TUGAS 1 – PENCARIAN ARTIKEL

STRUKTUR KRISTAL

KULIAH MATERIAL TEKNIK

ZUL FAUZI FACHRI ABIDIN

(07525008)

JURUSAN TEKNIK MESIN

FAKULTAS TEKNOLOGI INDUSTRI

UNIVERSITAS ISLAM INDONESIA

YOGYAKARTA

2011

TUGAS 1 – PENCARIAN ARTIKEL

STRUKTUR KRISTAL

ZUL FAUZI FACHRI ABIDIN

( 07525008 )

JURUSAN TEKNIK MESIN

FAKULTAS TEKNOLOGI INDUSTRI

UNIVERSITAS ISLAM INDONESIA

YOGYAKARTA

2011

The Structure of Metal

By Bob Capudean

April 24, 2003

Let's start with the obvious: Molten metals have no particular structure. The atoms that make up

that metal are just whipping around helter-skelter—at a high rate of speed—with no real orderly,

defined pattern.

As you think about molten metal, keep a couple of points in mind. First, heat flows to cold-

always. And that becomes more understandable when you consider that warm atoms are moving

faster than cold atoms. And those fast-moving atoms are bumping into other atoms, causing them

to move quickly.

Furthermore, the warmer a metal-or any material, for that matter-is, the faster the atoms

composing that metal are moving. Yes, there are internal attractions that help keep the atoms in a

puddle, preventing them from just vaporizing, but the fact is, if they get moving fast enough-that

is, get hot enough-they eventually will evaporate, just like hydrogen and oxygen do when water

boils.

As thermal energy is transferred to another part, the atoms give up energy, slowing down and

cooling. What evaporates is still water, in the form of steam.

As a molten metal cools, atomic forces begin to pull or force the atoms into solid particles called

nuclei, which take on specific and identifiable crystal structures. Because the nuclei have the

metal's crystal structure, additional atoms join the nuclei. As these nuclei get bigger, they form

grains. This orderly arrangement of the atoms is called a lattice.

But as the metal solidifies and the grains grow, they grow independently of each other, which

means eventually these different areas of growing grains have to meet. When they do, the

arrangement of the atoms in the grain structure is disrupted at that meeting point. This is called a

grain boundary. Grain boundaries form a continuous network throughout the metal, and because

of the disrupted structure at the boundary, the metal often acts differently at the boundary

locations.

Grain boundaries aside, each grain in a pure metal has the same crystalline structure as any other

grain, assuming the temperature is the same. This structure, which is identifiable under the

microscope, has a huge influence on the metal's characteristics.

Common Crystal Structures

For our purposes, all metals and alloys are crystalline solids, although some metals have been

formed in the lab without crystalline structure. And most metals assume one of three different

lattice, or crystalline, structures as they form: body-centered cubic (BCC), face-centered cubic

(FCC), or hexagonal close-packed (HCP). The atomic arrangement for each of these structures is

shown in Figure 1.

A number of metals are shown below with their room

temperature crystal structure indicated. And for the record, yes,

there are substances without crystalline structure at room

temperature; for example, glass and silicone.

Aluminum — FCC

Chromium — BCC

Copper — FCC

Iron (alpha) — FCC

Iron (gamma) — BCC

Iron (delta) — BCC

Lead — FCC

Nickel — FCC

Silver — FCC

Titanium — HCP

Tungsten — BCC

Zinc — HCP

Alloys and Atomic Arrangements

Everything covered so far applies to pure metals, which begs the

question, What happens when you add an alloy or two? After all, most common metals are alloys

containing residual and added metallic and nonmetallic elements dissolved in a base metal.

Of course, those added elements can have a dramatic effect on the resulting alloy's properties.

But how those elements dissolve, or in other words how they combine with the existing atoms in

the parent metal's crystal lattice, can also greatly influence both the physical and nonphysical

properties of the end product.

Basically, there are two ways the alloying element(s)-called solutes-combine with the base, or

parent, metal, which is also called the solvent. The alloy's atoms can combine through either

direct substitution, creating a substitutional solid solution, or they can combine interstitially,

forming an interstitial solid solution.

