As an advanced casting method, lost pattern casting is paid more attention to and applied to the production of high manganese steel. At present, most coatings used by manufacturers are water-based corundum, zircon powder coatings or alcohol based magnesia powder coatings. The cost of using water-based corundum coating is high, and the coating performance is not ideal. Because zircon powder is weak acid and high manganese steel is alkaline, water-based zircon powder coating often causes serious sand sticking at the hot spot or thick castings when it is used to produce high manganese steel parts. The strength of alcohol based magnesia powder coating with polyvinyl butyral (PVB) as binder is low. If the use of PVB is increased in order to increase the strength, PVB foams and pockmarks appear on the casting surface. Therefore, in order to obtain high quality castings, it is necessary to develop new coatings to meet the needs of production of high manganese steel parts, especially large-scale high manganese steel parts.
1 Test materials
1.1 the refractory aggregate used in the refractory aggregate coating is ferrochrome slag, a byproduct of ferrochrome produced by ferroalloy plant, which is crushed and ground into powder (size of 200 mesh). Because its a l 2O 3 content is more than 85% and c r 2O 3 content is more than 10%, it is called high chromium corundum. Its melting point is 1830-2000 ℃, density is 3.68g/cm 3, Mohs hardness is 9, thermal expansion coefficient is about half of quartz, thermal conductivity is twice of quartz. High chromium corundum is a kind of neutral material, which is not easy to react with high manganese steel, and has high fire resistance, which can resist the erosion of high temperature metal liquid. Other properties are conducive to the preparation of coatings with good performance, and the cost is low, only 1 / 3 of the price of corundum powder and zircon powder.
1.2 phenolic resin is used as the binder of other component α – alcohol based coating, which has good heat resistance and appropriate price. PVB is both a suspending agent and a binder. The coating strength can be improved by mixing PVB with phenolic resin. Both of them are dissolved in alcohol before use. The suspension agent is lithium bentonite, which can form alcohol sol in alcohol solvent. It is initiated by a small amount of water, and after standing for 24 hours, it becomes paste like. At this time, it is added with appropriate amount of ethanol and then dispersed in a high-speed mixer to make paste like suspension agent. The solvent is industrial ethanol, the auxiliary agent is micro surfactant, defoamer (n-octanol), etc. In order to make the coating layer easy to peel off and prevent sand sticking during sand falling, the flux Fe2O3 is added.
2 Test method for coating performance
The suspension property is measured in a 100 ml measuring cylinder by the method of standing for 24 hours. The air permeability was measured by STZ direct air permeability tester (with standard sample of air permeability). The specific operation and calculation method of dry air permeability of coating layer are shown in the literature. The determination of coating strength is from the viscosity cup to the coated glass plate, until the coating is scratched and exposed, and the total mass of sand falling is called as the quantitative index to judge the coating layer strength. The determination of high temperature crack resistance is to immerse the water glass sand sample < 50 mm into the coating to form a 2 mm coating, dry it and put it into a high temperature furnace, and take it out in 1200 for 2 min to observe the cracking of the coating. Thixotropy is to measure the change of apparent viscosity of paint with time by NDJ – 1 rotary viscometer, that is, thixotropy curve. Other performance tests of coatings shall be conducted with reference to the literature.
3 Test results
The determination of the best formulation of the coating is the basic performance of the EPC coating. Therefore, the orthogonal test was used to investigate the influence of the main components of the coating (phenolic resin, PVB, bentonite and iron oxide powder) on its performance, so as to determine the best coating formula.