Mech 473 Lectures
Professor Rodney Herring
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.
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.
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.
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.
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
Isothermal Transformation Diagram for “Water
Quenched” Carbon Tool Steel
256 grains
128 grains
32 grains
Higher austenitizing temperatures (790 – 870 C) promotes
grain growth (rippening), which results in fewer grains
per cm2 and reduced % of martensite upon cooling.
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.
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.
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.
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
>600 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.
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
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.
No decomposing region
In the temperature
interval of ~360-600 oC,
the metastable austenite
shows no sign of
decomposing even after a
period of weeks.
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
Types and Uses of Tools Steels
Types and Uses of Tools Steels
C Mn Cr V W Mo Co
*
*
* - See next slide
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 4%Cr, they
contain 5%Co.
•
The Tungsten steel contains 12%W. The Mo steel contains
6%W + 3.5%Mo.
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.
(cont’d)
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.
The End
(Any questions or comments?)