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Refractory Lining of Coreless Induction Furnace


Refractory Lining of Coreless Induction Furnace

In steel melting shops of low capacities, induction furnace is preferred as a convenient melting unit because of its high efficiency, low energy consumption, easy operational control, and good performance with various kinds of steel scrap. The induction furnace is an electrical furnace in which the heat is applied by induction heating of a conductive medium (usually steel scrap) in a crucible placed in a water-cooled alternating current solenoid coil. Induction coil is required to be protected from the liquid steel  by isolation material called refractory. A coreless induction furnace has a non-conductive refractory crucible, surrounded by a coil of copper tube.  Cross-section of induction furnace showing refractory lining is at Fig 1.

Fig 1 Cross-section of induction furnace showing refractory lining

The refractory lining practice for a particular induction furnace depends upon the capacity and design of the furnace, operation practice adopted during making of a heat, and furnace output. For successful and consistent performance of the lining, the important aspects are (i) use of proper grade and quality of the lining material, (ii) careful and systematic lining practice, and (iii) consistency in working conditions.

Refractory lining is consumable material which gets damaged during the operation of the induction furnace. The number of heats the lining last is known as lining life. When a certain amount of damage occurs, induction furnace operation is to be interrupted for repair or replacement of the refractory lining. Both of these activities increase the furnace downtime.

Refractory lining is the important part of induction furnace since the furnace performance is directly related to the performance of the refractory lining. Well stabilized refractory lining results in smooth working of furnace, optimum output and better metallurgical control. The lining practice best suited to particular induction furnace depends upon (i) the furnace capacity and design, (ii) raw material used for melting, (iii) quality and temperature of steel being melted, and (iv) capacity of furnace etc.  For successful and consistent performance of the refractory lining, important aspects are (i) use of proper quality of lining material, (ii) careful and systematic lining practice, and (iii) consistency in working conditions.

The reliability of the refractory lining of the induction furnace is dependent on several factors. These include (i) proper selection refractory material for application, (ii) proper method of refractory installation, (iii) sintering procedure followed for the refractory material, (iv) procedure of preheating used before regular operation, (v) improper monitoring of lining wear and allowing the lining to become too thin, (vi) sudden or cumulative effect of physical shock or mechanical stress, (vii) sudden or cumulative effect of excessive temperature in the furnace, (viii) Excessive build-up of slag in the furnace, and (ix) corrosion due to chemical reaction.

Refractory lining material is required to have certain characteristics such as (i) retaining of structural strength at high temperatures, (ii) chemical inertness with respect to the liquid steel, (iii) resistant to thermal shock, (iv) low thermal conductivity, (v) low coefficients of expansion, (vi) withstand the stresses developed by thermal cycles in operation, (vii)  high erosion resistance, (viii) ease of installation, (ix) to have reparability, (ix) ease of knocking, and (x) economical in cost. Normally it is very difficult to judge the suitability of particular lining under various conditions like operating temperature, liquid steel being melted, slag being formed, and furnace capacity.

Refractory lining material can be shaped or unshaped (monolithic material). Dry monolithic refractory material is normally used for the lining of induction furnace. Life of refractory lining depends on dryness of refractory material and also compaction ratio.

For the lining of induction furnace, refractory materials are normally monolithic refractories made of dry powder which is required to be compacted with homogenous density. Further, there are heat losses by conduction, convection and radiation, and hence the improvement in best refractory material and optimization in wall thickness of refractory material are important aspects for the lining of the induction furnace.

Proper installation of the refractory material is important for the safe operation of the furnace as well as for the attaining adequate life of the lining. If the refractory material is not compacted properly then voids and low density areas are created in the lining resulting into weak spots which are susceptible to the attack of the liquid steel. Also if the crucible shows out of roundness, it means that the thickness of the lining is uneven and this results into lower lining life.

The procedure for sintering the lining specified by the supplier of the lining material is required to be followed. If the refractory material is inadequately sintered, then the proper bond is not developed and the lining material is prone to attack by the liquid steel and slag. Further, the sintering schedule must be completed, once it has started.

