Tool steels:

Tools steels are special steels developed to form, cut or change the shape of a material into a finished or semi-finished product.

Properties of tool steels

• Slight change for form during hardening.
• Little risk of cracking during hardening
• Good toughness
• Good wear resistance
• Very good machinability
• A definite cooling rate during hardening
• A definite hardening temperature
• A good degree of through hardening
• Resistance to decarburization
• Resistance to softening on heating (red hardness)

Carbon Tool steel:

 It is used for light sections since hardness is low and it is brittle at high temperature.

0.6 - 0.75 % Carbon: Used for machine parts, chisels, set screws.

0.75 - 0.9 % Carbon: Forged dies, hammer sledge

0.9 - 1.1% Carbon: Drill, cutters, saw, shear blades, heavy duty cutter.

1.1 - 1.25% Carbon: Small drills, lathe tools, razor, light duty cutting edges where extreme hardness and sharpness is necessary.

They do not hold their hardness at elevated temperatures. Above 300° F, they soften rapidly.

Alloy Tool Steels:

They are employed by tool manufacturers when the tool life provided by carbon steel is insufficient.

1) Low alloy steels retain high hardness at temperatures up to 250°C.

2) Medium and high alloy steels, i.e. high speed steels retain high hardness at temperatures up to 620°C. They acquire high cutting properties only after suitable heat treatment.

Alloy tool steels are smelted in open-hearth and electric furnaces and belong to high quality classes.

Manganese Tool and die steels: They are oil hardened and non-deforming. Thus close dimensional tolerance can be maintained and tendency of cracking minimized.

Chromium Tool and die steels: They are divided into 2 groups. The low chrome steels are as much as plain carbon tool steels with chromium added to produce hardenability and toughness and some vanadium for grain size.

High Chromium group may be oil or air hardened. These steel must be annealed before they can be machined. After machining, they are hardened and drawn in the usual manner. They return their hardness up to 800° F. It is used for tool and dies which remain hard at elevated temperatures.

Forging dies, die casting die blocks and drawing dies are of this material.

Classification of Tool steels

The Joint Industry Conference (JIC), USA has classified steels as follows:

AISI Designations 

Types of Tool Steels

1)  Water Hardening Steels

Water hardening tool steels are widely used because of their low cost, good toughness and excellent machinability. They are shallow-hardening steel unsuitable for non-deforming applications because of high warpage, and possess poor resistance to softening at elevated temperatures. W grade high carbon plain carbon steels, Water-Hardening Tool steels include all class W tool steels, and while they do not retain hardness well at elevated temperatures, they do have high resistance to surface wear. Typical applications include blanking dies, files, twist drills, shear knives, chisels, hammers, forging dies, taps, countersinks, reamers, jewelry dies, and cold-striking dies.

Advantages:

•  Account for a large percentage of all the tool steels

•  Least expensive.

Disadvantages

• Usually the parts are quite small

• Not used in severe usage or elevated temperatures.

• Because their hardenability is low, they should be used only for thin sections.

• They are brittle, especially at their higher hardness.

• Prolonged exposure to temperatures over 300F usually results in undesired softening. Typical uses depending on the carbon content

• 0.60-0.75% carbon: medium hardness with good toughness and shock resistance. Examples: machine parts, chisels, setscrew

• 0.75-0.90%- forging dies, hammers, sledges

• 0.90-1.1% - general purpose tooling - good wear resistance and toughness. Examples of drills, cutters, shear blades, heavy duty cutting edges.

• 1.10-1.30% extremely hard, but little toughness. Examples are small drills, lathe tools, razor blades, and other light duty applications.

2) Shock Resisting steel

Shock resisting tool steels contain combination of chromium-tungsten, silicon-molybdenum, or silicon-manganese. These have good hardenability with outstanding toughness and wearing qualities. They are among the toughest of the tool steels, and are typically used for screw driver blades, shear blades, chisels, knockout pins, punches, and riveting tools. The most common type has 0.6% carbon, and tungsten, chromium, or vanadium.

Advantages

•  Low carbon content for toughness, but the alloys have carbide for good abrasion resistance, hardenability, and hot-work.

Disadvantage

• It has a tendency to distort easily, which can be minimized by oil quenching.

Typical Uses

•  Hot and cold impact use

3) Cold-work tool steels

It is further classified as oil-hardening; medium-alloy air hardening; and high carbon, high chromium. These possess high wear resistance and hardenability, develop little distortion, but at best are only average in toughness and in resistance to heat softening. Machinability varies from good in the oil-hardening grade to poor in the high-carbon, high-chromium steels

Air hardening Steels

Typical examples of these types of tool steel are grades ‘W’ and 'D' of AS1239.

Heat Treatment

These steels require adequate preheat at 780°C prior to austenitising and hardening is generally affected by still air cooling. Larger sections may need to be cooled in an airblast to achieve maximum hardness.

Tempering

These steels should be tempered when cooled to a handwarm condition and multiple tempering is sometimes necessary to achieve complete transformation and maximum toughness commensurate with hardness.

Air Hardening Hot Work Steels of H13 Type

These steels may be air hardened in sections up to 60mm. Above this thickness, whilst full hardening will occur, carbide precipitation at grain boundaries wilt lead to poor tool life and low impact strength.

Heat Treatment

The preferred procedure is to quench into a fluidised bed furnace or salt bath held just above the Ms point. This allows the cooling rate to miss the critical areas of the ‘S’ curve where carbide precipitation occurs. The tool is allowed to equalise at temperature in the quenching bath and then is removed and still air cooled to handwarm (approximately 50 - 60°C) for tempering.

