Microwave roasting of agglomerated flux for submerged-arc welding

2.1 Analysis of raw materials

Experimental materials used are from a welding materials plant in China. The materials are named SJ101, which are semi-finished products of agglomerated flux and stirred for granulation and dried without high-temperature sintering. The main chemical components are shown in Table 1. X-ray diffraction (XRD) which is measured by Empyrean (PANalytical, Netherlands) analysis of unroasted agglomerated flux is shown in Figure 2. As we can see, the phase containing crystal water is Ca₆Si₆O₁₇(OH)₂.

Figure 2:

The XRD pattern of unroasted agglomerated flux.

A thermal analysis of unroasted agglomerated flux is shown Figure 3, differential thermal – thermogravimetric experiment is performed by a STA449F3 analyzer (NETZSCH, Germany). It can be seen that samples begin to lose weight at about 380°C and the ratio of weight loss increases with the increase of temperature. The weight loss ratio decreases sharply when the temperature reaches 753°C. It can be noticed in DSC curve that the weight loss process is accompanied by a strong endothermic process at 380–753°C, indicating that the crystal water is decomposed and removed at this stage and its content is 0.7%. It is found that a strong exothermic peak occurs in DSC curve when the temperature is higher than 753°C. It may be due to the redox reaction of samples at this temperature. Because the main purpose of high-temperature roasting for agglomerated flux is the removal of the crystal water, therefore, the roasting temperature of semi-finished agglomerated flux should be no more than 750°C.

Figure 3:

The TG-DSC curves of unroasted agglomerated flux.

2.2 Experimental method and equipment

In the present study, MW reactor is made by the Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming University of Science and Technology, China, and the schematic is shown in Figure 4. The MW heating experiments are carried out in a laboratory-made MW muffle furnace, and the MW equipment consists of four sections: two magnetrons at the frequency of 2.45 GHz and 1.5-kW power, which are cooled by water circulation as MW sources; a waveguide for transporting MWs; a resonance cavity to manipulate MWs for a specific purpose; and a control system to regulate the temperature and MW power. The inner dimensions of the MW cavity are 260 mm in height, 420 mm in length, and 260 mm in width. Continuous temperature measurement during MW heating is a major problem, so a thermocouple (connected to the computer system) with a thin layer of aluminum shielding can be employed to measure temperature and placed at the closest proximity to the material.

Figure 4:

The schematic of microwave roasting experimental system.

For each experiment, the semi-finished agglomerated flux of 600 g is weighed and then placed in Al₂O₃ crucible, which does not absorb MW. The roasting experiments are carried out as follows: the agglomerated flux is placed in an Al₂O₃ crucible and put in the MW cavity and then stirred at different power levels. At the start of the roasting experiments and after each 1-min interval, the temperature of the samples is noted down until the temperature of agglomerated flux reaches the set temperature and heat preservation for 30 min. Then the materials are removed and cooled to room temperature and finally used for the welding experiments. In the paper, the roasting temperatures are 600°C, 650°C, 680°C, and 720°C.

3.1 Characteristics of MW heating

The heating curve of agglomerated flux semi-finished at different power levels in MW field is indicated in Figure 5. As can be seen, there was an increase on heat rate with the increase of MW power. The heating time of the samples from room temperature to 720°C is found to shorten from 31 min to 17 min when MW power increased from 1.5 KW to 2.5 KW. It is noteworthy that, with the extension of heating time, the temperature of samples increased rapidly under the same MW power. The samples’ temperature stagnated or even declined and then rapidly rose a few minutes later after 460°C. It may be caused by the characteristics of MW heating. Materials were heated directly via MW, and MW energy converted into thermal energy relying on dielectric properties of the materials themselves. The materials acted as a heat source. With the increase of temperature, the water of crystallization in materials began to decompose. It can be found in Figure 3 that it was a strong endothermic process; most quantity of heat was consumed in the decomposition of crystal water. Therefore, the materials’ temperature reflected stagnation or even decline. The crystal water of materials was completely decomposed with the extension of heating time, and then the samples’ temperature started to rise rapidly.

Figure 5:

Heating curves of agglomerated flux under different microwave powder.

3.2 Effect of MW roasting temperature on the weld surface

In order to investigate the effects of MW roasting, experiments with agglomerated flux of MW roasting with H08C welding wire were carried out. The welding wire experiments for the same type of agglomerated flux that manufacturers provided were also studied for comparison. Flux products were obtained using external heat electric rotary kiln at 860°C for 1-h roasting. The weld appearance is shown in Figure 6. As we can see, an obvious indentation can be found on the weld surface by MW roasting at 600°C and 650°C for 30 min; in addition, large pores are also observed at the crusts sections, indicating that the crystal water of flux has not been removed completely. With MW roasting at 680°C and 720°C for 30 min, the weld surface between MW roasting and conventional roasting of agglomerated flux had a neat appearance, smooth surface, and no indentation. However, the XRD spectra (in Figure 7) of MW roasting of agglomerated flux at different roasting temperature indicated that it contained the same phase and no phase of crystal water had been found. It may be explained that the phase of crystal water in agglomerated flux is less than the detectable trace amounts. Thus, in order to save energy, the agglomerated flux could meet the requirements at 680°C for 30-min roasting in MW field.

Figure 6:

The weld surface for agglomerated flux roasting by microwave heating.

Figure 7:

The XRD patterns of microwave roasting of agglomerated flux at different temperature for 30 min.