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Steel plates for ships and other welded structures havinghigh toughness of HAZ and base metal:fundamental studies and developments

Shuji Aihara1

1Nippon Steel Corporation, Japan

ABSTRACT

The present paper reviews fundamental studies on fracture, toughness and recent development of steel plates for ships

and other welded steel structures having high toughness of weld HAZ and plate at Nippon Steel Corporation. For

developing high toughness steel, identification of brittle microphase and its control and elimination by such

metallurgical measures as oxide metallurgy and microalloy control are necessary. Based on these fundamental studies,

steel plates for ships, offshore structures and buildings were developed. On the other hand, crack arrestability of base

plate is necessary for ensuring structural integrity against brittle fracture. For this purpose, high arrest toughness steelplates were developed through effective grain size control and ultra grain-refinement of surface layer by advanced

TMCP. It is also noted that further development of standards is necessary for establishing higher degree of structural

integrity.

Toughness, Heat-affected zone, Local brittle zone, Crack arrest, Oxide-meatallurgy

1 INTRODUCTION

Twenty first century started with a rapid growth of Asian economy, which should necessarily accompanies energy,

environment and other problems: energy will be explored and produced at severer areas, mass transportation of oil,LNG and other freights between the countries will be necessary. Because the welded steel structures used for these

purposes become larger, impact of possible failures of these structures to human lives and environment is much larger

than ever before: requirements for the structural integrity should certainly become stricter. The steel industries should

actively contribute to solving these problems.

Prevention of catastrophic failures of welded structures is of primary importance. Structural safety against fracture

should be maintained according to the double integrity concept, which is prevention both of brittle fracture initiation

and propagation. Maintaining sufficient toughness both at welds and base plates, together with optimal design,

fabrication and inspection, is essentially important. The present paper reviews fundamental studies on toughness

improvement by microstructural control, and development of steel plates having high toughness based on the studies at

Nippon Steel Corporation. Necessity for further development of standards of structural integrity is also commented.

2 CONTROL OF HAZ TOUGHNESS

ESW

EGW

SAW

(single

-pass)

GMAW

SMAW

0 10 20 30 40 50 60

8060

40

20

108

6

4

2

1

Thickness (mm)

HeatInput(kJ/mm)

100

SAW(multi-

pass)

ESW

EGW

SAW

(single

-pass)

GMAW

SMAW

0 10 20 30 40 50 60

8060

40

20

108

6

4

2

1

Thickness (mm)

HeatInput(kJ/mm)

100

SAW(multi-

pass)

Fig.1 Dependence of welding heat-input on

thickness.

2.1 Controlling factors of HAZ toughness

Figure 1 shows dependence of welding heat-input on plate

thickness. With increasing thickness due to increased size

scale of structures, high heat-input welding is more often

adopted. Maintaining sufficient toughness of welded joints is

becoming more difficult under these circumstances. Figure 2

shows factors controlling cleavage fracture initiation toughness

and associated microstructural factors. Elimination orminimization of brittle micropahses which can act as cleavage

crack initiator is essentially important. At the same time,

refinement of effective grain size should be accomplished.

Effective grain size is a size equivalent to ferrite grain size,which has an effect in resisting cleavage crack. Piled-up

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Matrix ToughnessMatrix Toughness

Effective Grain-SizeEffective Grain-Size

HardnessHardness

MA ConstituentMA Constituent

Coarse CementiteCoarse Cementite

Large InclusionsLarge Inclusions

Austenite Grain-SizeAustenite Grain-Size

Intra-Granular Ferrite /Aligned Ferrite Plate

Intra-Granular Ferrite /Aligned Ferrite Plate

Dislocation Cross-SlipDislocation Cross-Slip

Precipitation Hardening,Aging

Precipitation Hardening,Aging

FACTORS CPNTROLLING

INITIATION TOUGHNESS

MICROSTRUCTURAL

FACTORS

HardenabilityHardenability

Grain Boundary FerriteGrain Boundary Ferrite

Brittle MicrophaseBrittle Microphase

Matrix ToughnessMatrix Toughness

Effective Grain-SizeEffective Grain-Size

HardnessHardness

MA ConstituentMA Constituent

Coarse CementiteCoarse Cementite

Large InclusionsLarge Inclusions

Austenite Grain-SizeAustenite Grain-Size

Intra-Granular Ferrite /Aligned Ferrite Plate

Intra-Granular Ferrite /Aligned Ferrite Plate

Dislocation Cross-SlipDislocation Cross-Slip

Precipitation Hardening,Aging

Precipitation Hardening,Aging

FACTORS CPNTROLLING

INITIATION TOUGHNESS

MICROSTRUCTURAL

FACTORS

HardenabilityHardenability

Grain Boundary FerriteGrain Boundary Ferrite

Brittle MicrophaseBrittle Microphase

Fig.2 Microstructural factors controlling HAZ toughness.

