In this article we will discuss about:- 1. Meaning of Tool Steels 2. Important Requirements of Tool Steels 3. Selection 4. Components 5. Heat Treatment.
Contents:
- Meaning of Tool Steel
- Important Requirements of Tool Steel
- Selection of Tool Steel
- Components of Tool Steel
- Heat Treatment of High Speed Tool Steel (With Defects)
1. Meaning of Tool Steel:
Technically, a steel used for, so-commonly used tools such as, screw drivers, pliers, wrenches, hammers should be called a tool steel, but these are not tool steels. The term ‘tool steel’ has been reserved for a large group of high quality special steels used as tools for the shaping and working of other metals, or for cutting and shaping of paper, wood, rock, or concrete.
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The uses of these steels include:
(i) Hand tools such as chisels, hammers, punches, etc.
(ii) Machine tools as shears intended for cutting the metals and other materials.
(iii) Dies for extrusion, forging, deep drawing, hot drawing, die-casting, etc.
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(iv) Gauges to ensure satisfaction of dimensional tolerances.
Though, the specific requirements for tools vary with the service conditions, but in general, all tools must possess high hardness and wear resistance with economic tool life and dimensional stability. There are large variety of steels used for different applications ranging from the simple and cheap high carbon steels to more complex and expensive high speed steels.
The cost of tool steel increases as the amount of the alloying elements added in steel increases. The alloying elements are added to increase one, or more of the following properties of the steel depending on the service conditions.
2. Important Requirements of Tool Steel:
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A tool must possess one or more of the following properties depending on its service conditions:
1. Resistance to Wear and Abrasion:
The resistance to wear is the resistance to abrasion. These properties are, in general, related to the hardness of the steel, i.e., as the hardness increases, the resistance to wear and abrasion increases. As the hardness of the hardened steel mainly depends on the carbon content of the steel, high carbon steels have good resistance to wear and abrasion.
As the hardness of carbides of alloying elements like tungsten, chromium, vanadium, and molybdenum is higher than martensite, or even cementite, the presence of these elements as carbides in steels further improves the resistance to wear and abrasion.
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2. Toughness:
The toughness of tool steels means resistance to breakage. This property is of prime importance in shock-resisting and hot worked tool steels. As the carbon content of steel increases, (as well as vanadium) brittleness increases and toughness decreases. Thus, hot-forging dies are made, of hypo- eutectoid steels. A proper balance of silicon and other elements results in proper combination of toughness and hardness to obtain good shock-resisting steels.
3. Red Hardness or Hot-Hardness:
It is the resistance to softening when the steel gets heated to temperatures higher than 400°C. The presence of this property in a steel is reflected by a peak in the hardness versus tempering temperature curve above this temperature. Plain carbon steels and low alloy tool steels loose their hardness, and thus, the cutting edges when get heated above 200°c.
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Steels having alloying elements which form hard and stable carbides increase the resistance to softening at elevated temperatures (even when the steel tool tip becomes red-hot). Very finely dispersed precipitates of carbides of tungsten, vanadium, molybdenum and chromium give rise a peak in hardness curve when tempered at 550°C, called secondary hardening, and is the cause of red hardness in these steels.
4. Non-Deforming Property:
It is the resistance to distortion, or dimensional changes which takes place during quenching the steel, and no grinding is required after the quenching. Steels generally expand on hardening. Intricate-shaped tools and dies must maintain their shape after hardening as such tools are machined very close to size before heat treatment.
The non-deforming property is achieved in steels of high hardenability, that is, air-hardening steels in general exhibit the least distortion; those quenched in oil show moderate distortion but the water-hardening steels show the greatest distortion.
5. Hardenability:
The hardenability of a steel is defined as its ability to harden (by forming martensite) throughout its cross-section, without having to resort to drastic quenching. It refers to the depth to which hardening takes place along the cross section of the steel. Most of the alloying elements (except cobalt) increase the hardenability of the steels.
To develop high strength throughout a large section, it is thus important to select a high alloy steels, i.e., the steel with high hardenability. Deep-hardening steels can be quenched in a mild-quenching medium, which causes less distortion and quench-cracking. Shallow-hardening steels such as the carbon tool steels and several of the carburising steels are generally quenched in water.
6. Machinability:
It is the ability of a steel to be machined easily and freely to result in good surface finish. This properly is affected by its hardness, microstructure and the amount of hard carbides, that is, the machinability of the tool steels decreases with the increase of the carbon content and the amount of alloying elements.
The presence of strong carbides as of vanadium, chromium, and molybdenum reduces the machinability of the tool steels. Among the tool steels, water-hardening tool steels have the best machinability, whereas, high speed steels and high chromium steels have the lowest machinability.
3. Selection of Tool Steel:
The choice of tool steel depends on the requirements of the application. It is a difficult job to do, however, the chosen steel should be no more expensive than is needed to meet these requirements.
