Creep Resistant Steel - P9

Published in OMMI, 2009, Volume 6, Issue 2, August www.ommi.co.uk

1

TYPE IV DAMAGE MECHANISM DUE TO THE MICROSTRUCTURE WEAKENING IN THE HAZ OF A

MULTI-LAYER WELDED JOINT OF THE W CONTAINING 9%Cr FERRITIC CREEP RESISTANT STEEL

Yasushi Hasegawa1, Masaaki Sugiyama 2 and Kazuto Kawakami 2

1 Nippon Steel Corporation, Technical Development Bureau, Steel Research Laboratories

20-1, Shintomi, Futtsu, 293-8511 Japan 2 Nippon Steel Corporation, Technical Development Bureau, Advanced Technology Research

Laboratories, 20-1, Shintomi, Futtsu, 293-8511 Japan

Dr. Affiliated Professor Yasushi Hasegawa is a Chief Researcher at the

Nippon Steel Corporation, Technical Development Bureau, Steel

Research Laboratories in Japan.

His areas of interest include Creep Resistant Steels and Metallurgy.

Kazuto Kawakami is a Chief Researcher at the Nippon Steel Corporation,

Technical Development Bureau, Advanced Technology Research

Laboratories in Japan.

His areas of interest include Solid State Physics.

Dr Masaaki Sugiyama is a Chief Researcher at the Nippon Steel

Corporation,Technical Development Bureau, Advanced Technology

Research Laboratories in Japan.

His areas of interest include Metal Physics and Microscopy.

Abstract

A metallurgical model of a Type IV damaged welded joint was proposed through the

transmission electron microscope (TEM) and scanning electron microscope (SEM) analyses of

the fine grain region at the outer edge of the heat affected zone (HAZ) of the weld. In case of

multi-layer welding, there is only a fine grain microstructure at the outer edge of the HAZ. The

fusion line of the welded joint consists of only the coarse grain HAZ. Due to retained austenite


2

coalescence by rapid heating of subsequent welding, austenite memory affects the fusion line,

possibly inducing the microstructure constitution at the HAZ in multi-layer welding. In this

report, the microstructure constitution generation pathway is proposed by microstructure

analyses of a welded joint of tungsten containing 9% chromium ferritic creep resistant steel.

Three-dimensional scanning ion micro-analysis (SIM) observation by focused ion beam (FIB)

sectioning clarifies the geometry of the growing retained austenite. The multiple fine grain

thermal-cycled HAZ supposedly produces the weakest fine grain portions at the outer edge of

the HAZ, resulting in the typical interior crack of Type IV damage with creep void connection.

Key words: Type IV damage, creep rupture strength, austenite memory effect, Heat Affected

Zone, Welded joint, fine grain zone, thermal cycle simulation, SIM, FIB, TEM, SEM

1. Introduction

Creep strength enhanced 9% Cr steel has increased the energy-converting efficiency of the

recent coal fired power plants with a steam temperature of 600 degrees C or higher. New

ferritic creep resistant steel, known as Grade, (Gr). 92 in the ASME code, contains tungsten

replacing molybdenum. Boron also increases the creep resistance through the grain boundary

strengthening effect. However, its creep rupture strength at the welded joint is almost half that

of the base metal due to typical local creep damage, so-called Type IV and it is the inherent

creep resistance deterioration at the fine grain region of welded joint for all ferritic steels.

Type IV damage decreases the allowable stress of high temperature application elements due to

significant creep life deterioration. Therefore, mitigation of Type IV damage is an urgent

matter to solve for the high efficiency power plants. Many numerical analyses have tried to

simulate Type IV damage with the hypothesis of strain concentration at softened HAZ in an

over tempered HAZ, [1]. Modeling of creep crack initiation or creep void concentration is

available for explanation of fracture morphology with multiaxiality or constraint effect, [2] of

creep stress at the welded joint. On the other hand, such models do not explain the generative

mechanism of the Type IV crack initiation decisive microstructure. In order to prevent damage,

the microstructure degradation mechanism of crack initiation and creep void development,

must be accurately analyzed and clarified. In this report, first, experimental determination of

the microstructure constitution of the HAZ with austenite memory effect at the fusion line

explains the coarse grain zone formation mechanism. Second, the formation mechanism of the

deciding microstructure of Type IV damage, the fine grain HAZ, is precisely discussed. Finally,

the effect of multiple fine grain thermal cycles in the fine grain zone on an interior crack is


3

discussed. A possible metallurgical Type IV damage model is proposed as a result of

discussion.

