Accepted Manuscript Title: Effect of activator on mechanical properties and intercrystalline corrosion resistance of austenitic stainless steel weld Author: Yangchuan Cai Zhen Luo Mengnan Feng Zuming Liu Zunyue Huang Yida Zeng PII: DOI: Reference:
S0924-0136(16)30047-4 http://dx.doi.org/doi:10.1016/j.jmatprotec.2016.02.014 PROTEC 14728
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
27-11-2015 3-2-2016 22-2-2016
Please cite this article as: Cai, Yangchuan, Luo, Zhen, Feng, Mengnan, Liu, Zuming, Huang, Zunyue, Zeng, Yida, Effect of activator on mechanical properties and intercrystalline corrosion resistance of austenitic stainless steel weld.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2016.02.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of activator on mechanical properties and intercrystalline corrosion resistance of austenitic stainless steel weld
Yangchuan Caia, b, Zhen Luoa, b, *, Mengnan Fenga,b, Zuming Liua,b, Zunyue Huanga, b, Yida Zenga, b
School of Materials Science and Engineering, Tianjin University, Tianjin 300072,
Tianjin Key Laboratory of Advanced Joining Technology, Tianjin University, Tianjin
* Corresponding author at: 25-C-1201, School of Materials Science and Engineering, Tianjin University, No.92 Weijin Road, Tianjin 300072, China. Tel: +86-22-27406602. E-mail address: [email protected]
Abstract 5 mm thick austenitic stainless steel was welded using an A-TIG welding process with in-house developed activators. The microstructure, mechanical properties, and intergranular corrosion resistance of the weld joint were characterized. The microstructure of conventional TIG and A-TIG welds were all austenite with a small amount of δ-ferrite. The included angle between the fusion line and rolling direction of the conventional TIG and A-TIG welds were nearly 0° and 90°, respectively. The microstructure of the A-TIG weld had more and finer δ-ferrite under conditions of less heat input. Compared with the conventional TIG weld, the tensile strength of the A-TIG weld was enhanced approximately 10%, and the elongation was increased by 30%. The microhardness of the conventional TIG weld was found to be 280 HV, and the microhardness of the A-TIG weld was 210HV. The corrosion rates of substrate, conventional TIG and A-TIG welds were measured and found to be 0.056mm/h, 0.110mm/h and 0.065mm/h respectively. The chromic carbide content in the conventional TIG weld was higher than that of the A-TIG weld.
Keywords: Austenitic stainless steel; Conventional TIG welding; A-TIG welding; Intercrystalline corrosion resistance
1. Introduction Austenitic stainless steel is widely used for the welding of chemical containers because it confers good corrosion resistance. Austenitic stainless steel has a small 2
coefficient of thermal conductivity and a large linear expansion coefficient; as a consequence, the steel is prone to substantial deformation during the welding process. Therefore, welding methods with concentrated energy such as conventional TIG welding are necessary chosen to weld these steels. However, Arivazhagan et al. (2015) found that the highest penetration achieved in conventional TIG welding was about 3mm without a groove, which limited its production efficiency. In addition, Huerta et al. (2005) described that intercrystalline corrosion occurred along the austenite grain boundary of the weld in chemical media. The material surface maintained a metallic luster without any signs of being destroyed even after intergranular corrosion had occurred, as Timofeev et al. (2004) reported. The strength of materials can decrease if the binding force between two grains is significantly reduced. Therefore, intercrystalline corrosion is difficult to observe but could allow sudden destruction of the material. To avoid this, it is important to develop a new type of welding method to enhance the production efficiency over conventional TIG welding, and improve the intergranular corrosion resistance of the weld. Ding et al. (2001) found that the penetration of A-TIG weld could be increased 1-3 times compared with conventional TIG weld; groove preparation was not required for A-TIG using a plate of medium thickness. A-TIG may be an attractive welding method for austenitic stainless steels due to its high welding efficiency and low heat input compared with the conventional TIG welding. For these reasons, the successful application of A-TIG to the welding of austenite stainless steel is a current focus of welding technology research efforts. Liu et al (2015) utilized a self-developed 3
