Guide to Ferrous Metals

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Guide to Ferrous Metals

Summary:

The basic substance used in ferrous metals such as steel and cast iron (gray and
malleable) is iron. It is used in the form of pig iron.Iron is a base metal, meaning that it is an alloying agent in many different metals.

Iron is produced
from iron ore that occurs chiefly in nature as an oxide, the two most
important oxides being hematite and magnetite. Iron ore is reduced to
pig iron in a blast furnace, and the impurities are removed in the form
of slag (figure 7-5).
Raw materials charged into the furnace include iron ore, coke, and
limestone. The pig iron produced is used to manufacture steel or cast
iron.

Common ferrous metals include:

  • stainless steels
  • tungsten carbide
  • carbon, tool and alloy steel

Ferrous Metal Composition, Properties and Characteristics

Ferrous Metal Blast Furnace - Figure 7-5

Steel

Plain carbon steel are ferrous metals that consists of iron and carbon. Carbon is the
hardening element. Tougher alloy steel contains other elements such as
chromium, nickel, and molybdenum. Cast iron is nothing more than basic
carbon steel with more carbon added, along with silicon. The carbon
content range for steel is 0.03 to 1.7 percent, and 4.5 percent for cast
iron.

Steel is produced in a variety of melting furnaces, such as open-hearth,
Bessemer converter, crucible, electric-arc, and induction. Most carbon
steel is made in open-hearth furnaces, while alloy steel is melted in
electric-arc and induction furnaces. Raw materials charged into the
furnace include mixtures of iron ore, pig iron, limestone, and scrap.
After melting has been completed, the steel is tapped from the furnace
into a ladle and then poured into ingots or patterned molds. The ingots
are used to make large rectangular bars, which are reduced further by
rolling operations. The molds are used for castings of any design.

Cast Iron

Cast Iron Engine Block

 

Cast iron is produced by melting a charge of pig iron, limestone, and
coke in a cupola furnace.It is a brittle and hard metal with above average levels of wear resistance. It is widely used in machine tools and automotive parts such as engines.

It is then poured into sand or alloy steel
molds. When making gray cast iron castings, the molten metal in the mold
is allowed to become solid and cool to room temperature in open air.
Malleable cast iron, on the other hand, is made from white cast iron,
which is similar in content to gray cast iron except that malleable iron
contains less carbon and silicon. White cast iron is annealed for more
than 150 hours at temperatures ranging from 1500 to 1700°F (815 to
927°C). The result is a product called malleable cast iron.

The
desirable properties of cast iron are less than those of carbon steel
because of the difference in chemical makeup and structure. The carbon
present in hardened steel is in solid solution, while cast iron contains
free carbon known as graphite. In gray cast iron, the graphite is in
flake form, while in malleable cast iron the graphite is in nodular
(rounded) form. This also accounts for the higher mechanical properties
of malleable cast iron as compared with gray cast iron.

Iron Ore

Iron ore is smelted with coke and limestone in a blast furnace to remove
the oxygen (the process of reduction) and earth foreign matter from it.
Limestone is used to combined with the earth matter to form a liquid
slag. Coke is used to supply the carbon needed for the reduction and
carburization of the ore. The iron ore, limestone, and coke are charged
into the top of the furnace. Rapid combustion with a blast of preheated
air into the smelter causes a chemical reaction, during which the oxygen
is removed from the iron. The iron melts, and the molten slag
consisting of limestone flux and ash from the coke, together with
compounds formed by reaction of the flux with substances present in the
ore, floats on the heavier iron liquid. Each material is then drawn off
separately (figure 7-6).

Conversion of Iron Ore Into Cast Iron, Wrought Iron and Steel - Figure 7-6

All forms of cast iron, steel, and wrought iron consist of a mixture of
iron, carbon, and other elements in small amounts. Whether the metal is
cast iron or steel depends entirely upon the amount of carbon in it.

Table 7-7 shows this principle.

Conversion of Iron Ore Into Cast Iron, Wrought Iron and Steel - Figure 7-7

Cast iron differs from steel mainly because its excess of carbon (more
than 1.7 percent) is distributed throughout as flakes of graphite,
causing most of the remaining carbon to separate. These particles of
graphite form the paths through which failures occur, and are the reason
why cast iron is brittle. By carefully controlling the silicon content
and the rate of cooling, it is possible to cause any definite amount of
the carbon to separate as graphite or to remain combined. Thus, white,
gray, and malleable cast iron are all produced from a similar base.

