Practical Heat Treating

301 Pages·2006·3.973 MB·English

by  Boyer, Howard E., Dossett, Jon L

Practical Heat Treating: Second Edition Copyright © 2006 ASM International ®
Jon L. Dossett, Howard E. Boyer All rights reserved.
DOI: 10.1361/pht2006_FM www.asminternational.org
Practical
Heat Treating
Second Edition
Jon L. Dossett
Howard E. Boyer
ASMInternational(cid:1)
MaterialsPark,Ohio44073-0002
www.asminternational.org
Copyright(cid:2)2006
by
ASMInternational(cid:3)
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LibraryofCongressCataloging-in-PublicationData
Boyer,HowardE.
Practicalheattreating/HowardE.Boyer,JonL.Dossett.—2nded.
p.cm.
ISBN:0-87170-829-9
1.Metals—Heattreatment.I.Dossett,JonL.II.Title
TN672.B68 2006
671.3(cid:4)6—dc22 2005055552
ISBN:0-87170-829-9
SAN:204-7586
ASMInternational(cid:3)
MaterialsPark,OH44073-0002
www.asminternational.org
PrintedintheUnitedStatesofAmerica
CoverphotographcourtesyofMTHeatTreatingInc.,Mentor,Ohio
Contents
Preface ...........................................................................................v
CHAPTER 1 What is Heat Treating?
Importance and Classifications ................................1
CHAPTER 2 Fundamentals of the Heat Treating of Steel .............9
CHAPTER 3 Hardness and Hardenability ..................................27
CHAPTER 4 Furnaces and Related Equipment for
Heat Treating .........................................................55
CHAPTER 5 Instrumentation and Control of Heat
Treating Processes .................................................85
CHAPTER 6 Heat Treating of Carbon Steels ..............................97
CHAPTER 7 Heat Treating of Alloy Steels ...............................125
CHAPTER 8 Case Hardening of Steel ......................................141
CHAPTER 9 Flame and Induction Hardening ..........................159
CHAPTER 10 Heat Treating of Stainless Steels ........................175
CHAPTER 11 Heat Treating of Tool Steels ...............................191
CHAPTER 12 Heat Treating of Cast Irons ................................207
CHAPTER 13 Heat Treating of Nonferrous Alloys ...................231
CHAPTER 14 Assuring the Quality of Heat Treated Product ...243
iii
APPENDIX A Glossary of Heat Treating Terms ........................257
APPENDIX B Decarburization of Steels ..................................275
APPENDIX C Boost/Diffuse Cycles for Carburizing ................279
APPENDIX D Use of Test Coupons for Process Verification ....283
Index ..........................................................................................287
iv
Preface
Nearly 44 years ago, I started into practical heat treating by operating
furnaces and induction equipment at Ross Gear in Lafayette, Indiana,
while studying for my metallurgical degree at Purdue University. After
graduation,Ispentthenext17yearsinvolvedinheattreatingandfoundry
operationsofmajormanufacturingcompanies.In1976,Ibecameinvolved
incommercialheattreatingasDivisionManageroftheMelrosePark(IL)
plant for Lindberg Heat Treating Company and continued in commercial
heat treating until I sold Midland Metal Treating, Inc. (Franklin, WI) in
2004.
During my career, with the encouragement of my lovely wife, Gwen-
dolyn, I have remained involved with ASM International and the Heat
Treating Society. That work involved preparation and dissemination of
practical heat treating knowledge by working on technical papers and
Handbooks, teaching, serving on technical committees and boards, re-
viewing papers, organizing heat treating conferences, and helping to or-
ganizeandthenlaterservingasEditorofASM’sJournalofHeatTreating.
It is an honor to update Practical Heat Treating. The first edition was
compiled by Howard Boyer, who is now deceased. I had the pleasure of
knowing Howard and working with him on several ASM Handbook sec-
tions. Practical Heat Treating covers the fundamentals and practical as-
pects of the broad field of heat treating. Since many of the fundamentals
havenot changedinthepast30years,inthisupdatededitionweconcen-
tratedonaddinginformationaboutthenewprocessesandprocesscontrol
techniques that have been developed or refined during the intervening
period.
