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, it is a process vital to almost every industry, from woodworking to nuclear power generation.

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 to wear and abrasion a material is, the process of hardening is done to raise this level and to increase a part’s ability to perform in abrasive or wearing environments.

There are a few different types of hardening, including through hardening and surface hardening, they can be applied to many different metals, although one of the most common is low carbon steel.

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 is usually done as one of the last steps of the manufacturing process, as it makes machining and other work difficult.

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 power products used to add carbon, and electric furnaces to hold the parts at the required temperature.

Benefits of Carburizing

• Cost – it requires relatively little materials or equipment
• Scale – once the process parameters are established, it can be performed en-mass
• Versatility – Most steel and iron alloys can be hardened via carburization, even if they are low-carbon

Limitations of Carburizing

• Oxidization – the increased carbon content and heat can cause oxidization 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 to 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 is limited in its application.

Flame hardening works by heating the part with a flame to its austenitization temperature, this can be done with oxy-acetylene or oxy-propane torches. For steel, this can be anywhere from 600-800°C.

Once heated the steel is immediately quenched in water, this 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, that 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, this 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 used a lot in the industry, as an efficient way of heating a metallic part or piece of material, not only can it be more localized than flame heating, but it is also much more efficient, as little energy is lost 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

Typical Applications of Surface Hardening

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
• Induction hardening: Common uses include larger shafts and gears.

Purchase Metals For Your Project At Metals Supermarket

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