0

0.8

1.6

2.4

3.2

0 0.08 0.16 0.24 0.32%Si

FractionofMA(%)

Double

Triple(Tp3=350)

Triple(Tp3=450)

Triple(Tp3=550)

Fig.3 Influence of Si content on formation and

decomposition of MA (simulated HAZ).

dislocations at cementite or other hard phase on grain

boundary collapse into a microcrack to open up the crack andincrease driving force for crack propagation into ferrite matrix.

Because number of dislocations increases with ferrite grain

size, large grain microstructure promotes cleavage crack

initiation [1]. Same mechanism may be applied to other

complex microstructures than ferrite.

2.2 Elimination of brittle microphase

The MA is one of the most harmful micropahses in initiating

cleavage crack due to its low toughness and largeness.Although the mechanism of crack initiation from MA is not

fully understood, decohesion at MA-ferrite matrix or

microcracking of MA are possible explanations [2]. Internalstress associated with MA transformation may be another

reason to the proneness of crack initiation. Low toughness of

upper bainite or ferrite side-plate is partly attributed to the formation of MA: large MA easily forms at upper bainite lathor ferrite plate boundaries. In the case of multi-pass welding, MA is formed at a localized region which received

intercritical reheating by subsequent welding pass [2]. Many studies have been made for eliminating or reducing the

MA: e.g. reduction of Si is effective for reducing MA and its decomposition into cementite and ferrite, Fig.3 [3].

-100

-60

-20

20

0 20 40 60 8

Fraction IGF (%)

CTODTransitionTemp.()

0

Fig.4 Effect of IGF on cleavage initiation toughness(simulated HAZ, Tp=1400, t8/5=130s).

2.2 Control of effective grain size by intragranular ferrite nucleation (IGF)

Reducing the effective grain size can be made by a number of

ways. One is the formation of intragranular ferrite (IGF).

Evidently, reducing the effective grain size by increasing

volume fraction of IGF increases initiation toughness, Fig.4[3,4]. Some kinds of particles for IGF nucleation have been

proposed [5]. Although, exact mechanism of ferrite nucleation

from the particles has not been fully understood [6], it is

suggested that Mn depletion near particle-ferrite matrix

interface plays an important role [7]. Figure 5 shows Mn

depletion formed around a TiN-MnS complex particle of low

alloy steel subjected to simulated thermal cycles [8]. While Mn

is depleted in a sample held at 1373K after heated up to 1713K,

the sample held at 1523K showed no depleted zone. Depth of

the Mn depletion of these samples is correlated with volume

fraction of IGF.Fig.5 Mn depleted zones formed near TiN-MnS

particle, simulated HAZ.Ti-oxide, as a part of oxide-metallurgy, is also a strong IGF

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nucleation agency. Its mechanism is complex and needs

extensive studies. Figure 6 indicates that larger fraction of IGF is

formed from Ti-oxide which contains higher amount of Mn,irrespective of S content of steels. On the other hand, Mn content

in oxide depends on the precipitation of TiN on the oxide. Mn

depleted zone and hence IGF is formed from Ti-oxide without ahelp of MnS precipitation but needs TiN precipitation. From

these facts, it can be presumed that Ti ions are expelled from

Ti-oxide (Ti2O3) to form TiN on the oxide surface and cation

vacancies increases in the oxide, and consequently Mn is

absorbed from the matrix into the oxide to occupy the cation

vacancies [9].

Fig. 6 Influence of Mn content in oxide on

IGF formation.

Although IGF is very effective for improving toughness through

refinement of effective grain size, its transformation is

competitive with the transformation of ferrite side-plate grown

from grain boundary allotriomorphic ferrite. The ferrite side-plate

and grain boundary ferrite constitute a quite large effective

grain unit, which often becomes a cleavage crack initiationsite. Therefore, elimination of these microstructures is

important. Boron nitride precipitated on austenite grain

boundary change grain boundary ferrite from

allotriomorphic to idiomorphic thereby suppressing ferrite

side-plate. It also promotes IGF transformation [10,11].

