How Do You Thermally Strengthen Metal?

Metal is one of the most commonly used materials in the world, not only for its impressive strength to weight ratio but also for its ease of workability, allowing it to be formed and processed to suit millions of applications.

One of the standout features of certain metals is the ability to alter their properties with heat, this could be to make them more ductile, harder, or stronger. But how do you thermally strengthen metal?

Heat Treatment in Metals

Depending on the type of metal, its current state, and the temperature used, the heat treatment process can achieve very different things:

• Hardening: increases the metal’s hardness, strength and brittleness
• Tempering: reduces the hardness of a metal, to make it less brittle after hardening
• Annealing: reduces the hardness, but usually allows further processing
• Normalizing: evens out the internal stresses in a material after a process such as machining, forging or welding

Types of Hardening in Metals

Within the category of hardening, there are further sub-categories, which are applied to metals in differing ways:

• Quench Hardening: Used to greatly increase the hardness of metal – often used for blades, this is also known as Martensitic Hardening
• Precipitation Hardening: Used to increase the strength of structural grades of certain steels, aluminum and others
• Age Hardening: Similar to precipitation hardening, age hardening is often achieved over longer periods of time at lower temperatures

Strengthening Metals With Heat Treatment

Hardening is used in many metals to increase a part’s hardness and/or strength. It follows the same basic principles, though some alloys require different parameters.

Hardening is achieved by increasing the temperature of a component, holding it at that temperature, then rapidly cooling it. The last stage is done using a bath of oil, water, brine or even just air, depending on the alloy and desired outcome.

How Does Metal Hardening Work?

Metals are thermally strengthened using heat through tightly controlled processes that differ slightly from one material to another. The basic principles of hardening involve heating the part to a certain temperature and holding it for a certain amount of time.

Quench & Tempering Hardening Process

This is the typical process used by blacksmiths, knife makers and other metalworkers, it increases the strength and hardness, but requires tempering for the part to be useable:

1. The part is heated to above its critical temperature, often in an atmosphere of inert nitrogen or argon.
2. The part is rapidly cooled in oil, water or brine
3. The part is tempered by raising the part to a lower temperature and allowed to cool slowly

Precipitation Hardening Process

Precipitation Hardening is more involved, it requires three stages, solution treatment, quenching and aging. It is usually only done on a commercial scale with high-performance alloys.

1. The material is heated to create a supersaturated solution, this allows more alloying elements to dissolve at cooler temperatures
2. The material is quenched, this traps the supersaturated solution in an ordinarily un-stable phase
3. The material is heated again (aged), to a lower temperature than before, this is the aging process. It allows the extra elements to form precipitates in the material, increasing strength by forming strain fields

Some alloys can be aged at room temperature, and do not require a heated aging stage.

What Materials Are Suitable For Hardening

The process used will depend on what materials can be hardened, but typically the following types of metal can be hardened:

• High carbon steels
• 17-4 Stainless steel
• Aluminium 2000, 6000, 7000 series (precipitation hardening)
• Inconel
• Maraging Steel

Buy Your Metals From Metals Supermarkets

If you have a project that requires hardenable stainless steel, aluminum, or steel alloy, speak to your nearest Metals Supermarket store first. We stock hundreds of grades and shapes and offer to cut to length too.