What is Surface Hardening?

If you work with hand or machine tools, it’s almost a guarantee that you will have used something that has been hardened. Hardening creates a perfect balance between hardness and toughness that simply isn’t possible to achieve through other means. It is a process vital to virtually every industry, from woodworking to nuclear power generation.

Some hardening processes focus on enhancing just the outer layer of metal while preserving the core’s natural properties, while others harden the entire material. The result is a component that can withstand intense friction, impact, and stress without failing prematurely. There are a few different types of hardening, a major one being surface hardening. What is surface hardening, and how is it done?

What is Hardening?

Hardness is a measure of how resistant a material is to wear and abrasion. In industrial applications, hardness often directly correlates with a part’s lifespan and reliability in demanding conditions. The process of hardening is done to raise this level and to increase a part’s ability to perform in abrasive or wearing environments. Without proper hardening, many metal components would quickly deteriorate, leading to costly replacements and potential mechanical failures.

There are several types of hardening that can be applied to metals. Through hardening affects the entire cross-section of a metal piece, creating uniform properties throughout, while surface hardening selectively targets the exterior layers where wear most commonly occurs. Each approach offers distinct advantages depending on the intended application and the stresses the component will face during its service life.

What is Surface Hardening?

What is surface hardening? As the name suggests, surface hardening involves hardening the material on the outside faces of a part, leaving the inside at the material’s natural hardness level. This approach creates a combination of properties that would be impossible to achieve through uniform treatment. The hardened outer layer provides exceptional resistance to wear, abrasion, and surface fatigue, while the softer, more ductile core maintains the toughness needed to absorb impacts and resist cracking.

Surface hardening is usually done as one of the last steps of the manufacturing process, as it makes machining and other work difficult. Once a surface is hardened, its increased resistance to deformation makes it challenging to cut, shape, or modify using conventional tooling. The hardened layer typically ranges from a fraction of a millimetre to several millimetres in depth, depending on the specific process used and the requirements of the final application.

Surface Hardening Techniques

To allow parts of varying materials and shapes to be surface hardened, multiple techniques have been developed, each with its own set of pros and cons.

Carburizing

This is the oldest surface hardening technique and involves increasing the carbon content of the metal at the surface level, by heating to high temperatures in a carbon-rich environment. The hard layer is usually between 1 and 2mm thick.

It is thought that medieval blacksmiths learned that leaving a blade in the coals of a coal forge for many hours added significant hardness to the steel; this is a primitive version of carburizing.

Nowadays, the process is much more controlled, with specific paste and powder products used to add carbon, and electric furnaces to hold the parts at the required temperature.

Benefits of Carburizing

Cost – It requires relatively few materials or equipment.

Scale – Once the process parameters are established, it can be performed en masse.

Versatility – Most steel and iron alloys can be hardened via carburization, even if they are low-carbon.

Limitations of Carburizing

Oxidation – The increased carbon content and heat can cause oxidation in the wrong circumstances.

Distortion – High heat levels can introduce warping on certain shapes and thicknesses of parts.

Range – Carburizing is limited to steel alloys.

Nitriding

In a similar fashion to Carburizing, Nitriding hardens the surface of a part by exposing it to a high-nitrogen environment. The addition of nitrogen into the material forms nitrides, which are incredibly hard, increasing the wear and abrasion resistance of the part.

Nitriding is performed at a much lower temperature than Carburizing, around 520°C. This results in a process less likely to cause dimensional changes or warping. Nitriding results in a hardened layer between 0.05 and 0.5mm thick.

Benefits of Nitriding

Heat resistance – Nitrided parts have better thermal resistance than carburized ones.

Wide Range of Metals – Nitriding can be done on many metals, such as Aluminum, Titanium, Vanadium and more.

Less distortion – Lower process temperatures mean less warping.

Limitations of Nitriding

Thickness – Nitriding results in a much thinner layer of hardened material when compared to carburizing.