The compaction of refractory material is done by pneumatic vibrators usually operated manually. Doing this process by hand cannot guarantee homogenous ramming which can cause shorter lifetime of refractory. Besides manual method takes much longer time when installing the refractory.

The wear of the refractory lining in induction furnace is due to the (i) cutting action of sharp corners of loose scrap and impact during scrap charging, (ii) scraping action of liquid steel on the wall, (iii) intense wear at the slag/metal interface (so called Marangoni effect), (iv) high wear at the floor-wall junction, (v) wear due to high turbulence and high temperature, and (vi) wear at the less dense area due to poor installation of the lining. Besides the wear, the lining is also corroded due to the chemical action of the steelmaking slag. Chemical action of slag depends on porosity level of the lining, slag chemistry, high slag fluidity, and long operating time with highly oxidized slag.

Bridging is one of the reasons for the refractory failure in the induction furnace. If proper size of scrap metal charge and charging sequence are not maintained, it causes bridging or clogging. Bridging is shown in Fig 2. Slag bridging happens due to improper de-slagging. Refractory reaches failure temperature due to entrapped metal (or slag) in a high-energy induction field.

Fig 2 Bridging in induction furnace

The refractory lining of induction furnace is brittle in nature and has poor resistance to the tensile stresses. The sudden or cumulative effects of shocks and stresses can lead to failure of the refractory lining. In case of very heavy charge being dumped in the induction furnace, it is necessary to first have adequate light material in the furnace bottom to cushion the impact. Further, while charging the induction furnace, the charge is needed to be properly centred to avoid damage to the wall. Also, the metal and slag jamming in the furnace is to be avoided so as to reduce, the mechanical stress on the refractory wall.

Bath temperature has an adverse effect on the lining wear. Excessive temperature in the bath leads to softening of the lining surface and hence results into accelerated erosion. Excessive heating and improper cooling can lead to thermal shocks leading to the damage of the lining integrity by cracking and spalling.

Refractory wall of induction melting furnace is a key component which is used as insulation layer. It is made of acidic, neutral, or basic ramming mass. The refractory wall is directly influenced by the thermal cycling of the high temperature liquid steel in the furnace. Thermal fatigue failure is easy to happen for it because of the larger phase transformation thermal stresses and it has a shorter life. This can cause serious production accidents. Therefore, the service life problem of the refractory wall has always been a focus of attention in the application of this to the industry.

Proper and well-maintained refractory lining is important for the safe operation of the induction furnace. The choice of refractory material depends on the type of charge, i.e. acidic, basic or neutral. The durability of the crucible depends on the grain size, ramming technique, charge analysis, and rate of the heating and cooling the furnace. Refractory materials are to be resistant to thermal shock and have ranges of thermal conductivity. They are to retain their strength even at the high temperatures.

Silica (SiO2), magnesia (MgO) or alumina (Al2O3) based materials are the most common refractory materials used. The refractory materials are classified on the basis of chemical composition as acidic, basic or neutral. Silica based refractories are acidic, magnesia based refractories are basic, while alumina based refractories are neutral.

Normally, the selection of refractory is based on the type of slag generated during melting. If the slag contains high amount of acidic components then a silica lining is used. For slags with a high basicity index magnesite linings are appropriate. Silica lining has good endurance against thermal shock but poor resistance against steelmaking slags. It corrodes by the chemical interaction when there is basic slag made during steelmaking. Magnesite lining is more compatible chemically but has poor thermal shock resistance and develops vertical cracks during service. Neutral lining has advantage over both silica and basic lining in terms of chemical reaction and thermal shock resistance. The ramming refractory mass used for neutral lining in the induction furnace consists mainly of Al2O3 with the addition of MgO in the matrix which results in in-situ formation of spinel (Al2O3.MgO) at the steel melting temperature. This in-situ spinel formation is associated with a significant expansion of volume to provide a rigid structure. It also provides a hard sintered refractory surface layer to the liquid metal bath which results into a good resistance to erosion.

