The following points highlight the seven main types of tool steels. The types are: 1. Water-Hardening Tool Steels (Symbol W) 2. Shock-Resisting Tool Steels (Symbol S) 3. Mould Steels (Symbol P) 4. Cold-Work Tool Steels 5. Hot Work Tool Steels (Symbol H) 6. High Speed Steels 7. Special Purpose Tools Steels.
Type # 1. Water-Hardening Tool Steels (Symbol W):
These tool steels are essentially plain carbon tool steels (carbon varies from 0.60 to 1.40%), and thus are least expensive. These steels have low inherent hardenability. Section less than about 8 mm in thickness will through-harden in water.
For section heavier than 20 mm, the depth of hardening, i.e., distance from the surface to 550 VPN is about 4 mm, so that the interior has a softer but relatively tough fine pearlitic structure, which could be used to an advantage in tools operating under heavy blows, such as upsetting dies for cold-heading of bolts, or even coining and striking punches.
But shearing tools or small tools which are not subjected to heavy impact blows such as scissors, knives, or letter die-punches, because of their thin-section get through-hardened. Increased manganese with small amount of chromium and vanadium is thus added to some of these tool steels to improve the hardenability as well as hardness (and thus, wear resistance).
Water/brine-quenching has to be done to obtain hard martensite, but it produces considerable distortion. When the carbon content of these steels is raised more than eutectoid carbon, undissolved hard carbide invariably globular cementite, appear in the hardened matrix. This further increases hardness to increase the wear resistance at the expense of toughness. The carbon content is kept low if tools are to have high toughness and shock resistance.
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Depending on the combination of these properties, water hardening tool steels can be classified into three groups depending on the carbon content in them:
(a) High toughness with reasonable wear resistance such as for hammers, concrete breakers, rivet sets – 0.60 to 0.75 % C
(b) Good toughness and good wear resistance such as for chisels, shear blades, punches dies – 0.75 to 0.95 % C
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(c) High wear resistance with retention of cutting edge such as for wood-working tools, drills, reamers, turning tools – 0.95 to 1.40 % C
Water-hardening tool steels can be given spheroidisation annealing treatment to impart the best machinability state compared to all the tool steels. After the machining operation, or if the steel has coarse grains, then, normalising is done at 870°C for 1-2 hours. Red-hardness of water-hardening tool steels is poor, and thus, cannot be used as high speed cutting tools on harder materials.
Wood, brass, aluminium and low carbon steels can be cut by them. Water-hardening is done from 760- 840°C (in controlled atmosphere, or neutral salt bath) followed by tempering at 150-300°C for 1-1½ hrs, which depends on the type, shape, purpose of the tool.
Fig. 11.1 illustrates microstructure of hardened and tempered water hardening tool steel:
During austenitisation, carbides in these steels dissolve relatively rapidly, the soaking time as a result, is short. Thus, the heating of small tools often takes place without any extra precautions against atmospheric oxidation. Quenching step is the most critical operation in these steels, as too slow or fast a cooling rate may produce either soft spots, or quench-cracks. If hardening is needed around holes, or re-entrant angles, then drastic cooling should be done of this area, which may be done by manual stirring, or by spraying.
Table 11.2 illustrates the composition of some of these steels Table 11.3 shows comparison of heat treatment and characteristic properties.
Type # 2. Shock-Resisting Tool Steels (Symbol S):
The primary characteristics of these tool steels are high toughness and ability to bear repeated impacts. Thus, the carbon content of these steels are intentionally kept low between 0.45% to 0.60%, but then, these steels have moderate wear resistance. The main alloying elements added are silicon, chromium, tungsten. Silicon strengthens ferrite but its higher content tends to accelerate decarburisation necessitating suitable precautions to be taken to avoid this during heating.
Chromium increases the hardenability and hardness (as carbide), and thereby the wear resistance. Tungsten induces some red- hardness property. Molybdenum as sometimes added increases the hardenability. These steels thus are oil-hardening type but some steels require water-hardening to develop full hardness.
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Tempering temperature depends on the desired combination of toughness and hardness, i.e., higher tempering temperatures are used if toughness is of primary importance, but optimum combination of toughness and hardness is obtained by tempering at lower temperatures.
