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Nitriding is a heat treating process that diffuses nitrogen into the surface of a
metal to create a case-hardened surface. These processes are most commonly
used on low-carbon, low-alloy steels. However, they are also used on medium
and high-carbon steels, titanium, aluminum and molybdenum. Recently,
nitriding was used to generate unique duplex microstructure (Martensite-
Austenite, Austenite-ferrite), known to be associated with strongly enhanced
mechanical properties
Typical applications include gears, crankshafts, camshafts, cam followers, valve
parts, extruder screws, die-casting tools, forging dies, extrusion dies, firearm
components, injectors and plastic-mold tools.
Processes
The processes are named after the medium used to donate. The three main
methods used are: gas nitriding, salt bath nitriding, and plasma nitriding.
Gas nitriding
In gas nitriding the donor is a nitrogen rich gas, usually ammonia (NH3), which
is why it is sometimes known as ammonia nitriding. When ammonia comes into
contact with the heated work piece it disassociates into nitrogen and hydrogen.
The nitrogen then diffuses onto the surface of the material creating a nitride
layer. This process has existed for nearly a century, though only in the last few
decades has there been a concentrated effort to investigate the thermodynamics
and kinetics involved. Recent developments have led to a process that can be
accurately controlled. The thickness and phase constitution of the resulting
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nitriding layers can be selected and the process optimized for the particular
properties required.
The advantages of gas nitriding over the other variants are:
Precise control of chemical potential of nitrogen in the nitriding
atmosphere by controlling gas flow rate of nitrogen and oxygen.
All round nitriding effect (can be a disadvantage in some cases, compared
with plasma nitriding)
Large batch sizes possible - the limiting factor being furnace size and gas
flow
With modern computer control of the atmosphere the nitriding results can
be closely controlled
Relatively low equipment cost - especially compared with plasma
The disadvantages of gas nitriding are:
Reaction kinetics heavily influenced by surface condition - an oily
surface or one contaminated with cutting fluids will deliver poor results
Surface activation is sometimes required to treat steels with a high
chromium content - compare sputtering during plasma nitriding
Ammonia as nitriding medium - though not especially toxic it can be
harmful when inhaled in large quantities. Also, care must be taken when
heating in the presence of oxygen to reduce the risk of explosion