5 Techniques for Hardening Steel

August 14, 2023

Steel is one of the most popular alloys used in engineering and manufacturing due to its high durability and toughness. However, heat treatments allow you to further harden steel and create optimized components for your intended applications. This post will explain five of the most commonly used techniques for hardening steel and the best processes for specific applications.

All metal hardening techniques are based on these three fundamental steps:

1. Heating: The first step involves heating the metal to extremely high temperatures.

2. Soaking: The second step requires holding the metal at the required temperature for a specific period of time. During this step, the steel may also be exposed to another substance to change its chemical composition, which allows it to achieve new properties.

3. Cooling: The final step requires cooling the metal down, which further alters the steel’s molecular structure, preventing it from changing further.

The required temperatures, materials, and time intervals will vary depending on the type of metal you are working with and the results you want. For example, you will need nitrogen or hydrogen to reduce surface oxidation in precipitation hardening applications.

Each technique differs in its requirements and the degree of hardening of steel. Consider your intended application for your component before choosing a steel hardening process.

Quenching and Tempering

Also known as martensitic transformation, quenching and tempering is a hardening process that works by heating a steel component- then rapidly cooling and reheating it until its molecular structure is sufficiently altered.

Heating the steel above the solvus temperature- which is the point when steel’s molecular structure begins to change to austenite. Metal with an austenitic grain structure is very soft, which is why you need to quench it.
Quenching is the process of submerging or “quenching” hot metal in a cooling medium. Gas, oil, water, and brine are common examples of quenching media. The medium you choose will largely depend on your desired results. For example, gas quenching steel in a gas such as argon or helium provides a high degree of control over the cooling process and eliminates the residue that is often left behind during water or oil quenching.
When austenitic steel is quenched, its grain structure transforms to a harder structure. The resulting structure may be martensite, ferrite, cementite, or pearlite depending on how quickly the metal cools. At this stage, the steel is hard but very brittle.
To increase ductility and remove internal stress, temper the steel, or reheat it to a lower temperature. Typically the component may be allowed to gradually cool to the steel hardening temperature instead of quenching it again.
Quenched and tempered steel has many potential applications, including:

• Heavy equipment parts
• Truck wear liners
• Building supports
• Storage tanks
• Cutting edges

Annealing

Annealing is a heat treatment process that reduces hardness, resulting in a component that is more ductile and easier to shape. It is usually done in between other metalworking processes to relieve internal stress buildup and refine the steel’s microstructure.

Heat the steel component above the solvus temperature and then allow it to soak for a set period of time before cooling. This allows the atoms in the grain to shift and create a lower-energy configuration. Maintaining a controlled environment is critical for managing impurities during the annealing process — nitrogen, for example, effectively prevents surface oxidation and leaves a brighter metal finish.
With ferrous metals like steel, it is best to allow it to slowly cool at room temperature. The metal recrystallizes and hardens as it cools, encouraging new grain growth and eliminating strain.

Precipitation Hardening

Precipitation hardening exponentially increases a component’s yield strength and durability, which makes malleable materials such as steel alloys able to withstand higher duress when in use.

Case Hardening

Case hardening, or carburizing, is a steel hardening process that creates a hard, wear-resistant surface layer with a tough underlying core.
To carburize steel, you heat it to a temperature above the solvus temperature and then allow it to soak. This temperature can change depending on the specific alloy being used. Expose the component to a gas atmosphere containing carbon introduced into the furnace and allow the part to soak.
At this temperature, the steel’s grain structure changes, allowing it to absorb the carbon from the atmosphere. Once the soaking period ends, you can either allow the part to cool over time or immediately quench it in oil.
A tightly controlled, reproducible, acetylene-based atmosphere is critical for successful case hardening. Accidental exposure to oxygen can cause surface oxidation, which prevents the steel from hardening as needed. Vacuum carburizing — also known as low-pressure carburizing (LPC) — is a technique that utilizes a vacuum furnace to heat and soak the metal, eliminating the risk of oxygen exposure.
Typical applications for case-hardened steel include automotive parts like gears and pistons. The hardening process protects these components against excessive wear, making them last longer.

The technique follows a three-step process:

• Solution treatment: Heat the steel above its solvus temperature and allow it to soak until it forms a uniform solid solution free of precipitates.
• Quenching: Having soaked the steel for the required amount of time, remove it from the heat source and immediately quench it before precipitates can form — quenching in gases such as argon, nitrogen or helium allows better control of the cooling process and reduces internal stress or distortion.
• Aging: Finally, reheat the component below the solvus temperature so that a thin layer of precipitate forms. This process creates a strong internal lattice, resulting in a more durable metal.

The following industries commonly use precipitation-hardened steel in their operations:

• Space
• Chemical
• Food and Beverage
• Oil and Gas

Flame Hardening

Flame hardening increases a steel component’s malleability while hardening its surface. You typically flame-treat metal before beginning other fabrication processes. Rapidly heat the metal through direct contact with a controlled high-temperature flame from a gas burner. Acetylene is an optimal fuel for flame hardening — when mixed with oxygen, it can reach temperatures up to 3,160 degrees Celsius. Propane is also effective for this purpose. After heating and soaking, immediately quench the component. This step enables it to cool quickly and develop a hard, wear- and corrosion-resistant surface.

Some advantages of the flame hardening process include:

• Improved surface wear resistance
• Reduced internal distortion
• Shorter process time
• Greater control over hardening localization
• Cost-effectiveness

Typically, flame-hardened steel is used in automotive and equipment components like gears and blades. Because it is so wear-resistant, it can withstand tough applications like construction for long periods of time.

At Linde, we are committed to making our world more productive. We provide high-quality industrial gases to help improve our customers’ process efficiency and profitability. For more information about which gases are right for your steel hardening needs, contact us today.