PROPERTIES OF SUMITOMO 347AP STEEL TUBE FOR HYDRO

PROPERTIES OF SUMITOMO 347AP STEEL TUBE
FOR HYDRO-TREATER IN COMPLEX REFINERY
Presenter:
YUYA MATSUDA
Specification & quality control, research and development for
seamless stainless steel tubes and pipe for 3 years in Sumitomo
Metal Ind., LTD. and continuing as a technical staff.
1
Properties of Sumitomo 347AP Steel tube for
hydro-treater in complex refinery
YUYA MATSUDA, JUNICHI HIGUCHI, and HIROYUKI ANADA
Sumitomo Metal Ind., LTD., Amagasaki, Japan
KEYWORDS: stabilized stainless steel, polythionic acid stress corrosion cracking,
intergranular corrosion stabilization, post-weld heat treatment, desulfurizing plant,
weldability, hot weld cracking, hydro-treater, hydro-cracker
Abstract:
Sumitomo 347AP(UNS S34751) tube has been widely used for hydro-treater in
Japanese complex refinery as a substitute for 9-Cr, 321 and 347 for more than 15
years since its especially excellent resistance for sensitization gives reduction of
maintenance cost and construction work to omit post-weld heat treatment and
neutralize cleaning.
This paper describes fundamental property of 347AP as well as its long term usage
review and advantages of its application on actual plant.
Foremost feature on Sumitomo 347AP is remarkable reduction of SCC sensitivity in
polythionic acid to prevent sensitization by lower carbon content up to 0.02%.
Resistance of Polythionic acid SCC (PTA-SCC) examinations (ASTM G35) reveal
that SCC did not occur even after 10000h aging at 550 deg. C.
Strength and creep rupture properties of 347AP are equaling or surpassing 321 or
347 to add optimum amount of nitrogen, which compensates for the reduction in
strength due to the lower carbon content.
Furthermore, lower Carbon resulted lower Niobium requirement achieves improved
weldability which is almost same as TP304 and significantly better than 347 by the
evaluation of Varestraint test.
Therefore 347AP has been applied as an optimum material for dozens of actual
hydro-treater without any treatment for the maximum of 15 years or more, and
actually, not only SCC but also any other troubles resulted from material property
haven’t been observed.
2
1 Introduction
Chemically stabilized austenitic stainless steels such as TP321 and TP347 are widely
used for hydro-treater and hydro-cracker in complex refinery, because of their
resistance to high temperature corrosion by desulfurizing process. However,
austenitic stainless steels are susceptible to stress corrosion cracking (SCC) after
welding. Among them, the SCC due to polythionic acid (H2SxO6, x = 3 - 6) is one of
the most serious corrosion issue affecting equipment for hydro-treater and hydrocracker in complex refinery. The PTA-SCC of stainless steels is generally caused by
the formation of chromium depleted zone in HAZ (Heat Affected Zone) of welded
joints due to the precipitation of chromium carbides on grain boundaries during
fabrication or operation. 1)
To prevent PTA-SCC, post-weld heat treatment (PWHT) at about 900deg.C is
necessary for TP321 and TP347 to stabilize TiC and NbC in HAZ. Table 1 shows a
modification of alloying elements and PWHT for conventional TP321 and TP347
steels in order to prevent the PTA-SCC for welded joints.
Table 1: Conventional steels used for polythionic acid condition
Recommendation
Steel
Base metal
PWHT
TP321
min.Ti/C: 7
Mandatory at about 900deg.C
TP347
min. Nb/C: 10
Mandatory at about 900deg.C
Furthermore, several methods have been well-known such as NACE RP-01-70
standard which lists the main shutdown procedure to reduce the probability of the
PTA-SCC during shutdown.
Although TP321 and TP347 are generally used to apply above methods in the
petroleum refinery industries, PWHT and shutdown procedure affect not only cost but
also production since they take certain period of time. Hence a steel tube material
which has excellent resistance for sensitization to prevent chromium depleted zone
on grain boundary without above methods is ideal for hydro-treater and hydro-cracker
in complex refinery.
