Welding TMCP Steels

8/20/2019 Welding TMCP Steels

http://slidepdf.com/reader/full/welding-tmcp-steels 1/7

Proceedings of The Twelfth 2002) International Offshore and Polar Engineering Conference

Kitakyushu, Japan, May 26-31, 2002

Copyright 0 2002 by The International Society of Offshore and Polar Engineers

ISBN l-880653-58-3 Set); ISSN 1098-6189 Set)

haracteristics of TM P steels and their welded joints used for hull structures

kfasahiro Toyosada

Department of Marine Systems Engineering, Kyushu University

Fukuoka, Japan

ABSTRACT

TMCP steels were developed in Japanese steel makers about 20 years

ago. They are extensively used in ship building industry and are

diversifying to other industries such as marine structures, bridges and

so on. Reentry European and Korean steel makers produce TMCP steel

also. They have not only great merits but also possible demerits. In this

paper, these merits and demerits are explained in addition to recent

topics in Japan about TMCP steel.

KEY WORDS: TMCP steel; low carbon equivalent; weldability;

toughness; fatigue strength; softening of HAZ; distortion; residual

stress

INTRODUCTION

For recent years, ships with extensive use of high tensile steels with

tensile strength of 5OOMPa class (HT-500) have increased more and

more in number in Japan for the purpose of reducing their hull weight,

thus leads to reduce its material cost. This is because Thermo-

Mechanical Control Process (TMCP) has been developed and TMCT

steels have good weldability due to their low C,,

Though Japanese steel makers developed TMCP steels that meet

the extensive demand of strength and toughness for line pipe had

become a center of attraction from Japanese shipbuilders. The

shipbuilders have utilized various highly efficient welding technologies

in their fabrication. Accordingly, a great deal of joint work among them

including university professors has been carried out to put such steel

and welding technology into practical use in the Shipbuilding Research

Association of Japan (SR193), that has resulted in the accumulation of

an abundance of useful data.

This paper shows the characteristics of TMCP steels and their

welded joints from the viewpoints of welding procedures, fracture

toughness and fatigue strength, which is summarized mainly in the

SR193 committee (1985). Moreover recent topics about TMCP steels

are also explained.

CLASSIFICATION AND METALLURGY OF TMCP STEEL

TMCP steels are basically classified into two types: non-accelerated

cooling (Non-AcC) and accelerated cooling (AcC) processes. Fig.1

shows a schematic diagram of TMCP in comparison with conventional

rolled process.

Time

Fig. 1 Schematic illustration of thermo-mechanical control

process (TMCP) (TMR: thermo-mechanical rolling, AcC:

accelerated cooling, AC: air cooling)

Non-AcC process consists of (1) low slab reheating temperature

and (2) intensification of rolling reduction in the austenite unre-

crystallized region. The controlled rolling is finished either in th

region of austenite or in the intercritical region, austenite + ferrite

(r+a).

In AcC process, accelerated cooling is carried out after controlled

rolling. Cooling rate and finishing-cooling temperature in the process

are controlled depending on required properties. Maximum available

thickness in AcC type TMCP steel is 100 to 200 mm at present.

385





Fig.5 (Machida et al, 1988) is the relation between the maximum

hardness in the HAZ and bead length for YP320 and YP360 AcC type

TMCP steels. The YP400 class steels are nearly equal with the Non-

AcC type TMCP manufactured YP320 and YP360 steels as regards the

hardening of HAZ. This

may

be because the C,, upper limit remained

almost same.

From the viewpoint of maximum hardness being not in excess of

400Hv, bead length may be allowed up to 30mm for YP400 AcC type

TMCP steel, and YP320 and YP360 Non-AcC type TMCP steel.

