Updated: Apr 28
What Happens When Steel Is Heated?
When steel is heated, its internal structure changes, taking a cubic crystalline shape called austenite. This form of steel has a more open cell structure, allowing the metal to absorb more carbon. If allowed to cool slowly, this austenite releases the carbon slowly, and the metal regains its standard properties.
However, if the metal is cooled rapidly while in its austenitic form, the carbon does not have time to diffuse back into the surrounding metal structure. Instead, the carbon is trapped by the crystal structure, forming a needle-shaped crystal structure within the steel known as martensite. Martensite is a phase of steel that can be surface hardened or through hardened.
Carbon steel is a robust material that is already quite strong in its base state, but its ability to be altered to fit different applications is what makes it so essential to modern industry.
Once carbon steel has been cast or forged into a final product and cooled, flame or induction hardening will result in surface hardened steel.
Because the heat is applied at the surface without penetrating fully to the core of the steel, the resulting steel has a hard crystalline exterior, while the core retains the more ductile structure it was forged with.
High carbon steel is ideal for use cases involving repetitive work where mild steel would wear out too quickly under load, or through contact with abrasive or corrosive materials.
What’s the Difference Between Hardened Steel and Mild Steel?
You can read all about the types of steel and their strengths and weaknesses on our blog, but the difference in steel formulations comes down to strength vs hardness. Hardened steel is prone to cracking under pressure. In similar situations, mild steel will simply flex and return to its original shape unless exposed to sufficient pressure to bend the metal permanently. Mild steel is therefore a more appropriate fit for applications such as construction, where the ability to flex and sway may save a building from collapse in the event of an earthquake or gale force winds. Hardened steel in this situation would not bend, instead forming cracks in the structure, compromising the integrity of the building.
However, in an application such as sprocket manufacturing, surface hardened steel teeth create a wear-resistant surface that greatly extends the useable life of the sprocket, while the ductile core keeps the sprocket from fracturing under the pressure of shock loads and heavy, repetitive work.
What is Flame Hardening
Steel can be heated for hardening purposes using either flame or by induction heat. In flame hardening, a high-intensity flame is channeled through a torch or a custom rig that dissipates the flame across the surface of the steel until the metal is heated to a specified temperature. Correctly assessing the temperature of the steel is one of the skills of an experienced steelworker, and with experience can be done by eye.
This process can be applied to a targeted area, such as the wear surface of a sprocket tooth, to harden only the required areas, retaining the resilience of the steel while extending the life of working surfaces.
Because the flame requires time to heat the metal, sometimes taking up to a half hour or more to heat a large surface, the underlying metal is heated as well, but to a lesser degree, alongside the surface. This restricts the minimum depth that flame hardening can achieve. Because of this, flame hardening is ideal for applications requiring a hardness depth of 1.5mm (1/16”) or more.
Advantages of Flame Hardening
Can be used to harden steel of any shape, including irregular or small items
Targeted flame can harden select parts while leaving others ductile
Hardness can be achieved on the surface or deep within steel
Cost effective for small batch, compared to making an induction hardening setup
Can be scaled up to achieve hardness on large items
Disadvantages of Flame Hardening
Slower than induction hardening
Difficult to achieve hardness depth <1.5mm
Requires skilled steelworker to accurately assess heat and depth
Overheating can result in brittle steel, or other unwanted results
Cost is not competitive with induction at scale
Open flame not permissible in some work environments
What Is Induction Hardening?
Induction hardening is a much quicker process than flame hardening. Induction involves shaping a highly conductive water-cooled copper coil around an object, and running an electrical current through it. This creates an electromagnetic field within the the coil. When a steel part, such as a sprocket tooth, enters the field, eddy currents are produced in the steel. These currents flow against the natural electrical resistance of the metal, causing it to heat up rapidly.
Advantages of Induction Hardening
Since the intensity of the current can be easily adjusted, heating steel with induction happens in seconds instead of minutes, applying heat evenly across the surface of the steel that is within range of the copper coil.
Once fabricated, this same induction setup can be used repeatedly with items of the same shape, making it ideal for hardening items in batches.
Disadvantages of Induction Hardening
The setup cost involved with induction is higher than a typical flame hardening setup, as each different item to be hardened will require a bespoke copper coil to be fabricated in a way that completely surrounds the surface to be hardened.
Additionally, because the induction coil must be bent to match the contours of the steel surface, the complexity of shape appropriate for induction hardening is restricted to fairly straightforward profiles.
Flame vs Induction Hardening Summary
Steel is heated using oxy-acetylene flame, or other high intensity flame
Steel is heated using a/c current passed through an induction coil
Temperature is gauged and controlled by the operator
Temperature is easily regulated by digitally controlling voltage
Heating is gradual, may require holding period
Heating happens quickly
Most effective for individual items or small batches
Work is easily automated and replicated at scale
Not effective for hardening depth less than 1/16”
Can be used to create ultra-shallow hardened layer, <1/16”
Low equipment and maintenance cost
High equipment and maintenance cost
Appropriate for targeted application, flat or very large surfaces
More difficult to use on isolated surfaces, or in very large applications
Can be used on items with irregular shape
Best used on items with basic shape
Requires skilled labor
Can be done by unskilled labor
Cost effective when small quantity required
Cost effective at scale
Overheating can result in damage due to warping, scaling, and decarburization
Easy to avoid damage due to finely attuned heat
Still unsure about what material and hardness will be the best for your application? Give us a call. Our qualified engineers and technicians will make sure you end up with the right solution for your application.