High Yield/Strength Steel

Harm Meelker Lincoln Smitweld BV, Netherlands

Content Introduction High Strength Steel (base material) Typical applications Welding high strength steel

• • • •

– Welding processes – Requirements – Procedure development

• Procedural effects on mechanical properties – Flux Cored Wires – Submergered Arc Welding

• Conclusions

High Strength Steels (HSLA) Low Carbon steels (0.04 - 0.12% C) + Small amount of alloying elements (Ni, Mo and Cr) + Precipitating elements (Nb, V, Cu, Ti) High strength (Yield > 550 Mpa) Metallurgical mechanisms;
Grain refining
Precipitation hardening
Controlled heat treatment (Q + T)
Weldability comparable with unalloyed steel
More efficient design, weight saving and as a result cost saving






High Strength Steels (HSLA) High strength ≥ 355 MPa

Welding ‘problems’


High strength by:
Grain refining by
Micro-alloying with Nb, Ti, Al, V, B
Thermo-mechanical treatment
Low impurities
Low Carbon content
Sometimes Cu-alloyed for higher strength


Dilution from base metal by Nb, Ti, V, etc..
Grain growth in CGHAZ
Weakening of HAZ
Sensitive for HIC
Limited use of CE and methods to calculate preheat temperature

High Strength Steel (base material)

S690x is from Dillinger Hütte Wel-Ten from Nippon steel Japan

S690 + Wel-Ten 780 Mod mm

C

Si

Mn

P

S

Cu

Ni

Cr

Mo

8

0,143

0,362

1,44

0,012

0,0009

0,025

0,028

0,051

0,022

60

0,158

0,28

1,31

0,010

0,0008

0,029

0,108

0,307

0,254

120

0,171

0,284

1,31

0,013

0,0005

0,033

0,323

0,771

0,405

178

0,12

0,26

1,10

0,006

0,001

0,27

2,50

0,59

0,54

178

0,12

0,26

1,10

0,005

0,001

0,28

2,44

0,59

0,54

Type + QT beh.

V

Ti

N

B

Zr

T-Al

V+Nb

Ceq

690E - 8 mm Q: 910±20 °C water 0,021 T: 590±10 °C lucht

-

0,013

0,0035

0,0015

0,0002

0,043

0,02

0,40

690E - 60 mm Q: 910±20 °C water 0,026 T: 610±10 °C lucht

0,001

0,002

0,0042

0,0019

0,0002

0,070

0,03

0,50

690T - 120 mm Q: 910±20 °C water 0,029 T: 630±10 °C lucht Wel-Ten 780 Mod - 178 mm Q: 930 °C / 1h T: 630 °C / 1h

Nb

0,002

0,003

0,0038

0,0021

0,0002

0,077

0,03

0,65

-

0,04

-

0,0041

0,0012

-

0,072

0,04

0,72

-

0,04

-

0,0040

0,0010

-

0,079

0,04

0,72

High Strength Steel (base material) • In European Standards, construction steel grades are classified according to their method of production. – Normalized fine grain steel grades (EN 10025-3) • Primary purpose of normalizing is grain refinement • Micro structure: ferrite + perlite

– TMCP fine grained steel grades

(EN 10025-4)

• Thermo-Mechanical Control Process • Final structure due to accelerated cooling • Micro structure: ferrite + bainite

– Q&T high strength steel grades

(EN 10025-6)

• Quenched & Tempered • Micro structure: tempered martensite

Relation strength/toughness commercial steels 27Joule transition temp. [°C]

40 High carbon C 0.8 %

20

Normalised 0

Coarse grained

Q

-20 -40 -60

Fine grained C-Mn-Al

Wear resistant

Q+T

Microalloyed C-Mn-Nb

-80

TMCP

-100 -120 -140 200

400

600

800

1000

1200

Yield Strength [MPa]

High Strength Steel (base material) Accelerated cooling responsible for the formation of bainite

1400

When is a steel a high yield steel?
Depends on people’s background!
In principle when the Yield is at least 460 MPa
Differentiate in;
Structural steels (S460 up to S1100)
Wear resistant steels (Hardox, Dillidur, Creusabro, etc…)

Unalloyed steel
The simple mild steels have a relative low C and Mn content
Yield: 185-355 MPa
80-90% of all applications at environmental temperature