Substitutional Solid Solution. When the alloy's atoms are similar to the parent metal's atoms,

they'll simply replace some of the parent metal's atoms in the lattice. The new metal dissolves in

Figure 1 Three crystal structures

favored by metals are (a) body-

centered cubic (BCC), (b)

face-centered cubic (FCC), and

(c) hexagonal close-packed

(HCP).

the base metal to form a solid solution. Examples include copper dissolved in nickel, gold

dissolved in silver, and carbon dissolved in iron (ferrite).

Interstitial Solid Solution. When the alloy's atoms are smaller than the parent metal's atoms,

they'll fit between the atoms in the parent metal's lattice. The alloy atoms don't occupy lattice

sites and don't replace any of the original atoms. Of course, this causes strain in the crystal

structure because the fit isn't perfect: There are atoms taking up space that was originally

unoccupied.

The end result is usually an increase in tensile strength and a decrease in elongation. Examples

include small amounts of copper dissolved in aluminum and carbon, and nitrogen dissolved in

iron and other metals.

Phases, Microstructures, and Phase Changes

Often neither direct nor interstitial solution can completely dissolve all the added atoms. And

when this happens, the result is mixed atomic groupings. In other words, different crystalline

structures exist within the same alloy. Each of these different structures is called a phase, and the

alloy-which is a mixture of these different crystalline structures-is called a multiphase alloy.

These different phases can be distinguished under a microscope when the alloy is polished and

etched. Pearlite is a good example of a multiphase alloy within the carbon-iron family.

The phases present in an alloy, along with the overall grain arrangements and grain boundaries,

combine to make up an alloy's microstructure. And the microstructure of an alloy is critical,

being largely responsible for both the physical and mechanical properties of that alloy.

For example, because the boundary areas are the last to freeze when an alloy cools, grain

boundaries contain lower-melting-point atoms compared to the atoms within the grains. These

foreign atoms cause microstructure distortion and harden the alloy at room temperature. But as

temperature goes up, alloy strength goes down because these lower-melting-point atoms begin to

melt sooner, allowing slippage between the grains.

Furthermore, foreign or odd-sized atoms tend to congregate at grain boundaries because the

atomic structure is irregular. This can lead to phases that reduce ductility and lead to cracking

during welding.

Consider this: Cold working a metal distorts its entire microstructure. The end result, in most

cases, is that the metal gets harder. Atoms from an alloying element distort the metal's

microstructure, and again, the metal gets harder. The same is true for alloy atoms that are

dissolved in a base metal and then precipitate out. The atoms leave, but a distortion remains, and

the metal is harder.

Grain size is also important. Generally speaking, fine-grained metals have better properties at

room temperature. And size is determined by cooling rate. Fast cooling leads to smaller grains,

and vice versa. But the fact is, grain size, grain boundary structure, and phases present all are

important. Overall, these characteristics in total determine a metal's capabilities and usefulness.

In short, a metal's overall microstructure determines its characteristics. Today just about every

metal we use is an alloy, with one or more elements added to modify, adjust, correct, or change

the base metal's microstructure, creating a multiphase system that can better serve our needs.

And every time we put torch to metal, we cause a phase change and influence that

microstructure.

This should give you an overview of how metals are structured and what happens when we melt

them to weld them together. Next time we'll consider phase transformations, carbon content,

hardening, the relationship between austenite and martensite, and the influence of welding on

metallurgical structure.

Microstructure of Ferrous Alloys

George F. Vander Voort, Director, Research & Technology, Buehler Ltd., Lake Bluff, IL

January 10, 2001

The microstructure of iron-base alloys is very complicated and diverse, being influenced by

chemical composition, material homogeneity, processing and section size. This article offers a

brief explanation of the terminology describing the constituents in ferrous alloys, and offers a

basic review of steel microstructures.