Steel SI (Table 11.2 & 3) has high wear resistance and high impact strength. Because of medium hardenability of the steel, parts having dimensions greater than 50 mm in diameter only get contour-hardened, the soil core of which further increases the toughness. Hardening temperature is 900°C, but using 950°C does not have risk of grain coarsening.
Quenching is to be done in oil, if desired hardness is more than 50 HRC in parts of dimensions up to about 60 mm diameter.
For thicker sections, water-oil quenching step is used. SI steel has best combination of toughness and wears resistance as punch for cold-punching of plates having thickness even greater than 3 mm and almost equal to the diameter of the hole. The suitable hardness of punch is 56-58 HRC. A 20-minute cyaniding treatment imparts increased wear resistance with good surface finish. This steel is also used to make impact hammer in nail- guns used for driving nails into concrete.
This steel also finds application for table-ware dies, shear blades for cold shearing of heavy plates. Even hot shears could be made from this steel having suitable hardness of about 45 HRC. Even chisels used to break hard-alumina crust in electrolytic reduction of alumina are made from this steel when the suitable hardness is about 350 BHN.
Shock-resisting steels are also used to manufacture forming tools. Table 11.2 gives composition of some of the steels, while their characteristic properties are shown in table 11.3.
Fig. 11.2 illustrates microstructure of one such steel in hardened and tempered state:
Type # 3. Mould Steels (Symbol P):
These tools steels are used to make moulds for plastics. The steels have to withstand heavy pressure along with abrasive action of the moulding powders. Thus, the surface of the moulds should be hard to withstand abrasive action. The core of the moulds should be tough to withstand shock during compression cycle. The surface of the moulds should have high degree of finish to impart good surface finish to the moulded products.
As illustrated in table 11.2, these steels have low carbon content (to have good machinability to shape to precise size and high toughness) and also contain chromium and nickel as the main alloying elements with molybdenum and aluminium as minor additions. Some of these steels may be used after giving a hard chrome-plated surface to impart good finish.
As most of these tool steels are alloy carburising grade steels, i.e., have low carbon content with low hardness in annealed state, these are suitable for hubbing operations. In hubbing, a master hub is forced into a soft blank. After the impression has been formed, or cut, the mould can then be carburised, and subsequently hardened to a surface hardness of 58 to 64 HRC to have good wear resistance.
As the composition of most of these steels as indicated in table 11.2 and 11.3 predict poor red-hardness property except P4, these steels are suitable for low temperature die-casting dies and for moulds for injection, or compression moulding of plastics.
Mould steels P20 and P21 are normally available in the heat treated state having a hardness of 30-35 HRc. The state is suitable for easy machining into large intricate dies and moulds, which is precisely preserved as no heat treatment is given further. But if the plastics is very abrasive, or if the pressure at the mating surfaces is extremely high, flame hardening of these mating surfaces results in good wear resistance of the surfaces of P20.
After air cooling from flame hardening temperatures, the hardness is 50-53 HRC. Flame hardening of edges of the moulds needs some precautions. A slight preheat even with flame is desired. A blue or green cone of oxy-acetylene flame is preferred initially, which then disappears as oxygen content is increased. The speed is 50 mm/minute with angle of torch kept at 60-90°. Tempering may be done at 160-200°C. Maraging steels exhibit better properties as mould materials.
Type # 4. Cold-Work Tool Steels:
This groups forms the most important group of tool steels as majority of tool applications are from this group, and is thus produced in the largest tonnage. As the name suggests, these steels are used for making tools for cold work applications when the tool surface temperature does not rise more than 200°C, and thus has wide range of applications. To meet the large variety of demands, these steels have wide range of compositions but the main alloying elements added are chromium, vanadium, chromium-tungsten, or chromium-vanadium, etc.
The prime properties of these steels are very high abrasion and wear resistance (that is, high hardness-thus, are difficult to machine), higher toughness and impact resistance. Sometimes, these steels are also called non-distorting steels as these steels show little change in dimensions during heat treatment and also harden to great depths.
