properties of matching filler metals for t 91/p 91
TRANSCRIPT
PROPERTIES OF MATCHING FILLER METALS FOR T 91/P 91
Herbert Heuser Thyssen Schweißtechnik Deutschland GmbH
Wilhelmstraße 2 59067 Hamm, Germany
Russel Fuchs Böhler Thyssen Welding USA Inc.
10401, Greenbough Drive Stafford, Texas 77477
Bernd Hahn Essener Hochdruck-Rohrleitungsbau GmbH
Wolbeckstraße 25
1. Abstract
Matching filler metals for Grade P 91 have been developed and are available for a number of years. However, experience gained through continued research and development efforts as well as practical application have brought certain refinements to the filler metals as compared to the early offerings.
This paper will explore the current state of the art for Grade P 91 filler metals, and present mechanical properties data for the all weld metal as well as welded joints in P 91 pipes. This data will also include actual creep rupture data for the SMAW and SAW processes for test durations to 30,000 hours. Discussions will include the effect of chemical composition on transformation temperatures and critical procedural considerations to obtaining optimum performance of the alloy in service. Considerations for dissimilar weld joints will also be addressed.
2. Introduction
The compulsion to improve the degree of efficiency in power stations requires materials which, under higher temperatures and pressures, must show at least the same duration characteristics as when using P 22 or X 20 CrMoV 12 1.
X 10 CrMoVNb 9 1 (1.4903) is a material which was developed in the USA and Japan and which according to the latest reports, has already provided interesting duration values in the 550 °C (1.022 °F) and above range. This material is ASTM A-335 standardized (1) and will generally be characterized as P91/T91/F91.
1
The central problem during the intensive development of the matching filler metal consisted first of all in guaranteeing sufficiently high impact values (minimum requirement: equivalent to parent metal) for the all weld metal in the shortest possible PWHT time. But the most important point is to demonstrate that the creep rupture properties of the weld metal fulfill the base metal requirements.
The following text will describe which areas influence the characteristics of the weld metal. Furthermore with the aid of results gained from welding procedure tests it will be proved that with matching filler metals from Thyssen Welding for P91/T91/F91(X 10 CrMoVNb 9 1), a sufficient property profile is present for the SMAW, GTAW and SAW welding methods.
Table 1: Requirements for P 91 All Weld Metal
Chemical composition of all weld metal
C Si Mn P S Cr Mo Ni V Nb N Cu Al min. 0.08 - - - - 8.0 0.80 - 0.15 0.04 0.030 � � max. 0.13 0.30 1.0 0.010 0.010 9.5 1.10 1.0 0.25 0.08 0.070 0.25 0.04 Mn + Ni < 1.5 % AC1b � 780 °C
Mechanical properties at RT: after PWHT at 760 °C (1,400 °F) / 2 h
YS MPa (ksi)
TS MPa (ksi)
Elongation %
CVN* J (FT-LB)
530 (77) 620 (90) � 17 � 41 (30) * acc. to (2)
3. Requirements for the Weld Deposit of P91/T91/F91
The development of suitable welding filler metal of a similar type which can be employed in the SMAW, GTAW and SAW welding methods; for the building of powerstations, has the P91 analysis as its reference point (2).
From this and from the needs of the processors regarding process safety, result the minimal requirements for the all weld metal listed in table 1.
These requirements fall within the AWS requirements for "B9" filler metal, but are much more stringent in order to guarantee in service mechanical properties requirements, including toughness, but especially creep rupture behaviour.
The optimizing of the chemical composition of weldments aimed at the improvement of the impact value in an economically viable annealing time.
The welding filler which was originally used resulted in a lower impact value, in line with those of (3) and (4). In the meantime weld fillers with sufficient impact values have been developed. Analytical influences on the impact properties are described in (5).
