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7/22/2019 Induction PWHT
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. . .. . .. . .-Publishedyhe PRl Center for Materialsabrication Vol. 2, No . 2, 1985ReprintedMarch, 1990
Using Induction HeatTreatment to ObtainSpecial PropertiesCost Effectively
Heat treatment is often one of themost important stages of metalprocessing because it determinesthe final properties that enablecqmponents to perform under such
demanding service conditions ashiah oad, hioh temperature, and
speed and selective heating capabil-ity, to produce quality parts costeffectively. It will answer such ques-tions as: What are the advantagesof induction heat treatment? Whatheat treatments can I conduct withinduction? What are some typicalparts and materials that are induc-tion heat treated? What propertiescan I obtain with nduction heat
treatment?The differences between induction
aiverse environment. This Tech- and conventional furnace-based
Commentary illustrates how you heat treating processes and some
can use the special features of advantages of induction heating are
nduction heat treatment, such as its discussed in TechCommentary
& . HEAT TREATING- PROCESS OVERVIEW
Heat treatment is the controlled heating and cooling f a solidmetal or alloy to obtain desired properties. Depending on the materialand its intended use, heat treatment can improve such characteris-tics as formability, machinability, and service performance. Typical
heat treating operations include:
Annealing-used to soften metals to improve formability and
m Hardening-used to increase the strength and deformation resis-
m Tempering-used to increase the toughness of hardened metals
machinability.
tance of metals (such as steels).
and thereby improve their resistance to brittle, catastrophic failurein high-stress. high-integrity applications.
A glossary at the end of this TechCommentary precisely definesthese and other important heat treating terms.
There are two broad categories of heat treating processes-thoseinvolving ind irect heating and those using direct heating. Withindirect methods, heat is produced in a furnace by burning a fuel or
by converting electrical energy into heat by passing a current
through resistance heating elements. This energy is then transferredto the workpiece by radiation, convection, or conduction.By contrast,
direct heating methods, including induction, direct resistance, andflame heating, supply heat directly to the workpiece. An indepthdiscussion of how induction heating workss included in TechCom-
mentary Vol. 2, No. 1. .
Vol. 2, N o. 1. Advantages specific toinduction heat treating are:
Speed- n heat treatment, thehigher heating rates play a centralrole n designing rapid, high-temperature heat treatingprocesses. Induction heat treating ofsteel may take as little as10 percent or less of the timerequired for furnace treatment.
Short heating times also lead to lessscaling for materials such as steelsthat oxidize readily at high heattreating temperatures.
Selective Heating-Controlling theheating pattern by selecting the rightinduction equipment allows surface
and selectwe heat treatments thatyield an attractive blend of proper-ties (e.g., high strength and tough-ness). Such treatments are notfeasible with furnace processes,which are slow and heat the entireworkpiece.
Energy Savings- n addition toeliminating dwell periods, nductionheat treating techniques put energyonly where needed, mproving
energy efficiency.
increased ProductionRates-Rapid heating often ncreasesproduction and reduces labor.
Types Of InductionHeat Treatments
You can utilize the speed andselective heating characteristics ofinduction processes for:
, Hardening of Steels-Steels arehardened by heating to austenitizingtemperatures and then quenching.The speed of induction heating andminimal soak time mandate higheraustenitizing temperatures thanthose used with furnace processes(Table 1). Depth of austenitization is
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70
$ 60
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X
0 40
3 30
20
10
0 )
C
Q
-al
0
0 0.20 0.40 0.60 0.80 1 oo 1.20 1.40.60
Com po s i t i on (pc t . c a rbon)
Figure 1 Hardness Of As-Quenched Martensite A s A FunctionOf Its Carbon Con tent
those heat treated n furnaces.
In many applications, however,
surface hardened and tempered
steel parts surpass their furnace
treated counterparts. This isbecause shrface nduction heating
leads to:
A hard case and a soft core,
which provide a good blend of
strength and toughness not attain-
able with furnace through heating.
