Cast iron trench drain grates are made by melting metal into a liquid form then pouring this liquid metal into a cavity shaped like the finished part. The cavity is shaped from a sticky sand. After the cast iron is poured into the cavity, it is allowed to cool. Once the cast iron drain grate has cooled the sand is broken and cleaned off of the cast iron leaving a trench drain grate.
During the melting and casting process the metallurgy can be altered by changing raw components and additives. Iron trench drain grates are categorized into two broad categories; Ductile iron and Cast iron. Cast iron is often called “pig iron”. This is the oldest technology and produces a strong iron part. Additives have become more popular in past years that create a spherical grain structure in the iron imparting ductility. These metals are called “Ductile Iron”. The ductility means that they can take more loading without cracking or breaking. Ductile iron trench drain grates are typically used when heavy loading such as Aircraft might be present.
Cast iIron trench drain grates can be cast in a variety of grades and each class of iron has several grades. Generally these are unimportant to the designer as long as the iron grate can reach the required loading. Cast iron grates are governed by AASHTO M105 class 35B or ASTM A48 Class 35B. Ductile irons are governed by ASTM A536.
Cast iron trench drain grates and ductile iron trench drain grates can be used in their natural state or they can be coated. Iron trench drain grates posses a natural corrosion resistance that is inherent to their material. When exposed to nature iron forms a protective surface coating of iron oxide which slows the corrosion. The early stages of this iron oxide is a reddish brown but with time ductile and cast iron trench drain grates will age to a dark brown similar to any other manhole casting.