Understanding Steel: Historical Uses, Crystal Structure, and Properties, Exams of Civil Engineering

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Understanding Steel as a Structural Material

Early constructions in

Steel

• Indus Valley, Greeks and

Egyptians ( 1000 BC)

• Wrought Iron as tie bars in

Haghia Sophia ( 6 th

Century)

• Galleries for house of

Commons and railings for

St.Paul’s cathedral ( 17 th

Century)

• 18 th Century refractory

furnace - Coalbrookadale

bridge

• 1855 Henry Bessemer

invented the modern steel

making

4 2.05 T 2 T A 2 T 2.05 + T B d d

1 T

2 T Beam A Beam B Ductility Load

d

Learning point : A structural component must posses adequate “strength” and “ductility” to carry loads safely. This point is more appropriate for flexural or bending members.

Metallurgy of Steel

Allotropy of Iron

(Ferrite)

200 400 600 800 1000 1200 1400 1600 Magnetic Non-Magnetic Heating Cooling 768 0 C 910 0 C 1400 0 C 1539 0 C bcc fcc bcc 0    d Temp 0 C Time

Ferrite or Iron has little mechanical strength

10

Steel gets its strength by shearing of its layers

Learning point : Steel is nothing but a solid solution of carbon in iron

Iron – Iron-Carbon phase diagram

Peritectic point 1.0 2.0 (^) 3.0 4.0^ 5.0^ 6.06. 910 0 C 1147 0 C 723 0 C 1493 (^0) C 0.02%  ferrite phase Austenite + Liquid Liquid Fe 3 C+ Liquid  + Fe 3 C Austenite + Cementite  + Fe 3 C Ferrite + Cementite Cementite Eutectoid point  + 

200 400 600 800 1000 1200 1400 1534 0 C 0.1% d ferrite 0.8% 4.3% Eutectic point % Carbon

Different stages of formation of Pearlite

Austenite Ferrite nuclei Austenite Ferrite Pearlite Ferrite C (a) (b) (c)

Microstructures of steels

(a) 100 % Ferrite in extra

low carbon steel

(b)Ferrite+Pearlite

(c) 100 % Pearlite in

Eutectoid steel

(d)Pearlite+Cementite in

hyper-Eutectoid steel

SOURCE: Thelning K.E., “Steel and its heat treatment”, Butterworths, ( 1984 ). Learning point : Structural steel or mild steel has a “Ferrite + Pearlite” micro-structure. The pearlite imparts the mechanical strength and ferrite gives ductility for steel.

Example of fire affected steel structures

Cross section of fillet welds (a) Coarse grained weld (b) Heat affected zone (c) original micro structure

0. 0 200 400 600 800 Temp 0 C Variation of microstructure as a function of cooling Time in Seconds 1 10 100 100 0 10 4 10 5 Eutectoid temperature Mantensite Mantensite+ Pearlite (^) Fine Pearlite Course Pearlite Full annealing Normalise Oil Water quench quench

20

200 400 600 800 Temp 0 C Rate of cooling Vs microstructure Time in Seconds 1 10 100 1000 104 105 Eutectoid temperature Martensite Martensite+B ainite Martensite Ferrite Bainite Pearlite Course Pearlite Austenite →Martensite Austenite → Bainite Martensite Ferrite Bainite Ferrite Pearlite 8.4^0 C/sec

1. 0230 C/sec
2. 00620 C/sec
3. 330 C/sec Bolts have martensitic microstructure Mild steel – 250 MPa Bolt 4.6 – 400 x 0.6 = 240 MPa Bolt 8.8- 800 x 0.8 = 640 MPa Bolt 10.9 = 1000 x 0.9 = 900 MPa