Basic characteristics of gray iron castings
The basic characteristic of high-density molding sand for gray iron castings is that the amount of pulverized coal added is also low. Therefore, the following points should be grasped during sand mixing.
(1) Clay content and mud content. The strength of molding sand with high clay content increases with the increase of specific pressure, and the effective bentonite is usually controlled at 7% to 10%. The sum of effective clay and dead clay in molding sand is equivalent to the mud content, which is generally controlled at 12% to 16%. Too high or too low mud content can affect various performance indicators of molding sand.
(2) Moisture. The water content in the molding sand used in lost foam casting is the main factor determining the plasticity and adhesion of the clay. When the moisture content is too high, it is easy to cause deterioration of the cohesive force of the clay, decrease in the fluidity of the molding sand, and failure to obtain a uniform mold density. When the moisture content is too low, the molding sand is not easy to mix evenly, the molding sand has low strength, high brittleness, and poor mold liftoff ability, which can easily cause sand sticking defects in the casting. Generally, the moisture content should be controlled to be 10% to 20% higher than the moisture content at the time of obtaining the peak wet compressive strength.
(3) Original sand particle size. During high-density molding, the density of the sand mold is high and the expansion is large during pouring. Therefore, the original sand particle size should not be excessively concentrated. The original sand particles are circular or polygonal, and generally three or four screen sand is selected.
Method for increasing tensile strength of gray iron castings:
(1) A small amount of carbon can be added to the raw material. In gray iron castings, the carbon content is mostly 2.6% to 3.6%, and the silicon content is 1.2% to 33.0%. When determining the wall thickness of gray iron castings, an appropriate amount of manganese should also be added.
(2) Using alloying methods to improve the tensile strength of gray iron castings, adding an appropriate amount of microalloy, chromium, molybdenum, tin, and other elements can also significantly improve the tensile strength of gray iron castings, and do a good job of inoculation treatment of molten iron.