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Standard BS 4659:1971 groups tool steels into six types:
1.
high speed,
2. hot work,
3. cold work,
4. shock resisting,
5. special purpose and
6. water hardening.
The
designations follow the AISI with the addition of B. Thus
BTI and BMI designates high speed steel of tungsten and molybdenum
grades respectively.
Non-Shrinking
Steels
This term refers to steels which show little change in volume
from the annealed state when hardened and tempered at low
temperatures. Usually the following volume changes occur.
 |
Pearlitic |
austenitic
state, contraction |
|
| |
austenitic |
martensitic
state, expansion |
|
| |
martensitic |
sorbitic
state, contraction |
|
In
non-shrinking steels the volume changes counterbalance each
other, and such steels are required for master tools, gauges
and dies which must not change size when hardened after machining
in the annealed condition. The cheapest non-shrinkage steel
contains 0,9% carbon and about 1,7% manganese. A better steel
is,
C,
1.0; Mn, 0.95; W, 0.5; Cr, 0.75; V, 0.2
Both
steels are oil quenched from 780° to 800°C and tempered
224-245°C. High carbon 5% and 12% chromium steels are
also used for non-distortion.
Finishing
Tool Steel
While high-speed steels are very efficient with heavy cuts
and high speeds they are incapable, at slow speeds and lighter
cuts, of holding the keen edge necessary for obtaining a very
smooth finish on certain articles. Special steels have been
produced for this purpose, known as finishing steels, which
are capable of retaining a keen cutting edge for much longer
periods than carbon steel used under similar conditions. The
usual type has the approximate composition:
C,
1.1 to 1.4; W, 4; Cr, 0.7 to 1.5; V, 0.3
After
preheating to 650°C it is water hardened at 820-840°C
and immediately tempered at 150-180°C. Anneal at 750°C.
Tungsten steels containing 1 to 5,5% and 1 to 1,3% carbon
are used for twist drills, taps, milling cutters, drawing
dies and also tools for rifling gun barrels, boring cylinders
and expanding tubes, which require long continuous cutting
without interruption for regrinding. They are tempered at
200-230°C.
Cold Die Steels
The standard oil hardening die steels contain 1 C, 1 Mn, 0,3-1,6
W, 0,5 Cr, hardened from 800°C and immediately tempered
at 170-250°C. For cold obtrusion punches high-speed steels
are satisfactory, e.g. 6W6 Mo.
High
carbon-chromium (A)
|
C
|
Cr
|
Mn
|
Si
|
Harden
°C
|
Temper
°C
|
|
2
|
13
|
0-25
|
0-6
|
OQ
950 or AC 1000
|
480-2
hrs
|
This
steel has good resistance to oxidation at elevated temperatures,
high hardness and good wearing properties. lt is suitable
for intricate sections, dies for blanking, coining, toller
threading and drop forging hard materials. The structure is
martensitic on cooling in air but the carbides can be precipitated
and the steel softened by very slow cooling from 840°C.
High
Tungsten-Chromium Steel
| C |
Mn |
W |
Cr |
V |
Mo |
Harden,°C |
Temper,°C |
Anneal,
°C |
| 0.3 |
0.3 |
10 |
3 |
0.3 |
0.3 |
OQ
1150 |
570 |
850 |
This
is the best type of steel for hot work except where resistance
to scaling or oxidation is important. lt is used for hot-drawing,
hot-forging, extrusion dies and dies for die casting aluminium,
brass and zinc alloys. Die-casting die steels often fall through
surface cracking caused by cyclic expansion and contraction,
aggravated by the erosive action of the molten metal. Increased
die life necessitates regular maintenance and careful preheating
before use.
Sensitivity
of die steels to distortion during heat-treatment is largely
affected by directionality and particle size of the carbides
in the microstructure. Expansion is greatest in the direction
of carbide stringers. Fine random distribution of carbides
are therefore desirable. For die casting and extrusion dies
molybdenum containing 0,5 Ti + 0,08 Zr is useful in critical
applications. Thermal conductivity, resistance to thermal
shock and attack by molten metal is high and no heat treatment
is required. Nimonic 80(a) and 90 have also been used satisfactorily
for dies and inserts. Die block steels for drop forging have
been standardised into four type. These are:
1) 0,6 carbon steel,
2) 1% nickel, 0,6 C,
3) 1,5 Ni, 0,7 Cr, 0,6 C,
4) 1,5 Ni, 0,7 Cr, 0,6 C, 0,25 Mo.
Hardness
ranges from 425/455 for dies with shallow impressions to 298/355
for very large forgings.
Sliear
Blades
Some examples of alloy steels used for shearing are given
in Table 3.
High-Speed
Steels
The evolution of high-speed cutting tools commenced with the
production of Mushet`s self-hardening tungsten-manganese steel
in 1860. The possibilities of such steels for increased rates
of machining were not fully appreciated until 1900, when Taylor
and White developed the forerunner of modern high-speed steels.
