CHAPTER 2 LITERATURE REVIEWshodhganga.inflibnet.ac.in/bitstream/10603/16955/7/07... · 2018-07-09 · basic steps: (1) powder manufacture, (2) powder blending,(3) compacting, (4)

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CHAPTER 2

LITERATURE REVIEW

2.1 HIGH STRENGTH LOW ALLOY STEEL

High Strength Low Alloy (HSLA) steel is a type of steel alloy that

provides better mechanical properties or greater resistance to corrosion than

carbon steel. HSLA steels vary from other steels in that they are not made to

meet a specific chemical composition, but rather to specific mechanical

properties. They have carbon content between 0.05–0.5% to retain formability

and weldability. Other alloying elements include up to 2.0% manganese and

small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium,

molybdenum, titanium, calcium, rare earth elements, or zirconium

(Huppmann and Mhirschragel 1976, Peter Lee and Howard Kuhn 1987).

Copper, titanium, vanadium, and niobium are added for strengthening

purposes (Peter Lee and Howard Kuhn 1987). These elements are intended to

alter the microstructure of carbon steels, which is usually a ferrite-pearlite

aggregate, to produce a very fine dispersion of alloy carbides in an almost

pure ferrite matrix. This eliminates the toughness-reducing effect of a pearlitic

volume fraction, yet maintains and increases the material's strength by

refining the grain size, which in the case of ferrite increases yield strength by

50% for every halving of the mean grain diameter. Precipitation strengthening

plays a minor role, too. Their yield strengths can be anywhere between 250–

590 MPa (36,000–86,000 psi). Due to their higher strength and toughness

HSLA steels usually require 25 to 30% more power as compared to carbon

steels (Peter Lee and Howard Kuhn 1987).

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Research on the development of new high-strength, high-toughness

and corrosion-resistant steels with outstanding fracture toughness and tensile

strength combination for landing gear materials has been a subject of intense

interest in the recent times. Improved Ni-Co, low carbon steels (most notably

Aermet100 and AF1410), have excellent combinations of properties and are

developed to the point where they are now being specified as replacements for

the standard landing gear steels 300M and 4140 (Wisker and Jones 1974).

These improved steels are used for landing gear on carrier-based

aircraft because they exhibit excellent damage tolerance and environmental

resistance. The steels can also find application as attach fittings, horizontal

stabilizer spindles, arresting-hook shanks, and catapult hooks. Other

aerospace applications under consideration include rotorcraft actuators and

masts, gas turbine engine shafts, and rocket motor casings. Non-aerospace

applications include ordnance, armor, high-strength fasteners, pump splines,

and automotive drive shafts (Huppmann and Mhirschragel 1976, Huppmann

1977).

The improved combination of strength, damage tolerance, and

stress corrosion cracking resistance provides significant benefits for

applications under severe service conditions, such as the naval aircraft

environment. An additional benefit includes fatigue strength superior to 300M

steel. These materials have good weldability because of low carbon content.

Testing has shown that UHSS (Ultra High Strength Steel) can be welded,

without preheat, with joint efficiencies approaching 100 percent.

It appears that the strength of these new steels cannot increase to

higher levels without corresponding decreases in ductility and toughness

(Kotschy et al 1976). However, increased strength can be achieved while

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keeping toughness at levels acceptable for many applications. Such a balance

of properties may be acceptable for landing gear for civil aircraft.

It has been observed that, as strength increases, toughness

decreases, therefore toughness of ultra high strength steel is an important

consideration. These super alloys meet these requirements offering excellent

fracture toughness and strength. The ultra high strength of this steel is

attributed to the presence and distribution of the fine second phase particles

such as (MoCr) 2C, (MoFe) xC, (Fe,Ni,Co) 3C (Antes 1971). Furthermore,

heat treatment, grain size, precipitates; dislocation structures size-shape and

distribution of inclusions and compositions of matrix can generate variations

on material properties. All of these parameters are very important in order to

enhance the toughness of steel. Large second phase particles (carbides) and

less distance between them can produce detrimental effect in toughness and

tensile strength of the material. These microstructural characteristics can

enhance the crack tip blunting and improve the resistance to fatigue crack

growth (Antes 1974).

Copper, silicon, nickel, chromium, and phosphorus are added to

increase corrosion resistance. Zirconium, calcium, and rare earth elements are

added for sulfide-inclusion shape control which increases formability. These

are needed because most HSLA steels have directionally sensitive properties.

Formability and impact strength can vary significantly when tested

longitudinally and transversely to the grain. Bends that are parallel to the

longitudinal grain are more likely to crack around the outer edge because they

experience tensile loads. This directional characteristic is substantially

reduced in HSLA steels that have been treated for sulfide shape control (Peter

Lee and Howard Kuhn 1987).

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They are used in cars, trucks, cranes, bridges, and other structures

that are designed to handle large amounts of stress or need a good strength-to-

weight ratio (Peter Lee and Howard Kuhn 1987).

2.2 POWDER METALLURGY

Powder metallurgy is the name given to the process by which fine

powdered materials are blended, pressed into a desired shape, and then heated

to bond surfaces (Figure 2.1).

Figure 2.1 Chart showing P/M Component production

There are three main processing stages of P/M techniques. First, the

primary material is physically powdered, divided into many small individual

particles (William et al 2001). Next, the powder is injected into a mold or

passed through a die to produce a weakly cohesive structure (via cold

welding) very near the dimensions of the object ultimately to be

manufactured. Pressures of 10-50 tons per square inch are commonly used.

Also, to attain the same compression ratio across more complex pieces, it is

often necessary to use lower punches as well as an upper punch. Finally, the

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end part is formed by applying pressure, high temperature, long setting times

(during which self-welding occurs), or any combination thereof.

