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Volume 243, October 2021, 167478
Optik

Effect of nitrogen content on the microstructure and properties of the laser-arc hybrid welding joint of high nitrogen steel

https://doi.org/10.1016/j.ijleo.2021.167478Get rights and content

Abstract

This paper deals with laser-arc hybrid welding of high nitrogen steel with 1.2 mm diameter filler wire. The effect of nitrogen content in the weld on the microstructure and properties was analyzed. Results show that as the nitrogen content in the weld increases, ferrite growth is inhibited and the average hardness of the weld increases. As the nitrogen content in the weld increases, the tensile strength and elongation of the joints first increase and then decrease. The tensile strength and elongation of the joint with a nitrogen content of 0.52% reach a maximum, which are 928.9 MPa and 7.3%, respectively. As the nitrogen content in the weld increases, the impact energy of the joints first increase and then decrease. The impact energy of the joint with a nitrogen content of 0.52% has the highest impact energy of 51.7 J. Increasing nitrogen content can enhance the solution strengthening effect, but higher nitrogen content will cause nitrogen pores in the weld to form.

Introduction

High nitrogen steel (HNS) is a newly developed material in which the nitrogen content is more than 0.4% [1], [2]. HNS are steels that combine high strength and ductility with good corrosion resistance specifically used in the arms, aerospace, cryogenic and medical device industries [3], [4], [5]. Nitrogen is one of the most important alloying elements in HNS because it can increase strength without significantly losing ductility [6], [7].

Many studies have been performed on the weldability of HNS, specifically related to nitrogen loss. In the welding process of HNS, nitrogen is easy to precipitate from the base metal, enabling lower nitrogen content in the weld than that in the base metal, which results in the degradation of joint properties [8], [9], [10]. To solve the problem of nitrogen loss in the weld, some researchers increase the nitrogen content in weld by controlling the composition of shielding gas, such as increasing the nitrogen partial pressure or adding a small amount of oxygen [11], [12], [13], [14]. In addition, researchers have used nitrogen-containing filler materials to increase the nitrogen content in the weld, or increased the content of manganese, chromium, and other alloy elements in the weld to improve nitrogen solubility in stainless steel [15], [16], [17], [18]. However, some studies have shown that when the nitrogen content of the weld increases, the mechanical properties of the joints do not significantly improve. Also, when the nitrogen content is too high, the mechanical properties of the joints begin to decrease [10], [19], [20], [21]. This is contradictory to the normal relationship between nitrogen content and performance in the base metal, so further research on this relationship is necessary.

Laser-arc hybrid welding technology has attracted wide-ranging attention for its advantages of large welding penetration, good bridging performance, and high welding speed [22], [23], [24]. It has been widely used in automobiles, ships, oil pipelines, and other fields [25], [26], [27]. Researchers have studied the influence of laser-arc hybrid welding process parameters on the structure and properties of HNS-welded joints. The study found that the weld structure is composed of austenite and a small amount of ferrite; large heat input makes the dendrites coarser, resulting in poor weld impact performance [28], [29]. Bai et al. [30] studied the corrosion behavior of laser-arc hybrid welded joints of HNS. It was found that the growth mode and density of dendrite were the most important factors affecting the corrosion behavior.

Current research on the welding technology of HNS primarily focuses on arc and laser welding. The published literature on laser-arc hybrid welding technology of HNS is less and typically focuses on the effects of process parameters on microstructure and properties. In previous research, the author's team solved the problem of nitrogen porosity in laser-arc hybrid welding of HNS through detailed process control methodologies. Based on the previous research, the present work studies the effects of nitrogen content in the weld on the resultant microstructure and properties.

Section snippets

Materials

The material used in this research is HNS. HNS plates with dimensions of 8 mm × 400 mm × 100 mm were cleaned with acetone to remove dirt and contamination. The test was conducted to obtain full penetration of 8 mm-thick specimens for a butt weld configuration with a Y-groove preparation. A schematic view of the welded sample is shown in Fig. 1. The welding wire with a diameter of 1.2 mm was used as the filler material for the hybrid welding test. The chemical compositions of the base material

The effect of nitrogen content on the microstructure of HNS welds

Fig. 4 shows the effect of different nitrogen content on the microstructure of the weld. When the nitrogen content is 0.38%, the microstructure of the weld is composed of coarse lath ferrite and austenite matrix. As the nitrogen content increase from 0.42% to 0.49–0.52%, lath ferrite gradually transforms into dendritic ferrite. Also, with increasing nitrogen content, the tertiary dendrite arms of the ferrite gradually disappear, and the trunk of the dendrite becomes thinner. Since nitrogen is a

Conclusion

In this study, to obtain HNS-welded joints with different nitrogen contents, HNS was welded by laser-arc hybrid welding with different various shielding gases. The effects of nitrogen content in weld on the microstructure and properties of the weld were studied, and the following conclusions were drawn:

  • (1)

    As the nitrogen content of the weld increases, the ferrite morphology changes and the size decreases. Nitrogen as an austenite-forming element can inhibit ferrite growth.

  • (2)

    As the nitrogen content

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding

2021 Doctoral (Talent) Research Startup Fund (Natural Science) of Jilin Agricultural Science and Technology University: [2021] No. 5001.

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