By comparison with the base metal P91 (2), differences arise in the analysis regarding the Ni and Nb contents. These differences are based on the following observations:
2
The improvement of the properties of P91 at high temperatures (> 540 °C/ > 1.004 °F) is achieved by NbV-Carbonitrides of type MX and Cr-carbides of type M23C6 /3/. For the base material, reliable long term values are available. Initially the same analysis range was used as a data base, with the exceptions of Ni and Nb, for the development of the weld metal. To prove the impact value of the weld metal the Nb-content was reduced from a minimum of 0.06 % to a minimum of 0.04 %. For the same reasons a max. Ni-content of 1.0 % was desired. To compensate for the higher Ni the Mn + Ni-content is not allowed to exceed 1.5 %, because not only Ni, but also Mn influences the lower transformation point Ac1b (3).
The reduction of the Nb-content to 0.04 % was carried out, according to current information because all hitherto known fractures in creep rupture tests occurred in the HAZ and not in the weld metal (6, 7). However, welding filler with less than 0.040 % Nb failed in the weld metal at low creep rupture time (9).
The following text details the areas of influence, which guarantee process-safety, with regard to sufficient mechanical properties in the welding joints of high temperature resistant steel P91.
0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5 6 6,5 7 7,5 8 8,5 9 9,5 10 10,5
1111,5
1212,51313,5Time
0
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Tem
pera
ture
[°C
]
250 - 300°C
760°C
Heating rate80 - 120°C/h
Cooling rate100 - 150°C/h
< 400°Ccooling in still air
+ 0 - 10
2 - 4 h
slowcooling rate
RT· ·
··
preheating weldingandsoaking
necessary cooling down after weldingor soaking
Figure 1: Heat Control during Welding and PWHT-Condition for P 91
4. Influence on the mechanical values
The mechanical properties of a welding joint can only be partly determined by successively matched alloy elements. The welding process and heat treatment factors exert a large influence, particularly on the impact values of weld metal. Figure 1 shows the typical heating cycle during and after welding. Preheating and welding occur at temperatures around 250 °C (482 °F). After welding it is essential to cool down to a temperature below 100 °C (212 °F) in order to allow a complete transformation into Martensite. Afterwards PWHT has to be performed, normally at a temperature between 750 °C and 760 °C (1.382 – 1.400 °F). Typical heating and cooling rates are 80 – 120 °C/ h (176 - 270 F).
In the case of storage after welding and before PWHT, the maximum time should be one week and during this time the components must be kept dry, to avoid potential problems with stress corrosion cracking.
3
The parameters can be slightly changed depending on the type of components to be welded. Weldments with low internal stresses, like butt welds of tubes, can be welded below 200 °C (392 °F) depending on the wall thickness. Up to a wall thickness of 80 mm (3.15") they can be cooled down to room temperature. On the contrary, heavy thick wall forgings or castings must not be welded below 200 °C (392 °F) and cooling after welding has to be limited to a minimum temperature of 80 °C (176 °F) in order to avoid cracking (2).
The following text describes which parameters in the SMAW, GTAW and SAW processes lead to optimal welding joint characteristics.
. . . . .0
10
20
30
40
50
60
70
80CVN [J]
0
10
20
30
40
50
60CVN [ft-lb]
Figure 2. Toughness at RT in Relation to the Welding Technique (SMAW-All Weld Metal for P 91)
4.1 SMAW
As with X 20 and other heat resistant CrMo-steels, the toughness of the P91 weld seam is influenced by the type of welding technique and thereby the type of heat-input. As far as the welding position allows, weave beads can be welded. Figure 2 shows clearly that weaving results in higher impact energy than with stringer beads. The weave beads lead to thinner layers, so that by the welding of subsequent passes, the lower layers undergo a greater „tempering effect“ than the thicker stringer weld beads. This is not necessary with the current analytically optimized stick electrodes to attain sufficient impact values. It shows however that improved impact values can be attained through the optimizing of welding technology.
A relatively high temperature and lengthy timespan is shown for the heat treatment of the base material in (2). This however does not have a negative influence on the material. With the optimizing of weld filler materials and improved welding technology, a sufficiently high impact value can be guaranteed in the areas of tempering temperature and stress-relief annealing temperature (740 - 780 °C/1.364 - 1.436 °F) with a simultaneously shorter holding time of 2 hrs.