Powder metallurgy is the process of blending fine powdered

materials, pressing them into a desired shape or form (compacting), and then

heating the compressed material in a controlled atmosphere to bond the

material (sintering). The powder metallurgy process generally consists of four

basic steps: (1) powder manufacture, (2) powder blending,(3) compacting,

(4) sintering. Compacting is generally performed at room temperature, and the

elevated-temperature process of sintering is usually conducted at atmospheric

pressure. Optional secondary processing often follows to obtain special

properties or enhanced precision. Two main techniques used to form and

consolidate the powder are sintering and metal injection molding. Recent

developments have made it possible to use rapid manufacturing techniques

which use the metal powder for the products. Because with this technique the

powder is melted and not sintered, better mechanical strength can be

accomplished.

2.2.1 Production Aspects of HSLA Steel Alloy

Production aspects of HSLA steel involves the following steps

Powder Production

Compaction

Sintering

Secondary and other finishing operations

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2.2.1.1 Powder production

The raw material for P/M components is the powder. These

powders are engineered materials in the sense they are manufactured to

precise specifications to facilitate subsequent processing (Jena et al 2010).

The powders used in P/M can be pure elements, elemental blends or pre-

alloyed powders. Several production methods are available for making

powders like atomization, reduction of compounds, electrolysis, etc.

Most metals are fabricated by atomization techniques. The use of

molten alloy and high velocity water or gas jet provides a major amount of

powder (Misra et al 2008). The gas and centrifugal techniques produce

spherical powder, while water atomization gives an irregular shape. Particle

size is controlled by the energy delivery per unit volume of molten metal. The

greater the energy delivers to the metal stream the finer the resulting particle

size.

The mechanical method produces coarse, irregular or angular

powders via machining, milling, crushing or impacting routes. Due to

increased concerns of energy efficiency, the mechanical fabrication

approaches are not attractive. Probably, the one major exception in the

mechanical alloying approach to the formation of alloy dispersion

strengthened alloys relieves on repeated milling fracture welding events on a

macroscopic scale (Brown 1971). Any fusible material can be atomized.

Several techniques have been developed which permit large production rates

of powdered particles, often with considerable control over the size ranges of

the final grain population. Powders may be prepared by grinding, chemical

reactions, or electrolytic deposition. Powders of the elements titanium,

vanadium, thorium, niobium, tantalum, calcium, and uranium have been

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produced by high-temperature reduction of the corresponding nitrides and

carbides. Iron, nickel, uranium, and beryllium submicrometre powders are

obtained by reducing metallic oxalates and formates. Exceedingly fine

particles also have been prepared by directing a stream of molten metal

through a high-temperature plasma jet or flame, simultaneously atomizing and

comminuting the material. On earth various chemical- and flame-associated

powdering processes are adopted in part to prevent serious degradation of

particle surfaces by atmospheric oxygen.

2.2.1.1(a) Apparent Density

The mass of the unit volume of powder in the loose or unpacked

condition, usually expressed in g/cc. For powder metallurgy powders, it is

often 50% of the theoretical density for the materials.

Apparent density is not only of importance in P/M but also in other

fields such as point, pigments, chemical reagents and pyrotechnical

applications. The apparent density of a powder, sometimes called packing

density or loading weight, is defined as the mass per unit volume of loose or

unpacked powder. Thus it includes internal pores but excludes external pores.

It is governed by chemical composition, particle shape, size distribution.

2.2.1.1(b) Tap Density

The density of powder obtained when it is vibrated for a prolonged

period. The tap density represents the highest packing density possible for a

powder without the application of pressure.

Tap Density is always higher than the free flow apparent Density.

Tap density is the apparent density of the powder after it has been

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mechanically shaken down or tapped until the level of the powder no longer

falls .it appears to be widely used for storage, packing or transport of

commercial powder and also as a control test on mixed powders .since

tapping or vibration tends to segregate fines, different sized particles and

materials with different specific gravities, it interferes with the uniform

packing of particles.

2.2.1.1(c) Relative Density

It is a dimensionless quantity, as it is the ratio of either densities or

weights

R =

where, R is relative density, - Actual is the density of the substance being

measured and - theoretical is the density of the reference.

Relative density can be calculated directly by measuring the density

of a sample and dividing it by the (known) density of the reference substance.

The density of the sample is simply its mass divided by its volume. Although

mass is easy to measure, the volume of an irregularly shaped sample can be

more difficult to ascertain.

Relative density or specific gravity is the ratio of the density (mass

of a unit volume) of a substance to the density of a given reference material.

Specific gravity usually means relative density with respect to water. The

term "relative density" is often preferred in modern scientific usage.

If a substance's relative density is less than one then it is less dense

than the reference; if greater than 1 then it is denser than the reference. If the

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relative density is exactly 1 then the densities are equal; that is, equal volumes

of the two substances have the same mass. If the reference material is water

then a substance with a relative density (or specific gravity) less than 1 will

float in water. For example, an ice cube, with a relative density of about 0.91,

will float. A substance with a relative density greater than 1 will sink.

Temperature and pressure must be specified for both the sample

and the reference. Pressure is nearly always 1atm equal to 101.325 kPa.

Where it is not, it is more usual to specify the density directly. Temperatures

for both sample and reference vary from industry to industry. In British

brewing practice the specific gravity as specified above is multiplied by 1000.

Specific gravity is commonly used in industry as a simple means of obtaining

information about the concentration of solutions of various materials such as

brines, sugar solutions (syrups, juices, honeys, brewers wort, must, etc.) and

acids.

2.2.1.1(d) Green Density

Green Density is density of Green compact. The weight per unit

volume of an un sintered compact

In general, green density has been found to

(i) Increase of compaction pressure

(ii) The increase of particle size or apparent density

(iii) The decrease of particle hardness and

(iv) The decrease of compacting speed. Improvements in green

density may also be affected by employing smooth and

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regularly shaped annealed particles with high particle

densities.

In case of regular shaped compacts eg. Cube, cylinder tetragon, bar,

etc, the green density is readily determined from their weight and dimensions

.however, for compacts with irregular contours it measured by weighting in

air and water. In the latter case impregnation with oil or other lubricant is the

usual procedure transparent lacquer coating may also be employed but it is

not widely used.

