Group 4 Steels: Tools Steels and Their Uses

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Group 4 Steels Tools Steels and Their Uses
High speed (HS) tools steels are used for high
cutting speeds such as drills, mill cutters, taps
and others. Hot-worked tool steels are intended
to withstand combinations of heat, pressure, and
abrasion associated with shearing, punching or
forming of metals during manufacturing at high
temperatures. Cold-work tool steels are those
intended for applications that do not require
prolonged or repeated heating in the range above
205-260 oC.
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Group 4 Steels Tools Steels and Their Uses
Shock resisting tool steels are those intended
for applications requiring toughness and
resistance to shock-loading such as hammers,
chisels, punches, driver bits and others. Water
hardening tool steels are shallow hardened and
have relatively low resistance to softening.
They are suitable for woodworking tools,
hand-metal cutting tools such as taps and reamers
and cutlery.
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Tool Steels for Room Temperature Use
The primary requirement of these tool steels is a
room temperature (RT) hardness of at least RC60
for a plain carbon steel. These steels are
classified according to their quenching media. W
water hardened grades are plain carbon steels
with 0.6-1.0 C. These have a low hardenability,
ie., martensite only to a depth of 0.5 in. V can
be added to improve the hardness of these
steels. Vanadium forms a very hard carbide V4C3,
although relatively expensive, a small amount
goes a long way for improving wear resistance. S
shock resistant grades contain small amounts of
Cr or Mo and are quenched in oil. They have
lower C contents (0.5) to improve impact
strength. - Recall - A value of Rockwell C-54
the hardness of 50/50 P/M in a eutectoid steel,
which was used to determine the Ideal Quench
Diameter, DI.
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Tool Steels for Room Temperature Use
O oil hardened grades have small percentages of
Cr and W with 0.9 C. The have medium hardness
and are used to short run cold forming dies. A-
air hardening and the addition of greater amounts
of Cr and Mo and 1 C are used for complicated
shapes and thread rolling. Mo and W are
relatively expensive so they are only added in
small amounts to give much improved
hardenability. D high carbon, high Cr grades
with 12 Cr and 1.5-2.25 C are extremely wear
resistant and used for long run dies and for
gauges. Chromium is a relatively low cost
addition for increasing hardenability with the
excess Cr, Cr23C6 is also formed, which improves
wear resistance.
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Isothermal Transformation Diagram for Water
Quenched Carbon Tool Steel
These steels must be cooled very rapidly past 500
C to avoid pearlite. Additional Mn 0.5 - is
added for hardening saw blades by oil quenching.
Note the Martensite as a function of temperature
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Isothermal Transformation Diagram for Water
Quenched Carbon Tool Steel
128 grains
32 grains
256 grains
Higher austenitizing temperatures (790 870 C)
promotes grain growth (rippening), which results
in fewer grains per cm2 and reduced of
martensite upon cooling.
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Isothermal Transformation Diagram for Oil
Quenched Carbon-Chromium Tool Steels
Proeutectoid cementite curve
g ? pearlite
The slower cooling rate of oil results in small
thermal gradients and more uniform
transformations through the cross sections eg.,
a ball bearing steel.
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Isothermal Transformation Diagram for Oil
Quenched Carbon-Chromium Tool Steel
256 grains
32 grains
16 grains
2 grains
Again, higher austenitizing temperatures result
in larger grain size and reduced amounts of
martensite upon cooling.
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Tool Steels for High Temperature Use
These steels are intended to withstand
combinations of heat, pressure, and abrasion
associated with shearing, punching or forming of
metals during manufacturing at high temperatures.
H Chromium hot working steels ? 5-7 Cr,
0.4-1.0 V, 1.5-7.0 W, 1.5 Mo, 0.35 C Medium hot
working for Mg and Al extrusion die-casting
dies. H Also for Tungsten hot working steels ?
9.5-12 W, 3.5-12.0 Cr, 0.35 C Hot working
extrusion and forging dies for brass, nickel and
steel. T Tungsten high speed steel ? 12-18 W,
4.0 Cr, 1-5 V, 0.7-1.5 C. Original high speed
(HS) cutting steel with excellent HT wear
resistance. M Molybdenum HS steel ? 3.5-8.0 Mo,
1.5-6.0 W, 4.0 Cr, 1-5 V, 5 Co, 0.8-1.5 C Used
for 85 of US cutting steels before the advent of
ceramic cutting tools.
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Tool Steels for High Temperature Use
Although Mo and W are expensive, when present in
large amounts they form very hard carbides,
(Mo-W)6C , which precipitate as fine particles
during secondary tempering of martensite.
(Mo-W)6C resists growth at dull red temperatures,
which is the basis of High Speed (HS)
steels. Vanadium forms V4C3, which is the hardest
of all the carbides. It resists dissolving into
solution in austenite and thus remains unchanged
in the microstructure after successive heat
treatment cycles producing the hardest
alloys. For high speed machining operations, the
tool tip may be heated to gt600 oC (red hot) by
the friction heat for long periods. Present
developments are to coat these tool steels used
as bits with TiC, or use ceramic tool tips.
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Pseudo-Binary Phase Diagram for High Speed Steels
This diagram below indicates temperature of phase
changes but not the phase compositions, except
for the carbon content. The 18W 4Cr 1V raises the
eutectoid temperature from 727 to 840 oC. Also,
the eutectoid composition is reduced from 0.8 to
0.25 C. the solubility of C in austenite is
reduced from 2.1 to 0.7 C.
The increase in eutectoid temperature means that
the HS steels have to be heat treated at higher
temperatures than plain carbon or low alloy
steels. In addition, higher temperatures are
needed to dissolve the carbides.
Note
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Isothermal Transformation Diagram for Air
Hardened High Speed Tool Steels
High speed tool steels are used for high cutting
speeds such as drills, mill cutters, taps and
others.
The pearlite knee is displaced upward and well to
the right for these steels. In the temperature
interval of 360-600 oC, the metastable austenite
shows no sign of decomposing even after a period
of weeks.
No decomposing region
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Isothermal Transformation Diagram for Air
Hardened High Speed Tool Steels
High speed tool steels are used for high cutting
speeds such as drills, mill cutters, taps and
others.
One can use hot-quenching (like martempering) in
which the steel is quenched in a salt bath at
500-550 oC, held until a uniform temperature and
then quenched in oil. The advantage is economic
as there is less time to process the steel
compared to slow air cooling the steel.
No decomposing region
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Types and Uses of Tools Steels
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Types and Uses of Tools Steels
C Mn Cr V W Mo Co