In addition to tungsten, chromium was found to be essential
and a high hardening temperature to be beneficial. The -steel
resisted tempering up to 600°C. This allowed the tool
to cut at speeds of 80-50 meters per minute with its nose
at a dull red temperature and it was one of the astonishing
exhibits at the Paris Exhibition of 1900.
Table
3. Shear blade Steel
| Type
of Work |
C
|
Cr
|
V
|
W
|
| Cold
shearing for heavy materials |
0.85
|
|
0.2
|
|
| |
0.55
|
Mn=0.8
|
Mn=0.8
|
| Cold
shearing for light materials |
1.0
|
-
|
0.2
|
| |
0.7
|
0.9
|
0.2
|
-
|
| |
0.6
|
4
|
1
|
18
|
| |
2.2
|
12
|
-
|
-
|
| Shears
for hot work |
0.5
|
1.2
|
0.2
|
2
|
| |
0.4
|
3.5
|
0.4
|
10
|
The
main constituents in high-speed steel are 14 or 18% tungsten,
3 to 5% chromium and 0,6% carbon. Other elements are frequently
added to modern steels which vary considerably in composition
and cost. 0,09-0,15% sulphur is sometimes added to give free
machining for unground form tools, e.g. gear hobs in 6,5×2
M2S.
Vanadium
improves the cutting qualities of the tools and increases
the tendency to air hardening. Cobalt, often added to the
"super high-speed" steel, raises the temperature
of the solidus and enables a higher hardening temperature
to be used, with consequent greater solution of carbon. Secondary
hardness is marked in such steels, and this permits the use
of deep cuts at fast speeds. The molybdenum steel is susceptible
to decarburisation. The high vanadium steel is somewhat brittle,
but is excellent for cutting very abrasive materials.
The
study of the structures of such highly alloyed steels is complex,
but it can be simplified by converting the amounts of the
various elements to an equivalent percentage of tungsten as
regards the effect on the closed g-loop:
| 1%
OF |
Mo |
V |
Cr |
| Equivalent
percentage of tungsten |
1.5 |
5.0 |
0.5 |
Hence
18 W, 4 Cr, 1 V is equivalent to 25% tungsten and the section
of the FE-W-C equilibrium diagram is shown in Fig. 1.
Figure
1. Section of the Fe-W-C equilibrium diagram at 25% tungsten
In
the ingot the structure is similar to cast iron, but the cementite
consists of mixed carbides (Fe, W Cr, V),C with the balance
of the elements in solution in the ferrite. In this condition
the steel is extremely brittle and the eutectic net-work has
to be broken up into small globules, evenly distributed by
careful annealing, followed by forging. "Strings"
or laminations of carbides should be avoided, otherwise cracks
are liable to form during hardening.
Annealing
High-speed steel is softened by annealing at 850°C for
about four hours, followed by slow cooling. The steel must
be protected against oxidation. After forging, tools should
be heated to 680°C for -If hour and air cooled before
hardening in order to reduce risk of fracture. The annealed
structure consists of carbide globules in a matrix of fine
pearlite.
Hardening
From Fig. 1 it will be seen that on heating, austenite forms
at about 800°C, but contains only 0-2% carbon (eutectoid
E). Quenching produces martensite, which tempers readily and
has no advantage over carbon tools. More carbide dissolves
on heating, as indicated by line EB, and quenching produces
structures of increasing red-hardness, due to the effect of
the larger amounts of alloying elements in solution, which
render the steel sluggish to tempering. Even at 1300°C,
when melting occurs, only 0,4% carbon (B) is dissolved and
the remainder exists as complex carbides. It will be seen,
therefore, that to attain maximum cutting efficiency sufficient
carbon and alloying elements must be dissolved in the austenite
and this necessitates temperatures little short of fusion,
usually 1150-1350°C.
Grain
growth and oxidation occur rapidly at such temperatures. Hence
the tools are carefully preheated up to 850°C, then heated
rapidly to the hardening temperature and quenched in oil or
cooled in an air blast without soaking. To reduce the severe
stresses set up by quenching, the following modifications
can be used to reduce the temperature gradient from outside
to center prior to the austenite-martensite transformation:
a) cool in salt bath at 600°C until temperature is uniform;
then quench in oil, or
b) oil quench to 425°C, then air cool to room temperature.
Tempering
When quenched from high temperatures high-speed steels contain
an appreciable amount of retained austenite which is softer
than martensite. This is decomposed by tempering, or by sub-zero
cooling to -80°C. Multi-tempering is often more effective
than a single temper of the same duration.
Tempering at 350-400°C slightly reduces the hardness but
increases toughness. Tempering at 400-600°C increases
the hardness, frequently to a value higher than that produced
by quenching. This phenomenon is known as secondar hardening.
The structure of the hardened high-speed steel consists of
isolated spherical carbides embedded in an austenite-martensite
matrix.
Dark etching grain boundaries are frequently evident. Tempering
produces a general darkening of the matrix. "Stellite"
type alloys consist of a cobalt base with about Cr, 30; W,
15 with other additions, including carbon. The structure consists
of a cobalt matrix with complex tungsten-chromium carbides.
lt has a high resistance to corrosion and to tempering and
is used for tools, gauges, valve seatings and hard facing.
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