C-equivalent is always low
Easy to weld, no preheating

High quality unalloyed steel
Mainly S355 types
Way of steel producing is variable
Classical or modern


C eq. can vary from 0.3 up to ….
This means take care:
Hardening
Cold cracks
Preheat
Interpass temperature

Actual deliveries S355J2H + N en S355J2H + QT (60 mm) S355J2H+N

S355J2H+N

S355J2H+QT

S355J2H+QT

C

0,14

Y = 363 (335)

0,18

Y = 432 (335)

Si

0,30

TS = 550 (470)

0,28

TS = 560 (470)

Mn

1,33

EL = 29 (19)

1,42

EL = 28 (19)

P

0,008

Impact -20ºC

0,008

Impact -20ºC

S

0,002

134 - 25 - 195

0,002

181 - 225 - 210

Cu

0,02

N = 880 - 910ºC

0,01

Q = 880 - 910ºC

Ni

0,009

0,02

T = 490 - 510ºC

Cr

0,04

0,06

Mo

0,003

0,005

Al

0,03

0,015

V

0,001

Nb

0,001 CE = 0,37

0,001

Ti

CE = 0,43

0,014

High yield steel and low alloyed steel
S420, S460, S500, S690, ….
With these the welding is very important
H.I. / consumables / Tp and Ti / layers build up


Low alloyed high temperature
Defined in codes


Difficult to weld steels
Depending on type and material thickness Tp from 150°C up to 350°C

EN 10025, hot rolled structural steels overview Designation Part 2 according EN Hot rolled 10025 Part...

Part 3

Part 4

Part 5

Part 6

Normalized fine grain

Thermo mechanical rolled fine grain

Atmospheric corrosion resistance

High Yield strength quenched and tempered

S235 S275

JR, J0, J2 JR, J0, J2

N, NL

M, ML

S355

JR, J0, J2, K2 N, NL

M, ML

N, NL

M, ML

N, NL

M, ML

N= 40 J at -20°C NL = 27 J at -50°C

M = 40 J at -20°C ML = 27 J at -50°C

S420 S450 S460 S500 S550 S620 S690 S890 S960 Note that:

J0W, J2W J0WP, J2WP J0W, J2W, K2W

J0

JR = 27 J at +20°C J0 = 27 J at 0°C J2 = 27 J at -20°C K2 = 40 J at -20°C

Q, QL, Q, QL, Q, QL, Q, QL, Q, QL, Q, QL, Q, QL W = atmospheric corrosion resistance P = greater P-content

QL1 QL1 QL1 QL1 QL1 QL1

Q = at -20°C QL = at -40°C QL1 = at -60°C For all 30 J long. and 27 trans.

S235J2G3 (1.0116) EN 10025

S355N (1.0545) EN 10025-3

S690QL (1.8928) EN 10025-6

C

≤ 0,19

≤ 0,22

≤ 0,22

Mn

≤ 1,5

≤ 0,85 – 1,75

≤ 1,80

Si

-

≤ 0,55

≤ 0,86

Cr

-

≤ 0,35

≤ 1,6

Ni

-

≤ 0,55

≤ 2,1

Mo

-

≤ 0,13

≤ 0,74

Cu

-

≤ 0,60

≤ 0,55

V

-

≤ 0,14

≤ 0,14

Ti

-

≤ 0,06

≤ 0,07 ≤ 0,17

Zr

-

-

B

-

-

≤ 0,006

Nb

-

≤ 0,06

≤ 0,07

Yield

≥ 235 250/500 360 – 510 = 0,5 ≥ 17 (22)

≥ 355

≥ 690

Tensile Elongation

360/520 470 – 630 = 0,7 ≥ 22

700/800 770 – 940 = 0,88 ≥ 14

S690x is from Dillinger Hütte Wel-Ten from Nippon steel Japan

S690 + Wel-Ten 780 Mod mm

C

Si

Mn

P

S

Cu

Ni

Cr

Mo

8

0,143

0,362

1,44

0,012

0,0009

0,025

0,028

0,051

0,022

60

0,158

0,28

1,31

0,010

0,0008

0,029

0,108

0,307

0,254

120

0,171

0,284

1,31

0,013

0,0005

0,033

0,323

0,771

0,405

178

0,12

0,26

1,10

0,006

0,001

0,27

2,50

0,59

0,54

178

0,12

0,26

1,10

0,005

0,001

0,28

2,44

0,59

0,54

Type + QT beh.