JonL.Dossett,P.E.
v
Practical Heat Treating: Second Edition Copyright © 2006 ASM International ®
Jon L. Dossett, Howard E. Boyer, p1-8 All rights reserved.
DOI: 10.1361/pht2006p001 www.asminternational.org
1
CHAPTER
What Is Heat Treating?
Importance and
Classifications
THE GENERALLY ACCEPTED TERM for heat treating metals and
metal alloys is “heating and cooling a solid metal or alloy in such a way
soastoobtainspecificconditionsand/orproperties.”Heatingforthesole
purpose of hot working (as in forging operations) is excluded from this
definition. Heat treatments sometimes used for nonmetallic products are
also excluded from coverage by this definition.
Importance of Heat Treatment
It is difficult to imagine how our lives would be changed if the prop-
erties of metals could not be altered in a variety of ways through the use
ofheattreatment.Withthosebenefitsderivedfromheattreating,manyof
the products manufactured bythetransportation,aerospace,construction,
agricultural, mining, and consumer goods sector of our economy would
not be available for use.
The village blacksmith performed crude heat treatments in years past,
which improved the standard of living for society at that time. However,
the understanding of the science and underlying principles of various
metal and metal alloy systems and their related heat treatments has been
significantly developed only during the past 75 to 100 years.
Almost all metals and alloys respond to some type of heat treatment,
inthebroadestsenseofthedefinition.Theresponseofvariousmetalsand
alloys, however, is by no means equal. Almost any pure metal or alloy
can be softened (annealed) by means of a suitable heating and cooling
cycle;however,thenumberofalloysthatcanbestrengthenedorhardened
by heat treatment is far more restricted.
2/PracticalHeatTreating:SecondEdition
Practicallyallsteelsrespondtooneormoretypeofheattreatment.This
is the main reason that steels have been so extensivelyusedinthemanu-
facturing sector of oureconomy during the twentiethcentury.Theunder-
lying principles of the heat treatment of steel are discussed in Chapter 2,
“Fundamentals of the Heat Treating of Steel.”
Many nonferrous alloys—namely aluminum, copper, nickel, magne-
sium, and titanium alloys—can be strengthened to various degrees by
specially designed heat treatments, but nottothesamedegreeandnotby
the same techniques as steel. The heat treatment of nonferrous metals is
covered in Chapter 13, “Heat Treating of Nonferrous Alloys.”
Classification of Heat Treating Processes
In some instances, heat treatment procedures are clear cut in terms of
techniqueandapplication,whereasinotherinstances,descriptionsorsim-
ple explanations are insufficient because the same technique frequently
may be used to obtain different objectives. For example, stress relieving
and tempering are often accomplished with the same equipment and by
useofidenticaltimeandtemperaturecycles.Theobjectives,however,are
different for the two processes.
The principal heat treating processes are described subsequently. Ad-
ditional information on these processes as well as the terminology asso-
ciatedwiththemcanbefoundinAppendixA,“GlossaryofHeatTreating
Terms.”
Normalizing
The term normalize does not characterize the nature of this process.
Moreaccurately,itisahomogenizingorgrainrefiningtreatment,withthe
aim being uniformity in composition throughout a part. In the thermal
sense, normalizing is an austenitizing heating cycle followed by cooling
in still or slightly agitated air. Typically, work is heated to a temperature
of approximately 55 (cid:1)C (100 (cid:1)F) above the upper critical lineof theiron-
iron carbide phase diagram, and the heating portion of the process must
produce a homogeneous austenitic phase. The actual temperature used
depends on the composition of the steel, but the usual temperature is
approximately 870 (cid:1)C (1600 (cid:1)F). Because of characteristics inherent in
cast steel, normalizing is commonly applied to ingots prior to working,
andtosteelcastingsandforgingspriortohardening.Air-hardeningsteels
arenotclassifiedasnormalizedsteelsbecausetheydonothavethenormal
pearlitic microstructure typical of normalized steels.
