Surface hardening of metals is a process that improves the mechanical properties of only the surface layer of metal parts, while the internal structure remains unchanged. The method aims to increase the hardness and wear resistance of metals while preserving their internal ductility and ability to absorb impact loads. As a result, parts become more durable while retaining their basic performance characteristics.
Surface hardening methods
Carburizing
Carburizing is a process of diffusion saturation of the surface layer of a steel part with carbon. The part is heated in a carbon-rich atmosphere (for example, carbon monoxide or gaseous hydrocarbons), and carbon atoms penetrate the metal surface, forming a hard, wear-resistant structure. Carburizing is usually followed by hardening to create a strong surface layer.
Applications: carburizing is widely used to strengthen parts subject to heavy mechanical loads, such as gears, shafts, axles and engine cams.
Carbonitriding
Carbonitriding is a process of saturating the metal surface with carbon and nitrogen. Unlike carburizing, carbon and nitrogen penetrate the surface simultaneously, creating a layer that is more resistant to corrosion and wear. The process is carried out at relatively low temperatures (500–600 °C), which reduces the risk of part distortion.
Applications: carbonitriding is used to strengthen small and medium-sized parts that require high wear resistance, such as pistons, rods and fasteners.
Nitriding
Nitriding is a process of saturating the metal surface with nitrogen at around 500–600 °C. Nitrogen forms nitrides with elements of the metal, significantly increasing surface hardness and wear resistance. Unlike carburizing, nitriding does not require subsequent hardening, and the part can be used immediately after cooling.
Applications: nitriding is used for parts that require high wear and corrosion resistance, such as turbine blades, hydraulic cylinder rods, gears and gearbox components.
Induction hardening
Induction hardening involves rapidly heating the surface layer of the metal by electromagnetic induction, followed by quick cooling (usually in water or oil). This creates a hard surface layer with a soft, ductile core.
Applications: induction hardening is used for parts operating under impact loads, such as axles, gears, cams and guides.
Chrome plating
Chrome plating is a method of applying a thin layer of chromium to the surface of metal parts. The process not only gives parts an attractive appearance but also significantly increases hardness and corrosion resistance and reduces friction.
Applications: chrome plating is used to strengthen parts exposed to high friction, such as piston rings, hydraulic cylinders, suspension parts and machine mechanisms.
Boriding
Boriding is a process of saturating the metal surface with boron, which forms a very hard layer of borides on the surface. This layer is highly resistant to corrosion and wear. Boriding is carried out at around 900–1000 °C and, like nitriding, does not require subsequent hardening.
Applications: boriding is used to produce parts that require high hardness and abrasion resistance, such as turbine blades, fuel pump parts and tools for working with abrasive materials.
Where is surface hardening used?
Surface hardening methods are used across industries where parts need greater wear resistance and durability without a significant increase in weight or changes to their internal properties. Key areas include:
- Mechanical engineering: gears, shafts and bearings exposed to heavy mechanical loads and wear are hardened to extend their service life.
- Automotive industry: engine, gearbox and suspension parts, cams and pistons operating under high temperatures, impact loads and friction require hardening to improve wear resistance.
- Aerospace industry: turbine blades, engine casings and other highly loaded components are hardened to increase their strength with minimal weight gain.
- Agricultural machinery: plough components, blades and other parts that come into contact with aggressive environments and are subject to mechanical wear require hardening to improve abrasion resistance.
- Tool industry: cutting tools such as drills, milling cutters and knives are hardened to extend their service life and reduce the need for frequent replacement or sharpening.
Examples of surface hardening
Automotive gearbox gears
Gears in cars are subjected to heavy loads, friction and temperature fluctuations. Carburizing or induction hardening is often used to extend their service life. This allows the gears to withstand high contact loads while remaining wear-resistant.
Car suspension springs
Springs exposed to cyclic loads and deformation undergo shot peening to increase their fatigue strength. This process helps prevent cracking and extends the service life of the springs.
Engine pistons
Pistons are chrome-plated to reduce friction and increase wear resistance. The chrome layer protects pistons from wear under high temperatures and pressures, extending their service life.
Cutting tools
Steel milling cutters and drills undergo laser hardening to create a hard cutting edge. This helps tools stay sharp longer and reduces the need for frequent replacement or sharpening.
Surface hardening of metals is an important process that significantly improves the performance of metal parts by increasing their wear resistance, strength and durability. Various hardening methods make it possible to optimally combine a hard surface layer with a ductile core, making such parts ideal for use under heavy loads and in aggressive environments.