P16
Fe–Cr 合金融体の表面張力に及ぼす組成と酸素吸着の影響
Effects of Composition and Oxygen Adsorption onSurface Tension of Molten Fe–Cr Alloys
堀内豪暉, 朝見海斗, 西村美咲, 清宮優作, 小澤俊平
Goki Horiuchi, Kaito ASAMI, Misaki NISHIMURA, Yusaku SEIMIYA, and Shumpei OZAWA
千葉工業大学，Dept. Adv. Mater. Sci. Eng., Chiba Institute of Technology

1. Introduction

Welding of stainless steel is indispensable for the manufacture of structures in various fields. In
heavy and medium industries, such as shipbuilding, automotive, pipelines, and pressure vessels,
welding is often performed using shielding gases like CO 2, typically in combination with other gases.
This technique, known as Metal Active Gas (MAG) welding, is used to protect the weld pool fromatmospheric contamination. The presence of CO2 gas stabilizes the welding arc through a thermal pinch
effect, as its decomposition reaction is endothermic. The gas also contributes to achieve deeper weldpenetration. This is because the adsorption of oxygen, generated by the decomposition reaction,induces an inward flow in the weld pool by altering its surface tension. However, the effect of oxygenadsorption on the surface tension of high-melting-point materials, such as stainless steels, has not been
fully investigated. This is primarily due to the difficultly of preventing contamination of the sample
from the supporting materials at elevated temperatures when using conventional container techniques.
In this study, we measured the surface tension of molten Fe–Cr alloy, which are the main
components of stainless steels, under a containerless condition by the oscillating droplet method using
electromagnetic levitation. The purpose of this study was to examine the effects of sample composition,temperature, and oxygen adsorption on the surface tension of the molten sample.

2. Experimental procedure
An ingot of Fe–Cr alloy was prepared from high-purity iron and chromium, each with a purity of 99.9
mass% or higher. The ingot was cut into a nearly cubic shape with a mass of about 1 g, and then chemicallyetched in a Nital solution using an ultrasonic cleaning machine, followed by cleaning with acetone. Thesample was electromagnetically levitated and then melted under a mixed gas of Ar–He–H2–CO2 flowing at arate of 2L/min. The oxygen partial pressure of the gas (Po2) was controlled through the gas phaseequilibrium between H2 and CO2. To confirm the Po2 of the inlet gas, a zirconia-type oxygen sensor was used,operating at 1008 K, considering the temperature dependence of the Po2 of the gas. The temperature of thedroplet was controlled by varying the partial pressures of argon and helium gases, which have different
thermal conductivities, using a monochromatic pyrometer. After the indicated temperature and Po2 values
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had become constant, the oscillation behavior of the droplet was observed from above using a high-speed
video (HSV) camera. The frequencies of the surface oscillations and those of the center of gravity wereanalyzed from time-sequenced data of the HSV images using fast Fourier transformation (FFT). The surface
tension of molten Fe-Cr alloy was calculated from these frequencies using the Rayleigh equation 1), calibrated
with the Cummings and Blackburn equation 2).

3. Results and Discussion
Figure 1 displays the surface tension of molten Fe–Cr alloys, measured by EML at a Po2 of 10-6. As thesample temperature rises, the surface tension of molten iron increases up to about 2100 K, after which it
begins to decrease. Although oxygen adsorption decreases the surface tension of molten iron at lower
temperatures, oxygen desorbs from the melt surface with increasing temperature due to the exothermic
nature of its adsorption reaction. This leads to a rise in surface tension, approaching the pure state value ofsurface tension. Consequently, the surface tension decreases above about 2150 K, when oxygen adsorption
becomes less effective.
In the Fe-Cr alloy, an increase in chromium content leads to decrease in surface tension. However, anincrease in the surface tension with temperature is consistently observed regardless of the samplecomposition. This result can be linked to the inward flow observed in MAG weld pool. The surface tensionof molten alloy is the highest at the high temperature region of the pool center and lowest at the lowtemperature region of the pool edges. In this case the thermocapillary Marangoni convection flows radiallyfrom the edge to the center in a weld pool.

4. Summary
The effect of temperature, sample composition and oxygen adsorption on the surface tension of molten
Fe-Cr alloys were measured by oscillating droplet method using EML at Po2 of 10-6 Pa. The surface tension ofmolten alloy increases with increasing temperature due to oxygen adsorption and desorption reaction,regardless of sample composition. The surface tension of molten Fe–Cr alloy was decreased as the chromium
content was increased.

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Surface tension σ, 10-3 N·m-1

2000
Po2 ≈ 1.0×10-6 Pa

1800
Fe1600Fe-25at.%Cr140050at.%Cr75at.%Cr1200
1700

190021002300
Temperature T, K2500

Figure 1. Surface tension of molten Fe-Cr alloys measured under the Po2 of 1.0×10-6 Pa, as functions oftemperature and sample composition

Acknowledgement
This work was financially support by JSPS KAKENHI under Grant No. P20H02453
References
1） Lord Rayleigh: On the Capillary Phenomena of Jets. Proc. R. Soc. Lond., 29 (1879) 71,DOI:
https://doi.org/10.1098/rspl.1879.0015
2） D.L. Cummings, D.A. Blackburn: Oscillations of magnetically levitated aspherical droplets. J. Fluid Mech,224 (1991) 395, DOI: https://doi.org/10.1017/S0022112091001817© 2023 by the authors. Submitted for possible open access publication under theterms and conditions of the Creative Commons Attribution (CC BY) license(http://creativecommons.org/li censes/by/4.0/).

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