PowerPoint to accompany
Technology of Machine Tools
6th Edition
Krar • Gill • Smid
Heat Treatment of
Steel
Unit 86
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
86-2
Objectives
• Select the proper grade of tool steel for
a workpiece
• Harden and temper a carbon-steel
workpiece
• Case-harden a piece of machine steel
86-3
Heat Treatment of Steel
• Process of heating and cooling metal in its
solid state in order to obtain desired changes
in its physical properties
• Important mechanical properties of steel
– Hardened to resist wear and abrasion
– Softened to improve ductility and machinability
– Heat treated to remove internal stresses, reduce
grain size or increase toughness
86-4
Heat Treating Equipment
• Done in specially controlled furnaces which
may use gas, oil or electricity to provide heat
• Equipped with safety devices and control
devices to maintain temperature
• Equipped with fume hood and exhaust fan
– Air switch in exhaust duct operates solenoid
valve which permits main gas valve to open
• Should fan fail, air switch also fails, main gas supply
close down
86-5
Furnace Temperature
• Controlled by thermocouple and indicating
pyrometer
Temperature
Currenttemperature
When
Temperature
conducted
in
drops
furnace
in
to
below
furnace
pyrometer
rises,
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thermocouple
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amount
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andbecomes
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seton
onpyrometer,
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pyrometer,
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to dissimilarity
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opens,
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of
gas
wires,
permitting
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actuated
full
furnace
electrical
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and
ofcurrent
flow
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produced
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
86-6
Types of Furnaces
• Low-temperature furnace
– Temperatures up to 1300ºF
• High-temperature furnace
– Temperatures up to 2500ºF
• Pot-type furnace
– Used for hardening and tempering by
immersing part in molten heat-treating medium
which may be salt or lead
– Parts do not come into contact with air
86-7
Heat-Treatment Terms
• Heat treatment
– Heating and subsequent cooling of metals to
produce desired mechanical properties
• Decalescence point
– Temperature where carbon steel transforms from
pearlite to austenite; 1300ºF for 83% carbon steel
• Recalescence point
– Temperature where carbon steel transforms from
austenite to pearlite when being slowly cooled
86-8
• Lower critical temperature point
– Lowest temperature where steel may be
quenched in order to harden it
– Coincides with decalescence point
• Upper critical temperature point
– Highest temperature where steel may be
quenched to attain maximum hardness and
finest grain structure
• Critical range
– Temperature range bounded by upper and lower
critical temperatures
86-9
• Hardening
– Heating of steel above its lower critical
temperature and quenching in proper medium
(water, oil, air) to produce martensite
• Tempering (drawing)
– Reheating hardened steel to desired temperature
below its lower critical temperature, followed
by any desired rate of cooling
– Removes brittleness and toughens steel
• Called tempered martensite
86-10
• Annealing (full)
– Heating metal to just above upper critical point
for required period of time, followed by slow
cooling in furnace, lime, or sand
– Will soften metal, relieve internal stresses and
strains and improve machinability
• Process annealing
– Heating steel to just below lower critical
temperature, followed by any suitable cooling
method
– Soften work hardened metals for further cold
working
86-11
• Normalizing
– Heating steel to just above its upper critical
temperature and cooling it in still air
– Done to improve grain structure and remove
stresses and strains (back to normal state)
• Spheroidizing
– Heating steel to just below lower critical
temperature for prolonged period of time
followed by cooling in still air
– Produces grain structure with globular-shaped
particles (spheroids) of cementite which
improves machinability of metal
86-12
• Alpha iron
– State in which iron exists below lower critical
temperature; atoms form body-centered cube
• Gamma iron
– State in which iron exists in critical range
– Molecules form face-centered cubes
– Nonmagnetic
• Pearlite
– Laminated structure of ferrite
– Iron and iron carbide
– Condition of steel before heat treatment
86-13
• Cementite
– Carbide of iron (Fe3C) which is hardener in steel
• Austenite
– Solid solution of carbon in iron, which exists
between lower and upper critical temperatures
• Martensite
– Structure of fully hardened steel obtained when
austenite quenched
– Characterized by needlelike pattern
86-14
• Tempered martensite
– Structure obtained after marensite has been
tempered (also known as troosite and sorbite)
• Eutechtoid steel
– Steel containing just enough carbon to dissolve
completely in iron when steel heated to critical
range (0.80% to .85% carbon)
86-15
• Hypereutectoid steel
– Steel containing more carbon than will
completely dissolve in iron when heated to
critical range
• Hypoeutectoid steel
– Steel containing less carbon than can be
dissolved by iron when steel is heated to critical
range
86-16
Problems that Arise in Selection and
Heat Treatment of Tool Steel
1.