Microstructures of castings look different from those of wrought products, even if they have the

same chemical composition and are given the same heat treatment. In general, it is easiest to

identify heat-treated structures after transformation and before tempering. For example, if a

mixed microstructure of bainite and martensite is formed during quenching, these constituents

will become more difficult to identify reliably as the tempering temperature used for the product

increases toward the lower critical temperature. Further, ferrous metallographers tend to use nital

almost exclusively for etching, but nital is not always the best reagent to use to properly reveal

all microstructures. Picral is an excellent etchant for revealing certain micro-structural

constituents in steel, but the use of picral is prohibited by some companies because picric acid

can be made to explode under certain conditions. However, picral-related accidents are less

common than for nital. Vilella's reagent, which also contains picric acid, is exceptionally

valuable for certain compositions and microstructures.

Because of misuse and confusion regarding certain terms, there is a need to discuss the

terminology describing the constituents in ferrous alloys. Certain terms, such as sorbite and

troostite, were dropped from the metallographic lexicon in 1937 because they referred to

microstructural constituents inaccurately. However, such terms still are occasionally used. The

term phase often is used incorrectly in reference to mixtures of two phases, such as pearlite or

bainite. A phase is a homogeneous, physically distinct substance. Martensite is a phase when

formed by quenching but becomes a constituent after tempering as in decomposes from body

centered tetragonal (bct) martensite to body centered cubic (bcc) ferrite and cementite.

Definitions will be given in this article in the process of describing and illustrating various

phases and constituents in ferrous alloys.

SPECIMEN PREPARATION

Ferrous metals must be properly prepared to observe their microstructures. Many view this task

as a trivial exercise, yet its proper execution is critical to successful interpretation. The first step

in the process is to select the test locations to be sampled. The specimens selected must be

representative of the lot; this is critical if the interpretation is to be valid for the part or lot being

evaluated. The plane of polish may be oriented in different directions relative to the piece being

sampled. For example, for a casting, the test plane may be perpendicular or parallel to the

solidification axis and may be located anywhere between the surface (where solidification

begins) and the center (where solidification ends). In a small casting, the structure will not vary

greatly over the cross section. However, this is not the case for large castings. Also, the use of a

separately cast keel block (a block of metal from which test coupons are taken) for test

evaluations may be highly misleading, as its solidification characteristics may be quite different

from that of the casting.

Wrought alloys are sampled in a similar manner, using either longitudinally or transversely

oriented cutting planes, which may be taken in any location from the surface to the center. The

midradius location is often selected as being representative of the overall condition, which may

be true in many cases. Additional processing alters the microstructure, usually producing greater

homogeneity and finer structures. But, problems still can arise.

Sectioning is almost always required to obtain a test piece of the proper size and orientation for

metallographic examination. An abrasive cutoff saw is the most commonly used device for

sectioning, producing a good surface having minimal damage when the proper blade is used with

adequate coolant. More aggressive sectioning methods often are used in production operations.

These produce greater damage to the structure that must subsequently be removed if the true

structure is to be revealed.

After obtaining a specimen, it may be mounted in a polymeric material to facilitate handling, to

simplify preparation, to enhance edge retention, and for ease of identification of the specimen

(by scribing identification information on the material). Mounting may be done in a press using a

thermosetting or thermoplastic resin or with castable resins that do not require external heat and

pressure for polymerization.

The use of automation in specimen preparation has grown enormously over the past twenty-five

years. Automated devices produce better results than can be achieved manually. They yield more

consistent results, better flatness and better edge retention, and offer greater productivity. Many

procedures for successfully preparing ferrous specimens could be listed; there is no one correct

procedure. Some methods favor certain types of specimens or problems. There also are many

different products that give successful results. Tables 1 and 2 list procedures that can be used to

prepare most steel specimens. These methods give consistent results with good specimen edge

retention. For the most difficult specimens, a 1-Km diamond step can be added after the 3-Km

diamond step, using the same materials, speeds and direction, but somewhat less time. Other

variations are possible depending on particular needs and specimens.

The first step, often called planar grinding, can be done using several products. Traditional

silicon-carbide (SiC) paper always is satisfactory, and aluminum-oxide (Al2O3) paper also may

be used. The process should always start using the finest possible abrasive that can remove the

damage from cutting and get all of the specimens in the holder co-planar in a reasonable time.

