material selection & quality assurance
TRANSCRIPT
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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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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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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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