Silica‐based refractories have long been the standard for lining of the coreless induction furnaces for steelmaking. Traditional lining materials include a high purity silica aggregate with a boron‐ based, heat-set binder, generally in the form of either boron oxide or boric acid. The typical chemical composition of silica ramming mass is SiO2 – minimum 99 %, Al2O3 – maximum 0.6 %, iron oxides – maximum 0.2 %, and calcium oxide (lime) – maximum 0.1 %. The size fraction of silica ramming mass is typically in the range of 33 % in the range of – 4 mm to 1 mm, 30 % in the range of – 1 mm to 0.20 mm, 17 % in the range of – 0.20 mm to 0.06 mm, and 20 % is below 0.06 mm. The bulk density of the refractory is in the range of 2.0 tons/cum to 2.2 tons/cum. The softening point of the refractory is around 1280 deg C. The PCE value is typically ASTM number 31-32.

One of the main weaknesses of SiO2 based refractory is the formation of low temperature melting corrosion products, causing severe and rapid erosion. At high temperature under low oxygen pressure, SiO2 dissociates into SiO (g) and oxygen (g). Due to this dissociation, lining is consumed and there is increase of oxygen level of the liquid steel in the bath.

The typical chemical composition of neutral ramming mass is Al2O3 – 86 % to 88 %, MgO – 7 % to 13 %, chromium oxide (Cr2O3) – 4 %, SiO2 – maximum 0.5 %, Fe2O3 – maximum 0.2 %, and TiO2 – maximum 0.2 %. The size fraction of the neutral ramming mass ranges from 0 mm – 5 mm. It has spinel ceramic bond and has application temperature of 1750 deg C maximum.

The Al2O3 based neutral refractories have spinel based refractory bond which has high refractoriness, no low temperature liquid formation with slag, and superior corrosion resistance to chemical attack. Also Al2O3-MgO spinel has additional capability to absorb FeO and MnO (manganese oxide) on the free vacancies present in the crystal structure, which causes increase in the slag viscosity and hence lesser slag infiltration.

The basic lining is produced from either naturally occurring carbonate ore of magnesium, or is synthetically produced from sea water magnesia. Two types of bonds are used- either spinal bond (MgO.Al2O3) or silicate bond (2MgO.SiO2). The typical chemical composition of basic ramming mass having silicate bond is MgO – around 88 %, and SiO2 – maximum 8 %. The size fraction of the basic ramming mass ranges from 0 mm – 5 mm. It has application temperature of 1750 deg C maximum. The sintering temperature is 800 deg C.

Basic refractories are very sensitive to thermal cycling. In these refractories, spalling is the main wear mechanism. Because of high thermal expansion, thermal cycles generate considerable stresses which can exceed the material acceptance level and lead to early failure.

The comparison of other properties of the three types of refractory materials is given in Tab 1.

Tab 1 Comparison of properties of the refractory materials
Type of material   Silica based Alumina based Magnesia based
Property Unit      
Nature   Acidic Neutral Basic
Melting point Deg C 1723 2050 2800
Free energy at 1450 deg C kj/mol -594 -758 -732
Average thermal conductivity between 0 deg C and 1200 deg C W/mk 1.7 2.6 4
Expansion co-efficient between 0 deg C and 1200 deg C x 1000000 12.2 8.2 1.8
Relative cost per ton of material   Low High High

Choosing of a right refractory material for a given melting and holding application is important. Selection of correct refractory material depends on several factors such as (i) melting temperature, (ii) holding time, (iii) volume, (iv) induction stirring, (v) scrap quality, size and shape, (vi) additives and alloying agents etc.

The refractory lining of the induction furnace is required to be kept at the correct thickness to avoid any rupture due to liquid metal penetration. In induction furnace earth detector helps in monitoring the condition of the furnace lining. This essentially monitors the potential between current carrying coil and the liquid metal which is earthed through antenna and it gives signal in case of (i) liquid metal film which has penetrated the lining and solidified near to the coil, (ii) liquid metal has penetrated through the lining and touching the coil, (iii) metal streak or metal dust on the coil from outside getting earthed through physical contact between coil and yoke, and (iv) sweating of the coil during sintering of the crucible and/or water leakage through the coil.

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