Steel is a ubiquitous resource to modern-day industry, with numerous applications of its strength and durability. The composition of steel can vary to suit a wide range of demands in engineering, construction and other sectors. Understanding whether to use carbon steel or black steel, for instance, is vital to industrial plans.
How is carbon steel different from black steel?
The main difference between carbon steel and black steel can be observed in their alloy composition and resistance to corrosive agents. Carbon steel alloy is predominantly composed of carbon and iron; due to the alloy’s susceptibility to rusting, it is usually galvanized. Black steel, in contrast, refers to steel that is non-galvanized, and instead blackened to inhibit corrosion.
What is Carbon Steel?
Carbon steel alloy is defined by its especially high concentration of carbon, with the element composing up to 2-2.5% of the alloy compared to the 0.05-0.3% content found in other steel alloys.
Added carbon content strengthens the molecular structure of the alloy, enabling it to better withstand stress and high pressures. These qualities make carbon steel desirable as a durable material for automobile parts, construction materials, railway tracks, high-strength wiring, and many more.
The increased strength of carbon steel can also be a disadvantage, as it makes the alloy difficult to shape and mold properly. Compared to other steel types, carbon steel also rusts and corrodes more readily, necessitating the addition of chromium as a protective coating.
Note that carbon steel can be further categorized based on its carbon content:
- Low-carbon steel. This type of carbon steel is relatively weaker and contains no more than 0.25% carbon. Because it is easier to weld and turn into thin wires, it is the most commonly used form of carbon steel.
- Medium-carbon steel. These steels are somewhat stronger than low-carbon steel, containing 0.25-0.6% carbon and further strengthened with manganese and heat.
- High-carbon steel. Usually containing 0.6-1.25% carbon or more, this steel is the most physically durable type of carbon steel. It is hardened and resistant to wear, at the cost of low ductility and weldability.
What is Black Steel?
Black steel pertains to steel that has not undergone galvanization to protect against corrosion. Instead, the steel is subjected to a chemical conversion process called blackening to form a resilient black iron oxide, or magnetite (Fe3O4), which gives the material its name.
During blackening, a black oxide solution containing oxidizing salts is applied to the steel. Depending on the type and temperature of the treatment, magnetite usually develops in 15-45 minutes. The initial reaction adds some resistance to corrosion and wear, although oil-based post-treatments are often applied for further strengthening.
Due to its resistance to corrosion, black steel is popular as a material for piping. Black steel pipes are used for gas and water transportation, electrical wire protection as conduits, and sewage delivery systems.
Black steel is a versatile material. Other applications can be found in firearms and ammunition, aerospace vehicles, automotive components, architecture, and medical equipment.
Differences between Carbon Steel and Black Steel
Alloy Composition
As an alloy, carbon steel is aptly named because it consists primarily of only carbon and iron. It contains far less chromium than other types, such as stainless steel. The amount of carbon varies among carbon steel’s many varieties, generally ranging from 0.25-3% carbon.
Black steel contains a much smaller amount of carbon, although the element is still present as it is the primary hardening element to make steel. It can also contain manganese and phosphorus for strengthening, and other alloying elements such as chromium, silicon and vanadium.
Note that many types of steel can become black steel so long as they undergo the blackening process.
Physical Properties
Carbon steel is distinguished by its superior strength. It is less likely to succumb to stress and high pressures than other types of steel, at the cost of being harder to weld, mold or turn into wires. It is brittle; attempts to shape it may result in the steel shattering into pieces.
Black steel is also strong, but not to the level of carbon steel. While their exact physical properties may vary depending on the steel used in blackening, the material generally has higher weldability, ductility and malleability than carbon steel.
Resistance to Corrosion
Carbon steel is susceptible to corrosive agents, including bleach, chlorine and strong acids. It can even corrode when exposed to air if atmospheric humidity reaches 70 to 80%.
In contrast, black steel is resistant to corrosion and abrasion due to the resilience of the magnetite on its surface as well as its oil-based treatment.
Treatment
To offset corrosion, carbon steel is often galvanized, or coated with a protective layer of zinc, although this process can be expensive. Note that black steel is never galvanized.
Black steel is initially treated in the blackening process, in which an oxidizing solution causes magnetite – or black iron oxide – to form on the surface. Because magnetite can corrode over time, an oil-based treatment is applied afterward to leave a protective heavy oil film on the surface.
Varieties
Black steel can be made from many types of steel so long as they undergo blackening. As a result, one can find varieties such as black stainless steel, black rebar, black tool steel, and even black carbon steel.
Carbon steel varieties differ in their carbon content. Low-carbon steels are weaker but more ductile and weldable, high-carbon steels are stronger but difficult to mold, and medium-carbon steels are a balance between physical strength and malleability.
Appearance
Despite its strength, carbon steel has an unsightly appearance. It has a matte finish with an uneven texture, and is easily discolored or stained due to its susceptibility to rust.
Black steel is quite popular due to its aesthetic appeal, possessing a smooth, glossy grey-black finish that does not rust. It is used as a decorative finish for chairs, countertops, railings, tables, refrigerators, and much more.
Comparison Chart: Carbon Steel Vs Black Steel
| Areas | Carbon Steel | Black Steel |
| Alloy Composition | Mainly carbon and iron. | Primary: Iron, carbon. Additives: Manganese, phosphorus, chromium, etc. |
| Physical Properties | Very high structural strength. Low malleability, ductility and weldability. | High structural strength. High malleability, ductility and weldability. |
| Resistance to Corrosion | Susceptible, unless galvanized. | Resistant. |
| Treatment | Galvanization. | Blackening: Oxidizing solution. Post-treatment: Oil-based solution. |
| Varieties | High-carbon steel, medium-carbon steel, low-carbon steel. | Many, i.e. black rebar, black carbon steel. |
| Appearance | Matte, uneven, easily stained. | Smooth, glossy, grey-black. |
How are Carbon Steel and Black Steel similar?
Despite their differences, carbon steel and black steel are still chemically categorized as steel. As alloys, they are both primarily composed of iron with added carbon for strengthening.
Being an alloy of iron makes them susceptible to corrosion, necessitating treatments such as galvanization and blackening.
Owing to their strength, hardness and durability, both types of steel are mainstays as materials in modern-day construction, engineering and consumer products. They are used as components in several applications, from tools and structural parts, to automobiles, firearms and kitchenware.
FAQ
What is black steel pipe used for?
Black steel pipes are popular for their durability and low maintenance needs. Their primary use is in delivering gas, water or fossil fuels across long distances, transporting steam and air under high pressures. They are also used as conduits to cover electrical wires.
What are the applications for carbon steel?
Varieties of carbon steel are used for different purposes:
- Low-carbon steel: flattened into thin sheets or strips for constructing automobiles, ships, panels for gates, or fences. Their higher ductility makes them suitable as wire material.
- Medium-carbon steel: machined to form the structural material for bridges, buildings, rails, trucks, pipelines, and gears.
- High-carbon steel: processed into heavy-duty blades, punches, cutting tools, and wiring.
Conclusion
Black steel and carbon steel are two common types of steel.
They differ primarily in the composition of their alloys, with carbon steel containing mostly carbon and iron, while black steel can be made from any steel subjected to blackening.
Due to its weakness to corrosive agents, carbon steel often goes through galvanization with a protective layer of zinc. Black steel, on the other hand, gains resistance through the formation of magnetite (Fe3O4) on its surface.