2. Experimental procedure

2.1. Chemical composition and manufacturing process of the specimen

Table 1 shows the nominal chemical composition of the specimen in this report, Gr.92 steel in

the ASME code. The specimen was soaked at 1180 degrees C for 1 hour and hot rolled into a

plate specimen of 20mm in thickness. The plate specimen was normalized at 1050 degrees C

for 1 hour and tempered at 780 degrees C as per specified in ASTM A335 and A369. A

welding groove was cut and formed with a 45 degree of groove angle. The welding process

was tungsten inert gas arc welding (TIG) with 1 kJ/mm heat input forming a sound joint with

similar welding material. Post-weld heat treatment (PWHT) at 740 degrees C for 1 hour

relieved the residual stress of the joint. Table 2 shows the nominal chemical composition of the

welding material. According to X-ray inspection, no significant welding defect was detected

due to accurate inter-layer temperature control.

2.2. Thermal cycle simulation and sampling geometry

In order to evaluate the creep resistances of HAZ constitution microstructures, a thermal cycle

applied to a round bar-type specimen supplied the simulated microstructure through the

high-frequency-induced heating facility. Fig.1 is a thermal cycle heating diagram indicating a

schematic image of the high-frequency-induced heating facility. The round bar-type specimen

with a 5-mm homogeneous temperature-region length extracted a sufficient amount of

specimen for the various tests in this report. Heat input of 1kJ/mm was traced in 15-sec.

cooling time between 800 to 500 degrees C for 20-mm-thick plate welding. The extracted

round bar-type block of 5 mm in length was effective for microstructure investigation.

However, the size was insufficient for enough to the uniaxial-type creep specimen. Therefore,

the slim bar-type specific specimen is available for comparison of relative creep resistances.

Table 2. Chemical composition of the weld metal

C Si Mn Ni Cr Mo W Nb V N

0.06 0.25 0.50 0.40 9.0 0.50 1.50 0.06 0.20 0.05

Table 1. Chemical composition of the specimen, base metal

C Si Mn Cr Mo W Nb V N B

0.09 0.25 0.50 9.0 0.50 1.80 0.06 0.20 0.05 0.002


4

Typical thermal cycle condition of the simulated specimen for analyses

The soakinglength is 5 mm

Image of high-frequency-induced heating of sample

Ac3 by rapid heating


960℃

890℃

2 sec.

100℃/s

800℃

500℃

15sec.

Time

Tem

pera

ture

Peak temperature variation 700～1300℃

Typical thermal cycle condition of the simulated specimen for analyses

The soakinglength is 5 mm

Image of high-frequency-induced heating of sample



960℃

890℃

2 sec.

100℃/s

800℃

500℃

15sec.

Time

Tem

pera

ture




960℃

890℃

2 sec.

100℃/s

800℃

500℃

15sec.

Time

Tem

pera

ture


Fig. 1. Thermal cycle simulation diagram and

high-frequency-induced heating test image

4mmφ

M14

M14 M14

M146mmφ

4mmφ

30mm

(a)

(b)

M14

M14 M14

M146mmφ

4mmφ

30mm

(a)

(b)

10mm

4mmφ

M14

M14 M14

M146mmφ

4mmφ

30mm

(a)

(b)

M14

M14 M14

M146mmφ

4mmφ

30mm

(a)

(b)

10mm

Fig. 2. Slim- and short-gauge-type creep specimen (b) of the simulated HAZ measuring the extracted microstructure creep properties comparing the geometry with that of an ordinary-type creep specimen (a)

Fig. 2 shows the design of a slim bar-type short-gauge length compared to that of an ordinary

creep specimen. The specific specimen extracted the peak temperature-induced microstructure

effect on the creep properties by thermal cycle simulation.

