Induction Heat Treating
Taking the Crank Out of Crankshaft Hardening
Gary Doyon, Doug Brown, Valery Rudnev, Glen Desmier, Jeffrey Elinski –
Inductoheat, Inc., Madison Heights, Mich.
Induction heat treatment is traditionally a popular choice for hardening and
tempering of quality crankshafts. This article will explain how continuous
process improvement makes a good process even better.
rankshafts are widely used in
r crankshaft requirements. Most of these rate of the clamshell coils are some of
internal combustion engines, attributes are augmented by the induction- the main drawbacks of these inductors.
pumps, compressors, etc. and
p hardening process. The short coil life resulted from inherent
belong to the group of the
b necessity of breaking the current pass by
most critical auto components typically
l Induction Technologies of the Past having the high-current contacts.
weighing between 30-85 pounds depending Inasmuch as the diameters of a crank’s When the inductor is closed, it must be
upon the engine. At the same time, the journals (mains and pins) are much small- clamped with sufﬁcient pressure to ensure
weight of some crankshafts (ships and er compared to the external dimensions that good electrical contact is made
power generators) exceeds 2,000 pounds. of the counterweights (webs), the con- between the movable parts. Realistically,
A crankshaft, typically cast or forged, ventional encircling-type coils could not there are no contact surfaces of a coil that
comprises a series of crankpins (pins) and freely pass from one heat-treated journal are perfectly smooth.
main journals (mains) interconnected to another. This feature dictates having a Regardless of the amount of contact-
by webs/counterweights (Fig. 1). Steel speciﬁc inductor design. surface polishing and cleaning, air pockets
forgings, nodular-iron castings, micro- and contaminated islands of the contact
alloy forgings and austempered ductile- Clamshell or Split Inductors area will force the coil current to ﬂow
iron castings are among the materials Specially designed clamshell or split through the localized solid-to-solid contact
most frequently used for crankshafts. inductors (Fig. 2) were developed and points. What results is the appearance
High strength and elasticity, good wear extensively used for induction hardening of a localized increase of current density
resistance, light weight, low vibration, of crankshafts in the 1950s. No rotation and an increase in electrical resistance
geometrical accuracy, short length and of the crankshaft was required. Short coil of the contact area compared with solid
low cost are some of the most important life, poor reliability and low production copper areas of the coil because electrical
resistance of the contact surfaces is usually
more than tenfold that of solid copper, and
heat generation is directly proportional to
change in electrical resistance.
The clamping area of the coil also con-
tributed to short inductor life due to wear
and contaminants, which led to excessive
overheating and even arcing and, ultimate-
ly, to premature coil failure. The quality of
the electrical contact degrades apprecia-
bly after multiple openings and closings
of the coil in a production environment.
Contaminants quickly build up on contact
surfaces, which further increase electrical
resistance of the contact area.
These factors caused an increase of the
Fig. 1. A crankshaft comprises a series of
crankpins (pins) and main journals (mains) electrical resistance of transitional areas
interconnected by webs/counterweights Fig. 2. Clamshell or split inductors between contact surfaces to continuously
IndustrialHeating.com - December 2008 41
Induction Heat Treating
change during coil operation, resulting in sensitivity and short tooling life radiated from the journal surface and
poor reliability and variation in the power • Appreciable equipment downtime (short a moist working environment acceler-
induced within the heated part. Heat treat- coil life and wear out of ﬂexible cables) ates coil copper deterioration due to
ers often were required to increase the con- • Safety concerns due to the presence of stress-corrosion and stress-fatigue fail-
tact pressure to compensate for a clamshell multiple moving heavy machine parts ure modes.
coil surface deterioration with time. This and cables • U-shaped coils are fabricated using
practice resulted in coil copper deforma- one of two techniques: copper banded
tion in clamping areas, unpredictable coil More detailed shortcomings related to or brazed. In both cases, precision and
performance and its premature failure. use of the U-shape inductors include: repeatability of fabrication of complex-
• Carbide guides (locators) are required. geometry coils (Fig. 3 C,D) is always a
U-Shaped Inductors Carbides “ride” on the pin/main sur- concern.
