Research On The Detection Of Ooliticization Rate Of Green Sand And Its Influence On The Properties Of Sand And Casting Defects

In green sand casting production,the old sand produced after casting must be recycled,so the quality of recycled sand is an important factor of the molding sand performance and casting quality.For green sand,bentonite layers,which fail under the action of high temperature for many times,will adhere tightly to the surface of sand grains and accumulate to form oolitic layers.The oolitic layer can not be removed by mechanical or wet regeneration,but gradually thickened oolitic layer is formed with the increase of regeneration times.It not only affects the properties of sand such as wet compression strength,air permeability and compactness,but also affects the formation of casting defects such as porosity,clay sand and sand hole,which leads to fluctuations in the properties of sand and casting quality.At present,developed countries pay much attention to the oolitization rate of wet sand,which is a key index,and it is widely used in casting production.However,in China,there is no report on the detection method of oolitization rate of wet sand and its application.Therefore,this paper takes the wet sand in the vertical molding production line of a domestic foundry enterprise as the object to carry out the research on the oolitization rate detection method of wet old sand and its influence on the properties of wet sand and casting quality,which has great practical value.The main work of this paper is as follows:Firstly,the main performance indexes,such as mud content,effective bentonite content and effective coal powder content,of the old wet sand in five molding lines were tested.The results show that the mud content is 13.39%?23.57%,the effective bentonite content is 8.29%?9.02%,and the effective pulverized coal content is 3.42%?7.95%.In terms of the detection method of oolitization rate,firstly,the influences of core sand mixing and metal particle content on the accuracy of oolitization rate are analyzed,and the sampling conditions of old sand are proposed.The methods of oolitization rate detection reported in literature were summarized,and hydrochloric acid-phosphoric acid method and hydrochloric acid-potassium hydroxide method were selected for experiment and optimization.The heating method was changed to heating with furnace to avoid coke film appearing on the sample surface.In order to remove oolitic layer more completely,the boiling process was changed to boiling + stirring process.The hydrochloric acid in hydrochloric acid-phosphoric acid method is cancelled and simplified to phosphoric acid method to reduce the loss of metal elements and the measurement value is more accurate.The reaction time of hydrochloric-potassium hydroxide method was extended from 25 min to 30 min,and the problem of unwashed sand was solved.A small amount of hydrochloric acid is dropped into the solution mixed with the precipitation and sand particles,and the precipitation is effectively removed,which solves the problem that the two are difficult to separate.Then,the oolitization rate of the tested old sand is analyzed with its mud content,effective bentonite content and effective coal powder content.When the mud content in the old sand is high(low),the oolitization rate content is also high(low).The trend is the same but not strictly linear.In this experiment,the effective bentonite and coal additives have little influence on the oolitic rate.Finally,the relationship between the oolitization rate of wet sand and the properties of wet sand and casting defects was established,and the mechanism was preliminarilyanalyzed.When the oolitization rate is between 10% and 17.5%,the wet compressive strength decreases and the air permeability increases.When the oolitization rate is above 17.5%,the wet compressive strength and the air permeability basically remain stable.When the oolitic rate is below 12.6%,the compaction rate remains stable,and when the oolitic rate is between 12.6% and 21%,the compaction rate gradually decreases.The water absorption and water retention of oolitic layer are the key factors affecting the properties of the above molded sand.In terms of casting defects,when the oolitization rate is 10%?21.5%,the porosity decreases first and then increases,and the porosity is the least when the oolitization rate is12.6%.When the oolitization rate is between 10% and 12.6%,there is almost no adhering sand defects in the casting,but when the oolitization rate is above 17.5%,the casting will begin to adhering sand defects.When the oolitization rate is between 10% and 12.6%,the sand hole rate decreases,and when the oolitization rate is above 12.6%,the sand hole rate basically remains stable.During casting production,the oolitic ratio of molding sand should be kept near 12.6% to reduce the porosity,adhering sand defects and sand hole defects.