Nb

V

Ti

N

B

Zr

T-Al

V+Nb

Ceq

690E - 8 mm Q: 910±20 °C water 0,021 T: 590±10 °C air

-

0,013

0,0035

0,0015

0,0002

0,043

0,02

0,40

690E - 60 mm Q: 910±20 °C water 0,026 T: 610±10 °C air

0,001

0,002

0,0042

0,0019

0,0002

0,070

0,03

0,50

690T - 120 mm Q: 910±20 °C water 0,029 T: 630±10 °C air Wel-Ten 780 Mod - 178 mm Q: 930 °C / 1h T: 630 °C / 1h

0,002

0,003

0,0038

0,0021

0,0002

0,077

0,03

0,65

-

0,04

-

0,0041

0,0012

-

0,072

0,04

0,72

-

0,04

-

0,0040

0,0010

-

0,079

0,04

0,72

The Graville weldability diagram Crack sensitivity in the Heat Affected Zone .40

Carbon [%]

.30

ZONE II Depends on conditions

ZONE III High under all conditions

.20

.10

.00

A, B, D, E

36 EH HSLA 65

HY 80/100 HSLA 80/100

ZONE I * Safe under most conditions .30

.40

.50

.60

.70

.80

.90

Mn + Si Cr + Mo + V Ni + Cu CE = C + + + 6 5 15

* High Strength welding consumables may require additional care

Why high yield steels?
Always because of economical advantage
During fabrication and/or use


Smaller wall thickness due to higher allowable stresses
Lower fabrication costs?
Welding and machining costs
Cheaper support structures
Material costs?
Inspection costs?


Lower company costs, especially transport
As a total result lower energy costs

High yield steels, consequences!
Manufacturing costs up or down Tp, Ti < 225°C
Welding C, Ceq
Preheat; in case a high cooling speed is not desired

Why high yield steels?

Why high yield steels? • Application of high strength steels may offer advantages • Compared to S355, reduction in plate thickness, can be 40% for S690 and 50% for S890 grades • The challanges related to welding activities increase with the strength of the base material • Challanges include – Weld layer thickness • (stringer vs slight weave) – Controlling base metal dilution – Heat input control – Cooling rate ∆t 8/5 – Post Weld Heat Treatment

Some applications

Typical offshore application Legs for Jack-up rigs Common joint types for which high strength consumables are applied “Rack-to-rack” “Cord-to-rack”

Cord

Rack

Typical offshore application Rack

Cord

Typical offshore application • Split Rack-to-Chord Fabrication • Solid Rack-to-Chord Fabrication

Weld Metal Requirements & Properties • The use of high strength steel type S690 in offshore applications continuous to increase • Requirements shift to higher impact toughness at lower temperatures • A gradual decrease in impacts in transition curve • Customer requirements might specify a higher minimum tensile strength.

Welding high yield steel • Influences on the welding procedure
Welding process
Preheat and interpass temperature
Heat Input
Welding sequence
Welding consumable
Heat treatment

Welding processes

Preheat and interpass temperature Defined by:


Cooling time 800°C→500°C (∆ ∆t8/5)
Material thickness
Heat Input
Carbon Equivalent
Moisture (hydrogen) in environment and consumable(s)

Influence of cooling time on hardness and transition temperature

Heat input

Carbon equivalent (IIW)

Mn C-eq. = C +

Cr + Mo + V +

6

Cu + Ni +

5

15

Bead sequence

43

46

30

Be aware off;
The fatigue strength of HYS structures is not higher than from conventional S355
Thinner walls can result in less stability of a structure
Change from S355 in production to S690 requires a change in production process
A higher yield gives higher shrinkage stresses
In principle all welding processes are possible
Do not heat up QT and TM steels above their annealing temperature
Sensitive to hydrogen induced cracking
Sensitive for brittle fracture

Guidelines for welding HY-Steels • Preheat when tack welding too, prevent hardening cracks • Arc-strike is forbidden! • Work very accurate by building together to prevent unnecessary stress concentrations, use mechanical tools (no tack welds) • Use extremely dry consumables • In thicker materials use for the root run a consumable of a lower strength level • Take care of the bead sequence, more thinner runs, controlled weaving • Use for the cap layer the Temper-Bead technique