2.2.1.1(e) Sintered Density

The methods used for determination of sintered density are similar

to those which have already been described for the determination of green

density. The property facilitates rendering of information or porosity of the

finished product. The mechanical properties, in general, increase with the

decrease in porosity of the sintered compact. It is expressed in g/cc.

2.2.1.1(f) Flow Rate

The time required for a powder sample a standard weight to flow

through an orifice in a standard instrument according to a specified procedure.

A related test used to determine the ease of flow of powders is the

flow time, measure the time taken for a known weight of powder sample to all

through a specially designed funnel called the Hall funnel.

The flow rate is a very important characteristic of powder which

measures the ability of a powder to be transferred .it is defined as the rate at

which a metal powder will flow under gravity from a container through an

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orifice both having the specific shape without bridge formation for obtaining

a rapid rate of production, consistent compacts and economy.

It is the amount of a substance which passes through a specified

surface per unit time. The flow rate can be based on mass (mass flow rate),

volume (volume flow rate) or mole (mole flow rate)

2.2.1.1(g) Aspect Ratio

The aspect ratio (i.e. height to diameter ratio) is the density

variations created by friction effects between the powder feedstock and the

tooling mean that, even with double-ended compaction, aspect ratio should be

limited to no more than 3:1.

2.2.1.1(h) Porosity

The amount of pores (voids) expressed as a percentage of the total

volume of the powder metallurgy part.

The presence of porosity has a much greater influence on the

elongation, and impact and fatigue strength and a rapid increase in these

values is obtained with the density approaching the theoretical. Porosity acts

as stress raiser and sintered component does not indicate truly elastic

behavior, rather it appears to function in a similar manner as graphite in cast

iron.

2.2.1.2 Compaction

Compaction of powder mixtures is generally carried out using dies

to close tolerances. Dies are made typically from die steels or cemented

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carbides. Equipment used for compaction includes mechanical or hydraulic

presses.

The main purpose of compaction is to

Consolidate the powder into desired shape

Impart desired level and type of porosity

Produce green compacts with sufficient strength to withstand

further handling operations

Die compaction represents the most widely used method and is

considered as the conventional technique. These involve rigid dies and special

mechanical or hydraulic pressures (Hirschhorn and Bargainnier 1970).

Density of up to 90% of full solid density can be achieved. Following the

compaction cycle the duration may be of order of just a few seconds for very

small parts. Mechanical process is now available in capacity up to 20 MN

force or several thousand tons force for hydraulic press: these capacities are

determined by the size of part that can be pressed to a given green density.

Powder compaction is the process of compacting metal powder in a die

through the application of high pressures. Typically the tools are held in the

vertical orientation with the punch tool forming the bottom of the cavity. The

powder is then compacted into a shape and then ejected from the die cavity.

In a number of these applications the parts may require very little additional

work for their intended use; making them very cost efficient.

There are four major classes of tool styles: single-action

compaction, used for thin, flat components; opposed double-action with two

punch motions, which accommodates thicker components; double-action with

floating die; and double action withdrawal die. Double action classes give

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much better density distribution than single action. Tooling must be designed

so that it will withstand the extreme pressure without deforming or bending.

Tools must be made from materials that are polished and wear-resistant.

Better workpiece materials can be obtained by repressing and re-sintering.

2.2.1.3 Sintering

In this process, heating of the 'green' compact is carried out in a

protective atmosphere within a furnace at a temperature below the melting

point of the base metal. These temperatures are generally 75% of the absolute

melting temperature. Sintering thus will be decreased in the number of

process, the pore shape will tend to become smooth and the grain growths will

take place. It is possible that some melting or liquid phase sintering of other

powder additives may take place during sintering.

‘Sintering’ is a method used to create objects from powders. It is

based on atomic diffusion. Diffusion occurs in any material above absolute

zero, but it occurs much faster at higher temperatures. In most sintering

processes, the powdered material is held in a mold and then heated to a

temperature below the melting point. The atoms in the powder particles

diffuse across the boundaries of the particles, fusing the particles together and

creating one solid piece. Because the sintering temperature does not have to

reach the melting point of the material, sintering is often chosen as the

shaping process for materials with extremely high melting points such as

tungsten and molybdenum. Sintering is traditionally used for manufacturing

ceramic objects but finds applications in almost all fields of industry. The

study of sintering and of powder-related processes is known as powder

metallurgy. A simple, intuitive example of sintering can be observed when ice

cubes in a glass of water adhere to each other.

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The process of sintering is generally the result of atomic motion

stimulated by the high temperatures. The initial strains, surface area and

curvatures existing in a pressed powder, compact drive the atomic motions

responsible for sintering (Kahlow 1971). Several different patterns of atomic

motion can contribute to the effect including evaporation and condensation,

volume diffusion, grain boundary and surface diffusion, and plastic flow. In

most cases, the sintering Kinetics is determined by several parameters

including pressed density, material, particle size, sintering atmosphere,

temperature and even the degree of sintering. As material transport takes

place, the geometric progression can be divided into a number of stages

representing the driving forces.

These are

Initial bonding among particles

Neck growth

Pore channel closure

Pore rounding

Pore shrinkage

Pore coarsening

To enhance the overall densification process, it is common to

impose an external force through pressure or to provide a high active kinetic

path through the addition of a second phase. Hot pressing techniques use

super imposed temperature and pressure effects to give rapid, controlled

densification. The form of hot pressing can be manifested as either uniaxial or

hydrostatic forces (Kaufman and Mocarski 1971). In recent, years hot forging

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in static pressure has been the most rapidly expanding commercial form of

pressure enhanced sintering.

Presence of a liquid phase during all or part of the sintering cycle of

material also represents and enhances sintering method. There are two

variations of the process.

(a) Normal liquid phase sintering, for which the formation of the

liquid phase is associated with one or more components

contained in the original green compact.

(b) Infiltration of the original green compact with a liquid formed

outside the compact during the very early period of sintering.

