In order to determine substances, which causes welder's lung, metalic fever, and irritation of the mucous membrane, the mechanism of dust development from the coating materials of electrodes being heated. Heating four kinds of coating materials of electrodes in the electric furnace continuously from room temperature to 1, 100°C, we conducted differential thermogravimetric analysis (DTG) and differential thermal analysis (DTA). And the following results were obtained. 1) The mineral components of the coating materials determined by polarizing microscopy and X-ray diffraction are shown in the table. 2) Characteristics of the DTG curve:DTG makes it possible to observe simultaneously the state of weight change and the reaction velocity of heated samples. In other words, DTG includes the functions of both thermogravimetric analysis and DTA, being an advantageous method for studying the multi-component system such as coating materials of the electrode. When chemical reactions occurring in the sample are endothermic, the peak of the differential values of weight decrement on the DTG curve appears at a little higher temperature than that of endothermic reactions on the DTA curve. In the case of exothermic reactions on the other hand, this appears at a little lower temperature. 3) Generation of dusts from the electrode: The temperatures at which the dusts are generated during the arc-welding are classified into three groups, i. e. low (below 300°C), medium (from 450 to 670°C), and high (over 1, 000°C). The particles given forth at the low temperature are mainly caused by the evaporative pressure of water. The dusts at the medium temperature are caused by falling off the glutinosity of the coating materials and by changes of minerals contained in the coating materials such as α-β type invertion of quartz, dehydration of water of crystalization, decomposition of MgCO₃, etc. At the high temperature only low hydrogen type electrode produces dusts and CaF₂ accelerates the flow of the coating materials. The particle of the dust is coarse in the case of the low temperature, fine at the medium temperature, and forming lumps at the high temperature.

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Coating materials of electrodes are rapidly heated during arc-welding. In order to study how thermal stress generated by the are-welding effects the production of dusts, samples of coating materials were rapidly thrown into an electric furnace kept at constant temperatures (every 100°C, from 100°C to 1000°C) and were heated for 20 minutes. Obtained results were shown by the time-differential value of weight change curves. Whether the heating temperature was high or low, the thermal stress was generated ad maximum 30∼40 seconds after the beginning of heating the samples and caused to produce a lot of dusts. Heatings at 300°C∼400°C and 500°C∼800°C generated extreme thermal stresses. These temperatures were in accordance generally with those (below 300°C and 450°C∼670°C) already reported. When the coating materials was rapidly heated, the thermal stress generated even below 1, 000°C could be a thermal shock, producing dusts. The thermal shock at 300°C∼400°C caused mainly by the vaporization of hygroscopic water, which acts between structures and between crystals, developing coarse dusts. The dusts produced at 500°C∼800°C are caused by decreasing glutinosity of the coating materials and by various changes of the crystal system. This thermal shock acts between crystals and between the internal parts of the crystal, producing fine dusts.

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Coating materials of four types electrodes (low hydrogen type, high iron oxide type high titania type and ilmenite type) and quartz, as a raw material for the coating, were examined by a thermobalance in order to recognize the temperature and state of solid dusts which were produced. Causes and the prevention of dust development during arc-welding were considered by both the present results and those already reported. 1) Temperatures at which dusts were produced from the electrodes. The coating materials of the low hydrogen type electrode produced abruptly a large quantity of solid dusts at both 600 and 1, 000°C. In the high iron oxide type, solid dusts were slowly produced in a wide range of temperatures, however somewhat abruptly at 100, 450, and 1, 000°C. In the high titania type, solid dusts were also slowly produced in a wide range of temperatures and somewhat abruptly at 250 and 680°C. In the ilmenite type, solid dusts were abruptly produced at 100 and 680°C. These temperatures, at which a large quantity of solid dusts were produced, are generally in accordance with temperatures at which minerals contained in the coating materials react severely turning the thermal stress to the thermal shock. 2) Scattering or fine quartz particles. When quartz, a raw material of the coating of the electrode, was heated at temperatures continuously from the room temperature to 900°C, it did not show a constant weight even at 900°C because it contains a little alkali salts as impurities. Thermobalancecurve of the quartz is composed of two factors, i.e. the weight decrease shown as a straight line, which means dehydration of the bound water, and the weight decrease shown as steps, which means giving out fine quartz particles by extrication of liquid inclusions. The more conspicuous the latter, the thinner the layer of the sample. 3) The cause and the prevention of dust production during arc-welding. Causes of dust production are classified into three groups: a) partial pressures of the gases, b) changes of the thermal expansion of minerals contained in the coating materials, c) lacking glutinosity. As to the prevention for a), it is necessary to use dried electrodes, to give grinding, and to give heat treatment to the raw materials of the electrode. For b), to heat beforehand the electrode, and for c), to use contractibitity-bond instead of water glass.

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