Wrought Iron

Wrought Iron Home Decor

Wrought iron is one of the ferrous metals that is an alloy that is almost pure iron. It is made from pig
iron in a puddling furnace and has a carbon content of less than 0.08
percent. Carbon and other elements present in pig iron are taken out,
leaving almost pure iron. In the process of manufacture, some slag is
mixed with iron to form a fibrous structure in which long stringers of
slag, running lengthwise, are mixed with long threads of iron. Because
of the presence of slag, wrought iron resists corrosion and oxidation,
which cause rusting.

Uses: Wrought iron is used for porch railings, fencing,
farm implements, nails, barbed wire, chains, modern household furniture,
ornaments and decorations.

Capabilities: Wrought iron can be gas and arc welded, machined,plated, and is easily formed.

Advantages: Wrought iron is bends easily when cold or when heated. It is easy to weld and rusts slowly.

Limitations: Wrought iron has low hardness and low fatigue strength.

Properties: Wrought iron has Brinell hardness number of
105; tensile strength of 35,000 psi; specific gravity of 7.7; melting
point of 2750°F (1510°C); and is ductile and corrosion resistant.

Appearance test: The appearance of wrought iron is the same as that of rolled, low-carbon steel.

Fracture test: Wrought iron has a fibrous structure due to
threads of slag. As a result, it can be split in the direction in which
the fibers run. The metal is soft and easily cut with a chisel, and is
quite ductile. When nicked and bent, it acts like rolled steel. However,
the break is very jagged due to its fibrous structure. Wrought iron
cannot be hardened.

Spark test: When wrought iron is ground, straw-colored
sparks form near the grinding wheel, and change to white, forked
sparklers near the end of the stream.

Torch test: Wrought iron melts quietly without sparking.
It has a peculiar slag coating with white lines that are oily or greasy
in appearance.

Cast Iron (Gray, White and Malleable)

Cast iron is a man made alloy of iron, carbon, and
silicon. A portion of the carbon exists as free carbon or graphite.
Total carbon content is between 1.7 and 4.5 percent.

Uses: Cast iron is used for water pipes, machine tool castings, transmission housing, engine blocks, pistons, stove castings, etc.

Capabilities: Cast iron may be brazed or bronze welded, gas and arc welded, hardened, or machined.

Limitations: Cast iron must be preheated prior to welding. It cannot be worked cold.

Properties: Cast iron has a Brinell hardness number of 150
to 220 (no alloys) and 300 to 600 (alloyed); tensile strength of 25,000
to 50,000 psi (172,375 to 344,750 kPa) (no alloys) and 50,000 to
100,000 psi (344,750 to 689,500 kPa) (alloyed); specific gravity of 7.6;
high compressive strength that is four times its tensile strength; high
rigidity; good wear resistance; and fair corrosion resistance.

Other types of cast iron ferrous metals are described below:

Gray Cast Iron

If the molten pig iron is permitted to cool slowly, the chemical
compound of iron and carbon breaks up to a certain extent. Much of the
carbon separates as tiny flakes of graphite scattered throughout the
metal. This graphite-like carbon, as distinguish from combined carbon,
causes the gray appearance of the fracture, which characterizes ordinary
gray cast iron. Since graphite is an excellent lubricant, and the metal
is shot throughout with tiny, flaky cleavages, gray cast iron is easy
to machine but cannot withstand a heavy shock. Gray cast iron consists
of 90 to 94 percent metallic iron with a mixture of carbon, manganese,
phosphorus, sulfur, and silicon.

Special high-strength grades of this
metal also contain 0.75 to 1.50 percent nickel and 0.25 to 0.50 percent
chromium or 0.25 to 1.25 percent molybdenum. Commercial gray iron has
2.50 to 4.50 percent carbon. About 1 percent of the carbon is combined
with the iron, while about 2.75 percent remains in the free or graphitic
state. In making gray cast iron, the silicon content is usually
increased, since this allows the formation of graphitic carbon. The
combined carbon (iron carbide), which is a small percentage of the total
carbon present in cast iron, is known as cementite. In general, the
more free carbon (graphitic carbon) present in cast iron, the lower the
combined carbon content and the softer the iron.

