chapter 7 alloy steels - dr. norhayati ahmad · 7-4 chapter 7 copyright ©2007 dr. ali ourdjini....

Chapter 7 Copyright © 2007 Dr. Ali Ourdjini.

Alloy SteelsAlloy Steels

77

7-2Chapter 7 Copyright © 2007 Dr. Ali Ourdjini.

At the end of this lesson students should be able to:

• Classify alloy steels

• Explain: effects of alloying elements to steel properties

• Discuss: composition, microstructure, mechanical

properties and engineering applications of

various type of alloy steels

• Select : suitable steel for appropriate purposes


Metal Alloys

Ferrous Nonferrous

Steels Cast Irons

Low Alloy

Low-carbon Medium-carbon High-carbon

High Alloy

PlainHigh

strength,

low alloy

Heat

treatablePlain ToolPlain Stainless

Gray

ironWhite

ironMalleable

iron

Ductile

iron

Classification of Ferrous Alloys


� Steels (0.008 ~ 2.14wt% C)

In most steels the microstructure consists of both a and Fe3C phases.

Carbon concentrations in commercial steels rarely exceed 1.0 wt%.

� Cast irons (2.14 ~ 6.70wt% C)

Commercial cast irons normally contain less than 4.5wt% C

Classification of Ferrous Alloys

• Based on carbon content

– Pure iron (< 0.008wt% C)

From the phase diagram, it is composed almost exclusively of the ferrite phase at room temperature.


• The carbon content is normally less than 1.0 wt%.

• Plain carbon steels: containing only residual concentrations of impurities other than carbon and a little manganese

About 90% of all steel made is carbon steel.

• Alloy steels: more alloying elements are intentionally added in specific concentrations.

• Stainless steels

Ferrous Alloys — Steels


• Low-carbon steels

� Less than 0.25 wt%C

• Medium-carbon steels

� 0.25 ~ 0.60 wt%C

• High-carbon steels

� 0.60 ~ 1.4 wt%C

Classification of Steels

According to Their Carbon Contents


• A four-digit number:

� the first two digits indicate the alloy content;

� the last two, the carbon concentration

• For plain carbon steels, the first two digits are 1 and 0;alloy steels are designated by other initial two-digit combinations (e.g., 13, 41, 43)

• The third and fourth digits represent the weight percent carbon multiplied by 100

For example, a 1040 steel is a plain carbon steel containing 0.40 wt% C.

The Designation of Steels


• A four-digit number: the first two digits indicate

the alloy content; the last two, the carbon

concentration

4141 40 40

Identifies

major alloying

element(s)

Percentage

of carbon

The Designation of Steels


• AISI: American Iron and Steel Institute

• SAE: Society of Automotive Engineers

• UNS: Uniform Numbering System

Table 11.2a AISI/SAE and UNS Designation Systems


SteelsSteels

Stee l Numerica l Name Key Alloys

10XX, 11 XX Carbon only

13XX Manganese

23XX, 25 XX Nickel

31XX, 33XX, 303XX Nickel-Chromium

40XX Mo

41XX Cr-Mo

43XX & 47XX Ni-Cr-Mo

44XX Mn-Mo

48XX Ni-Mo

50XX, 51XX, 501XX, 521XX, 514XX, 515XX

Cr

61XX Cr-V

81XX, 86XX, 87XX, 88XX Ni-Cr-Mo

92XX Si-Mn

93XX, 98XX Ni-Cr-Mo

94XX Ni-Cr-Mo-Mn

XXBXX Boron

XXLXX Lead 94XX N i-


• Less than 0.25 wt%C

• Unresponsive to heat treatments intended to form martensite;

strengthening is accomplished by cold work

• Microstructures: ferrite and pearlite

• Relatively soft and weak, but having outstanding ductility and

toughness

• Typically, sy = 275 MPa, sUT = 415~550 MPa, and ductility = 25%EL

• Machinable, weldable, and, of all steels, are the least expensive to

produce

• Applications: automobile body components, structural shapes, and

sheets used in pipelines, buildings, bridges, etc.

Low-Carbon Steels


TTT Diagram of Hypoeutectoid Steel


Table 11.1b

Mechanical Characteristics of Hot-Rolled Material and

Typical Applications for Various Plain Low-Carbon Steels


Applications

- automobile body components.

- structural shapes (I-beams, channel and angle iron)

- sheets (used in pipelines, buildings, bridges, tin cans)


• 0.25 ~ 0.60 wt%C

• May be heat treated by austenitizing, quenching,

and then tempering to improve their mechanical

properties

• Stronger than low-carbon steels and weaker than

high-carbon steels

a Classified as high-carbon steels

Typical Tensile Properties for Oil-Quenched and Tempered Plain Carbon

Medium-Carbon Steels


Applications

- railway wheels and tracks

- gears

- crankshafts


• 0.60 ~ 1.4 wt%C

• Used in a hardened and tempered condition

• Hardest, strongest, and yet least ductile; especially wear

resistant and capable of holding a sharp cutting edge

• Containing Cr, V, W, and Mo; these alloying elements

combine with carbon to form very hard and wear-resistant

carbide compounds (e.g., Cr23C6, V4C3, and WC)

• Applications: cutting tools and dies for forming and

shaping materials, knives, razors, hacksaw blades,

springs, and high-strength wire

High-Carbon Steels


Applications

- cutting tools

- drills

- high-strength wires

- spring materials

- wires and Springs

Applications of High Carbon Steels


Carbon Steel Alloy Steel

� Lower cost � Higher strength

� Greater availability � Better wear

� Toughness

� Special high temperature behavior

� Better corrosion resistance

� Special electrical properties

94XX Ni-

• Alloy steel is more expensive than carbon steel; it

should be used only when a special property is needed.

Comparison of the Advantages

Offered by Carbon Steels and Alloy Steels


• Plain carbon steels are relatively cheap, but have a number of

Property limitations. These include:

i) Cannot be strengthened above about 690 MPa without loss of

ductility and impact resistance.

ii) Not very hardenable i.e. the depth of hardening is limited.

iii) Low corrosion and oxidation resistance.

iv) Must be quenched very rapidly to obtain a fully martensitic

structure, leading to the possibility of quench distortion and

cracking.

v) Have poor impact resistance at low temperatures.


� Hardenability - Alloy steels have high hardenability.

� Effect on the Phase Stability - When alloying elements are added to steel, the binary Fe-Fe3C stability is affected and the phase diagram is altered.

� Shape of the TTT Diagram

�� Increase strengthen, hardness and toughness of the steelIncrease strengthen, hardness and toughness of the steel

�� Improve corrosion and wear resistanceImprove corrosion and wear resistance

Effect of Alloying Elements


Effect of Alloying Elements on TTT diagram


Alloying ElementsAlloying Elements

•• Alloying elements combine in one of two ways:Alloying elements combine in one of two ways:

�� Form solid solution with ferrite: strengthen the ferrite Form solid solution with ferrite: strengthen the ferrite

�� Combine with carbon to form carbides: retard the softening rate,Combine with carbon to form carbides: retard the softening rate,resulting in greater toughnessresulting in greater toughness


αα -- stabilisersstabilisers

γγ -- stabilisersstabilisers

γγγγ

chapter 7 alloy steels - dr. norhayati ahmad · 7-4 chapter 7 copyright ©2007 dr. ali ourdjini....

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