In the final analysis, it is the cost per unit part made by the tool that determines the proper selection. Commonly used tool steels for some common applications are- Wood working tools, files, axes, taps, reamers and the common drill-bits show good performance when made from water-hardening tool steels.
Battering tools such as chisels, railways track tools, punches and rivet sets are made from shock-resisting tool steels as shock-resistance (toughness) is their main requirement. Steels used for making moulds for plastics should be easily machinable, and are then carburised to develop wear resistance, and thus tool steels, P2-P6 are used, hut tool steels P20 and P21 are pre-hardened, and then machined as die-casting dies as these have resistance to thermal shock.
Cold-forming dies such as stamping and trimming dies, and even, thread rolling dies, punches must be finish-machined to close tolerances, and these tolerances must be maintained after heat treatment. These components are thus, made of cold-work tool steels, which because of high carbon and high chromium contents are air-hardening or oil-hardening in nature to have high degree of dimensional stability.
The components like forging dies, extrusion dies, die-casting dies, piercing tools are made from steels which have high resistance to softening, deformation at high temperatures.
Hot-work tool steels are used to make them. These steels have lower carbon content and high alloy content resulting in high inherent hardenability and high toughness. The components like lathe tools, milling cutters and other cutting tools are to cut other metals at high speeds, even when, the cutting edge becomes red hot.
The steels used in making them should have high wear resistance, abrasion resistance and high resistance to softening at elevated temperatures to maintain cutting ability. Such tools are made from high speed steels. These are the most complex and important of the tool steels containing high carbon, tungsten, or and molybdenum with additions of elements like chromium, cobalt and vanadium.
4. Components of Tool Steels:
Tool steels are invariably high carbon steels and one, or more alloying elements may be added to it for specific structural, or property effects. Except the water-hardening plain carbon steels, tool steels have sufficient alloying elements to increase the hardenability to permit quenching either in oil, or air and still result in martensitic structure.
Metallurgically, the functions of some of the alloying elements are:
i. Carbon:
Tool steels invariably have at least 0.6% carbon in order to assure attainment of a martensitic hardness of at least HRc 60. Carbon more than this minimum is used only, to have undissolved carbides in the martensitic structure to increase the wear resistance and abrasion resistance. Carbon is always kept as minimum as possible, but just to obtain the desired results, because carbon increases brittleness due to the formation of cementite. Shock-resisting steels which require toughness and increased impact strength as prime properties have carbon below 0.5%.
ii. Chromium:
It is a relatively low-cost element added to increase the hardenability, and when present along with carbon forms chromium carbide, Cr23C6 which increases hardness and wear resistance. Depending on the requirement, the chromium content varies in tool steels from 0.5 to 12%.
iii. Manganese:
It is the cheapest element added to tool steels to increase the hardenability to make them oil, or air-hardening type to reduce distortion during heat treatment. Its content normally varies between 0.6 to 2.0%.
iv. Tungsten and Molybdenum:
These elements are normally present in between 0.2% to 18%. When present in smaller amounts, these elements resist softening on heating, but when present in larger amounts, cause secondary hardening, or red hardness by forming fine precipitates of their carbides in martensite. These elements increase hardness, hardenability and resist grain growth at high temperatures. M- type tool steel are cheaper than the equivalent T-type tool steels as molybdenum is less expensive.
v. Vanadium:
As vanadium forms a very hard and stable vanadium carbide, V4C3, which remains unchanged during heat treatment as it resists solution in austenite. V4C3 is the hardest of all the carbides.
Vanadium increases hardness, wear resistance, abrasion resistance, hardenability, (due to whatever vanadium dissolves in austenite) and keeps the steels fine grained. As vanadium is an expensive element it is present normally in amounts between 0.1 to 2.0% depending on functions to be performed by it.
Silicon is normally kept low but in some steels, it may be as high as 2.0 (shock-resisting steels). It increases hardness and strength without decreasing the ductility and resist softening as it stabilises epsilon carbide in steels. Cobalt is commonly added in high speed steels to increase the cutting efficiency of the tools.
As a large variety of steels are used as tools, their classification becomes difficult. The classification and designation system adopted by AISI (American Iron and Steel Institute) has been widely used.
Here, tool steels have been divided into seven main groups with sub-groups. Each of them has been designated with an alphabetical letter as a symbol as illustrated in table 11.1.
5. Heat Treatment of High Speed Tool Steel (With Defects):
1. Austenitisation:
As the high speed steels have high alloy contents, their thermal conductivity is poor. As the austenitisation temperatures in general are high, heating the steels directly to these temperatures in one step, particularly the thick and complex shaped tools, may cause distortion, or even cracks due to differential expansion of surfaces and cores.