Weld Metal Requirements • Distinctions need to be made between the different stages in the total chain and the requirements that are applicable according to the Rules • Rules take into account that the conditions for welding get progressivly worse – And so will the mechnical properties obtained • Approval requirements and Procedure requirements are therefor different

Weld Metal Requirements Type Approval Program

• Ideal conditions • At producers weld lab

Welding Procedure • Favorable conditions • Typically fabricators weld school Qualification Welding Production Test

Weld Metal Requirements

• Reality •

Production

Weld Metal Requirements Overview of all weld metal requirements • AWS A5.59 : E111T1-K3M-JH4 • EN ISO 18276-A : T 69 4 Z M 2 H5 Class

Yield

Tensile

Elongation

CVN@-29C

AWS

Min 680

760-900

Min 15%

Min 27J

CVN @ -40

ISO

Min 690

770-970

Min 17%

Min 47J

“4Y69” *

Min 690

770-940

Min 17%

Min 69J (48)

Customer

Ref GL

Min 790

Ref GL

Min 69J (69)

* Classification grade according to DNV-GL-LR-ABS etc

Weld Metal Requirements Example of welded joint requirements for initial approval • DNV TAP 401 Table 12-1 DNV Approval

Yield [MPa]

Tensile [MPa]

Elongation [%]

CVN [J]

4Y42

420

530-680

20

47

4Y46

460

570-720

20

47

4Y50

500

610-770

18

50

4Y55

550

670-830

18

55

4Y62

620

720-890

18

62

4Y69

690

770-940

17

69

Weld Metal Requirements Example of welded joint requirements for WPS • DNV OS-B101 Table D2 DNV Steel grade

Yield [MPa]

Tensile [MPa]

Elongation [%]

CVN [J]

NV X420

420

530-680

18

28

NV X460

460

570-720

17

31

NV X500

500

610-770

16

33

NV X550

550

670-830

16

37

NV X620

620

720-890

15

41

NV X690

690

770-940

14

46

X = A(0°C ) D (-20°C) E(-40°C) F(-60°C)

Weld Metal Requirements What happens in reality • DNV OS D-101 / Chapter 3 Sect 1. D 400

WPT = Welding Production Test

Welding Process Selection • Offshore construction companies typically utilize the following welding processes for the fabrication: – Submerged Arc Welding SAW – Flux Cored Arc Welding FCAW – Shielded Metal Arc Welding SMAW • The slag systems that are used in these processes determine to a great extend what the expected toughness can be – Basic slag systems (low oxygen) – Rutile slag systems (medium oxygen)

Welding process Selection & Properties

Impact (J)

200 180 160 140

GTAW

120 100

Basic SAW,

80 60 40

Basic SMAW Rutile FCAW

20 0 0

200

400

600

oxygen content (ppm)

800

1000

Welding Process Selection Weldability 1G 3G

Toughness

Slag release

Efficiency +++

SAW Basic

+++

n.a.

+++

+++

SMAW Basic

++

++

++ / +++

++

+

FCAW Rutile

+++

+++

++

+++

+++

FCAW Basic

+ / ++

+

+++

+ / ++

+

Efficiency is an overall rating of : -spatter -post weld clean-up time -overall deposition rate -required skill -etc

+ = Poor ++ = Good +++ = Excellent

Consumables Development Challenges C Mn

:

Basis as low as possible

:

Yield strength ↑ CVN ↓ above 560 N/mm2

Ni Mo Cr V

:

CVN ↑ , Yield strength ↓ / ↑

:

CVN ↓ / ↑ , Yield strength ↑

:

CVN ↓ , Yield strength ↓ / ↑

:

Strength ↑ most potent element Low resistance to hydrogen cracking ↓ Poor properties when PWHT ↓

Micro alloying elements for Cored Wires: Nb, Ti and B, depending on concentration and ratio ↑ or ↓

Consumables Development Challenges • Influence of essential individual elements – C, Mn, Cr, Mo, Ni, V, Cu,Ti, B • Microstructure – Aim is formation of acicular ferrite • Hardness • Preheat • Crack resistance • Mechanical properties – Yield, Tensile, Impact toughness, (CTOD )

Consumables Development Challenges Importance of Acicular ferrite in weld metal • Needle shaped ferrite • Formed inside of the original austenite grains • Nucleates on inter granular inclusions (hence micro alloyed FCW’s) • Fine grained acicular ferrite is randomly orientated providing maximum resistance to inter granular “cleavage” crack propagation • Resulting in improved impact toughness

