Carburization
is the formation of metal carbides in a material as a result of exposure to a
carbon containing atmosphere. In some cases, materials are intentionally
carburized to impart a high surface hardness and wear resistance. However, in
service, it can result in loss of mechanical properties over time in addition
to wastage of the material. The corrosion process referred to as "Metal
Dusting" is a carburization process which results in surface attack and
the formation of a powdery residue. Carburizing environments are commonly found
in process piping and furnace tubes that contain an excess of carbon-containing
species which includes hydrocarbon gases or carbon monoxide. Pyrolysis piping
in ethylene and olefins plants are prime examples of equipment that must be
designed to resist severe carburization.
Resistance
to carburization generally centers around two mechanisms:
- the establishment of an
effective barrier on the surface of the materials that limits the ingress
of carbon into the material.
- tying up carbon in the material
using alloying elements which are strong carbide formers.
In
the first case, the barrier is usually a Cr2O3 layer on
the material. This is accomplished through the use of Cr as an alloying
element. Secondly alloying additions of Ni also help by reducing carbide
diffusion in the material.
Under
severe carburizing conditions up to 800 C, the use of Type HK Alloys
(Fe-25Cr-20Ni), Type HP (Fe-25Cr-35Ni) and Alloy 800H is common. Si additions
have been found to be beneficial in enhancing these alloys through formation of
a protective SiO2 layer on the surface of the material over the
range of 1 to 2 wt percent Si (See Figure). Additions of
niobium, a carbide forming element, is also used to extend the performance of
these alloys by tying up carbon in the form of stable carbides to reduce its
reaction with Cr. Carburization
can also occur in Cr-Mo steels at low temperatures (450 to 650 C). Several
methods of reducing carburization can be utilized. These include the use of
barrier coatings, reducing the carbon activity of the environment, lowering the
service temperature by cooling and introduction of sulfur or other compounds
which poison surface adsorption sites and thereby limit the entry of carbon
into the material.