The Formation Mechanism And Preventing Methods Of Hot Tears In The Thin-Wall Steel Castings With Furan Nobake Resin Sand

The thin-wall steel castings are prone to hot tears when furan no bake sand is used as molding and core material. To solve hot tears in the thin-wall steel castings is a difficult problem for many foundry factories. The purpose of this paper is to explore the formation mechanism of the hot tears in the thin-wall steel castings made by furan no bake sand process and to formulate the methods for eliminating hot tears.In order to show the appearance and location of the hot tears in the steel castings, Railway bolster and side frame were first made by furan no bake sand according to the old sodium silicate sand process. And then, four kinds of test castings were designed referring to the geometric and the thickness of the bolster and side frame. These test castings were made by three kinds of sand: furan no bake sand, polycyclic sodium sand and sodium silicate sand. The macro-appearance of hot tears was analyzed. The hot tears were divided into two kinds: shrinkage hot tears and stress hot tears.According to the results of the analysis and the characteristic of the furan no bake sand, The new viewpoint about formation mechanism of the hot tears in the thin-wall steel casting was presented. The research showed that the shrinkage hot tears and the stress hot tears have different formation reasons. The main formation reason of the shrinkage hot tears is that the hot spots have bad thermal defused condition and sulfur pickup in the casting surface. Therefore, the liquid solidification in there will take a longer time. When the liquid feed to other section and there can not be feed enough, the shrinkage hot tears occur. Because of the bad yield of the furan no bake sand, when the casting cool to the temperatures near the solids, the bigger stress will occur. When the stress exceed the strength of the steel at that temperature, the micro-crack first form in casting surface where the supplied concentrate. With the increasing of the stress, the micro-cracks propagate and stress hot tears form.The bad thermal conductivity of the furan no bake sand is the main reason of the shrinkage hot tear formation. The bad yield ability of the furan no bake sand is the main reason of the stress hot tear formation. The sulfur pickup in the surface of the casting increases either the tendency of the shrinkage hot tears or the tendency of the stress hot tears.Based on the new viewpoint about the hot tears, the fundamental rule of eliminating the hot tears was proposed. The different methods of preventing shrinkage hot tears and stress hot tears were given out. These methods were applied to foundry process design of the bolster and side frame. The hot tears were significantly reduced. Finally the qualified castings were manufactured.