SiC paper does have a short life. Continuing to grind after the paper has lost its cutting efficiency

will generate heat and damage the specimen. The Ultra-Prep disks recommended in Table 1 are

excellent for obtaining flatness and edge retention and yield high stock removal rates. The disk

surface is covered with diamond in small pads, and diamond-free regions surrounding the spots

reduce surface tension and increase cutting efficiency. These disks have a long life. The metal-

bonded disks used for the harder ferrous alloys and the resin-bonded disks for the softest.

BuehlerHerculesT rigid grinding disks (RGD) offer an alternative grinding possibility; they

produce a very flat surface and are recommended when edge retention is critical. Two types of

RGD are available: type H and type S. In general, all steels can be prepared with the H disk, but

it is best to use the S disk for the softest steels. These disks do not contain embedded abrasive;

diamond is periodically added to the surface, usually as a suspension. There are cloth alternatives

that work well for the second step, but they have a shorter life than a rigid grinding disk. Ultra-

PadT and Ultra-PolT are two excellent cloths for the 9-Km diamond step. The former is more

aggressive and heavier and has a longer life, while the latter yields a better surface finish and is

recommended for the most difficult to prepare metals and alloys of any composition.

ETCHANTS

A steel specimen that is to be examined for inclusions or nitrides should not be etched. To see

the other microstructural constituents, etching is needed. Nital (usually 2%) is most commonly

used. It is excellent for revealing the structure of martensite, and also is very good for revealing

ferrite in a martensite matrix and to bring out ferrite grain boundaries in low-carbon steels.

Picral, on the other hand, is better for revealing the cementite in ferritic alloys and the structure

of ferrite-cementite constituents, pearlite and bainite. Nital and picral both dissolve ferrite but

nitalns dissolution rate is a function of crystal orientation, while picralns rate is uniform. Other

reagents have specific uses, especially when dealing with higher alloy grades, such as tool steels

and stainless steels, or when trying to selectively reveal certain constituents or prior-austenite

grain boundaries. Etchants for steels are listed in many standard text books (1) and handbooks,

and in ASTM E 407.

MICROSTRUCTURES

Fig. 1. Ferrite grain structure of a lamination steel; 2%

nital etch.

Alpha iron, strictly speaking, refers only to the bcc form of pure iron, which is stable below

912C (1674F) while ferrite is a solid solution of one or more elements in bcc iron. Often these

terms are used synonymously, which is incorrect. Ferrite may precipitate from austenite in

acicular form under certain cooling conditions. Acicular means the shape is needle-like in three

dimensions. However, this is not the actual shape of acicular ferrite in three dimensions. Figure 1

shows the appearance of ferrite grains in a carbon steel used for laminations. There are also

ferritic stainless steels, which contain high chromium contents and very little carbon. Ferrite is a

very soft, ductile phase, although it looses its toughness below some critical temperature.

Gamma iron, as with alpha iron, pertains to only the face-centered cubic (fcc) form of pure iron

that is stable between 912 and 1394C (1674 and 2541F) while austenite is a solid solution of one

or more elements in fcc iron. Again, these terms are often used interchangeably, which is

incorrect. For heat-treatable steels, austenite is the parent phase for all transformation products

that make ferrous alloys so versatile and useful commercially. Austenite is not stable at room

temperature in ordinary steels. In chrome-nickel (Cr-Ni) steels, know as stainless steels, there is

a family of very important grades where austenite is stable at room temperature.

Fig. 2. Austenite grains, with annealing twins, in AISI

type 316 austenitic stainless steel; Kalling?s number 2

etch.

Figure 2 shows an example of the microstructure of AISI type 316 austenitic stainless steel.

Austenite is a soft, ductile phase that can be work hardened to high strength levels, particularly in

the fully austenitic Hadfield manganese steels.

In high-carbon, high-alloy steels, such as tool steels, use of an excessively high austenitizing

temperature will depress the temperatures where martensite begins and completes its

transformation. These martensite start and end temperatures are depressed to such an extent that

the austenite is not fully converted to martensite during quenching and the remaining austenite,

called retained austenite, is present (but not necessarily stable) at room temperature.

