Cast iron and cast steel are engineered metals used in the casting process.
The difference between cast iron and cast steel is in their composition. Cast iron has more than 2% carbon content, while cast steel only has up to 0.75% carbon. This carbon content variation modifies their structural properties, affecting their castability, machinability, weldability, abrasion resistance, compressive strength, etc. In common, both metals have a larger composition of iron.
This article discusses cast iron and cast steel by looking into their differences, applications, etc.
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Difference Between Cast Iron and Cast Steel
| Property | Cast Iron | Cast Steel |
|---|---|---|
| Castability | Easy to cast | Difficult to cast |
| Cooling rate | Fast non-uniform cooling. | Slow uniform cooling |
| Fracture | Rare instances of fracture | Easily fractures |
| Machinability | Easy to machine | Difficult to machine |
| Weldability | Develops cracks while welding | Doesn't develop cracks while welding |
| Vibration Damping | High damping capacity | Low dampening capacity |
| Compressive Strength | High compressive strength | Less compressive strength |
| Impact resistance | Less impact resistance | High impact resistance |
| Wear resistance | High wear resistance | Low wear resistance |
| Cost | Cheap | Expensive |
Castability
The major difference between cast iron and cast steel is in their castability.
Cast iron can be easily made into a quality casting compared to cast steel. This is due to their material shrinkage property.
As cast steel shrinks, it becomes tedious to calculate the shrinkage rate and plan the manufacturing as it requires more melted steel to make the casting.
Cooling Rate
Comparatively, cast steel has a lower but uniform cooling rate. Cast iron cools faster, but the cooling rate varies throughout its surface.
Fracture
Cast iron has less chance of getting internal or external fractures as it doesn't shrink much in the casting process.
Unfortunately, cast steel is susceptible to fracture as it shrinks during the casting process.
Machinability
The state of carbon in both cast iron and cast steel also plays an important role in their machinability.
In cast iron, carbon exists in the form of graphite flakes, making it easier to break while machining. But in cast steel, the carbon is in the form of sphere grains, making it difficult to process.
However, you can machine steel by following the proper machining procedure and ensuring good process control.
Weldability
Because of the high carbon content of cast iron, you can't weld it without cracking, but you can easily weld cast steel.
Vibration Damping
In the case of vibration dampening, cast iron shows high resistance to vibration than cast steel. This is why cast iron tools are frequently employed in mining jobs.
Compressive Strength
The compressive strength or the ability to maintain its structure with loading is higher for cast iron than cast steel. This also aids it in machining.
Impact Resistance
Impact resistance is the ability of a material to absorb immediate shock.
Cast steel shows high impact resistance due to its tough nature.
Comparatively, cast iron has less impact resistance due to its brittle nature.
Wear Resistance
Cast iron has high resistance to wear, and abrasion as its graphite content acts as a dry lubricant, allowing smooth friction contact without causing wear.
Comparatively, cast steel is more susceptible to wear, but some of its alloyed variants show better wear resistance. They are usually formulated with more amount of silicon, manganese, etc.
Cost
In terms of cost, cast iron is way more affordable than cast steel as its raw materials are cheaper than building steel.
Also, the steel casting process requires more labor and time, which adds to the cost.
How to tell the difference between Cast Iron and Cast Steel
It's hard to visually identify cast iron and cast steel as they show similar surface characteristics.
Most cast iron alloys tend to have a grey color with a tint of silver, but cast steel primarily has a silver color.
Don't only rely on color for identification as it changes on various alloys.
You can also identify them by examining their grain structure.
Because of the cast iron's graphite content, its grains have a flake-type structure, which is why it produces small chips when machined.
The cast steel gains have a globular structure that produces curly chips when machined.
Some other techniques to identify cast iron and cast steel includes chemical analysis and spark test.
Cast Iron: What You Need to Know
Cast Iron is commercial iron that is used for casting jobs. It is very hard and brittle in nature.
Generally, cast iron would have 93-97% iron, 1-3% alloying elements, and 2-4% carbon.
| Materials | Composition |
|---|---|
| Iron | 93 - 97% |
| Alloying elements (silicon, magnesium, etc.) | 1 - 3% |
| Carbon | 2 - 4% |
The color of parts made using cast iron will be greyish-black. Sometimes they maintain a color that is more of a black shade.
Cast iron is divided into various types depending on the composition of carbon and other alloys.
This includes grey cast iron, white cast iron, ductile cast iron, compacted graphite cast iron, malleable cast iron, abrasion cast iron, spheroidal cast iron, and austenitic cast iron.
Grey Cast Iron
| Carbon | Silicon | Manganese | Sulfur | Phosphorus |
|---|---|---|---|---|
| 2.5 - 4.0% | 1.0 - 3.0% | 0.2 - 1.0% | 0.02 - 0.25% | 0.02 - 1.0% |
Grey cast iron is inexpensive compared to other types of cast iron. It has good machinability and is highly brittle.