4
SMAW: Thyssen Chromo 9 V
IS TV TZWL
: 160 A: 200°C: 250 - 300 °C
0 2 4 6 8 10 12 14 16 18
PWHT time [h]
0
20
40
60
80
100C
VN [J
]PWHT:
780°C (1463°F) 760°C (1400°F) 740°C (1364°F) 720°C (1328°F)
CVN
[ft-l
b]
10
20
30
40
50
60
70
0
Is: 160 ATp: 200°CTi: 250 - 300°C
Figure 3: Influence of the PWHT-Conditions on the Toughness of Matching Filler Metal to P 91 (All Weld Metal SMAW)
SMAW: Thermanit MTS 3; ø: 4,0 mm
0 2 4 6 8 10 12 14 16 1
Holding time [h]
200
220
240
260
280
300
Har
d nes
s [H
V 10
]
PHWT: 720°C (1328°F) 740°C (1364°F) 760°C (1400°F) 780°C (1436°F)
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Figure 4. Influence of the PWHT-Conditions (Temperature/Time) on the Hardness at RT
Figure 3 shows the influence of the tempering-temperature and the holding time on the toughness of the weld metal Thyssen Chromo 9 V. For economic reasons a shorter holding time is called for. From figure 3 it can be seen, that with a 2 hours holding time, the tempering-temperature should be higher than 740 °C (1.364 °F) to achieve the impact values shown in table 1. However a tempering-temperature greater than 760 °C (1.436 °F) must not be exceeded. If the temperature does rise above 760 °C (1.436 °F), the Ac1b-Point could also be exceeded (dependent of the chemical composition of the weld metal) (3). The Ac1b – temperature of the all weld metal Thyssen Chromo 9 V has been measured at 795 °C. If the PWHT would be above the Ac1b -temperature of the weld metal untempered Martensite could be produced, which would have a negative influence on the long term creep rupture properties and the impact value. The holding time must be extended correspondingly at tempering-temperatures lower than 740 °C (1.364 °F). This must be given particular attention with one-sided inductive annealing because a temperature difference (outside/inside) of 20 ° - 30 °C (68 - 86 °F) could result depending on wall thickness.
5
The tempering conditions will also have an effect on the weld metal hardness. To reduce the weld metal hardness at low annealing temperatures, the holding time must be extended. If the goal of optimizing tempering parameters is to satisfy the minimum impact value (CVN > 41 J (> 30 FT-LBS) at RT), then this will always result in a maximum weld metal hardness of well under 300 HV 10 (figure 4).
Table 2. Results of Welding Procedure Tests on P 91-Pipes with Matching Filler Metal Thyssen Chromo 9 V (SMAW) in Comparison to the All Weld Metal
C Si Mn P S Cr Mo Ni V N Nb 0.09 0.23 0.65 0.005 0.005 9.11 1.08 0.76 0.20 0.03 0.06
Welding parameter Mechanical properties at RTPipe dim. EL Is Us TInterpass PWHT Weld YS0,2 TS A5 CVN (ISO-V)
��x t � positionmm mm A V °C °C/h MPa MPa % J
a.w.m. 3.2 130 25 250/280 760/2 1G 621 729 19.4 62 67 74260 x 60 3.2 140 27 170/190 760/4 2G (89) 666* 104 108 102
G.W. 760/10 2G 664* 102 130 100
a.w.m. 3.2 130 25 250/280 760/2 1G 643 754 19.0 65 70 621G 690* 79 99 80
280 x 22 3.2/4.0 130/180 200/250 750/2 5G 686* 75 70 64G.W. 4G 679* 86 83 85
2G 687* 76 118 84
a.w.m. 3.2 130 25 250/280 760/2 1G 611 729 19.2 68 73 661G (25) 684* 98 70 82
316 x 32 2.3/4.0 125/170 250/290 760/2 5G (23) 693* 70 81 74G.W. 4G (18) 695* 65 86 65
2G (41) 692* 82 70 72 all weld metal (a.w.m.) Girth weld (G.W.) * Fracture in B.M. ( ) Number of beads
A number of procedure qualification tests were carried out on P91 using the optimized stick electrodes. Procedure qualification test results are shown in table 2 in comparison with the listed values of the all weld metal. This shows clearly that the minimum requirements listed in table 1 have been achieved not only in the all weld metal but also in the welded joint. All impact test specimen taken from the joint (Notch-vertical weld metal-middle) show higher impact energy values than those from the all weld metal. (Refer separately to the explanation on submerged-arc welding). This is regardless of the welding position. Weave beads were welded in the 1G, 5G and 4G positions. Only string beads were welded in position 2G. Here, the values are of the highest standard. That no low impact values were detected (see figure 2) can be attributed to the fact that in the horizontal position a higher welding speed was used (longer length of bead deposited), thereby resulting in thinner beads, and owing to a large number of successive beads, a multi-tempering effect taking place in an even higher degree.
