Effect of cerium oxide flux on active flux TIG welding of 800 MPa super steel

doi:10.1016/j.jmatprotec.2015.11.008

Journal of Materials Processing Technology

Volume 230, April 2016, Pages 80-87

https://doi.org/10.1016/j.jmatprotec.2015.11.008 Get rights and content

Abstract

The effect of cerium oxide on penetration, microstructure and mechanical properties of BS700MC super steel weld was investigated. Cerium oxides changed the penetration of the weld via changing the Marangoni convection in the weld pool, and the heat input was different with different content of cerium oxide. The results demonstrate a gradual decrease of the heat input as the content of cerium oxide increased from 0 to 15%. The heat input increased significantly for the content of cerium oxide surpassed 15%. A certain amount of the cerium oxides changed the microstructures of the welds by transiting the cerium element from the flux layers to the weld metal. The content of acicular ferrites reached the maximum when the cerium oxide content was 15%, the strength, low-temperature impact and micro-hardness achieved their optimal value accordingly. As the content of cerium oxide is less or more than the optimal value, the content of proeutectoid ferrites increased, and the values of the mechanical properties of the weld deteriorated.

Introduction

Activating flux TIG (A-TIG) is a method that can improve weld penetration by brushing a thin layer of activators onto the surface of the work piece before welding with conventional TIG. A-TIG welding, compared with conventional TIG welding, provides an advantage, which is that the penetration and productivity can be increased 1–3 times. Ding et al. (2001) reported that groove preparation is not needed with A-TIG for 3–10 mm thick plates.

Lu et al. (2002) found that rare-earth oxides used as metal oxide activators may increase the penetration. The arc contraction theory and the reversal of the Marangoni convection in the weld pool are the two main theories used to account for the mechanism of penetration increasing. Specific researches are needed because different activators have the different mechanism of penetration increasing. The microstructure and mechanical properties of super steel will deteriorate because the grains of the heat-affected zone grow during the welding thermal cycle. Using rare-earth oxides, the weld receives less heat input compared to conventional TIG; therefore, the microstructure and mechanical properties of the weld may be improved. The proper content of rare-earth oxides is different for different materials. 800 MPa super steel, as a new type of low-alloy high-strength structural steel, is widely used in bridges, ships and engineering machineries. So, the explicit content of rare-earth oxides used to weld 800 MPa super steel is important and necessary. Also, Rare-earth elements can promote the nucleation of acicular ferrites to enhance the mechanical properties of the weld joints, as described by Song et al. (2015). Tang et al. (1987) found that a certain quantity of rare-earth elements may transit to the weld pool from the flux layers during the welding process. But Tang et al. did not do any experimental verification to support this viewpoint.

Consequently, cerium oxide was chosen to study the mechanism of penetration increase. Then, the influence of heat input and cerium element on the microstructure and mechanical properties of 800 MPa super steel were also studied systematically.

Section snippets

Materials

The 800 MPa BS700MC super steel scaled 300 mm × 150 mm × 5 mm in bulk was used as a substrate in the study. The chemical composition of the BS700MC super steel is shown in Table 1. The chemical compositions of the activators are shown in Table 2, and the particle size and purity level of each component in the fluxes are shown in Table 3.

To study the effect of cerium oxide on weld pool convection direction, the work-pieces were milled to create two 1 mm-width grooves, each of which was 1.5 mm away from

Penetration of weld

The macroscopic morphology of the welds and the penetration curves of the welds are illustrated in Fig. 3, Fig. 4. The penetration of the welds increases on a gradual basis as the cerium oxide content increases to 15%. These results, however, demonstrate a slower decline as the cerium oxide content surpasses 15%. Also, the penetration of A-TIG welding is apparently higher than those of conventional TIG welding.

Penetration increase mechanism

Vasudevan et al. (2010)illustrated that, the arc contraction theory and the reversal

Conclusions

(1)
Cerium oxide can increase the penetration of super steel weld by changing the Marangoni convection in the weld pool. As cerium oxide content is optimized at 15%, the penetration reaches the maximum value.
(2)
Transiting proper amounts of the cerium element from the flux layers to weld metals could effectively refine the microstructure of the weld and promote the nucleation of acicular ferrites.
(3)
As cerium oxide content is optimized at 15%, the microstructure of the weld generates a lot of acicular

Acknowledgments

The authors wish to acknowledge Dr. Wang Fei and Mr. Feng Yue Qiao for useful technical discussions and support during the process of this project.

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Effect of cerium oxide flux on active flux TIG welding of 800 MPa super steel

Abstract

Introduction

Section snippets

Materials

Penetration of weld

Penetration increase mechanism

Conclusions

Acknowledgments

Scripta Materialia

Mater. Lett.

A study of microstructure and mechanical properties of grade 91 steel A-TIG weld joint

J. Mater. Eng. Perform.

Effect of flux on A-TIG welding of mild steels

Trans. JWRI

Surface active element effects on the shape of GTA, laser, and electron beam welds

Weld. J.