72 1 Introduction Oxygen plasma arc cutting (PAC) is a high-performance method used for cutting mild steel 6 to 32 mm-thick. It is a widely used cutting process in various manufactur- ing fields such as construction, shipbuilding, bridges and other welded structures [1]. Such industrial fields require improved plasma arc cutting to enable either cutting thicker material or cutting faster. Increasing electrical cur- rent improves the performance; however, it significantly reduces the life of consumables electrodes and nozzles. The life of a consumable is measured either by the time span it is used for or by the number of cuts possible with one unit. Figure 1 shows the relationship between electric current and maximum cutting thickness, and the relation- ship between electric current and consumable life. The data was obtained from the PAC machines manufactured by Komatsu working in actual industrial fields. The con- sumable life indicates the number of cuts possible with 20 s of arc-on time. The maximum cutting thickness indi- cates the performance. Increasing the current allows PAC to cut thicker material; however, it reduces the life of con- sumables significantly. As shown in Figure 1, at 400 A we can cut a plate 40 mm-thick; however, this does not provide sufficient consumable life, it could only cut about 50 parts. Therefore, 300 A is the practical maximum cur- rent. Long life of consumables has become the most important subject for PAC, long life is required in order to increase the current and reduce operating costs, which are mostly the cost of consumables. Therefore, many studies for that have been conducted on increasing consumable ABSTRACT An experimental study was conducted on the consumption of hafnium electrodes during oxygen plasma arc cutting (PAC). Hafnium (Hf) is used as a refractory cathode material for thermionic emission in a high- temperature region. The consumption of an Hf electrode was investigated by measuring the concavity depth on the Hf electrodes after operations. These measurements were performed under several operating condi- tions. The consumption properties are separated into two consumption rates: Hf consumption during transient periods, when the arc current is increasing or decreasing such as at start-up and shut-off; and Hf consump- tion during the stable arc phase, when the arc current is held at the operating current. The consumption rates are estimated from the experimental data. Spectroscopic measurements of the arc plasma jet coming from a nozzle were conducted to study the consumption behaviour of Hf electrodes. A line of Hf at λ = 340 nm was observed using a monochromator. Characteristic intermittent Hf radiations in addition to the background continuum were observed in transient periods when the arc current or gas pressure changed. The intermit- tent radiation is considered to arise from droplets of Hf electrode from the surface of a molten electrode. The ejection of droplets is thought to be caused by rapid consumption during transient periods, and to be induced by a disruption in the balance of the fluid dynamics on the melted electrode. IIW-Thesaurus keywords: Consumables; Consumption; Erosion; Hafnium; Oxygen; Plasma cutting; Spectroscopy. Doc. IIW-2194, recommended for publication by Commission XII “Arc Welding Processes and Production Systems.” EXPERIMENTAL STUDY OF CONSUMPTION OF HAFNIUM ELECTRODE IN OXYGEN PLASMA ARC CUTTING Y. Yamaguchi, K. Yoshida, Y. Uesugi, Y. Tanaka, S. Morimoto, M. Minonishi and K. Saio EXPERIMENTAL STUDY OF CONSUMPTION OF HAFNIUM ELECTRODE IN OXYGEN PLASMA ARC CUTTING Figure 1 – Relation between arc current and consumable life and maximum cutting thickness