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Taking the Crank Out of Crankshaft Hardening

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					                                                                                               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.




C
             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.[2]
             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.[1]     inductors (Fig. 2) were developed and         points.[2] 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

 A                                B




                                   C



                                   D


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-
                                                  pattern
                                                  patter drift. There are far fewer
                                                  patte
                                                  components involved in the patented
                                                  comp po
                                                  coil design, meaning higher reliability
                                                        d
                                                  because
                                                  becau of the smaller number of parts
                                                        u




                                                                                                   Fig. 6. Non-rotational hardening process
                                                        Fig. 5. Patented non-rotational
                                                                                                   provides crankshaft pins and mains with
                                                        hardening inductor
                                                                                                   superior microstructural properties




 A                                                                                                B
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-
               Non-rotational
                                                   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: gdesmier@inductoheat.com; web:
with rotational hardening process.                 tures of non-rotational technology and         www.inductoheat.com

44 December 2008 - IndustrialHeating.com

				
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