I ’

I

100

P

0

t

-1

0% Q

00

L I

I

0 26

0 3

0 34

0 38

C,, (WES)=C+Si/24+Mn/6+Nil40+Cr/5+Mol4+V/14 [%]

01 ' I ' ' ' 1

0

10 20 30 40 50

Heat input [MJ/m]

Fig.7 Relationship between heat input and HAZ toughness at fusion

line in comparison with AcC and non-AcC type of TMCP steel

plates and conventional steel plates

While many methods of cold cracking test have been proposed, JIS

small Y-groove cracking test is used here for summarizing the data on

the cold cracking susceptibility of the HAZ for HT-500 steels. Fig.6

shows the effect of C,, on the critical preheating temperature to prevent

cold cracking of conventional steels and AcC and Non-AcC type

TMCP steels by using small Y-groove cracking test.

As the small Y-groove cracking test specimen has very large

constraint, the critical preheating temperature to prevent cold cracking

in the small Y-groove cracking test is higher by about 75°C than that

applicable to welded joint in general steel structures. Therefore it can

be said that in actual welding operation using low-hydrogen electrodes,

preheating may be unnecessary at the ambient temperature of over 0°C

Moreover the application of medium hydrogen electrodes becomes

possible as shown in Table 1.

NO

3M-1

3M-2

1M-3

-

,M-4

-

Table 1 Weld cracking susceptibility of TMCP steels

-

stee

-

-

iH36

-

IH36

-

H36(

-

N no crack

rhdaess

[mm1

14

35

Hydrogen

content

[rn/lOOg]

4

20

20

20

(**) (I) At startmg end

(II) At mddle

(III) At timshmg end

The reduction of the C,,

served effectively for toughness

improvement of the HAZ, particularly when high heat input welding is

used. This is generally proved by the fact that the welded joint

toughness of mild steels is superior to that of high tensile steels. This is

due to the formation of fine ferrite by the lowered hardenability as well

as reduced volume fraction of island martensite and also reduced

amount of cementite. However, the reduction in C,, generally brings

about a reduction in strength of the plates. Therefore, in conventional

manufacturing methods such as as-rolled or as-normalized processing,

efforts were made to adjust the MnK ratio or to replace C by Ni or Cu

387



to suppress C,, as low as possible. By these techniques, however, no

substantial reduction of C,, would be achieved to improve HAZ

toughness. As mentioned previously, the reduction of C,,

can

be

achieved by applying TMCP.

Fig.7 shows an example of the relationship between heat input and

HAZ toughness at fusion line in comparison with AcC and Non-AcC

type TMCP steel plates and conventional steel plates. AcC type TMCP,

in particularly, brings about further improvement in toughness and

makes possible to be applied for very high heat input welding. It is

natural that the improvement of HAZ toughness leads to increase

fracture toughness such as critical CTOD and Kit for the welding joints.

F HAZ ::<I I Base metal

fusion HAZ

line bondary

a) Afler single thermal cycle

g------’,,,

I I

I I

I I

./I-‘

I

I I

I I

I I

j I

I coarse :

: grain I

fine grain I

I realm I

rwon ;

1A,,

fusion HAZ

line bondary

t

.It-

li

L

I

IP

-

-Lnt ion.1 steel

. intercritical region

fusion

line

HAZ

bondary

a) Afler single thermal cycle

+ HAZ :j:

HAZ :j:

I TMCP steel i I

grains and intercritical region is the second deteriorated part. On th

other hand, in the TMCP steel treated by adjusting microalloying and

gaseous elements, typical deterioration is not observed after single

thermal cycle. However after 2nd thermal cycle, new deteriorated

microstructure generally generates in the intercritical region for 2nd

thermal cycle at the coarse grain HAZ due to the first thermal cycle.

This sudden deterioration is caused by the formation of th

substructure named martensite-austenite constituent (M-A) o

martensite islands. This deteriorated area is very small and is scattered

around in the HAZ near the fusion line. Succeed thermal cycle of which

peak temperature is over about 400 degree decomposes martensite

islands, and then the toughness recovers. Although there ar

deterioration parts in toughness in the HAZ of TMCP steel, the HAZ

toughness of TMCP steel is further high compared with that o

conventional steel as shown earlier in Fig.7.