Although simultaneous infiltration in sintering appears to be

dominant, infiltration of a previously sintered material is also

practiced.

During liquid phase sintering, three stages such as rearrangement

'or' liquid flow 'accommodation' or 'dissolution and precipitation' and

'coalescence' or 'solid phase bonding' take place. These stages follow the

approximate order of their occurrence but there may be significant over

tapping of any specific system.

2.2.1.4 Secondary and other finishing operations

A sintered part may be subjected to one or more secondary or

finishing operations. These are used to ensure close dimensional tolerances,

improve surface finish, increase density and corrosion resistance. Typical

operations include repressing, grinding and plating.

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2.3 POWDER FORGING

As in conventional P/M, powder forging begins with custom-

blended metal powders being fed into a die, then being compacted into a

“green” shape, which is then ejected from the die. This compact, called a

“preform,” is different from the shape the final part will acquire after being

forged. Again as in the conventional P/M process, the green compact is

sintered (solid-state diffused) at a temperature below the melting point of the

base metal in a controlled atmosphere furnace, creating metallurgical bonds

between the powder particles and imparting mechanical strength to the

preform.

The heated preform is withdrawn from the furnace, coated with a

high-temperature lubricant, and transferred to a forging press where it is

close-die forged (hot worked). Forging causes plastic flow, thus reshaping the

preform to its final configuration and densifying it, reduces its porosity to

nearly zero (Koczak and Chung 1975).

Powder forging produces parts that possess mechanical properties

equal to wrought materials. Since they are made using a net-shape

technology, PF (Powder forged) parts require only minor secondary

machining and offers greater dimensional precision and less flash than

conventional precision forgings.

It is a process in which an unsintered, presintered, or sintered

powder metal preform is forged in a confined or trapped die. Usually, the

preform is heated, but powder forging may be performed using a preform in

the warm or cold state. The process is sometimes referred to as P/M (powder

metallurgy) forging, P/M hot forming, or simply by the acronym P/F. When

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the preform has been sintered, the process is often referred to as “sinter

forging.” Powder forging is a natural extension of the conventional press and

sinter (P/M) process, which has been recognized as an effective technology

for producing a great variety of parts to net or near-net shape. In essence, a

porous preform is densified by forging in a single blow. Heat facilitates the

densification and deformation aspects of the process in producing a net or

near-net shape. Typically, forging is performed in heated, totally enclosed

dies, and virtually no flash is generated.

This contrasts with forging of cast/wrought metals where multiple

blows are often necessary to form a forging from bar stock and considerable

metal is wasted in the form of flash. The shape, quantity, and distribution of

porosity in P/M and P/F parts strongly influence the resultant mechanical

performance. Powder metallurgy parts have porosity levels of 10 to 20 vol%.

The aim in powder forging parts is to eliminate porosity, especially in the

most highly stressed regions of a part. Any remaining porosity in a P/F part is

referred to as residual porosity. Powder forging is a deformation process that

aims at achieving or exceeding wrought properties through the elimination of

porosity. There are two basic forms of powder forging: Hot upsetting, in

which the preform experiences a significant amount of lateral metal flow

during forging Hot repressing, in which metal flow during forging is restricted

so that the process is similar to a hot coining process .In hot upset powder

forging, the extensive unconstrained lateral flow of metal results in a stress

state around collapsing pores that is a combination of normal and shear

stresses. The combination of normal pressure and shear stress allows this

method of P/F to be applied to hot, warm, and cold states. A spherical pore

becomes flattened by the normal pressure and elongated in the direction of

lateral flow. The shear stress across the contacting pore surfaces acts to break

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up any residual interparticle oxide films and leads to strong metallurgical

bonding across collapsed pore surfaces.

This enhances dynamic properties such as fracture toughness and

fatigue strength. The lateral flow also imparts some mechanical anisotropy

due to grain flow. The stress state during hot repress powder forging consists

of a small difference between vertical and horizontal stresses, which results in

very little metal movement in the horizontal direction and limited lateral flow.

As densification proceeds, the stress state approaches a pure hydrostatic

condition. A typical pore flattens, and the opposing sides of the pore are

brought into contact under pressure. Hot repress forging requires higher

forging pressures than does hot upset forging to achieve a comparable density

level. The decreased interparticle movement compared with upsetting reduces

the tendency to break up any interparticle oxide films, and this may result in

lower toughness and fatigue strength. Mechanical anisotropy is minimized,

however. Most P/F processes incorporate some of each forging mode. The

design of the preform shape, however, is decidedly different for the two

forging modes.

Parts fabricated through the PF process are subject to certain

limitations. Tooling and the maximum press tonnage capabilities impose size

and shape constraints on parts, just as in impression die hot forging. Annual

production quantities in excess of 25,000 pieces are typically required to

amortize the development costs of tool set-ups and maintenance. Finally,

material systems are somewhat limited (all commercial PF products are steel).

Typical Powder Forged Products—fly wheel, crank shafts, cams,

bearing races, transmission components and so on.

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Typical Markets Using Powder Forged Parts—automotive,

truck, air craft equipments, power tools and so on.

Production of metal components via P/M techniques embraces the

high rate of conventional processes with low losses due to negligible

machining requirements and excellent material utilization. However, the

attained density in P/M products is the predominant factor while considering

their performance aspects. Shaping powder preforms to different desired

shapes with high fractional theoretical densities had been the attempt of

investigators in the past (Ramakumar et al 1988), but, 100% dense

components could not be produced. Associated reasons for the above being

the parts of the components in contact with die or punch surfaces do exhibit

particulate structure indicating the existence of dead metal zones where the

flow of the material or porosities for all practical purposes is nil (Skelly

1977). The resultant densification in these proportions of the component is

negligibly small and hence with certainty it cannot be claimed that cent

percent dense products can be produced through P/M route unless otherwise

combinations of two, or more than two, deforming processes are adopted, i.e.,

one following the other.

