Guide to Ferrous Metals

What are Ferrous Metals?

Ferrous metals include all forms of iron, iron-based alloys (steel), cast iron (malleable and gray)  and other elements added to achieve desirable properties. Ferrous metals are typically magnetic. The word ferrous means iron (Fe).

The primary substance used to make ferrous metals such as steel and cast iron (gray and malleable) is iron. On Earth, iron is the most common metal (by mass). Iron is in the soil on Mars, on rocks, and in the ground. In a star, iron is produced from nickel decay (Ni).

Most welding occurs using ferrous metals, and the most common type is steel, which is considered to be an iron alloy.

Video on Ferrous Metals

Iron

Properties of Iron

• Physical Properties
  • Magnetic
  • Brittle (when not alloyed)
  • Conducts electricity (but not as good as other common metals)
  • Conducts heat with average thermal conductivity
  • Melting point: 15380C
  • Boiling point: 4428⁰C
  • Density: 7.87 g/cm³
  • Average ductility
  • To raise the temperature of one gram of iron by 1⁰C requires .45 J of heat energy
• Mechanical Properties
  • Forms rust when oxidizing in moist air
  • Solution is produced of hydrogen gas and ferric chloride when the iron is added to hydrochloric acid
  • Not as reactive as magnesium, but more reactive than silver

The principal raw material in all ferrous metals is pig iron, which is produced in a blast furnace. Carbon is added (at least 1.8%) to reduce the iron ore to pig iron. Iron ore occurs chiefly in nature as an oxide, the two most important oxides being hematite and magnetite. Raw materials charged into the furnace include limestone,  iron ore, and coke. The pig iron produced is used to manufacture steel, wrought iron or cast iron.

The characteristics of ferrous metals can be altered by heat treatment or by adding an alloy.  Properties of ferrous metals change based on the amount of carbon in the metal, making them less brittle and harder. Carbon makes it harder by holding iron molecules in the crystal structure of the carbon.  All types of steel and iron are ferrous metals.

What are the types of iron?

• Cast iron
• Carbon steel
• Tool steel

What are iron alloys?

What are Iron Ores?

Metallic iron is extracted from iron ore minerals and rocks.  Ores rich in iron oxides vary in color from bright yellow, dark gray, deep purple, rusty red to bright yellow.

Pig iron is made from the raw material iron ore, which is one of the main raw materials used to make steel. Steel is made from ninety-eight percent of the mined iron ore.

How is Iron Produced?

A blast furnace produces Iron by converting iron ore to pig iron. Pig iron is the intermediate product of smelting iron ore with coke, usually with limestone as a flux. The carbon content of Pig iron is high (typically 3.5–4.5%). This makes the pig iron not directly useful due to its’ brittleness, except in a small number of applications.

Blast Furnace Diagram

Iron ore is smelted in a blast furnace with limestone and coke to eliminate any oxygen (the process of reduction) and to remove any foreign matter.

• A liquid slag forms when Limestone combines with earth matter.
• The carburization and reduction of the ore with coke.

Charged into the top of the furnace are coke, iron ore, and limestone charge into the top of the furnace. A blast of preheated air and rapid combustion in the smelter results in a chemical reaction. The reaction causes the oxygen removal from the iron.  The iron then melts, and the molten slag (coke ash and limestone flux) combines with compounds resulting from substances in the ore, floats on the heavier iron liquid. Each material is separately drawn off.

Cast Iron (Malleable, Gray, and White)

What is cast iron?

Cast iron, a human-made iron alloy,  is nothing more than basic carbon steel with more carbon added, along with trace amounts of silicon, manganese, phosphorus, and sulfur. A portion of the carbon exists as graphite or free carbon. Between 1.7 and 4.5% carbon content.

The carbon content range for steel is 0.03 to 1.7 percent, and 4.5 percent for cast iron. Any iron that contains greater than 2% carbon is a carbon alloy. Cast iron has so much carbon that with the exception of malleable cast iron, it tends to be brittle. A lengthy annealing process is used to produce malleable cast iron.