Tempering

These steels must be adequately preheated at 650°C and 850°C prior to austenisation and soaking at 1010°C. As these steels are subject to secondary hardening effects, the maximum hardness is not achieved until the first temper has been carried out at 550°C. Subsequent multiple tempers are necessary to complete transformation of a sluggish austenite and achieve the desired working hardness.

Air Hardening High Speed Steels as1239 Grades 'T' and 'M'

Light section tools made from high speed steel may be satisfactorily quenched by air cooling although with flat tools it may be necessary to air harden between plates to minimise distortion. HSS may be quenched in a salt bath or fluidised bed furnace at 550°C, allowed to equalise and then still air cooled to handwarm prior to tempering. HSS is a secondary hardening steel achieving maximum hardness after the first temper. A second or third temper is necessary to reduce the hardness to the desired working level.

 Oil Hardening Steels

An example of oil hardening tool steel is AS1239 grade S1A-5 which is hardened from 800 – 840°C by quenching into oil.

Applications

This steel is normally used for heavier section punches than the ‘W’ series tool steels and possesses good dimensional stability.

Heat Treatment

Preheating at 650 – 700°C is recommended to allow the tool to equalise at a sub critical temperature prior to raising to the austenitisation temperature. This procedure helps to maintain dimensional stability.

Tempering

Tempering is recommended in the range 170 – 200°C which will give harnesses in excess of 60HRc. Tempering in the range 250 – 350°C can result in a reduction of impact strength.

4) Hot work Steel

Hot work tool steels (either chromium based or tungsten based) possess fine non-deforming, hardenability, toughness and resistance to heat softening characteristics, with fair machinability and wear resistance. These are used in blanking, forming, extrusion and casting dies, hot blanking dies, hot punching dies, forging and die-casting dies, where temperature may rise to 540° C

5) High Speed Steel:

They are used for cutting of metals where hardness must be retained at elevated temperature. Sir Robert Mushat in 1868 discovered that steel containing 2.2% carbon, 2% manganese, 5% tungsten were self hardening when cooled in air from high temperature.

In 1898, Frederick W. Taylor & Maunsel White found that steel retain hardness at elevated temperatures if high tungsten - high chrome steel is annealed at 1550 - 1650° F, and heated rapidly to 2250 - 2350° F, followed by quenching usually in oil but sometimes in air, then drawn by heating at 1050 - 1,150° F and then cooled in air.

Commonly used High Speed Steel is 18-4-1. Another type of High Speed tool steel has cobalt added to improve red hardness called Super High speed Steel. Its toughness is less than that of 18-4-1 type and is rather difficult to forge.

Properties of High speed steel

1. Excellent red hardness
2. Good wear resistance
3. Good shock resistance
4. Fair Machinability
5. Good non-deforming property
6. Poor resistance to decarburization.

Types of High Speed steels

There are two main types of high speed steels

1) Tungsten base

2) Molybdenum base

Molybdenum base is more commonly used because it is cheaper than tungsten base. Besides tungsten and molybdenum as the primary heat resisting additive, some other elements are also present in high speed steel. Carbon for high hardness, chromium for ease of heat-treating, vanadium for grain refining and cobalt for hardness and resistance to heat softening.

Molybdenum High Speed Steel:

It is developed to reduce the amount of tungsten and chromium required in steel. It is not as suitable as 18-4-1 or cobalt type but for many jobs perform nearly as well. It has excellent toughness and cutting ability. It has 6 per cent molybdenum, 6 per cent tungsten, 4 per cent chromium and 2 per cent vanadium.

Cobalt High-Speed Steel:

This is known as super high speed steel. Cobalt is added from 5 to 8 percent to increase hot hardness and wear resistance from 18:4:1 type.

Vanadium High-Speed Steel:

This steel contains 0.7 per cent carbon and more than 1 per cent vanadium. It has excellent abrasive resistance and is superior to 18:4:1 type for difficult to machine materials.

Effect of Alloying elements on High speed steel

Carbon produces carbides and a hardenable matrix. Melting point is decrease with increase in carbon content. A low carbon content increase the impact strength but reduces the matrix hardness. Chromium reduces tendency to scaling. It is mainly present in the ferritic matrix and is largely responsible for the air hardening of High speed steel. Vanadium increases the abrasion resistance, cutting quality of the tools and the tendency to air hardening. Tungsten provides hot hardness by forming carbides and form-stability. Molybdenum increase hardenability, while cobalt improves hot hardness and makes the cutting tool more wear resistant.

Application of High speed steel:

1) They are used for making all types of cutting tools such as drills, taps, reamers, milling cutters, broaches, power-saw blades, lathe, shaper and planer tool bits etc.

2) They are used for making forming dies, inserted heading dies, knives, chisels, high temperature bearings and pump parts

Ultra high speed steels have longer tool life, can do severe cutting, can cut even gritty material containing hard particles and posses higher cutting efficiency. They maintain a very fine cutting edge. They contain large amount of vanadium (upto 4%) and cobalt ( 5-12%)

6) Special purpose tool steels

Special purpose tool steels are comprised of the low-carbon, low-alloy, carbon-tungsten, mould and other miscellaneous types.

Spring Steels

This type of steel is used in manufacturing of springs. Steel is supplied in the form it requires no heat treatment except perhaps a low temperature annealing to relieve forming stresses. The spring wire has Brinell hardness of 350 -400. Steel for both helical and flat springs, which is hardened and tempered after forming, is supplied in an annealed condition. For small springs, plain carbon steel can be used. For large springs, alloy steel can be used.