2.
3.
4.
Not be tough or strong enough for job
Not offer sufficient abrasion resistance
Not have sufficient hardening penetration
May warp during treatment
Because of these problems, steel producers forced
to manufacture many types of alloy steels to cover
range of most jobs.
86-17
Table 86.1 Tool steel selection
guide*
Quench
Group
Type Medium
Toughness
Wear
Resistance
High-speed M
O, A, S
Very high
Low
T
O, A, S
Very High
Low
Hot-work Portion
H
A,
Fair
of O
table in textbook
Cr base
Good
W base
W
A, O
Fair to good
Good
More
detailedMdescription
and specifications
Mo
base
O, A, of
S qualities
High
Mediumof
all types of tool steels, see Table 18 in appendix of text.
86-18
Tool Steel
• Classification
–
–
–
–
Water-hardening
Oil-hardening
Air-hardening
High-speed steels
• Identified by manufacturer by trade name
– Alpha 8, Keewatin, Nutherm, or Nipigon
86-19
Water-Hardening Tool Steels
• Contain from 0.50% to 1.3% carbon, along
with small amounts (0.20%) of silicon and
manganese
– Silicon facilitates forging and rolling of material
– Manganese helps make steel more sound
• Achieve maximum hardness for depth of
about 18 in.
• Increase hardenability, toughness and wear
resistance if add chromium or molybdenum
86-20
Water-Hardening Tool Steels
• Heated to around 1450ºF to 1500ºF
• Used where dense, fine-grained outer casing
with touch inner core required
– Typical applications: drills, taps, reamers,
punches, jig bushings, and dowel pins
• Problems
– Distortion and cracking when quenched
86-21
Oil-Hardening Steels
• Contains about 0.90% carbon, 1.6%
manganese, and 0.25% silicon
– Manganese (>1.5%) increases hardenability of
steel up to 1 in. from each surface
• Hardening rapid so less severe quenching
medium (oil) must be used
– Retards cooling rate and reduces stresses and
strains in steel which cause warping and
cracking
86-22
Oil-Hardening Steels
• Chromium and nickel added to increase
hardness and wear resistance
– Higher hardening temperatures1500ºF to 1550ºF
• Typical applications
–
–
–
–
Blanking, forming, and punching dies
Precision tools
Broaches
Gages
86-23
Air-Hardening Steels
• Due to slower cooling rate, stresses and
strains that cause cracking and distortion
kept to a minimum
• Full hardness throughout
• Contain about 1.00% carbon, 0.20% silicon,
0.70% manganese, 5.00% chromium, 1.00%
molybdenum, and 0.20T vanadium
86-24
Air-Hardening Steels
• Higher temperatures: 1600ºF to 1775ºF
• Applications: large blanking, forming,
trimming, and coining dies; rolls; long
punches; precision tools; and gages
86-25
High-Speed Steels
• Used in manufacture of cutting tools such as
drills, reamers, taps, milling cutters, and
lathe cutting tools
• Retain hardness and cutting edges even
when operating at red heat
• Contains 0.72% carbon, 0.25% manganese,
0.20% silicon, 4% chromium, 18% tungsten,
and 1% vanadium
86-26
High-Speed Steels
• Preheated slowly to 1500ºF to 1600ºF in
neutral atmosphere and then transferred to
another furnace and quickly brought up to
2300ºF to 2400ºF
• Generally quenched in oil
– Small intricate sections may be air cooled
86-27
Classification of Steel
• Society of Automotive Engineers (SAE) and
American Iron and Steel Institute (AISI)
Classification Systems
– Use series of four or five numbers
– First digit indicates predominant alloying
element
– Last two or three digits indicate average carbon
content in points (hundredths of 1% or 0.01%)
86-28
Difference in the Two Systems
• AISI system adds prefix indicating the
steelmaking process used
–
–
–
–
–
A:
B:
C:
D:
E:
basic open-hearth alloy steel
acid-Bessemer carbon steel
basic open-hearth carbon steel
acid-open-hearth carbon steel
electric furnace steel
86-29
First Number in Series:
Type of Steel
1.
2.
3.
4.
Carbon
Nickel
Nickel-chrome
Molybdenum
5.
6.
7.
8.