It has been well established that the pores present in the sintered

P/M products act as the sites of weaknesses from where the cracks initiate

during the application of these components in service and the crack initiation

and its propagation would depend upon the applied load and therefore, the

pore closure is the necessity to achieve full density (Bernd Gunther 1999,

Antes 1972, Zygmunt Marclesa et al 1988). Hence, the secondary deforming

operations become essential to attain full density and these processes include

rolling, forging, and extrusion forging (Pandey et al 1986). However, the

applications of shear type of loading on densification are highly pronounced

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when the same is superimposed on hydrostatic loading as demonstrated by

Koerner (Kuhn and Downey 1974). The actual stress and strain data on

densification have been reported elsewhere. Material and void response

during powder preform forging are reported by Kuhn and Kahlow

respectively. Kuhn et al (Kuhn and Downey 1971) have presented a

comprehensive application of preform design for powder preform forging and

the structural changes during densification are described elsewhere. Basically

two modes of powder preform forging is carried out, one involving

compression without inducing any change in the cross-section right from the

initial to the final part is called repressing and the other involving

compression of a relatively simple shaped preforms into a complex product is

addressed as true forging. The first part of the argument involves densification

with no lateral deformation while the second part involves densification as

well as shape change simultaneously through a large degree of plastic

deformation coupled; with a good level of lateral flow of material. However,

in true forging, the lateral flow subjects the pores to experience the

combination of normal and the shear stresses. The shearing action assists in

the pore closure and thereby reducing the pressure required for the

densification. In other words, the combined action of the hydrostatic and

deviatoric components of the stress plastically deforms the void/pores. The

hydrostatic stress promotes the shrinkage of the pores so as to bring the

internal surface of the pores into intimate contact. This phenomenon enhances

the bond strength between the adjacent particles and produces a sound

metallurgical structure.

The important feature that distinguishes the deformation behaviour

of porous material from the fully dense material is the continued change in

volume, which affects the Poisson's ratio (Koermer 1971). For all porous

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materials, the Poisson's ratio has been reported to be always less than half

with the inherent tendency due to mass constancy principle and the variable

volume. It has been shown that Poisson's ratio was influenced by the current

density of the deforming preform and not the initial density. However, the

cold deformation enhances the density as well as strength and hardness with

the resultant drop in ductility. The excess of cold deformation, i.e., beyond

40-50 % of height reduction would result into the appearance of cracks on the

free surfaces of the deforming preform. Similar results have been reported

during hot axial deformation of ferrous P/M preforms and the same are

described elsewhere (Lee Peter and Kuhn Howard 1987). Therefore, it is

obvious to introduce intermediate annealing operation to soften the cold

worked P/M preforms in order to avoid early cracking and also to induce

sufficient ductility.

Friction plays a critical role in any deformation process. Under the

frictional constraints, the P/M preforms deform in-homogeneously, which

results in barreling. Tearing and cracking are caused by high frictional

constraints. However, the detrimental effects of friction can be reduced by the

application of lubricants like graphite, molybdenum - di-sulphide and zinc

stearate etc., but, the beneficial effects of lubrication are reduction in

barreling, improved surface finish, and decrease in the wear and tear of the

die. In the present investigation, an attempt has been made to study the

behavior of sintered HSLA P/M steels under the unlubricated conditions.

Some important literature on cold deformation of sintered P/M preforms can

be found elsewhere.

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2.4 DEFORMATION PARAMETERS

Deformation parameters are basically the forging temperature,

deformation rates and contact time of the work piece with the tools. In the

present investigation, it is cold deformation. Therefore, temperature is

ambient. And in cold deformation the parameters, which influence

deformation of porous materials, are the following:

1. Preform material

2. Initial preform density

3. Sintering temperature and time maintained

4. Powder characteristics by which the porous body was made

5. Frictional constraints employed

6. The preform aspect ratio, and

7. Mechanism of deformation

2.5 MECHANISM OF DEFORMATION

Deformation processes on sintered powder material typically result

in homogeneous deformation, i.e., the degree of deformation varies from

point to point throughout the material. Non-uniform distributions of density

usually occur and stress states involving tensile stresses may develop, which

increases the potential for fracture (Luo Liao and Tsai 1999). Each of these

detrimental effects can be minimized by careful control of the deformation to

the final shape. This is accomplished through proper design of the processing

details such as preform shape, friction conditions and the die shape. When a

material is subjected to a metal forming process, its properties can be

improved or be degraded depending on the original properties, the amount of

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deformation and the mean pressure accompanying deformation. Perfectly

dense material cannot be made denser by forming, it can only deteriorate.

High receiving pressure can minimize the deterioration however. Normally,

porous material undergoing gross plastic deformation may increase or

decrease in density. Logic demands and experience shows that with the

exception of strain induced transformation of material, an increase in density

is associated with a closure of pores. Similarly, a decrease in density is

associated with an increase in the size of existing pores and/or with the

creation of new pores. As pores become larger, or as new pores are introduced

during the deformation process, they decrease the ductility of the product and

eventually cause fracture by coalescence of the voids. However, if pressure is

appropriately applied, the deformation process can also close the existing

pores while improving the ductility of the product (Mocarski and William

Hall 1987). Powder metallurgy preforms fall in to the third category, very

porous materials. The larger the reduction, the denser becomes the product.

However, beyond a certain reduction, a drop in density is feasible. When the

density of a material decreases at high reductions, it is assumed that the flow

pattern of the material, which permits void creation and/or growth, is

energetically more favorable than sound flow with no void formation.

Stassi's expression for relative critical pressure required to cause

contraction of spherical voids by pressure alone was given as,

cr = 2 o ln( Ro/Ri)

where o is the flow stress of the material

Rois the outside radius (equivalent to mean space between voids)

Ri is the hole radius (equivalent to void radius).

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From the above equation, it is clear that voids of larger diameter

(larger Ri) require reduced magnitude of pressure for densification than small

voids and that, as the void diameter approaches zero, the pressure approaches

infinitely large for complete densification. Thus, it can be stated that under

hydrostatic pressure, the void simply changes size but not shape due to the

fact that the pressure remains uniformly same in all directions.