2.3. Microstructure analyses

Optical microstructure observation with accurately determined Ac1 and Ac3 transformation

temperature at the HAZ by rapid heating dilatometry at 100 degrees C / second decontrolled

the precise microstructure constitution at the HAZ. TEM analysis confirmed the dislocation

microstructure of the HAZ that had a lath martensite microstructure, retained austenite at the

lath boundary, and carbide distributions.


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１mm

Welded Metal

Base Metal

D.P.ＨＡＺ

F.G.ＨＡＺ

C.G.ＨＡＺ

Creep Voids

Fusion Line

１mm１mm１mm

Welded Metal

Base Metal

D.P.ＨＡＺ

F.G.ＨＡＺ

C.G.ＨＡＺ

Creep VoidsCreep Voids

Fusion Line

Fig.3. Microstructure constitution of Type IV ruptured welded joint showing the creep voids at the fine grain zone.

2.4 Austenite memory effect verification

The HAZ microstructure in multi-layer welding joints contains the austenite memory effect at the

fusion line (FL). In this report, corroborative evidence of it was confirmed through TEM and SEM

analyses using thin foil. Furthermore, SIM analyses on rectangular solid of 10-μm square and

its thickness of 3 μm by the FIB serial sectioning method in 30 nm pitch generated a 3-D

image of the specimen interrupting the subsequent welding heat for multi-layer welding.

Austenite memory effect was observed three dimensionally.

3. Experimental results

3.1 Optical microstructure and creep crack propagation position of the HAZ at a Type IV

fractured welded joint

Figure 3 is a Type IV ruptured welded joint

of ASME GR.92 showing the microstructure

constitution of the HAZ and generation of

creep voids in the fine grain (FG) HAZ. The

coarse grain (CG) zone is located adjacent to

the welded metal and the dual phase (DP)

HAZ is adjacent to the base metal. Creep

cracks seem to propagate in the FG zone

with creep voids, as indicated by the arrows

in the figure.

The microstructure of a multi-layer welded joint is composed of inhomogeneous phases which

are expected due to the various thermal cycle mixtures. However, creep cracks never intrude

unexpectedly into the FL or welded metal in multi-layer welding.

3.2. Creep lives and room-temperature strength of welded joints

Figure 4 shows the thermal cycle peak temperature dependence of tensile strength at room

temperature. The lowest strength is observed at Ac1, a typical temperature for rapid heating

simulating the heating of the HAZ, or little lower than the thermal cycle peak temperature. It is

well-known that the “HAZ-softening” phenomenon is observed in conventional carbon steels.


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Fig.6.Initial microstructure, dislocation lath martensite with carbides before thermal cycle simulation.

1μm Fig. 7. Wide lath martensite with vaguely outlined and coarsened carbides after the FGHAZ thermal cycle.

1μm

Fig. 8. Polygonal sub-grain microstructure and coarse carbides of the FGHAZ and PWHT thermal cycle.

1μm

600

650

700

750

800

850

900

950

1000

800 900 1000 1100 1200 1300

Peak temperature of HAZ thermal cycle (℃)

Tensi

le s

tren

gth a

t R

.T. (N

/mm

2 ) Ac1 Ac3

600

650

700

750

800

850

900

950

1000

800 900 1000 1100 1200 1300

Peak temperature of HAZ thermal cycle (℃)

Tensi

le s

tren

gth a

t R

.T. (N

/mm

2 ) Ac1 Ac3

Fig 4. Room-temperature tensile strength of thermal cycle simulated specimens.

HAZ

The ordinary hypothesis says that the strain concentration at the HAZ-softening region is the

decisive factor in Type IV damage. Figure 5 indicates the thermal cycle peak temperature

dependence of creep lives at of constant loads at 700 degree C at 60 MPa (solid symbols), and

at 70 MPa (open symbols). The shortest creep lives were observed at Ac3 for the HAZ or little

higher than the thermal cycle peak temperature. Figure 5 indicates a typical Type IV

phenomenon, deterioration of creep life, confirming the microstructure and fractured geometry

of creep specimens. Therefore, the peak temperatures for the HAZ-softening phenomenon and

those of the Type IV phenomenon do not coincide. This study disagrees with the hypothesis

that Type IV damage initiates at the HAZ-softening region, [3]. A Type IV damaged

microstructure is composed of only fine prior austenite because the peak temperature is just

above the Ac3 transformation temperature at the HAZ.

