Steels and Cast Irons Applications and Metallurgy Metallurgy for the Non-metallurgist

Learning Objectives After completing this lesson, students will be able to: o Describe steel o List some of the properties of steel and explain how they are different from those of other materials o Tell how steel is produced o Describe cast irons o Tell how cast iron differs from steel

Introduction: Steels and Cast Irons Steel as an engineering material Production of steel and steel shapes Some mechanical properties/microstructures Production of cast irons Microstructures of cast irons

Steels Enormous variety, compositions & micros Plain carbon, low alloy, alloy, specialty Range from ELC to 2 % C, plus Si, Mn, Al, S Plastic for forming, elastic for use Modulus: 30 million PSI Properties largely dictated by amount and distribution of Fe 3 C; heat treatment Up to 600+KSI BCC: ductile to brittle transition

Elasticity Steel is Elastic σ = Eε Elastic Limit—maximum stress steel can withstand without permanent deformation Elastic Modulus is not appreciably affected by carbon content, amount of alloying elements or processing variables

Stress-strain diagram plotted from tensile test results. (Upper curves show other possible variations.)

Strength Plasticity—elongation due to load Effect of Composition—effects the shape of the stress-strain curve High carbon steels resist deformation and are more elastic

Hardness of Steel

Approximate relationship between hardness and tensile strength for steel

Impact Resistance Charpy V-notch Impact Test Ductile to Brittle Fracture Transition

Effect of carbon content on notch toughness

Effect of microstructure on notch toughness

Fatigue Resistance

Effect of shot peening on fatigue behavior

Flow diagram showing the principal processes involved in converting raw steel into mill product forms (excluding coated products)

Cast Iron Contain more than 2% carbon Types of Cast Iron o White Iron o Gray Iron o Ductile Iron o Malleable Iron

Schematic illustration of the mechanical deforming action that occurs during hot rolling

Types of graphite flakes in gray iron (AFS-ASTM)

Structure of as-cast white iron

Gray cast iron, as-cast. Structure is Type A graphite flakes (dark) in a matrix of pearlite (gray lamellar structure of ferrite and pearlite).

Typical ductile (nodular) iron as-cast. Spheroidal nodules of graphite (dark) surrounded by an envelope (bull’s-eye) of ferrite (white) in a pearlite (gray) matrix. With slower cooling, the ferrite envelope would be larger until eventually the entire matrix would be ferrite.

Ferritic malleable iron two-stage annealed by holding 4 h at 954 °C (1750 °F), cooling to 704 °C (1300 °F) in 6 h, air cooling. Type III graphite (temper carbon) nodules in a matrix of granular ferrite; small gray particles are MnS

Typical stress-strain curves for three classes of gray iron in tension

Effect of section diameter on tensile strength at center of cast specimen for five classes of gray iron

Tensile properties of ductile iron vs. hardness

Stress-strain curve for as-cast ductile iron used for crankshafts

Effect of composition and microstructure on Charpy V-notch impact behavior of ductile iron

Fatigue properties of ductile iron (a) ferritic and (b) pearlitic

Rubber wheel abrasion test results

Wear of grinding balls

Relation between gouging wear and carbon content for various types of steel and cast iron

Melting and Casting

Sectional view of conventional cupola

Sectional view of a coreless induction furnace. (Arrows in crucible show direction of stirring action.)

Design of a bottom-pour ladle used for pouring large steel castings

Typical teapot-type ladle used for pouring small to medium-size castings

Summary Both steels and irons show wide range of compositions and properties Anisotropy from working or from cast structure Small alloy additions have dramatic effects Heat treatments applicable to both