The melting point of cast iron is lower than steel. Most castings are gray cast iron.

How is cast iron produced?

By melting a charge of coke, limestone, and pig iron in a cupola furnace, you can produce cast iron. It is then poured into alloy steel or sand molds.

• Gray cast iron castings:  the molten metal in the mold is allowed to become solid and cool to room temperature in the open air.
• Malleable cast iron:  Malleable cast iron is similar in content to gray cast iron except that malleable iron contains less carbon and silicon. It is made from white cast iron.
• White cast iron: When annealed for 150 hours or more at temperatures ranging from 1500 to 1700°F (815 to 927°C), the resulting product is called malleable cast iron.

Cast irons desirable properties, because of the difference in structure and makeup are less than those of carbon steel. Cast iron contains free carbon called graphite, while hardened steel has carbon in a solid solution.

The graphite is in flake form in gray cast iron, while in malleable cast iron the graphite is in nodular (rounded) form. This also includes the poor mechanical properties of gray cast iron as compared to the higher mechanical properties of malleable cast iron.

Uses

Cast iron is found in:

• water pipes
• machine tool castings and parts
• transmission housing
• automotive parts: engine blocks, pistons, gearbox cases, cylinder blocks
• stove castings

Welding Capabilities of Cast Iron?

Cast iron may be bronze welded or brazed, arc and gas welded, hardened, or machined.  Characteristics that make it valuable to engineers are:

1. Excellent machinability
2. Excellent wear resistance
3. High compressive strength
4. Low Cost
5. Good casting characteristics

Cast Iron Limitations

Cannot be worked cold. Cast iron must be preheated before welding. The main limitation is little tensile strength and brittleness, so it cannot be used where there are shocks or vibrations.

Properties of Cast Iron

• Hardness: Brinell hardness: number of 150 to 220 (no alloys) and 300 to 600 (alloyed)
• Strength:
  • No Alloys: Tensile strength of 25,000 to 50,000 psi (172,375 to 344,750 kPa)
  • Alloys: 50,000 to 100,000 psi (344,750 to 689,500 kPa)
• Gravity: Specific gravity of 7.6
• Strength: High compressive strength that is four times its’ tensile strength; high rigidity; good wear resistance; and fair corrosion resistance.

Alloy Cast Iron

Cast Iron alloying elements help to overcome any deficiencies in ordinary cast iron. Alloy cast iron is wear-resistant and tough.

Iron Alloying Element	Description
Boron (B)	Boron increases harden-ability, useful when alloyed with low carbon steel.
Cobalt (Co)	Improves hardness of high speed steel When added to high strength steel, improves magnetic properties, thermal resistance, tensile strength, and toughness and hardness.
Chromium (Cr)	When added to steel improves corrosion resistance, hardness and toughness.
Molybdenum (Mo)	Improves thermal resistance, hardness, wear resistance and the ability to retain at elevated temperatures mechanical properties. Helps the inhibition of temper brittleness.
Nickel (one of the most important - Ni)	Improves corrosion resistance, toughness, ductility and tensile strength.
Niobium (Nb)	Improves impact strength significantly, decreases hardenability and improves ductility. Helps to promote fine grain growth.
Titanium (Ti)	Promotes grain growth an dis a good deoxidizer. It is the strongest former of carbides. It is used to fix carbon in stainless steel, preventing precipitation of chromium-carbide.
Tungsten (W)	Improves ability at elevated temperatures to retain hardness. It improves toughness, magnetic reluctance, shock resistance, wear resistance and toughness. It also adds abrasion resistance properties to steel.
Vanadium (V)	Increases shock resistance, ductility, elastic limit and tensile strength. When added to molten steel acts as a degaser. It improves the hardenability of steel.

