Carburization of Steel: Why Use High Temp?

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In summary, carburization is the process of implanting carbon atoms into the surface layers of a metal, which is achieved through diffusion. This diffusion process is dependent on temperature and follows Fick's law for bcc and fcc iron. Despite the lower diffusion rate in fcc iron, carburization is carried out at a higher temperature due to its higher solubility in gamma iron. This allows for a desired microstructure to be obtained through quenching and tempering, resulting in improved mechanical properties with wider engineering applications.
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Rafa
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The process of carburization works via the implantation of carbon atoms into the surface layers of a metal. As metals are made up of atoms bound tightly into a metallic crystalline lattice, the implanted carbon atoms force their way into the crystal structure.

The carburization is produce for difusion of C in the Steel. I read that temperature for carburization are around 900ºC.

The difusion of C depends of Fick'a law. For:
bcc Fe , the difusion of C is D=220x10^-6*exp(122000/RT)
fcc Fe , the difusion of C is D=20x10^-6*exp(142000/RT)

Then difusion in austenitic steel (fcc Fe) is lower than ferritic steel (bcc Fe)

¿Why do it use a higher temperature (900ºC) if the diffusivity in fcc-fe is lower than in bcc-fe?
 
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  • #2
It boils down to obtaining a desirable microstructure than kinetics of diffusion.

Carbon is far more soluble in gamma iron than in alpha iron (where the maximum solubility is about 0,02%). If you make carbon react with alpha iron, you would expect to get a thin, brittle surface layer of iron carbide and, with longer times, large iron carbide precipitates just below the surface. This might be OK if you just want a hard surface but it would crack readily under a tensile load.

If your carbon is in solution, you can quench and temper your alloy to get a fine distribution of carbides and the properties obtained from this microstructure have a much wider engineering application.
 
  • #3
An additional thought. Whether or not you actually get iron carbide on the surface will, of course, depend on the temperature. Above the eutectoid temperature, the alpha iron transforms to gamma iron
 
  • #4
Thanks Macon, now I understand. The diffusion is lower but the solubility is higher and improves the posibility of treatment.
 
  • #5


Thank you for your question. The reason for using a higher temperature (900ºC) in carburization of steel is to increase the rate of diffusion of carbon atoms into the metal. While it is true that the diffusivity of carbon in fcc-Fe is lower than in bcc-Fe, the higher temperature allows for more energy to be available for the diffusion process, leading to a faster rate of diffusion.

Additionally, the increased temperature also helps to break down the metallic crystalline lattice, making it easier for the carbon atoms to penetrate and diffuse into the surface layers of the metal. This results in a more uniform and thorough carburization process, leading to a stronger and more durable end product.

Furthermore, the higher temperature also allows for longer exposure times without risking the formation of undesirable phases or structures in the metal. This is especially important in industrial settings where large quantities of steel need to be carburized quickly and efficiently.

In summary, the use of a high temperature (900ºC) in carburization of steel is essential for achieving a fast and effective diffusion of carbon atoms into the metal, resulting in a stronger and more durable end product.
 

1. What is the purpose of carburizing steel?

The main purpose of carburizing steel is to increase its surface hardness and wear resistance. This is achieved by introducing carbon atoms into the surface of the steel, creating a high-carbon layer known as a "case." This allows the steel to withstand high stress and wear, making it suitable for use in applications such as gears, bearings, and tools.

2. How does high temperature affect the carburization process?

High temperature is essential for the carburization of steel because it allows the carbon atoms to diffuse into the surface of the steel more quickly. This results in a thicker and more uniform case, which improves the hardness and wear resistance of the steel. Additionally, high temperature can also help to reduce the amount of time required for the carburization process.

3. What are the common methods of achieving high temperature for carburization?

The most common methods of achieving high temperature for carburization include gas carburizing, liquid carburizing, and pack carburizing. Gas carburizing involves heating the steel in a furnace with a carbon-rich gas atmosphere, while liquid carburizing involves immersing the steel in a molten salt bath. Pack carburizing, on the other hand, involves surrounding the steel with a carbon-rich material, such as charcoal, and heating it in a furnace.

4. Are there any drawbacks to using high temperature for carburization?

One potential drawback of using high temperature for carburization is the risk of distortion or warping of the steel. This can occur if the steel is not properly supported during the heating and cooling process. Additionally, high temperature carburization can also increase the risk of grain growth in the steel, which can affect its mechanical properties.

5. How can the effects of high temperature carburization be controlled?

To minimize the risk of distortion and grain growth, it is important to carefully control the heating and cooling process during high temperature carburization. This can be achieved through the use of proper fixtures and supports, as well as precise temperature and time control. Additionally, the choice of carburizing method and atmosphere can also affect the overall outcome of the process.

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