On a P91 pipe joint (� 130 x 17 mm/5.1 x 0.67 in) the influence of the welding position, the interpass temperature and post-weld cooling temperature on the impact energy was investigated. These tests showed clearly, that neither an interpass temperature of 150 °C (302 °F) or 250 °C (482 °F), nor a post-weld cooling to room temperature or tempering at
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100 °C (212 °F) with subsequent PWHT had a significant influence on the toughness with a wall thickness of 17 mm (0.67 in).
Table 3. SMA Weld Metal Thyssen Chromo 9 V Quenched + Tempered
Chemical composition all weld metal for P 91
C Si Mn P S Cr Mo Ni V Nb N 0.10 0.21 0.68 0.008 0.008 8.91 1.05 0.81 0.21 0.041 0.041
Mechanical properties all weld metal at RT after quenching + tempering:
Heat treatment YS MPa
TS MPa
A4 %
CVN-(ISO-V) J
Hardness HV 10
1,050 °C / 30 min + 760 °C / 2 h 548 701 23 118 / 121 / 117 236
1,070 °C / 30 min + 760 °C / 2 h 548 693 20 127 / 124 / 128 236
Welding parameter SMAW: El-� 4.0 mm; TPreheat = 200 °C; TInterpass = 250 °C; IS = 170 A
Table 4. All Weld Metal – and Procedure Qualification Tests GTAW, Thermanit MTS 3
Welding parameter Mechanical properties at RTPipe dim. Wire IS US TInterpass PWHT YS0.2 TS A5 CVN (ISO-V)
��x t �
[mm] [mm] [A] [V] [°C] [°C/h] [MPa] [MPa] [%] [J]
a.w.m. 2.4 240 16 200/230 750/2 669 769 19.8 191 194 250490 x 20 2.4 180 200/250 750/2 703 230 236 265
G.W.
a.w.m. 2.4 240 16 200/250 740/2 752 852 17.0 65 72 124pulsed
260 x 60 1.0 250 250/300 760/4 673 162 - 227G.W. GTA narrow gap orbital weld
all weld metal (a.w.m.) Girth weld (G.W.)
It has been explained in numerous publications (e.g. (3, 6, 9)), that neither the weld metal nor the unaffected base material represent the weak point with regards to the long term creep rupture properties, but is instead the heat affected zone. This is proven by the hardness profile over the weld cross section.
One procedure qualification test (pipe dimension � 260 x 60 mm/10.24 x 2.36 in) and one sample joint (� 130 x 17 mm/6.3 x 0.67 in) were welded under the same conditions (welding parameters, heat input, PWHT 760 °C/2 h / 1.400°F/2 h). The investigated hardness profile (HV 10) shows that the 60 mm (2.36 in) thick pipe joint had a reduction of hardness of only max. 17 HV 10 at the HAZ. Hardness values of about 20 HV 10 under the hardness of the unaffected base metal were detected in the 17 mm (0.67 in) thick pipe sample.
