Steel carburizing process

Steels for carburizing - introduction

Carburizing steel is a special type of steel that, after a carburizing and hardening process, achieves a hard and wear-resistant surface while maintaining a ductile and strong core. Thanks to these properties, it is widely used in the automotive, engineering, aerospace and tooling industries, especially for gears, shafts, pins and bearings that need to be resistant to abrasion and impact.

Carburizing allows for a controlled increase in the carbon content of the surface layer of a steel product, allowing it to be hardened later by quenching. The core, on the other hand, remains ductile and resistant to cracking, which ensures a long service life for parts subjected to high dynamic loads.

What is carburizing steel?

Carburizing steels can be divided into alloyed and unalloyed steels.

Non-alloy steels contain less than 0.45% C and a maximum of 0.75% Mn, in addition, they may contain increased S content to improve machinability. After the carburizing operation, they are subjected to hardening and low tempering to obtain a hard surface layer and a soft ductile core. Due to its limited hardenability, it is most often used to make small machine parts. Steels delivered in the annealed softened state are subjected to machining, followed by carburizing at a temperature of about 890°C and quenching directly from this temperature, and in the last stage the steel parts are tempered at a temperature of 150°C d0 200°C.

Alloy steels for carburizing are characterized by high strength properties of the carburized surface layer and high ductility of the core. In addition, they are distinguished by resistance to overheating, good machinability, low susceptibility to deformation during heat treatment, and hardenability suitable for the cross-section, loads and geometric characteristics of finished products.
They are characterized by low carbon content (0.10-0.25% C) and usually Cr additions of 0.8-1.7% and additions of Mn, Ni, Mo, Ti, W, V. In alloyed steels, elemental additions play a very important role, providing high strength properties of the carburized layer and a ductile core.
Cr, Mn and Ni additives improve hardenability and reduce the hardening stresses of the carburized layer. The temperature at which carburizing is carried out often depends on the method of this heat treatment and is, for example, 1050°C for vacuum carburizing. In the final stage of heat treatment, low tempering is carried out.

As a result of these operations, the core retains good ductility and fracture resistance. After carburizing and quenching, the surface layer reaches a hardness of up to 60 HRC, which provides high wear resistance.

The most commonly used steels for carburizing are:
🔹 16MnCr5 (1.7131) - a popular carburized steel with good mechanical properties.
🔹 20MnCr5 (1.7147) - a version with higher manganese content, increasing hardenability.
🔹 16/20MnCrS5 (1.7131/1.7147)
🔹 18CrNiMo7-6 (1.6587) - high alloy steel with high core strength.
🔹 20CrMo5 (1.7264) - carburized steel with molybdenum addition for improved creep resistance.
🔹 C15 (1.0401); C15E (1.1141); C15R (1.1140 )
🔹 20CrMo5 (1.7264) - carburized steel with molybdenum addition to increase creep resistance.

Chemical composition of steels for carburizing?

The chemical composition of carburizing steels is dominated by elements that improve hardenability, fatigue and wear resistance.

Carbon (C) - Responsible for the hardness of the surface after carburizing
Manganese (Mn) - Improves core strength
Chromium (Cr) - Improves wear and corrosion resistance
Molybdenum (Mo) - Increases creep resistance and stabilizes the structure
Nickel (Ni) - Improves ductility and resistance to dynamic loads

With various combinations of these elements, carburizing steels can be optimized for core strength and surface wear resistance.

Mechanical properties of carburizing steels?

Carburizing steels before and after heat treatment exhibit different mechanical properties.

After carburizing and quenching, the steel obtains a hard surface and a resilient core, making it a very good material for parts operating under high dynamic loads.

Carburizing and heat treatment process

The heat treatment process consists of several steps:
Carburizing is the operation of heating the steel to 850-950°C in the presence of a carbon-bearing medium (e.g., gas, liquid or solid), which allows carbon to diffuse into the surface layer.
Hardening is rapid cooling in oil or water, which hardens the surface.
Tempering is heating to 150-200°C to improve the material's resistance to cracking and stress relief.
Thus, the combination of a hard surface and a flexible core makes carburizing steels extremely resistant to wear and fatigue.

Applications of carburizing steels

These steels are used wherever hard surfaces and flexible cores are required to guarantee high component life. Typical applications include:
1. Automotive industry

• Gears, camshafts and piston pins
• Gearbox and differential components
• Bushings, cams and suspension parts

2. Machinery industry

• Gears and drive shafts
• Guideways, rollers and bearings
• Components of hydraulic pumps and precision mechanisms

3. Aerospace and power generation industry

• Turbines and shafts in power plants
• Rotor components and aircraft gears
• Parts operating under high dynamic loads

Advantages of steel for carburizing?

✅ Very hard surface - provides wear resistance and long service life.
✅ Plastic core - resistance to fracture and material fatigue.
✅ Surface hardening capability - fine-tuning of mechanical properties.
✅ Good resistance to dynamic loads - ideal for automotive and industrial applications.
✅ Widely used in drive technology - used in shafts, gears and gears.

🚫 Limitations:
❌ Requires complex heat treatment - precise carburizing and quenching is necessary.
❌ Not suitable for structures requiring uniform hardness - differences between surface and core can cause uneven loading.

Summary
Carburizing steel is a key material used in automotive, engineering and aerospace applications where the combination of a hard surface and flexible core is important. The carburizing and quenching process gives these steels exceptional abrasion and fatigue resistance, so they are used for drivetrain and transmission applications.
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