The active deformation mechanism in a material depends on the

homologous temperature, confining pressure, strain rate, stress, grain size,

presence or absence of a pore fluid, presence or absence of impurities in the

material. Note that these variables are not fully independent e.g. for a pure

material of a fixed grain size, at a given pressure, temperature and stress, the

strain-rate is given by the flow-law associated with the particular

mechanism(s). More than one mechanism may be active under a given set of

conditions and some mechanisms cannot operate independently but must act

in conjunction with another in order that significant permanent strain can

develop. In a single deformation episode, the dominant mechanism may

change with time e.g. recrystallization to a fine grain size at an early stage

may allow diffusive mass transfer processes to become dominant. The

recognition of the active mechanism(s) in a material almost always requires

the use of microscopic techniques, in most cases using a combination of

optical microscopy, SEM and TEM. Using a combination of experimental

deformation to find the flow-laws under particular conditions and from

microscopic examination of the samples afterwards it has been possible to

represent the conditions under which individual deformation mechanisms

dominate for some materials in the form of deformation mechanism maps.

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2.6 MECHANISM OF DENSIFICATION

The densification process is combined with main two processes.

They are, pore filling and vacancy destruction processes. The former is due to

the diffusion of atoms, mainly through grain boundaries, the latter occurs by

means of dislocation sinks. Thus, the densification process represents a kind

of teamwork between pores and dislocations, the former being vacancy

donors, the latter vacancy acceptors. In order to function, this kind of system

requires the simultaneous existence of pores and nearby dislocations

(Nanninga et al 2010). Densification of the preforms by plastic deformation is

strongly influenced by the stress states created in the material by the

application of the external loads. During the deformation, the stress state is

not uniform throughout the work piece and densification consequently is also

not uniform. The densification in powder preform during deformation is a

function of the mode of loading, the preform geometry, the shape, the size and

the location of the pores inside the preform. Densification can be obtained by

upsetting type of deformation. By applying hydrostatic type of load, the

volume decrease is achieved (Narayana Rao 1993). The pressure needed to

close the voids completely will be much high for any particle depending on

the model that is chosen.

2.7 PURPOSE OF ALLOYING ELEMENTS

The main advantages of alloying elements added to steels are given

below :

1) Increased wear resistance

2) Enhanced Hardenability

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3) Improved mechanical properties at either high or low

temperatures

4) Improved strength at ordinary temperatures

5) Increased corrosion resistance

6) Improved magnetic properties

2.8 HEAT TREATMENT

Heat Treatment is the controlled heating and cooling of metals to

alter their physical and mechanical properties without changing the product

shape. Heat treatment is sometimes done inadvertently due to production

processes that either heats or cools the metal such as welding or forming.

Heat Treatment is often associated with increasing the strength of

material, but it can also be used to alter certain manufacturability objectives

such as improved machining, improved formability, and to restore ductility

after a cold working operation. Thus, it is a very enabling production process

that can not only help other production process, but can also improve product

performance by increasing strength or other desirable characteristics.

Steels are particularly suitable for heat treatment, since they

respond well to heat treatment and the commercial use of steels exceeds that

of any other material. Steels are heat treated for one of the following reasons:

1. Softening

2. Hardening

3. Material Modification

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Heat treatment is an important operation for most of the ferrous

P/M parts. The major heat treatment operations are quenching and tempering,

case hardening and annealing in special cases. The heat treatment procedure

of P/M structural parts, though similar to those of conventional cast or

wrought products involves some basic limitations. While heat-treating P/M

steel part, salt baths and cyanide baths are generally not used for austenizing

to avoid the possibility of trapping salt in the porous structure. By giving

carburizing and carbonitriding treatments, the surface hardness of P/M steel

parts can be improved. In P/M parts, the carburizing or carbonitriding

atmosphere penetrates into the pores and produces a case depth as a function

of the part density. Heat-treating can improve strength, hardness and make the

surface wear-resistant.

Many of the P/M structural parts are subjected to quenching and

tempering. Important considerations in quenching and tempering of P/M parts

include the dependence of heat treatment response on the combined carbon

(rather than on total carbon), the influence of sintering time and temperature

on the amount of carbon to be added (Ogrodink and Karpinski 1977). The

hardenability of the parts, in the case of alloy steels, depends on the

composition as in the case of similar wrought compositions. Alloy content in

the P/M part will determine the austenizing as well as quenching temperature.

Austenizing is usually done with slightly higher temperatures and much

longer times (as high as 50% higher) than conventional parts because of the

limited homogeneity of the P/M parts. The austenizing time is also dependent

on the part section thickness.

Furnaces used include both batch and continuous type, but the

choice depends on the size of the parts, and total part quantity. Usually an

endothermic and sometimes vacuum atmosphere is used for heating the parts.

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Typical quenching media employed are water (plain water, brine etc.) and oil.

However, oil is the preferred medium since it avoids cracking (due to drastic

cooling) as well as corrosion. Tempering of quenched parts is done

immediately in a forced air circulation furnace with temperatures not below

150°C to obtain a tempered martensitic structure.

Annealing is done mainly for P/M forgings to improve

machinability, the temperature depending on the carbon content. Major aim of

annealing is to obtain a coarse ferrito-pearlitic structure which gives good

machining characteristics.

P/M parts are surface hardened by carburizing, carbonitriding,

nitrocarburizing and nitriding processes. Carburizing is applicable to parts

with carbon content of 0.1 to 0.2% without any free graphite. Gas carburizing

and pack carburizing processes are used. Liquid carburizing is not used

because of difficulties in cleaning. Carburizing of P/M parts requires that the

parts have a minimum density (7.2 g/cc) to have a good case development. In

case of low-density parts, gas carburizing may lead to gas penetration. Case

depth depends on temperature and time of carburization.