Chromium
Chromium-vanadium
Triple alloy
Manganese-silicon
86-30
Examples of Steel Identification
A I S I System
A2360
Indicates alloy steel made by basic open-hearth process
Indicates steel contains 3.5% nickel
Indicates 0.60% carbon content
S A E System
4170
Indicates chromium-molybdenum steel
Indicates 0.70% carbon content
86-31
Table 86.2 SAE classification of
steels
Carbon steels
1xxx
Plain carbon
10xx
Free-cutting (resulfurized screw stock) 11xx
Free-cutting manganese
X13xx
High-manganese
T13XX
Nickel steels
2xxx
Portion of table
0.50% nickel
20xx
taken from textbook
1.50% nickel
21xx
3.50% nickel
23xx
5.00% nickel
25xx
86-32
Heat Treatment of Carbon Steel
• Steel heated from room temperature to upper
critical temperature and then quenched,
several changes take place
– Critical points when change of state of metal
• Decalescence point
• Recalescence point
86-33
Experiment to Determine Critical
Points of 0.83% Carbon Steel
1. Select piece of 0.83% carbon steel about 1
½ in. x 1 ½ in. x 2 in. long and drill small
hole in one end for most of length
2. Insert thermocouple in hole and seal end of
hole with fireclay
3. Place block in furnace and run
thermocouple wire to voltmeter
86-34
4. Light furnace and set temperature for
about 1425ºF on pyrometer
5. Plot readings of voltmeter needle at regular
time intervals
6. When furnace reaches 1425ºF, shut down
and let it cool
7. Continue to plot readings until temperature
in furnace drops to approximately 1000ºF
86-35
Observations and Conclusions
• Steel at room temperature consists of
laminated layers of ferrite and cementite
– Called pearlite
• As steel heated from room temperature,
time/temperature ratio climbs uniformly
until temperature of about 1333ºF reached
– Temp of steel drops although temperature of
furnace rising: Declaescence point
86-36
•
At decalescence point several changes take
place in steel
1. If observed in furnace, dark shadows in steel
disappear
2. Becomes nonmagnetic
3. Changes caused by change in atomic structure
of steel (atoms rearranged)
•
•
Heat (energy) for change drawn from metal so
slight drop in temperature
Layers of iron carbide completely dissolve in iron
to form solid solution known as austenite
86-37
4. At this point, steel if quenched in water,
would show first signs of hardening
5. If steel examined under microscope, notice
grain structure gets smaller
•
•
Past decalescence point, get smaller until
reach upper critical temperature
As steel cools, grain size gradually get
larger until point of 1300ºF reached
–
Recalescence point
•
Reverse of atom changing and austenite reverts
back to pearlite and becomes magnetic
86-38
Another Experiment to Demonstrate
Decalescence and Recalescence Points
Decalescence Point
1. Place magnet on firebrick
2. Select ½ to 5/8 in. round piece of 0.90 to
1.00 carbon steel and place it on magnet
3. Place can of cold water under magnet ends
4. Heat piece held to magnet using small flame
•
Do not allow flame to come into contact with
magnet
86-39
5. When temperature reaches its critical
point, steel will drop into water and
become hardened
Recalescence Point
1. Remove can of water from under magnet
2. Place flat plate under work held on
magnet
3. Heat steel until it drops from magnet onto
plate
4. When steel cools, it will become attracted
by magnet
86-40
Summary
• When steel loses its magnetic value, drops
into water and change in steel trapped or
stopped
– Steel hardens because it does not have time to
revert to another state
• When steel not quenched but allowed to
cool gradually from decalescence point,
regains its magnetic value
– Steel does not change, merely acquires
temporary characteristics
86-41
Hardening of
0.83% Carbon Steel
• Heat uniformly to about 50ºF over upper
critical temperature and held long enough to
allow sufficient carbon to dissolve and form
solid solution
– At this point steel will have smallest grain size
and when quenched, will produce maximum
hardness
• Increase in carbon content beyond 0.83%
will not increase hardness
– Does increase wear resistance
86-42
Quenching
• After steel heated throughout, quenched in
brine, water, or oil to cool it rapidly
• During operation:
– Austenite transferred into martensite (brittle)
– Steel cooled rapidly, austenite prevented from
passing through recalescence point so small
grain size of austenite retained in martensite
• Rate of cooling affects hardness of steel
• Cracking may occur when quenching
medium too cool
86-43
Method of Quenching
• Affects the stresses and strains set up in
metal
– cause warping and cracking
• Long, flat pieces held vertically above
medium and plunged straight into liquid
– Part moved in figure 8 motion
• Keeps liquid at uniform temperature and prevents air
pockets from forming on steel
86-44
Metcalf's Experiment
•
Simple experiment demonstrates effect
various degrees of heat have on grain
structure, hardness, and strength of tool
steel
Experiment
1. Select piece of SAE 1090 about 12 in. in
diameter and about 4 in. long
2. With sharp, pointed tool, cut shallow
grooves approximately 12 in. apart
86-45
3. Number each section
4. Heat bar with an oxyacetylene torch,
bringing section 1 to white heat
5. Keep section 1 at white heat, and heat
sections 4 and 5 to cherry red
•
Do not apply heat to sections 6 to 8
6. Quench in cold water or brine
7. Test each section with edge of file for
hardness
8. Break off sections and examine grain
structure under microscope
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
86-46
Results of Experiment
• Sections 1 and 2 have been overheated
– Break easily and grain structure very coarse
• Section 3
– Requires more force to break and grain structure
somewhat finer
• Sections 4 and 5
– Greater strength and resistance to shock with finest grain
structure
• Sections 6 to 8
– Underheated, require greatest force to break, bends
86-47
Tempering
• Process of heating hardened carbon or alloy
steel below its lower critical temperature
and cooling it by quenching in liquid or air
• Imparts toughness to metal but decreases
hardness and tensile strength
• Modifies structure of martensite, changing
it to tempered martensite, which is softer
and tougher than martensite
86-48
Tempering and Drawing
•
Factors affecting tempering and drawing
temperature
1.