This paper describes alloy design, performance properties, and service experience of
347AP which expected to be the alternative to TP321 or TP347 to have excellent
resistance for sensitization to omit PWHT and shutdown procedure.
3
2 Design of Alloy Composition of Sumitomo
347AP (UNS S34751)
Table 2 shows chemical compositions of 347AP (ASTM TP347LN / UNS S34751).
347AP have a low carbon content with less than 0.02mass% and optimum nitrogen
content with 0.06-0.10mass%. The addition of nitrogen compensates for the
reduction in strength due to the lower carbon content. Low carbon content results
lower niobium requirement for stabilization, and lower niobium content contributes
improved weldability.
Figure 1 shows the effect of carbon and nitrogen on the precipitation of carbonitride
for type347 aged at 700deg.C for 24 hours. When the carbon content is 0.02mass%
or more, the chromium carbides precipitate continuously along the grain boundaries.
When the nitrogen content is 0.1mass% or more, the chromium nitrides precipitate on
the grain boundaries. To prevent carbide and nitride precipitation on grain
boundaries of welded joint without PWHT, the 347AP has been optimized carbon
content with less than 0.02mass% and nitrogen content with 0.06 to 0.10mass%.
Table 2: Typical composition of 347AP (UNS No.S34751)
C
Max.
TP347
0.08
347AP
0.005 [TP347LN UNS No.S34751] 0.020
Heat A
0.009
Si
Mn
P
S
Ni
Max. Max. Max. Max. 9.0 0.75 2.00 0.040 0.030 13.0
Max. Max. Max. Max. 9.0 0.75 2.00 0.040 0.030 13.0
0.39 1.45 0.012 0.001 9.9
Cr
17.0 20.0
17.0 20.0
18.4
Nb
Max.
1.00
0.2 0.5
0.32
5µm
(mass%)
Nb/C N
10
min
Min. 0.06 15 0.10
36 0.09
5µm
700 deg.C x 24 h
NbC
N (%)
0.15
CrN
Cr2(C,N)
NbCrN
0.10
Cr23C6
Cr7C3
NbC
0.05
NbCrN
NbC
347AP
0
0.01
0.02
0.03
5µm
C (%)
Figure 1: Effect of C and N on precipitation of Type 347
(Solution heat treatment + 700deg.C x 24hrs aging)
4
3 Corrosion resistance of GTAW welded joints
Table 3 shows the GTAW matching filler for 347AP. TP321 and TP347 steel
welded joints were also prepared with conventional TP347 GTAW filler and welding
condition is shown in Table 4. Test pieces were cut from welded joints of TP321,
TP347 and 347AP steels precisely shown in Figure 2. U-bend specimen was
prepared for the corrosion test.
Two corrosion tests were conducted for these welded joints without and with
stabilization at 900 deg.C for one hour. Intergranular corrosion tests were conducted
by the copper sulfate-sulfuric acid test in accordance with ASTM A262 Practice E for
72h(CuSO4-H2SO4, solution ) 2). The polythionic acid stress corrosion cracking tests
were conducted by Wackenroder liquid in accordance with ASTM G35.3), 4)
Table 3: Chemical composition of filler for welding Sumitomo 347AP (%)
Chemical
C
Si
Mn
P
S
Ni
Cr
Nb
N
composition
#STG347AP <0.03 <0.65 1.0-2.5 <0.03 <0.03 9-11 19-21.5 10xC-1.0 <0.1 GTAW filler
#SM347AP <0.03 <0.65 1.0-2.5 <0.03 <0.03 9-11 19-21.5 10xC-1.0 <0.1 MIG filler
Actual case 0.014 0.45 1.70 0.005 0.001 9.53 20.3
0.58
0.077
Table 4: Welding conditions
(Manual TIG welding)
Figure 2: Position of sampling test piece
from welded portion
5
Figure 3 shows the results of intergranular corrosion test following ASTM A262
Practice E on 347AP, TP321, and TP347 without stabilization. Serious intergranular
corrosion were observed in TP321 welded joints without stabilization after sensitized
at 550 - 650deg.C for less than 10 hours. Also, intergranular corrosion was observed
in TP347 welded joints without stabilization after sensitized at 550deg.C for 30 hours
or more. 347AP welded joints without stabilization were not observed any
intergranular corrosion even after sensitized at 500 - 700deg.C for 104 hours. After
stabilization at 900deg.C for one hour, no intergranular corrosion was observed in
TP321 and TP347 welded joints.
