Machinability of Tool steels

-Gaurav Bhati

10BME037

Machinability

• Machinability of an alloy is similar to the palatability of wine -easily appreciated but not readily measured in quantitative terms

• Machinability of a work material can often be measured in terms of the numbers of components produced per hour; the cost of machining the component; or the quality of the finish on a critical surface.

Many grades and compositions of tool steels are known. Most tool steels

can be classified according to the American Iron and Steel Institute

(AISI). According to the classification the tool steels are divided in the

following groups:

• Water-hardening tool steels

• Shock-resisting tool steels

• Oil-hardening cold work tool steels

• Air-hardening, medium alloyed cold work tool steels

• High-carbon, high-chromium cold work tool steels

• Mould steels

• Hot work tool steels, alloyed with chromium, tungsten, and/or

molybdenum

• Tungsten high-speed tool steels

• Molybdenum high-speed tool steels

• Some of the new high performance tool steels are difficult to machine.

• Machining dominates the cost in tool production as shown in chart. Hence, enhanced machinability would reduce the cost of machining operations through less cutting tool consumption, power consumption and operation time.

• Machinability of tool steels is influenced by many factors, such as chemical composition, microstructure, inclusions and thermo-mechanical properties.

65%

20%

5%

10%

Cost distribution for manufacturing of adie casting tool

Machining

Work material

Heat treatment

Assembly and adjustment

Combined effect of alloying, heat treatment, stress, and temperature:

• To permit higher removal rates, steel work materials are often heat treated to reduce the hardness to a minimum.

• The heat treatment for medium or high carbon steel often consists of annealing just below the transformation temperature (about 700 C).

• This “spheroidizes” the cementite – the form which it has least strengthening effect.

• For some operations a coarse pearlite structure is preferred. This structure is obtained by a full annealing treatment in which the steel is slowly cooled from above the transformation temperature.

Effect of size and distribution of carbides

• The HIGH CARBON and alloy contents that make tool steels serviceable as tools also make them more difficult to machine than the lower-carbon and the lower-alloy constructional steels.

• Several of the alloying elements used in tool steels, especially chromium, tungsten, molybdenum and vanadium readily form carbides that have adverse effect in machining.

• These effects are markedly influenced by the size, shape, and distribution of the carbide particles in the matrix of steel.

Effect of properties of ferrite matrix

• Most tools steels are easiest to machine when they have been annealed to a microstructure that consist of small spheroidal carbides uniformly distributed in a matrix of ferrite.

• In tool steels containing less than about 0.75% C, the spheroids are more likely to become large and widely dispersed in relatively large areas of ferrite.

• Carbides dispersed in this manner cause poor finish and low tool life.

• If the alloy content of the steel is low, the ferrite is characteristically gummy;

• and if the alloy content is high, the ferrite is tough.

• The preferred structure usually is a mixture of spherodite and lamellar pearlite obtained by controlled annealing.

• As the carbon content is increased to approximately 1% in unalloyed tool steel, the spheroid become finer, more numerous , and more closely spaced.

• To provide a basis for comparing the relative machinability of different types of tools steel, carbon tool steel containing 1% carbon is rated 100;

• Other tool steels are rated as a percentage of 100, as shown in table

Approximate machinability ratings for annealed tool steels

Steel or group (a) Machinability rating (b)

W…………………………………………………………………..100

S…………………………………………………………............60-70

A (except A7)…………………………………………………45-60

D, A7……………………………………………………………..30-40

H10-H19………………………………………………...........60-70

H20-H43………………………………………………...........45-55

M2, T1…………………………………………………............40-50

TURNING

• Tools made from high speed tool steel, carbide, and cast Co-Cr-W alloy are all used for turning tool steels.

• For continuous cutting with sufficiently rigidity to prevent chatter, carbide tools usually provide the greatest productivity at the lowest tool cost per piece.

• However, for shock loads (as in interrupted cutting), for the turning of forged drill blanks, or for setups lacking rigidity, high-speed tool steel tools are most suitable than carbide,

• Cast alloys tool are specially suited for turning application in which the use of cutting fluids is impractical.

• Indexable coated carbide, cubic boron nitride (CBN), and ceramic

tooling inserts have also been used for turning because of their

higher resistance to wear and hence longer tool life.

• Si3N4 – base ceramics are unsuitable because of chemical

interactions that occur at the cutting interface.

• Cubic Boron Nitride Tools It is now possible to turn and face mill almost any type of tool steel in the fully hardened condition. Typically 63 to 65 HRC with CBN.

• Tool angles: Many form tools are difficult to grind because of their shape or because they must maintain high accuracy.

• It is economical to use slower speeds and lighter feeds to prolong the life of such tools.

• Conversely, when the maximum rate of metal removal is the primary objective, as in rough turning, tool grinding is usually simpler.

• For these condition it is more economical to remove metal at the maximum rate and grind tools more frequently.