Thus, at least one preheating step of heating at 815°C may be used, but at times 3 to 4 preheating steps may be used, according to which, tools are put first in forced- air circulation furnace to heat them uniformly to 400°C. In second step, salt bath at 815°C heats the tools with soaking time of 8-9 min./cm.
For complex shapes, and also to reduce time at the final hardening temperature, a third step of salt bath heating kept at 1050-1100°C for same time (though otherwise reduced) as at the final temperature may be used. Final temperature and time at the temperature depends on the composition, size, purpose of tool and temperature of heating.
On an average final hardening temperatures for the two types of steels are:
1. Molybdenum base high speed steels 1170°-1250°C
2. Tungsten base high speed steels 1260°-1290°C
The exact temperature controls the ultimate hardness, red-hardness, wear resistance, toughness and resistance to crumbling of cutting edge.
Higher the austenitising temperature:
(a) More carbides dissolve in austenite, which ultimately causes increasing amount of finely divided precipitates of carbides during tempering to result in increased tool hardness, red hardness, wear resistance, increased tempering temperature as well as increased heat resistance during cutting operations.
(b) Lower as-quenched hardness (see Fig. 11.7)
(c) More retained austenite
(d) Increased grain size to decrease the toughness as well as resistance to crumbling of cutting edges.
Toughness is also decreased due to precipitation of carbides at grain boundaries between 760°-650°C, due to very high super-saturation of austenite obtained due to large dissolved carbides.
If high toughness as well as fine grains are desired, or for multi-point tools, or for thin-edged tools, or tools subjected to high dynamic loads and are to resist chipping of cutting edges, or intricate shaped tools, or very large size tools of diameter more than 75-80 mm, then lower limit of hardening temperatures should be used. As the steel stock dimension increases, the amount of carbide segregation increases, which in turn lowers the temperature of incipient fusion (i.e., burning).
The service conditions and applications thus control to a large extent the hardening temperature. For example, tools for machining, i.e., turning and planning tools, or for rough milling should be hardened from the highest temperature in order to be sure that they attain highest hot-hardness, since the cutting edges in service may attain as high temperature as 600°C. Tools to be used at lower temperatures or that require good impact strength, such as cold-upsetting tools, can be hardened from temperatures as low as 1050°C. By this treatment, the resistance to tempering is reduced.
2. Process:
In annealed state, high speed steels contain carbides as much as -40% of structure (as in V-high speed steels), consisting of M23C6, M6C and MC (where M represents a metal atom) in ferrite matrix, where, M23C6 is principally chromium carbide (though it could also be iron-tungsten- molybdenum carbide), M6C, the most voluminous carbide is principally of tungsten-molybdenum, and MC is predominantly a carbide of vanadium (with tungsten and molybdenum in solution).
When high speeds are heated to hardening temperature, ferrite changes to austenite above about 840°C and carbides begin to get dissolved in austenite. Fig. 11.8 illustrates a pseudo-binary phase diagram of 18/4/1 steel.
This diagram illustrates some changes from Fe- cementite phase diagram such as:
(i) A1 (eutectoid temperature) increases from 727°C to 840°C.
(ii) Eutectic temperature is raised from 1147°C to 1330°C.
(iii) Eutectoid carbon decreases from 0.77% to 0.25%,
(iv) Maximum solid solubility of carbon in austenite decreases from 2.11%% to 0.70%.
Chromium carbides start dissolving around 900°C to get completely dissolved by 1100°C. Dissolution of M6C, tungsten- molybdenum carbide starts around 1050°C and by 1225°C, a larger proportion gets dissolved. VC hardly dissolves below 1200°C but does dissolve around and above this temperature.
The amount of undissolved VC depends strongly on the vanadium content of the steel. The amount of undissolved carbide is nearly constant after 10 minutes at the hardening temperature but generally soaking time at this temperature is less than that. Generally, undissolved carbides constitute 10% of the volume of steel.
Tool steels (except water hardening group) have high hardenability, and thus need not be quenched drastically. Though small tools may be air cooled from hardening temperatures but may precipitate proeutectoid carbides in temperature range 900-700°C in thicker steels at grain boundaries to increase brittleness.
Fig. 11.10 illustrates TTTdiagram of high speed steel (18/4/1), which illustrates a deep bay in the temperature range of 600-400°C in which austenite is stable, and cooling can be delayed here but the precipitation of carbides in temperature 900-700°C should be prevented by oil quenching or salt bath quenching i.e. tools may be quenched in warm oil, held there until they reach a temperature of about 500°C, and then cooled in air, or quenched in salt or lead bath at 500-600°C, and allowed to reach the temperature of the bath before cooling is completed in air.
