Alloy Steel vs. Carbon Steel Bars: What's the Difference

It is why you will typically see an alloy grade used in the frame of an airplane or the structure of an oil well and a carbon grade in construction equipment or automotive parts. But why? Let’s dig into the chemical makeup of these two products.

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What is the Difference Between Carbon Steel and Alloy Steel?

Carbon steel contains mostly iron and carbon (less than 2%) as its primary alloying elements, while alloy steel contains such elements as manganese, nickel, chromium, vanadium, and molybdenum, among others.

These additional elements help improve the strength, hardness, hardenability, and toughness of alloy steel. This makes it ideal for use in more high-performance applications like those in aerospace, defense, and oil and gas.

In addition, these elements also improve the corrosion resistance, heat resistance, and wear resistance of alloy steel.

Is Alloy Steel Stronger than Carbon Steel?

Alloy steel is stronger than carbon steel, due to various levels of manganese, nickel, chromium, vanadium, and molybdenum, contained within these steels.

The specific strength range depends on the specific composition of an alloy. The level of strength of alloy steel is further enhanced by the processing and heat treatment methods used on the material.

For example, thermal treatment of quench and tempering can be applied to many alloy grades, which improves the strength of the material.

What are Some Common Alloy Grades?

4140/4150 are widely used alloy grades. They are considered to be the ultimate “general purpose” grades and are low in cost. The typical composition of these grades is roughly .95% chromium and .20% molybdenum.

A broad range of strength and toughness is attainable through variations in heat treatment. These grades also exhibit good hardenability, strength, wear resistance, toughness, and ductility.

In terms of fee-machining alloy bars, 41L40 is among the most common. This chrome-molybdenum alloy contains .40 carbon and .15/.35 lead. It can be quenched and tempered to a broad range of strength levels.

8620 is the most widely used of all case hardening alloys. Considered to be “general purpose” 8620 is low in cost and contains good properties. Its composition is well balanced, nominally .55% nickel, .50% chromium, .20% molybdenum.

4340 is a highly alloyed steel, nominally 1.80% nickel, .80% chromium, and .25% molybdenum, assuring deep hardness when quenched and tempered, with high strength characteristics attained throughout the section. This grade can be used for heavily stressed parts operating under heavy-duty conditions.

If you’re looking for a metal alloy, you’ll be spoiled for choice. From brasses and bronzes to titanium alloys and nickel-based superalloys, there are a mindblowing number of metal combinations out there. However, one alloy in particular is more widely used than any other.

Because of its high strength and low cost, steel generally offers “the best bang for your buck” to designers and builders. Steel is everywhere! But there’s a lot to know about how this popular alloy works and this article will help break down one commonly misunderstood topic: What are the differences between alloy steel and carbon steel?

Preface: Common Steel Industry Terminology

Truthfully, the distinctions between alloy steels, carbon steels, stainless steels, and other steel alloys, like microalloyed steels, are somewhat arbitrary. They are generally based on the amounts of certain alloying elements (usually expressed as a percent by weight) included in the steel and how said steel behaves. This can make it hard to agree on a hard cutoff, but generally accepted definitions are as follows.

Steel (and Steel Alloy) – Traditionally and for the purposes of this article, “steel” refers to an iron-based alloy that includes between 0.05% and 2% carbon, and could also include a number of other elements up to certain limits. The term “steel alloy” is a bit like saying the same thing twice, but it is still commonly used when referring to specific chemical compositions in steel.

Carbon Steel – Carbon steel, sometimes distinguished as “plain carbon steel”, is an iron-based alloy containing between 0.05% and 2% carbon, and relatively low amounts of any other elements. Specifically, it cannot have more than 1.65 wt% manganese, 0.60 wt% silicon, 0.60 wt% Copper, or similar amounts of any other elements.

Cast Iron – Refers to an iron-based alloy with greater than 2% carbon, which makes it very hard and brittle.

Alloy Steel – Not to be confused with “steel alloys”, alloy steels are essentially any steel that does not count as carbon steel because they have meaningful quantities of elements other than iron and carbon. However, there are still limits in terms of how much of a certain element is allowed in alloy steel.