Carbonitriding of P/M ferrous parts is suitable for both high and

low-density parts. The processing temperatures and times are lower than for

carburizing. The process involves the heating of parts in an atmosphere of

carbon and ammonia (Pandey 1992). Nitrogen formed by the dissociation of

ammonia diffuses into the part along with carbon. Carbonitriding is usually

carried out at temperatures of 790°C. Continuous and batch type furnaces are

used. Parts are oil quenched to obtain a predominantly martensitic structure.

After heat treatment the parts are tempered at a temperature of 200°C.

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In nitrocarburizing, an atmosphere consisting of a mixture of

ammonia and endothermic gas is used. The treatment is carried out on

sintered parts at temperature between 500 and 570°C, much lower than

carbonitriding. The microstructure of the case (after quenching in oil) consists

of a thin layer of iron carbonitride and iron nitride (epsilon nitride, the "white

layer"), with a diffused layer of iron nitride, alloy nitrides and nitrogen. Gas

nitriding of P/M parts is used only to a limited extent. Nitriding gives best

results when the parts have a tempered martensitic structure.

2.8.1 Homogenization

Homogenization is a generic term generally used to describe a heat

treatment designed to correct microscopic deficiencies in the distribution of

elements and (concomitantly) modify the intermetallic structures present at

the interfaces. Accepted results of a homogenization process include the

following:

1. The elemental distribution within a grain becomes more

uniform.

2. Any low melting point constituent particles that formed at the

grain boundaries during sintering are dissolved back into the

grains.

3. Certain intermetallic particles undergo chemical and structural

transformations.

4. Large intermetallic particles that form during sintering may be

fractured and rounded during heat-up.

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2.9 TESTING OF MATERIALS

Materials are tested for diverse purposes: to determine the

properties of the raw material, to check quality at intermediate stages in

production processes, to check finished products, and to aid research.

Mechanical, physical, and chemical methods are used to study the properties

of materials. Destructive methods damage or completely destroy the object of

the tests; nondestructive ones permit further use of the object.

Broadly, there are four types of tests. Mechanical methods test the

material’s ability to resist tension, compression, impact, bending, twisting,

and shearing and its hardness and fatigue level (that is, its ability to withstand

alternating mechanical loads without breaking). Physical tests determine

electrical conductivity, heat conductivity, cold resistance, and magnetic and

other properties. Chemical tests determine chemical composition and ability

to withstand chemical action, especially oxidation. Structural tests (called

metalomicrographic tests) determine the mac-rostructure, or structure visible

to the naked eye; the microstructure, visible through a microscope; and the

crystalline structure, determined primarily by X rays. The determination of

the technological and operational properties of materials, primarily by the use

of machines and instruments.

Various technological samplings used to check the ability of a

material to resist deformation and the actions of temperature and chemicals or

its machinability may also be considered testing of materials.

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Materials are tested for the following purposes:

1. To assess numerically, the fundamental mechanical properties

of ductility, malleability, toughness, etc.

2. To determine data, i.e., force-deformation (or stress) values to

draw up sets of specifications which the engineer can base his

design upon.

3. To determine the surface or sub-surface defects in raw

materials or processed parts.

4. To check the chemical composition.

5. To determine the suitability of a material for a particular

application.

Considering the utmost utility of the material testing, various tests

for determining the properties and flaws in materials have become well

standardized. Tests on materials may be broadly classified as:

(a) Destructive testing - After being destructively tested, the

component or the specimen either breaks or remains no longer

useful for future use. Examples of destructive testing are:

tensile test, impact test, bends test, torsion test, fatigue test and

so on.

(b) Non-destructive tests - A component does not break in non-

destructive testing and even after being tested so, it can be

used for the purpose it was made. Examples of non -

destructive testing are radiography, ultrasonic inspection and

so on.

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2.9.1 Tensile Testing

A tensile test, also called as tension test, is probably the most

fundamental type of mechanical test one can perform on material. Tensile

tests are simple, relatively inexpensive, and fully standardized. By pulling on

something, you will very quickly determine how the material will react to

forces being applied in tension. As the material is being pulled, you will find

its strength along with how much it will elongate.

Tensile test is widely used to determine strength, ductility,

resilience, toughness and several other material properties. A test material of

circular, square or rectangular cross section of a suitable size (according to

ASTM E8 standard) is prepared from the material to be tested. During

preparation of the specimen, care has to be taken to avoid sharp changes in

section to redress stress concentration. This reduces the chances of failures of

specimen at low stress values. The specimen is held by suitable means

between the two heads of a testing machine and subjected to a progressively

increasing tensile load till it fractures (Pandey 1989). A record of load acting

on a specimen with progressive extension of the specimen is obtained.

The common machine for tensile test is universal testing machine,

Hounsfield tensometer, instron and MTS (material testing system). Tensile

test are done on fixed lengths called as gauge lengths. Before the

commencement of the test, two permanent marks are made on the specimen at

appropriate distance called as original gauge length.

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2.9.2 Hardness Testing

Hardness is the resistance of a material to permanent indentation. It

is important to recognize that hardness is an empirical test and therefore

hardness is not a material property. This is because there are several different

hardness tests that will each determine a different hardness value for the same

piece of material. Therefore, hardness is test method dependent and every test

result has to have a label identifying the test method used.

Hardness is, however, used extensively to characterize materials

and to determine if they are suitable for their intended use. All of the hardness

tests described in this section involve the use of a specifically shaped

indenter, significantly harder than the test sample that is pressed into the

surface of the sample using a specific force. Either the depth or size of the

indent is measured to determine a hardness value. The hardness of metal or an

alloy may be defined in one or more of the following forms.

Indentation hardness: It is the resistance offered by metal to

indentation, which is mainly a measure of plasticity and

density.

Rebound hardness: It is the resistance offered by metal to

strike and rebound, which is principally a measure of elastic

properties.

Scratch hardness: It is the resistance offered by metal to

scratching, which is principally a measure of strength and

plastic properties.

Cutting hardness: It is the resistance of metals to various

cutting operations.