Appearance Test

The unmachined surface of gray cast iron castings is a very dull gray in
color and may be somewhat roughened by the sand mold used in casting
the part. Cast iron castings are rarely machined all over. Unmachined
castings may be ground in places to remove rough edges.

Fracture Test

Nick a corner all around with a chisel or hacksaw and strike the corner
with a sharp blow of the hammer. The dark gray color of the broken
surface is caused by fine black specks of carbon present in the form of
graphite. Cast iron breaks short when fractured. Small, brittle chips
made with a chisel break off as soon as they are formed.

Spark Test

A small volume of dull-red sparks that follow a straight line close to
the wheel are given off when this metal is spark tested. These break up
into many fine, repeated spurts that change to a straw color.

Torch Ferrous Metals Test

The torch test results in a puddle of molten metal that is quiet and has
a jelly like consistency. When the torch flame is raised, the
depression in the surface of the molts-puddle disappears instantly. A
heavy, tough film forms on the surface as it melts. The molten puddle
takes time to harden and gives off no sparks.

White Cast Iron

When gray cast iron is heated to the molten state, the carbon completely
dissolves in the iron, probably combining chemically with it. If this
molten metal is cooled quickly, the two elements remain in the combined
state, and white cast iron is formed. The carbon in this type of iron
measures above 2.5 to 4.5 percent by weight, and is referred to as
combined carbon. White cast iron is very hard and brittle, often
impossible to machine, and has a silvery white fracture.

Malleable Cast Iron

Malleable cast iron is made by heating white cast iron from 1400 to
1700°F (760 and 927°C) for abut 150 hours in boxes containing hematite
ore or iron scale. This heating causes a part of the combined carbon to
change into the free or uncombined state. This free carbon separates in a
different way from carbon in gray cast iron and is called temper
carbon. It exists in the form of small, rounded particles of carbon
which give malleable iron castings the ability to bend before breaking
and to withstand shock better than gray cast iron. The castings have
properties more like those of pure iron: high strength, ductility,
toughness, and ability to resist shock. Malleable cast iron can be
welded and brazed. Any welded part should be annealed after welding.

Appearance Test

The surface of malleable cast iron is very much like gray cast iron, but
is generally free from sand. It is dull gray and somewhat lighter in
color than gray cast iron.

Fracture Test

When malleable cast iron is fractured, the central portion of the broken
surface is dark gray with a bright, steel-like band at the edges. The
appearance of the fracture may best be described as a picture frame.
When of good quality, malleable cast iron is much tougher than other
cast iron and does not break short when nicked.

Spark Test

When malleable cast iron is ground, the outer, bright layer gives off
bright sparks like steel. As the interior is reached, the sparks quickly
change to a dull-red color near the wheel. These sparks from the
interior section are very much like those of cast iron; however, they
are somewhat longer and are present in large volume.

Torch Test

Molten malleable cast iron boils under the torch flame. After the flame
has been withdrawn, the surface will be full of blowholes. When
fractured, the melted parts are very hard and brittle, having the
appearance of white cast iron (they have been changed to white or
chilled iron by melting and fairly rapid cooling). The outside, bright,
steel-like band gives off sparks, but the center does not.

Steel

Steel Tool Ferrous Metals

A form of iron, steel is one of the ferrous metals that contains less carbon than cast iron, but
considerably more than wrought iron. The carbon content is from 0.03 to
1.7 percent. Basic carbon steels are alloyed with other elements, such
as chromium and nickel, to increase certain physical properties of the
metal.

Uses: Steel is used to make nails, rivets, gears, structural steel, roles, desks, hoods, fenders, chisels, hammers, etc.

Capabilities: Steel can be machined, welded, and forged, all to varying degrees, depending on the type of steel.

Limitations: Highly alloyed steel is difficult to produce.

Properties: Steel has tensile strength of 45,000 psi
(310,275 kPa) for low-carbon steel, 80,000 psi (551,600 kPa) for
medium-carbon steel, 99,000 psi (692,605 kPa) for high-carbon steel, and
150,000 psi (1,034,250 kPa) for alloyed steel; and a melting point of
2800° F (1538°C).