3.3. TEM analyses of the microstructure with FGHAZ generation thermal cycle simulation


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The dislocation microstructure of thermal cycle simulated specimens was analyzed after the

FGHAZ thermal cycle and also after PWHT compared with that of before the thermal cycle.

Figures 6 is a TEM image of the specimen before the FGHAZ thermal cycle. It is typical

tempered lath martensite with fine carbides on the boundaries. Figure 7 shows a lath martensite

microstructure with coarsened carbides after the FGHAZ thermal cycle. The lath width is

larger than that of Figure 6. The carbides are coarsened through the thermal cycle compared

with that of Figure 6. The vaguely outlined and coarsened carbides precipitate independently

of the wide lath structure. Figure 8 is a TEM image of the microstructure of the FGHAZ

thermal cycle and PWHT thermal cycle. The lath structure disappeared and recovered to the

polygonal sub-grain microstructure. Sharply defined coarsened carbides are also observed.

4. Discussion

4.1. Decisive dislocation microstructure at the FGHAZ for Type IV damage

Microstructure development through the simulated FGHAZ thermal cycle can be explained

through thermal cycle simulation and TEM analysis of it as follows:

The initial microstructure before the HAZ thermal cycle consists of dislocation lath martensite

and carbides precipitating in a line on high-angle boundaries. If the initial microstructure is

heated up around Ac3 temperature or higher at the HAZ at a rapid heating rate, fine austenite

grains nucleate under reverse transformation from the high-angle boundaries. Fine carbides

resolve into a matrix. On the other hand, coarse carbides do not resolve completely and remain

as carbides on the prior high-angle boundaries due to the brief holding time of the FGHAZ

thermal cycle and following a relative high cooling rate. The vaguely outlined and coarsened

carbides in Figure 7 are remaining carbides and decomposing carbides. Carbon shortage in the

matrix due to incomplete carbide resolution extended the lath width in Figure 7. The carbon

shortage and fine austenite grain size possibly decreased the hardenability of lath martensite.

The martensite with low hardenability was completely recovered to a polygonal sub-grain

microstructure as shown in Figure 8 due to carbide coarsening and low dislocation density.

Reproduction of carbides on the remaining carbides formed sharply defined coarsened

carbides[4]. Therefore, the microstructure in Figure 8, a polygonal sub-grain microstructure

with coarsened carbides, is possibly the decisive microstructure in Type IV damage. A

polygonal sub-grain microstructure with coarsened carbides cannot resist creep deformation.

Figure 9 is a comparison of creep rupture strength curves of welded joints and those of base


8

10

100

1000

1 10 100 1000 10000 100000

Time (h)

Applie

d s

tress (M

Pa)

.

600℃

650℃700℃

● Base metal

○ Welded joint

10

100

1000

1 10 100 1000 10000 100000

Time (h)

Ap

pli

ed

str

es

s (M

Pa)

.

600℃

650℃700℃

● Base metal○ Welded joint

10

100

1000

1 10 100 1000 10000 100000

Time (h)

Applie

d s

tress (M

Pa)

.

600℃

650℃700℃

● Base metal

○ Welded joint

10

100

1000

1 10 100 1000 10000 100000

Time (h)

Applie

d s

tress (M

Pa)

.

600℃

650℃700℃

● Base metal

○ Welded joint

10

100

1000

1 10 100 1000 10000 100000

Time (h)

Ap

pli

ed

str

es

s (M

Pa)

.

10

100

1000

1 10 100 1000 10000 100000

Time (h)

Ap

pli

ed

str

es

s (M

Pa)

.

600℃

650℃700℃

● Base metal○ Welded joint

Fig. 9. Comparison of the creep rupture strength of a welded joint and that of base metal of ASME Gr.92 steel.