Acicular ferrite with IG-inclusions

Consumables Development Challenges

The variables to come to an approved procedure    

Code & Customer requirements Base material specifications Actual properties of delivered base material Tempering temperature used during base material production

        

Welding consumable selection Welding parameters : volt, amps, travel speed ∆t8/5 Preheat temperature Interpass temperature PWHT to requirements (heating rate, holding time & temp, cooling rate) Method of PWHT Sample preparation (smooth to avoid crack initiation points) People that can follow instructions Common sense and a little bit of luck….…..

(PWHT Temp ≤

T Temp-30°C)

Calculating ∆t8/5 • The importance of ∆t8/5 increases with the base material strength • All major phase transformation that determine mechanical properties happen in the 800-500°C temperature range

∆T8-5 = (HI and Tp) ∆T8-5

Heat Input in kJ/mm Tp 150°C

Tp 175°C

Tp 200°C

10 sec

1.14

1.02

0.91

15 sec

1.72

1.53

1.36

20 sec

2.29

2.04

1.81

Procedural effects on mechanical properties • Procedural effects vary from process to process • In general ; close attention need to be paid to welding procedures and the practical execution of the welding process. • The higher the strength of the steel, the higher the level of attention required to achieve good properties • Extra care should be taken when weld metal properties such as toughness do not provide high levels of cushion.

Procedural effects on mechanical properties • The selected / purchased base material plays an crucial role in setting welding procedures in more than one way. – The thickness plays an important role on the chemical composition selected by the mill – The chemical composition allows that the steel has sufficient mechanical properties throughout the whole thickness of the plate as it determines the critical cooling rate during Quenching (part of Q+T) – The composition (Cev) in combination with cooling rate (∆t8/5) determines the hardness. – The Tempering temperature is crucial information in case a PWHT needs to be applied

Procedural effects on mechanical properties • So key elements to a successful procedure are: – Determine the required preheat based on Cev – Determine the heat input / ∆t/85 for a procedure with a given base material thickness / composition – Determine QA/QC methods to control these parameters in your workshop

Procedural effects on mechanical properties PF / 3G



Fully restrained condition Root on ceramic backing

Procedural effects on mechanical properties

PF/3G



Fully restrained – Root on ceramic backing

Procedural effects on mechanical properties

Plate

Yield [MPa]

Tensile [MPa]

Elongation [%]

EV32 EV34

763 782

824 827

Requirement

≥690

≥790

EV32 8mm root

18 17

CVN -40°C [J] 57 78

CVN -50°C [J] 58 52

CVN -60°C [J] 40 40

≥17

≥69

n.a.

n.a.

EV34 4mm root

Procedural effects on mechanical properties • Procedural effects using SAW are obviously there but compared to rutile cored wire to a lesser extend

Procedural effects on mechanical properties • LAC 690- 888 mechanical results at -60°C • Various weld thicknesses at similar Heat Input level • Visible effect of AC current vs DC current

Procedural effects on mechanical properties SAW effect of polarity on impact toughness (F8A8/P8-ENi5-Ni5) 200 180 160 140

Joules

120 DC+

100

AC

80 60 40 20 0 -40°C

-50°C

-60°C

Test temperature

Procedural effects on mechanical properties • High strength steels are often applied in thick and highly restrained conditions. • Due to high cooling rates is the root weld often critical (hardness – restrained - crack issues) • A generally accepted root welding solution is using a lower strength consumables for the root;

– Higher ductility in weld metal in the root absorbing stresses

Controlling hydrogen in consumables • The use of low hydrogen consumables are essential for high strength steel. • Low hydrogen consumables for high strength steels are required by the Rules

Controlling hydrogen in consumables • Consumables are regularly classified per AWS with the “H4” designation in the as supplied conditions • Hermetically sealed consumables are recommended for – SMAW – SAW – FCAW • SMAW electrodes with the optional AWS “R” designator are preferred and recommended in combination with guaranteed exposure properties

Controlling hydrogen in consumables

FCAW : Vacuum AluBag

SMAW : Sahara ReadyPack SAW : Sahara ReadyBag

Controlling hydrogen in consumables • In Sahara ReadyPack high strength electrodes are guaranteed to meet: •