Selection of crucible for medium frequency furnace and sintering process of furnace lining

May 13, 2022

1. Crucible of medium frequency furnace and refractory mastic lining

Refractory mastic is a kind of heat-resistant and insulation resistant cement. A circle of 1.5-2.5cm thick is coated on the inner wall of the induction coil, which is a material to protect the induction coil and improve the mechanical strength of the coil. At the same time, it can make the furnace lining expand and contract smoothly in the vertical direction. At present, the domestic refractory mastic can withstand half an hour at 1800 ℃ and can effectively prevent furnace leakage.

2. Selection of furnace lining (crucible) materials

There are three kinds of crucibles for medium frequency furnace: alkaline crucible, acid crucible and neutral crucible. The material used to make alkaline crucible is mainly magnesium oxide; The only material used to make acid crucible is silicon oxide; The materials used to make neutral crucible are only alumina, MgO · Al2O3 composite oxide, graphite, etc.

The crucible of induction furnace shall have the following characteristics:

(1) high fire resistance. The steel-making crucible is required to be 1700 ℃, and the melting cast iron crucible is required to be 1500 ℃ ~ 1600 ℃;

(2) stable physicochemical properties. Crucible products are required to be resistant to molten slag and liquid steel corrosion;

⑶ good thermal shock resistance and high temperature strength;

(4) have certain insulation performance;

(5) small thermal conductivity;

(6) low cost and no pollution.

There are three kinds of refractory materials commonly used to make crucibles: magnesia, magnesia alumina spinel and quartz sand. They are introduced as follows:

Magnesia alumina spinel is a kind of synthetic refractory with theoretical composition of ω (Al2O3)=71.5%, ω (MgO)=28.5%; The melting point is 2135 ℃, and the fire resistance is about 1900 ℃. It has good thermal shock resistance and can be used as crucible materials for various induction furnaces. The production method is to mix Al2O3 and MgO and synthesize them by sintering or arc and electroslag. Quartz sand is an acid refractory, which is used to make acid crucible. After natural quartz sand is selected, high purity quartz sand can be obtained. The harmful impurity of quartz sand is alkaline oxide. The refractoriness of quartz sand is about 1650 ℃, and the working temperature of the crucible is 1550 ℃. The crucible has excellent thermal shock resistance and long service life below 1500 ℃.

In addition to refractory materials, there are a small amount of additives in the materials for making crucibles. Boric acid (H3BO3) is commonly used. The main function of boric acid is to reduce the sintering temperature of sand, promote the formation of magnesia alumina spinel, and change the volume change rate of crucible. Adding different amounts of boric acid can adjust the volume change rate of the crucible. Different boric acid addition has different effects on the volume change rate of quartz sand. As an additive, the particle size of boric acid is generally less than 0.5mm, which can reach industrial purity. The addition amount of boric acid is related to the working temperature and service conditions of the crucible. The addition of boric acid will reduce the working temperature of the crucible. When melting in vacuum induction furnace, adding boric acid into the crucible will increase boron in the molten metal, which should be paid attention to when melting superalloys.

Crucible forming

Before the induction furnace crucible is formed, it is necessary to mix the ingredients first, mainly to make the particle size ratio. A crucible with small porosity, good sintering property, high strength and good quench and heat resistance can be obtained by reasonable particle size ratio. The porosity of the crucible is usually about 20%. In the particle size ratio of crucible material, the more coarse particles, the better the quench and heat resistance. The sand material for making the crucible can be divided into three particle size grades, namely coarse particle size (2 ~ 6mm), medium particle size (0.5 ~ 2.0mm) and fine particle size (≤ 0.5mm). Coarse grained sand plays a skeleton role in the crucible, making the crucible have a certain strength. Medium size sand fills the gap of coarse particles, increases the bulk density, improves the sintering performance of crucible and improves the strength. The function of fine-grained sand is to ensure the sintering performance and quality of the crucible and the continuity of the sintering network, so that the crucible has good compactness. The crucible with a capacity of less than 500kg generally uses sand with a particle size of 0 ~ 4mm, and the crucible with a large capacity uses sand with a particle size of 0 ~ 6mm.

