Because stainless steel castings cool and solidify faster in metal molds than in sand molds, and metal molds lack flexibility, the casting stress generated in the casting during metal mold casting is greater than that in sand casting, resulting in a higher tendency for cracking and making defects such as incomplete filling, cold shuts, and white iron (for cast iron castings) more likely to occur. Generally:
(1) In order to prevent incomplete pouring and cold shut in stainless steel casting, the following structural issues should be noted: A. The shape of the casting should be as streamlined as possible, avoiding sharp corners and abrupt changes in the structural shape, so as to facilitate the flow of molten metal; B. The wall thickness of the casting should be appropriate and not too thin, especially when the outline size of the casting is large; C. Large horizontal surfaces should be avoided, because they make the molten metal rise very slowly during pouring, and the surface in contact with air is large, which is easy to oxidize. At the same time, due to the rapid heat dissipation of the metal mold, the molten metal quickly loses its fluidity, which can easily cause defects such as incomplete pouring, cold shut, and slag inclusions.
(2) The following structural issues should be noted in order to prevent cracks in stainless steel casting: A. Ribs, bosses, etc. should be provided to strengthen the weaker parts of the structure to prevent casting cracks; B. When arranging reinforcing ribs on the casting, the influence on the shrinkage of the casting should also be considered; C. Protruding parts such as bosses, ribs, and flanges that hinder the free shrinkage of the casting should be minimized as much as possible; D. The requirements for uniform wall thickness, smooth transition and connection between walls, and appropriate rounded corners should be stricter than those for sand casting; E. Vertically connected walls should be changed to inclined connections.
When designing the basic structural units and selecting their parameters for stainless steel casting, the following should also be noted:
(1) In order to prevent the formation of white iron in gray cast iron parts, in addition to taking measures in terms of process, the wall thickness must not be too thin. (Some data indicate that when the wall thickness is above 15mm, the corners of the castings made by metal mold casting must be rounded. For aluminum alloy and magnesium alloy metal mold castings, the casting roundness should not be less than 3-4mm. For cast iron, cast steel and copper alloy metal mold castings, the casting roundness can be selected from Table 1.1-32.)
(2) Since the metal mold and the core are not flexible, in order to facilitate the removal of castings and the extraction of molds, the casting draft angle of metal mold castings should be appropriately larger than that of sand castings, generally 30%-50% larger. It should be pointed out that the casting draft angle is related to the alloy type and wall height, as well as the position of the casting surface. Casting surfaces that tend to separate from the metal mold surface when the casting cools and shrinks can be designed with a smaller draft angle, while casting surfaces that tend to press against the metal mold when the casting shrinks should be given a larger draft angle. The casting draft angle of stainless steel castings of various alloys;
(3) Since metal molds dissipate heat quickly, the minimum wall thickness of stainless steel castings should be larger than that of sand castings. The minimum wall thickness of various casting alloys and castings of different sizes is also important. (4) The thickness of the inner wall and inner ribs of the casting should generally be 0.6-0.7 times the thickness of the connected outer wall. Otherwise, because the inner wall (ribs) cools slowly, cracks are likely to occur at the junction of the inner and outer walls when the casting shrinks.
When designing stainless steel castings with complex shapes, if the production process presents significant difficulties, the shape of the casting should be kept as simple as possible and the structure modified to facilitate its removal from the mold, without affecting its use.