Figure 4 shows the results of polythionic acid stress corrosion cracking test
following ASTM G35 on 347AP, TP321, and TP347 without stabilization. TP321
welded joints without stabilization were observed SCC after sensitized at 550 600deg.C for 100 hours or more. 347AP welded joints without stabilization were not
observed any polythionic acid stress corrosion cracking after sensitized at 500 700deg.C for 10,000 hours.
6
700
700
Temperature(℃)
750
650
600
550
650
600
550
500
500
450
450
10
30
100
300 1000
Holding time (hr)
10
3000 10000
30
○ : No SCC
○ : No intergranular corrosion
100
300 1000
Holding time (hr)
3000 10000
(a)347AP
750
750
700
700
Temperature(℃)
Temperature(℃)
(a)347AP
650
600
550
650
600
550
500
500
450
450
10
30
100
300
10
1000
○: No IGC ■: Slight IGC ●: Heavy IGC
100
300
1000
○:N o SC C ●:SC C
(b)TP321
750
30
Holding time (hr)
Holding time (hr)
(b)TP321
750
700
700
Temperature(℃)
Temperature(℃)
Temperature(℃)
750
650
600
550
500
650
600
550
500
450
450
10
30
100
300
1000
10
Holding time (hr)
30
100
300
1000
Holding time (hr)
○:N o IG C ■:Slight IG C ●:H eavy IG C
(c)TP347
Figure 3: Effect of aging conditions
on IGC resistance of weldment
(ASTM A262 Practice E)
(c)TP347
Figure 4: Effect of aging conditions
on SCC resistance of weldment
(ASTM G35)
7
Photo 1 and Photo 2 shows extraction replicas of HAZ in TP321, TP347 and
347AP welded joints after sensitized at 550deg.C for 300 hours and 600deg.C for
1,000 hours. In case of conventional TP321 and TP347 steels, chromium carbides of
M23C6 type were observed continuously along grain boundaries. 347AP was free
from carbide and nitride precipitation on grain boundaries.
Cromium carbide
precipitation
Cromium carbide
precipitation
10µm
(a) TP321
(b) TP347
(c) 347AP
Photo 1: Extraction replicas of HAZ sensitized at 550deg.C for 300 hrs
(a) TP347
(b) 347AP
Photo 2: Extraction replicas of HAZ sensitized at 600deg.C for 1000 hrs
4 Tensile properties
The elevated temperature tensile properties of 347AP are shown in Figure 5.
Although the carbon content of the steel is lower to prevent stress corrosion cracking,
the tensile strength at each temperature exceed allowable stress/0.9 of ASME TP347
because of the addition of nitrogen.
700
TS, YS (N/mm2)
600
TS
500
400
YS
(0.2% proof stress)
300
200
100
ASME TP347
Allowable stress/0.9
0
0
200
400
600
Testing temperature (deg.C)
800
Figure 5: Elevated temperature tensile properties of 347AP
8
5 Creep rupture properties
The creep rupture properties of 347AP are shown in Figure 6. Creep rupture tests
of 347AP were conducted for over 10,000hours at 550 to 750deg.C. It is concluded
that the 100,000 hours extrapolated strength at 550 to 700deg.C satisfies allowable
stress/0.83 of the JIS SUS347 (equivalent to conventional TP347). It is estimated
that finely dispersed precipitation such as NbCr-nitride and Ni-nitride contribute to
improve the creep rupture strength for 347AP.
