Heat Treatments
Heat
treating is the general name for a set of technologies in
which the product is heated to give the proper alloy
properties (tensile strength, hardness, toughness) or to
modify the surface. These types of technologies are also
known as diffusion treatments when they also involve the
diffusion of metal atoms from a packed bed into the surface
– primarily Al, Cr and B.
The most
widely used surface modifications are nitriding, carburizing
and nitrocarburizing. Older processes used to involve the
use of molten salt baths (frequently cyanides) or packed
beds, but these methods have been largely displaced by gas
methods of nitriding, carburizing and nitrocarburizing, and
by plasma nitriding. New technologies such as ion
implantation have been adopted into the surface treatment
industry from the semiconductor industry, but have found
only small niches.
Because
they cannot be used to build up worn material, heat
treatments are not used in repair. Their primary usage is
to create wear-resistant surfaces on products (carburizing
of gears, for example), or to make surfaces resistant to
corrosion or oxidation (e.g. boiler tubes, hot section
turbine blades). The exception to this is ion implantation,
which is a room temperature process in which ions are fired
into the surface to form a very thin (0.1
mm) layer. Although the layer affected is extremely
thin, the process appears to change the wear mechanism so
that it has an impact on wear long after the thin surface
has been worn away.
Facts
about heat treatments:
-
Heat
treatments generally take place at temperatures of
500-1,000°C (930–1,830°F).
This limits the alloys and products that can be treated.
-
At
these temperatures, some alloys can grow as they go
through phase changes, and products can distort.
-
These
treatments are not coatings. They cannot come off as
coatings can, but they also cannot be used to build up
worn surfaces.
-
Not
all alloys can be nitrided or carburized. These
processes are usually used on nitridable or carburizable
steels, but can be used on some other alloys.
-
The
exception to this is ion implantation, which is done at
room temperature, usually using nitrogen ions. It has
very limited market niches (medical prostheses such as
hip and knee joints, certain molds, dies and metal
forming tools). The method has been shown to reduce
wear in hard chrome plate, and so some companies have
used the process to improve the life of plated products.
Nitriding, carburizing and implantation are clean processes
and the surfaces are RoHS-compliant. Metallizing treatments
(chromizing, aluminizing, boronizing) can product chlorine
and HCl gas and leave the packed bed a hazardous waste that
must be disposed of properly. Some newer metallizing
methods have been developed to alleviate these problems. |