activated flux to weld 8 mm thick AISI304 stainless steel with excellent formation and mechanical properties. Ahmadi et al. (2015) investigated the effect of the coating density of activating flux on the resulting weld pool shape and oxygen content in the weld after welding. The effect of activator on the penetration and mechanical properties of weld has been investigated, but the effect of A-TIG on intergranuar corrosion resistance remains unclear. Consequently, self-developed activated flux was utilized to weld austenitic stainless steel. The microstructure, mechanical properties, and intergranular corrosion resistance of the weld were studied systematically. The mechanism of activators on the intercrystalline corrosion resistance was also explored. 2. Experimental procedures 304 austenitic stainless steel scaled 300 mm × 150 mm × 5 mm in bulk were used as a work-piece in the study. The chemical composition of 304 austenitic stainless steel are shown in the Table 1. The chemical composition of tested activators are shown in the Table 2. The particle size of each component in the fluxes was about 50μm, and the purity level was 99%～99.5%. The substrates were grounded with sand paper and then cleaned with alcohol to remove any organic elements. Then, the fluxes were mixed with alcohol, and the concentration of the fluxes was similar to the cream, namely, about 2.6 g fluxes were added in 1 mL alcohol. Finally, the fluxes were coated on the surface of the work-piece with a brush, and the width of the fluxes was about 80mm; the thickness of the flux layer was about 0.2 mm (about 7.6 g). Table 3 lists the welding parameters 4
used in the experiments. 1# parameters were used for A-TIG; 2# and 3# parameters were used for conventional TIG; Wires were used in the conventional TIG welding process, the diameter was 1.2 mm, and the wire feed rate was 800 mm/min. Metallographic samples were mounted, polished and etched by an etchant (the ratio of nitric acid, hydrochloric acid and glacial acetic acid was 1:1:1) in line with standard procedures. The microstructure of welds was observed using the Olympus GX51 optical microscope. The size and content of δ-ferrite and austenite within conventional TIG and A-TIG welds were determined using an IPS500 image analyzer. The distribution of chromium element and carbon element at the austenitic grain boundary was studied via S4800 scanning electron microscope (SEM). The kind of chromic carbide in the weld was detected by D/MAX-2500 X-ray diffraction (XRD). Referring to ISO 6892-1 (2009), the tensile test was measured by a WE-1000B tensile machine, and the dimension of the tensile specimen is illustrated in Fig.1. S4800 scanning electron microscope was used to observe the micro-morphology of tensile fractures after tensile test. The microhardness of the weld was measured by a MHV2000 type digital microhardness tester with a 100 g load and 10 s dwell time, the distance between two points was 0.2 mm. The specific location of microhardness is shown in the Fig.1. An intergranular corrosion test was performed according to ASTM A 262-02a standard to study the difference in the extent of corrosion resistance between a conventional TIG weld and an A-TIG weld, using the set-up as shown in Fig.2. A test solution of 600 mL ferric sulfate-sulfuric acid was used at approximately 50%. The 5
dimension and sampling position is shown in Fig.1. The thickness of samples was about 3.6 mm after being machined on the milling machine. The weight before intergranular corrosion of substrate, conventional TIG and activating flux TIG specimens was 16.312g, 16.318 g and 16.308 g respectively. Corrosion rates were calculated as millimeters of penetration per month, as follows:
Where t is time of exposure, h; A is area, cm2; W is weight loss, g; d is density, g/cm3, for chromium-nickel steels, d=7.9 g/cm3. The microstructure of welds after intergranular corrosion test was observed using the Olympus GX51 optical microscope. 3. Results and discussion 3.1. Microstructure of welds Fig.3 shows the microstructure of 304 austenitic stainless steel welds after conventional TIG welding and A-TIG welding. The microstructure of the two welds is austenite with a small amount of δ-ferrite, with dendritic δ-ferrite distributes among the austenite. Table 4 shows the concrete size and content of δ-ferrite and austenite within conventional TIG and A-TIG welds. The A-TIG weld has more δ-ferrite and finer grain compared with the conventional TIG weld. Furthermore, A-TIG weld shows higher penetration and less width than the conventional TIG weld. As a result, the fusion line of the conventional TIG weld is nearly parallel to the rolling direction, as shown in Fig.3 (a). In contrast, the fusion line of the A-TIG weld is perpendicular to the rolling direction, as shown in Fig.3 (b). 6
