Hardenen

Hardening (Surface Hardening) – Strengthening Surface Durability in CNC Machining

Surface hardening is based on the principle of changing the microstructure of the metal surface by heat treatment. This is done by diffusing elements such as carbon or nitrogen into the outer layer of the metal, followed by controlled cooling (quenching) to fix the hardness. The core remains unaffected, retaining its original mechanical properties, such as toughness and flexibility.

At high temperatures, carbon or nitrogen atoms penetrate the outer layer of the metal. With rapid cooling, this often leads to the formation of martensite, a hard and brittle phase. This diffusion can be achieved by various methods, such as carbonizing, nitriding and carbonitriding, depending on the application and material. In carbonizing, for example, the metal is exposed to a carbon-rich environment at high temperatures, followed by quenching, which traps the carbon in the surface microstructure, resulting in a hardened outer layer.

Surface hardening temperatures typically range from 850°C to 950°C, depending on the type of steel and desired properties. Quenching media, such as oil, water or gas, are chosen based on the desired cooling rate; faster cooling usually yields higher hardness.

Types & Variants of Surface Hardening

There are several types of surface curing, each with specific advantages depending on the required properties of the final product:

• Carbonation: This process involves heating the part in a carbon-rich atmosphere, causing carbon atoms to diffuse into the surface. It is widely used for low-carbon steel parts, creating a hardened outer layer while the core remains ductile.
• Nitriding: In this process, nitrogen diffuses into the surface at lower temperatures than carbonizing, typically between 500°C and 550°C. Nitriding provides a very hard and wear-resistant outer layer without the need for quenching, making it suitable for parts that must withstand high surface loads without deformation.
• Carbonitriding: A hybrid process in which both carbon and nitrogen are diffused at a slightly lower temperature range (around 820°C to 900°C). Carbonitriding is used to harden materials with a thinner hardened layer and increases the wear resistance of fasteners, gears and small automotive parts, for example.
• Induction hardening: This localized surface hardening uses electromagnetic induction to heat the surface of the metal, followed by quenching. It is suitable for large parts or specific areas, such as gear teeth or bearing races.

Each of these processes offers unique advantages in terms of depth of cure, surface finish and possible applications.

Material suitability & Preparation

Surface hardening is most commonly applied to low-carbon steels. After diffusion of carbon or nitrogen, a hard outer layer is formed while the core remains soft. Medium- and high-carbon steels can also be treated, but the effect is weaker due to the higher initial carbon content.

Proper preparation is crucial to a successful hardening process. The parts must be thoroughly cleaned to remove any surface contaminants that may interfere with the diffusion of carbon or nitrogen. In some cases, preheating is required to ensure uniformity in the hardened layer.

Materials such as aluminum and most nonferrous metals are unsuitable for traditional surface hardening processes because they do not contain the necessary carbon or do not undergo the required phase transformations. Alloys such as titanium, however, can undergo alternative surface hardening processes such as nitriding.

Performance Enhancements & Benefits

The main benefit of surface hardening is the significant increase in surface hardness, leading to improved wear resistance. The process can increase the hardness of steel components to as much as 60 HRC (Rockwell hardness scale). This makes surface-hardened components exceptionally durable under abrasive conditions, extending their life in harsh working environments.

In addition to hardness, surface hardening also improves fatigue resistance, as the tough inner core absorbs impact and prevents cracks from propagating through the material. This is especially beneficial for components such as gears and shafts, which are exposed to cyclic loads and high rotational stress.

Applications & Relevance by Industry

Surface hardening is essential in industries where components must endure harsh conditions. In the automotive sector, surface-hardened gears, camshafts and crankshafts ensure long-term performance under constant wear and stress. In aerospace, components such as bearings and landing gears benefit from increased surface hardness and durability, which is critical for safety and reliability. Hardened parts, such as heavy tools and vehicle parts, are also widely used in construction and defense because of their high surface resistance and toughness.

Customization & Process Optimization

We offer extensive customization options within our surface curing processes. We customize temperature profiles, waiting times and quenching media to meet the specific requirements of each project. Our precise control of carbon and nitrogen diffusion allows us to precisely match the depth and hardness of the hardened layer to the customer’s needs. Whether you need a thin, hard layer for small, high-speed components or a deep hardened layer for heavy components, we can tailor the process.