Gray Cast Iron Foreign and Domestic Grades
Gray cast iron is an important commercial material used for a variety of engineering applications due to its unique combination of high strength, wear resistance, castability and machinability. While the performance of gray cast iron depends on the quality of the raw materials and the casting process, one of the key elements is the grade or type of iron used. Two distinct categories of iron used in casting are “foreign” (or foreign equivalent) and “domestic” (or domestic equivalent).
Foreign Grade Gray Iron
Foreign grade gray iron is typically used to describe gray iron with a tensile strength range between 12,000 psi and 22,000 psi produced to American Society for Testing and Materials (ASTM) or European standards. Foreign cast iron includes grades such as Ductile Iron (EN-GJS or ASTM A536), Austempered Ductile Iron (ADI or ASTM A897) and Compacted Graphite Iron (CGI or ASTM A395). Although the standard numbers are different, the grades are generally cross-referenced, and foundry production facilities may be able to meet one standard with a grade compliant with another.
Domestic Grade Iron
Domestic grade iron typically describes iron produced to ASTM A48 or ASTM A395 specifications. A wide range of mechanical properties is available through various types and classes, including “gray iron class 25”, “gray iron class 30” and “Compacted Graphite Iron (CGI) class 30”. The class designation is directly correlated with the minimum tensile strength. “Gray iron class 25” has a minimum tensile strength of 25,000 psi, while “CGI class 30” will have a minimum of 30,000 psi.
In recent years, Iron Foundries have tended to favor foreign grade iron due to its higher strength, longer wear life and improved machinability. The increased use of foreign grade iron has resulted in the widespread adoption of “foreign equivalent” grades (sometimes indicated simply as “foreign” or “F”) for domestic castings. This is a beneficial solution as it allows foundries to use the same process, equipment and procedures to produce high quality castings, while still providing superior strength and performance.
Conclusion
Gray cast iron is an important engineering material due to its many beneficial properties and is commonly used in a variety of industrial applications. When it comes to the suitability of an iron grade, two distinct types are in use – foreign and domestic. Although there are clear differences in the characteristics and strengths of these two grades, Iron Foundries often opt for foreign-equivalent grades due to their superior strength and machinability. It is important to be familiar with both categories of gray cast iron to ensure that the best grade is selected for each particular application.