Cost – Nitriding is usually done in smaller batches than carburizing, as it requires tighter temperature control, which results in increased processing cost.

Flame Hardening

If your material has sufficient carbon, it can be hardened without the addition of any other elements such as Carbon or Nitrogen. This hardening process is one of the simplest ways to harden a high-carbon steel, but it is limited in its application.

Flame hardening works by heating the part with a flame to its austenitization temperature, which can be achieved with oxy-acetylene or oxy-propane torches. For steel, this temperature range is typically between 600 °C and 800 °C.

Once heated, the steel is immediately quenched in water, which increases the amount of martensite that forms in the resulting material. Martensite is an extremely hard phase of steel that would not form under gradual cooling.

This adds a hardened layer to the material or part, which is anywhere between 0.1mm and 6mm thick, depending on the level of heating and thickness of the part.

Benefits of Flame Hardening

Cost – Flame hardening can be done with minimal equipment and is easy to control at scale.

Control – Flame hardening allows tight control of where the part is to be hardened, which allows the rest of the part to retain its toughness.

Limitations of Flame Hardening

Range of materials – Hardening only works on medium or high carbon steels, where there is sufficient carbon present to form martensite; it will not work on mild or low carbon steel.

Warping – The high temperature can warp or distort certain parts

Scale – The flame hardening process causes a heavy scale to form, often requiring further processing, such as sandblasting or grinding, to remove.

Induction Hardening

Induction hardening uses the same principles as flame hardening, except the heating is provided by an induction coil, rather than a flame. This allows the heating to be much better controlled and localized.

Induction heating is widely used in the industry as an efficient method for heating metallic parts or pieces of material. It offers several advantages over flame heating, including more localized heating and significantly reduced energy loss through radiation.

Induction hardening is limited to the same range of materials as flame hardening, i.e. those that have sufficiently high carbon content. Thanks to the controllability of the process, the thickness of the hardened material can be closely controlled.

Benefits of Induction Hardening

Control – The induction coil heating process allows higher control than heating with a flame, including both the thickness of the hardened material and the area to be hardened.

Speed – Thanks to the efficiency of the induction heating element and the large amounts of energy that can be transferred, the process of heating is faster than with flame hardening.

Less scale – As the process does not use a flame to heat the material, less scale is produced on the surface.

Limitations of Induction Hardening

Scale-induction coil heating requires the part to be inserted into the coil aperture, meaning batch heating is not possible unless many coils are used.

Cost – Both the equipment purchase and the energy are higher than with flame/oxy-gas heating.

Material – As with flame hardening, induction hardening can only be used on medium or high carbon steel.

What is Surface Hardening Used For?

Hardening is used across all sorts of industrial applications, from hand tools to heavy-duty machinery. Each hardening process is suited for different components, as described below.

Carburising: Common uses include shafts, gears and other parts that need high wear resistance and resistance to impact loads.

Nitriding: Engine valves and camshafts are often hardened with a nitride process, as this can retain good dimensional consistency.

Flame Hardening: Larger objects, such as hand tools and implements, are often flame hardened, as warping is less of an issue with thicker parts, and the process can be automated for high-volume production.

Induction hardening: Common uses include larger shafts and gears.

Surface Hardening vs. Through Hardening

Surface hardening and through hardening serve different purposes in metal treatment. Surface hardening is the preferred option when components need to withstand intense wear, friction, and surface stress while maintaining internal flexibility and shock absorption capabilities. This makes it ideal for parts like gears, camshafts, crankshafts, and cutting tools that experience high contact stresses but also require toughness to prevent catastrophic failure.

Through hardening is chosen when uniform hardness throughout the entire cross-section is required. This process is typically selected for smaller components that need consistent properties from surface to core, such as ball bearings, punches, dies, and high-stress fasteners.

Purchase Metals for Your Project at Metals Supermarket

Whether you need to hard enable tool steel, malleable mild steel, or anything else, Metals Supermarket has a wide range of common metals, shapes and sizes, all with cut-to-length services available.

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