From the 1960s to the year 2000, the face at high temperature. The pro- • U-shaped coils produce a non-
majority of existing induction crankshaft- cess demanded having critically small symmetric heating pattern at any given
hardening machines utilized U-shaped “journal-to-coil” air gap (0.25-0.4 mm) time because heat is applied to less than
inductors, which rode on while a crankshaft making it difﬁcult to monitor the wear half portion of the crankshaft journal
rotated during heating. According to that of the carbide. The small gap requires (Fig. 3 B). The rest of the pin/main
process, each crankpin and main journal time-consuming setup training and undergoes a “soaking-cooling” mode.
was heated by bringing a U-shaped experience in the proper adjustment of The non-symmetrical heating nature of
inductor close to the pin or main bearing the locators, and it still allows for hu- U-shaped inductors can result in non-
surface while the crank was rotated about man error. Due to the small air gap and uniform hardness proﬁles and requires
its main axis. Since the pins’ axes were uncontrollable wear of carbide guides having relatively prolonged heat times
offset radially from the main axis, the pins (locators), the U-shaped coil often ac- (7-20 seconds), which in turn leads
orbited the main axis. The circular orbital cidentally touches a rotating crank to heating appreciable metal masses,
motion of such a heavy system should be surface. This results in coil water leaks resulting in excessive shape distortion.
maintained quite precisely with a special and premature coil failure and nega-
control tracking system providing a power tively affects the quality of heat-treated Better Technology – SHarP-C
modulation for each heated crank’s feature journals (i.e., pattern shifting, an ap- In order to utilize induction hardening
during its rotation (Fig. 3 A,B). pearance of “soft” spots on as-hardened while not having to rotate the crankshaft,
There are several obvious drawbacks as- surface). Besides, each locator is simply a patented non-rotational technology,
sociated with this technology, including: one more part that can go wrong. SHarP-C, was introduced in early 2000.
• High maintenance cost • The necessity of having critically small Since ﬁrst appearance, this technology
• Bulky and noisy system design “journal-to-coil” air gap in combina- was further “ﬁne-tuned” to become a
• Poor pattern repeatability, high system tion with appreciable amount of heat proven advanced process that eliminates
Fig. 3. Rotational hardening process utilizes a number of U-shaped Fig. 4. CrankPro machine implements Inductoheat’s patented
inductors. Note: Carbide guides (locators) are required. non-rotational crankshaft hardening and tempering technology
42 December 2008 - IndustrialHeating.com
the need to rotate the crankshaft during electromagnetically coupled to a top coil, that can go wrong.
heating and quenching cycles while at the a current ﬂowing in the bottom coil will • No wearing of the locators/guides
same time eliminating drawbacks of high- induce the eddy currents that start to ﬂow involved. The SHarP-C process utilizes
current contacts associated with clamp- in the top coil. Those induced currents inductors, which do not require contact
type coils. Figure 4 shows a CrankPro will be oriented in the opposite direction guides or complex and expensive
machine, which implements SHarP-C compared to a source current similar to a non-contact coil-positioning tracking
technology, providing high-production transformer effect. Any heated feature of systems of any kind.
hardening and tempering of crankshafts. the crankshaft “sees” the SHarP-C inductor • On average, required heat time has been
According to a patented non-rotational as a classical fully encircling coil[1-4] having reduced fourfold – being in the range of
hardening process, an inductor consists a very discrete heat pattern possible. 2-4 seconds – providing several princi-
of two coils (Fig. 5) – a top (passive) coil This patented non-rotation induction- pal beneﬁts, including energy reduction
and a bottom (active) coil. The bottom hardening and tempering technology and improving shape/size distortion.
coil, being active, is connected to a provides several principle beneﬁts such Reduction of total indicated runout
medium- or high-frequency power supply, as simple operation, superior reliability,
while the top coil (passive) represents a quality, maintainability and cost reduction.
short circuit (a loop). The bottom coil is Other beneﬁts:
a stationary coil, while the top coil can • Heat patterns are “locked in place”
be opened and closed. Each coil has two and very repeatable since neither the
semicircular areas where the crankshaft’s crankshaft nor the coils are moving.
features will be located. The same pattern is achieved over
Following robot loading of a crankshaft many cycles.
into the heating position, the top coil • Induction coils are much more robust,
pivots into a “closed” position, and the rigid and repeatable, being CNC-
power is applied from the power supply machined from solid copper without any
to the bottom (active) coil. The current brazed or banded parts. This eliminates
starts to ﬂow in the top coil. Being
p g inductor distortion and hardness-
patter drift. There are far fewer
components involved in the patented
coil design, meaning higher reliability
becau of the smaller number of parts
Fig. 6. Non-rotational hardening process
Fig. 5. Patented non-rotational
provides crankshaft pins and mains with
superior microstructural properties
Fig. 7. Computer modeling of temperature proﬁles during spray quenching of crankshaft journal (A) and prediction of austenite
transformation (B). Courtesy of Deformation Control Technology, Inc.