In grey cast iron, carbon exists in the form of graphite. This is what provides a grey color to the metal.
White Cast Iron
| Carbon | Silicon | Manganese | Sulfur | Phosphorus |
|---|---|---|---|---|
| 1.8-3.6 | 0.5-1.9 | 0.25-0.8 | 0.06-0.2 | 0.06-0.2 |
White cast iron has a high compressive strength, helping it resist longitudinal deformation. It also has good resistance to wear.
Unfortunately, white cast iron is difficult to machine as it has carbon as iron carbide (cementite), a hard material.
Ductile cast iron
| Carbon | Silicon | Manganese | Sulfur | Phosphorus |
|---|---|---|---|---|
| 3.0 - 4.0% | 1.8 - 2.8% | 0.1 - 1.0% | 0.01 - 0.03% | 0.01 - 0.1% |
Ductile cast iron is known for its ductility. This is because carbon added to it has a spherical shape helping it radically re-distribute applied stress.
It also is highly machinable and has a good toughness character.
Malleable Cast Iron
| Carbon | Silicon | Manganese | Sulfur | Phosphorus |
|---|---|---|---|---|
| 2.0 - 2.9% | 0.9 - 1.9% | 0.15 - 1.2% | 0.02 - 0.2% | 0.02 - 0.2% |
Malleable cast iron is known for its machinability. It can be bent and worked for a long time without causing fatigue and fracture to the material.
Heat treatment processes like annealing and tempering help it acquire a highly machinable nature.
Spheroidal Cast Iron
| Carbon | Silicon | Manganese | Sulfur | Phosphorus |
|---|---|---|---|---|
| 3.0 - 3.6% | 2.0 - 2.8% | 0.1 - 0.2% | 0.005 - 0.2% | <0.4% |
Spheroidal cast iron has carbon in the form of nodules (spheres), providing high tensile and elongation properties.
It is highly machinable and has a low hardness.
Cast Steel: What You Need to Know
Cast steel has less carbon content, in a range of 0.1 - 0.75%.
It is known for its high hardness, machinability, and toughness. Unfortunately, it is not resistant to abrasion.
You can work cast steel in high and low temperatures as it alters specific mechanical properties to provide various outcomes.
Cast steel is broadly classified into two categories based on carbon and alloy content.
- Carbon cast steels
- Alloyed cast steels
The grading system classifies them. There are standard grading systems from ASTM (American Society for Testing and Materials) and grading from foundries to produce special castings.
Carbon Cast Steel
Carbon cast steels are again divided into three sub-types based on the amount of carbon content.
| Type of Carbon Cast Steel | Carbon Content |
|---|---|
| Low | <0.2% |
| Medium | 0.2 - 0.5% |
| High | 0.5 - 0.75% |
Alloyed Cast steel
Alloyed cast steel is subdivided into two types based on the alloy content.
| Type of Alloyed Cast Steel | Alloy Content |
|---|---|
| Low | <8% |
| High | >8% |
What should you choose? Cast Iron or Cast Steel?
Depending on your application, you can use either cast iron or steel.
When to Choose Cast Iron
Choose cast iron
- If you require a hard, but not tougher material.
Hardness is the ability of the material to resist scratches, load, etc. Toughness is the property that involves the ability to conduct energy before undergoing breakage.
- To machine complex parts.
- If you need a material with very less impact resistance.
- You need an economical option.
- You need the material to have high compressive strength.
Cast iron is commonly used to make parts like gearbox cases, pans, cylinder blocks, pipes, etc.
When to Choose Cast Steel
Choose cast steel
- If you need a tougher material.
- If the application faces impact forces regularly (needs more impact resistance).
- If the cost is not a constraint.
- If the application requires good wear resistance and decent compressive strength.
Cast steels are used to make parts for high-pressure valves, high-speed train tracks, mining equipment, etc.
Final Thoughts
Cast iron and cast steel are great materials to work with, but to optimize the part better, choose them based on the job requirement.
Cast Iron is the best option when you need a hard material with high compressive strength. Unfortunately, it doesn't offer good impact resistance.
If the part needs to resist impact, go with cast steel. It is a tougher material, but it costs more than cast iron.
Though they look almost similar, they have differences in their material composition and properties.
Frequently Asked Questions
Do cast iron pans get better with time?
Yes, cast iron pans get better with time. When a cast iron pan is oiled well and maintained properly, it ages well. The surface properties of the pan will alter a little bit, but ultimately, it benefits you.
Do cast iron and cast steel conduct electricity?
Yes, cast iron and cast steel conduct electricity. The majority of the composition of cast iron and cast steel is iron, a pure conductor of electricity. So be careful when handling these materials in an environment exposed to electricity.
What is casting?
Casting is a manufacturing process in which a material is molten beyond its melting point and poured into a pre-designed mold or “cast,” from which the final product is separated. It is an important manufacturing technique with a wide range of applications. From transport to machine tools, utensils to architecture, the casting process is used in many industries to make numerous products.
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