JFE TECHNICAL REPORT

No. 18 (Mar. 2013)

Development of
SA-738 Gr. B High Strength Steel Plate
with Excellent Toughness
for Power Generating Plants†
OHTSUBO Hirofumi*1   ARAKI Kiyomi*2   MORIYA Yutaka*3

Abstract: SA-738 Gr. B under the ASME Code (ASME Boiler &
JFE Steel has developed ASME SA-738 Gr. B (ASME: Pressure Vessel Code, ASME: The American Society of
The American Society of Mechanical Engineers) high Mechanical Engineers) has been designated as a steel
strength steel plate for reactor containment vessels in plate for reactor containment vessels.
response to great demand for new construction of Since the volume of a reactor containment vessel is
nuclear power plants mainly in America, China, and extremely large and construction involves many parts
developing countries. The developed steel has excellent which are performed at the site, post-weld heat treat-
toughness to meet the requirement of exemption rule of ment (PWHT) of field welds increases both construction
post-weld heat treatment (PWHT) in ASME with thick- costs and the work load. On the other hand, the ASME
ness 44.5 mm or less, and also has good weldability Code, Section III, Division I, Subsection NE, Class MC
through the reduction of preheat temperature. Heavy Components (Rules for Construction of Nuclear Facility
section plate around 100 mm thickness has good tough- Components) recognizes the possibility of omitting
ness by redeucing impurity levels, and excellent internal PWHT (exemption rule) for SA-738 and certain other
qualities can be obtained through the application of a carbon steels with thicknesses of 44.5 mm or less based
forging and plate rolling process to continuous casting on strict toughness requirements in cases where limits
slabs. These developed steels have already been adapted on the C content and preheat temperature are applied.
to some nuclear power plants. Accordingly, development of steel plates with excellent
low temperature toughness of the base material and
welded joints, which satisfy the requirement in PWHT
1. Introduction
exemption rule within the specified composition range,
In order to prevent global warming due to CO2 emis- has been demanded.
sions and respond to increasing global energy demand, In addition to the above-mentioned materials, heavy
construction of a large number of new nuclear power section plates with 100 mm class thickness are used in
plants is being planned, centering on the United States, equipment hatches and airlocks in reactor containment
China, and the emerging economies. It is generally vessels. When these heavy section plates are produced
assumed that the main stream in nuclear power plants from continuous casting slabs by a plate rolling process,
constructed in the future will be Generation III (III+) deterioration of mechanical properties due to the effects
nuclear reactors1). In the new 1 000 MW class pressur- of center porosities which were not closed in the center
ized water reactor, which is one of these new reactors, of plate thickness is a concern. To improve the internal

†
Originally published in JFE GIHO No. 29 (Feb. 2012), p. 48–53 *2
Staff Manager,
Plate Business Planning Dept.,
JFE Steel

*1
Senior Researcher Deputy Manager, *3
Staff General Manager,
Steel Products Res. Dept., Plate Business Planning Dept.,
Steel Res. Lab., JFE Steel
JFE Steel

50
 Development of SA-738 Gr. B High Strength Steel Plate with Excellent Toughness for Power Generating Plants

Table1  Target properties of the developed steel

Thickness
C Si Mn P S Cu Ni Cr Mo V Nb Ceq
(mm)
t ≤38.1 ≤0.20
0.90/
≤0.48
SA-738 38.1<t ≤63.5 0.15/ 1.50
≤0.20 ≤0.030 ≤0.030 ≤0.35 ≤0.60 ≤0.30 ≤0.07 ≤0.04
Gr. B 0.55 ≤0.30
0.90/
t >63.5 —
1.60

Base plate Welded joint

Charpy impact Drop weight Charpy impact
Thickness Tensile properties Tensile properties
properties properties properties
(mm)
YS TS El vE TNDT TS vE
(MPa) (MPa) (%) (J) (°C) (MPa) (J)
vE−60°C vE−40°C
38.1<t ≤63.5 ≥414 587/703 ≥20 ≥48 (Min.) — ≥587 ≥48 (Min.)
≥55 (Ave.) ≥55 (Ave.)
vE−40°C vE−40°C
t >63.5 ≥414 587/703 ≥20 ≥62 (Min.) ≤−25 ≥587 ≥62 (Min.)
≥68 (Ave.) ≥68 (Ave.)
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5
YS: Yield strength    TS: Tensile strength    El: Elongation    vE: Absorbed energy    TNDT: Nil-ductility transition temperature

