Carburization of 8620H steel alloy (of dimensions 100 in diameter and 0:62500 thick) was
demonstrated using the AtmoPlasTM process and acetylene as the source gas [110]. The steel
sample was heated by microwave plasma to a temperature of 920 C in an argon atmosphere
and acetylene gas was then introduced for carburization. The temperature of the sample was
maintained at a desired temperature for a period of time by adjusting the microwave power
and then oil quenched and tempered for approximately 1 hour at 177 C [110]. Table 4.37
lists the processing parameters for the carburizing and microhardness of the steel samples.
The microhardness values of the microwave-plasma-processed samples are reported to be
comparable to conventional carburized samples [110].
4.4.8 Other Applications
Other applications of microwaves related to metals include the production of nanophase
metals using microwave-assisted synthesis [113–115], the combustion synthesis of intermetallics
[38] using metal powders and waste remediation of electronic circuitry for the
reclamation of precious metals [116].
Looking at the current trends, it can be anticipated that improvements will be made to
existing applications and new and innovative applications will emerge to exploit the many
advantages of microwave energy for the processing of metals in the near future.
4.5 Summary
Contrary to general belief that metals cannot be heated significantly by microwaves, it has
been demonstrated through both experiments and theoretical calculations that microwave
energy can be used to heat metals in the powder form rapidly to high temperatures.
Due to the skin effect, the penetration depth of microwaves in most metals at room
temperature and common microwave frequencies of 915 MHz and 2.45 GHz is limited to
only a few microns.
For effective coupling of metal particles and microwaves, different recommendations have
been made on the minimum particle size of the metal powder to be used:
— Sheinberg suggested that the particle diameter must be less than or equal to the skin
depth at the applied microwave frequency and the presence of an oxide layer around
the particles is essential for the coupling of metal compacts [2].