Chilled Cast Iron

Iron produced with quick cooling is called “chilled cast iron.” All castings are chilled to a limited depth (1 to 2 mm) during solidification and pouring of molten metal after coming in contact with the cool sand of the mold. At times the casting is intentionally chilled to become chilled accidentally to a small depth.

Chills are used when castings need hardness to withstand friction and wear.

Uses

• Wheel cam followers
• Crushing rolls railway
• Stamping dies

Gray Cast Iron

How Gray Cast Iron is Made

The chemical compound of carbon and iron breaks up to some extent when molten pig iron cools slowly. A good amount of carbon separates into small graphite flakes that become scattered throughout the metal. The graphite-like carbon (different from combined carbon), results in a fracture with a gray appearance, which is seen in regular gray cast iron.

Since graphite acts as a lubricant, metal shoots throughout with tiny, flaky cleavages.  It is easy to machine gray cast iron, but it cannot withstand a massive shock.

Characteristics

Free graphite in the metal’s structure acts as a lubricant. The lubricant property is ideal for components where sliding is required.  Gray cast iron also has:

• Good machinability
• No ductility
• Low tensile strength
• High compressive strength

It is also low cost, making it ideal when high strength and flexibility are not required.

Uses

• Machine tool bodies
• Automobile cylinder blocks
• flywheels

Properties

There is 90% to 94% metallic iron in gray cast iron with a mixture of silicon, sulfur, phosphorus, manganese, and carbon. Special high-strength grades of this metal contain  0.25 to 0.50 percent chromium, 0.75% to 1.50% nickel and/or 0.25% to 1.25% molybdenum.

Commercial gray iron has 2.50% to 4.50% carbon. About 1% of the carbon is combined with iron, while about 2.75% remains in the graphite or free state. Silicon content is frequently increased In making gray cast iron to allow the creation of graphitic carbon. The combined carbon (iron carbide) is cementite, which is a small percentage of the total carbon present in cast iron.

The more free carbon (graphitic carbon) present in cast iron, the softer the iron and the lower the amount of combined carbon content.

It is the iron used most frequently in foundries.

Identification Tests

• Appearance: Dull gray is the color of the un-machined surface of gray cast iron castings. It may be slightly roughened by the sand mold used in casting the part. It is rare to the machine to overcast iron castings. Un-machined castings may be ground in places to remove rough edges.
• Fracture Test: When conducting a fracture test nick a corner all around with a hacksaw or chisel and with a sharp hammer blow strike the corner. The dark gray color of the broken surface is caused by fine black specks of carbon present in the form of graphite (hence the name gray cast iron.) When fractured cast iron breaks short, brittle, small chips made with a chisel break off as soon as they are formed.
• Spark Test: When this metal is spark tested, a small straight line close to the wheel of dull-red sparks is given off.  The sparks disperse into a stream of multiple fine, repeated spurts that change to a straw color.
• Torch Test: The torch test causes a puddle of molten metal with a soft jelly-like consistency, and that is quiet. When raising the torch flame, the depression in the surface of the molts puddle immediately disappears. A tough, heavy coating forms on the surface as it melts. The molten puddle gives off no sparks and takes time to harden.

White Cast Iron

When gray cast iron is heated to a molten state, carbon dissolves completely and combines chemically with the iron. If the molten metal cools too quickly, white cast iron is formed as the two elements remain in a combined state. The carbon in this type of iron (iron carbide or cementite – Fe₃C)  measures above 2.5 to 4.5 percent by weight. It is called combined carbon. White cast iron is difficult to machine since it is brittle and hard.

White cast iron gets its name from the appearance of the metal when fractured.  It has limited applications due to poor machinability and mechanical properties.  It is a raw material used in the production of malleable cast iron.

It is used for inferior castings and areas where a hard coating is needed such as car wheel outer surfaces.

Difference Between Gray and White Cast Iron

The difference between white and gray cast iron is the color of the surface after it has been fractured and the composition.