soundness of heavy section products, JFE Steel devel- test temperatures for Charpy impact properties and drop
oped a process in which forging is applied to continuous weight properties were calculated based on the ASME
casting slabs before plate rolling2–5), and has supplied a Code, assuming construction in a cold region, and secur-
large number of plates for pressure vessels and other ing toughness at −60°C and −40°C was then set as a
applications. In the developed steel, heavy section mate- development target, also considering a safety allowance.
rials with excellent internal properties were developed In particular, for Charpy impact properties, when the
using continuous casting slabs by applying the combined above-mentioned PWHT exemption rule is to be applied,
forging and plate rolling process. it is necessary to satisfy the specified absorbed energy at
This paper introduces the base material performance, a temperature 5.6°C lower than the specified tempera-
weldability, and welded joint performance of the SA-738 ture, or to increase the absorbed energy at the specified
Gr. B high strength steel plate which was developed as a temperature by 7 J.
material for reactor containment vessels.
2.2 Development Concept
2. Development Targets for SA-738 Gr. B for To achieve the target properties, a study was carried
Reactor Containment Vessels out based on SPV490 6–8), which is a JIS material (JIS:
Japanese Industrial Standards) of tempered 600 MPa
2.1 Target Properties of SA-738 Gr. B
grade steel, and has a record of use as a steel plate for
Table 1 shows the target properties of SA-738 Gr. B reactor containment vessels at nuclear power plants in
high strength steel plate, which was developed for reac- Japan.
tor containment vessels, based on the ASME Code for Regarding the manufacturing method, although both
chemical composition and mechanical properties. SA-738 Gr. B and SPV490 are Q-T materials, if the tem-
SA-738 Gr. B is a 600 MPa class steel which is pro- per temperature is set to an elevated temperature of
duced by quench-temper (Q-T) treatment. Upper limits 650°C or more, recovery of dislocations, precipitation
for the contents of alloy elements and carbon equivalent coarsening, etc. generally proceed. As these changes
Ceq are specified by plate thickness. In order to guaran- reduce the strength of the material, consideration of the
tee properties after applying PWHT (615°C×10 h), one composition design is necessary, for example, measures
target in material development was to secure the to improve resistance to temper softening, etc. in order
required mechanical properties with a temper tempera- to secure the required strength when applying high tem-
ture of 650°C or higher. perature tempering followed by PWHT.
The toughness guarantee temperature was set based To satisfy the PWHT exemption rule with steel plates
on the lowest service metal temperature, and was modi- having thicknesses of 44.5 mm or less, it is important to
fied based on the reactor structure, location of construc- prevent increased hardness of the heat affected zone
tion, plate thickness, etc. In material development, the (HAZ) by reducing the C content and weld-crack sensi-

JFE TECHNICAL REPORT No. 18 (Mar. 2013)  51

 Development of SA-738 Gr. B High Strength Steel Plate with Excellent Toughness for Power Generating Plants

750 0.06%C-Si-Mn-Ni-Cr-Mo-V Steel

transition temperature, vTrs (°C)

Conventional : (0.09–0.11)%C-Si-Mn-Ni-Mo-V −50

Charpy fracture appearance

Developed: 0.06%C-Si-Mn-Ni-Cr-Mo-V
700
−60
Tensile strength (MPa)

650 Developed −70

steel

600 587–703 MPa −80

Conventional 0 0.01 0.02 0.03
steel
(SPV490) P Content (mass%)
550
Fig. 2 E
 ffect of P content on toughness after post weld heat
treatment (PWHT)
500
0.16 0.18 0.20 0.22 0.24
ments such as Mo, V, Cr, etc., which have large resis-
PCM(%)
tance to temper softening, were also added to secure
PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B
strength after PWHT, and in addition, reduction of the
Fig. 1 R
 elationship between tensile strength and PCM after impurity elements P and S was reflected in the composi-
post weld heat treatment (PWHT) tion design as a measure to improve low temperature
toughness.
tivity composition PCM.
Figure 1 shows the results of a laboratory study of 3. Properties of Developed SA-738 Gr. B Steel
toughness after high temperature tempering (670°C) and
3.1 Base Material and Welded Joint Properties
PWHT (615°C×10 h) based on a 0.11% C conventional
of Developed Steel (Thickness: 47.6 mm)
SPV490 and a 0.06% C steel. In order to compensate for
the strength reduction due to the low C content of the The chemical composition of the developed steels
developed steel in comparison with the conventional (also including steels with thicknesses of 44.5 mm and
SPV490 steel, a composition design which makes it pos- under to which the PWHT exemption rule under ASME
sible to secure strength with a low PCM ( 0.20%) was Sec. III is applicable) are shown in Table 2. Weldability
adopted by optimum addition of elements with large is improved by holding the C content to a low level and
temperature softening resistance, such as Mo, V, Cr, Si, reducing PCM to 0.20% or less. To suppress the strength
etc.9). drop due to PWHT, the composition design actively uses
The effect of reducing impurity elements in order to elements (Mo, V, Cr, Si, etc.) which increase resistance
improve low temperature toughness was also studied. to temper softening, and to improve low temperature
Using the 0.06% C steel as a base, the effect of the phos- toughness, the impurity elements P and S are reduced in
phorus content on base material toughness after PWHT the steelmaking process.
was studied. The results are shown in Fig. 2. It is possi- The mechanical properties of the base plate of the
ble to suppress toughness decrease in the matrix and developed steel, which is manufactured by the quench-
improve base material toughness after PWHT by reduc- temper (Q-T) process, are shown in Table 3. Strength
ing the phosphorus content. satisfying the requirement of SA-738 Gr. B and excel-
With heavy thickness materials with thicknesses of lent low temperature toughness at −60°C are obtained
100 mm class, securing toughness in the center of thick- before and after PWHT.
ness area using continuous casting slabs is an issue. The results of a maximum hardness test and the
Therefore, a combined forging and plate rolling process results of a y-groove weld cracking test, which were
which had been established previously by JFE Steel was performed to evaluate weldability, are shown in Table 4
applied. As in the study of plates of 40 mm class thick- and Table 5, respectively. Because PCM was reduced to
ness described above, the optimum amounts of alloy ele- no more than 0.20%, maximum hardness is low, at 275