500 deg.C
x105hr
Stress (kgf/mm2)
100
10
550 deg.C
x 105hr
600 deg.C
x 105hr
650deg.C
x 05hr
700 deg.C
x 105hr
Temperature
(deg.C)
550
600
650
700
750
: SUS 347TF/0.83
Allowable stress
1
17
18
19
20
21
22
23
T(18.9478 + log t) x
24
25
26
27
10-3
Figure 6: Creep rupture strength curve of 347AP (Larson-Miller parametric curve)
6 Effect of aging on toughness
The effect of aging on charpy impact values of 347AP and TP347H is shown in
Figure 7. Aging at 500 to 650deg.C up to 10,000hours reduces the impact value at
0deg.C for both 347AP and TP347H, but the impact values of 347AP is still enough
high compared with the conventional TP347H.
35
2
Impact value (kgf-m/cm )
30
25
20
15
10
TP347H 347AP Aging temp.(℃）
5
500
550
○
●
△
▲
■
□
600
650
5x10x2V(mm)
0
0
100
300
1000
3000
10000
Aging time (hr)
Figure 7: Charpy impact values at RT of 347AP and TP347H after long-term aging
9
7 Weldability
The Varestraint weld-cracking test is one of the methods for evaluation of hot weldcracking. Figure 8 gives an outline of the test method. Figure 9 shows the result
of the test describing crack length of 347AP, TP347, TP321, TP304 and TP316.
Whereas TP347 and TP316 indicate relatively longer crack length than others,
347AP indicates improved susceptibility of weld-cracking since the crack length was
same as TP304 and significantly smaller than TP347. It’s suggested that lower
niobium content up to 0.5mass % contribute improved weldability.
20
Varestraint crack length in weld metal (mm)
・Test plate thickness(mm)：12
・Strain added(%)：2.0
・Weld heat input(J/cm)：12,000
・Filler metal：none
・Shield gas：Ar
Added strain = 2%
15
10
5
0
347AP
347
321
304
316
Figure 8: Outline of Varestraint test
Figure 9: Result of Varestraint test
(welding crack susceptibility)
8 Service experience in practical plants
Sumitomo 347AP steel tubes have been installed in Japanese practical plants since
1985. 1,500 metric tons in total of 347AP have been used for furnace tubes, pipes
and heat exchanger tubes in hydro-treater and hydro-cracker units. Table 5 shows
some examples of the service experience in the petroleum refinery industry. In most
of cases, 347AP steel tube have been applied under the temperature higher than
400deg.C without PWHT and no trouble resulted from material property has been
reported.
10
Table 5: Service experience of 347AP
Plant
Plant A
Plant B
Application
Furnace Tube
Furnace Tube
Size (outer diameter
101.6 x 13.2mm
165.2 x 7.0mm
x wall thickness)
139.8 x 11.9mm
190.7 x 7.5mm
Service conditions
Metal temperature
Feed product
S (wt%)
PWHT
Year delivered
Nitrogen purge
Alkaline wash
Plant
Application
Size (outer diameter
x wall thickness)
Service conditions
Metal temperature
Feed product
S (wt%)
PWHT
Year delivered
Nitrogen purge
Alkaline wash
Plant
Application
Size (outer diameter
x wall thickness)
Service conditions
Metal temperature
Feed product
S (wt%)
PWHT
Year delivered
Nitrogen purge
Alkaline wash
Plant C
Furnace Tube
165.2 x 6.3mm
500 deg.C
Residue of crude oil
3% max.
N/A
1996
applied
N/A
460 deg.C
Naphtha
0.2% max.
N/A
1996
applied
N/A
320 deg.C
Naphtha
0.2% max.
N/A
1994
applied
N/A
Plant D
Heat Exchanger
19.0 x 2.1mm
Plant E
Furnace Tube
165.2 x 7.1mm
Plant F
Furnace Tube
216.3 x 11.1mm
388 deg.C
Light oil
2.3% max.
N/A
1996
applied
N/A
460 deg.C
Heavy oil
1.0% max.