- Base metal

Temperature 1 / TK [x 1 Om3K’]

Fig.9 Comparison of crack arrestability

Moreover it is observed that Non-AcC type TMCP steel plates

especially the plate manufactured by TMCP which the controlled

rolling is finished in the region in the intercritical region ( y + a ), have

excellent brittle fracture arrestability through the refinement of grains

as shown in Fig.9.

Fig.8 Schematic illustrations about changes of fracture toughness in

HAZ near the fusion line

By the way, it is well known that many different microstructures

generate in HAZ depending on thermal cycles due to welding and

chemical composition of steel. Fig.8 shows the typical schematic

changes of toughness in HAZ near the fusion line considered from the

Charpy test results for the plate with simulated welding thermal cycle.

Fig.Sa) shows the schematic changes of toughness after single thermal

cycle. Generally speaking, HAZ near the fusion line is the most

deteriorated part in conventional steel due to the generation of coarse

Microstructure in Mid-thickness

Mmostructure in Surface Layer

with Ultra-Fine grains (SIJF)

Photo 1 Microstructure of SUF steel

388



By the way, temperature at the layer close to plate surfaces rises

after stopping AcC with high intensity of cooling. Grains of

microstructure after rolling in the intercritical region under temperature

rising process become finer than under temperature cooling process.

Then Rolling technique after finishing AcC had been studied and plates

with layer with quite fine grains of 1 to 3 b m close to the surface, as

shown in photo 1, are then developed. They are called SUP steel plate

and show ultra high arrestability for brittle crack propagation (Ishikawa

et al, 1997)

POSSIBLE DEMERITS OF TMCP STEELS

Since TMCP steels obtain sufficient strength and toughness without

heat treatment, the problems in strength will mainly create due to

reheating such as by hot working and PWHT. Although some

countermeasures for the problems due to hot working and PWHT have

been already prepared, the explanation about these is intended to delete

here because hot working and PWHT are usually not applied for ship

hull structures.

TMCP steel plates, due to their low C,,, tend to decrease their

welded joint strength through the softening of HAZ caused by high heat

input welding with heat input greater than about 70kJ/cm. On the

contrary the softening of the HAZ does not appear in the normal steel

because of reinforcement by alloying elements.

However, the decreased HAZ strength stays at a level of about 90%

or more of that of the base plate and the width of the softened HAZ is

less than about 70% of the plate thickness, even when the very high

heat input welding, say 300kJ/cm, is applied. This comes from the

restriction of lower limit of C,, as shown in Table 2.

Table 2 Range of C,, for HT-500 steel

Therefore, no problems will arise in securing the required strength.

Moreover if the width of the tensile test specimen is 10 times the

specimen thickness, the joint strength is far greater than that of the

standard small width specimen as shown in Fig.10.

I

cC ype iMCP steel

5~0 (AH320. C,,=O 25%. t=25mm)

Solid marks Flush

~ 520

a

;5001 -----? 1

iYl

a,

=

2 480

P

:

a

________________________________________------

0

NK rule ( 490 [MPa] for YP320 steel )

460 ________________________________________----------------------------

Tensile strength of softened HA2

NK rule ( 490 [MPa] for YP360 steel )

I. I-. I. I. I I

0 100

200 300 400

Specimen width , W [mm]

Fig. 10 The effect of plate width on the tensile strength of

welded joint

The softened HAZ may affect the buckling strength as well as th

tensile strength. However if the softened HAZ is located away from

highly strained zone such as plastic hinge lines that are located in th

transverse direction in the middle of the panel plate, the reduction o

the bucklina strenath due to the softened zone is nealiaiblv small.

CO, sm-automatic CO, Gas shlelded Arc Process

0.1

IO4

IO5 IO6 IO'

Number of cycles to fracture : N,

Fig. 11 S-Nf curves (Base metal, butt welded joints)

0

Stress intensity factor range : A K [MPa m’“]

Fig. 12 Relationship between crack growth rate and stress intensity

factor range

The existence of the softened HAZ in case of high heat inpu

welding is also considered to decrease its fatigue strength. Fig. 11 shows

an example of fatigue test results obtained from YP400 AcC typ

TMCP steel and its butt welded joints by CO2 gas shielded arc welding

and FCB welding. The heat input for the FCB welding was 149kJ/cm.

