Q235 carbon steel

Journal of Materials Processing Technology 145 (2004) 294–298

Microstructure and performance in diffusion-welded joints of Fe3Al/Q235 carbon steel Juan Wang, Yajiang Li∗ , Peng Liu, Huiqiang Wu Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University, Jinan 250061, PR China Received 30 October 2002; received in revised form 4 June 2003; accepted 22 July 2003

Abstract Fe3 Al and Q235 carbon steel were joined successfully at 1080 ◦ C for 60 min by vacuum diffusion welding technology. A Fe3 Al/Q235 diffusion-welded joint was thus formed. The microstructure features in the Fe3 Al/Q235 diffusion-welded joint were analyzed by a variety of characterization techniques such as scanning electron microscope (SEM) with energy-dispersive spectroscopy (EDS), electronic probe microanalysis (EPMA) and an X-ray diffractometer. The micro-hardness from Fe3 Al to Q235 carbon steel was measured with a micro-sclerometer. The shear strength in the Fe3 Al/Q235 diffusion-welded joint was evaluated with a strain and stress machine. The experimental results indicated that Al atom content decreases from 27 to 1% and Fe atom content increases from 73 to 96% from Fe3 Al base metal to Q235 carbon steel. The micro-hardness was 420–200 HM and the shear strength was 71.4 MPa in the Fe3 Al/Q235 joint. There were Fe3 Al, ␣-Fe (Al) solid solution and FeAl in the Fe3 Al/Q235 diffusion-welded joint without brittle phases and with high micro-hardness. Thus, the ideal Fe3 Al/Q235 joint can be obtained, which will meet the requirement for engineering structures. © 2003 Elsevier B.V. All rights reserved. Keywords: Fe3 Al/Q235 diffusion-welded joint; Microstructure; Shear strength; Phase constitution

1. Introduction Recently, Fe3 Al intermetallic compound has attracted a lot of researchers’ attention because of its unique properties such as low density, excellent resistance to oxidation and corrosion and high strength at elevated temperatures [1,2]. Therefore, it is expected that Fe3 Al can be applied to the petrochemistry industry, pressure vessels, electronic equipment and so on. Up to now, many researchers have successfully developed Fe3 Al intermetallic compounds with excellent mechanical properties and resistance to corrosion through studying the crystal structure, dislocation features and machinability of Fe3 Al [3,4]. However, Fe3 Al can easily suffer from weld cracks when being welded by melting welding processes, such as arc welding and gas-shielded welding. This is because the plasticity and toughness of Fe3 Al at room temperature is low [5–7]. So, the major obstacle to apply Fe3 Al at high temperatures and on structural material is its weldability. Q235 carbon steel belongs to low carbon steel with carbon content 0.25% and without alloy elements. Q235 car∗ Corresponding author. Tel.: +86-531-6656082; fax: +86-531-2609496. E-mail address: [email protected] (Y. Li).

0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2003.07.007

bon steel has excellent toughness and weldability because its carbon equivalent is less than 0.4%. So research on the joining technology of Fe3 Al and Q235 carbon steel may have many advantages in engineering. However, vacuum diffusion welding technology is used widely in joining dissimilar materials and brittle materials are hard to weld by common welding processes. The diffusion-welded joint of Fe3 Al and Q235 carbon steel is a good combination of the high temperature performance and resistance to corrosion. It may be used in valves and oil pipes under conditions of high temperature and unfavorable environment. The objective of this study is to investigate microstructure and performance in the Fe3 Al/Q235 diffusion-welded joint obtained by vacuum diffusion welding technology. Microstructure characteristics in Fe3 Al/Q235 joint were observed and the micro-hardness from Fe3 Al to Q235 carbon steel was measured. The shear strength in the Fe3 Al/Q235 joint was evaluated, and the phases formed in the Fe3 Al/Q235 joint were analyzed by means of X-ray diffractometer. These experimental results provide an important theoretical and practical base for improving the microstructure and performance of diffusion-welded joints and may open a new way to accelerate the wide application of Fe3 Al in engineering structures.