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Abrasive hardness: It is the resistance of metals to wear

subjected to sliding or rotating motion.

The various hardness-testing methods are based on one of the

above principles. These testing methods are widely used in the field of

research and industry.

2.9.2.1 Brinell hardness test

Brinell invented the Brinell test in Sweden in 1900. The oldest of

the hardness test methods which is in use today, the Brinell test is commonly

used to determine the hardness of forgings and castings that have a grain

structure too coarse for Rockwell or Vickers testing. Therefore, Brinell tests

are frequently done on large parts. By varying the test force and ball size,

nearly all metals can be tested using a Brinell test. Brinell values are

considered test force independent as long as the ball size/test force

relationship is the same.

In the USA, Brinell testing is typically done on iron and steel

castings using a 3000 kg test force and a 10 mm diameter carbide ball.

Aluminum and other softer alloys are frequently tested using a 500 kg test

force and a 10 or 5 mm carbide ball. Therefore, the typical range of Brinell

testing in this country is 500 to 3000 kg with 5 or 10mm carbide balls. In

Europe, Brinell testing is done using a much wider range of forces and ball

sizes. It is common in Europe to perform Brinell tests on small parts using a

1mm carbide ball and a test force as low as 1kg. These low load tests are

commonly referred to as baby Brinell tests.

The Brinell hardness number is a function of the test force divided by

the curved surface area of the indent. The indentation is considered to be spherical

43

with a radius equal to half the diameter of the ball. The average of the two

diagonals is used in the following formula to calculate the Brinell hardness.

2 2

PBHND (D D d )2

where P = applied load, kg

D = diameter of ball, mm

d = average measured diameter of indentation, mm

2.9.2.2 Rockwell Hardness Test

Stanley Rockwell invented the Rockwell hardness test. He was a

metallurgist for a large ball bearing company and he wanted a fast non-

destructive way to determine if the heat treatment process they were doing on

the bearing races was successful. The only hardness tests he had available at

time were Vickers, Brinell and Scleroscope. The Vickers test was too time

consuming, Brinell indents were too big for his parts and the Scleroscope was

difficult to use, especially on his small parts.

To satisfy his needs he invented the Rockwell test method. This

simple sequence of test force application proved to be a major advance in the

world of hardness testing. It enabled the user to perform an accurate hardness

test on a variety of sized parts in just a few seconds.

Rockwell test methods are defined in the following standards:

ASTM E18 Metals

ISO 6508 Metals

ASTM D785 Plastics

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Types of the Rockwell Test

There are two types of Rockwell tests:

Rockwell: The minor load is 10 kgf, the major load is 60, 100, or

150 kgf.

Superficial Rockwell: The minor load is 3 kgf and major loads are

15, 30, or 45 kgf.

In both tests, the indenter may be either a diamond cone or steel

ball, depending upon the characteristics of the material being tested.

Rockwell Scales

Rockwell hardness values are expressed as a combination of a

hardness number and a scale symbol representing the indenter and the minor

and major loads. The hardness number is expressed by the symbol HR and the

scale designation.

There are 30 different scales. The majority of applications are

covered by the Rockwell C and B scales for testing steel, brass, and other

metals. However, the increasing use of materials other than steel and brass as

well as thin materials necessitates a basic knowledge of the factors that must

be considered in choosing the correct scale to ensure an accurate Rockwell

test. The choice is not only between the regular hardness test and superficial

hardness test, with three different major loads for each, but also between the

diamond indenter and the 1/16, 1/8, 1/4 and 1/2 in. diameter steel ball

indenters.

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If no specification exists or there is doubt about the suitability of

the specified scale, an analysis should be made of the following factors that

control scale selection:

Type of materialSpecimen thicknessTest locationScale limitations

Principle of the Rockwell Test

The indenter moves down into position on the part surface. A

minor load is applied and a zero reference position is established. The major

load is applied for a specified time period (dwell time) beyond zero. The

major load is released leaving the minor load applied. The resulting Rockwell

number represents the difference in depth from the zero reference position as

a result of the application of the major load.

Hardness is a characteristic of a material, not a fundamental

physical property. It is defined as the resistance to indentation, and it is

determined by measuring the permanent depth of the indentation. More

simply put, when using a fixed force (load) and a given indenter, the smaller

the indentation, the harder the material. Indentation hardness value is obtained

by measuring the depth or the area of the indentation using one of over 12

different test methods.

2.9.2.3 Vickers Hardness Test

The Vickers hardness test method, also referred to as a

microhardness test method, is mostly used for small parts, thin sections, or

case depth work. The Vickers method is based on an optical measurement

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system. The Microhardness test procedure, ASTM E-384, specifies a range of

light loads using a diamond indenter to make an indentation which is

measured and converted to a hardness value. It is very useful for testing on a

wide type of materials as long as test samples are carefully prepared. A square

base pyramid shaped diamond is used for testing in the Vickers scale.

Typically loads are very light, ranging from a few grams to one or several

kilograms, although "Macro" Vickers loads can range up to 30 kg or more.

The Microhardness methods are used to test on metals, ceramics, composites -

almost any type of material.

Since the test indentation is very small in a Vickers test, it is useful

for a variety of applications: testing very thin materials like foils or measuring

the surface of a part, small parts or small areas, measuring individual

microstructures, or measuring the depth of case hardening by sectioning a part

and making a series of indentations to describe a profile of the change in

hardness. The Vickers method is more commonly used.

Sample preparation is usually necessary with a microhardness test

in order to provide a small enough specimen that can fit into the tester.

Additionally, the sample preparation will need to make the specimen’s

surface smooth to permit a regular indentation shape and good measurement,

and to ensure the sample can be held perpendicular to the indenter. Usually,

the prepared samples are mounted in a plastic medium to facilitate the

preparation and testing. The indentations should be as large as possible to

maximize the measurement resolution. (Error is magnified as indentation

sizes decrease) The test procedure is subject to problems of operator influence

on the test results.