Fig.7 Oxide and sulfide for pinning grain growth in

HAZ.

2.3 Control of effective grain size by austenite grain

refinement

Control of austenite grain growth is effective for reducing effective

grain size of HAZ. It is especially useful in very high heat-input

welding. Precipitates in steel effectively suppress grain boundary

migration. The pinning effect can be achieved by the precipitateswhich are finely dispersed and thermally stable. TiN has been widely

used for this purpose [12]. However, it is dissolved if exposed for long

period at high temperature like that of fusion boundary of very high

heat-input weld HAZ. Although, oxide is very stable even at high

temperature, it was difficult to finely disperse. To overcome these

contradictory natures, a new kind of particles in steel was proposed

(HTUFF) [13,14], Fig.7.

As Fig.8 shows, these

particles effectively

suppress grain growth

of austenite even if the

sample is held for long

period at 1400 .

Refinement of prior

austenite grain size is

effective for reducing

the size of grain

boundary ferrite and

ferrite side-plate, Fig.9.

The steels with strong

pinning particlesreduce the effective

grain size, thereby

increasing the

toughness. It is also

mentioned that

Fig.8 Comparison of grain growth

characteristics of simulated HAZ.

grain size (m)

LengthofGBForFSP

vE0

(J)

10kJ/mm SAW simulation70kJ/mm SAW simulation

grain size (m)

LengthofGBForFSP

vE0

(J)

10kJ/mm SAW simulation70kJ/mm SAW simulation

Fig.9 Effect of austenite grain refinement

on effective grain size and toughness ofsimulated large heat-input weld HAZ. Fig.10 IGF nucleation from Mg-containing oxide in HAZ.

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Mg-containing oxide acts as IGF nucleation site, Fig.10 [15].

2.4 Development of steel plates having high HAZ toughness

Based on the above mentioned fundamental studies, kinds of steel plates having high HAZ toughness have been

developed and widely used. Examples are:YP 390MPa steel plate for large heat-input welding for large container ships [16],

YP350 to 420MPa offshore structural steel plates for ultra low temperature service (-50)

having high HAZ CTOD property [17,18],

YP500MPa offshore structural steel plate having high HAZ CTOD property [15],

YP 350 to 440MPa steel plates for high heat-input box column welding [19].

It should be mentioned that TMCP [20] is indispensable in these steels for promoting low hardness and reducing brittle

microphase at HAZ. It is also mentioned that HTUFF is applied to line-pipe steels for achieving high toughness of seam

weld HAZ [21].

3 CONTROL OF CRACK ARRESTABILITY

3.1 Controlling factors on crack arrest toughness

While some metallurgical factors controlling toughness of cleavage crack initiation and arrest are the same, others are

different. Grain size is the most influencing factor for crack arrest toughness, as well as initiation toughness. Ridges

which are formed between cleavage facets shield stress concentration at the propagating crack-tip, thereby promoting

crack arrest. Controlled rolling is an effective way to refine ferrite grains. However, texture may develop at the same

time: randomization of crystallographic orientation is necessary, Fig.11 [22]. Crack arrest toughness can be related to

the cleavage facet size or effective grain size, rather than optically determined grain size [22].

Shear-lip, which is a shear fracture zone formed near

plate surface during crack propagation, reduces crack

driving force, thereby increasing crack arrest toughness.By applying advanced TMCP, ultra-fine grain layers near

plate surfaces (SUF) can be formed. The HIAREST steelplate having SUF exhibits extremely high crack arrest

toughness due to the formation of shear-lips, Fig.12 [23],

and contributes to increased double integrity of e.g. ships.

Grain refinement Randomization ofcrystallographic orientationGrain refinementRandomization of

crystallographic orientation

Fig.11 Cleavage crack paths in relation to grain size and

crystallographic orientation, schematic.

1000/T (K-1)

Kca(N/mm1.5)

1000/T (K-1)

Kca(N/mm1.5)

Fig.12 Effect of shear-lip by SUF on crack arrest

toughness.

Applied K (large-scale duplex test) (kgf/mm3/2)

CrackArrestTo

ughnessKca(kgf/mm3/2)

Applied K (large-scale duplex test) (kgf/mm3/2)

CrackArrestTo

ughnessKca(kgf/mm3/2)

Fig.13 Comparison of crack arrest tests between

standard-size and large-scale tests (solid: arrest,

open: propagate).

