Question:
We are facing a decarburization problem in gas-carburizing process for low-carbon steel parts. The atmosphere we are using is nitrogen/(methanol and acetone). We do not understand why the parts are decarburizing. It will be a great help for us if you suggest anything in this matter.

Answer:
Decarburization is a change in the steel’s microstructure in which some of the carbon in the surface layer is lost. In total decarburization, the upper layer of the steel is primarily ferrite. In partial decarburization, the ferrite layer is not continuous. Decarburization on low-carbon steel parts is typically an indication that either (1) your furnace atmosphere is out of control or (2) that air is infiltrating into your furnace.

Decarburizing of steel requires an understanding of the furnace (and metal) temperature and atmosphere concentration (CO and CO2 values).

Steps you can take to avoid this problem include making sure that you have enough atmosphere flow and that both your acetone and methanol are properly dissociating (that is, breaking down from a liquid to a gas). You are probably introducing them through an injector we call a "sparger," and it may be damaged or clogged. If you do not properly atomize these liquids (that is, if you are getting large drops rather than a fine mist), your furnace atmosphere will not have the proper carbon potential, which could also be the problem.

Here are some useful conversions for you:[1]

1) 1 gal of methanol = 3.785 liters = 241 scf of dissociated methanol = 161 scf of H2 + 80 scf of CO.
2) Typical endothermic gas (1,000 scf) is approximately equal to 400 scf of N2 + 400 scf of H2 + 200 scf of CO.
3) Equivalent N2 + methanol = 40% N2 + 60% dissociated methanol
4) 1,000 scfh = 400 scfh N2 + 600 scfh of dissociated methanol = 400scfh of N2 + 2.48 gph of liquid methanol (9.42 liters per hour)
5) 1 gallon per hour liquid = 63.1 cc/min liquid
6) 1 cc/min liquid = 3.83 scfh of dissociated gas.

You may also find the two figures to the right useful. The top figure notes methanol breakdown, while the bottom is the breakdown of methanol at various temperatures.