Casting coatings affect the internal 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 we all know, casting coatings are composed of five basic components: carrier, refractory filler, suspending agent, binder and auxiliary additives, among which refractory filler is the main component, which determines the refractoriness, chemical stability and heat insulation of the coating. It also has an important influence on the gas permeability and sintering peeling property of the coating. Therefore, it can be said that the quality of refractory fillers is a key factor in the quality of casting coatings.
In lost foam casting, how to choose the refractory filler of the coating mainly considers the following properties of the filler:
(1) Refractoriness;
(2) Density;
(3) Thermal expansion coefficient;
(4) Grain type and particle size distribution;
(5) Chemical reactivity with molten metal;
(6) gas output;
(7) Miscibility with other ingredients in the paint;
(8) Whether it affects the health of operating workers and the environment.
1. Bauxite powder
Bauxite has been widely used in ordinary casting steel coatings and cast iron coatings in recent years because of its high refractoriness, good chemical stability, small expansion coefficient, abundant sources, and low price.
1.1 Chemical composition
The content of Al2O3 in bauxite, the sintering temperature and the content of impurities such as Ti02 and FeO3 have a great influence on the coating. Therefore, the ore with high Al2O3 content is selected and calcined at 1450°C-1500°C, and the clinker is selected and ground to make fine powder. Finally, the pickling treatment is still a key step. In addition to the high content of Ti02 and FeO3 in the bauxite without pickling treatment, the dust and mechanical iron mixed in the calcination and grinding process increase, (currently the bauxite fine powder Most of the processing is based on Raymond mills, and most of the wear-resistant parts are high-manganese steel castings, and the mechanical iron mixed in is difficult to remove by magnetic separation.) The prepared coating is firstly affected by refractoriness, and secondly, by the increase of floating objects and sediments, which affects Paint brushability, suspension and storage time.
1.2 Granularity
The particle size and particle size distribution of bauxite are important factors affecting the density and air permeability of the coating, especially the lost foam coating. Air permeability is a major feature that distinguishes it from other sand casting coatings. According to different needs, Xiangyu Company formulates the particle size gradation of bauxite into two different specifications for use in sand casting coatings and lost foam coatings respectively.
The particle size distribution of bauxite powder used in sand casting coatings is:
-270 mesh to 320 mesh, the concentration rate is 96%
+270 mesh no more than 1%
The particle size distribution of bauxite used in lost foam coatings is:
-270 mesh to -320 mesh 78%
-200 mesh to -270 mesh 15%
-100 mesh to -200 mesh 5%
+100 mesh not more than 2%
The above particle size distribution is also applicable to the gradation 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 stability, high refractoriness and no chemical effect on most alloys at high temperature. It is a good refractory filler and is mostly used in stainless steel. , high-alloy steel castings, imported zircon powder is mostly ground by jet mill abroad, the Zr02 content is greater than 65%, and the impurity content is low. Domestically processed zircon powder is mostly ground by Raymond mill, and the impurity content is relatively high. In recent years, zirconium The source of Huan powder is relatively complicated, and the price has been rising all the way. Many coating manufacturers are looking for substitute products for zircon powder to reduce the cost of cast steel coating. Xiangyu company has successfully developed a new artificially synthesized refractory filler N-3 powder, which has been Widely used in cast steel coatings.
N-3 powder is synthesized from several natural refractory materials in proportion, electromelted, pulverized, and acid-washed. It is beneficial to the anti-sanding property of the coating, and the price is only one-third of that of zircon powder.