Fig. 3. Coarse plate martensite (black ?needles?),

retained austenite (white areas between martensite

?needles?), and some cementite (arrows) in the

carburized case of AISI type 8620 alloy steel; 2% nital

etch.

Figure 3 shows an example of retained austenite in the carburized case of AISI type 8620 low-

alloy alloy steel. The retained austenite is white and lies between the plate martensite "needles."

However, there are also a few white particles of cementite in the micrograph (arrows). Excessive

retained austenite in tool steels usually is detrimental to die life, because it may transform to

fresh martensite and cause cracking in the die, or reduce die wear resistance. In the case of a

carburized gear tooth, retained austenite usually is not detrimental because the gear teeth

typically are not shock loaded, so the retained austenite would transform to martensite and the

toughness of the austenite, when stabilized, could be beneficial. There are grades of stainless

steel where the composition is balanced to produce approximately equal amounts of ferrite and

austenite (dual phase) at room temperature.

Fig. 4. Ferrite (dark) and austenite (white) in 2205

dual-phase stainless steel; etch: 20% NaOH in water, 3

V dc, 12 sec.

Figure 4 shows the microstructure of such a stainless steel.

Delta iron is the bcc form of pure iron that is stable above 1394C (2541F) to the melting point,

1538C (2800F), while delta ferrite is the stable high-temperature solid solution of one or more

elements in bcc iron. Delta ferrite may be observed in as-cast austenitic stainless steels (it is put

into solution after hot working and solution annealing), in some precipitation hardened stainless

steels (for example, 17-4 PH) when the composition is not balanced to avoid it, in some

martensitic stainless steels and in some tool steels. Delta ferrite usually is considered detrimental

to transverse toughness when it is present in a hardened structure.

Fig. 5. Delta ferrite (dark) stringers in AM 350 PH

(precipitation hardenable) stainless steel; etch: 20%

NaOH in water, 3 V dc, 5 sec.

Figure 5 illustrates delta-ferrite stringers (longitudinal plane) in AM350 precipitation hardenable

stainless steel.

Carbon in iron exists either as graphite or as cementite. Graphite is the stable form of carbon in

iron (mainly observed in cast iron), while cementite is metastable and can transform to graphite

under long-term, high-temperature exposure. Cementite is a compound of iron and carbon with

the approximate formula Fe3C and has an orthorhombic crystal structure. Some substitution of

other carbide forming elements, such as manganese and chromium, is possible. Therefore, it is

more general to refer to the formula as M3C, where M stands for metal. Only small amounts of

the various carbide forming elements can be substituted before alloy carbides of other crystal

structures and formulae are formed.

Fig. 6. Cementite (white) and pearlite (dark) in white

cast iron; 4% picral etch.

Figure 6 shows cementite in white cast iron. The carbon content of cementite is 6.67 wt%, which

usually is the terminus for the iron-carbon (Fe-C) phase diagram. Cementite is hard but brittle

(about 800 HV, or Vickers hardness, for pure Fe3C, and up to about 1400 HV for highly alloyed

M3C).

Carbon are alloy steels are in the austenitic condition when they are hot worked. Subsequent

cooling results in the transformation of austenite to other phases or constituents. If a carbon or

low-alloy steel is air cooled after hot rolling, a diffusion-controlled transformation occurs where

ferrite first precipitates, followed by pearlite. Pearlite is a metastable lamellar (plate-like)

aggregate of ferrite and cementite that forms at temperatures below the lower critical temperature

(the temperature where austenite starts forming from ferrite upon heating). With time and

temperature, the cementite in the pearlite will become spheroidized; that is, it changes from a

lamellar to a spheroidal shape. This reduces the strength and hardness of the material, while

increasing its ductility. The degree of change is a function of the carbon content of the alloy.

Pearlite forms by a eutectoidal reaction. A eutectoid transformation is an isothermal, reversible

reaction in which a solid solution (austenite) is converted into two intimately mixed solid phases,

ferrite and cementite. All eutectoidal products are lamellar, even in nonferrous systems.

