Material Selection & Quality Assurance

8/18/2019 Material Selection & Quality Assurance

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Material Selection and Quality Assurance

Chiew Sing-PingSchool of Civil and Environmental Engineering

Nanyang Technological University, Singapore


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Material Desi n & Execution

Desi nBS 5950

BS EN 1993

Material Execution s on y BS EN 1090

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ni uel in a ore!

Desi nBS 5950

BS EN 1993

Material Execution s on- s

(ASTM/JIS/AS/NZS/GB)BS EN 1090

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Material Issues ?

• Steel material production standards aresubstantial documents covering mechanical,

• One piece of steel is not necessary the same

• We are not the only ones using steel• e never uy s ee y we g

• Testing a batch of steel from different ‘parents’

is meaningless• Material failure can be sudden and disastrous

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Performance Re uirements for Structural

Applications

• Strength – ability to carry load

• Ductility – ability to sustain permanent

–

without fracture

• Weldability – ability to transfer load

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–

Strength Toughness

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The performance of structural steel can be

enhanced through three basic mechanisms, i.e.

• the introduction of interstitial and

• the generation and concentration ofdislocations at the grain boundaries (work or

strain hardenin

• the formation of additional grain boundaries.

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any ypes o ruc ura ee

• Carbon (non-alloy) steel

• Alloy (fine-grain) steel

• -

• -

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Steel Structure at Macro, Micro and Nano

Level

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–

molecules

The scanning tunneling

possible to image the electron

cloud associated individualatoms at the surface of a

material. Right is an STM

showing the regular alignmentof atoms.

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Linear Defects (Dislocations) - Source of

Plasticity

the crystal structure. The movement of dislocations when a

– .

Discovered by Taylor, Orowan

& Polyani in 1934 with the aid

of TEM

Dislocation as seen under TEM (transmission electron microscope)

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-

Pure iron is soft and weak. By dissolving

carbon and other elements into molten iron,

steel with much superior engineering propertiesover pure ron can e ac eve .

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• Carbon steel: carbon and manganese the main

interstitial alloys, with Mn (< 1.65%), Si (< 0.6%) & Cu

< . . improving strength with addition of carbon and

manganese.

• Alloy steel: essentially low carbon-manganese steelalloyed with addition of strong carbide or nitride

forming elements, e.g. Nb, Ti or V.

increasing strength by grain refinement andprecipitation hardening.

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• s ee may e c ass e as a car on s ee e

maximum content specified for alloying elements does

- . , - . , -0.60%; (2) the specified minimum for Cu does not

.

• Carbon steel differ from low-alloy and alloy steel in that

.

alloying elements are not specified.

• ncreas ng e percen age o car on ra ses e y e

strength and hardness but reduces ductility and

a verse y a ec s we a y.

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Hi h Stren th Low-Allo Steel

• HSLA steel have moderate amount of alloying elements

other than carbon. The term low-alloy is used to describe

steel for which the total of all the alloying elements does

not exceed 5% of the total composition.• These steel have been develo ed as a com romise

between the convenient fabrication characteristics of the

low cost mild carbon steel and the high cost of heat-treatedalloy steel.

• HSLA steel have ield stresses ran in from 275 to 460

MPa, and well-defined yield points like mild carbon steel.They are used in the as-rolled or heat-treated in the

normalized conditions.

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Work Hardening -

Work hardening also known as strain hardening or

cold workin is a wa of stren thenin b lasticdeformation

dislocations at the grain boundaries.

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–

deformation

(b)

(a)

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Dislocation Pile-ups at Grain Boundaries

s oca on p e-ups a gra n oun ar es n ca e ese oun ar es are very s rong o s ac es ofurther dislocation motion.

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Theory of Work Hardening

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-

Cold forging for rebars Cold for in for headin s

Cold flat rolling Cold roll bending

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Heat Treatment -

Improving the properties through control of grain size

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Heat Treatment• Although the properties of steel are greatly affected

,

furnace can also affect the mechanical properties

• Most of these treatments involve changing the

,treatment is used generally to cover all these

.

