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What is Carburizing Bearing Steel Tube?

Carburizing bearing steel is a high-quality, low-carbon alloy structural steel with excellent wear resistance and strength. Simultaneously, a high-hardness carburized layer can be produced on the surface via carburizing treatment, increasing surface wear resistance and fatigue strength. 


Carburizing bearing steel is often created from carbon steel or low-alloy steel using heat treatment procedures like carburizing, quenching, and tempering.

What is the carburizing treatment?
Carbon atoms are forced to enter the surface of the bearing steel using certain methods, generating a carburized hardened layer with a set depth on the surface, which, together with the cores good toughness, achieves the goal of strengthening and toughening. 

Carburizing procedures can be classified as follows based on the carbon-containing media:
 
Solid Carburizing
Place the bearing steel and solid carburizing agent (charcoal + carbonate) in a closed container, then heat and keep warm in a high-temperature furnace for a set period of time. During the insulating period, the carbonate decomposes gradually, eventually yielding active carbon atoms. When a large number of active carbon atoms are adsorbed on the surface of the steel, a concentration difference forms between the surface and the inside of the steel. The surface carbon atoms will permeate into the interior along the concentration gradient. After a long period of insulation, a carburized layer with a specific depth will form.

- Advantages: simple equipment and easy operation
- Disadvantages: limited output and poor carburization quality

⚑ Liquid carburizing

Generally, it is salt bath carburizing, where the bearing steel is immersed in a carbon-containing liquid (usually a salt solution) for carburizing.

There is highly toxic cyanide in it, which not only harms human health but also pollutes the environment. Although some scholars are committed to exploring non-toxic liquid carburizing agents, they have never been able to completely eliminate cyanide (CN-), so this carburizing method has rarely been used.

⚑ Gas carburizing
The fundamental carburizing process is to transmit the carbon source through the gas medium, allowing the metal workpieces surface to absorb carbon and produce a carburized layer.

The workpiece is placed in an atmosphere containing a carbon source and heated to a specific temperature, causing the carbon source in the gas (typically methane, ethylene, or other carbon-containing gases) to react with the workpieces surface, producing carbon atoms that penetrate the metal surface to form a high-carbon area. The carburizing process is often carried out in a gas flow environment to guarantee uniform distribution of the carbon source.

- Carburizing temperature: 880-930℃

⚑ Vacuum carburizing
Carbon source gas (such as acetylene or propane) is decomposed at high temperature in a low pressure (vacuum) atmosphere to liberate carbon atoms, which penetrate into the surface of the workpiece to form a carburized layer.

- Carburizing temperature: 920-1050℃, making it appropriate for both high temperature and thorough carburizing.

⚑ Ion Carburizing
Ion carburizing is based on the pulse glow discharge mechanism. The gas ion flow forcefully bombards the material surface due to the accelerating voltage. The vacancies in the surface iron atoms are sputtered off and replaced by carbon atoms until the surface carbon concentration is saturated.

- Carbon sources: methane and propane
- Common carrier gases: hydrogen and nitrogen
- Carburizing temperature: 850-980℃

Carburized bearing steel heat treatment organizational flaws:
Network carbide
During carburization, insufficient diffusion of surface carbon atoms to the inside can easily result in an oversaturation of carbon concentration on the steel surface. Excessive carbon concentration precipitates along the austenite grain boundaries in the form of carbides, enclosing the austenite grains and presenting a network under the microscope, known as network carbide

- Hazards: It is in the brittle phase, which significantly reduces the toughness and fatigue capabilities of carburized bearing steel and can easily cause quenching cracking.

How to avoid it?
- Properly raise the carburizing temperature.
- Reduce carbon potential during the diffusion stage.
- Shorten the intense carburization cycle or increase the diffusion time.

✔ Surface decarburization
When carbon steel is heated and kept warm, the carbon in the surface layer is fully or partially lost as a result of the surrounding atmosphere.

Decarburization is the process by which carbon in steel combines with oxygen and hydrogen at high temperatures to produce gases like methane, carbon monoxide, carbon dioxide, or water, lowering the carbon content of the steels top layer.

Features: 
- Decreased carbon concentration in the surface layer.
- The surface hardness of the workpiece reduces after quenching.
- A dazzling white decarburized layer with decarburization properties can be seen under a microscope.

What are the factors that cause decarburization on the surface of carburized bearing steel?
★ Chemical composition of steel:
-Carbon content: The higher the carbon content in the steel, the easier it is to decarburize
-Alloying elements: Different alloying elements have different effects on decarburization.

For example, elements such as W, Al, Si, and Co will increase the decarburization tendency of steel, while elements such as Cr and Mn can prevent the decarburization of steel.

 Heating conditions:
-Heating temperature: Heating temperature is one of the most important factors affecting decarburization.

The higher the heating temperature, the deeper the decarburization. When the temperature exceeds a certain value, the oxide scale loses its protective ability and the decarburization rate will increase significantly.

-Hot holding time: The longer the stay at high temperature, the thicker the decarburization layer.

The depth of the decarburization layer does not increase completely in direct proportion to time, but long-term heating will significantly increase the depth of the decarburization layer.

 Furnace gas composition:
-Oxidizing atmosphere: will cause oxidation and decarburization of steel, among which H2O is the medium with the strongest decarburization ability
-Reducing atmosphere: may increase carbon content in steel

 Cooling rate:
When the cooling rate is low, the steel stays at high temperature for a longer time, which is conducive to the diffusion of carbon atoms and the increase of the decarburization layer.

When the cooling rate is high, the steel is quickly cooled to low temperature; the diffusion degree of carbon atoms is small, so only slight decarburization occurs or even no decarburization occurs.

✔ Excessive residual austenite on the surface
After quenching, carburized bearing steel leaves a high amount of untransformed austenite on the surface, resulting in low surface hardness and a significant impact on the bearings structural stability.

Advantages of carburized bearing steel
- High surface hardness and good wear resistance
- Good core toughness and impact resistance
- Carburized hardened layers can form high residual compressive stress, and surface cracks are not easy to expand inward.
- High-rolling contact fatigue strength
- Carburizing depth and concentration can be adjusted as needed.

✔ Standard: GB/T 3203, ASTM A534
✔ Grade: G20CrNiMo, 4118H, 8620H
✔ Metallurgy method: vacuum degassing or electroslag remelting

Chemical Composition (%)

Grade

C

Si

Mn

P

S

Ni

Cr

Mo

Cu

O

Al

G20CrNiMo

0.17-0.23

0.15-0.40

0.60-0.90

0.020

0.015

0.40-0.70

0.35-0.65

0.15-0.30

0.25

0.0015

0.050

4118H

0.17-0.23

0.15-0.35

0.60-1.00

0.025

0.015

-

0.30-0.70

0.08-0.15

0.30

0.0020

0.050

8620H

0.17-0.23

0.15-0.35

0.60-0.95

0.025

0.015

0.35-0.75

0.35-0.65

0.15-0.25

0.30

0.0020

0.050


Hardness
✔ Surface hardness: >60 HRC
✔ Core hardness: >33-48 HRC

Heat treatment process of carburized bearing steel
Preheating
Carburizing
Diffusion
Quenching + low-temperature tempering

Application of carburized bearing steel tube
Bearings and rolling elements subjected to high loads and high speeds
Bearings
We can provide wear-resistant carburized bearing steel tubes that can be used to make bearing rings. There are several material grades and specifications available.
 
Carburizing Bearing Steel Tube

Carburizing Bearing Steel Tube

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