For steels having carbon contents below the eutectoidal value (0.77% carbon), ferrite precipitates

before the eutectoidal transformation and is called proeutectoid ferrite.

Fig. 7. Proeutectoid ferrite and pearlite structure of

plate from the ship RMS Nomadic; 2% nital etch.

Figure 7 shows proeutectoid ferrite and lamellar pearlite in a piece of plate steel from the ship

RMS Nomadic, a tender for the RMS Titanic. The ferrite is white and the pearlite is dark becasue

the lamellae are much too finely spaced to be resolved at the 200X magnification in Figure 7.

Fig. 8. Coarse pearlite and proeutectoid ferrite in fully

annealed AISI type 4140 alloy steel; 4% picral etch.

Figure 8 shows coarse pearlite in a fully annealed specimen of AISI type 4140 alloy steel where

the lamellae can be resolved. The cementite lamellae appear dark while the ferrite remains white.

In steels having carbon contents above the eutectoidal composition, cementite will precipitate in

the grain boundaries before the eutectoid reaction occurs and is called proeutectoid cementite.

Pearlite increases the strength of carbon steels. Refining the interlamellar spacing also increases

the strength, and toughness, as well. In a slowly cooled specimen, the amount of pearlite

increases to 100% as the carbon content increases to the eutectoidal carbon content. The

hardness of a fully pearlitic eutectoidal steel varies with the interlamellar spacing from about 250

to 400 HV for the finest spacings. Pearlite can be cold drawn (cold worked) to exceptionally high

tensile strengths, as in piano wire, which also has considerable ductility.

If the cooling rate is faster than that achieved by air cooling, or if alloying elements are added to

the steel to increase hardenability, a different two-phase constituent may be observed, called

bainite. Bainite is a metastable aggregate of ferrite and cementite, which forms from austenite at

temperatures below where pearlite forms and above the temperature where martensite starts to

form. The appearance of bainite changes with the transformation temperature, being called

"feathery" in appearance at high temperatures and "acicular" at low transformation temperatures.

The feathery appearance of "upper" bainite also is also influenced by carbon content and is

common in grades having high carbon contents. The term acicular is not a perfect description of

the shape of "lower" bainite.

Fig. 9. Upper bainite (dark) and martensite (light) in a

partially transformed (1525?F - 30 min, 1000?F - 1

min, water quench) specimen of AISI type 5160 alloy

steel. The austenite which had not transformed to

upper bainite after 1 minute formed martensite in the

quench; 2% nital etch.

Figures 9 and 10 show the appearance of upper and lower bainite, respectively, in partially

transformed AISI type 5160 alloy steel specimens.

Fig. 10. Lower bainite (dark) and martensite (light) in

a partially transformed (1525?F - 30 min, 650?F - 5

min, water quench) specimen of AISI type 5160 alloy

steel. The austenite which had not transformed to

lower bainite after 5 minute formed martensite in the

quench; 2% nital etch.

If the cooling rate from the austenitizing temperature is rapid enough (a function of section size,

hardenability and quench medium), martensite will form. Martensite is a generic term for the

body-centered tetragonal phase that forms by diffusionless transformation, and the parent and

product phases have the same composition and a specific crystallographic relationship.

Martensite can be formed in alloys where the solute atoms occupy interstitial sites, such as

carbon in iron, producing substantial hardening and a highly strained, brittle condition. However,

in carbon-free alloys having high nickel contents, such as maraging steels, the solute atoms (Ni)

can occupy substitutional sites, producing martensites that are soft and ductile. In carbon-

containing steels, the appearance of the martensite changes with carbon in the interstitial sites.

Low-carbon steels produce lath martensites, while high-carbon steels produce plate martensite

(often incorrectly called "acicular" martensite) when all of the carbon is dissolved into the

austenite.

Fig. 11. Lath martensite in AISI type 8620 alloy steel;

2% nital etch.

Lath martensite is shown in Figure 11 (see Figure 3 for plate martensite).

