Micro-structural analysis, an example of the blast furnace trough |
| A strong tool to determine and to improve the performance of materials is to look at the microstructure before and after use with a light microscope. In combination with X-ray diffraction, micro analysis (SEM EDS) and physical tests (when needed), light microscopy enables to clarify attack mechanisms and generate possibilities for improvement. An example from a wrecked iron/slag trough lining after 14 days in operation is shown. The wrecked piece includes the hot and cold faces and has the appearance as shown on photograph 1. |
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| For the determination of the attack mechanism in this case the hot side has been sampled and compared with a sample from the cold side using light microscopy and X-ray diffraction. The material consists of very large white grains and almost black small grains. After polishing the white grains are identified as corundum and the black grains as SiC. Examples of the microstructure made with reflected light microscopy are given hereafter. For light microscopy the sample areas (2577 and 2567) have been mounted in resin and polished to 1 µm using diamond on a lead wheel. The samples (rd4006 and rd4007) for X-ray diffraction have been grounded to < 63 µm. |
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Photopgraph 1: Cut piece of the worn lining of the trough with sample positions |
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| The piece of the lining is still 250 mm thick and has only a very small zone (2mm) of slag containing material at the hot side. The material between the white grains has turned black in the vicinity of the hot face. The results of the diffraction analyses are listed in Table 1. |
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rd4006
cold side |
rd4007
hot side |
| Corundum |
Al2O3 |
10 |
9 |
| Moissanite (2 polytypes) |
SiC |
6 |
5 |
| Mullite |
3Al203.2SiO2 |
0 |
2 |
| Diayudaoite |
Na2O.11Al2O3 |
1 |
0 |
| Aluminium |
Aluminium |
1 |
0 |
| Anorthite |
CaO.Al2O3.2SiO2 |
0 |
1 |
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Table 1: Results XRD |
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| Cold side |
| At the cold side of the piece from the trough the composition and the grain size of the original material is observed. The material is a castable and contains sinter corundum in a very wide size range up to 12 mm, and SiC grains up to 1 mm in grain size. Silicon is present as relict in the SiC grains. |
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Micrograph 1: Microstructure at the cold side of the lining |
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| Hot side |
| At the hot side two temperature related phenomena have changed the microstructure. First the SiC is oxidized to SiO2 and secondly this SiO2 has reacted with corundum to form mullite (3Al2O3.2SiO2). The matrix in between the remaining corundum and SiC now consists of mullite. The attack by slag takes place on all three constituents. |
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Micrograph 2: Microstructure at the hot side of the lining |
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| The Microscopy and XRD reveals that oxidation of SiC and formation of mullite and attack by slag at the hot face are the main changes in the castable of the trough. The oxidation of SiC results in an increase of the occupied space by 107 volume%. Fortunately, SiC is not directly oxidized to SiO2 but to the intermediate and gaseous SiO. The volumetric expansion is now realized with filling the open space inside the castable without destruction of the structure but has increased the sensitivity for slag attack. Change in the recipe with a decrease of the amount of SiC with 10% and decreasing the grain size of corundum is recommended. |
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