3.2. Mechanical properties 3.2.1 Effect of activators on strength Fig.4 shows the tensile test results of the A-TIG and conventional TIG welded parts. The tensile strength of the conventional TIG weld is about 630 MPa, and that of A-TIG weld is about 690 MPa. Compared to the conventional TIG weld, the tensile strength of A-TIG weld is enhanced approximately 10%, and the elongation is increased by 30%. Many cleavage fracture morphologies are observed in the conventional TIG weld fracture, as shown in Fig.4 (b). Although some dimples are found, they are quite small. There are more dimples of larger size observed in the A-TIG weld fracture, and the shear lip of the dimples is longer, as shown in Fig.4 (c). The microstructure of the weld can be used to interpret the difference of tensile test results between the conventional TIG weld and the A-TIG weld. The grain size (i.e. δ-ferrite and austenite) of A-TIG weld is less than that of the conventional TIG weld. The tiny grain has the effect of fine-grain strengthening on the weld joint. Furthermore, A-TIG weld has more δ-ferrite compared with conventional weld. The presence of the δ-ferrite could disorganize the directionality of the coarse austenite grain leading to more highly refined grains. Additionally, δ-ferrite could dissolve more sulfur and phosphorus than γ-austenite, as Goncalves et al. (2015) concluded. Therefore, the additionaly and finer δ-ferrite in the A-TIG weld could improve the mechanical properties and crack resistance. 3.2.2 Effect of activators on microhardness The microhardness of the conventional TIG weld is higher than that of the 7
A-TIG weld, as shown in Fig.5. The maximum microhardness of A-TIG weld is approximately 210 HV, compared to a maximum microhardness of conventional TIG weld of about 280 HV. Arivazhagan et al. (2013) found that the mechanical properties of the weld can be reduced as a result of high heat input that could deteriorate the microstructure. The Eq2 is used to evaluate the heat input (KJ/mm):
Where V is arc voltage, I is arc current, S is welding speed, and 0.9 is the arc efficiency. In the welding process, A-TIG requires less current and voltage, and higher welding speed, as shown in Table 3. Hence, the A-TIG welding has less heat input than conventional TIG welding. 3.3. Intergranular corrosion The weight after intergranular corrosion test of substrate, conventional TIG and A-TIG intergranular corrosion specimens was 16.301g, 16.296 g and 16.295 g respectively. The degree of corrosion measured as millimeter per month was calculated after the physical parameters were substituted into Eq1. The corrosion rates of substrate, conventional TIG and A-TIG welds were calculated to be about 0.056 mm/h, 0.110 mm/h and 0.065 mm/h respectively, indicating that the intergranular corrosion resistance of the A-TIG weld is superior to the conventional TIG weld. The microstructure of conventional TIG weld, A-TIG weld, and the substrate after the intergranular corrosion test can intuitively indicate the ability of intergranular corrosion resistance, as shown in Fig.6. The austenite in the conventional TIG weld presents mutual independence because of the original grain boundary disappeared 8
after intergranular corrosion test, as shown in Fig.6 (a). The influence of the corrosion medium on the austenitic grain boundary in the A-TIG weld is not obvious, and the corrosion condition is similar to the substrate, as shown in Fig.6 (b and c). The poor chromium theory can be used to interpret the intergranular corrosion of the austenitic stainless steel weld, as shown in Fig.7. Taiwade et al. (2013) found that the solid solubility of the carbon in the austenite increased with increased temperature. The carbon is in the hyper-saturated state after the solution treatment of austenitic stainless steel, as shown in Fig.7 (a). The hyper-saturated carbon will diffuse to the grain boundary quickly when the austenite stainless steel is heated to 500～700℃, because the average solid solubility of the carbon in the austenite is less than 0.01% at 500～700 , as shown in Fig.7 (b). The carbon will consume the chromium near the grain boundary to form chromic carbide, as shown in Fig.7 (c). Timofeev et al. (2004) calculated that the diffused activation energy of chromium within the grain and grain boundary was about 540 KJ/mol and 240 KJ/mol respectively; thus, the diffused speed of chromium in the grain was lower than that of grain boundary. Results indicate that most of the chromium element in chromic carbide comes from the grain boundary because there is insufficient time for the chromium in the grain to diffuse to the grain boundary during the cooling process. The amount of chromium near the grain boundary will be reduced observably. The austenitic stainless steel becomes corrosion resistant when the amount of chromium is at least 10～12%. So, when the content of the chromium in the grain boundary is lower than this, a “chromium deficiency area” will exist. A corrosion cell will be formed due to the difference of the 9
