Chapter 5

Ferrous Alloys

Objectives

• Upon completion of this chapter you should understand:• Applications of steel and cast iron.• Compositions of plain carbon steel, alloy steel, high alloy steel

and cast iron.• The basic process of making iron and steel.• Applications of plain carbon steel, alloy steel, and cast iron.• Heat treatment of steel.• Surface hardening and its applications.

Key Concepts

• Ferrous alloys generally refer to iron and steel.• Approximately 85% of all metals used for

industrial applications in the United States are iron based.

• The popularity of iron and steel is due to low cost, mechanical properties, and high modulus of elasticity.

Composition• Ferrous alloys are basically iron with a certain amount of carbon

added.• Steel contains less than 1.7% of carbon.• Cast iron has 2 to 4% of carbon.• Plain carbon steels are alloys of only iron and carbon.• Alloy steels have less than 5% alloy elements.• High alloy steels have large amounts of alloys - for example

stainless steel has 18 % chromium and 8 % nickel.• Super alloys are plain carbon steel with more than 50% alloys.• Cast iron has high content of carbon and silicon.• Cast iron containing chromium and nickel is abrasion resistant.

Creating Iron and Steel• The first step in iron and steel making is to convert iron ore into pig iron.• Iron ore is converted into pig iron in a blast furnace (fig 5-1, page 71).• Pig iron is about 4% carbon.• Since most steel is less than 1% carbon, excess carbon must be removed.• An oxygen furnace can be used – fig 5-2, page 72).• A high velocity stream of oxygen is directed down the molten mixture.• Fig 5-3, page 73 shows a steel tube production process.• Cast iron is converted from pig iron in a smelting furnace.• Most iron foundries use electric induction furnaces.

Plain carbon steel

• Contains a maximum of 1.5% carbon.• The higher the carbon content, the harder and stronger the

steel – fig 5-5, page 76.• 3 categories (fig 5-6, page 77):• 1. Mild steel or low carbon steel: Maximum of 0.25 % of

carbon. Used in construction and are inexpensive.• 2. Medium Carbon Steel: Carbon content between 0.25 and

0.65%. Used in automobile parts – crankshafts, pinions, struts, and tie rod ends.

• 3. High Carbon Steel: Contains between 0.60 and 1.0% carbon. Used in cutting tools such as lawn mower blades.

Alloy Steels• Alloy steels contain 1 to 4% alloy elements (fig 5-7, page 78):• Manganese increases the strength and hardness of carbon steels - used

in high strength bolts.• Silicon strengthens the strength of steel – used in leaf springs for trucks,

electrical transformer cores.• Chromium increases strength, fatigue resistance, and hardness – used for

gears, axles, shafts, springs, ball bearings, and metal working rollers.• Nickel has a strengthening effect – used in airplane landing gears and

heavy truck parts.• High strength Low Alloy Steels (HSLA Steels) are used in structural

applications where welding is a major requirement – bridges, towers, railings, and stairs.

High Alloy Steels• High alloy steels have alloy content of 10% or higher.• Stainless steel is selected for excellent corrosion resistance. This is due to

high chromium content (12%). • Ferritic Stainless Steels: Contain 12 to 30 % chromium (fig 5-9, page 81).• Martensitic Stainless Steels: Are cooled by quenching in water. Used

where high hardness is required – cutlery, scissors, and surgical tools. • Austenitic stainless steels: 65 to 70 % of US stainless steel production.

Iron-chromium-nickel alloys containing 16 to 25% chromium and 7 to 20 % nickel. High corrosion resistance (fig 5-11, page 83).

• Tool Steels: Used in dies, shear blades, punches, rollers, and cutting tools. Slow cooling protects the steel from cracking and distortion (fig 5-12, page 84).

Cast Iron• Cast Iron usually contains 2 to 4% carbon and 1 to 3% silicon and other alloys to

control other specific properties.• The advantage of cast iron is low cost and ease of casting.• They are used for engine blocks, heads, camshafts, piston rings and crankshafts.• White Cast Iron: carbon and silicon content are low – white fracture surface on

breaking. Used for grinding balls, farm equipment, and roller dies.• Grey Cast Iron: Grey when broken. Have excellent machinability and are used for

machine bases.• Malleable Cast Iron: White cast iron heated to a high temperature. Stronger and

more ductile. Used in automotive differential housings.• Ductile Cast Iron: Used in automobile engine crankshafts because of high strength

and toughness.• High Alloy Cast Iron: Large amounts of alloy for special properties. 30% chromium

for corrosion resistance used in mining and agricultural applications. High silicon content for corrosion resistance used in acid containers and chemical pumps.

Heat Treatment• Heat treatment is a process of heating and cooling certain alloys to obtain

desired properties.• Quick cooling increases the strength of steel.• Quenching: Heating steel above critical temperature, holding for a period

of time, and rapidly cooling the metal.• Tempering: Tempering is when a quenched part is reheated to a

temperature lower than quenching temperature, held for a long period, and then cooled. Must be done soon after quenching to reduce risk of cracking.

• Annealing: Heating to high temperature and then slow cooling. Softer material for good machinability.

Surface Hardening• Flame Hardening: Rapidly heat the surface with a flame and rapidly

quench. For knifes and gears. • Induction Hardening: Heat source is a magnetic field that induces current

flow within the steel surface. This is converted to heat. Subsequent quenching results in a hardened surface layer. For crankshafts, axle shafts, and gears.

• Laser Hardening: Intense localized heat. For bores in an engine block.• Carburizing: Adding carbon to the outer surface by diffusion. For gears,

bearings, and shafts.• Nitriding: Nitrogen is diffused in the surface. Prevents oxidation and

scaling. Harder than carburized steel.

Summary• Approximately 85% of all metals used for industrial applications in the

United States are iron based.• The popularity of iron and steel is due to low cost, mechanical properties,

and high modulus of elasticity.• The first step in iron and steel making is to convert iron ore into pig iron.• Iron ore is converted into pig iron in a blast furnace.• The higher the carbon content, the harder and stronger the steel.• Stainless steel is selected for excellent corrosion resistance. This is due to

high chromium content (12%). • The advantage of cast iron is low cost and ease of casting.• Heat treatment is a process of heating and cooling certain alloys to obtain

desired properties.• Quick cooling increases the strength of steel.

Home Work

• 1. Why are iron and steel popular?• 2. What furnace is used to covert iron ore into pig iron?• 3. What are 3 categories and uses of plain carbon steel?• 4. What are the advantages and uses of adding manganese,

silicon, chromium, and nickel to alloy steels?• 5. What is the advantage of stainless steel?• 6. What is the advantage of cast iron?• 7. What is the advantage of heat treatment?