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ENSC1004 - Lecture 19

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ENSC1004 - Lecture 18
Engineering Alloys - Slides

Introduction

  • Most engineering metallic materials are alloys

Pure metals are alloyed to:

  • Enhance properties, such as strength, hardness or corrosion resistance
  • Create new properties, such as shape memory or magnetic properties

Classification of Engineering Alloys

Ferrous Alloys

  • Iron is one of the most used elements in the world
    • 2.5 billion tons of iron ore mined in 2019
  • Key element in steel

Steels

Nearly all steels produced as either:

  • Cast products produced by casting, produces near-finish components
  • Wrought products semi-processes raw materials for further fabrication into components

Steels are roughly Iron (Fe) + Carbon (C) alloys

  • Increasing C content leads to:
    • Increased strength and hardness
    • Decreased ductility and toughness

Main microstructure characteristics in steels:

  • Two dominant phases:
    • Ferrite: Fe-C
      • Solid solution; appear as equiaxial grains
      • A soft & ductile phase
    • Cementite: FEC
      • Typically of platelike morphology
      • A hard & brittle phase
  • Unique microstructure:
    • Pearlite: layered structure of ferrite and cementite
    • Good strength & toughness

Low Carbon Steels (Mild Steels)

Carbon: 0.1-0.3 wt%

Properties:

  • Strength: relatively low (𝜎𝑦 =250-400 MPa)
  • Ductility: high (~30% elongation)
  • Weldability: excellent
  • Relative cost: set as 1

Used where low cost, high formability are needed

  • Typical applications: structural panels (cars, train carriages…), containers, pipes, panels, sheets, wires, etc.
Medium Carbon Steels (Structure Steels)

Carbon: 0.35-0.55 wt%

Properties:

  • Strength: 1.5 times of low carbon steel (400800 MPa)
  • Ductility: ~75% of low carbon steel (~25% elongation)
  • Weldable, weldability decreases with increased C%
  • Relative cost: 1.1

Used where high strength/toughness is required

  • Typical applications (load bearing): crankshafts, bolts, gears, link rods, structures, heavy-duty machinery, mining equipment, pressure vessels, railway tracks, train wheels, axles, chassis…
Higher Carbon Steels (Spring and Tool Steels)

Carbon:

  • 0.6-0.8 wt% (spring)
  • 0.8-1.2 wt% (tool)

Properties:

  • Strength: ~ 2-7 times of low carbon steels
  • Ductility: < 50% of low carbon steels
  • Weldability: poor
  • Machinability: poor
  • Relative cost: 1.2 – 1.5

Used where high strength/high hardness is required, often ductility/toughness sacrificed

  • Typical applications: springs, hammers, chisels, drill bits, knives, saw blades, extrusion dies, punches, cutting tools and other wear resistant applications
Stainless Steels
  • Has 12 wt% Cr by definition
    • The Cr lets the steel form CrO which protects it from corrosion
    • Reduced level of C to prevent CrC forming (causes cracks)

Properties:

  • Strength: 𝜎𝑦 ~ 200-1600 MPa
  • Ductility: %EL~ 2-40%
  • Relative cost: 4 to 10

Cast Irons

  • Iron (Fe) + Carbon (2.5-4.3 wt%)
    • Much higher carbon content than steels
    • Higher C → lower melting temperature of the alloys
      • Enables easier casting
    • Higher C → Formation of graphite

Four Main Types of Cast Irons:

  • Grey irons
  • Nodular irons
  • White irons
  • Malleable irons

Grey Cast Irons

Microstructure:

  • Graphite flakes embedded in ferrite and/or pearlite matrix, ‘cutting’ the continuity of the matrix and rendering the grey irons of high brittleness

Mechanical/processing characteristics:

  • High strength in compression, but brittle in tension
    • Graphite flakes act as cracks
  • High damping capacity
  • Good metal-metal wear resistance when lubricated
  • Excellent castability
  • Cheap to produce

Typical applications: engine cylinders, pistons, gear box casing, transmission casing, machine tool bases, balance weight of large cranes, large diameter underground pipework…

  • Casting enables making of complex shapes that are difficult to machine
Ductile (Nodular) Cast Irons

Microstructure:

  • Graphite nodules in ferrite and/or pearlite matrix, which significantly improve the toughness and ductility

Mechanical/processing characteristics:

  • Much stronger and tougher than grey irons
  • More expensive than grey irons

Typical applications: produced for higher specification applications, including gears, crankshafts, pump bodies, pressure valves, rollers, …

White Cast Irons

Microstructure:

  • Cementite plates (light areas) embedded in pearlite matrix (dark areas)

Production:

  • Produced with low Si (< 1.0 wt%) cast irons under rapid cooling (thus stabilizing FeC and preventing graphite formation)
  • Mostly produced as an intermediary to produce malleable irons

Properties:

  • High hardness & brittleness

Typical applications: suited for abrasion wear resistant applications, such as ball mill lining tiles, slurry pipeline elbows, slurry pump bodies, earth moving equipment and jaw crushers …

Malleable Cast Irons

Microstructure:

  • Graphite rosettes embedded in ferrite or (ferrite + pearlite) matrix

Production:

  • Heat treatment of white cast irons to decompose FeC into ferrite and graphite, as following:
    • Heat to 900 °C - 950 °C for 40 hours
    • Cool to 700 °C & hold for another 40 hours
  • This extensive heat treatment procedure makes malleable irons expensive to produce

Properties:

  • Stronger, tougher, much more ductile than grey irons and comparable to nodular irons
  • Have certain capacity to take shock loading, bending and tension
  • Suitable for thin-wall casting

Typical applications: gear box casing, transmission casing, differential casing, …

ENSC1004 - Lecture 20

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