Furnace knotting forming method: the first step of this method is to prepare sand according to the specified particle size and mix it evenly with additives. If wet knotting is adopted, 1% ~ 2% water can be added, and then placed for 1 ~ 2H after mixing. The second step is to lay asbestos cloth on the inner side of the sensor. For acid crucible, two more layers can be added. The third step is to knot the furnace bottom. After adding sand in batches, tamp repeatedly with a pointed steel drill with a diameter of 12 ~ 14mm each time. The number of knotting and feeding at the bottom of the furnace is 4 ~ 5 times, and each knotting time is at least 10min. The thickness of the furnace bottom is generally knotted to the specified height. The fourth step is to put the crucible core, that is, the mold that controls the shape and volume of the crucible. When sintering at low temperature, the core is generally made of steel plate. When the core is placed, it shall be centered, and the steel drill head shall not pierce the glass cloth. During the knotting process, it is also necessary to check whether the core is in the center, so as to avoid the uneven thickness of the crucible wall caused by the skew of the core and reduce the service life. When the hard sand layer reaches the position of the second induction ring from above, turn to the knotting operation at the furnace mouth. The fifth step is to knot the furnace mouth. As the furnace mouth area is not easy to sinter, the proportion of fine powder must be increased in the sand, or an appropriate amount of clay and water glass must be added to obtain a stronger nozzle.

The crucible of large capacity induction furnace is formed by manual knotting, which is difficult to ensure the quality. Vibration forming method is often used. This not only improves the working conditions, but also saves the production time. The equipment used in vibration forming method is vibration furnace building machine. The utility model is composed of a pneumatic vibrator, a furnace bottom vibrating block and a furnace wall vibrator. The vibration furnace builder uses the vibration generated by the air motor driven by compressed air as the vibration source, which can produce a pressure of 0.3 ~ 0.5MPa. The vibration force is transmitted to the sand material through the furnace bottom vibration block and furnace wall vibrator, so as to make the sand material reach high filling density.

3. Selection of crucible materials

① Alkaline crucible material is suitable for melting special steel, precision alloy and alloy steel. Such as stainless steel.

② Acid crucible material is suitable for smelting carbon and low alloy steel, and it is not suitable for smelting steel grades containing active wave elements such as Al, Ti, B and rare earth.

4. Sintering of induction furnace crucible

The sintering process is to combine the liquid on the contact surface of the sand material at high temperature to form a continuous sintering network, through which the whole sand material is connected as a whole. After sintering, the crucible section should be divided into non sintered layer, semi sintered layer and sintered layer from the outside to the inside. The sintering layer requires high density, high strength, few surface cracks, all the sand particles melt, and the sintering network is uniform and complete. The thickness of the sintered layer accounts for about 20% ~ 30% of the total wall thickness. Transverse cracks in the sintered layer should be avoided as far as possible.

The semi sintered layer is the transition zone between the sintered layer and the non sintered layer. In this zone, the contact surface of some particles begins to melt and the sintering network is incomplete. The function of this layer is to buffer the stress of the sintering layer and prevent the cracks in the sintering layer from extending outward. The thickness of the semi sintered layer accounts for about 25% ~ 35% of the crucible wall thickness. The non sintered layer is a layer of raw sand without sintering between the semi sintered layer and the induction ring. This layer of sand plays the role of heat insulation. When the crucible is heated and cooled, it also plays the role of volume expansion and contraction. The thickness of non sintered layer accounts for about 30% ~ 35% of the crucible wall thickness. The furnace drying process and furnace curve must be implemented according to the provisions of the manufacturer of furnace lining materials or the supplier of furnace lining materials.

After the crucible is made, pay attention to maintenance during use, repair and clean the inner wall of the crucible in time to maximize the service life of the crucible.