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Microstructure

High strength, high

tou hness, low ducti li t

c anges ur ng

heat treatment

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• ,

structures

• emper ng: re n ng e m cros ruc ures an par a yrelieving residual stresses

• Annealing: stress relieving, a treatment opposite to

hardening• Normalizing: refining grains which have been

deformed through cold work

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•

anywhere to 815 to 9000C for most steel and then

suddenl coolin it in water brine oil or molten lead.

• The rapid cooling causes the formation of fine grained

.

fabricating steel, it is most commonly used to harden

steel b introducin martensite a ver hard but brittlemicrostructure.

• ,

residual stresses and distortion. Besides, the

problem with rapid quenching.

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emper ng

• To refine the microstructures and partially relieve

residual stresses, quenching is normally followed by

emper ng.• Tempering consists of normally reheating the steel to

370-6500C and cooling it in air.

• As a result, the internal stresses are partially relievedand the ductility as well as toughness are improved

remarkably, without great reduction in the strength.

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Annealing

• Annealing, also called stress relieving, is a treatment

opposite to hardening (quenching).

• It is achieved by heating the steel to a temperatureabove the transformation ran e hi her than

tempering), and after maintains the specific

temperature for a sufficient time, cooling the steelvery slowly in the furnace.

• This rocess im roves the ductilit of the steel and

decreases residual stresses but on the other hand,reduces the yield strength, tensile strength and

hardness accordingly.

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orma z ng

• Normalizing includes heating a ferrous alloy to a

suitable temperature above the transformation

temperature range and cooling in air.• It is used to refine rains which have been deformed

through cold work. During normalizing, small grains

are formed which lead to a tough metal with normalstrength, but it is not so ductile as steel achieved by

annealing.

• Strictly speaking, normalizing is an annealing process.

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Difference between Tempering, Normalizing


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Difference between Tempering, Normalizing

an nnea ng

Tempering• To toughen previously hardened steel.

• The steel is heated uniforml below lower critical tem erature

and then cooled in air.

• To relieve stresses and increase ductility.

• .

temperature, and then cooled in the furnace.

orma z ng

• To remove coarse grained structures in forgings or castings.

• The steel is heated to 37.8-93.30C above upper critical

temperature, and then cooled in air.

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ea rea e ee

• Heat-treated steel, mainly quenched and tempered (QT)

steel differ from alloy-steel in that they have a higher

percen age o a oy ng e emen s an ey re y on eatreatment to develop high strength and improve

mec an ca proper es.

• They have very high strength (620~690 MPa) and poorductility compared to carbon or alloy steel and are only

available in plates.

• QT steels do not exhibit well-defined yield points. They

are generally weldable but special welding techniques

are usually required.

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-

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Thermo Mechanical Controlled Process


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Thermo-Mechanical Controlled Process

(TMCP)TMCP combines controlled rolling with accelerated cooling

for grain refinement.

• The decrease in strength due to less alloy elements arecom ensated b accelerated coolin rocess

• Improved weldability thanks to lower CEV

•

• TMCP steel cannot be normalized (or annealed)

After hot rolling the deformed structure

prevented from growing by precipitation of

extremely small carbides and nitrides.

33Controlled Rolling

TMCP Steel Plate by OLAC


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y

Facility LayoutHotLeveler Finisher Rougher Cooling

Accelerated Water-cooling

Furnace

ee

On-Line Accelerated CoolingHot Rolling

Fine grain

50μm50μm

High strengthExcellent toughness

Coarse grain

50μm50μm

34(1) Advanced TMCP

(2) Conventional process

Low strengthPoor toughness

Com arison of DQT RQT and TMCP


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Com arison of DQT RQT and TMCP

Steel Plates

DQT: rolling=> cooling => rolling twice => quenching => tempering => cooling in air

RQT: rolling => quenching => reheating => quenching => tempering => cooling in air

TMCP: rollin => coolin => rollin twice => accelerated coolin without tem erin

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Comparison of Production Process


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p

Temperature As-RolledControlled Rolling

(Normalized)

TMCP

Water-cooled TypeDirect Quenched & Tempered

Slab reheating

RollingRolling

Rolling

Off-lineheat treatment

QuenchingRolling

Rolling

+Rolling

(Ar 3) Water Cooling

Tempering

Strength: TS 400-500(MPa) 400-500(MPa) 500-590(MPa) 550-800(MPa)

Thickness: t max. 50mm max. 50mm max. 100mm max. over 100mm

Toughness

Weldability◎

◎

×

Product Cost

t>50mm: Decrease Strength

Countermeasure: Increase Carbon

On-line heat

treatedOff-line heat treated

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Increase Strength

Decrease Toughness & Weldabili ty

and

low-alloy steelalloy steel

So many different Steel Products!