Table 2  Chemical compositions of the developed steels

(mass%)
Thickness
C Si Mn P S Others Ceq PCM
(mm)
Cu, Ni, Cr,
47.6 0.05 0.39 1.44 0.003 0.001 0.46 0.20
Mo, V, etc.
Ni, Cr, Mo,
103.1 0.12 0.30 1.54 0.003 0.001 0.52 0.25
V, etc.
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5
PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B

52  JFE TECHNICAL REPORT No. 18 (Mar. 2013)

 Development of SA-738 Gr. B High Strength Steel Plate with Excellent Toughness for Power Generating Plants

Table 3  Mechanical properties of base plate of the developed steel

Tensile properties Charpy impact properties

Thickness
Position Direction PWHT YS TS El vE−40°C vE−60°C vTrs
(mm)
(MPa) (MPa) (%) (J) (J) (°C)
— 573 642 28 431 380 −97
47.6 1/4t T
615°C×10 h 565 642 27 343 303 −80
Tensile test specimen: ASME SA-370, Φ12.7 mm (GL 50.8 mm)
Charpy impact test specimen: ASME SA-370, Full-size
ASME: The American Society of Mechanical Engineers
PWHT: Post weld heat treatment    YS: Yield strength    TS: Tensile strength    El: Elongation
vE−40°C: Absorbed energy at −40°C    vE−60°C: Absorbed energy at −60°C    vTrs: Fracture appearance transition temperature

Table 4  Results of maximum hardness test (Thickness: 47.6 mm)

Preheat temperature (°C) Maximum hardness, HV10

R.T. (25) 275
50 266
100 275
150 260
Test method: JIS Z 3101 (JIS: Japanese Industrial Standards)
Welding condition: SMAW    SMAW: Shielded metal arc welding    Heat input: 1.7 kJ/mm    R.T.: Room temperature

Table 5  Results of y-groove weld cracking test

Preheat Cracking ratio (%)

Thickness
Welding condition temperature
(mm) Surface Section Root
(°C)
R.T. (25) 0 0 0
SMAW
Welding consumable: LB-62UL* 50 0 0 0
47.6
Heat input: 1.7 kJ/mm 75 0 0 0
Atmosphere: 30°C-80%
100 0 0 0
Test method: JIS Z 3158 (JIS: Japanese Industrial Standards)
*Supplied by Kobe Steel, Ltd.
SMAW: Shielded metal arc welding    R.T.: Room temperature    SMAW: Shielded metal arc welding

Table 6  Welding condition and mechanical properties of welded joints of the developed steel

Tensile properties Charpy impact properties

Thickness Groove
Welding condition PWHT TS Fracture vE−40°C
(mm) configuration Position
(MPa) position (J)
WM 147
— 638 WM 1/4t FL 176
GMAW
Welding consumable: HAZ 289
47.6 X
MG-S3N* WM 144
Heat input: 3.8 kJ/mm
615°C×10 h 619 WM 1/4t FL 176
HAZ 244
*Supplied by Kobe Steel, Ltd.    GMAW: Gas metal arc welding    PWHT: Post weld heat treatment    TS: Tensile strength
WM: Weld metal    FL: Fusion line    HAZ: Heat-affected zone    vE−40°C: Absorbed energy at −40°C

points, even under a condition of no preheating, and the strength satisfying the specification of the base material
plate possesses excellent weldability, having a cracking and excellent weld toughness were obtained before and
prevention preheat temperature of room temperature or after PWHT. In case the PWHT exemption rule is
less. applied, it is necessary to satisfy the specified toughness
The mechanical properties of welded joints of the value at a temperature 5.6°C lower than the lowest ser-
developed steel are shown in Table 6. Welded joint vice metal temperature, or to increase the absorbed