Cold-work tool steels are further divided into three sub-groups:
1. Oil-hardening steels
2. Air-hardening steels
3. High carbon high chromium steels.
1. Oil-Hardening Steels (Symbol O):
These steels are hardenable by oil-quenching, and contain high carbon with reasonable manganese content with occasional low alloy additions of chromium and molybdenum to improve hardness, hardenability and wear resistance. These steels are relatively inexpensive. These steels possess very good non-distorting properties, and are less likely to bend, twist, sag, distort, or crack during heat treatment as compared to water-hardening steels.
This steel-group has best machinability of the cold-work steel group enabling to obtain complicated shapes by machining to high degree of precision such as required for reamers, taps, blanking and forging dies, gauges, stamping dies. Fig. 11.3 illustrates microstructure of hardened and tempered steel of this group. Steel O 6 has high silicon content to graphitise a part of the carbides to improve the machinability of the steels in the annealed state. Such steels are used for taps, solid threading dies, form-tools and expansion reamers.
2. Air-Hardening Steels (Symbol A):
These steels are hardened by air cooling. The carbon content is about 1% with manganese, chromium, molybdenum as the main alloying elements with sometimes addition of tungsten.
These elements impart high wear resistance and induce high hardenability to confer air-hardening properties to the steels resulting in negligible dimensional changes during hardening (i.e., non-distorting property). These steels have fair trimming, thread-rolling dies. Fig. 11.4 illustrates microstructure of one such steel.
3. High Carbon High Chromium Steels (Symbol D):
These steels are either oil, air-hardened. Normally these steels contain 1.4-2.3% carbon and 12-14% chromium. Vanadium, molybdenum, cobalt may be present as alloying elements. Vanadium as carbide does not allow these steels to show grain coarsening even when heated to about 1040°C.
Presence of large amount of chromium (also due to presence of other elements), increases the hardenability of these steels to obtain martensite even by air cooling, which also imparts non-deforming properties so that the steels could be used for intricate shaped parts of thicker dimensions.
Tempering results in chromium carbide (as well other carbides) which imparts high hardness, wear and abrasion resistance. These steels are used for blanking and piercing dies (see Fig. 11.5); drawing dies for wires, bars, tubes; thread-rolling dies; master gauges; bushings; coining dies; trimming dies; shear blades, cutting tools, cold-forming rolls, punches.
For example, steel D2 having hardness of 52-54 HRC is used as mandrel for tube rolling by Pilger rolls and as forging dies for stainless steel knives. D3 is used for valve seats for I.C. engines of cars.
Type # 5. Hot Work Tool Steels (Symbol H):
These tool steels are used mainly for high temperature metal forming operations (except cutting) such as hot stamping, piercing, hot-forging, hot drawing, hot extrusion, upsetting, swaging, die-casting dies of aluminium and copper alloys, hot pressing dies for copper alloys, etc., where the operating temperature is above 200°C to around 800°C.
Apart from the common main properties such as high hot yield strength, high red-hardness, wear resistance, toughness, erosion resistance, resistance to softening at elevated temperatures, good thermal conductivity, these steels should have resistance to heat checks on surface (heat checks are thermal cracks which appear on bitch surface due to thermal shocks as tools come in repeated contacts with hot metal being processed.
Differential dilations of the surface and the interior cause alternating stresses to give rise to cracks, which are usually in a network ‘alligator’ pattern. Chill cracks too, appear due to frequent temperature changes in surface regions).
The steels have high toughness as the carbon content is on the lower side around 0.3 to 0.5% and small content of alloying elements. Other properties are imparted to the steels by the additions of alloying elements like tungsten, molybdenum, chromium, vanadium, cobalt, etc. Red-hardness is mainly increased by elements like chromium, molybdenum and tungsten, and this property becomes appreciable when the sum of these elements is at least 5%.
The larger the tungsten content, higher becomes the red-hardness with high resistance to abrasive wear, but lowers thermal shock resistance (heat check resistance), i.e., high tungsten steels have excellent properties when in constant contact with hot metals such as for extrusion dies, hot forging dies, die-casting dies, etc., but inferior properties when in intermittent contact with hot metals. To develop good thermal shock resistance, choice would be between molybdenum-base steels and chromium-base steels, or lower tungsten steels.