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2. Experimental Fe3 Al was prepared by induction melting in the vacuum furnace. Fe3 Al casting was hot rolled to a final plate, whose stress strength is about 455 MPa and extension ratio is 5%. The product was identified to be 80.20% Fe, 15.70% Al, 0.78% Cr, 1.93% Nb, 0.58% Zr, 0.41% B and 0.40% Ce. Fig. 1(a) shows the microstructure of Fe3 Al, which was polished and etched with a solution consisting of 70% HCl and 30% HNO3 . The grains have columnar shapes ranging from 150 to 200 ␮m in dimension. The bright small and white particles are second phase precipitation rich in Cr and Nb determined by EPMA. The sample was cut into a 100 mm × 20 mm × 20 mm size. The chemical compositions in Q235 carbon steel was: 0.25% C, 0.2% Si, 0.6% Mn and 98.95% Fe. Q235 carbon steel was machined out to the dimension of 100 mm × 20 mm × 8 mm. As seen in Fig. 1(b), ferrite is distributed with isometric grain after etching. Oxidizing film on all sample surfaces was removed by mechanical and chemical methods prior to vacuum diffusion welding. Fe3 Al and Q235 carbon steel were joined together using Workhorse-II vacuum diffusion welding equipment. Heating power was 45 kVA, vacuum degree 6.65 × 10−4 Pa. The joining was carried out in 12 MPa pressure at temperature 1080 ◦ C for a holding time of 60 min. The heating rate was 15 ◦ C/min in each run, followed by furnace cooling. Thus, the Fe3 Al/Q235 diffusion-welded joint was formed. The microstructure in the Fe3 Al/Q235 diffusion-welded joint was observed by means of an optical microscope and JXA-840 scanning electron microscope (SEM) with energy-dispersive spectroscopy (EDS). The composition in the microstructure was identified by electronic probe microanalysis (EPMA). The micro-hardness from Fe3 Al to Q235 carbon steel was measured by a micro-sclerometer, with a test loading of 25 g and a loading time of 10 s. The shear strength was evaluated with a strain and stress machine. The phases formed in the Fe3 Al/Q235 joint were analyzed with an X-ray diffractometer.

Fig. 2. Microstructure feature in the Fe3 Al/Q235 diffusion-welded joint (using SEM).

3. Results and discussion 3.1. Microstructure features Fig. 2 shows typical microstructure features in the Fe3 Al/Q235 diffusion-welded joint experiencing vacuum diffusion welding. The left side is Fe3 Al intermetallic compound with large columnar grain and the right side is Q235 carbon steel with isometric grain of ferrite. In the middle is the transition zone. A diffusion interface was obviously observed. The microstructure on the side of the Fe3 Al stretches into the side of Q235 across the joint continuously and they engage each other at the interface. This is explained by the diffusion of Fe and Al atoms during heating. Fig. 3 shows the line scan analysis of EPMA in the Fe3 Al/Q235 joint. The result indicates that the transition zone is about 0.15 mm. With the transition from Fe3 Al to Q235 carbon steel across Fe3 Al/Q235 interface, Al atom content decreases from 27 to 1% and Fe atom content increases from 73 to 96%. There is no obvious brittle phase in the Fe3 Al/Q235 joint according to Fe–Al equilibrium phase diagram. At the interface, there aggregate the second phase precipitations, whose composition was measured by means of EDS. The measured results are shown in Table 1.

Fig. 1. Microstructure feature of Fe3 Al and Q235 carbon steel: (a) Fe3 Al and (b) Q235 carbon steel.

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Fig. 3. The distribution of Fe and Al atoms content from Fe3 Al to Q235 carbon steel. Table 1 Content microanalysis of metallic elements at the interface in the Fe3 Al/Q235 joint (%) Number

Fe

Al

C

Cr

Si

Mn

1 2 3 4

79.66 78.90 78.04 78.77

14.31 15.90 15.45 13.10

0.65 0.61 0.50 0.22

1.18 1.18 1.32 1.26

0.07 0.10 0.10 0.06

0.21 0.23 0.23 0.20

It can be found that Fe and Al contents are nearly similar to Fe3 Al base metal, but the contents of C and Cr are obviously more than that in Fe3 Al base metal. This is favorable to improve the strength at the interface because C and Cr are elements, which can strengthen the property of the microstructure [8,9]. 3.2. Micro-hardness and shear strength The micro-hardness from Fe3 Al to Q235 carbon steel across the Fe3 Al/Q235 joint was measured in order to study the performance in the whole Fe3 Al/Q235 diffusion-welded joint. Measured results show that Fe3 Al intermetallic compound has a micro-hardness value of about 420 HM in its fully dense state. Close to Q235 carbon steel, the micro-hardness becomes lower due to an increasing number of Fe atoms and a decreasing number of Al atoms. Nearby Q235 carbon steel, the micro-hardness is lowered to 200 HM after diffusion welding. That is to say that the micro-hardness decreases from 420 to 200 HM in line, which fact might be attributed to the disordering transition of the intermetallic compound. This is related to the concentration of the changes in the iron and aluminum elements during diffusion welding. Shear strength in the Fe3 Al/Q235 joint was measured and the relation between loading and time when shearing the Fe3 Al/Q235 joint is shown in Fig. 4.