JFE TECHNICAL REPORT No. 18 (Mar. 2013)  53

 Development of SA-738 Gr. B High Strength Steel Plate with Excellent Toughness for Power Generating Plants

Photo 1  orging reduction in widthwise of continuous casting

F Photo 2  orging reduction in thicknesswise of continuous
F
slab casting slab

Table 7  Mechanical properties of base plate of the developed steel

Tensile properties Charpy impact properties Drop weight

Thickness
Position Direction PWHT YS TS El vE−20°C vE−40°C vTrs properties
(mm)
(MPa) (MPa) (%) (J) (J) (°C) TNDT (°C)
— 591 674 28 271 242 −69 −50
1/4t
615°C×10 h 566 663 28 295 258 −69 −40
103.1 T
— 551 651 22 187 199 −43 —
1/2t
615°C×10 h 536 646 24 219 174 −38 —
Tensile test specimen: ASME SA-370, Φ12.7 mm (GL 50.8 mm)    Charpy impact test specimen: ASME SA-370, Full-size
ASME: The American Society of Mechanical Engineers
Drop weight test specimen: ASTM E208 type P-3    ASTM: The American Society for Testing and Materials
PWHT: Post weld heat treatment    YS: Yield strength    TS: Tensile strength    El: Elongation    vE−20°C: Absorbed energy at −20°C
vE−40°C: Absorbed energy at −40°C    vTrs: Fracture appearance transition temperature    TNDT: Nil-ductility transition temperature

Table 8  Results of y-groove weld cracking test

Thickness Welding Preheat temperature Cracking ratio (%)

(mm) condition (°C) Surface Section Root
SMAW 75 15 23 94
Welding consumable: LB-62UL*
103.1 100 0 17 67
Heat input: 1.7 kJ/mm
Atmosphere: 30°C-80% 125 0 0 0
Test method: JIS Z 3158 (JIS: Japanese Industrial Standards)    *Supplied by Kobe Steel, Ltd.    SMAW: Shielded metal arc welding

energy at the specified temperature by 7 J. However, the manufactured by the quench-temper (Q-T) process, are
developed steel has excellent low temperature toughness shown in Table 7. The heavy thickness plate assumes
which amply exceeds the specified value at −40°C even PWHT. Before and after PWHT, this plate possesses
without PWHT. The above-mentioned base material strength amply satisfying the specified value at both the
toughness and welded joint toughness results confirmed 1/4t and 1/2t positions and excellent low temperature
that the developed steel has excellent low temperature toughness at −40°C.
toughness satisfying the PWHT exemption rule. The results of the y-groove weld cracking test with a
103.1 mm full thickness test specimen of the developed
3.2 Base Material and Welded Joint Properties
steel are shown in Table 8. The crack prevention preheat
of Developed Steel (Thickness: 103.1 mm)
temperature is 125°C, and the plate displays sufficiently
The chemical composition of the heavy thickness high weldability as a heavy section plate of 600 MPa
plate used as a material for equipment hatches and air- class steel.
locks of reactor containment vessels is shown in Table 2. The mechanical properties of welded joints of the
As shown in Photos 1 and 2, forging is applied to con- developed steel are shown in Table 9. Before and after
tinuous casting slabs in the width and thickness direc- PWHT, the plate possesses welded joint strength satisfy-
tions, followed by plate rolling. The mechanical proper- ing the specification of the base material and excellent
ties of the base plate of the developed steel, which is low temperature toughness in the weld at −40°C.

54  JFE TECHNICAL REPORT No. 18 (Mar. 2013)

 Development of SA-738 Gr. B High Strength Steel Plate with Excellent Toughness for Power Generating Plants

Table 9  Welding condition and mechanical properties of welded joints of the developed steel

Tensile properties Charpy impact properties

Thickness Groove
Welding condition PWHT TS Fracture vE−40°C
(mm) configuration Position
(MPa) position (J)
WM 118
— 678 WM 1/4t FL 124
GMAW
Welding consumable: HAZ 303
103.1 X
MG-S3N* WM 162
Heat input: 3.8 kJ/mm
615°C×10 h 623 WM 1/4t FL 172
HAZ 303
*Supplied by Kobe Steel, Ltd.    GMAW: Gas metal arc welding    PWHT: Post weld heat treatment    TS: Tensile strength
WM: Weld metal    FL: Fusion line    HAZ: Heat-affected zone    vE−40°C: Absorbed energy at −40°C

4. Conclusion thinner plates, etc.

A steel plate with high weldability and excellent low

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JFE TECHNICAL REPORT No. 18 (Mar. 2013)  55