Effect of furnace atmospheres in carburising

[smurali1]

Dear all,

I am interested in knowing effects of furnace atmosphere in carburising heat treatment.

The material in question is 8615H (with C=0.1-0.14 and B=10-30ppm) Effective case depth required: 0.45-0.78mm. Case hardness: 79-82HRA Core hardness=32-44HRC

The atmospheres being considered are LPG+CO2(CO=35%, H2=Balance) & N2+Methonol+LPG (CO=20% H2=40% N2=40% LPG=1-2%) in a Sealed Quench Furnace with fully automatic control of Cp, furnace temperature and Quenchant temperature.

What will be effect of either of the atmospheres on 1. Case depth capability 2. Micro structures 3. Skin hardness 4. Core hardness 5. Any other load bearing property of the part
6. Cleanliness of the part. (we cannot allow shot blasting/vibro finishing after HT)

Please note we do a tempering after carburising hardening.

Thanks in advance for the help.

 

[Carburize]

The atmosphere effects case microstructure - excessive carbon potential can cause formation of damaging carbides. Low carbon potential can cause low surface hardness and reduced effective case depth.

 

[Goahead]

At any given temperature, a furnace atmosphere is in a different dynamic equilibrium with iron and its surface carbon content.
Furthermore if the furnace has fully automatic control of Carbon Potential, equilibrium is reached, at any temperature, by increasing or decreasing the
quantity of the carburizing agent relative to the carrier gas.
Case depth depends on carbon potential temperature and time.
In principle at the same temperature, dew point, time and carbon potential you should find almost equal characteristics on the same type of steel.
Please note that, if carburising temperature is high, it is customary to quench from lower temperature or even to cool slowly and reheat for quenching, to improve properties. Core hardness will not be influenced by carburising, only by austenitizing temperature, time, quench and temper, for
any given size.

http://www.welding-advisers.com/

 

[smurali1]

Let me rephrase the question a bit:
Will change in atmosphere from LPG+CO2 to N2+CH3OH+LPG affect 6 properties I have mentioned? If yes, how? (All other background details are same)