１mm

Base metal

Welded metal

F.G.ＨＡＺ

10μm

Layi

ng d

irect

ion

１mm１mm１mm１mm

Base metal

Welded metal

F.G.ＨＡＺ

10μm10μm

Layi

ng d

irect

ion

Fig. 10. The fusion line of multi layered welded joint does not contain fine grain microstructure even affected by the subsequent fine grain thermal cycle.

metal for ASME Gr.92 steel. The open marks show the effect of Type IV damage on creep

rupture strength.

4.2. Coarse grain zone generation mechanism at the fusion line of the welded joint

The coarse grain zone is located adjacent to the welded metal due to a higher thermal cycle

peak temperature than 1300 degrees C even if the holding time is short. However, in the case

of multi-layer welding, the subsequent FGHAZ of welding intrudes into the precedent HAZ

including the CGHAZ. Therefore, a microstructure mixture of coarse and fine grains should be

observed. However, no fine grain microstructure was observed at FL of the welded joint at any

part of the thickness as shown in the magnified micrograph of Figure 10.

Coarse grain zone generation at the fusion line affects the creep damage concentration in the

fine grain zone, the outer edge of the HAZ. Therefore, simulation of the CGHAZ affected by


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the subsequent FGHAZ thermal cycle is an important research step to clarify the Type IV

damage mechanism.

Figure 11 shows the thermal cycle diagrams of single-layer welding at the FGHAZ (a) and at

the CGHAZ followed by subsequent FGHAZ thermal cycle (b) and multi-layer welding,

showing each optical microstructure. The initial microstructure is tempered lath martensite of

20μm in mean diameter. In the case of single-layer welding thermal cycle simulation, the

assumed outer edge of the HAZ showed a typical fine grain microstructure. On the other hand,

in the case of multi layer welding, the assumed fusion line showed a coarse grain

Assumed outer edge of the HAZ Assumed FL of the welded joint

(a)

Ac3

Time

Tem

pera

ture

Ac3

Time

Tem

pera

ture

Ac3

Time

Tem

pera

ture

Ac3

Time

Tem

pera

ture

(b)

20μm

20μm

Fig. 11. Thermal cycle diagrams and simulated microstructures of the assumed outer edge of the HAZ and those of the fusion line of multi-layer welding showing the significant difference in prior austenite grain size.


10

Fig. 15 Growing retained identical austenite crystal orientation by TEM dark field image.

1μm

1μm1μm1μm

Fig. 13 TEM image of lath martensite and retained austenite on the boundary as a typical FL thermal-cycled microstructure.

0.2μm0.2μm0.2μm

Fig. 14 Lath structure with retained austenite and austenite growth front between the lath boundaries.

Retained austenite Lath boundaries

Austenite growth front

Retained austenite growth direction

Retained austenite

Lath boundaries 1μm 0.2μm

Fig. 16 3D-image of growing austenite by memory effect through SIM analysis on the FIB serial sectioning method.

microstructure through CGHAZ and FGHAZ thermal cycle simulation. The subsequent

thermal cycle did not decide the microstructure in the case of FL of multi-layer welding for

ASME Gr.92. This coarse grain zone is also observed in other Type IV crack problems inherent

in ferritic creep resistant steels including Cr-Mo steels. Grain refining through the FGHAZ

thermal cycle was confirmed. Therefore, the precedent FL thermal cycle possibly resulted in

the reproduction of coarse grains at the fusion line. In order to analyze the mechanism,

subsequent heating interruption and quenching of the microstructure at various temperatures of

the FL thermal cycle by multi-layer welding simulated the microstructures as illustrated in

Figure 12.

TEM analyses detected the lath structure

and retained austenite on lath boundaries

by electron micro diffraction in the fusion

line thermal cycled specimen as shown in

Figure 13.