Understanding Carbon Steel

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Carbon steels have been around for centuries. And from ancient blacksmiths to modern production facilities, metalworkers have been fine-tuning their processes to create the desired results. Modern production methods are much more precise, so finer ranges can be established. This allows metallurgists to group carbon steels into several subcategories based on their properties: low-carbon steels, medium-carbon steels, and high-carbon steels.

Low-carbon steel – These are carbon steels containing no more than about 0.25% carbon and 0.4% manganese. While they are the weakest of the carbon steels, low-carbon steel alloys are more ductile, easier to weld, easier to machine, and easier to form than carbon steels with a larger carbon content.

Medium-carbon steel – These carbon steel alloys contain between 0.25% and 0.5% carbon, along with higher limits for manganese. Because of this, medium-carbon steel alloys are somewhat stronger and more wear-resistant, which also makes them slightly harder to machine and weld. The higher carbon and manganese content improves hardenability during heat treatments.

High-carbon steel – Much harder to weld, machine, or form, high-carbon steels have a carbon content between 0.5% and roughly 1%. They are very heat-treatable, which allows designers to target very specific physical properties if the additional processing challenges are worth it.

Another group that is seldom used are ultra-high-carbon steels, which fill the gap between high-carbon steels and cast iron, but are generally not the best material for the job.

Keep in mind that these ranges are more suggestions than hard rules, and they may not even agree between sources. Instead, they provide a general idea of the way certain groups of carbon steel alloys behave.

Understanding Alloy Steel

With improved control over additives during manufacturing, modern steel producers can develop new steel alloys with precise amounts of other elements to improve the properties of the steel. These are generally separated into low alloy steels and high alloy steels depending on whether the total weight percentage of alloying elements is below or above 8% respectively.

Depending on which alloying elements are used and in what amounts, alloy steels can achieve a number of unique properties that can’t be obtained by carbon steels. While the key effects of these alloying additions are outlined in Degarmo’s Materials and Processes in Manufacturing, five of the most commonly used elements added to make alloy steels are:

Chromium – Essential in making stainless steels. The right amount of chromium can significantly improve corrosion resistance and positively impact hardness.

Manganese – Improves hardenability during heat treatment while also reducing the likelihood of cracking.

Molybdenum – Mainly useful for improving toughness and strengthening the alloy steel. It is essential for high-loading applications.

Nickel – Also useful for enhancing the toughness of the steel, but nickel can also contribute to corrosion resistance when used in quantities higher than 12%.

Vanadium – Very useful for improving the high-temperature performance of steels. Vanadium controls grain size during heat treatment to increase strength while retaining ductility.

Characteristics of Alloy Steel vs Carbon Steel

With a stronger understanding of what distinguishes carbon steels from alloy steels, it makes sense to wonder, “which one is better?” Unfortunately, that question is impossible to answer. As a whole, these two classes of steel alloys have more in common than differences between them.

One glaring exception is cost. Carbon steels, especially low and medium-carbon steels, are cheaper to produce and easier to process. This is likely why they are the most widely used classes of steel alloy.

Some people may also point out that certain alloy steels are more corrosion-resistant or stronger or better-suited to high-temperature service than carbon steels. Meanwhile, carbon steel is easier to work with all around. While these statements may be true for specific alloys, they are not universally true for all alloy comparisons between the two classes of steel.

Making Parts from Different Alloys

Generally speaking, low and medium-carbon steel alloys, along with certain low-alloy steels, are cheaper to produce and easier to work with. This makes them ideal for the vast majority of manufactured steel parts and components. Everything from simple steel parts used in automotive assemblies and kitchen appliances, to construction equipment and piping relies on low-alloyed, low-cost steel. Meanwhile, advanced aerospace components and high-temperature chemical processing equipment will likely need to rely on more heavily alloyed steels that are hand-picked for that specific application.

In addition to selecting the right alloy, there are a large number of different shaping and treatment processes available to choose from. It’s important to find an experienced and reliable manufacturing partner to ensure your parts are processed according to the specific needs of the material and part design. Whether you’re using widely available carbon steels or advanced alloy steels, Gensun has the tools and expertise to deliver top-quality machined products on schedule and at an affordable cost.

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