Chromium increases red-hardness (up to temperature 500°C’), oxidation resistance and increases resistance to heat checking. Steels containing more than 5% chromium are readily air-hardened. Molybdenum and vanadium improve hardness and high temperature properties. These steels need higher hardening temperatures due to the presence of stronger carbides, but hardening from higher temperatures may lead to distortion. Erosion resistance is increased by increasing the carbide content of the steels.
Cobalt, when present, improves the resistance to erosion and heat checking during severe thermal shocks. Hot work tool steels, when tempered around 500°C are not allowed to attain hardness higher than 50-52 HRC, otherwise the impact strength becomes very low.
Hot work tool steels are divided in three groups depending on the principal alloying element present as:
1. Hot Work Chromium Type Tool Steels (H 11 to H 19):
Table 11.2 illustrates their compositions. Apart from the minimum 3.25% chromium, these steels also contain small amounts of vanadium, tungsten, molybdenum. Thus, these steels have high red- hardness and very high hardenability such that sections up to 300 mm get hardened by air-cooling, which also imparts non-distorting property.
These steels find applications for hot dies particularly for extrusion, forging, mandrels, punches, die-casting dies, hot shears, etc. H 11 finds applications in highly stressed structural parts of super-sonic aircrafts, as it can resist softening when continuous exposed to temperatures up to 500°C. H 13 is the most commonly used of all hot work steels. Hardening temperature is 1000-1050°C.
High hardening temperatures, if used, dissolve larger amount of carbides which quickly precipitate at grain boundaries to decrease the toughness and impact strength if cooled slowly like air cooling. Thus, steels have to be cooled faster, or dissolve lesser amount of carbides by using lower hardening temperatures. Oil-quenching is resorted to when requirements of dimensional stability are not of primary importance. Large and intricate dies such as die-casting dies for aluminium made out of H 13 have longer life if martempered.
2. Hot Work Tungsten Type Tool Steels (H 21 to H 26):
These steels too contain low carbon, otherwise resemble high speed steels. These have at least 9% tungsten and 2-12% chromium. As the alloy content increases, resistance to high temperature softening i.e., red-hardness increases but brittleness increases and also heat checks tendency, even for working hardness of 45-55 HRC. These steels are used for punches, mandrels and extrusion dies for brass, steel and nickel alloys.
3. Hot Work Molybdenum Type Tool Steels (H 41 to H 43):
These steels contain more carbon with 8% molybdenum, 4% chromium with some tungsten and vanadium. These steels have similar properties as hot-work tungsten type steels but are cheaper and have higher toughness. These have high heat check resistance than tungsten type tool steels but need precautions during heat treatment as these suffer from decarburisation.
Type # 6. High Speed Steels:
High speed steels are highly alloyed tools steels developed initially to do high speed metal cutting (that is why the derived name). First high speed steel was 18/4/1 (W/Cr/V)-Grade T1, and is still being used. Later, cobalt (5-10%) was added to the steels to increase hot wear resistance.
As tungsten is an expensive element, its replacement at least of a part by cheaper molybdenum, as has been the case for hot work tool steels, resulted in a new variety of high speed steels. Now-a-days, a wide variety of high speed steels are available to cover a wide variety of machining operations.
However these steels can be divided into two groups depending upon the principal alloying element, and the compositions are given in table 11.2.
I. Tungsten High Speed steels (Symbol T) contain high amount of tungsten with chromium, vanadium and at times cobalt.
II. Molybdenum High Speed steels (Symbol M) contain in addition to molybdenum, tungsten, chromium, vanadium and occasionally cobalt. Meanwhile, though cemented carbides gained wide use, still high speed steels are economical and have superiority where brittleness of tools is undesirable due to vibrations present and cutting edge is subjected to shocks.
The prime important properties of high speed steels are high hardness, hot strength, high red-hardness, wear resistance and reasonable toughness. These steels maintain a high hardness of 60-65 HRC even at service temperature of 600-650°C.
High speed steels have good hardenability (and thus, non-deforming properties), and thus, these may be quenched in oil/air/salt bath, but high speed steels having 10% cobalt have somewhat reduced hardenability and thus sections less than 30 mm in diameter can only be air-hardened to maximum hardness.
