Carburizing

Carburizing:

 Is the most widely used method and is used on steels having a carbon content of 0.1-0.25%. The process is generally carried out in a temperature range of 900-930— C. The surface layer is enriched with 0.7-0.9% of carbon. The diffusion of carbon is done by heating the steel above the transformation temperature in contact with a material rich with carbon. This material may be solid liquid or gaseous. Since the solubility of carbon in austenite is more, a fully austenite face is desired for the process. The process is divided in to different categories according to the carbon source used.

i)       Pack Carburizing: 

This method is also known as solid Carburizing. It is the oldest method of Carburizing. In this process the steel components, which are to be treated, are packed with 80% granular coal and 20% Barium Carbonate (BaCO3), in heat resistant boxes. Barium Carbonate is used as an energizer. The box containing steel component and the carbon source is heated to 930—C in an electrical chamber furnace. The holding time depends on the case depth required. The following reactions take place on heating.

1. Energizer decomposes to give Carbon Monoxide gas to the steel surface.

                  BaCO₃                                      BaO +CO₂,

CO₂+ C               2CO 

2. Carbon monoxide reacts with surface f the steel.

            2CO + Fe             FeC + CO₂

3. Diffusion of carbon into steel.

4. CO₂ formed in step 2 reacts with carbon in the coal.

CO₂+ C                2CO

            For a given temperature the depth of penetration depends on diffusion and can be related to time “t” by the equation    Case depth = k¨ t    where k is a constant. Generally the time varies from 6 to 8 hours and the case depth obtained is 1 to 2mm. Since the result depends on the quality of coal, there is a factor of uncertainty.

 ii)       Gas Carburizing:

This is the most widely used method of Carburizing. It is carried out in retort type, sealed quench type or continuous pusher type furnace. These are either gas fired or electrically heated. Temperature varies from 870-950— C. Gas atmosphere is produced from liquid (methanol) or gaseous (Propane & Methane). An endothermic gas generator is used to supply endothermic gas. A mixture of propane or methane with air is cracked in hot retort of an endogas generator to form carrier gas. Furnace chamber is purged with this gas to maintain a slight positive pressure. This prevents the infiltration of atmospheric air from entering the furnace. The presence of this gas prevents the oxidation of steel surface. When the material is heated up to Carburizing temperature, propane and methane is introduced in to the furnace to maintain specific carbon potential. Measuring the dew point or carbon dioxide by infrared absorptiometry or measuring oxygen by an electrolytic potential technique using a zirconia probe computes carbon potential.

1. 
   C₃H₈                      2CH₄ + C (cracking of hydrocarbon)
2. 
   CH₄ + Fe              Fe(C) + 2H₂
3. 
   CH₄ + CO₂           2CO + 2H₂
4. 
   2CO + Fe             Fe(C) + CO₂

Carburizing occurs due to the conversion of CO to CO2 through reaction no.4. Hydrogen created during the step 2 and 3 reacts with CO2 and increase CO concentration.

H₂+ CO₂             CO + H₂O

iii)      Liquid Carburizing:

This process is also known as salt bath Carburizing. In this process Carburizing occurs through molten Cyanide (CN) in low carbon steel. Pot type furnace, which is heated by gas or oil, is used for this process. Bath temperature is maintained between 815 and 900— C. the life of the pot depends on the quality of material, operating temperature and mode of operation. This may be continuous or intermittent. Continuous process gives better end results. Bath surface is covered with graphite or coal to reduce radiation losses and excessive decomposition of cyanide. In this process heating time is shorter and heat transfer is rapid. There is complete uniformity of the carburized layer. The process results in a thin and clean hardened layer (0.08mm). Extensive safety precautions are needed to avoid explosion. Other than Sodium Cyanide and Potassium Cyanide the bath contains chlorides of Sodium and Potassium and Barium. Barium Chloride acts as an activator.

1. 
   BaCl₂  + 2NaCN       Ba(CN)₂  + 2NaCl
2. 
   Ba(CN)₂  + Fe          Fe(C) + Ba(CN)₂

iv)     Vacuum Carburizing:

It is the process of Carburizing carried out either in vacuum or in reduced pressure. The main advantage of this process is tremendous energy saving. The process is carried out in two stages. The job is introduced into the furnace and   the furnace is then evacuated and then heated up to Carburizing temperature, which normally lies between 925-1050— C.  Then a gaseous hydrocarbon such as methane or propane is introduced in to the furnace. Compared with the conventional Carburizing only 1% of gases are required. The surface of the steel cracks when it comes in contact with the gas, and as a result carbon is deposited on the surface, which is immediately absorbed by the surface.  The process continues till the required case depth is achieved. It is an energy saving process and the Carburizing cycles are shorter than conventional method.