Figure 14 shows the microstructure of the

heating-interrupted-and-quenched

specimen from 700 degrees C of

subsequent welding. Retained austenite

and some decomposed carbides were

observed on the lath boundaries. Retained

austenite and carbide-free trail boundaries

were also identified between the lath

boundaries. The trail boundary seems to


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be the austenite growth front. Retained austenite possibly grows perpendicularly to the lath

boundary due to the rapid heating of subsequent welding as explained in Figure 14. Retained

austenite grows to satisfy the phase ratio for elevated temperature. Retained austenite

memorizes the prior austenite crystal orientation with large grain size. Therefore the identical

crystal orientation of retained austenite could be confirmed by TEM dark-field image analysis

as shown in Figure 15. Bright regions are growing austenite by rapid heating of subsequent

welding. Figure 16 is the three-dimensional scanning ion micrograph of growing austenite as

identified in Figure 15. The image was generated by accumulation of the sliced SIM in 30nm

pitch through the serial sectioning by FIB fabrication method. The extracted uniform crystal

orientation portions, growing austenite by memory effect, extend along the lath boundary and

also perpendicular to the lath boundary at the same time. The growing austenite is in the form

of thick plates with a roughly constant space, the lath width.

Figure 17 shows a lath martensite microstructure with large restored prior austenite grains as a

result of coalescent growing retained austenite due to the memorized identical prior austenite

crystal orientation. The observed coalescence of retained austenite is the so-called “austenite

memory effect”, [5]. The austenite memory effect explains the coarse grain microstructure at

the

fusion line of a welded joint in Figure 10. The fusion line microstructure always consists of

coarse grains. Therefore, the creep life of the FL microstructure is almost the same as that of

the base metal, independent of the subsequent thermal cycle as compared with that of the

FGHAZ in Figure 18.


12

100

1000

10000

100000

800 900 1000 1100 1200 1300

Peak temperature of thermal cycle simulation (℃)

Cre

ep

life

at

700

℃，

60M

Pa

(h) CGHAZ & FGHAZ cycle

FGHAZ cycle

Ac1 Ac3

Creep life of base metal

100

1000

10000

100000

800 900 1000 1100 1200 1300

Peak temperature of thermal cycle simulation (℃)

Cre

ep

life

at

700

℃，

60M

Pa

(h) CGHAZ & FGHAZ cycle

FGHAZ cycle

Ac1 Ac3

Creep life of base metal

Fig. 18 Creep life of FL of a welded joint compared with that of the FGHAZ.

The reason why fusion line of welded joint consists only of coarse grain microstructure was

explained through the TEM analyses and the austenite memory effect. Therefore, fine grain

zone is always formed only at the outer edge of HAZ.

4.3. Creep strength evaluation of the duplicated FGHAZ thermal cycle

The FGHAZ consists of a polygonal sub-grain microstructure and coarsened carbides through

the welding thermal cycle. The creep deformation resistance at the FGHAZ decreased

compared with that of the base metal. Additionally, the thermal cycle of FGHAZ is duplicated

in the fine grain zone in places due to the subsequent welding heat effect as shown in Figure

19.


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Double F.G. HAZ thermal cycle

Tem

per

atu

re

Time

PWHT(740℃×1hr)

Ac3

Double F.G. HAZ thermal cycle

Tem

per

atu

re

Time

PWHT(740℃×1hr)

Ac3

Fig. 20 Thermal cycle simulation of the duplicated FGHAZ with PWHT in Fig. 18.

Thermal cycle simulation with PWHT is illustrated in Figure 20 as a diagram.

The slim- and short-gauge-type creep specimen evaluated the creep lives at various peak

temperatures. The creep lives of double thermal cycle-applied specimens were compared with

those of single thermal cycle-applied specimens in Figure 21.

The double thermal cycle-applied specimens also had the shortest creep life at a peak

temperature around Ac3, and the double FG thermal-cycled specimen showed a shorter creep

life compared with that of the single FG thermal-cycled specimen. Therefore, the creep life of

the double FGHAZ thermal-cycled specimen is the shortest.

4.4 Metallurgical Type IV damage model proposal


14

Base metal

Welded metal

F.G.HAZC.G.HAZ

Creep void

Base metal

Welded metal

F.G.HAZC.G.HAZ

Creep voidCreep void

Fig. 22 Image of the duplicated FG thermal-cycled portions and creep void generation during the creep.

