HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELSCORROSION, FIRE PROTECTION AND FATIGUE CONSIDERATIONS

Dr. Shamsher Bahadur Singh, PEGroup LeaderCivil Engg. GroupBITS Pilani-333031, (Raj.)E-mail: sbsingh@bits-pilani.ac.in

Layout of the LectureHistorical Development of SteelMetallurgy of steelProduction ProcessMechanical propertiesSpecial steelsCorrosion aspectsFire Engineering of steel structuresFatigue considerations

Early constructions in SteelIndus Valley, Greeks and Egyptians (1000 BC)Wrought Iron as tie bars in Haghia Sophia (6th Century)Galleries for house of Commons and railings for St.Pauls cathedral (17th Century)18th Century refractory furnace - Coalbrookadale bridge1855 Henry Bessemer invented the modern steel making

Iron & Steel Tradition of IndiaOur Great EpicsIron Pillar of MehrauliDwaja Stamba - a rustless wonder for 1.5 millennia !Rabindra Sethu!Numerous Steel monuments of colonial pastSecond Hooghly and Jogighopa Bridges- Steel intensive constructionsVisionaries like Tata, Pandit Nehru and Visweswaraya have sown the seed for development in Steel

Into the Lecture ...Historical Development of SteelMetallurgy of steelProduction ProcessMechanical propertiesSpecial steelsCorrosion aspectsFire Engineering of steel structuresFatigue considerations

What do we do in these situations? And Why should we teach the students a little bit of metallurgy?Bhai Sahib, I saw some pipes lying in your back door, can I use them as purlins for your shop floor extension shed?A failure investigation report say ..the bolts have matensitic structure in microstructure analysis.After exposure to fire the steel member a hard ness value of 345 PHN.

The world has become cramped ? ! !So do Engineering disciplinesNo Single conventional Eng. discipline can exist on its ownStructural Steel Construction, Metallurgy and Mechanical Engineering disciplines crisscross each other !Air Crossing !

(b) Face centred cube (fcc)(a) Body centred cube (bcc)Crystal structure of Iron

Interstitial solid solution of Carbon in Iron Steel is alloy of Iron and Carbon ! (Point to ponder)More Carbon more strength? - YesBut what happens to ductility? Adding Carbon - Is it the only way to get strength?Is there a limit for carbon content in structural steel? Carbon content in Structural steel- 0.12-.25%

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 steelSOURCE: Thelning K.E., Steel and its heat treatment, Butterworths, (1984).

Are there any parameters which increases strength of steel? Yes, Grain sizeUsing temperature control we can achieve small grain sizeUsing Mechanical pressure during rolling we can get smaller grain size

Substitution solid solution of Manganese in Iron-Manganese-FerriteAre there any parameters which increases strength of steel? Yes, Additive metalsHow much can we add? It Should not exceed Ceq(0.45)Ceq =%C + % Mn/6 + (% Cr + % Mo + % V)/5+(% Ni + % Cu)/15 What happens then (Ceq >0.45)? Welding becomes a problem !!

Points to Ponder about steel!More carbon : more strength; but negatively affect the important property of ductility!Smaller the grain size greater the strength (Production control)Smaller additive metals also increases the strength of steel - HSLA steels

Into the Lecture ...Historical Development of SteelMetallurgy of steelProduction ProcessMechanical propertiesSpecial steelsCorrosion aspectsFire Engineering of steel structuresFatigue considerations

Iron ore, Limestone and coal is fed into the BFLiquid pig iron collects at the bottom of BF(C-4.0%;Mn-.5%;P-0.12%;S-0.05%)Hot metal is charged into Steel making vessel. Oxygen is blown in a controlled manner to reduce carbon and impurities like silicon, manganese, phosphorous and sulphur . Final steel is produced. (C-0.1-.25%;Mn-.4-1.2%;P-0.05%;S-0.05%).Oxygen less than 30 ppm - fully killed steel; 150 ppm semi-killed steel.

Source:Adams P.F., Krentz H.A. and Kulak G.L., Limit state design in structural design SI Units, Canadian Institute of Steel Construction (1979).

Primary rolls for plates Primary rolls for structural shapesMolten steel

Into the Lecture ...Historical Development of SteelMetallurgy of steelProduction ProcessMechanical propertiesSpecial steelsCorrosion aspectsFire Engineering of steel structuresFatigue considerations

FStandard tensile test specimenFrtdLcArea=S0-LTensile strength of steelFDeformed regionsMoving edges of Luders bandLuders bands in tensile test specimenTensile test specimen before ruptureFArea=S-LFF

fyysh 10yfyElastic rangePlastic rangeStrain hardening rangeEsh Stress strain curve for sharp yielding structural steelsVariation due to Luders bands Stress strain curve for continuously yielding structural steels

Hardness

Hardness is regarded as the resistance of a material to indentations and scratching. This is generally determined by forcing an indentor on to the surface.