Low-carbon steel (carbon content up to 0.30 percent. This
steel is soft and ductile, and can be rolled, punched, sheared, and
worked when either hot or cold. It is easily machined and can readily be
welded by all methods. It does not harden to any great amount; however,
it can easily be case hardened.

Appearance Test

The appearance of the steel depends
upon the method of preparation rather than upon composition. Cast steel
has a relatively rough, dark-gray surface, except where it has been
machined. Rolled steel has fine surface lines running in one direction.
Forged steel is usually recognizable by its shape, hammer marks, or
fins.

 

Fracture Test

When low-carbon steel is fractured,
the color is bright crystalline gray. It is tough to chip or nick. Low
carbon steel, wrought iron, and steel castings cannot be hardened.

Spark Test

The steel gives off sparks in long
yellow-orange streaks, brighter than cast iron, that show some tendency
to burst into white, forked sparklers.

Torch Test

The steel gives off sparks when melted, and hardens almost instantly.

Medium-carbon Steel (carbon content ranging from .30% to .50%)

This steel may be heat-treated after fabrication. It is used for
general machining and forging of parts that require surface hardness and
strength. It is made in bar form in the cold-rolled or the normalized
and annealed condition. During welding, the weld zone will become
hardened if cooled rapidly and must be stress-relieved after welding.

High-carbon Steel (carbon content ranging from .50% to .90%)

High-carbon steel (carbon content ranging from 0.50 to 0.90
percent). This steel is used for the manufacture of drills, taps, dies,
springs, and other machine tools and hand tools that are heat treated
after fabrication to develop the hard structure necessary to withstand
high shear stress and wear. It is manufactured in bar, sheet, and wire
forms, and in the annealed or normalized condition in order to be
suitable for machining before heat treatment. This steel is difficult to
weld because of the hardening effect of heat at the welded joint.

High-Carbon Steel Tests

Appearance Test

The unfinished surface of
high-carbon steel is dark gray and similar to other steel. It is more
expensive, and is usually worked to produce a smooth surface finish.

Fracture test

High-carbon steel usually produces a
very fine-grained fracture, whiter than low-carbon steel. Tool steel is
harder and more brittle than plate steel or other low-carbon material.
High-carbon steel can be hardened by heating to a good red and quenching
in water.

Spark test

High-carbon steel gives off a large volume of bright yellow-orange sparks.

Torch test

Molten high-carbon steel is brighter
than low carbon steel, and the melting surface has a porous appearance.
It sparks more freely than low-carbon (mild) steels, and the sparks are
whiter.

High Carbon Tool Steel

Tools Steels

Tool steel (carbon content ranging
from 0.90 to 1.55 percent) is one of the ferrous metals that are used in the manufacture of chisels, shear
blades, cutters, large taps, wood-turning tools, blacksmith’s tools,
razors, and similar parts where high hardness is required to maintain a
sharp cutting edge. It is difficult to weld due to the high carbon
content. A spark test shows a moderately large volume of white sparks
having many fine, repeating bursts.

The advantages of tool steels are their ability to hold a cutting edge. Frequently used for applications such as drill bits punches, dies and cutters.

Cast Steel

Welding is difficult on steel castings containing
over 0.30 percent carbon and 0.20 percent silicon. Alloy steel castings
containing nickel, molybdenum, or both of these metals, are easily
welded if the carbon content is low. Those containing chromium or
vanadium are more difficult to weld. Since manganese steel is nearly
always used in the form of castings, it is also considered with cast
steel. Its high resistance to wear is its most valuable property.

Cast Steel Tests

Appearance test

The surface of cast steel is brighter than cast or malleable iron and sometimes contains small, bubble-like depressions.

Fracture test

The color of a fracture in cast steel is
bright crystalline gray. This steel is tough and does not break short.
Steel castings are tougher than malleable iron, and chips made with a
chisel curl up more. Manganese steel, however, is so tough that is
cannot be cut with a chisel nor can it be machined.

Spark test

The sparks created from cast steel are much
brighter than those from cast iron. Manganese steel gives off marks that
explode, throwing off brilliant sparklers at right angles to the
original-path of the spark:

Torch test

When melted, cast steel sparks and hardens quickly.