Annealing
Annealing is a generic term denoting a treatment consisting of heating
toandholdingatasuitabletemperature,followedbycoolingatasuitable
Chapter1:WhatIsHeatTreating?ImportanceandClassifications/3
rate; the process is used primarily to soften metals and to simultaneously
producedesiredchangesinotherpropertiesorinmicrostructures.Reasons
for annealing include improvement of machinability, facilitation of cold
work,improvementinmechanicalorelectricalproperties,andtoincrease
dimensional stability. In ferrous alloys, annealing usually is done above
theuppercriticaltemperature,buttime-temperaturecyclesvarywidelyin
maximum temperature and in cooling rate, depending on composition of
thesteel,conditionofthesteel,andresultsdesired.Whenthetermisused
without qualification,fullannealingisimplied.Whentheonlypurposeis
relief of stresses, the process is called stress relieving or stress-relief an-
nealing.
In full annealing, steel is heated 90 to 180 (cid:1)C (160 to 325 (cid:1)F) above
theA forhypoeutectoidsteelsandabovetheA forhypereutectoidsteels,
3 1
and slow cooled, making the material easier to cut and to bend. In full
annealing, the rate of cooling must be very slow to allow the formation
of coarse pearlite. In process annealing, slow cooling is not essential be-
cause any cooling rate from temperatures below A results in the same
1
microstructure and hardness.
In nonferrous alloys, annealing cycles are designed to: remove part or
all of the effects of cold working (recrystallization may or may not be
involved); cause substantially complete coalescence of precipitates from
solidsolutioninrelativelycoarseform;orboth,dependingoncomposition
andmaterialcondition.Specificprocessnamesincommercialusearefinal
annealing, full annealing, intermediate annealing, partial annealing, re-
crystallization annealing, and stress-relief annealing. Some of these are
“inshop” terms that do not have precise definitions.
Stress Relieving
Residual stresses can be created in a number of ways, ranging from
ingot processing in the mill to the manufacture of the finished product.
Sources include rolling, casting, forging, bending, quenching, grinding,
and welding. In the stress-relief process, steel is heated to approximately
595 (cid:1)C (1105 (cid:1)F), ensuring that the entire part is heated uniformly, then
cooled slowly back to room temperature. The procedure is called stress-
relief annealing, or simply stress relieving. Care must be taken to ensure
uniform cooling, especially when a part has varying section sizes. If the
cooling rate is not constant and uniform, new residual stresses, equal to
or greater than existing originally, can be the result. Residual stresses in
ferritic steel cause significant reduction in resistance to brittlefracture.If
a steel, such as austenitic stainless steel, is not prone to brittle fracture,
residual stresses can cause stress-corrosion cracking (SCC). Warping is
the common problem.
4/PracticalHeatTreating:SecondEdition
Surface Hardening
These treatments, numbering more than a dozen, impart a hard, wear-
resistantsurfacetoparts,whilemaintainingasofter,toughinterior,which
givesresistancetobreakageduetoimpacts.Hardnessisobtainedthrough
quenching, which provides rapid cooling above a steel’s transformation
temperature. Parts in this condition can crack if dropped. Ductility isob-
tained via tempering. The hardened surface of the part is referred to as
the case, and its softer interior is known as the core. Four of the more
popular surface hardening treatments are carburizing, carbonitriding, ni-
triding, and ferritic nitrocarburizing.
Carburizing consists of absorption and diffusion of carbon into solid
ferrous alloys by heating to some temperature above the upper transfor-
mation temperature of the specific alloy. Temperatures used forcarburiz-
ingaregenerallyintherangeof900to1040(cid:1)C(1650to1900(cid:1)F).Heating
is done in a carbonaceous environment (liquid, solid, or gas). This pro-
duces a carbon gradient extending inward from the surface, enabling the
surface layers to be hardened to a high degree either by quenching from
the carburizing temperature or by cooling to room temperature followed
by reaustenitizing and quenching. Carburizing is discussed in greaterde-
tail in Chapter 8, “Case Hardening of Steel.”