Further, because the hardness of
as-quenched steels depends only
on carbon content (Figure l ) , his
combination of properties can be
obtained n nexpensive carbon
steels. Induction heat-treated
carbon steel parts can be used in
many applications that require
alloy steels with good toughness,
which are through heat treated in
furnaces.
m Compressive residual stresses at
the surface, which are important
in combination with the surface
hardness. These residual stresses
arise primarily from the density
difference between the hard
martensite layer and the softer
interiorayer of pearlite r ainite
in surface hardened parts.
The combination of a hard surface,compressive residual stresses, and
a soft core results in excellent wear
and fatigue resistance. Improvement
in bending fatigue when compared
to furnace treatments is shown in
Figure 2 for axle shafts. The attri-
butes of induction treatment are
particularly attractive in bending
fatigue in which high levels of
tensile stress are generated at the
surface, and no stresses are
imposed at the center. Here, the. .
property distribution in the surface
hardened part s well matched with
the demands placed on it in service.
Another example is the wear resis-
tance afforded gear teeth by,'-selective surface hardening. Proper-
ties of induction surface hardened
parts are discussed further in Tech-Commentary (Vol. 2, No. 3) .
Heat Treating Processes
That Compete With
Induction
heat treating requirements, and
Considerations of speed, selective
process economics are often suffi-
cient to establish whether you
should use nduction or conventional
furnace-based methods for through
heat treatment. On the other hand, if
you've determined that surface
hardening s necessary, there are a
number of other processes toconsider. These include conven-
tional carburizing and nitriding,ion-nitrid ing, laser hardening and
electron beam hardening.
Carburizing and nitridi ng are
welLestablished technologies n
which surfaces are alloyed with
carbon or nitrogen by placing parts
in a gaseous or liquid environment.
The alloying results n surface
hardening. Cost data from commer-
cial heat treaters ndicates that the
overall cost for nduction hardening
is much ess than for carburizing,
salt bath nitriding, or gas nitriding.
(The cost ratio for the 4processes in the order listed is
0.1 1 :2.5:1.75:8.) Thus, if your part
geometry and production volume
allow the use of induction, it s the
preferred surface hardening method
from a cost standpoint.
Ion-nitriding, laser hardening,and electron-beam (EB) hardeningare emerging technologies that are
70 IMedium-carbon s teel
axle shaf ts
60I
10 I t I0.01 0.1 1 10
M i l l i o n s of Cy c les to Failure .
Figure 2 Bending Fatigue Response Of Medium-Carbon SteelTractor Axles Which Were Either Furnace Hardened OrInduct ion Hardened
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used to obtain shallow case-
hardened depths (0.02 n. or less).
lon-nitriding is similar to other
nitriding processes except that a
glow-discharge method s employed.
Laser and EB techniques both use
extremely high-energy nput rates
that austenitize a very thin surface
layer in a fraction of a second. The
bulk of the substrate remains cooland provides an adquate heat sink
for “self-quenching”. Advantages
include:
Minimal workpiece distortion
m Ability to selectively harden
portions of a surface and tocontrol the process in general
m Ability to harden areas inacces-
sible to conventional nduction
techniques
Repeatability
High speed.
According to a recent American
Society for Metals (ASM) survey, on-
nitriding offers the greatest
challenge to induction heating for
purposes of surface hardening.
Laser and EB processes are also
expected to satisfy some of the
applications now handled by induc-
tion, particularly those for which
induction coils are difficult to design.
Nevertheless, in situations in which
case depths of 0.02 to 0.04 in. are
required, induction systems that
produce very high-power inputs perunit of surface area (i.e., “high
intensity” nduction setups) can
compete effectively because of
lower equipment cost, higher
productivity, ess maintenance, and
lower floor space requirements. For
example, n typical ndustrial
applications, a 2 to 3 kilowatt aser,
costing approximately $250,000 is
required. This is about 3 times the
cost of comparable induction equip-
ment.lectron-beamardeninglsohas some important limitations, such
as the need for a vacuumatmosphere.
