Carbon Steel vs. Steel Alloy: What’s the Difference?

Huyett Marketing Department
10/14/2020
Updated: 07/27/2023

Carbon Steel vs Alloy Steel Blog Hero Image

Steel is an essential material in various industries, and it exists in many different forms. Each form provides different mechanical properties that make them unique and suited to different applications, with some of the most common forms being carbon steels and alloy steels. However, steel’s fundamental structure applies across the board to these variations.

At its core, steel is primarily a mixture of iron with carbon added to it. Technically, it isn’t a metal –
metals
are naturally occurring elements, like aluminum or copper. Steel, even at its most basic level, is always an
alloy
(a combination of several elements and/or components). While iron is an element, iron does not transform into steel until carbon is added.

To understand the difference between carbon steel and alloy steel, it’s important to first understand the role of iron and carbon in steel.

The Role of Iron in Steel

Iron is the base of all steel and provides the foundation from which all different types of steel are formed, depending on which elements are added to it. It is very strong and resistant to heat and corrosion, which are properties it passes on to steel in the manufacturing process.

The basic process of converting iron to steel involves removing impurities (there are several ways to do this), refining the carbon content, and adding elements to create specific types of steel.

The Role of Carbon in Steel

Carbon is the element that controls the hardness and ductility of steel (remember, diamonds are the hardest natural substance in existence, and they are 100% carbon). The higher the carbon content, the more difficult steel is to machine. Iron on its own is malleable and soft; carbon changes that, and it doesn’t take much to do so.

Carbon Content

Carbon steel is divided up into three categories: high, medium, and low. High-carbon steel only has a carbon content of around 0.60% – 1%; any steel with a carbon content higher than 1% is significantly brittle, and it only takes 2% – 3% carbon content to transform steel into cast iron. High-carbon steel is commonly used for cutlery and applications that require sharp, hard edges.

Medium-carbon steel features anywhere from 0.30% – 0.60% carbon. This steel is not as strong or hard as high-carbon steel, but it is more workable. It is used to make train wheels, train tracks, machinery and automotive parts, and applications that require strength, toughness, and decent machinability.

Carbon steel below 0.30% is low-carbon steel, which typically requires carburizing when heat treated. During carburization, carbon molecules are introduced so that a hardened “skin” develops on the surface, creating a “case.” This is where the concept of case hardening is found. These steels work well in applications that require cold-forming or machinability, which makes them suitable for fasteners like clevis pins and machine keys.

Steel Phase Development

Steel, like water, can go through phase changes. With water, the phases are solid, liquid, and gas. With carbon steel the phases are liquid, austenite, and ferrite. Similar to how salt changes the transition temperature of water, carbon changes the transition temperature of steel.

The more carbon that is added (to a point), the lower the temperature at which the phase change will occur. Carbon also creates new phases that don’t exist in iron by itself; for instance, pearlite is a mixture of cementite (Fe3C) plus ferrite.

The Four-Digit AISI Numbering System

Before learning the difference between carbon and alloy steel, it’s important to understand how they are named. The first two numbers of any steel’s name in the four-digit AISI Numbering System indicate the type of steel.

For instance, cold‑drawn plain carbon steel is typically numbered with the prefix “10.” The next two numbers in
any
steel name indicate the nominal percentage of carbon in the product (up to 1.00%). For example, 1018 has 0.18% carbon, 1045 has 0.45%, and 4140 has 0.40%. All 10 grades also have manganese, phosphorus, and silicon.

Names sometimes also feature letters – the letter “L” means it contains lead for machinability, and “B” means it has boron for increased hardenability. If you know the chemistry of the alloy, you will have an idea of its hardness, strengths, and if a thermal treatment will work at all.

The following table lists the four-digit AISI Alloy Numbering System for steel alloys:

A Note on Mechanical Properties: Hardenability

Different types of steel feature different
mechanical properties, which are the characteristics that make a metal behave a certain way. These properties include a metal’s ability to reach a certain hardness, to be cut, to resist corrosion, to deform without breaking, etc. One of the more confusing properties to understand is a metal’s
hardenability
.

The hardness of a metal is determined by its resistance to deformation, indentation, or scratching. Rockwell hardness is the most common measure of a metal’s hardness. Soft steels are usually measured using the Rockwell B scale while harder steels and deep case-hardened steels are usually measured on the Rockwell C scale.

