary solidification phase, and the solid-state transformation of austenite to martensite would process after the completion of primary austenite solidification. In regard to single-phase martensite region from the FA solidification mode, after both the primary and eutectic ferrite completely transformed into austenite, all austenite transformed into martensite under the cooling rate exceeding the critical cooling rate. With the dilution decreasing, the value of Cr eq/ Ni eq increased and the solidification mode changed to the F mode. The M + δ and γ + M + δ regions were both F solidification mode in which the primary ferrite was the only phase. Then the solid-state transformation of austenite to martensite proceeded after some primary ferrite transformed into austenite. The transformation of austenite to martensite proceeded fully in the M + δ region, while that of the γ + M + δ did not.
The variation in microhardness across the transition zone was closely related to the microstructural evolution, as is illustrated in Fig. 5. The softened region in the base metal adjacent to the fusion line formed as a result of the decomposition of pearlite owing to the formation of proeutectoid ferrite. The formation of the single-phase martensite zone contributed to the sharp increase in microhardness. Then the microhardness decreased with the evolution of microstructures from M + δ to γ + M + δ.
5. Conclusions
This study addressed the microstructural evolution within the transition zone of
DSS2209 cladding on pipeline steel X70. The main conclusions are as follows:
(1) The main growth mode of the transition zone was the epitaxial nucleation with planar growth adjacent to the fusion line that shifts to cellular growth.
(2) The major alloy elements Fe, Cr, and Ni had a bond of steep composition gradient within the transition zone. The dilution estimated by the Fe element within the transition zone ranged from approximately 99% to 14%.
(3) The transition zone was composed of regions of α + M, M, M + δ, and γ + M + δwith the variation of the value of Cr eq / Ni eq from 0.36 to 2.55. The regions of α + M and M were narrower while the M + δ and γ + M + δ regions were broader.
(4) A dramatic fluctuation in the microhardness existed within the transition zone, which was consistent with the microstructural evolution. A softened region was obtained in the base metal adjacent to the fusion line while a hardened region was observed in the clad layer.
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