IndustrialHeating.com - December 2008 43
Induction Heat Treating
(TIR) distortion is traditionally one of continuous improvement, this undesir-
the most important factors in the heat able phenomenon has been eliminated.
treating of crankshafts. It directly affects Figure 8 shows the results of SEM anal-
the amount of metal required to grind. ysis of similar regions located in close
One of the most important factors that proximity to an oil hole using rotational
has a pronounced effect on distortion and non-rotational technology.
is the amount of heat generated within • The SHarP-C coil-to-journal air gap is
the crankshaft body. The greater the noticeably larger compared to air gaps
amount of heated metal, the greater the required by the rotational crankshaft-
metal’s expansion, which in turn causes hardening process. This creates a
Rotational greater distortion. Appreciable reduction favorable condition to reduce stress-
of the heat time associated with this pat- corrosion and stress-fatigue induction-
ented process leads to only a small mass coil copper failures and allows tooling
of metal being heated. The heat-affected life to be dramatically increased.
zone is minimized, resulting in corre- • Accurate CNC coil shaping and
spondent reduction of metal expansion utilization of a “quick-change” pallet
and, obviously, a minimization of size approach guarantee that coils are
and shape distortion (typically distor- automatically aligned with respect to
tion is less than 25 microns). the crankshaft after coil replacement.
• Crankshaft pins and mains have superi- No time-consuming process adjustments
or microstructural properties. These in- are required to “tweak” each coil after
clude the noticeable reduction of grain replacement. Unitized construction
growth, decarburization and oxidation allows quick, error free, production-
of the pin/main surface. The hard- ready factory installation and start-up,
ened zone is clearly deﬁned and “crisp” substantially reducing downtime.
(Fig. 6) without the “fuzzy transition • Since there is no rotation of a crankshaft
zone” that is present when longer heat required, it is not necessary to move
times are employed. The case depth heavy structures often weighing over
consists of a ﬁne-grain martensitic mi- 2,000 pounds through the orbital path
crostructure with a negligible amount during heating. There are no high-
of retained austenite and without any current electrical contacts or ﬂexible
traces of free ferrites. Essential surface cables to wear out. There is only “open-
compressive stresses obtained when ap- close” action. All of which improve the
plying SHarP-C technology are impera- safety of operating equipment.
tive for prevention of any surface-crack
development. Intensive theoretical and Conclusion
computer-modeling studies have been In addition to superior production
conducted in cooperation with leading improvements, SHarP-C technology offers
world experts such as Dr. Lynn Fergu- crankshaft designers more ﬂexibility to
son of Deformation Control Technol- optimize hardness and wear properties
ogy, Inc. to provide an ideal distribution where needed and not where formerly
of transitional and residual stresses in limited by other processes. IH
heat-treated cranks (Fig. 7).
References available online only.
• The necessity of having the surface of
crankshaft journals at high temperature For more information: The authors are as
follows: Gary Doyon, Inductotherm Group VP
for prolonged times, as required by ro-
& Chairman/CEO Inductoheat; Doug Brown,
tational technology, is often associated
President/COO; Dr. Valery Rudnev, FASM
with such an undesirable metallurgical – Group Dir. Science & Technology; Glen
phenomenon – grain boundary liqua- Desmier, Product Mgr. - Crankshaft Systems;
tion. This phenomenon substantially Jeffrey Elinski, Chief Metallurgist; Inductoheat,
Fig. 8. The results of SEM analysis of similar increases brittleness and sensitivity to Inc., 32251 N. Avis Dr., Madison Heights, MI
regions near oil hole using rotational and
non-rotational technology. Note: There is
intergranular cracking, in particular of 48071; tel: 800-624-6297; fax: 248-589-1062;
indication of a grain-boundary liquation the oil-hole area. Thanks to process fea- e-mail: email@example.com; web:
with rotational hardening process. tures of non-rotational technology and www.inductoheat.com
44 December 2008 - IndustrialHeating.com