Elsevier

Vacuum

Volume 121, November 2015, Pages 159-165
Vacuum

Microstructure and corrosion behavior of Zr-702 joined by electron beam welding

https://doi.org/10.1016/j.vacuum.2015.08.005Get rights and content

Highlights

  • High vacuum EBW can effectively protect Zr-702 alloy from being polluted.

  • Needle-like α-Zr, residual β-Zr forms the weld.

  • The BM possesses lower corrosion resistance than HAZ and FZ.

  • The disappearing of Zr3Fe is responsible for the improving of corrosion resistance.

Abstract

Zirconium alloy (Zr-702) joints welded by electron beam welding technique were characterized by studying the microstructures and corrosion tests. X-ray diffraction and transmission electron microscope analysis showed that the joints were consisted of α-Zr and β-Zr phases. Small β-Zr blocks were distributed along the edges of lamellar α-Zr in the fusion zone. Analysis of corrosion behavior of the joints showed that the Zr3Fe intermetallic was presented in the base metal strongly affected the corrosion resistance. The heat-affected zone and fusion zone showed better corrosion resistance than that of the base metal.

Introduction

Zr-702 alloy is one of the most popular structural materials which is characterized as high corrosion resistance, excellent mechanical properties and good machinability. It is widely used in the fields of nuclear energy and petrochemical [1], [2], [3]. There are some problems in welding zirconium alloys despite its good weldability. Brittle phases would generate in the TIG welded joints due to the reaction between high activity Zr and H, N, O elements which could severely weaken the plasticity and toughness of the joints [4], [5]. So the joints should be shielded strictly during the welding process. Besides, the grains of fusion zone and heat-affected zone would tend to coarsen because of the poor thermal conductivity (0.167 W/(cm °C)) of zirconium alloy which is detrimental to the performance of the joints [6].

The high vacuum electron beam welding (EBW) can protect Zr-702 alloy from the contamination of N, H and O. Meanwhile, the low heat input and high energy density characteristic of EBW can prevent grains coarsening during the welding process [7], [8], [9], [10]. Thus, electron beam welding is a potential method in joining Zr-702 alloy. However, the welding of Zr-702 alloy by EBW has not yet been reported. The main aim of this work is to study the influence of electron beam welding process on the microstructures and corrosion behavior of Zr-702 alloy.

Section snippets

Experimental procedure

The Zr metal used in this study is a rolled sheet of Zr-702 alloy with a thickness of 4.3 mm which has been subjected to an annealing treatment. Chemical composition of the plate was given in Table 1. The microstructures of the material are composed of equiaxed α grains with their size ranging from 10 to 50 μm as shown in Fig. 1.

Zr-702 alloy was cut into 50 mm × 20 mm × 4.3 mm pieces. Prior to joining, all the surfaces of the Zr-702 alloy specimens were ground on SiC abrasive papers and then

Appearance of welded joint

The parameters used for welding Zr-702 alloy were shown in Table 2. Fig. 2a and Fig. 2b show the appearance of butt welded joint. All the welds obtained at different welding parameters appeared silvery white with densely and uniform ripples at the surface. It is demonstrated that the high vacuum degree of electron beam welding could prevent Zr-702 alloy from being contaminated. Fig. 2c, d and e show the macrosections obtained at different welding parameters. It can be found that sound weld was

Conclusions

On the basis of experimental researches carried out in present work, we can draw the following conclusions.

  • (1)

    The sound weld was achieved at the optimum parameters with the beam current of 16.5 mA and welding speed of 300 mm/min. And the surface of the obtained-weld appeared silvery white which indicates that electron beam welding can effectively protect Zr-702 alloy from being polluted.

  • (2)

    The XRD and TEM results show that the electron beam welded joints are consisted of α-Zr and β-Zr. The small β-Zr

Cited by (0)

View full text