Martensite starts forming at Ms temperature, which is dependent on the temperature of austenitising as the Ms temperature is lowered as more elements dissolve at higher temperature. It starts around 220°C (see Fig. 11.11) and 65 to 70% martensite forms as steel reaches room temperature, i.e., 20 to 25% retained austenite is obtained, which at this stage allows any straightening of parts to be required.
If the tool steels are given sub-zero treatment, i.e., if cooling is continued to -70 to – 80°C, then 5 to 6% of retained austenite may be obtained and the rest transforms to martensite and at around – 110°C, there may be reduction further of retained austenite by a maximum 1%, Tempering has to be done even after this treatment.
Tempering is then immediately done of the high speed steels, which on an average have 65-70% martensite, 20-25% retained austenite and around 10% undissolved carbides. The as-quenched hardness decreases as the tetragonality of martensite decreases and ε-carbide forms, and decreases further when total loss of tetragonality occurs and cementite forms. No martensite or bainite forms from retained austenite on heating. Alloy carbides starts forming around 450-500°C as below this temperature, alloying elements cannot diffuse sufficiently rapidly to nucleate carbides.
First precipitation occurs of carbides of chromium, and then above 550°C, (M2C) carbides of tungsten-molybdenum and then, MC, carbide of vanadium precipitates. The last two types of complex carbides in finely dispersed state are responsible for secondary hardening, which is equal or greater than as quenched hardness with improved toughness and wear resistance. Fig. 11.7 illustrates effect of tempering parameter on hardness of high speed steel.
3. Multiple Tempering:
In the first tempering, martensite decomposes and precipitation occurs of carbides in it as it is a supersaturated solid solution of carbon and alloying elements, but there is little tendency for retained austenite to transform due to deep bay in TTT curve of these steels. During first tempering, however, austenite is said to be ‘conditioned’ for at least some of it transforms to martensite on cooling from tempering temperature.
During conditioning, retained austenite loses carbon to other martensitic regions from where carbon has been depleted due to precipitation of alloy carbides, i.e., martensite becomes the sink of carbon.
The Ms temperature of retained austenite is increased, and thus on cooling, it transforms to martensite. This martensite mast be tempered by second heating to the same tempering temperature. Double tempering may not be sufficient in some cases, and thus 3 to 4 tempering are required to bring down the retained austenite to acceptable level.
Two microstructures of high speed steel (type 6-5- 4-1), where as-quenched state from 1230°C at x 1000 (nital) having carbides in martensite (not noticeable), the same steel after tempering at 565°C for 2 hours (nital etch and 1000 x ).
Tempering temperature of a tool must be adjusted to its toughness requirements (though hardening temperature too has to be adjusted accordingly). Cutting tools, for which the highest hardness is required, are invariably tempered at 550°C. Impact strength is highest when the steel is tempered in range of 250- 450°C and lowest at the temperature that imparts maximum hardness.
As red-hardness is further increased by adding cobalt and increasing vanadium content such as in high speed steel of M 42 grade, these are the best to cope with the high cutting speeds and high cutting-edge temperatures, which machining tools such as lathe and planning tools, are subjected to. In milling cutters, toughness also is important, and thus, it is hardened to obtain a hardness of 65 HRC and not 68 HRC.
High speed steels, though expensive, can work as hot-work tools for performing shear work as hot shears and punches in hot- punching machines that produce nuts (hardness is then 50-60 HRC). These steels are also used to some extent as punching and blanking tools.
Defects in Heat Treatment of High Speed Tool Steels:
i. Low Hardness:
If hardness of tools in below 60 HRC then the usual 64-62 HRC, the reasons could be:
(a) Very high austenitising temperature to cause grain growth, which results in coarser martensite to decrease the hardness. If burning occurs, then not only the hardness decreases, but tool is very, very brittle. Grinding cracks develop easily.
(b) Low austenitising temperature. As less carbides are dissolved in austenite, the secondary hardening effect gets reduced to decrease the hardness. Decarburisation should be avoided which can also cause lower surface hardness.
(c) Faulty grinding of tools- When heavy cuts are made in one pass, temperature during grinding may become much higher than even the highest tempering temperature. Coagulation of carbides (overageing) reduces the surface hardness.
ii. Cracks & Distortion:
The reasons could be:
(a) Very high rate of heating of tools.
(b) Over-heating and burning of steels to increase brittleness
(c) Rapid quenching- Large and intricate parts due to differential contraction and expansion may develop cracks and distortion.
(d) Cracks due to grinding due to use of unsuitable grinding wheels, either too hard, or glazed, or due to too deep a cut, or improper tempering was done so that untempered martensite was present, or larger amount of retained austenite was present which by the heat, expanded non-uniformly due to transformation to martensite.
(e) Due to stress raisers (faulty tool design) such as sharp edges in tooth roots, key ways, etc.
(f) Decarburised surface layers get stressed when central part gets hardened. The surface is having lower strength due to loss of carbon and may give in.