D.P.HAZ

Base metal

Welded metal

F.G.HAZC.G.HAZ

Creep void

Type IV crack

D.P.HAZ

Base metal

Welded metal

F.G.HAZC.G.HAZ

Creep void

Base metal

Welded metal

F.G.HAZC.G.HAZ

Creep voidCreep void

Type IV crackType IV crack

Fig. 23 Creep void density increase the FGHAZ and interior creep crack initiation as a result of the void connection.

Integrating all the discussions and through Figures 22 and 23, a metallurgical Type IV model

can be proposed. From the beginning of the creep deformation, the HAZ at welded joints

consists of the CGHAZ, FGHAZ and DPHAZ. In the FGHAZ, duplicated FG thermal-cycled

parts are as the indicated shaped portions in Figure 22. Creep voids generate from portions

with low creep resistance, the duplicated FG thermal-cycled parts with a shaped area in the

figures, under the constant stress of creep deformation due to plastic constraint. Creep voids

also generate at the other fine grain zone of welded joints with creep duration.

Finally, the creep voids connect with each other and initiate the interior creep cracks shown in

Figure 23. An interior crack is a typical Type IV fracture of a welded joint. The proposed

metallurgical Type IV damage model explains the possible interior cracks without numerical

analysis. However, in order to predict the creep life of a welded joint, a numerical approach

with finite element analysis is necessary as further work.


15

5. Conclusions

(1) The decisive microstructure for Type IV damage was confirmed through the thermal cycle

simulation and TEM observation. Type IV cracks initiate at the fine grain microstructure of

the HAZ. The conventional numerical hypothesis of Type IV damage, strain concentration

at HAZ-softening zone, did not explain the creep life of the fine grain zone.

(2) The coarse grain zone did not contain a fine grain microstructure at any part of the fusion

line of the welded joint due to the austenite memory effect in multi-layer welding.

(3) Duplicated fine grain thermal cycle portions in the fine grain zone indicated the shortest

creep life according to the thermal cycle-simulated specimen creep tests with the slim- and

short-gauge-type specimens for measurement of relative creep properties.

(4) As integrated research results, a metallurgical Type IV damage model was proposed

reflecting the austenite memory effect at the fusion line and the shortest creep life at

duplicated fine grain thermal-cycled portions. The proposed model explains typical interior

crack initiation of Type IV damage without numerical analysis.

Acknowledgement

This report was accomplished as a part of research activities of "Fundamental Studies on

Technologies for Steel Materials with Enhanced Strength and Functions" by Consortium of

JRCM (The Japan Research and Development Center of Metals). Financial support from

NEDO (New Energy and Industrial Technology Development Organization) is gratefully

acknowledged．

* This paper was previously presented at the 2nd. ECCC International Conference on Creep,

April 21st. – 23rd. 2009, Zurich, Switzerland.

References

[1] E. Letofsky, H. Cerjack and P. Warbichler, Prakt. Metallogr., 2000, 37, pp509-521.

[2] T. Ogata and Y. Yaguchi, “High-Temperature Strength Property Evaluation of Heat Affected

Zone in Boiler Weldment Parts of 2.25Cr-1Mo steel”, J. Soc. Mat. Sci. Japan, 1998, 47, pp

253-259.

[3] T. Ogata and Y. Yaguchi, “Type IV cracking and Life Evaluation of Weldments on

2.25Cr-1Mo Steel under Creep-Fatigue Loading Condition”, J. Soc. Mat. Sci. Japan, 1999,

48, pp137-143.

[4] Y. Hasegawa, T. Muraki, Ohgami, “Metallurgical investigation of a Type IV damage at the

Heat Affected Zone of Weld for Tungsten Containing Martensitic Heat Resistant Steels”,

Int. Conf. of ASME-PVP, 2004, 476, pp45-56.


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[5] M. Sugiyama, G. Shigesato, T. Hara, H. Asahi, “Mechanism of abnormal α to γ

transformation-2 (In-situ observation of α to γ transformation by scanning ion

microscopy”, 2004, 17, p1269.

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Creep Resistant Steel - P9