High speed steels have at least 0.60% carbon to impart hardness of at least 60 HRC of martensite-formed. Excess of carbon than this helps to get carbides in martensite to increase the resistance to wear and abrasion.
Tungsten and molybdenum have almost similar effects. When present in good amounts form hard carbides like M6C and M2C, where M stands for metal, or may form (Mo.W)6C which has high hardness. Table 11.4 illustrates hardness of various carbides. Finely dispersed precipitates of M2C carbides are responsible for the red-hardness, or secondary hardening effects in high speed steels.
These carbides dissolve at high temperatures 1150 to 1300°C during heating for austenitisation of high speed steels, but during tempering of these steels, fine precipitate of these carbides (mainly M2C type) form in martensite at 540 to 570°C and which do not grow much in size in this range of tempering, causing secondary hardening of tool steels.
Larger is the amount of these finely dispersed carbides, higher is the red-hardness of high speed steels, which requires the use of highest possible austenitisation temperature (without causing grain growth of austenite and burning of steels) to dissolve as much of the carbides as possible.
Molybdenum is less expensive than tungsten and has a greater volume per unit weight, and hence, molybdenum-base high speed steels are less expensive than equivalent tungsten types, but more precise temperature and atmosphere controls are needed during hardening of molybdenum-base steels as these are more prone to grain growth and decarburisation.
Chromium is normally present in amounts around 4% in all high speed steels, and is a relatively low-cost element which increases hardenability and corrosion resistance. It forms with carbon carbides Cr7C3 and Cr23C6. Cr23C6 dissolves during heating in range 950-1000°C while austenitising high speed steels, but precipitate again during tempering at temperature 400-500°C, and increases hardness and wear resistance.
Vanadium is a very strong carbide former and forms VC (normally V4C3) at 550-650°C during tempering to produce marked secondary hardening peak and it maintains its fine dispersion even at tempering temperature of 700°C. But as VC resists dissolution in austenite during heating for austenitisation, it generally remains unchanged during heat treatment.
This vanadium as vanadium carbide keeps the austenetic grain size fine even at a high temperature of 1290°C in 18/4/1 high speed steel. VC is the hardest of all carbides (~ 3000 VPN) and thus causes greatest wear resistance and hardness. It is an expensive element. Super high speed steels (V = 5%) have very high (70-72 HRC) hardness, red-hardness, wear resistance.
Cobalt is present in some high speed steels in amounts 5 to 12%. Cobalt does not form carbides but dissolves in matrix and increases thermal conductivity of high speed steels. Cobalt increases the melting point of high speed steels enabling to use higher hardening temperatures during austenitisation without the risk of grain growth, and thus, permits more of the alloy carbides to go into solution in austenite. This increases red-hardness and wear resistance of the high speed steels.
Cobalt thus permits higher cutting speeds with 2 to 3 times more life of the tools. Vanadium and carbon content are increased in cobalt added steels as V4C3 also dissolves and precipitates.
Type # 7. Special Purpose Tools Steels:
There are certain steels for specific applications but are too expensive for other applications having specific requirements, but do not fall into usual categories, and are called special- purpose tool steels.
We divide them into two types:
1. Low Alloy Types (Symbol L):
These steels have characteristics similar to water-hardening steels. Chromium is the main alloying element with small additions of molybdenum, vanadium and nickel. Chromium forms hard iron-chromium carbides which give high wear resistance to the steels. Nickel increases toughness, whereas vanadium as vanadium carbide refines the grains.
All these elements increase the hardenability, so that these steels are oil-hardening types, and thus some dimensional changes of components occur during heat treatment. These are basically used where high wear resistance and high toughness are of prime importance such as clutch plates, cams, bearings, rollers, wrenches, collets, etc. Steels with higher carbon content find applications for taps, dies, drills, knurls and gauges.
2. Carbon-Tungsten Type (Symbol F):
These steels have high carbon (> 1.0%) with tungsten as the alloying element, which forms hard tungsten carbides to impart high wear resistance to the steels, which is many times of the plain carbon ‘W’ group tool steels, but are quite brittle. These are water hardening type tool steels to be used at low temperatures and for low-impact applications, but with high wear resistance, such as broaches, reamers, burnishing tools, taps, paper-cutting knives, wire-drawing dies, plug gauges, etc.