Heat Treatment of Steel
Heat treatment is a process of heating that is used on different materials to achieve varying goals depending on the treatment and material. In steel manufacturing, the rate of heating and afterwards cooling can achieve different mechanical properties on the end result material. Heat treatment to steel has been an ongoing process for several decades and has been refined over time.
Modern heat treatment processes include but are not limited to annealing, quenching and tempering, normalizing, case hardening, surface hardening, QPQ and nitrocarburizing.
Why Do You Need to Heat Treat Steel?
Exposing steel to heat does not necessarily result in the same mechanical properties every time. The amount of heat and time a component is exposed highly affect the end result. Some processes are performed pre-machining to “soften” the material, while others are performed post-machining to “harden” the material.
Types of Heat Treatment
There are many heat treatment methods available for steelworking. Read below to find out more about them and how they can benefit a given project.
Annealing Heat Treatment Methods
Annealing is a process performed before manufacturing to soften the material for better malleability and ductility. Annealing is used to relieve stress, increase ductility, soften the metal and improve the metal's grain structure before it becomes a product. Annealing metal before it is welded prevents accidental weakening of the material and inhibits the development of brittle spots.
The steel annealing process begins with slowly heating the metal to an appropriate temperature before soaking it and allowing it to slowly cool. Depending on the steel's type and mass, it will require a longer or shorter soak cycle. Low-carbon steel requires the highest annealing temperature. As carbon content increases, the necessary annealing temperature tends to decrease.
Quenching and Tempering Heat Treatment Process
On the other hand, quenching and tempering is a process of heating steel to a very high temperature and then rapidly cooling it to increase hardness. Different methods of performing heat treatment can be used depending on desired mechanical properties. Most steel uses quenching-style rapid cooling, yet some steels can be air-cooled.
As a general rule, steel cools faster as more alloys are added to it. With quenching, the slower the cooling rate, the less the risk of the steel warping or cracking. Carbon steel has a particularly quick hardening rate — it must be cooled to under 1,000 degrees F in less than a second, which is a tricky task to accomplish. By adding additional alloys to this steel, you can select a slower quenching medium and reach a specific hardness much more easily.
Tempering begins after the steel has already been quenched and helps reduce the steel's brittleness and relieve its internal stresses. Tempering has three steps:
- Steel is heated to a set temperature below its hardening temperature.
- Steel is held at that temperature for a specified period based on its alloy.
- Steel is cooled in the still air.
The steel tempering process is near identical to the three stages of heat treatment and provides you with steel that has higher strength, hardness and ductility.
Heat Treatment of Steel by Induction Hardening
Induction hardening is most commonly used for surface hardening of bearing inner raceways and gear teeth. Induction hardening refers to the process of quickly hardening the surface of a metal part, usually with a copper coil that carries an alternating current. This process generates heat and directs the quenching process at the submerged component of the inner raceway or gear teeth.
The most common type of induction hardening is the scanner method, which consists of holding a part between two centers, rotating it and passing it through a progressive oil heat quench. Induction hardening gives your operation the advantages of increased wear resistance, improved strength and enhanced fatigue life.
Thermal Diffusion Heat Treatment of Steel
Thermal diffusion in a heated oven through nitrocarburizing and nitriding can also be used when other elements like Carbon and/or Nitrogen need to be diffused into steel to achieve desired mechanical properties. The diffusion process occurs only between low carbon content steel and a carbon-rich environment. It can thicken the steel's surface while keeping the core's original mechanical properties intact.
Completing the Process With Heat Treatment Post Shaping
Upon machining, a semi-finished component may require further heat treatment process to harden steel after it has been cut and shaped. Different compositions of steel are heat treated differently, but the end goal of hardening steel remains consistent.
Steel’s hardness is essentially its ability to resist friction or its abrasion resistance. When unhardened steel is used in an application involving heavy friction, wear and tear may result in premature failure depending on the application. For example, low-quality heads of screws are often unhardened or poorly heat treated. The force required to unscrew an unhardened screw will cause the metal to deform and the screw to strip and become unusable.
When steel is used to construct something such as a slew ring, it is first shaped and cut in its untreated soft form, allowing cutting tools and machines to shape the steel much more easily. In the case of slew rings, the teeth are then hardened in a process that involves heating the steel to a high heat and then cooling it quickly in a quenching medium, followed by heating the steel up to a medium heat and allowing it to air dry to soften it slightly from its peak hardness.
The harder a steel is, the more brittle it becomes, so steel is generally not used at its peak hardness in order to retain toughness and durability. Heat treatment will also sometimes include a stage called stress relief to relieve stresses that are created within the steel from high-speed cutting tools causing localized heating in portions of the steel.