Case hardening is a thermo-chemical process. Within the framework of this process, the surface layer of components is enriched with carbon, through a method known as carburization. This process is utilized to improve the mechanical properties of the component surface layer.
the case hardening process
The process of case hardening typically involves the sequential steps of carburizing, hardening, and tempering. In the first step, the workpieces are exposed to a carbon-rich environment at a temperature ranging from 800 °C to 1,050 °C. The second step involves quenching, either directly from the carburizing temperature or after intermediate cooling and re-heating to a specific hardening temperature. The third step, tempering, is primarily aimed at relieving the highest stresses in the material structure and reducing the susceptibility to grinding cracks.
1. process: carburizing
In case hardening, carburizing is a diffusion-controlled process used for surface treatment of workpieces. It involves enriching the surface of the workpiece with carbon to improve its hardness and wear resistance.
Carburizing can be carried out through gas carburizing or salt bath carburizing. In gas carburizing, the workpiece is exposed to a carbon-rich gas atmosphere, while in salt bath carburizing, the workpiece is immersed in a molten salt bath. Through the diffusion process, carbon penetrates the surface of the workpiece and forms carbide-rich layers.
2. process: hardening
Hardening in the context of case hardening encompasses the process of rapidly cooling a heated workpiece to produce a martensitic structure. It involves reaching the austenitizing temperature, followed by rapid quenching in a medium such as water, oil, or air. This results in high surface hardness and strength by transforming the austenite structure into a martensitic structure.
3. process: tempering
Tempering is a heat treatment process carried out after hardening in case hardening. The workpiece is heated to a specific temperature and then slowly cooled. The aim of tempering is to reduce hardness while improving the toughness and ductility of the workpiece. Through controlled heat treatment, internal stresses are relieved, and the mechanical properties are optimized. The tempering process allows for the adjustment of the workpiece properties to meet the specific requirements of the application.
advantages of case hardening
✓ Increased lifespan: Case hardening improves the lifespan of the workpiece, allowing it to withstand repeated loads without experiencing material fatigue or failure.
✓ Enhanced resistance: Case hardening improves the workpiece’s resistance to wear, abrasion, and deformation. This enables the workpiece to withstand higher loads and makes it more durable.
✓ Increased tensile strength: Case hardening increases the tensile strength of the workpiece, making it more resistant to tensile forces and increasing its load-bearing capacity.
✓ Improved heat resistance: Case hardening enhances the workpiece’s ability to withstand high temperatures without losing its strength or hardness. This makes it well-suited for applications where high temperatures are involved.
Case hardening steel (C ≤ 0.25 %)
easy to machine
easy to weld
Core zone (C ≤ 0.25 %)
tough and ductile
enhanced service properties
(toughness and, if applicable, strength)
Surface layer (C = 0.70…0.90 %)
hard and wear resistant
improved fatigue strength under
which metals are suitable for case hardening?
Case hardening is typically performed on alloy steels that contain specific alloying elements. Here are some suitable materials for case hardening:
Low-carbon steels: Steels with a carbon content of less than 0.3% are often suitable for case hardening. They have a relatively low hardness and are easily machinable, but they can be hardened on the surface to improve wear resistance. Examples include 1018 and 1020 steels.
Medium-carbon steels: Steels with a carbon content ranging from 0.3% to 0.6% can be case hardened. They have higher strength and hardness compared to low-carbon steels, making them suitable for applications requiring greater toughness and wear resistance. Examples include 1045 and 4140 steels.
Alloy steels: Alloy steels containing additional elements such as chromium, molybdenum, or nickel can also undergo case hardening. These alloying elements enhance the hardenability of the material, allowing for deeper and more uniform hardening. Examples include 8620 and 9310 steels.
Some non-ferrous metals: While case hardening is predominantly associated with ferrous materials, there are certain non-ferrous metals that can be case hardened to a limited extent. These include aluminum alloys and certain copper alloys, such as bronze.
properties and fields of use
Case hardening, as one of the most widely used heat treatment processes worldwide, finds extensive application in gears, machinery, automotive, and aerospace industries.
It is particularly beneficial for gears and similar components that are subjected to high wear in harsh environments. This heat treatment process is especially useful if there is a requirement for a hard, wear resistant surface, overlying a much softer and tougher core. A good example for its application is a gear wheel. The surface of the teeth needs to be extremely hard so that they can withstand constant metal-to-metal contact, without undue wear. The underlying material needs to be tough so that the teeth can tolerate occasional impact loads, without the risk of fracture.
Case hardening, properly known as carbon case hardening, is used to give a hard, wear and indentation resisting surface to mild and low alloy steels, up to depths of 4-5 mm.
Steels suitable for case hardening are unalloyed or alloyed steels with a carbon content below approx. 0.25%. Partial case hardening is achievable through suitable isolation techniques.
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