2.
3.
4.
Toughness required for part
Hardness required for part
Carbon content of steel
Alloying elements present in steel
86-49
Tempering Facts
• Hardness obtained after tempering depends
on temperature used and length of time
workpiece is held at temperature
• Generally, hardness decreases and toughness
increases as temperature increased
• Length of tempering time increased for a
specific part, hardness of metal decreases
86-50
More Tempering Facts
• Tempering time too short, stresses and
strains set up by hardening not totally
removed and metal brittle
• The cross-sectional size of the workpiece
affects its tempering time
86-51
Tempering Colors
• When steel heated from room temperature
to red heat, passes through several color
changes caused by oxidation of metal
• Color changes indicate approximate
temperature of metal and used as guide
when tempering
86-52
Table 86.3 Tempering colors and
approximate temperatures for carbon
steel
Color
°F
Pale yellow
430
220
Lathe tools, etc.
Light straw
445
230
Milling cutters, drills,
reamers
Dark straw
475
245
Taps and dies
Brown
490
255
Scissors, shear blades
Brownish-purple
510
°C
265
Use
Axes and wood chisels
Purple
525
275
Cold chisels, center punches
Bright blue
565
295
Screwdrivers, wrenches
Dark blue
600
315
Woodsaws
86-53
Annealing
•
Heat-treating operation used to soften metal and
improve its machinability
• Relieves internal stresses and strains caused by
previous operations, such as forging or rolling
Procedure
1. Set pyrometer approximately 30ºF above upper
critical temperature and start furnace
2. Place part in furnace; bring to temperature and
allow to soak 1 hour per inch of thickness
3. Shut off furnace and allow part to cool slowly
86-54
Normalizing
•
Performed on metal to remove internal stresses
and strains and to improve its machinability
Procedure
1. Set pyrometer approximately 30ºF above upper
critical temperature of metal and start furnace
2. Place part in furnace, bring to temperature, allow
part to soak for 1 hour per inch of thickness
3. Remove part from furnace and allow to cool
slowly in still air (may pack in lime to retard
cooling rate)
86-55
Spheroidizing
• Process of heating metal for extended
period to just below lower critical
temperature
• Produces special kind of grain structure
whereby cementite particles become
spherical in shape
• Done on high-carbon steel to improve
machinability
86-56
Procedure for Spheroidizing
1. Set pyrometer approximately 30ºF below
lower critical temperature of metal and
start furnace
2. Place part in furnace and allow it to soak
for several hours at this temperature
3. Shut down furnace and let part cool slowly
to about 1000ºF
4. Remove part from furnace and cool in still
air
86-57
Case-Hardening Methods
• Cheaper than heat treating carbon steel
• Produces hard outer case with soft inner core
– Often preferable to through-hard parts
• Several methods
– Carburizing
– Carbonitriding
– Nitriding
86-58
Carburizing
• Process whereby low-carbon steel, when
heated with some carbonaceous material,
absorbs carbon into its outer surface
• Depth of penetration depends on time,
temperature, and carburizing material used
• Three methods
– Pack carburizing
– Liquid carburizing
– Gas carburizing
86-59
Procedure for Pack Carburizing
•
Generally used when hardness depth of
penetration of .060 in. or more required
• Parts to be carburized packed with
carbonaceous material such as activated
charcoal in sealed steel box
Procedure
1. Place 1 to 1 ½ in layer of carbonaceous
material in bottom of steel box to fit in
furnace
86-60
2. Place parts to be carburized in box, leaving
about 1 ½ in. between parts
3. Pack carburizer around parts and cover
parts with about 1/ ½ in material
4. Tap sides of box to settle material and to
pack it around workpieces
–
Excludes most of air
5. Place metal cover over box and seal
around joint with fireclay
6. Place box in furnace and bring temperature
up to 1700ºF
86-61
7. Leave box in furnace long enough to give
required penetration
•
Rate generally .007 to .008 in./h; decreases as
depth of penetration increases
8. Shut down furnace and leave box in
furnace until it cools (may be 12 to 16 hr)