Carbonitridingisacase-hardeningprocessinwhichaferrousmaterial
(mostoftenalow-carbongradeofsteel)isheatedabovethetransformation
temperature in a gaseous atmosphere of such composition as to cause
simultaneous absorption of carbon and nitrogen by the surface and, by
diffusion, create a concentration gradient. The process is completed by
cooling at a rate that produces the desired properties in the workpiece.
Carbonitriding is most widely used for producing thin, hard, wear-
resistant cases on numerous hardware items. For more detailsoncarbon-
itriding, see Chapter 8.
Nitriding.Theintroductionofnitrogenintothesurfacelayersofcertain
ferrousalloysbyholdingatasuitabletemperaturebelowthelowertrans-
formationtemperature,Ac ,incontactwithanitrogenousenvironmentis
1
known as nitriding. Processing temperature is generally in the range of
525to565(cid:1)C(975to1050(cid:1)F),andthenascentnitrogenmaybegenerated
by cracking of anhydrous ammonia (NH ) or from molten salts that con-
3
tain cyanide. Quenching is not required to create a hard, wear-resistant
and heat-resistant case (see Chapter 8 for more details on the nitriding
process).
Nitrocarburizing is used to describe an entire family of processes by
which both nitrogen and carbon are absorbed into the surface layers of a
widevarietyofcarbonandalloysteels.Thesourcesforcarbonandnitro-
genmaybeeithermoltensaltorgas,andtemperaturesaregenerallybelow
the lower transformation temperature of the alloy; that is, below Ac .
1
Nitrocarburizing not only provides a wear-resistant surface, but also in-
Chapter1:WhatIsHeatTreating?ImportanceandClassifications/5
creases fatigue strength. Retention of these properties dependslargelyon
the avoidance of finishing operations after the heat treatment becauseni-
trocarburized casesareextremelythin.Thereareanumberofproprietary
processes that comprise the family of nitrocarburizing processes (see
Chapter 8 for further details on and applications of nitrocarburizing).
Quenching/Quenchants
Steelpartsarerapidlycooledfromtheaustenitizingorsolutiontreating
temperature, typically from within the range of 815 to 870 (cid:1)C (1500 to
1600 (cid:1)F). Stainless and high-alloy steels may be quenched to minimize
the presence of grain-boundary carbides or to improve the ferrite distri-
bution, but most steels, including carbon, low-alloy, and tool steels, are
quenched to produce controlled amounts of martensite in the microstruc-
ture.Objectivesaretoobtainarequiredmicrostructure,hardness,strength,
or toughness, while minimizing residual stresses, distortion, and thepos-
sibilityofcracking.Theabilityofaquenchanttohardensteeldependson
the cooling characteristics of the quenching medium. Quenching effec-
tiveness is dependent on steel composition, type of quenchant, or quen-
chant use conditions. The design of a quenching system and its mainte-
nance are also key to success.
QuenchingMedia.Selectionheredependsonthehardenabilityofthe
steel, the section thickness and shape involved, and the cooling rates
neededtogetthedesiredmicrostructure.Typically,quenchantsareliquids
or gases.
Common liquid quenchants are:
(cid:127) Oil that may contain a variety of additives
(cid:127) Water
(cid:127) Aqueous polymer solutions
(cid:127) Water that may contain salt or caustic additives
Most common gaseous quenchants are inert gases, including helium,
argon, and nitrogen. They are sometimes used after austenitizing in a
vacuum.
A number of other quenching media and methods are available, in-
cludingfogs,sprays,quenchingindrydies,andfluidizedbeds.Inaddition,
some processes, such as electron-beam hardening and high-frequency
pulse hardening, are self-quenching. Very high temperatures are reached
inthefractionofasecond,andmetaladjoiningthesmall,localizedheating
area acts as a heat sink, resulting in ultrarapid cooling.
Tempering
In this process, a previously hardened or normalized steel is usually
heated to a temperature below the lower critical temperature and cooled