7
The spread range of the hardness traverse survey, apart from the tempering conditions, appears to be influenced principally by the heat input during welding. The dominant influence is the wall thickness, which influences the heat dissipation and thereby the tempering effect.
Table 3 shows the mechanical properties of the all weld metal at RT after quenching and tempering, where excellent results are obvious in that all test results fall within accepted ranges.
wire 3,7 3,1 2,6 1,8 1,3Basicity acc. Boniszewski
0
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0,04
0,06
0,08
0,1
0,12C-content [%]
Ther
man
it M
TS 3
Mar
a tho
n 54
3
Figure 5. Influence of the Flux Basicity to the C-Burn-Off
4.2 GTAW
Table 4 shows results of all weld metal tests and procedure qualification tests. There are no problems regarding the toughness requirements because this welding process tend to the lowest oxygen content in the weld due to the inert arc atmosphere. However, it is obvious that also for the GTA-process 10 °C difference in the PWHT-temperature has a great influence to the toughness; compare the CVN-values at 740 °C with 750 °C (all weld metal).
4.3 SAW
The biggest problem in the development of a suitable wire/flux combination for the welding of P91 involved guaranteeing a sufficient impact value in the pure weld metal with a minimal tempering time (2 hours). A further problem consisted of the retention of the C-content at a minimum of 0.08 %. When using a wire with a 0.08 - 0.13 % Carbon-content, a flux must also be used which is neutral to carbon burn-off and carbon pickup (figure 5). For this purpose the highly basic flux Marathon 543 with good welding characteristics was developed, which is to be classified according to EN 760 as follows: SA FB 255 DC.
With this flux in combination with wires which have a chemical composition corresponding to those in table 1 sufficient impact values even with a 2 hours tempering time (at 760 °C/1.400 °F) can be met. Table 5 lists the mechanical properties for pure submerged arc weld metal and procedure test joint weldment. Noticeable are the particularly higher CVN-values in
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the joint in comparison with the all weld metal, which has already been described for SMAW. The reason for this is the different wall thickness.
Table 5. Results of a SAW-Procedure Qualification Test on a P 91 – Pipe in Comparison to the All Weld Metal
C Si Mn P S Cr Mo Ni V N Nb 0.09 0.22 0.32 0.005 0.006 8.72 0.91 0.71 0.22 0.036 0.05
Welding parameter Mechanical properties at RTPipe dim. Wire Is Us vs TInterpass PWHT YS0,2 TS A5 CVN (ISO-V)
��x t �
mm mm A V cm/min °C °C/h MPa MPa % J
a.w.m. 3.0 380 30 45 200/250 760/2 581 738 20.0 65 74 77
260 x 40 3.0 380 30 45 200/250 750/2 682* 110 126 123G.W.
a.w.m. 2.5 330 32 50 200/250 750/4 649 772 19.0 35 45 52
300 x 56 3.0 400 29 50 250/300 760/4 676* 62 64 64G.W.
* Fracture in B.M. all weld metal (a.w.m.) Girth weld (G.W.)
All weld metal according to EN 760 (nearly AWS 5.23) will be determined for the 3.0 mm � wire with 20 mm (0.78 in) thick plates. The SAW process is used for P91 to weld circumferential seams of thick walled pipes. The increased seam volume results in the higher cooling rate of each bead, and leads to an especially high continual tempering effect through the subsequent beads and simultaneous deep penetration. From this it can be deduced that if CVN-values in the pure weld metal according to EN 760 (nearly AWS 5.23) of 40 to 50 J (30 - 37 FT-LBS) and with a PWHT 760 °C/2 h (1.400 °F/2 h) are obtained, then in the SAW joint with a wall thickness of � 25 mm (1 in) a CVN-value of minimum 50 J (37 FT-LBS) can be achieved. This is heavily influenced by the welding parameters. The impact value is dependent on the tempering effect of the successive beads, in the same way as was shown with SMAW. This means the welding of as wide and as flat beads as possible, is essential. The optimal parameters for the 3.0 mm diameter wire are:
Is = 380 A – 400 A; Us = 30 V; vs = 45 cm/min. (17.7 in/min.).