Fig. 12. Plate martensite in a fine-grained, properly

austenitized AISI type 52100 bearing steel specimen

(fine white, spheroidal particles are undissolved

cementite) is virtually featureless at 1000?; 2% nital

etch. (Compare with coarse plate martensite in Figure

3.)

When quenched from the proper temperature, so that the correct amount of cementite is

dissolved (see discussion following) and the grain size is quite fine, martensite will appear

virtually featureless by light microscopy, as shown in Figure 12 for AISI type 52100 bearing

steel.

Fig. 13. Soft, carbon-free martensite in low-residual

18Ni250 maraging steel; 500?, modified Fry?s reagent

etch.

Figure 13, for comparison, shows the structure of martensite in nearly carbon-free 18Ni250

maraging steel.

The strength and hardness of martensite varies linearly with percent carbon in austenite up to

about 0.5% C. As the carbon in the austenite increases beyond 0.5%, the curve starts to flatten

and then goes downward due to the inability to convert the austenite fully to martensite (the

amount of retained austenite increases). Therefore, when high-carbon steels are heat treated, the

austenitizing temperature is selected to dissolve no more than about 0.6% C into the austenite.

There are other minor constituents in steels, such as nonmetallic inclusions, nitrides,

carbonitrides, and intermetallic phases, such as sigma and chi phases. Nonmetallic inclusions are

of two types: those that arise from the restricted solubility of oxygen and sulfur in the solid phase

compared with the liquid; and those that come from outside sources, such as refractories in

contact with the melt. The former are called indigenous and the later are called exogenous. Many

poor terms are used in reference to inclusions. Nitrides and carbonitrides result when certain

nitride forming elements are present in adequate quantities, aluminum, titanium, niobium, and

zirconium, for example. A certain amount of nitrogen always is present in the melt and this

varies with the melting procedure used. Electric-furnace steels usually have around 100 ppm

(parts per million) nitrogen while basic oxygen-furnace steels have about 60 ppm nitrogen.

Aluminum nitride is extremely fine and can be seen only after careful extraction replica work

using transmission electron microscopy (TEM). The other nitrides often are visible in the light

microscope, although submicroscopic size nitrides can also be present. Sigma and chi phases

(not shown in this article) can be produced in certain stainless steels after high temperature

exposure.

SUMMARY

The microstructure of ferrous alloys is very complicated and this review has only touched the

surface of knowledge about steel microstructures. It is a basic tenet of physical metallurgy that

composition and processing establishes the microstructure, and that microstructure influences

most properties and service behavior. To maintain control of the quality of steel products and to

diagnose problems in processing, testing, or service, the microstructure must be identified and, in

some cases, quantified. This can only be accomplished when the metallographer can properly

distinguish the phases or constituents present, which depends on proper specimen preparation

and etching.

References

1. G.F. Vander Voort, Metallography: Principles and Practice, ASM International, Materials

Park, OH, 1999.

Daftar Pustaka :

Bienias.J, Walczak.M, Surowska.B, Sobczak.J. ( 2003 ). Microstructure And Corrosion

Behaviour Of Aluminum Fly Ash Composites. Journal of Optoelectronics and Advanced

Materials Vol. 5, No. 2, June 2003, p. 493 – 502.

Masyrukan. ( 2006 ). Penelitian Sifat Fisis dan Mekanis Baja Karbon Rendah Akibat

Pengaruh Proses Pengarbonan dari Arang Kayu Jati. MEDIA MESIN, Vol. 7, No. 1, Januari

2006, 40-46.

Saptono.R. ( 2008 ). Pengerahuan Bahan 2008 : Bab 3 Logam dan Paduan Berbasis Besi.

Departemen Metalurgi dan Material FTUI 2008.

Capudean.B. ( 2003 ). The Structure Of Metal. [ cited 2011 Oct 18 ]. Available from : URL :

http://www.thefabricator.com/article/metalsmaterials/the-structure-of-metal

George.F.( 2001 ). Microstructure of Ferrous Alloys. [ cited 2011 Oct 18 ]. Available from :

URL :

http://www.industrialheating.com/Articles/Cover_Story/93096f835cbb7010VgnVCM100000f93

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