9. Remove box from furnace and take out
parts and clean them
10. Heat parts to proper critical temperature in
furnace and quench in oil or water
86-62
Procedure for Liquid
Carburizing
1. Place carburizing material into pot furnace; heat
until molten and reaches proper temperature
2. Preheat part to be carburized to approximately
800ºF in low temperature drawing furnace
3. Suspend parts in liquid carburizer and leave them
for time required to give desired penetration
•
From .015 to .020 in. for first hour; .010 in. each
succeeding hour
4. Use dry tongs to remove parts; quench parts
immediately in water
86-63
Cautions: Liquid Carburizers
• Some contain cyanide – Use Extreme Care
• Avoid letting any moisture come in contact
– Cause explosion
• Heat jaws of tongs before using to remove
moisture or oil
• Avoid inhaling fumes; they are toxic
• Wear protective clothing when removing
and quenching parts
86-64
Gas Carburizing
• Used on parts where over .060 in. depth of case
hardening required and where necessary to grind
parts after carburizing
• Requires special types of furnaces
• Process:
– Parts placed in sealed drum, natural gas introduced,
workpieces heated, gas exhausts at one end and burned
to prevent air from entering chamber, carbon from gas
absorbed by workpiece
– Parts remain in drum for time to give desired
penetration, removed and quenched, then repeated
86-65
Carbonitriding Processes
• Both carbon and nitrogen absorbed by
surface of steel workpiece when heated to
critical temperature to produce hard, shallow
outer case
• Done by liquid or gas methods
– Cyaniding – liquid carbonitriding
– Carbonitriding – gas cyaniding
86-66
Cyaniding
• Process uses salt bath composed of cyanidecarbonate-chloride salts
• Carried out in pot-type furnace
• Parts suspended in liquid cyanide bath,
temperature above lower critical point of
steel being used; penetration about .005 to
.010 in. in 1 hour at 1550ºF; parts quenched
in water or oil
• After hardening, wash to remove cyanide
86-67
Carbonitriding
• Carried out in special furnace
• Workpieces put into inner drum of furnace
• Mixture of ammonia and carburizing gas
introduced and circulated through chamber;
heated externally to 1350ºF
– Workpiece absorbs carbon from gas and
nitrogen from ammonia
• Parts removed from furnace and quenched
in oil
• Penetration .030 in. in 4-5 h at 1700ºF
86-68
Nitriding Processes
• Used on certain alloy steels to provide
maximum hardness
• Two methods
– Salt bath nitriding
• Hardened part suspended in molten nitriding salt at
900ºF to 1100ºF
• Improves durability on high-speed taps, drills,
reamers
– Gas nitriding
86-69
Gas Nitriding
• Uses atmospheric furnace
• Parts placed in airtight drum; heated
externally to temperature of 900ºF to 1150ºF
– Ammonia gas circulated through chamber
• Decomposes into nitrogen and hydrogen
– Nitrogen penetrates outer surface of workpiece
• Slow process
• No quenching of part required
• Used on parts that have been hardened and
ground
86-70
Surface Hardening of MediumCarbon Steels
• Need medium- or high-carbon content to be
surface hardened
– Retain soft inner core
• May be surface hardened by flame or
induction hardening
– Depends on size of part and application
86-71
Induction Hardening
• Part surrounded by coil through which highfrequency electrical current is passed
• Current heats surface of steel to above
critical temperature in few seconds
• Automatic spray of water, oil or compressed
air used to quench and harden part
• Only surface heated, so hardness localized
at surface
86-72
Induction Hardening
• Depth of hardness governed by current
frequency and heating-cycle duration
• Frequencies vary from 1 kHz to 2 MHz
– Higher produce shallow hardening depths
– Lower produce hardening depths up to ¼ in.
• Used for selective hardening of gear teeth,
splines, crank shafts, camshafts, and
connecting rods
86-73
Flame Hardening
• Used extensively to harden ways on lathes
and other machine tools, as well as gear
teeth, splines, crank shafts, etc.
• Surface of metal heated very rapidly to
above critical temperature and hardened
quickly by quenching spray
– Immediate tempering removes strains created
by hardening process