Effect of refractory filler on the quality of foundry coatings

Abstract : Casting coatings affect the intrinsic and surface quality of castings, especially the lost foam coating is one of the key factors to control and ensure the quality of lost foam casting. As is known, the foundry coating is composed of five basic components: carrier, refractory filler, suspending agent, binder and auxiliary additive. The refractory filler is the main component, which determines the refractoriness, chemical stability and thermal insulation of the coating. It also has an important influence on the gas permeability and sintering peelability of the coating. Therefore, it can be said that the quality of the refractory filler is a key factor in the quality of the cast paint.

Key words: lost foam casting, refractory filler, coating quality

In the lost foam casting, how to choose the refractory filler of the coating, mainly consider the following properties of the filler:

(1) Refractoriness;

(2) density;

(3) coefficient of thermal expansion;

(4) particle size and particle size distribution;

(5) chemical reactivity with molten metal;

(6) the amount of gas generated;

(7) compatibility with other components in the coating;

(8) Whether it affects the health and environment of the operators.

1. Alumina bauxite powder

Bauxite is widely used in ordinary cast steel coatings and cast iron coatings in recent years due to its high refractoriness, good chemical stability, small expansion coefficient, abundant source and low price.

1.1 chemical composition

The Al2O3 content, sintering temperature and the content of impurities such as Ti02 and FeO3 in bauxite have a great influence on the coating. Therefore, the ore with high Al2O3 content is calcined at 1450°C-1500°C, and the selected clinker is ground to make fine powder. After that, pickling treatment is still a key step. In addition to the high content of Ti02 and FeO3, the bauxite without pickling treatment has increased dust and mechanical iron mixed during calcination and grinding. (Currently, bauxite fine powder The processing is mostly Raymond mill, the wear parts are mostly high-strength steel castings, and the mechanical iron mixed in is difficult to remove by magnetic separation.) The prepared coatings are affected by the refractoriness, and the second is the increase of floating objects and sediments. Paintability, suspension and storage time of the paint.

1.2 granularity

The particle size and particle size distribution of bauxite are important factors affecting the density and permeability of the coating, especially the lost foam coating. The pyrolysis products of the foam molding during the sintering process can be smoothly discharged through the coating. Breathability is a major distinguishing feature from other sand casting coatings. According to different needs, Xiangyu Company uses the particle size of bauxite into two different specifications for sand casting paint and lost foam coating.

The particle size distribution of bauxite powder used in sand casting coatings is:

-270 mesh to 320 mesh, with a concentration rate of 96%

+270 mesh no more than 1%

The particle size distribution of bauxite used in the lost foam coating is:

-270 mesh to -320 mesh 78%

-200 mesh to -270 mesh 15%

-100 mesh to -200 mesh 5%

+100 mesh is not more than 2%

The above particle size distribution is also suitable for the grading of other refractory fillers.

2, zircon powder and N-3 powder

Zircon powder is processed from zircon sand. It has small linear expansion coefficient, good thermal shock resistance, high refractoriness and chemical resistance to most alloys at high temperatures. It is a good refractory filler and is used in stainless steel. High-alloy steel castings, imported zircon powders are mostly milled by airflow, Zr02 content is more than 65%, and the impurity content is low. The domestically processed zircon powder is mostly Raymond mill grinding, and the impurity content is relatively high. The source of glutinous powder is more complicated, and the price is rising all the time. Many paint manufacturers are looking for substitute products of zircon powder to reduce the cost of cast steel coating. Xiangyu Company has successfully developed a new synthetic refractory filler N-3 powder. Widely used in cast steel coatings.

N-3 powder is prepared by proportional synthesis, electrofusion, pulverization and acid water washing of several natural refractory materials. The refractoriness, linear expansion coefficient and chemical stability are close to zircon powder, and it is not alkaline oxidized with FeO3. The reaction of the material is beneficial to the anti-sticking property of the coating, and the price is only one third of the zircon powder.

3. Magnesia powder and forsterite powder

Magnesia powder and forsterite powder are usually used as the preferred refractory materials for the preparation of coatings for alkaline steel castings. The main components of magnesium sea powder are calcined by natural magnesite ore (MgCO2), which are crushed, ground and magnetically processed. Cheng, it is worth paying special attention to the following points:

(1) The agglomerate must be a so-called burnt block calcined at 1500 ° C - 1600 ° C. The particles are dense and hard, and the MgO content is greater than 90%. Otherwise, the coating is prone to cracking at high temperature pouring.

(2) The magnetic material such as iron and iron oxide in the agglomerate is quite high. Xiangyu Company uses magnetic magnetic separation of 5000 high magnetic double-roller magnetic separator twice, and the magnetic material is selected up to about 25%. Therefore, in order to obtain high-quality magnesia powder, magnetic separation is a key step.

(3) Magnesia powder is easy to absorb moisture and moisture, and special attention should be paid to the moisture content in the powder.

The content of iron oxide should be mainly controlled in the forsterite powder, not more than 10%. The smaller the content of serpentine (hydrous magnesium silicate), the better, generally not more than 20%, otherwise the refractoriness will decrease, the amount of ignition and the amount of gas generated will increase. Big.

4, brown corundum powder and white corundum powder

Brown fused alumina is made from bauxite as raw material and treated by high temperature at 2000 °C-2400 °C. White corundum is obtained by electrofusion recrystallization of alumina powder. Its basic mineral is a-Al2O3, the former is more than 94%. The latter is more than 97%. The corundum has high hardness, high melting point, high thermal conductivity, good thermal stability, no cracking, good acid and alkali resistance, and brown corundum powder is suitable for formulating ordinary cast steel and alloy steel coatings. Due to its high price, corundum powder is often used as a composite filler for large steel castings. It should be noted that the particle size distribution and label of the corundum powder for abrasive and the corundum powder for casting coating are inconsistent, and should be graded according to the particle size composition of the refractory filler for coating. In addition, there are waste sand wheels on the market to make corundum powder, which has high impurity content and should be paid special attention when purchasing.

5. Quartz powder and fused silica powder

Quartz powder is one of the traditionally used refractory fillers. It is still used by many manufacturers because of its low price and abundant resources. However, its low refractoriness and large coefficient of linear expansion are only applicable to cast coatings and small cast steel coatings. Xiangyu Company uses fused silica powder and ordinary quartz powder to be used in lost foam casting steel and cast iron coatings. It effectively transmits cracking and peeling of paint due to phase change of quartz powder. The effect is very good. The characteristics of fused silica powder are: low thermal expansion rate, good thermal stability and high temperature creep resistance, especially suitable for lost foam coatings.

6, other refractory fillers

In addition to the above refractory fillers, there are also mullite powder, chromite powder, high alumina steel jade powder, diatomaceous earth powder, talc powder, mica powder, feldspar powder, etc., depending on the functional requirements of the coating, Used alone or as a refractory filler, it is not detailed here, and Xiangyu has a supply. In summary, the refractory filler is the main part of the coating and plays a decisive role in the effect of the coating. The selection of refractory fillers should focus on the main indicators such as mineral composition, purity, impurity content and particle size composition of refractory fillers.

Schedule: Several commonly used refractory filling properties.

Several common refractory filling properties

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