- See next slide
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Tool Steels Based on Undissolved Carbides

• Super-Speed Steels
• Compositions are given under T15 and M15
• These steels contain higher carbon (1.5)
  compared to the original High Speed Steels, and
  in addition to the 4Cr, they contain 5Co.
• The Tungsten steel contains 12W. The Mo steel
  contains 6W 3.5Mo.
• These steels have undissolved V4C3 present in the
  quenched state and this is not affected by the
  double tempering treatment to develop (W,Mo)6C.
• Their overall hardness is thus in the region of
  C70 - C72 and they have a much longer tool life
  than the normal high speed steels.
• (contd)

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Tool Steels Based on Undissolved Carbides
Cemented Carbide Tool Tips Made of 90 WC (or
WC TiC) embedded in a ductile matrix of Co
metal. There are 3 Steel Bonded Carbide Tool
Tips (Compositions listed) 1) 45 TiC 55
Cr-Mo alloy steel by volume 26 Ti 7 C 2
Mo by weight C36-43 in annealed state or C69-72
in oil quenched and tempered state 2) 45 (WC
TiC) 55 high speed steel by volume 34 W
10 Ti 5 C 3 Cr 1 V by weight C44-46 in
annealed state or C71-72 oil quenched and
tempered 3) Co-based Hard Alloys referred to
as Stellites 30 Cr 19 W 2 C 3.5 Ni
balance Co Hardness of C60 65 Also very
resistant to corrosion. Used as blade edges in
mining equipment.
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