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So many different Steel Products!

Hot-rolled coils Cold-rolled coils Coated coils Slitted coils

Plates Sheet piles Hot-rolled sections Rail sections

Reinforcing bars Wire rods UOE pipes Spiral-welded pipes

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Some Current Material Issues -

• Boron-treated Steel

• TMCP, DQT & RQT Steel Plates

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oron – ar ena y n ancemen

• Boron is a potent alloy for hardenability enhancement,

-

the other more expensive alloying elements (e.g.

.

• Heat treated low alloy steel with boron extremely

high strength, e.g. quenched and tempered boron steel

ar or ve c es w y e s reng o - a.

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Influence of Weldin in Boron-treated Steel


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Influence of Weldin in Boron treated Steel

(HAZ)

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C Mn Cu P S Al Ti Si Cr Mo V Ni B

B 0.19 0.53 0.002 0.035 0.007 0.026 0.001 0.23 0.012 0.006 0.005 0.008 0.0014

BW 0.19 0.51 0.003 0.033 0.007 0.026 0.001 0.22 0.009 0.004 0.003 0.008 0.0014

C 0.21 1.6 0.6 0.04 0.04 - - - - - - - -

FG 0.18

0.5-

0.55 0.03 0.025 0.02 0.05 0.40 0.30 0.10 0.05 0.30 -.

RQT-

S6900.2 1.6 0.20 0.025 0.01 0.06 0.04 0.50 0.25 0.20 0.08 0.70 0.005

B: Boron steel, S275BW: Boron steel after welding, S275

Welding will not change thechemical composition

C: BS EN 10025-2, carbon steel, S275

FG: BS EN 10025-3, normalized fine grain low alloy steel, S275

RQT-S690: Reheated, quenched and tempered steel, S690

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Tensile Test Results


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Tensile Test Results

600 Obvious necking

500

+0.0%

300

400

200

+12.3%-9.4%

Almost no necking

100Boron steel

Boron steel - welding affected

0 5 10 15 20 25 30 35 40

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Im act Test Results


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Im act Test Results

B B-W hr hr-w cf cf-wmpac

Value (J)228 228 280 245.8 255.4 265.7

Average228 262.9 260.6

Boron steel Boron steel - welded

- -

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-


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-

800Boron steel

hot-formedRQT: Extremely high strength

& low ductility

500

600

RQT-S690

Boron -> little influence

300

400

Typical mild steel:

o m s ee

100

200average strength & good ductility

0

0 5 10 15 20 25 30 35 40

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• oron s a e o car on or a oy s ee , on y n

heat-treated quenched and tempered steel to enhance

.

• There is no product standards for Boron-treated carbon

or a oy s ee , m s crea e e r own s an ar s

such as ASTM A36B or A36 Modified because it is NOT

.

• We do NOT have Boron-treated welding electrodes and

we o no un ers an su c en y e e av or o oron-

treated carbon or alloy steel (HAZ) under the influenceo we ng.

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2. Form-Square Weld-Square Process

1. Electric Resistance Welding Process 3. Submerged Arc Weld Process

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Re-Forming Stage – Hot vs. Cold Formed


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Hot-Formed-

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Hot-formed & Hot-finished Cold-formed

• BS EN10210: 2006

•

• BS EN10219: 2006

•

and heat treated and welding

same in BS/EN design codes)

Similar in appearance,Hot-formed / hot-finished: 180mm x 180mm x 12.5mm

Cold-formed: 200mm x 200mm x 12.5mm

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different in properties


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The cold-formed hollow section had the largest corner radii, followed by hot-finished and hot-formed hollow

sections.