The vertical axis in the figure represents a dimensionless stress range

obtained by dividing the cyclic nominal stress range oR by the base

metal tensile strength oB. The solid line and the dot dash lines show

the mean fatigue strength and the range of data scatter for CO2 ga

389



welded joints and FCB welded joints of conventional HT-500 steels

respectively. Similar results were obtained for YP320 and YP360

TMCP steel welded joints. It is obvious that the effect of softened HAZ

on fatigue strength for TMCP steel is almost the same as that for

conventional steel with the same tensile strength.

Fig. 12 shows the test results of fatigue crack propagation rate in the

softened HAZ for AcC type TMCP steel. These joints were prepared by

very high heat input welding as shown in the column in the figure. In

this figure, the data for conventional steel welded joint is also included.

From the figure, it can be also said that the fatigue crack propagation

rate in the softened HAZ is almost the same as that in the HAZ for

conventional steel.

As explained above, the effect of the softened HAZ on fatigue

strength of the joint is negligibly small. However it is well known that

the fatigue limit of high tensile steel does not increase depending on its

static strength at highly stress concentration area. So we must take care

of this factor when high tensile steel is used. From the point of view,

when high tensile steel, especially YP400 steel, is used, designers must

carry out the detail stress analysis such as 3-D FEM and higher grade of

detail design standards must be applied to reduce stress concentration

of local areas where considered critical, such as (1) bracket end of

bottom transverses, (2) openings, holes, cut-outs, (3) radius comers at

connections, (4) toes of tripping brackets, and (5) tapering of faces in

transition arrears.

In the manufacturing process of AcC type TMCP steel plate, if the

uniformity of temperature within the plate can not be maintained, flame

cutting a plate into strips releases residual stress, thus leading to

distortion as shown in Fig. 13.

Fig. 13 An example of deflection after plate stripping

To prevent this, high finishing cooling temperature is usually

applied in addition to uniform cooling by controlled water pouring

distribution for width direction of steel plate. This technique has been

already established. But engineers in assembly sections on shipbuilding

factories have been feeling empirically and vaguely that TMCP steels

have larger scatter of distortion after thermal processes such as flame

cutting and welding than conventional steel. Cold leveler with high

press are developed for removing residual stress in TMCP plates after

compressive yielding in almost half zone of a plate in thickness

direction (Tani et al, 2001). Fine control of alloying elements, rolling

timing and cooling rate in TMCP is of cause achieved for the purpose

of getting sufficient strength, toughness and weldability.

Fig.14 (Tani et al, personal communication) is an example of

testing results of deflection after flame gas heating on lines parallel to

the diagonal of 16mm thick steel plate with 2500mm width and

3000mm long. Heating pitches of 300mm were applied for both of

normal AcC type TMCP steels and above developed TMCP steels.

Heating conditions are the same. It is seen that scatter of deflection is

very small each other in the developed TMCP steel after the reinforced

cold leveler in comparison with usual TMCP steel. This steel is now

under development with expectation of realization for block

construction with high accuracy-of its shape and scale.

loo-

F

.k

60

E

-

P

g 40

80

20

0

I -

Steel 1 Steel 2 Steel 3 Steel 4 Steel 5

I

I I I

Usual TMCP Developed TMCP

Fig. 14 Comparison of measuring deflection after flame line heating

10000

z

. 22

g 5050

mm

5

6

0

8 0.40.4

7

‘&-&-

.P EP E

540.240.2

EE

mEE

(I)-I)-

0

z 200200

B

5

5

-0 1000 100

2

b

2

Q

0

-100100 0 10000 2 IO

Tempetture [Deg]