Steel Forgings

Steel forgings may be of carbon or alloy steels. Alloy steel forgings are harder and more brittle than low carbon steels.

Steel Forging Tests

Appearance test

The surface of steel forgings is smooth.
Where the surface of drop forgings has not been finished, there will be
evidence of the fin that results from the metal squeezing out between
the two forging dies. This fin is removed by the trimming dies, but
enough of the sheared surface remains for identification. All forgings
are covered with reddish brown or black scale, unless they have been
purposely cleaned.

Fracture test

The color of a fracture in a steel forging
varies from bright crystalline to silky gray. Chips are tough; and when a
sample is nicked, it is harder to break than cast steel and has a finer
grain. Forgings may be of low-or high-carbon steel or of alloy steel.
Tool steel is harder and more brittle than plate steel or other
low-carbon material. The fracture is usually whiter and finer grained.
Tool steel can be hardened by heating to a good red and then quenching
in water. Low-carbon steel, wrought iron, and steel castings cannot be
usefully hardened.

Spark test

The sparks given off are long, yellow-orange
streamers and are typical steel sparks. Sparks from high-carbon steel
(machinery and tool steel) are much brighter than those from low-carbon
steel.

Torch test

Steel forgings spark when melted, and the sparks increase in number and brightness as the carbon content becomes greater.

Alloy Steel

Steel Alloy Examples

Machines made with Steel Alloy (left to right, vandium, tungsten and chromium)

Alloy steel is one of the ferrous metals is frequently recognizable by its
use. There are many varieties of alloy steel used in the manufacture of different types of equipment . They have greater strength and durability than carbon
steel, and a given strength is secured with less material weight.
Manganese steel is a special alloy steel that is always used in the cast
condition (see cast steel above).

Nickel, chromium, vanadium, tungsten, molybdenum, and silicon are the most common elements used in alloy steel.

  1. Chromium is used as an alloying element in carbon steels to
    increase hardenability, corrosion resistance, and shock resistance. It
    imparts high strength with little loss in ductility.
  2. Nickel increases the toughness, strength, and ductility of
    steels, and lowers the hardening temperatures so than an oil quench,
    rather than a water quench, is used for hardening.
  3. Manganese is used in steel to produce greater toughness,
    wear resistance, easier hot rolling, and forging. An increase in
    manganese content decreases the weldability of steel.
  4.  Molybdenum increases hardenability, which is the depth of
    hardening possible through heat treatment. The impact fatigue property
    of the steel is improved with up to 0.60 percent molybdenum. Above 0.60
    percent molybdenum, the impact fatigue property is impaired. Wear
    resistance is improved with molybdenum content above 0.75 percent.
    Molybdenum is sometimes combined with chromium, tungsten, or vanadium to
    obtain desired properties.
  5. Titanium and columbium (niobium) are used as additional
    alloying agents in low-carbon content, corrosion resistant steels. They
    support resistance to intergranular corrosion after the metal is
    subjected to high temperatures for a prolonged time period.
  6. Tungsten, as an alloying element in tool steel, produces a
    fine, dense grain when used in small quantities. When used in larger
    quantities, from 17 to 20 percent, and in combination with other alloys,
    it produces a steel that retains its hardness at high temperatures.
  7. Vanadium is used to help control grain size. It tends to
    increase hardenability and causes marked secondary hardness, yet resists
    tempering. It is also added to steel during manufacture to remove
    oxygen.
  8. Silicon is added to steel to obtain greater hardenability
    and corrosion resistance, and is often used with manganese to obtain a
    strong, tough steel. High speed tool steels are usually special alloy
    compositions designed for cutting tools. The carbon content ranges from
    0.70 to 0.80 percent. They are difficult to weld except by the furnace
    induction method.
  9. High yield strength, low alloy structural steels (often
    referred to as constructional alloy steels) are special low carbon
    steels containing specific small amounts of alloying elements. These
    steels are quenched and tempered to obtain a yield strength of 90,000 to
    100,000 psi (620,550 to 689,500 kPa) and a tensile strength of 100,000
    to 140,000 psi (689,500 to 965,300 kPa), depending upon size and shape.
    Structural members fabricated of these high strength steels may have
    smaller cross sectional areas than common structural steels, and still
    have equal strength. In addition, these steels are more corrosion and
    abrasion resistant. In a spark test, this alloy appears very similar to
    the low carbon steels.

