Gases and their behavior:

1. Nitrogen is the primary component of atmospheric air . Oxygen is . Rest could be other gases, which is of very little significance. Nitrogen is considered to be chemically inert and is used as a carrier gas for reactive furnace atmosphere, for purging etc.  At high temperatures, nitrogen may show reaction with  Cr and .
   
   
2. Hydrogen is a reducing gas and is used where reducing atmosphere is required. It may be used to prevent oxidation of iron

        Hydrogen can also be used to decarburize the steel for certain applications.  At the material         temperature is greater than 973K, the following reaction occurs:

         

        Hydrogen may be absorbed by the metal at elevated temperatures and cause hydrogen
        embrittlement.

      3.   Carbon monoxide: it is also a reducing gas and is used to create a reducing atmosphere

      4.   Carbon dioxide: it is a mild oxidizing gas. It forms oxides with iron at elevated temperatures. At .          temperatures greater than 540 , the following reaction may occur

          and at temperatures lower  than  , the following reaction may occur
           

          Decarburization may also result by the reaction like
           and
           

    5   Argon and Helium: Both are inert gases and are used to maintain inert atmosphere. In some         applications argon is used for purging.

    6    Steam: it is used to provide  blueing effect in steel between 573K and 923K. Blueing effect is due          to the formation of either  or . The formation of oxide of iron depends on          temperature,and ratio by partial pressure of  to partial pressure of  in the atmosphere.

          "Dew point” quantifies the concentration of  vapor in the atmosphere. Dew point is a             temperature at which gas is saturated with water vapour (100% relative humidity). In a furnace,           water-gas reaction controls the concentration of  and  according to the following           reaction:

          .