• Gray Cast Iron: (2.5% to 4% carbon, 1% to 3% silicon, remainder iron)
• White Cast Iron: (1.7% to 4.5% carbon, .5% to 3% silicon, can also contain trace amounts of phosphorus, manganese, and sulfur

Malleable Cast Iron

When white cast iron is heated from 1400 to 1700°F (760 and 927°C) for about 150 hours in boxes containing iron scale or hematite ore, malleable cast iron is made. This heating (called annealing) causes a part of the combined carbon to change into an uncombined or free state. Temper carbon is formed, which happens when free carbon separates in a different way from the carbon in gray cast iron.  The temper carbon exists in the form of rounded, small carbon particles which give malleable iron castings the ability to bend before breaking. It can also withstand shocks better than gray cast iron.

Properties

The castings have properties more like those of pure iron: high strength, ductility, toughness, and ability to resist shock. Tensile strength is greater than gray cast iron and is excellent for machining.

Uses

Malleable cast iron is used when creating components utilized in the place of parts where intricate shapes create a forging problem or in place of forged steel.  It is used in pipe fittings, automobiles, and rail.

Welding Capabilities

Malleable cast iron can be brazed and welded. After welding the part should be annealed.

Identification Tests

• Appearance Test: The malleable cast iron surface is free from sand in a way that is similar to gray cast iron. The color is dull gray and a bit lighter than gray cast iron.
• Fracture Test: When fractured, the edges have a bright steel-like band while the central portion of the broken surface is dark gray. The fracture takes on a picture frame appearance. Good quality malleable cast iron is tougher than other cast iron and does not break when nicked.
• Spark Test: When ground, the bright, outer layer gives off sparks like steel. When reaching the interior, sparks near the wheel change to a dull-red color. Interior section sparks are like cast iron sparks; however, they are in a great volume and are somewhat longer.
• Torch or Flame Test: When exposed to a torch, molten malleable cast iron boils. After withdrawing, the surface will be full of blowholes. When fractured, the melted parts are very brittle and hard, having the appearance of white cast iron (they have been changed to chilled iron or white by melting and relatively rapid cooling).

Nodular Cast Iron (High Strength Cast Iron, Ductile Iron, or Spheroidal Graphite Iron)

Nodular cast iron is made when molten cast iron has added magnesium. Magnesium converts cast iron graphite from flake to nodular or spheroidal form. This improves the strength of the metal while increasing the yield point and lowering brittleness. Castings can replace steel.

Characteristics

One of the key features of nodular cast iron is high fluidity, enabling intricate shape castings.

Uses

• Cylinder heads for compressors and diesel engines
• Pipe fittings
• Pipes
• Valves
• Hydraulic cylinders

 Wrought Iron

Wrought iron is almost pure iron. It is made from pig iron with some slag mixed in during manufacture; it is almost pure iron. Wrought means that the metal has sufficient ductility to permit cold and hot deformation.

In the late 19th century, utilization of Wrought iron fell as mild steel became easier to obtain. Wrought iron resists corrosion and oxidation because of the presence of slag. It can be easily formed, arc or gas welded, and plated or machined.

In the late 19th century wrought iron usage declined as mild steel became widely available.  It is the purest iron (99%) plus a small amount of slag in the fibers. The 1% contains trace amounts of slag, sulfur, silicon, manganese, phosphorus or carbon.

How Wrought Iron is Made

Wrought Iron is made from pig iron in a puddling furnace and has a carbon content of less than 0.08 percent. It is the oldest form of iron made by man. Carbon and other elements present in pig iron are taken out, leaving almost pure iron.

In the manufacturing process, iron is mixed with some slag to create a fibrous structure in which long iron threads are combined with long lengthwise stringers of slag. Because of the presence of slag residue, wrought iron resists corrosion and oxidation, which cause rusting.  It is also can combat fatigue caused by vibrations and shocks.

