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Technical Paper


prevent graphite oxidation, and (c) the formation of low melting phases by reaction of Al2


O3 with e.g. SiO2 of slag viscosity [6].


This paper investigates in the first part the effect of calcium magnesium aluminate (CMA) addition to MgO-C bricks in comparison to MA-spinel- doped and un-doped MgO-C bricks. Different from alumina and spinel addition, the use of CMA (MagArmourTM


, Imerys Aluminates) brings also


small amounts of a lime source into the matrix. But unlike doloma addition, the lime in CMA is combined in calcium aluminate phases that are much less sensitive to humidity than the free lime of doloma. CMA grains consist mainly of microcrystalline MA-spinel embedded in calcium aluminate phases and has proven to improve corrosion and slag penetration resistance of castables for steel ladles [7]. In the second part of this report results from industrial steel ladle trials with CMA-doped MgO-C and MgO- Al2


O3 -C bricks are discussed.


2: Laboratory corrosion experiments with CMA-doped MgO-C model bricks


2-1: Test material and procedures


A low carbon containing MgO-C reference material (MgO-C-REF) has been prepared by mixing MgO fractions up to 4 mm particle size with 2.2 wt.% carbon (graphite and carbon black in equal parts). Sample MgO-C-CMA contains 5% coarse grained Calcium Magnesium Aluminate (MagArmour™, Imerys Aluminates), as replacement for 5% of the same fraction of fused Magnesia. To evaluate if the calcium containing CMA has a different effect than a calcium-free traditional sintered spinel, the mix MgO-C-MA has been prepared with 5% MA spinel (76% Al2O3). Both CMA and MA were used in the fraction <2 mm and replaced MgO <2 mm. All three mixes used 3 wt.% resin as binder. No anti-oxidants were used in these samples. The compositions of the three formulations are shown in table 1. The mineralogical compositions of the sintered MA spinel and of CMA are listed in table 2.


Material (wt.%) Fused MgO


CMA (MagArmourTM MA-spinel


Graphite + Carbon Black


Resin (powder + liquid)


Hexamethylene- tetramine


)


MgO-C REF MgO-C CMA MgO-C MA 97.8 - -


92.8 5.0 -


2.2


+3.0 +0.3


Table 1: Composition of MgO-C model systems CMA


Al2 O3


CaO MgO


SiO2


Fe2O3 TiO2


Mineral phases


(MagArmourTM 70 10 20 tr tr tr


MA, CA, CA2 ) 2.2


+3.0 +0.3


92.8 -


5.0 2.2


+3.0 +0.3


www.ireng.org and CaO from the slag resulting in increase


After mixing, bar shaped sample bodies (25 x 25 x 150 mm³) have been prepared using a uniaxial press (RUCKS engine building GmbH, Glauchau, Germany) with an applied pressure of 120 MPa. Afterwards, the samples were hardened in a three steps schedule with a maximum temperature of 180°C. The carbonization of the sample bodies took place in a steel box filled with pet-coke heated at 1400°C for 5 hours in order to provide a reducing atmosphere. After carbonization, the samples consist of a residual carbon content of 3 %. Since mechanical and thermomechanical properties after carbonization didn’t reveal significant differences between the three materials [8] the main objective of this present research work was to determine the corrosion resistance of the different compositions against molten steel and slag. Therefore, a melting test has been performed using a metal casting simulator (SYSTEC, Karlstadt, Germany). The principle setting of the metal casting simulator has been presented by Dudczig et al. [9]. The three different samples were fixed to an equipment and attached to a crucible (figure 1). Afterwards, the crucible was filled with approx. 30 kg of steel 18CrNiMo7-6 (material number 1.6587, Deutsche Edelstahlwerke GmbH, Witten, Germany).


Figure 1: Equipment for corrosion test of samples


Next to steel as corrosion medium, a synthetic basic slag has been prepared by mixing 19 wt.% Ciment Fondu, 52 wt.% LDSF RG, 6 wt.% CMA 72 (all Imerys Aluminates, Paris-La Defense, France), 3 wt.% Microsilica Elkem 971 (Elkem AS, Oslo, Norway), 3.5 wt.% MgO Nedmag 99 (Nedmag B.V., Veendam, The Netherlands), and 31.5 wt.% CaCO3


(Carl Roth GmbH,


MA-spinel 77


tr


23 tr tr tr


MA tr = traces Table 2: Composition of raw materials and synthetic slag (wt.%)


Synthetic ladle slag


32.5 51 5 6 4


1.5


Karlsruhe, Germany). The targeted chemical composition of the synthetic slag can be found in table 2. In order to prevent dust formation and reactions during the metal casting simulator test run, the mixed synthetic slag has been granulated, heated at 1000°C, and sieved to 100 – 500 µm. In the metal casting simulator, the steel was melted at approx. 1580°C under a fully controlled argon atmosphere without any further steel treatment. Afterwards, the slag was stepwise added to the top of the steel melt via a sluice and a slide. Investigations in a hot stage microscope revealed a melting temperature (hemisphere temperature) of the synthetic slag of 1304°C. Due to metal melt flow caused by induction field heating, the slag melts and first accumulates at the side wall of the crucible, where the samples are located. After adding more slag, the whole steel melt becomes covered by the synthetic slag (see figure 2). This condition has been kept constant so that the three test materials were equally exposed to steel and slag at 1580°C for 30 minutes, followed by shutting down the metal casting simulator. With the samples fixed to the crucible, it was not possible to pour the slag and molten steel out of crucible and after shutting down the induction heating, the samples effectively froze in the steel and slag. However, slag and steel could easily be separated and consequently broken bits of the samples with contact to slag could be extracted to perform the investigations. The corrosion has been evaluated at macroscopic scale, with aid of digital light microscopy, and with SEM equipped with EDX.


16


ENGINEER THE REFRACTORIES


November 2018 Issue


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