(a)

Brinell hardness

Square based diamond pyramids of 135O included angle

(b)

Vickers Hardness

Diamond core with 120 O included angle

h1h2cExperimental set up for notch toughness testTest specimen for notch toughness test

1.21.110-51.010-410-310-210-1Strain rate in seconds-1Effect of strain rate on the yield strength of steel

Some Mechanical properties of structural Steel

Type of steel

Design-ation

UTS (MPa)

Yield strength (Mpa)

Elongation Gauge

Charpy V -notch values Joules (min)

Thickness (mm)

40

Standard structural steel

Fe 410A

410

250

240

230

23

27

Fe 410B

410

250

240

230

23

27

Fe 410C

410

250

240

230

23

27

High tensile structural steel

St58HT

580

360

0.05

1.00

20

-

ST55-HTW

550

360

.05

1.00

20

-

Into the Lecture ...Historical Development of SteelMetallurgy of steelProduction ProcessMechanical propertiesSpecial steels Corrosion aspectsFire Engineering of steel structuresFatigue considerations

Special steelsStainless steels

Stainless Steel grades and their usage

Grade of stainless steel

Usage

316 (18% Cr)

Profiled roofing, cladding, gutters, facades and hand railingsin highly polluted environments

304 (18% Cr-(% Ni)

Decorative elements in areas near coast line. Also for kitchen and sanitary warescoastal and less polluted areas

430 (17% Cr)

Roofing, gutters, decorative wall tiles, hallow structural sectionsnon-polluted environments

409 (11% Cr)

Painted roofing-- non-polluted environments

Cold rolled steel and cold forming of steelUnheated slabs are rolled at ambient temperature into thin sheets (.3mm-8mm)Results in smooth surface and improved mechanical propertiesCold forming of hold /cold rolled sheetsCF sheets are basically low carbon steels which are reheated to 6500C to recrystalise ferrite to get finer grain size

TMCP steels and HSLA steelsThermo-Mechanically Controlled Process (TMCP) steels are becoming popular in obtaining high strength steels of yield strength of 800 MPa and thickness even beyond 40 mm.

High Strength Low Alloy(HSLA) also are very popular which requires very good process control.

Fire resistant steelsVery cost effective compared to structural steelFRS are available in IndiaVery popular and cost effective - Japanese experience

Chemical composition of fire resistant steel

C

Mn

Si

S

P

Mo+Cr

FRS

(0.20%

(1.50%

(0.50%

(0.040%

(0.040%

(1.00%

Mild Steel

(0.23%

(1.50%

(0.40%

(0.050%

(0.050%

-

Into the Lecture ...Historical Development of SteelMetallurgy of steelProduction ProcessMechanical propertiesSpecial steels Corrosion aspectsFire Engineering of steel structuresFatigue considerations

Drop of WaterThe mechanism of fretting corrosionGalvanic corrosionHydrogen embritlementBacterial corrosionTypes of corrosion

Simple orientation of membersMethods of prevention corrosion - Simple proceduresSimple rule:Eliminate the electrolyteAvoid simultaneous presence of water and oxygen

Is Corrosion a real Problem?Indian designers feel that steel corrodes most in India. Is it true?Steel corrodes all over the world! But they are better managed in the western countries!Excellent protective coatings which retain their life even up to 20 years are available!Corrosion-where does it matter? Normal inland there is no problem! Exposed conditions ofcourse do need attention.Corrosion is no more a disincentive for not using steel in housing sector!

Into the Lecture ...Historical Development of SteelMetallurgy of steelProduction ProcessMechanical propertiesSpecial steels Corrosion aspectsFire Engineering of steel structuresFatigue considerations

Positive points of steel as a construction material under fireDamage to strength of steel due to fire is reversible in most of the casesUsing the principle if the member is straight after the fire - the steel is O.K many of the members could be salvaged.Up to about 2150C steel retains its strength In the case of concrete, at 2350C turns pink; 5900C turns red and irreversible damage after 6000CSteel exposed to 6000C could be strengthened and reused.

Typical fire loads and behaviour of steel under fire

Examples of fire load in various structures

Type of steel structure

Kg wood / m2

School

15

Hospital

20

Hotel

25

Office

35

Departmental store

35

Textile mill show room

>200

Typical fire loads and behaviour of steel under fire

Mechanical properties of steel at elevated temperatures

Fire Engineering of steel structures- very well developed in the literature

Methods of fire protectionSpray protection

Board protection

Intumescent coatings

Concrete encasement?

Into the Lecture ...Historical Development of SteelMetallurgy of steelProduction ProcessMechanical propertiesSpecial steels Corrosion aspectsFire Engineering of steel structuresFatigue considerations

Modified Goodman diagram for fatigue resistant design of steel structures

THANK YOU VERY MUCH !