Learn about the performance characteristics of gray iron castings in one minute.

Gray iron castings are produced using high-density molding sand, characterized by their high clay content but low moisture and coal powder levels. This results in relatively low strength for gray iron parts, thereby limiting their applications. However, gray iron castings remain a cost-effective and versatile product. To enhance their strength, a small amount of carbon—or trace elements like manganese or chromium—can be added to the raw materials, effectively addressing the issue of insufficient strength. That said, gray iron castings are prone to loosening, often leading to defective products. Fortunately, these defects can be successfully repaired using several conventional welding techniques, yielding excellent results.

　　The basic characteristics of high-density molding sand used for gray iron castings are: high clay content, low moisture content, and a relatively low addition of coal powder. Therefore, when mixing the sand, it's important to keep the following points in mind.

　　1. Clay and silt content: The strength of molding sand with high clay content increases as compaction pressure rises, while the effective bentonite content is typically maintained between 7% and 10%. In molding sand, the combined amount of "effective" and "dead" clay particles roughly corresponds to the silt content, which is usually controlled within the range of 12% to 16%. Both excessively high or low silt content can negatively impact the various performance characteristics of the molding sand.

2. Moisture. The moisture content in lost-foam casting molding sand is the primary factor determining the plasticity and binding strength of the clay. When the water content is too high, it can easily lead to poor clay adhesion, reduced flowability of the sand, and uneven mold density. Conversely, if the moisture level is too low, the sand becomes difficult to mix uniformly, resulting in low sand strength, increased brittleness, and poor mold-release performance—conditions that often cause sand sticking defects in the castings.

3. The particle size of the original sand. When producing high-density molds, the sand mold has higher density and expands more significantly during pouring; therefore, the particle size of the original sand should not be overly concentrated. Ideally, the original sand particles should be either round or polygonal in shape—commonly, three-sieve sand or four-sieve sand is selected.

　　Performance Analysis of Gray Iron Castings:

　　1. Machinery

　　Mechanical property data for gray iron castings are typically obtained from single casting test bars. Since the microstructure and mechanical properties are significantly influenced by the cooling rates during both the solidification range and the eutectoid transformation zone, the properties measured on these test bars cannot accurately represent those of actual gray iron castings with varying shapes, wall thicknesses, and test sections. Wedge-shaped specimens are used to illustrate how differences in cooling rates—caused by variations in wall thickness across different areas—affect the resulting microstructure and mechanical performance. Therefore, the data derived from test bars reflect only the microstructure and mechanical properties under the specified testing conditions. For gray iron castings with specific requirements, casting test bars or attached test bars with cooling rates closely matching those of the critical sections can be selected for performance evaluation, based on the wall thickness of the key areas. The test bar dimensions recommended by British standards, meanwhile, correspond to an average thickness.

　　2. Physics

　　The density of gray iron castings depends on the relative content of each component in their microstructure, with carbon and graphite content having a significant impact. Different grades of gray iron castings typically exhibit densities ranging from 6.95 to 7.35 grams per cubic centimeter. The specific heat capacity, meanwhile, is influenced by both the material composition and the heating temperature. As for the coefficient of linear thermal expansion, it primarily varies with the temperature range; however, within typical operating ranges, the amounts of carbon, silicon, manganese, phosphorus, and sulfur have relatively little effect.