NOTE:
This type of steel is much tougher than low carbon
steels, and shearing machines must have twice the capacity required for
low carbon steels.

Alloy Steel Tests

Appearance test

Alloy steel appear the same as drop-forged steel.

Fracture test

Alloy steel is usually very close grained; at times the fracture appears velvety.

Spark test

Alloy steel produces characteristic sparks
both in color and shape. Some of the more common alloys used in steel
and their effects on the spark stream are as follows:

  1. Chromium. Steels containing 1 to 2 percent chromium
    have no outstanding features in the spark test. Chromium in large
    amounts shortens the spark stream length to one-half that of the same
    steel without chromium, but does not appreciably affect the stream’s
    brightness. Other elements shorten the stream to the same extent and
    also make it duller. An 18 percent chromium, 8 percent nickel stainless
    steel produces a spark similar to that of wrought iron, but only half as
    long. Steel containing 14 percent chromium and no nickel produces a
    shorter version of the low-carbon spark. An 18 percent chromium, 2
    percent carbon steel (chromium die steel) produces a spark similar to
    that of carbon tool steel, but one-third as long.
  2. Nickel. The nickel spark has a short, sharply
    defined dash of brilliant light just before the fork. In the amounts
    found in S. A. E. steels, nickel can be recognized only when the carbon
    content is so low that the bursts are not too noticeable.
  3. High chromium-nickel alloy (stainless) steels. The
    sparks given off during a spark test are straw colored near the grinding
    wheel and white near the end of the streak. There is a medium volume of
    streaks having a moderate number of forked bursts.
  4. Manganese. Steel containing this element produces a
    spark similar to a carbon steel spark. A moderate increase in manganese
    increases the volume of the spark stream and the force of the bursts.
    Steel containing more than the normal amount of manganese will spark in a
    manner similar to high-carbon steel with low manganese content.
  5. Molybdenum. Steel containing this element produces a
    characteristic spark with a detached arrowhead similar to that of
    wrought iron. It can be seen even in fairly strong carbon bursts.
    Molybdenum alloy steel contains nickel, chromium, or both.
  6. Molybdenum with other elements. When molybdenum and
    other elements are substituted for some of the tungsten in high-speed
    steel, the spark stream turns orange. Although other elements give off a
    red spark, there is enough difference in their color to tell them from a
    tungsten spark.
  7. Tungsten. Tungsten will impart a dull red color to
    the spark stream near the wheel. It also shortens the spark stream,
    decreases the size, or completely eliminates the carbon burst. Steel
    containing 10 percent tungsten causes short, curved, orange spear points
    at the end of the carrier lines. Still lower tungsten content causes
    small white bursts to appear at the end of the spear point. Carrier
    lines may be anything from dull red to orange in color, depending on the
    other elements present, if the tungsten content is not too high.
  8. Vanadium. Alloy steels containing vanadium produce
    sparks with a detached arrowhead at the end of the carrier line similar
    to those arising from molybdenum steels. The spark test is not positive
    for vanadium steels.
  9. High speed tool steels. A spark test in these steels
    will impart a few long; forked sparks which are red near the wheel, and
    straw-colored near the end of the spark stream.
  10. Special steel. Plate steel is used in the manufacture of
    built-up welded structures such as gun carriages. In using nickel plate
    steel, it has been found that commercial grades of low-alloy structural
    steel of not over 0.25 percent carbon, and several containing no nickel
    at all, are better suited to welding than those with a maximum carbon
    content of 0.30 percent. Armorplate, a low carbon alloyed steel, is an
    example of this kind of plate. Such plate is normally used in the "as
    rolled" condition. Electric arc welding with a covered electrode may
    require preheating of the metal, followed by a proper stress-relieving
    heat treatment (post heating), to produce a structure in which the
    welded joint has properties equal to those of the plate metal.
  11. For Additional Reading

  12. Recycling Iron Based Metals

  13. << PREVIOUS: Metal Composition
  14. Next: Non-Ferrous Metals >>



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