Fig. 4. Relation between loading and time when shearing the Fe3 Al/Q235 joint.

Fig. 5. Micro-voids morphology formed in the Fe3 Al/Q235 diffusionwelded joint.

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Fig. 6. X-ray diffraction diagram in the Fe3 Al/Q235 joint: (a) near to Fe3 Al and (b) near to Q235. Table 2 Result of X-ray diffraction diagram in the Fe3 Al/Q235 diffusion-welded joint Measured values

d (nm)

0.2881

0.2063

0.2024

0.1567

0.1454

0.1433

0.1331

0.1271

0.1169

0.1013

d (nm) hkl

– –

0.2040 220

– –

0.1670 222

– –

– –

0.1330 331

0.1290 420

– –

– –

␣-Fe (Al)

d (nm) hkl

– –

– –

0.2027 110

– –

– –

0.1433 200

– –

– –

– –

0.1013 220

FeAl

d (nm) hkl

0.2890 100

0.2040 220

– –

– –

0.1450 200

– –

– –

– –

0.1180 211

– –

Data from JCPDS Fe3 Al

With the increase of loading time, the pressure on the Fe3 Al/Q235 joint is increasing. When the pressure is up to maximum 5712 N, the Fe3 Al/Q235 joint is fractured. So the shear strength is calculated to be 71.4 MPa. EDS and XRD analyses of the fracture surface morphology have revealed that the fracture occurs near the side of Q235 carbon steel. This is because micro-voids were formed during diffusion welding and the micro-voids morphology is shown in Fig. 5. When Fe3 Al and Q235 carbon steel were diffusion welded, the irregularities of the surface flattens into a grain boundary at the points of contact with micro-voids. With the application of pressure and the elevation of heating temperature, Fe and Al atoms begin to diffuse into each other and the grain boundary and micro-voids disappear. But there retain a few of micro-voids near the side of Q235 carbon steel because of the hamper by grain boundary.

dards (JCPDS) indicate that there are Fe3 Al, ␣-Fe (Al) solid solution and FeAl in the Fe3 Al/Q235 joint. And there is no obvious brittle phase containing higher aluminum such as FeAl2 (HM1030), Fe2 Al5 (HM820) or FeAl3 (HM990) [10]. At the interface to the Fe3 Al base metal, there is little of FeAl with high aluminum content whose brittleness is large. ␣-Fe (Al) solid solution has B2 type b.c.c. lattice, this is because the number of Al atoms is reducing with the diffusion of the aluminum element. The change from FeAl, Fe3 Al to ␣-Fe (Al) is attributed to a transfer of the Al atoms from unstable interstitial sites in the iron lattice to a more stable and ordered B2 structure, in which they occupy specific iron lattice sites [11]. This helps to avoid weld cracking when Fe3 Al intermetallic compound and Q235 carbon steel are diffusion welded.

3.3. Phase constitution

4. Conclusions

The phase constitutions formed in the Fe3 Al/Q235 joint were further researched by means of X-ray diffractometry using a copper target. The working voltage is 40 kV and current is 150 mA. X-ray diffraction diagram of phase is shown in Fig. 6. The X-ray diffraction results (Table 2), compared with the data from Joint Committee Powder Diffraction Stan-

1. The interface with second phase precipitations rich in C and Cr was observed. Al atom content decreases from 27 to 1% and Fe atom content increases from 73 to 96% from Fe3 Al base metal to Q235 carbon steel. 2. The ideal joint of Fe3 Al/Q235 can be obtained by vacuum diffusion welding technology at 1080 ◦ C for 60 min and under the pressure of 12 MPa. The micro-hardness was

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420–200 HM and the shear strength was 71.4 MPa in the Fe3 Al/Q235 joint. 3. There were Fe3 Al, ␣-Fe (Al) solid solution and FeAl in the Fe3 Al/Q235 diffusion-welded joint without brittle phases and with high micro-hardness. This helps to avoid weld cracking of the Fe3 Al/Q235 joint. Acknowledgements The work was supported by the foundation of the National Key Laboratory of Advanced Welding, Technology, Harbin Institute of Technology, PR China. References [1] D.H. Kim, B. Cantor, Structure and decomposition behavior of rapidly solidified Fe–Al alloys, J. Mater. Sci. (29) (1994) 2884–2889. [2] C.G. Mckameey, J.H. Devan, P.E. Tortorelli, et al., A review of recent development in Fe3 Al-based alloys, J. Mater. Res. 8 (6) (1991) 1779–1804.

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