electrochemical performance between the chromium deficiency area and the surrounding matrix, where the chromium deficiency area acts as the positive pole, and the matrix acts as the negative pole. The chromium deficiency area will become corroded in the intergranular corrosion test, as shown in Fig.7 (d). X-ray diffraction was used to test and analyze the kind and content of phases in the two welds before the intergranular corrosion test, as shown in Fig.8. Both of the two welds contain CrFe7C0.45 and Fe3Ni2 solid solutions, ferric carbide (Fe3C) and some chromic carbide, as shown in Fig.8. The chromic carbide in conventional TIG weld is Cr23C6, and A-TIG weld has Cr3C2. Heat insulation method (i.e. Eq 3 and Eq 4) was used to calculate the mass fraction of the chromic carbide in the two welds, as Ying-Ji Chuang reported. ∑
(Eq 4) Where Xi is the mass fraction of the phase i, I is diffraction intensity, and R is the reflection ability of phase. The values of I and R in the conventional TIG and activating flux TIG welds were obtained via analyzing the PDF cards in jade5 software, as shown in Table 5. The mass fraction of the chromic carbide in the conventional TIG and A-TIG weld is about 1.036% and 0.095% respectively. The analytic result suggests that the conventional TIG weld contains more chromic carbide than the A-TIG weld. Distribution of chromium carbide in the weld before the intergranular corrosion test was measured by scanning electron microscope, as shown in Fig.9. The 10
distribution law of the C element and the Cr element in the conventional TIG weld is similar to A-TIG weld, namely the two elements are uniformly distributed in the grain, and the content at the grain boundary increases significantly. Results indicate that the chromic carbides detected from XRD are mainly distributed in the austenite grain boundary. Further analysis finds that the variation of C and Cr in the grain and grain boundary of conventional TIG weld is higher than that of A-TIG weld. As a result, the conventional TIG weld would have a higher potential difference between the grain and grain boundary. Eventually, the grain boundary of the conventional TIG weld becomes quite corroded in the intergranular corrosion test. Based on the above data analysis, the A-TIG weld shows improved intergranular corrosion resistance over conventional TIG weld due to the following reasons. (1) The segregation of supersaturated carbon in the austenite of A-TIG weld is reduced due to the welding process with less heat input. The potential difference between the grain boundary and the matrix decreases because of lower chromic carbide concentration in the grain boundary. (2) The amount of δ-ferrite in the A-TIG weld is greater than the amount in the conventional TIG weld. The δ-ferrite has greater solubility of chromium and expansion speed compared to austenite. As a result, the poor chromium phenomenon will be improved when δ-ferrite distributes along the austenite boundary.
4. Conclusions (1) The microstructure of both the conventional TIG and A-TIG welds is found to be austenite with a small amount of δ-ferrite. The included angle between the fusion line and rolling direction of the conventional TIG and A-TIG welds is close to 0° and 90° respectively. (2) The tensile strength and elongation of the A-TIG weld are increased compared to the conventional TIG weld. The microhardness of the conventional TIG weld is higher than A-TIG weld. (3) The A-TIG weld shows better intergranular corrosion resistance than the conventional TIG weld due to the higher content of δ-ferrite and less heat input during the welding process.
Acknowledgments This research is supported by National Nature Science Foundation of China (Grant 51275342 and 51405334).
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Fig.1 Dimensions of the tensile specimen, intergranular corrosion specimen and microhardness specimen.
Fig.2 Apparatus for ferric sulfate-sulfuric acid test.
Fig.3 Microstructure of welds: (a) Conventional TIG weld; (b) Activating flux TIG weld.
Fig.4 (a) Variation curves of strength; (b) Micro-topography of conventional TIG welds fracture; (c) Micro-topography of activating flux TIG welds fracture. 16
Fig.5 Microhardness distributions of welds.
Fig.6 Microstructure of welds after intergranular corrosion test: (a) Conventional TIG weld; (b) Activating flux TIG weld; (c) Substrate.
Fig.7 Schematic diagram of intergranular corrosion.
Fig.8 XRD analysis of the conventional TIG and activating flux TIG welds.
Fig.9 Distribution of C element and Cr element in the grain and grain boundary
Table 1 Chemical composition of 304 austenitic stainless steel (wt.%). C
Table 2 Chemical composition of activators (wt.%). Boron Oxide and Chromium Silica Oxide and Titanium Sodium Oxide
Table 3 Welding parameters. Welding
Shield gas and
Welding Group Current ( A)
Table 4 Size and content of δ-ferrite and austenite within conventional TIG and A-TIG welds. Group
Table 5 Values of I and R in the conventional TIG and activating flux TIG welds. Phase
100, 17.1 and 38.9
100, 17.1 and 38.9
3.2, 100 and 9.7
3.2, 100 and 9.7