Design Of Induction Heat
Treatment Processes
To make induction heat treatment
work for you, carefully select equip-
ment and understand the metallurgi-
cal variables that control heat treat-
ing response. The most mportant
equipment parameters are coil
design, generator frequency, and
applied power density.
0 .05 .1 .2 .5 1 2
Diameter , (in.)
Figure 3Relation Between Diameter o fRound Steel Bars And Minimum
Generator Frequency For Efficient
Austenitizingsingnduction ~
Heating
Coil Design -Solenoidal coilsare easily designed for round parts.
For more complex parts, design
procedures are described n nduc-
tion heating textbooks or can be
provided by equipment
manufacturers.
determined by the material, part
size, and the need for through or
surface heating. Low frequencies,
which provide arge “penetrat ion”
depths of the induced eddy currents,
are used for through heat treating.
High frequencies are used for
surface heat treating. The minimum
frequencies for efficient through
austenitizing of steel bars are shown
in Figure 3. Frequencies for surface
hardening of steels are chosen toensure penetration depths of about
twice the required hardened depth.
Other variables, such as power
density, are mportant n selecting
frequency in these instances as well
and are discussed in TechCommen-
tary, Vol. 2, No. 3.
Generator Frequency -This is
Power Densit y (or power per unit
of surface area)-The same power
input can ead to a ow heating rate
fo r a arge part but a high heating
rate for a small part. As with
frequency, power density level is
selected based on the need for
through or surface heat treating.
The schematic diagram in Figure 4
illustrates the typical temperatureversus time behavior for an nduc-
tion heated part and s useful n
explaining power density effects.
Because the magnitude of the eddy
currknts are greatest at the surface
and east at the center, the surface
heats more rapidly. After an nitial
transient, the difference n tempera-
ture remains fixed. This difference
represents an equilibrium between
heat nput via nduction and heat
transfer between the surface and
center by conduction. As such, it is
a function of the applied powerdensity and workpiece electrical and
thermal properties. For through
heating and heat treating, ower
power densities of 0.1 to 0.5 kWlin?
are recommended to ensure a more
or less uniform heating pattern. High
power densities (approximately 10 to20 kWlin.2) are needed for surface
heating and heat treatments of
steel.
Using data on coil design and
power supply requirements, you can
figure out where the heating power
is going to be located in your heattreating process. To ensure
success, however, it s important to
understand the metallurgical proper-
ties of the workpiece and how these
interact with your Heating cycle. Of
utmost importance is the design of
the necessary time-temperature
cycles, which are typically quite
different from those for furnace heat
treatments.
The data n Table 1 reveal that
higher temperatures are employed
for induction austenitizing of steels.
You need these to compensate forshort or zero soak times. Similarly,
short-time tempering treatments
involving short or zero soak time
can be designed around induction
heating processes by using the so-called tempering parameter,
T(14.44+ og,&,
where T is the tempering tempera-
ture in degrees Rankine and t is the
tempering time in seconds. An
induction tempering treatment
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La,
n
5I-
!?!Lm
8lossary Of Heat Treating Terms*
Figure 4
Schematic Illus tration Of TheSurface And Center TemperatureHistories Of A Bar Heated ByInduction (Note that, followingan ini tial transient, the surf ace-to-center temperature difference isconstant during the heating cycle.)
whose tempering parameter isequivalent to that for a longer-time,
lower-temperature treatment willgive a part with equivalent proper-ties. The application of this concept
is discussed further in TechCom-mentary, Vol. 2, No. 4.
consideration in process design isthe starting condition of the work-
piece. This is particularly important
for steels that are to be hardened. In
these cases, austenitizing behavior
will be affected greatly by the start-
ing microstructure. An example ofthis microstructure effect is seen in
Figure 5. As shown here, whenusing the same induction heating
parameters, the case hardeneddepth for a 1070 steel bar increasesas the starting structure becomesfiner: quench and tempered = finemartensite structure, normalized ='ine pearlite with thin lamellarcarbides, and annealed = structure
with coarse, difficult-todissolve
spheroidal carbides.