Hollowsections

bm(mm) tm (mm)

r o,m(mm)

r i,m(mm) b/t r o,m/tm r i,m/tm

Cold-formed 200.53 12.76 30.00 21.75 15.72 2.35 1.71

Hot-formed 180.27 12.72 25.00 12.13 14.17 1.97 0.95

Hot-finished 180.34 12.88 26.75 14.00 14.00 2.08 1.09

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• - -

cold-formed zone, provided:

- the cold-formed zones are normalized after cold-forming but before

welding;- the r/t ratio satisfy the relevant values below.

-

BS EN1993-1-8 Table 4.2

r forming (%)

(mm)

>3.0 2.0


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ress - ra n urves

600 a )

400

500

t r e s s (

M

300

S

100

Cold-Formed

Hot-Formed

0

0 5 10 15 20 25 30 35

-

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Residual Stress in the Hollow Sections


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Residual Stress in the Hollow Sections

Cold-formed

200mm 210mm

Hot-formed

180mm 180mm

Hot-finished

180mm 186mm

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1 / f y

Cold-formed SHSHot-finished and cold-formed:

0.8

f

Hot-formed SHS

Hot-finished SHS

Similar !!!

0.4

.

0.2

-0.2

0 45 90 135 180 225 270 315 360

-0.4

Angle (

)

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ompar son o esu s


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ompar son o esu s

• Residual stress distributions in cold-formed section

.

• The amount of residual stress:

co - orme o - n s e o - orme

• The hot-finished section also has very high residual

stress

• The hot-finished section is not fully-annealed as a

hot-formed section

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onc u ng emar s


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onc u ng emar s

• The cold-formed section contains the highest residual

stress with the biggest variance while the hot-formed

contains the least.• The residual stress distribution of the hot-finished

section is similar to the cold-formed section.

• Treatin hot-finished as the same as hot-formedhollow sections in current BS5950 / EC3 steel design

codes needs to be revisited ur entl .

• The restrictive Table 4.2 of EC3 Part 1.8 needs to berevised ur entl .

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High strength lowHigh CEV

Alloy

elements

alloys

High strengthCarbon steel

Fe + C

Low alloy

mild steelGood ductility

Heat

treatment Heat-treated steel

(TMCP, QT)

Extremely high strength

Low CEV

Plates only

Vulnerable to heat

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800.0

.

r e s s ( M p a )

25°C

300°C

°

600.0

. S t

450°C

500°C

Strength drops rapidly

from 300 to 700°C

400.0

500.0600°C

700°C

°

200.0

300.0

Ductility is improved

dramatically

0.0

100.0

. . . . . . . .

Elongation (%)

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Weldin of Hi h tren th T and TM P teel


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Loss of strength

and ductility in the

HAZ region

Strength

Ductility

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A36 (Most common)

g reng ow oy

A242, A572 (Co-V HSLA steel), A588 (Thicker weathering

steel), A945 (Low carbon and restricted sulphur), A992

(Rolled wide flange shape steel)

Heat Treated Carbon and Low Alloy Steel

A913 QT low allo sha e steel A1066 TMCP late

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BS EN 10025-1 General technical delivery conditions

BS EN 10025-2 Non-allo structural steel

BS EN 10025-3 Normalized / normalized rolled weldable

BS EN 10025-4 Thermo-mechanical rolled weldable fine

BS EN 10025-5 Structural steel with improved

BS EN 10025-6 Flat products of quenched and tempered

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: - es gn u e

on use of Alternative

Steel Materials to

. . . _ _ _ _ _ _ .http://www.bca.gov.sg/Publications/others/Explanatory_Notes_for_BC1-2008.pdf

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-

on use of Alternative

Structural Steel to

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Conclusions• A huge variety of steel microstructures, hence different


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A huge variety of steel microstructures, hence different

engineering behavior and properties can be obtained byusing and combining various strengthening and heat

treatment processes.

• Boron-treated carbon and alloy steel should not be usedbecause it is not possible to be certified.

• Be more careful with hot-finished rectangular hollow

sections.

• Select your steel from the list of certified steel materials in

BC1: 2012.

• , s ee p a es are cer e or e.g.

ASTM A1066, BS EN10025-4).• or qua y assurance, ensure your cer e s ee

materials can be classified as CLASS 1 under BC1: 2012.

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Material Selection & Quality Assurance