The influence of the interpass temperature on the toughness could not be proven for the all weld metal. Variations of the interpass temperature with an average of 100 °C (212 °F), 200 °C (392 °F) and 300 °C (572 °F) by otherwise constant welding parameters, result in no noticeable changes in the impact energy. To insure the economy of the SAW process the interpass temperature and the preheating temperature should be brought into line with the geometry of the construction. An interpass temperature and preheat temperature of > 200 °C (> 392 °F) is recommended for wall thicknesses > 20 mm (0.78 in) /8/. This recommendation should be followed for SAW (also for GTAW of root passes of large cross-sections) because the deformation properties of the non-tempered P91 joints can be improved through higher interpass temperatures.
9
In figure 6 it is apparent, that in a non-tempered state, the deformation property, characterized by the impact energy and hardness, is not very large. In this graph the dependence of the impact energy on the sample position in a 40 mm (1.57 in) pipe wall thickness is shown. In the root area the impact energy > 70 J (> 52 FT-LBS) is to be noted. On the one side the sample lays in the GTAW and SMAW fill runs, and on the other side the amount of remelted base metal is higher than in the cap pass. This means that the Nb-content in the final weld pass is lower and the Ni-content higher than in the root area. This causes a higher toughness in the final weld pass with CVN-values of > 110 J (> 81 FT-LBS).
Owing to increased heat input, SAW results in a wider HAZ than with SMAW or GTAW. Nonetheless the reduction of hardness in the HAZ / base metal transition is not more than with SMAW.
IS TV TZWL
: 160 A: 200°C: 250 - 300 °C
A B as welded0
20
40
60
80
100
120
140
CVN
[J]
CVN
[ft-l
b]
20
40
60
80
100
0
B
1 ,57
inch
A
Figure 6: Toughness in Relation to the Specimen Location at a SAW – Pipe Joint P 91
5. Experimental Determination of Creep Rupture Strengths in P 91 Weld Joints
Tests were carried out to determine the creep rupture strength of P91 weld joints.
Dependent upon the influencing parameters of the welding process, the chemical analysis and the heat treatment the following points were investigated:
�� Creep rupture strength of the all weld metal �� Creep rupture strength of the weld joint
Tested weld metals and weld joints are shown in table 6. Information regarding heat treatment and testing temperatures is also included.
The chemical analyses of the all weld metals are listed in table 7.
A maximum exposure period of more than 30.000 hrs was achieved in long term tests. This allows a reliable extrapolation from the creep curve up to 100.000 hrs. From the creep rupture behaviour of the all weld metal after standard heat treatment at 760 °C (1.400 °F) / 2 hrs, the following statements can be made with regard to the scatter band for base metal (Base metal sheet 435R for P91).
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Table 6. Test Temperatures for Creep Rupture Tests
Welding Test Temperature °C (°F)
process Filler Metal All weld metal Pipe joint SMAW Thyssen Chromo 9 V 550 / 600
(1,022 / 1,112) -
SAW Thermanit MTS 3 / Marathon 543
550 / 600 / 650 (1,022 / 1,112 / 1,202)
550 / 600 / 650 (1,022 / 1,112 / 1,202)
PWHT: 760 °C / 2 h (1,400 °F / 2 h)
Table 7. Chemical Composition; All Weld Metal and Base Metal for Creep Rupture Tests
All weld metal C Si Mn P S Cr Ni Mo Nb V N SMAW Thyssen Cromo 9 V
0.092 0.22 0.72 0.007 0.007 8.88 0.75 1.05 0.054 0.207 0.034
SAW Thermanit MTS 3 / Marathon 543
0.110 0.19 0.52 0.017 0.001 8.47 0.60 0.90 0.040 0.205 0.056
Base metal C Si Mn P S Cr Ni Mo Nb V N P 91 acc. to min. 0.08 0.20 0.30 - - 8.0 - 0.85 0.06 0.10 0.030 (ASME-SA 335) max. 0.12 0.50 0.60 0.020 0.010 9.5 0.40 1.05 0.10 0.25 0.070
Pipe 0.104 0.27 0.46 0.014 0.003 8.17 0.16 0.90 0.064 0.194 0.055
�� At 550 °C (1.022 °F) the weld metal lies within the scatter band, in the middle. �� At 600 °C (1.112 °F) (figure 7) the weld metals are distributed across the lower half of the
scatter band. �� At 650 °C (1.202 °F) the weld metals lie between the middle of the lower half of the
scatter band and the lower border (i.e. approx. 10 % under the base metal-average).
