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.