N/A
1996
applied
N/A
410 deg.C
Diesel oil
1.7% max.
N/A
1996
N/A
N/A
Plant G
Furnace Tube
165.2 x 7.0mm
216.3 x 9.2mm
400 deg.C
Light oil
5.0% max.
applied
1996
N/A
N/A
11
To see the resistance for sensitization of long term applied 347AP, furnace tube of
Plant E which was used for 12 years under 460deg.C condition without PWHT
(described in Table 5) has been evaluated.
Photo 3 shows furnace tubes appearance after penetration test on weld joint. No
indication has been observed.
Photo 3: 347AP Furnace tube appearance after penetration test
Photo 4 and Photo 5 show the microstructure and TEM examination of HAZ taken
by extraction replicas, and they were free from precipitation on grain boundaries.
These results suggest no sensitization has occurred on 347AP since it has applied.
Photo 4: Microstructure of 347AP HAZ on furnace tubes of plant E
12
Grain boudary
5µm
Photo 5: TEM examination of 347AP HAZ on furnace tubes of plant E
Figure 10 shows the result of identification of precipitates in the grain. Finely
dispersed NbCr nitride were detected. Any harmful precipitates such as chromium
carbides were not found.
Table 6 shows the result of EPR (Electrochemical Reactivation) test which has
conducted according to ASTM G108 on HAZ of the 347AP tube. Pa value 0.0
coulombs/cm2 suggests no sensitization since G108 interpret unsensitized
microstructure when Pa value was less than 0.10 coulombs/cm2 in 304 and 304L. 5)
Nb
200nm
Cr
Bright field
EDX analysis
Electron diffraction pattern
Result
Figure 10: Precipitate identification of 347AP HAZ on furnace tubes of plant E
13
Table 6: EPR test results of 347AP on furnace tubes of plant E
Portion
Initial Open Circuit
Q
Ir
Grain
Pa
2
Potential (mV vs. SCE) (coulombs) (mA/cm ) Size (coulombs/cm2)
HAZ
-385
0
0
8.0
0.00
Base metal
-404
0
0
8.0
0.00
Many products forms of 347AP, such as pipe and plate have been successfully used
in desulfurizing plant in Japan and its applications are expanding to other chemical
plants under sensitization condition in recent years. 347AP steel tube has been
designated in ASME cord case 2196 and in ASTM A312/A418 as TP347LN (UNS
S34751).
9 Conclusion
Sumitomo 347AP stainless steel has improved properties and advantages over
conventional TP321 and TP347 stainless steels. The main conclusion can be
summarized as follows.
(1) 347AP has high resistance to PTA-SCC on the welded joint without PWHT with
optimized chemical composition (low carbon content less than 0.02mass %, high
Nb/C min.15).
(2) 347AP has equivalent elevated temperature strength to that of TP347 with
optimum contained nitrogen of 0.60-0.10mass %.
(3) Weldability of 347AP is better than TP347 and equal to that of TP304 with low
niobium less than 0.5mass %.
(4) 347AP steel tubes have been applied since 1985 mainly as heat exchanger
tubes in hydro-treater and hydro-cracker units without PWHT, and no trouble
resulted from material property has been reported.
(5) No sensitization has been observed on 347AP furnace tube in actual hydrotreater which has used for 12 years without PWHT.
14
10 References
1) M. Kowaka “Metal Corrosion Damage and Protection Technology” (1983)
2) ASTM A262 “Detecting Susceptibility to Intergranular Attack in Austenitic
Stainless Steels” (2005)
3) T. Kudo, Y.Tarutani, M. Miura, Y.Sawaragi and M.Nishi “The Sumitomo Search 3”
(1988), No36, p73
4) ASTM G35 “Determining the Susceptibility of Stainless Steels and Related NickelChromium-Iron Alloys to Stress-Corrosion Cracking in Polythionic Acids”, (2004)
5) ASTM G108 “Electrochemical Reactivation (EPR) for Detecting Sensitization of
AISI Type 304 and 304L Stainless Steels” (2004)
15