Fig. 15 Results of V-notch Charpy impact test

The last possible demerit is concerning about separation. Because

of the enhanced strength of TMCP steel by controlled rolling, so-called

separation or splitting is often observed in the fractured specimens

taken from the longitudinal or the transverse direction of steel plates,

especially in steel plate controlled rolled in the intercritical temperature

region. Cracks called separation propagate in planes perpendicular t

the main fractured surface and parallel to the plate surface during the

final fracture process due to restriction force exposed in the thickness

direction. Separation starts from an origin such as micro-orientation

texture boundary or an elongated non-metallic inclusion such as MnS

390



which would be formed or elongated during TMCP rolling. Separation

is particularly observed in the fracture surface of V-notch Charpy

impact specimens.

Separation index is usually used for the purpose of quantifying the

degree of separation, which is defined by

SI =Cei/A

end>

1)

where SI: separation index (mm/mn?),

, : Each separation length (mm),

A: Area of main fracture surface.

Fig. 15 shows an example of Charpy transition and separation index

curves for a Non AcC type TMCP steel. Separations are observed only

in the transverse or the longitudinal directional specimens. Separation

depends clearly on the difference of toughness transition phenomena

between transverse (or longitudinal) and plate thickness directions. The

separation index indicates a maximum value at a temperature where the

difference in toughness between in transverse (or longitudinal) and in

plate thickness direction becomes evident as shown in Fig. 15.

5

H

0

F

.g

8 -5

6

I

t?

t -10

g

-15

O-

O-

O-

O-

OL

-100 -50 0 50

TsCC02( - Direction ) [Deg]

Fig.16 shows the comparison between the plate thickness

directional toughness and longitudinal directional toughness in CTOD

tests. The horizontal axis and vertical axis in the figure show

temperature where the critical CTOD becomes equal to 0.2mm for th

longitudinal and thickness direction in CTOD tests respectively. Th

small triangle marks are for AcC type TMCP steels of which V notch

Charpy specimens showed no separation. The small circular marks ar

for conventional HT-500 steels. Ductility in the plate thickness

direction has a mutual relation with sulfur content. So in the figure, the

range of sulfur content of steels used is also shown.

It is obvious from the figure that maximum separation index doe

not depend on sulfur content. It can be also seen that the degree of th

deterioration in critical CTOD in plate thickness direction compared

with in longitudinal direction for TMCP steels is almost the same a

that for conventional steels except for the steel having excessively high

maximum separation index of 0.6-0.7mm/mm2. Then it comes t

conclusion that clean steel with a maximum separation index o

0.5mm/mm2 or less may not create any problem in its practical

application.

CONCLUDING REMARKS

TMCP steels are excellent in terms of steel properties and ease o

fabrication as compared with conventional steels. However there ar

some possible demerits. Then we must use these steels with carefully

consideration about these demerits.

REFERENCES

Ishikawa, T., Imai, S., Inoue, T., Watanabe, K., Tada, M. and

Hashimoto, K., (1997).“Practical Assessment of Structural Integrity

of Ships Attained by the Use of SUF Steel Having Crack-

Arrestability (SUF: Surface-layer with Ultra-Fine grain) “, 16t

OMAE, Vol.111, p.301-308,

Machida, S., Kitada, H., Yajima, H. and Kawamura, A. (1988)

“Extensive Application of TMCP Steel Plates to Ship Hulls 40

kgf/mm’ Class Yield Stress Steel”, Int. J. ofMarine Structures, Vol.

No 3

SR193 committee (1983,1984 and 1985). “A study of effective

application for TMCP steels”, Shipbuilding Research Association o

Japan, No.367 (1983), No.374 (1984) and ISSN 050-1480(1985)

Tani, T., Okada, N., Ohe, K. and Miyazaki, M. (2001). “Effects o

“Residual Stress Controlled TMCP Steel Plate” on Accuracy Ship

Blocks”, J. Sot. Naval Architects of Japan, Vo1.189, p.299-307

391

Documents

Welding TMCP Steels