Iron Casting – White iron and Gray iron castings
White iron and gray iron are two ordinary cast iron, so what is the difference between these two materials? This article will introduce these two types of cast iron materials from several aspects, such as appearance, characteristics, and applications.
Appearance of Gray Iron Castings
Gray iron is the most common type of iron, also we can call it ordinary cast iron. This material has a raspy gray cross-section, and its internal carbon is present in the form of flaky graphite.
Characteristics of Gray Iron Castings
Gray iron usually formes during the melting process due to the slow cooling rate. However, it is mainly due to a large number of silicon elements prompting the graphitization of carbon. Therefore, the silicon content in gray iron is high, generally requiring up to 1%-3.5%.
Gray iron is an inoculated iron (also called metamorphic iron) obtained by inoculation-treatment of ordinary iron with higher strength. The carbon content of gray iron is more than 4.3%, and most or all of the carbon element in this iron exists as free state flake graphite.
Illustration-gray-iron-metallurgical-microstructure
Illustration-cross-section-of-gray-iron-castings
Applications of Gray Iron Castings
Gray iron is one of the most widely used iron. It has excellent castability, machinability, wear resistance, and vibration damping. The compressive strength and hardness of this iron are close to carbon steel, but the tensile strength and plasticity are low and brittle.
The production process of gray iron is inexpensive and straightforward. And this kind of iron usually applies in many mechanical engineering applications, mainly for wear-resistant components under medium loads.
Illustration-gray-iron-castings-flange-loop
Appearance
White cast iron is in bright or dark white color at the cross-section, with coarse grains and obvious directionality, in which all the carbon elements exist in the form of cementite(Fe3C).
Characteristics
The carbon content of white cast iron is about 2.5%, and the content of silicon element is less than 1%. Due to the large amount of hard and brittle Fe3C, white cast iron is hard, brittle, and difficult to be machined.
Ordinary white cast iron includes bainitic white cast iron and martensitic white cast iron based on its matrix.
Illustration-white-iron-grains
Illustration-cross-section-of-white-iron
Applications
Due to its high hardness, brittleness, and difficulty in machining, white cast iron is seldom used directly in industrial applications. Still, it is suitable for a few parts that require wear resistance and are not subject to impact, such as wire drawing dies, iron balls for ball mills, etc. And the most common usage of white cast iron is applied as billets for steel making and malleable iron.
Some gray cast iron parts require high surface hardness and wear resistance, such as train wheels, rolls, plowshares, etc. Usually, we can apply the chilling process onto the castings’ surface to obtain a white-iron layer while retaining the organization of gray iron in the center, so this kind of iron is also known as chilled-cast-iron.
Illustration-white-iron-cushion-plates
Comparison Table of White and Gray Iron Castings
Let’s summarize the differences between white and gray iron.
| White Iron | Gray Iron | |
|---|---|---|
| Color of Cross-Section | Silver White | Gray |
| Form of Carbon | Cementite | Flaky Graphite |
| Carbon Content | 2.5% | >4.3% |
| Silicon Content | <1% | 1%-3.5% |
| Wear Resistance | Good | Good |
| Hardness | Harder | Hard |
| Brittleness | Brittle | Brittle |
| Machinability | None | Good |
To find more information about metal casting processes, click here.