The wrought iron is press squeezed and rolled in the mill into billets. The result is materials that are pure iron that is separated by a thin layer of slag.

Wrought iron is very low in carbon with the iron silicate or slag distributed throughout the base metal in fibers. When broken these fibers give the material a stringy or woody appearance.

Uses

Many products still described as wrought iron, such as guardrails and gates, are made of mild steel. Some items traditionally produced from wrought iron include:

• rivets
• porch railings
• fences
• farm implements
• nails
• barbed wire
• chains
• railway couplings
• water and steam pipes
• nuts and  bolts
• handrails
• ornamental ironworks
• furniture
• decorations

Welding Capabilities

Wrought iron can be machined, plated, and easily formed, and arc and gas welded.  Billets can be reheated to create:

• cooling towers
• coal handling equipment
• barbed wire
• chains
• nuts and bolts
• railway couplings
• crane hooks
• forgings
• boiler tubing
• plates
• bars

Limitations of Wrought Iron

Wrought iron has low fatigue strength and low hardness.

Wrought Iron Properties

• Brinell hardness number of 105; tensile strength of 35,000 psi; specific gravity of 7.7; a melting point of 2750°F (1510°C); and is ductile and corrosion-resistant.  It cannot be tempered or hardened like steel.
• Tensile strength of 35,000 psi; specific gravity of 7.7; a melting point of 2750°F (1510°C); and is ductile and corrosion-resistant.  It cannot be tempered or hardened like steel.
• Specific gravity of 7.7
• Melting point of 2750°F (1510°C)
• Ductile
• Corrosion-resistant
• Cannot be tempered or hardened like steel.

Metal Test for Wrought Iron

• Appearance Test: Rolled, low-carbon steel has the same as wrought iron.
• Fracture Test: Wrought iron has a fibrous structure. As a result, it can be split in the direction in which the fibers run. The metal is very ductile, soft, and easily cut with a chisel. It looks like rolled steel when bent. However, due to the fibrous structure, the break is very jagged. Wrought iron cannot be hardened.
• Spark Test: Straw-colored sparks form near the grinding wheel, and change to white, forked sparklers near the end of the stream when wrought iron is ground.
• Torch or Heat Torch Test: Wrought iron quietly melts without sparking. It has a peculiar slag coating with white lines that appear greasy or oily.

Pig Iron

Pig iron and many other types of iron and steel are produced by the addition or deletion of carbon and alloys.

In a blast furnace, iron ore is reduced to pig iron. The impurities are removed in the form of slag. It is comparatively weak and brittle with limited use. Approximately ninety percent is used to produce steel, although cast-iron pipe and some fittings and valves are manufactured from pig iron.

Ingot Iron

Ingot Iron is a commercially pure iron (99.85% iron). It has properties that are similar to the lowest carbon steel, and it is quickly formed. In steel, the carbon content is considered to be an alloying element whereas, in iron, the carbon content is regarded as an impurity. The primary use for ingot iron is for galvanized and enameled sheets.

Difference Between Cast Iron and Steel Ferrous Metals

All the various forms of the ferrous metals cast iron, steel, and wrought iron consist of chemical compounds and mixtures of iron, carbon, and various other elements in small quantities. Whether the metal is classified as cast iron or as one of the steels depends entirely upon the amount of carbon in it.

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

Carbon Content of Steel and Cast Iron

Item	Approximate Percentage of Carbon	Condition of Incorporated Carbon
Low-carbon Steel	Up to .3	All Combined
Cast Steel	.15 to .6	All Combined
Medium-carbon steel	.3 to .5	All Combined
High-carbon Steel	.5 to .9	All Combined
Tool Steel	.9 to 1.7	All Combined
Malleable Cast Iron	2 to 3.5	Free and Combined
Gray Cast Iron	2.5 to 4.5	.6 to .9 Percent Free 2.6 to 2.9 Percent Combined
White Cast Iron	3.5	Mostly Combined
Pig Iron	4	Free and Combined