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Introduction
The Kolene QPQ(SM) Process is an adjunct to Kolene Corporation's salt
bath ferritic nitrocarburizing (SBN(TM)) treatment called
Nu-Tride(R), whereby a mechanical polish and post salt bath oxidative
treatment are provided to the nitrocarburized surface. This unique
combination of salt bath treatments with intermediate surface
finishing, produces a significant increase in corrosion protection
while maintaining all the engineering properties initially developed
through the SBN process. Additionally, the Kolene QPQ Process
provides a cosmetically appealing black surface on the treated
component.
Process
The term Kolene "QPQ" is based on a sequence of process events that
occur directly following the nitrocarburizing cycle. Referring to the
time-temperature profile in Fig. 1, the development of the Kolene QPQ
Process may be followed. It begins with the treating cycle of the
nitrocarburizing segment, i.e. pre-heat, Nu-Tride salt, Kolene KQ-5OO
salt bath quench (SBQ), which produces a layer of epsilon iron
nitride, Fig. 2.
Figure 1: Kolene QPQ Process Cycle
Figure 2: Low Carbon Steel, processed in Nu-tride plus Kolene
KQ-500 (500 X)
(view the full size 95K image)
The next step is a mechanical polish of the nitride layer,
thus restoring the original surface finish. This may be accomplished
by vibratory polishing, lapping, centerless polishing, or by other
similar means. Finally, to optimize the corrosion resistance, the
component is then reimmersed in the Kolene KQ5OO salt quench
bath for 20-30 minutes, rinsed and oil dipped. The resulting sequence
of quench-polish-quench operations is thus termed the Kolene
QPQ Process and may be expressed generically as
SBN/SBQ/polish/SBQ.
The change in surface chemistry of the workpiece resulting from the
Kolene QPQ treatment may be determined by tracking the nitrogen,
carbon, and oxygen pick-up through use of Auger analysis. Results,
shown in Fig. 3, denote the levels of nitrogen and carbon as a
consequence of the nitrocarburizing bath (SBN), and the extent of the
oxygen pick-up occurring from the re-immersion in the final oxidative
bath (SBQ). The oxygen is present in the form of iron oxide, Fe3O4, 3
to 4 microns thick. It is this layer of iron oxide that produces an
improved corrosion resistance for non-stainless steels and provides a
lustrous dark finish.
Figure 3: Auger analysis after quench-polish-quench treatment
Properties and Applications
Each engineering property developed through nitrocarburizing, i.e.
resistance to wear and corrosion, lubricity, and fatigue strength,
may alone satisfy a particular application. However, experience
indicates that it is more probable that two or more properties will
be utilized in combination to insure successful performance of a
part, and most often, this includes corrosion resistance. When
surface degradation due to corrosive environments is of primary
concern and the corrosion protection provided by nitrocarburizing
alone may not be adequate, the Kolene QPQ adjunct may be specified to
increase the protection.
The level of corrosion protection provided by SBN and the
SBQ/polish/SBQ adjunct is displayed in Fig. 4, as compared with other
surface treatments including deposition coatings. The degree of
corrosion is determined by the weight loss in g/m2 in 24 hours after
immersion in a NaCI/H202 solution. Results demonstrate that salt bath
nitrocarburizing (SBN) followed by the SBQ/polish/SBQ sequence
provides the maximum corrosion resistance, 0.3 g/m2, as compared to
chromium plating, 7.2 g/m2, nickel plating, 2.9 g/m2, and SBN/SBQ
(oxidizing salt quench), 7.1g/m2.
Figure 4: Comparison of corrosion resistances, based on field
immersion tests
Another comparative evaluation of corrosion resistance was based on the ASTM B-117 salt spray procedure. Three surface treatments were selected, hard chrome, electroless nickel, and SBN with the SBQ/polish/SBQ adjunct, and applied to spool shafts used in automotive steering columns. The results, shown in Fig. 5, demonstrate the superior protection provided by the SBN/SBQ/polish/SBQ treatment, even after 336 hours exposure to the salt spray testing environment.
Figure 5: Corrosion resistance evaluation, based on Salt Spray
Test, ASTM B-117
Exposure to corrosion and wear represent the most common
combination of operational conditions associated with applications
requiring engineered surfaces. Hydraulic and pneumatic systems are
examples of just such applications, particularly the piston rods. The
wear resistance (and lubricity) of the rod contact surface is
important so as to maintain the designed characteristics of the of
the sealing system, whether it involves a gas or liquid filled
cylinder. Furthermore, in many applications, the piston rod is
exposed to various degrees of environmental corrosion especially in
the extended position. The products of corrosion may not only
adversely change the surface contour but also may abrade the
seals.
