Module 4: Heat transfer calculations, miscellaneous topics and carbon credit
Lecture 35: Atmosphere in Furnaces
f) Ammonia base atmospheres (AGA 600)
Ammonia dissociation is used to prepare highest purity nitrogen which is free from oxygen.
Liquid ammonia is vaporized into a heat exchanger and is fed to dissociate in a reactor called as dissociater. The decomposition of ammonia to nitrogen and hydrogen begins at around . Rate of decomposition increases as temperature increases.
Its primary use is for bright annealing metals such as silicon to obtain electrical properties. Ferrous and non ferrous metals are bright annealed in ammonia atmosphere.
Bright silver brazing and copper brazing of steel are other applications.
Protective atmospheres applications
Composition(vol%)
Atmosphere
Lean exothermic
N2
86.8
CO2
10.5
CO
1.5
H2
1.2
CH4
-
Dew point
4.5
Applications.
Bright annealing of Cu, sintering of ferrites
Rich exothermic
71.5
5.0
10.5
12.5
5
10
Bright annealing low C steel, silicon steels/Cu brazing, sintering
Dissociated NH3
25
-
-
75
-
-50 to +60
Brazing sintering bright annealing
Endothermic
40.45
0-0.5
20
34-40
0.51
-10 to+10
Hardening, carburizing with CH4, sintering brazing
Nitrogen
H2
99.9
99.9
-60
-68
Natural for annealing
Reducing, sintering
Ar or He:These are pure and inert gases and are used to prevent oxidation during welding of stainless steel , aluminum etc. and heat treatment of special steels.
. Rate of decomposition increases as temperature increases. 