In some cases, one object may fall within more than one scale. A typical steel spring has a Rockwell hardness of 110 on the B scale and 38 on the C scale. In addition, several factors, including the size of the piece, can affect the hardness rating.

Carbon Steel vs. Steel Alloy: The Difference

Steel itself is an alloy because it’s a combination of multiple elements (iron and carbon). However, this mixture is steel’s most basic structure, and is referred to as
carbon steel
(this can have trace amounts of other elements, but not enough to call out).

Steel alloys build on the basic carbon steel structure and add other elements like sulfur, manganese, nickel, chromium, molybdenum, and other alloying elements. These are added to change, add, or subtract metal properties like machinability, tensile strength, hardness, corrosion resistance, etc.

In simple terms, here is the difference between carbon steel and steel alloy:

Carbon steel = iron + carbon
Steel alloy = iron + carbon + other elements

When reading about the different types of carbon and alloy steel, remember that the given information regarding material properties is an estimation and should only be used as a general guide. You should always do your own research to determine the specific properties of your desired material and the specific requirements of your application.

What is Carbon Steel?

Carbon steel is steel that relies upon carbon content for its structure. Carbon is the principal hardening element that determines the level of hardness or strength attainable by quenching (a process in many heat treatments that immediately cools the metal in water). It raises tensile strength, hardness, and resistance to wear and abrasion as the carbon content of steel is increased. It can also lower ductility, toughness, and machinability.

Types of Carbon Steel

Here are three common types of carbon steel that are used throughout a variety of industries and applications:

1018 Carbon Steel

Heat treating in contact with carbon (carburizing) hardens the surface of this low‑carbon steel. It is easy to cold‑form, bend, braze, and weld. Rockwell hardness is B71. Melting point is 2800°F. Yield strength is 53,700 psi. It is commonly used in shafts, spindles, pins, rods, sprockets, and component parts such as
clevis pins, straight pins,
machine keys, and
key stock.

1045 Carbon Steel

This medium‑carbon steel is stronger than 1018 and is slightly more difficult to machine and weld. Rockwell hardness is B84. Melting point is 2800°F. Yield strength is 45,000 psi. It is used in gears, shafts, axles, bolts, studs, clevis pins, machine keys, and key stock.

1095 Carbon Steel

This high-carbon steel is the strongest of the three grades and is also the most difficult to work. Rockwell hardness is typically B80. Melting point is around 2700°F. Yield strength is around 76,000 psi. It is used in cutlery, machine keys,
pins, and cutting tools.

Huyett offers a wide range of products in carbon steel, including
rivet nuts,
pin fasteners,
machine keys,
retaining rings,
lifting hardware components,
grease fittings,
hose clamps, and
bushings.

What is Alloy Steel?

Here are three common types of steel alloys that are used throughout a variety of industries and applications:

1144 Alloy

A medium‑carbon, resulferized steel with free‑machining qualities. 1144 steel heat treats better than 1045 steel. Stress relieving allows it to obtain maximum ductility with minimum warping. Rockwell hardness is B97. Melting point is 2750°F. Yield strength is 95,000 psi. This is frequently used in
keyed shafts,
clevis pins, and
detent pins.

4140 Alloy

Also called “chrome‑moly” steel. Ideal for forging and heat treating, 4140 alloy is tough, ductile, and wear resistant. Rockwell hardness is B92. Melting point is 2750°F. Yield strength is 60,000‑75,800 psi. It is the most common steel and is used in virtually all industries and applications. It is also used for clutch keys, clevis pins, and military clevis pins.

8630 Alloy

This alloy is tough yet ductile. It responds well to heat treating, exhibits superb core characteristics, and has good weldability and machining properties. Maximum attainable Rockwell hardness is C15. Melting point is 2800°F. Yield strength is 79,800 psi. This grade is frequently used in machine keys,
woodruff keys, gear drive keys, and keystock.

Huyett offers a wide variety of products in these steel alloys, including multiple types of key stock, machine keys, and
dowel pins.

The Bottom Line

Carbon steel and steel alloys are used to make a wide range of fasteners used in common applications in the automotive industry, agriculture, food and medical services, and more. Basic carbon steel on its own has a wide range of properties depending on its carbon and iron content ratio, while the properties of alloy steel are influenced by different elements added to the carbon steel base. This creates different types of metals with different structural components.

Huyett offers a range of products in these different types of metals that are suitable for a wide variety of applications.
Contact our Sales Team or
browse our range of products for more information.