The difference between cast steel and cast iron and how to distinguish them

Cast iron and cast steel are both iron-carbon alloys. However, cast steel and cast iron have different metallurgical structures after crystallization due to the different percentages of chemical elements such as carbon, silicon, manganese, phosphorus, and sulfur. Generally, in actual work, we call iron when the carbon content is higher than 2%, and it is called steel while lower than that.

Therefore, the essential difference between cast iron and cast steel is the chemical composition. And because of this, cast iron and cast steel have different organizational properties. Generally speaking, the plasticity and toughness of steel are better, demonstrated as good elongation, sectional shrinkage, and impact toughness. At the same time, the mechanical properties of iron show that it is hard and brittle, while some kinds of cast iron have some unique properties.

There are many differences in the mechanical and processing properties of cast iron and cast steel.

For example, cast iron’s elongation, section shrinkage, and impact toughness are lower than cast steel’s, while the compressive strength and vibration resistance of cast iron are better; The liquid fluidity of grey cast iron is better than that of cast steel, and it is more suitable for casting thin-walled castings with complex structure; Cast iron shows brittle fracture during the bending test, while cast steel deforms; etc.

Therefore, cast iron and cast steel are suitable for casting different parts with different requirements.

Cast iron is an iron-carbon alloy with a carbon element content of 2% or more. For industrial applications, cast iron generally has a carbon content of 2% to 4%. Carbon element mainly exists in graphite formation in cast iron, sometimes also in the form of cementite. And in addition to carbon, cast iron also contains 1% to 3% silicon as well as manganese, phosphorus, sulfur, and other elements. Moreover, alloyed cast iron also contains nickel, chromium, molybdenum, aluminum, copper, boron, vanadium, and other elements. Among them, carbon and silicon are the significant elements that affect the microstructure and properties of cast iron.

(Graphite form of grey cast iron)

The carbon content of grey iron is high (2.7% to 4.0%), and the carbon element exists mainly in the form of flaky graphite. As the fracture of the casting is in grey color, grey iron thus gets its name.

Due to its low melting point (1145-1250°C), grey iron has a low shrinkage during solidification. And its compressive strength and hardness are close to carbon steel, with good shock absorption.

(Graphite form of white iron)

White iron has a low carbon and silicon content, and the carbon element exists mainly in the form of cementite. The fracture of white iron castings is silvery-white, and it shrinks more during solidification, making it easy to create shrinkage holes and cracks. Due to the high hardness and brittleness, white iron castings cannot withstand impact loads.

(Graphite form of malleable cast iron)

Malleable cast iron comes from annealed white cast iron, in which the graphite is distributed in clusters. Sometimes we can also call it malleable steel because it has uniform organizational properties, is resistant to wear and tear, and has excellent plasticity and toughness.

(Graphite form of ductile cast iron)

As a result of the spherical treatment of the grey cast iron, the precipitated graphite is in the form of spheres, and that is what we call ductile iron. Compared to grey cast iron, ductile iron offers higher strength, better toughness, and plasticity.

(Graphite form of vermicular cast iron)

When grey cast iron goes through the vermicularizing treatment, the precipitated graphite appears worm-shaped. We call this vermicular cast iron. Its mechanical properties are similar to those of ductile iron, and its casting properties are in between those of grey iron and ductile iron.

Ordinary cast iron becomes alloyed cast iron after alloying with appropriate amounts of other elements (such as Si, Mn, P, Ni, Cr, Mo, Cu, Al, B, V, Sn, etc.). And the addition of the alloying elements changes the matrix organization of the cast iron. It has the corresponding characteristics of heat resistance, wear resistance, corrosion resistance, low-temperature resistance, or non-magnetic.

Ductile iron and malleable iron are two of the more common types of cast iron materials. Then what is the difference between ductile iron and malleable iron?

The hardness, wear resistance, and tensile strength of ductile iron are much greater than those of malleable steel. And the tensile strength of ductile iron can even reach 1000 MPa. So ductile iron can be used for various kinds of high-strength structural parts such as engine crankshafts and gears.

We can distinguish between ductile iron and malleable iron by listening to the sound. Malleable iron has a very sharp and short sound, while ductile iron has a loud and long echo.

Cast steel is the steel used to make castings. It is a casting alloy.

Cast steel can be categorized into cast carbon steel, cast low-alloy steel, and cast special steel.

Casting carbon steel alloyed with carbon as the main element and contains a small number of other elements.

Casting steel containing less than 0.2% carbon we call low carbon cast steel, 0.2% to 0.5% carbon is mid-carbon cast steel, while the carbon content of more than 0.5% is high carbon cast steel.

With the increase of carbon content, the strength of cast carbon steel grows, and the hardness rises. And cast carbon steel has high strength, plasticity, and toughness, with a lower cost. Components made in cast carbon steel can serve in the manufacture of heavy machinery to bear large loads.

Low-alloy cast steel contains Mn, Cr, Cu, and other alloying elements, but the total amount of alloying elements is generally less than 5%.

The low-alloy cast steel has an excellent impact toughness and can obtain better mechanical properties through heat treatment. It has better serviceability than carbon steel, and it can reduce the weight of parts and improve their service life.

Special cast steel is alloyed cast steel made to suit particular applications. There are various styles, usually containing one or more kinds of high-volume alloying elements to obtain a specific characteristic.

Cast steel is brighter, whereas cast iron is darker and greyer. Meanwhile, the ductile iron is brighter than the gray iron.

Cast steel is so dense that the grains are generally invisible to the naked eye. And the grains in both grey iron and ductile iron are visible, while the grains in grey iron are relatively larger.

There is a sound of “Dang Dang” from the collision of cast steel parts, but there is no such sound after the crash of cast iron parts.

The surface of cast steel parts is rough, and the area of riser and sprue are relatively large, so they must be removed by gas cutting. In contrast, ductile iron can not be cut with gas cutting.

The toughness of cast steel is close to that of steel plates. On the other hand, ductile iron is slightly less resilient, and some thin-walled parts can be bent to 20-30 degrees. Whereas grey cast iron has no toughness at all.