Drilling

• Most tool steels, particularly those with a carbon content of 1% or greater, respond best to drilling when the microstructure consists of ferrite and small spheroidized carbides.

• However, the smallest carbides should be large enough to be resolved clearly at a magnification of 1000X.

• For drilling more highly alloyed tool steels, such as high speed steels ( and particularly the high carbon high vanadium types) special drills are used.

• These highly alloyed tools are not only harder (often up to 280 HB ) but also more abrasive.

• Nitriding the drills will prolong the drill life.

• Drill points with the conventional included angle of 118° and normal lip relief angle (about 12° for a 13 mm or ½ inch diamdrill) are suitable for drilling tool steels of the W, O and L groups.

• However, drill points with a 130 included angle and lip relief about 2/3 the normal angle are some suitable for drilling steels of the D and A groups and the high speed steels, particularly when hardness of the work material is 260 HB or higher.

• A 135° crankshaft point is used on high speed tool steel of high hardness, such as M42 and T15.

Cutting Fluid

• Water-soluble or sulfurized oil should be used as a cutting fluid when drilling tool steels.

• The cutting fluid should be directed onto the cut, and the rate of floew must be sufficient to cool the drill point.

• In the vertical drilling of deep holes, an intermittent feed is best; that is the drill should be withdrawn at regular intervals to remove chips, cool the point and allow more cutting fluid to reach the cut.

• This is termed peck feed.

• In horizontal drilling, the cutting fluid is directed along flutes of the drill to the point.

• Intermittent withdrawal of the drill flushes away chips and allows the drill to cool.

• For deep hole drilling, regardless of workpiece composition, oil-hole drills should be used to permit a continuous supply of cutting fluid at the critical area.

Milling• In the production milling of flutes in carbon tool steel drills

rigidity of the milling machine and setup is essential.

• However, even with required rigidity, excessive variation in drill web and land dimension may be experienced if the steel is not properly annealed.

• Specifically, the presence of lamellar pearlite in the annealed microstructure is objectionable because it may contribute to large variation in web and land dimension and tearing of the surface matal.

• Because it is extremely difficult to produce lamellar peralite in suitably small amounts during annealing ,

• It is necessary that the annealed structure be completely spherodized.

Face Milling

• After being cold sawed to desired lengths, various sizes of hot rolled sections of W2 tool steel ate to be face milled.

• In milling a mixture of steel sizes clamping problem generally encountered.

• This is eliminated by equipping machine table with a magnetic chuck.

• However, the rough surface of the hot rolled sections not always make full contact with the surface of chuck.

• The feed rate has to be varied according to the operator ‘s judgment .

• Sometimes backup blocks had to be placed around the workpiece to prevent movement.

Hot Work Tool Steels• Machined in annealed condition

• Type H11 is used for structural parts in aerospace application.

• Such parts have to be machined in the quenched and tempered condition at higher hardness.

High-Speed Steels• Machinability of more highly alloyed high speed steels

decreases as hardness and abrasiveness increases.

• Especially true of the high carbon high vanadium types.

• Turning has to be done in annealed condition.

Tool steel Gears

• Gear that function in a hot or abrasive environment are sometimes made of a hot-work tool steel such as H11, H12, or H13.

• Milling is the gear production process most used.

• One of three procedures is followed, the choice of procedure depending largely on whether the primary requirement is abrasion resistance or heat resistance in service.

• One procedure is to mill the gear teeth in the fully annealed condition (200 to 250 HB)

• Followed by quenching and tempering to approximately 50 HRC.

• If this procedure causes excessive distortion, a final grinding operation can be included.

• When the gear must resist abrasion, common practice is to quench and temper the blanks to 325 to 375 HB, mill the teeth, and then nitride the machined gear.

• In some instances it is desirable to mill the teeth at higher hardness and employ no further finishing operations.

Grinding

• The wide difference between in composition among tool steels give rise to wide variation in grinding characteristics.

• The grindability index of a material is a measure of the case of removing stock by grinding, expressed in terms of wheel wear.

• Numerically it is equal to the grinding ratio obtained under a specified set of grinding condition, the grinding ratio being the volume of work material removed per volume of wheel wear.

• The grindability index is less of a factor with easy to grind materials.

• It is much more important when difficult-to-grind tool steels are considered.

• However, the development in CBN wheels with a resinoid or vitrified matrix and in metal-plated monolayer CBN wheels has advanced tremendously.

• This superabrasive has a very low wear rate and has increased both the metal removal rates and the wheel life significantly in many applications.

• Specifically form wheels with CBN abrasives have gained wide acceptance in numerous form tools for grinding (drills, end mills, and so on).

References

• ASME Handbook

Machining of Tool Steels

• Metal Cutting

By Paul K. Wright, E M Trent

• Uppsala dissertations “On the Machinability of High Performance Tool Steels”

By Natalia Sandberg