A final factor that warrants
Alloy Steel -Steel containing signifi-cant quantities of alloying elements(other than carbon and the commonlyaccepted amounts of manganese,silicon, sulfur, and phosphorous)added to effect changes in themechanical or physical properties.
Annealing -Heating to and holdingat a suitable temperature and thencooling at a suitable rate for suchpurposes as reducing hardness,improving machinability, facilitatingcold working, producing a desiredmicrostructure, or obtaining desiredmechanical, physical, or other proper-ties. Specific types of annealingorocesses include:
Recrystallization annealing "Anneal-ing cold or hot worked metal to
produce a new grain structurewithout phase change.
Spheroidization anneating- eating
and cooling to produce a spheroidalor globular form of carbide in steel.
Stress relief annealing-Heating to asuitable temperature, holding longenough to reduce residual stresses,and then cooling slowly enough to
minimize the development of new
residual stresses.
Austenitizing- orming of the face-centered-cubic austenite phase insteel by heating above the trans-formation temperature range.Process forms the basis of hardeningof steels.
Bainite- decomposition product ofaustenite consisting of an aggregateof ferrite and carbide. In general, itforms at temperatures lower thanthose where very fine pearlite formsand higher than that where marten-site begins to form on cooling. It sappearance is feathery if formed inthe upper part of the temperaturerange; acicular, resembling temperedmartensite, f formed in the- lowerpart.
Carbon Steel- ron alloy containingcarbon up to about 2 percent andonly residual quantities of other
elements except those added fordeoxidation. with silicon Usuallylimited to 0.60 percent and manga-nese to 1.65 percent. Also termedplain-carbon steel.
Cementite - A compound of iron andcarbon found frequently in steelsknown chemically as iron carbide andhaving the approximate chemicalformula Fe,C.
Ferrite-A solid solution of 1 or moreelements n body-centered cubic ron,the solute is generally assumed to becarbon unless designated otherwlse.
Hardenabi lity- In steels, theproperty that determines the depthand distribution of hardness nduced
by austenitizing and quenching.Hardenability is a function of alloycomposition and quenching medium.
Hardening- ncreasing thehardness by suitable treatment,usually involving heating and coolingFor steels, this typically consists ofaustenitizing followed by cooling to
form pearlite, bainite, or martenslte,or a combination of theseconstituents.
Martensite A metastable phase of
steel formed by a transformation ofaustenite below the M, temperature
and composed of a body-centeredietragonal attice. It s microstructureis characterized by an acicular, orneedle-like, pattern.
Microstructure-The structure ofpolished and etched metals asrevealed by a microscope at amagnification greater than 10
diameters.
Normalizing- eating a fe:rousalloy to a suitable temperature abovethe transformation range and thencooling in air to a temperaturesubstantially below the transformatior
range.Pearlite-A lamellar aggregate offerrite and cementite, often occurringin steel and cast iron.
Quench Hardening- ardening aferrous alloy, such as a steel, byaustenitizing and then cooling rapidlyenough so that some or all of theaustenite transforms to martensite.
Tempering -Reheating a normalizecor quench-hardened ferrous alloysuch as a steel to a temperaturebelow the transformation range andthen cooling at any rate desired.
Transformation Temperature-Thetemperature at which a change inphase occurs. The term is sometime:used to denote the limiting tempera-ture of a transformation range. ,
-Source: Melals Handbook. Vol. 1. EighthEdition.American SocietyFo r Melals.196
Such variations in induction heat
treatment results can be overcome
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70
60
50
v)
(0
mC
$ 40
X0
$ 30
-
x0
z
20
1 0 L I I
S 0.02 0.04 0.06 0.08 0.1 0
Distance Below Surface (in.)
