The term decarburization simply means the loss of surface carbon from the steels being treated. There are many causes of decarburization that occur as a result of rolling (hot-rolled products), forging, casting and heat-treatment procedures at high radiant temperatures without any surface protection to the steel.

The action of decarburization will change the carbon content at the surface of the steel. This means that the surface of the steel will react differently to the core when austenitizing and quenching simply because of the lack of surface carbon, which will not allow full transformation from austenite to martensite.

Decarburization is an Unwanted Metallurgical Condition

It means that the process control of temperature and gas analysis both for carburizing and neutral hardening is of paramount importance. Good control necessitates:
  • Good furnace maintenance
  • Good endothermic-gas-generation maintenance
  • Good atmosphere control and analysis
  • Good temperature control
  • Good pre-machining practice and engineering design
Decarburization will begin to occur at temperatures above 1250°F, and the rate of decarburization will increase as the temperature increases. The gases that will contribute to the cause of surface decarburization are CO2, H2O, H2 and O2.

These gas molecules will react with the carbon atoms as well as the iron atoms to extract carbon from the steel surface. It is believed that the following reactions are what occurs at the steel surface:

Reaction 1: CFe + CO2 = 2CO

Reaction 2: CFe + H2O = CO + H2

Reaction 3: CFe + 2H2 <- CH4 (this is of course you are using methane)

When the reactions occur left to right, decarburizing conditions will occur. When reacting right to left, carburizing conditions will occur.

What can affect the Process Gas to Become Decarburizing?
  • Changes in process temperature
  • Process-gas flow rate
  • Oxides being carried into the furnace process chamber
It is most important if using an endothermic gas generator to maintain the process chamber catalyst so as to be sure that the CO2 and water vapor are kept to a minimum.

This further means that the city gas to air ratios are maintained. The ratio of air to gas is generally 2.8 Vols (air) to 1 Vol (city gas). This will vary with: atmospheric pressure, atmospheric temperature and atmospheric humidity.

So, it is most important to control the endothermic generator as a consistent source of process gas for the protective atmosphere. Generally the carbon potential of the endothermic gas will be approximately 0.30%.