192 Lubricant Additives: Chemistry and Applications
the higher-cost materials are consistency, lubricating performance, and reduced oxidation rates of
graphite.
The chosen graphite should be of a specifi c particle size distribution to derive certain benefi ts in
performance. These benefi ts include the ease of dispersing graphite into a liquid carrier, the stability
of graphite within the concentrated product, the application and fi lm formation of the product onto
the workpiece, and the optimized lubrication for the deformation process.
Forging processes normally require a temporary bond of the lubricant onto the workpiece and
tool. This is achieved by the use of the type of bonding agents mentioned previously in this chapter.
The use of dry powder or simple liquid–powder mixes will not perform adequately because of the
poor adhesion onto the substrate.
To illustrate the value of graphite for hot-temperature metalworking applications, consider the
example cited in Table 6.13. A comparison is made between two formulated graphite products and
a nongraphite product tested under the same procedures of warm forging. In this example, the
degree of spike height and coeffi cient of friction generated by the forging process are determined.
The lower spike height and higher coeffi cient of friction for the nongraphite lubricant are indica-
tions of reduced lubrication capability in comparison to the graphite-containing materials.
In certain instances, graphite is not desirable due to either the operating temperature or concern
about housekeeping and cleanliness. Hexagonal boron nitride is a capable alternative to graphite for
these conditions. It is considered the “white graphite” due to its lamellar structure. It has a reason-
ably low coeffi cient of friction that approaches and sometime exceeds that of graphite. It is able
to withstand operating temperatures up to 1200°C in oxidizing environments. This makes boron
nitride an effective material for high-alloy isothermal forging, where extremely high temperatures
FIGURE 6.12 Deformed billet and spike.
TABLE 6.13
Lubrication Comparison of Forging Lubricants (800°C Forging Temperature)
Lubricant Spike Height (mm) Coeffi cient of Friction
Graphite A 1.5 0.05
Graphite B 1.3 0.08
Nongraphite lubricant 0.7 0.15