Applications that have realized increased performance through
SBNISBQ/polish/SBQ include piston rods for gas springs, cylinders and
rods for hydraulic systems, and shock absorber piston rods.
Components dealing with fluid flow e.g. pumps, shafts, valves, are
also treated to impart resistance to corrosion and wear. In many such
applications, SBN/SBQ/polish/SBQ has proven to be a successful
substitute for deposition coatings such as chrome plating.
One of the intrinsic properties of the epsilon iron nitride formed
during salt bath nitriding is its relatively low coefficient of
friction. By including the SBQIpolish/SBQ adjunct after SBN, surface
lubricity is still greater than a chrome plated surface or a case
hardened surface as determined both with and without lubrication,
shown in Fig. 6.
Figure 6: Frictional properties of various surface treatments
One application that makes use of the lubricity and corrosion resistance provided by the quench - polish - quench adjunct of SBN is the ball stud, Fig. 7, a part of the transmission linkage for the automotive windshield wiper system. Originally machined out of stainless steel, it was determined through testing that a cold formed, low carbon steel component, treated by SBN and the polish and re-oxidation adjunct produced an acceptable part with a significant saving in manufacturing costs.
Figure 7: Automotive components
(view the full size 63K image)
Cast iron parts are also Kolene QPQ treated to develop particular engineering properties. The ductile iron housing shown in Fig. 8 is part of a gear pump used in dehydrating natural gas as it comes from gas wells. The pump system handles glycol through a wide range of temperatures and viscosities. Since glycol has limited lubricity and can be highly corrosive especially at elevated temperatures, the treated surface provides the necessary frictional and corrosion resistant properties to enable successful pump performance.
Figure 8: Ductile iron pump housings
(view the full size 41K image)
Fatigue strength is another engineering property to consider,
especially regarding the influence of a corrosive environment. It is
well documented (Kolene Technology Update "Salt Bath Ferritic
Nitrocarburizing") that an increase in fatigue strength of SBN
components under bending or torsional loading is due to the presence
of nitrogen in the zone subjacent to the compound layer called the
diffusion zone. The level of strengthening is directly related to the
amount and in what form nitrogen occurs.
The importance of the quench-polish-quench adjunct to SBN is shown in
Fig. 9, a comparison of rotating-bending fatigue tests performed
under different atmospheric conditions. In a normal environment, the
initial fatigue strength is 180 N/mm2 increasing to 420 N/mm2 after
the SBN/SBQ/polish/SBQ process. As noted, this increase of over 100%
is attributable primarily to the strengthening effect of nitrogen in
the diffusion zone.
Figure 9: Influence of atmospheric conditions on rotating-bending
fatigue strength
The significant influence of a corrosive environment is first
evident when comparing fatigue strength levels of the non-treated
test specimens in the normal and the corrosive atmospheres, 180 N/mm2
and 7 N/mm2 respectively, a 96% decrease in strength. However,
a comparison of the relative fatigue strengths of the test specimens
after SBN/SBQ/polish/SBQ treatment shows a distinct reduction as to
the adverse effect of the corrosive environment, 420N/mm2 in a normal
atmosphere vs. 350 N/mm2 when tested in a corrosive atmosphere. This
is equivalent to about a 17% loss in strength resulting from
corrosion. It follows that performance of components subjected to
cyclic bending loads, e.g. support brackets, rotating shafts, etc.
may be improved significantly by the Kolene QPQ treatment,
particularly when operating in a corrosive environment.
Conclusions
The Kolene QPQ Process significantly enhances the corrosion
protection of cast iron and steel components and thus provides a more
effective and environmentally acceptable alternative to deposition
coatings, e.g. chrome and nickel plating. Furthermore, this process
used in conjunction with carbon steels has proven itself as a viable
replacement for stainless steels with an appreciable reduction in
material and manufacturing costs.
This technology update describes the theory and benefits of the
Kolene(R) QPQSM process. Based on Kolene's ferritic salt bath
nitriding (SBNTM) technology, Kolene QPQ offers a cost-effective
alternative to conventional plated metallic coatings for wear and
corrosion resistance.
For more information on Kolene QPQ, or to request other technical
information about Kolene SBN processes, please contact us.
Kolene(R) is a registered trademark in the U.S.A., Canada, and
elsewhere
SBN(TM) is a trademark of Kolene Corporation
Nu-Tride(R) is a registered trademark in the U.S.A.
Nu-Tride(TM) is a trademark in Canada
Kolene KQ-500(R) is a registered trademark in the United States and
Canada
Kolene QPQ(SM) is a service mark in the United States and QPQ(R)is a
registered trademark in Canada