Figure 5 Effect Of Starting Microstructur e In 1070 Steel Bars OnSurface Hardening Response Using A 450 kHzInduct ion Generator Operated At A Power Density Of2.5 Kilo watts Per Square Centimeter (15.9 kilowatts persquare n.)
by determining (on a trial-and-error
basis)modifiedeakemperaturesnductioneatreatment isppli-
or heating ates oryourparticularcable to hardening, empering,
alloy nd xpected ariationsn ormalizing, nd nnealingwide
startingondition.angefarts,articularlynerrous
In Summary
alloys-medium- and high-carbon
steels, alloy and stainless steels,and tool steels. I t is also inding
increasing application n the nonfer-
rous metals ndustry. Advantages
include high heating rates, selective
heating capability, mproved produc-
tion rates, and energy savings.
Induction heat treatment can
produce surface hardened partswith soft cores that exhibit excellent
wear and fatigue resistance. How-
ever, satisfactory heat treatment
requires careful selection of equip-
ment and good understanding of the
metallurgical variables nvolved. Fo
surface hardening applications,
there are a number of competing
Drocesses to consider.0
This TechCornrnentary provides
an overview. It is intended to famil-
iarize you with the importantapplications of induction heat treat
ment. If you are nterested n a more
detailed background, please refer tsubsequent issues of TechCorn-rnentary on “Surface and Selective
Heat Treatment” (Vol. 2, No. 3),“Induction Tempering” (Vol. 2, No.4), and he sources isted below.
The Center for Materials Fabrication (CMF) is operated by Battelle’s Columbus Division. Bast funding is provided by the ElectricPower Research Institute, a nonprofit institute that conducts research and development on behalf of the United States electricutili!y industry
The Center’s mission is to assistindustry in implementing cost- andenergy-efficient electric-basedtechnologies in the metals fabricationand related fields. TechCommentary isone communication vehicle that theCenter uses to transfer technology toindustry. The Center also conductsresearch in metal heating, metalremoval and finishing, and fabrication.
This issue of TechCommentary wasmade possible through the cooperationof Battelle staff members Lee Semiatin,author; Jack Mortland, editor; Laura
Cahill, publications coordinator.Sources used in this issue ofTechCommentary were:
Induction Hardening and Tempering,American Society for Metals, 1964, T.H.Spencer, et al.
Induction Heat Treatment of Steel,American Society for Metals, 1985, S.L.Semiatin and D.E. Stutz.
Metals Technology, Vol. 9, No . 12, pp.493-498, The Metals Society, 1982,“Transverse Flux Induction Heating ofAluminum-Alloy Strip,” R. Waggott, et al.
Sources for tables and figures were:
Transactions of ASM, Vol. 26, pp. 1-36,American Society for Metals, 1938,“Quantitative Hardenability,” J.L. Burns,T.L. Moore, and R.S. Archer.
Transactions o f ASST, Vol. 12, p. 871,1927, W.P. Sykes and Z. Jeffries.
High-Frequency Induction Heating,McGraw-Hill Book Company, 1950, F.W.
Curtis.Basics of Induction Heating, John F.Rider, 1960, C.A. Tudbury.
Induction Hardening and Tempering,American Society for Metals, 1964, T.H.Spencer, et al.
Appli cable SIC Codes
33- 12, 15, 16, 17, 21, 34, 41, 51, 53,54, 55, 56, 98, 99
52, 62, 63, 71.-,79, 83, 84, 89, 9395, 99
45, 46, 47, 62, 66, 67, 68, 92, 99
34- 11,12, 21, 23, 25, 29, 41, 43, 49,
35- 23, 24, 31, 32, 33, 36, 41, 42, 44,
36-2137- 11, 14, 21, 24, 28, 43, 51, 61, 64
69, 95, 99
For further information on Centerprograms, write or call
CENTER FOR MATERIALS FABRICATIO
An EPRl RBD Apphcallons Center
505 King Avenue - Columbus, Ohio 43201-2693
(614) 424-7737
Copyright 1985
Battelle Memorial InstituteColumbus,Ohio Reprinted39