Alloyed tool and die steels

Kivonat:

Tool steels are groups into six types: high speed, hot work, cold work, shock resisting, special purpose and water hardening. 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. 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. High Tungsten-chromium Steel 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.

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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