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3.2 Integrity for crack arrest

In 1970s and 1980s, series of experiments on fast crack propagation and arrest of welded steel plates for ships were

conducted. It was concluded that a crack initiated at weld deviates away from the weld due to the effect of welding

residual stress and propagates into base plate [24]. From comparison of standard-size crack arrest tests with large-scaleones, it was concluded that a long propagating crack can be arrested if the base plate has crack arrest toughness greater

than 40006000N/mm3/2 [25], Fig.13. However, a recent study showed that a cleavage crack possibly propagates along

large heat-input welded joint of very thick plate [26]. It has been pointed that extensive studies are needed for

establishing standards ensuring safety against fracture of large container ships [27].

4 FINAL REMARKS

(1) Risk of fracture initiation can be minimized by use of steel plates having high HAZ toughness, which can beachieved by microstructural control, e.g. decrease in effective grain size by IGF and grain growth pinning and

elimination of brittle microphase by microalloy control in conjunction with TMCP.

(2) Crack arrest toughness is increased by reducing effective grain size of plates. Formation of shear-lips by e.g. ultrafine-grained surface layer is very effective. Advanced TMCP is utilized for this purpose.

(3) Crack arrestability of steel plates should be necessary for ensuring structural double integrity. Further studies arenecessary for understanding dynamic crack propagation and arrest behavior and establishing optimal standards for

structural safety.

Prevention of catastrophic failures of welded structures is accomplished by an optimal combination of design, material,

fabrication and inspection. Fitness-For-Service evaluation is also important. It is noted that revision of FFS standard for

fatigue and brittle fracture [28] is under way at Japan Welding Engineering Society, which will definitely contribute to

higher safety of welded structures.

REFERENCES

[1] Petch, N.J., Acta Metall. vol.34 (1986), No.7, pp.1387-1393.

[2] Haze,T. et al, Tetsu to Hagane, vol.74 (1988), No.6, pp.1105-1112.

[3] Aihara,S., proc. OMAE99, OMAE99/MAT-2100, ASME.[4] Yamamoto, K. et al, ASTM STP 1042 (1989), p.266-284[5] Mabuchi, H. et al,Materia,

[6] Koseki, T. et al, Materials Science and Technology, vol.21 (2005),pp.867-879.

[7] Yamamoto K. et al, Tetsu to Hagane, vol.79 (1993), No.10, pp.1169-1175.

[8] Shigesato, G. et al, Tetsu to Hagane, vol. 87 (2001), No.2, pp.93-100.

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[12] Kanazawa, S. et al, Tetsu to Hagane, vol. 61 (1971), pp.2589.

[13] Uemori, R. et al, CAMP-ISIJ, vol.14 (2001)-1174.

[14] Kojima, A. et al, Nippon Steel Technical Report, No.90 (2004), p.2-6.

[15] Kojima, A. et al, proc. OMAE01, OMAE01/MAT-3241, 2001, ASME.

[16] Minagawa, M. et al, Nippon Steel Technical Report, No.90 (2004), p.7-10.

[17] Aihara, S. et al, proc. OMAE99, OMAE99/MAT-2100, 1999, ASME.

[18] Chijiiwa, R. et al, proc. OMAE99, OMAE99/MAT-2101, 1999, ASME.

[19] Kojima, A. et al, Nippon Steel Technical Report, No.90 (2004), p.39-44.[20] Tanaka, Y. ASME Pressure Vessels and Piping Conference, PVP2005-71258, 2005.

[21] Terada, Y. et al, proc. OMAE03, OMAE2003-37391, 2003, ASME.

[22] Ishikawa, T. et al, Nippon Steel Technical Report, No. 348(1993), p.3-9.

[23] Ishikawa, T. et al, Nippon Steel Technical Report, No. 75(1997), p.31-42.

[24] Ship Research Committee Report, SR-147 (1978).

[25] Ship Research Committee Report, SR-193 (1985).

[26] Ishikawa, T. et al, poster session, Fall Meeting Soc. Naval Architects Japan, (2004).

[27] Yamaguchi, K. et al, J. Japan Soc. Naval Architects and Ocean Eng., No.3 (2005), p.70-76.[28] WES-2805 (1997), Method of assessment for flaws in fusion welded joints with respect to brittle fracture and

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