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What’s the Difference Between Through-Hardening and Surface-Hardening?

Updated: Apr 28, 2023

Sprocket with visible signs of hardening
Sprocket with visible signs of hardening. Note the discoloration line near the teeth



One question we get all the time is “What’s the difference between surface hardened and through hardened steel?”. While both products are optimal when used in the right application, they’re rarely interchangeable, and it’s important to understand their key differences to get the most out of your components. In this article we’ll examine the difference between the two products, and outline the strengths and weaknesses they have in an industrial setting, such as sprocket manufacturing

Through Hardening

Surface Hardening (i.e Case Hardening)

Extremely hard throughout

Tough skin only, ductile core

Less suitable for impact loads

Resilient to impact loads

Very rigid, can fracture with too much pressure

Can flex slightly under pressure to prevent breakage

Abrasion and corrosion-resistant all the way through

After hardened skin is worn through, degrades from abrasion and corrosion quickly

Why Harden Steel ?

Carbon steel beginning the heating and quenching process
Carbon steel beginning the heating and quenching process

It’s important to know that not all steel is the same, even before it is hardened. Varying levels of carbon content in the steel affect its ability to be hardened when exposed to high levels of heat. Alloying can also shape the composition of steel to achieve different mechanical properties (such as stainless-steel’s resistance to rusting).

Low carbon steel, often referred to as plain steel or mild steel, cannot be hardened, but is a very strong material, able to flex under load without damage while offering exceptional performance. This property is referred to as ductility.

Medium and high carbon levels within steel offer increased options for hardening. Here’s an article comparing flame and induction hardening methods.

To illustrate the importance of selecting the right steel composition and hardening option, consider that a high carbon steel that has undergone a heat-treating process in this manner can be as much as three or four times harder than an unhardened mild steel. Hardening is ideal for creating steel components that will experience repeated exposure to workloads, providing a long-lasting friction-resistant surface.

What Is Through-Hardened Steel?

Through-hardened steel is created by heating carbon steel all the way through and rapidly quenching it, usually in water or oil. This causes the carbon to react throughout the metal as the crystal structure of the steel changes form. When the metal is quenched, the crystal structure is locked in. Steel intended for use in sprocket manufacturing often undergoes additional heat treatment called annealing. Annealing can help to offset the brittleness that results from hardening, and allows steel workers to tailor the mechanical properties of the steel to suit its intended purpose.

The resulting product offers exceptional hardness all the way through the steel, providing a tough, resilient surface that is abrasion resistant. This makes through-hardened steel an ideal material for use in high-contact applications where increased tensile strength is an asset, such as in springs, hand tools, and high-end knives.

While through-hardened steel offers an incredible degree of toughness and increased resistance to corrosive agents, the process makes the steel more brittle. This greatly reduces its ability to withstand impact, such as a sudden load. If subjected to abrupt application of force, hardened steel is prone to fracturing.

For industrial applications that require abrasion and corrosion resistance however, through-hardening can be an ideal choice, provided the work involves a relatively constant, low-impact load.

What is Surface-Hardened Steel?

Different depths of surface hardening
Different depths of surface/case hardening

Due to the carbon present in all steel (even mild steel), steel can be made with a hard surface while retaining a tough, flexible core. The mechanical change is more significant in medium and high carbon steel. Case hardening is accomplished by taking steel and heating its surface – either through extended exposure to flame or by heating it through electrical induction. This process causes a reaction with the carbon on the surface of the steel, resulting in a hardened skin.

Surface-hardened steel is able to handle the impact of shock loads while still providing a tough exterior that resists wear, ideal for applications involving heavy cyclical loading. This combination of flexibility and hardness makes surface-hardened steel ideal for use in industrial machine parts, roller bearings, theft-resistant chains, and automobile camshafts, to name a few examples.

Should I Use Surface-Hardened Sprockets or Through-Hardened Sprockets?

Sprocket with hardened adjustable teeth
Sprocket with hardened adjustable teeth

At DROP Sprockets, our sprockets are made-to-order with an intended purpose in mind, each requiring different base materials and hardening methods. While most sprockets are surface hardened to retain the ductile core of the material, the hardening method and depth of hardness are selected based on the required application.

Here's a few examples:

Unhardened Sprockets

Idler sprockets are often fitted with a round hub, often with a low-friction bushing or bearing insert to allow the sprocket to rotate easily as the chain goes by. These sprockets act like a spacer to support conveyor chain, allowing the contents to move freely along the length of the conveyor. Because idler sprockets rotate freely under a relatively light load, they can usually function without additional hardening. The ductile nature of the steel here is an asset, allowing these idler sprockets to absorb any sudden shock loads.

Surface Hardened Sprockets

Somewhere else in the machine, a drive sprocket is used to transmit power from the engine to the conveyor by use of roller chain. The sprocket attached to the drive shaft is placed under considerable stress, since it’s responsible for pulling the chain. The chain contacts the surface of the sprocket tooth while under tension from the motor, which is quite a considerable amount of force. These drive sprocket teeth are often hardened in order to reduce the wear from the chain over millions of revolutions. Generally surface hardening is preferred to keep the core of the sprocket ductile enough to handle shock loads without fracturing, while still providing durable teeth.

Through Hardened Sprockets

Through-hardening may be appropriate for work involving abrasive or corrosive environments with consistent workloads, where the risk of fracturing is outweighed by the benefits provided by the additional depth of hardness. This application is often seen in cement or aggregate plants, where a through hardened sprocket can often outlast a surface hardened sprocket by over 2-3 times! Some through hardened steel has superior strength/toughness properties than ductile case-hardened steel, but tends to be avoided due to cost. Creating a through hardened sprocket that has undergone sufficient annealing to act in place of a surface hardened sprocket is expensive, so the value-add this type of steel offers relative to case-hardened steel will be operation dependent.


At DROP Sprockets, we offer a wide selection of base steels ranging from mild to high carbon, and hardening options including flame, induction, and nitriding. If you’re interested in comparing options, or finding the best solution for your application, give our friendly support team a call today!

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