tool steel grade and standard designation

8/2/2019 Tool Steel Grade and Standard Designation

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Special tool steel

F F1 High carbon, low alloys

F2, F3 Tungsten

L L1, L3, L7 Carbon > 0.65%, Chromium

L2 Carbon <0.65%, Chromium

L6 Carbon > 0.65%, Nickel

S S1, S3 Tungsten

S2, S4, S5, S6 Silicon

S7 Chromium

P P1- P21 Low carbon mold steel

Tool Steel standard and equivalent

AISI ( USA) JIS ( Japan ) DIN ( Germany ) SS ( Sweden ) BS ( UK )

W1A SK2 1.1654 BW1A

1.1663

W1B SK3 1.1673 BW1B

1.1744

W2 SKS43 1.1645 BW2

SKS44 1.2206

1.2833

A2 SKD12 1.2363 2260 BA2

D2 SKD11 1.2201 2310 BD2

1.2379

1.2601

1.2609

D3 SKD1 1.2080 BD3

SKD2 1.2436

1.2884

O1 SKS3 1.2510 2140 BO1

SKS21

SKS93

SKS94

SKS95

H12 SKD62 1.2606 BH12

H13 SKD61 1,2344 2242 BH13

H14 SKD4 1.2567

H21 SKD5 1.2581 2730 BH21

M2 SKH9 1.3341 2722 BM2

SKH51 1.3343

1.3345

1.3553



T15 SKH10 1.3202 BT15

L2 SKT3 1.2235

SKC11 1.2241

1.2242

1.2243

L6 SKT4 1.2713

SKS51 1.2714

S1 SKS41 1.2542 2710 BS1

1.2550

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 and6. water hardening.

The designations follow the AISI with the addition of B. Thus BTI and BMI designates highspeed 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 whenhardened 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 arerequired for master tools, gauges and dies which must not change size when hardened aftermachining in the annealed condition. The cheapest non-shrinkage steel contains 0,9% carbonand 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 verysmooth finish on certain articles. Special steels have been produced for this purpose, knownas finishing steels, which are capable of retaining a keen cutting edge for much longer periodsthan carbon steel used under similar conditions. The usual type has the approximatecomposition:



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 at150-180°C. Anneal at 750°C. Tungsten steels containing 1 to 5,5% and 1 to 1,3% carbon areused 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 withoutinterruption 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 from800°C and immediately tempered at 170-250°C. For cold obtrusion punches high-speed steelsare 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 goodwearing properties. lt is suitable for intricate sections, dies for blanking, coining, toller threadingand drop forging hard materials. The structure is martensitic on cooling in air but the carbidescan 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 isimportant. lt is used for hot-drawing, hot-forging, extrusion dies and dies for die castingaluminium, brass and zinc alloys. Die-casting die steels often fall through surface crackingcaused by cyclic expansion and contraction, aggravated by the erosive action of the moltenmetal. Increased die life necessitates regular maintenance and careful preheating before use.

Sensitivity of die steels to distortion during heat-treatment is largely affected by directionalityand particle size of the carbides in the microstructure. Expansion is greatest in the direction ofcarbide stringers. Fine random distribution of carbides are therefore desirable. For die castingand 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 diesand 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 largeforgings.

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 increasedrates of machining were not fully appreciated until 1900, when Taylor and White developed theforerunner of modern high-speed steels. In addition to tungsten, chromium was found to beessential and a high hardening temperature to be beneficial. The -steel resisted tempering upto 600°C. This allowed the tool to cut at speeds of 80-50 meters per minute with its nose at adull 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 and0,6% carbon. Other elements are frequently added to modern steels which vary considerablyin 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 airhardening. Cobalt, often added to the "super high-speed" steel, raises the temperature of thesolidus and enables a higher hardening temperature to be used, with consequent greatersolution of carbon. Secondary hardness is marked in such steels, and this permits the use ofdeep cuts at fast speeds. The molybdenum steel is susceptible to decarburisation. The highvanadium 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 byconverting the amounts of the various elements to an equivalent percentage of tungsten asregards 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-Cequilibrium 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 thesteel 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 ofcarbides 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 slowcooling. The steel must be protected against oxidation. After forging, tools should be heated to680°C for -If hour and air cooled before hardening in order to reduce risk of fracture. Theannealed 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 noadvantage over carbon tools. More carbide dissolves on heating, as indicated by line EB, andquenching produces structures of increasing red-hardness, due to the effect of the largeramounts of alloying elements in solution, which render the steel sluggish to tempering. Even at1300°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 sufficientcarbon and alloying elements must be dissolved in the austenite and this necessitatestemperatures little short of fusion, usually 1150-1350°C.

Grain growth and oxidation occur rapidly at such temperatures. Hence the tools are carefullypreheated up to 850°C, then heated rapidly to the hardening temperature and quenched in oilor 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 tocenter prior to the austenite-martensite transformation:a) cool in salt bath at 600°C until temperature is uniform; then quench in oil, orb) 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 bysub-zero cooling to -80°C. Multi-tempering is often more effective than a single temper of thesame duration.Tempering at 350-400°C slightly reduces the hardness but increases toughness. Tempering at400-600°C increases the hardness, frequently to a value higher than that produced byquenching. This phenomenon is known as secondar hardening. The structure of the hardened

high-speed steel consists of isolated spherical carbides embedded in an austenite-martensitematrix.Dark etching grain boundaries are frequently evident. Tempering produces a generaldarkening of the matrix. "Stellite" type alloys consist of a cobalt base with about Cr, 30; W, 15with other additions, including carbon. The structure consists of a cobalt matrix with complextungsten-chromium carbides. lt has a high resistance to corrosion and to tempering and isused for tools, gauges, valve seatings and hard facing.

tool steel grade and standard designation

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