Figure 7 shows also the test results at 600 °C (1.112 °F) of weld joints (girth welds). The definition of the location of fracture is essential (3) in the assessment of the results of the test pieces taken transverse to the weld seam. All fractures of the girth welds take place in the HAZ.
The stress rupture curve, beginning on the same level as the parent metal, falls at a greater rate. The curve shows an S-shape i. e. the rate of fall decreases according to increasing running time. A premature shift in the position of the fracture from the parent metal to the outermost HAZ is connected with the steeper fall. After approx. 104 hrs the curve is approx. 10 % below the scatter band limit of the parent metal.
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100 1000 10000 100000Time [h]
20
200
Stre
s s [M
Pa]
SMAW Thyssen Chromo 9 V SAW Thermanit MTS 3 / Marathon 543 Pipe weld; fracture in HAZ Quenched and tempered girth weld Base metal P 91; VdTÜV - 511/2
- 20 %
600°C
100+ 20 %
Figure 7: Creep Rupture Strength Data for P 91 Matching Filler Metals
The results show:
�� The creep rupture strength drops with increasing fracture time in relation to the parent metal.
�� A tendency towards falling below the lower scatter band is revealed. �� The movement of the fracture position in the outer HAZ can be seen as the cause for the
drop in the creep rupture strength. �� The drop, partially indicated by an S-shape, can be estimated by the creep rupture strength
of the simulated structure in the outer HAZ. �� The trial temperature has a fundamental influence on these operations: a drop occurs
earlier with increasing temperature.
In comparison to the normal deviations in the parent metal of � 20 %, the creep rupture strength of the all weld metal shows only relatively small differences.
6. Dissimilar Welds
Dissimilar welds between T/P 91 and low-alloy ferritic steels e.g. T/P 22, as well as austenitic steels can be produced without problems. The behaviour is comparable to any other dissimilar weld of heat resistant ferritic steels with 9 – 12 % Chromium content like T/P 9, EM 12 or X 20.
For dissimilar welds with low-alloy ferritic steels, consumables matching T/P 91 or the low-alloyed ferritic steel can both be used (figure 8). There is a phenomenon that has to be encountered with such type of welds. Due to the difference in Chromium contents between the materials involved, Carbon diffuses during PWHT from the low Chromium material into the neighbouring high Chromium partner steel or weld metal. This decarborized zone cannot be avoided, unless a nickel-base filler metal is used.
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X20 CrMoV 12-1 P 9110 CrMo 9-1013 CrMo 4-5 P 91
Austenitee.g.: 1.4910
P 91E 911P 92
WM:Ni-base
e.g.: Tht. Nicro 82
WM:matching
WM:matching
Figure 8. Dissimilar Welds
These micro structural changes have an effect on room temperature toughness. However, the creep rupture strength of such a dissimilar weld usually is not affected. The fine grained intercritical HAZ of the low-alloy ferritic steel remains the weak zone after longtime service exposure.
PWHT has to be in accordance with the specifications of the low-alloy ferritic steel.
There is a tendency that more and more matching filler to P 91 is used for dissimilar weld P 91 to P 22. In this case it is not necessary to grind the root and cap layer smooth to the surface of the base metal. However, it is necessary to increase the PWHT time at the max. allowable temperature for P 22 to guarantee sufficient toughness in the weld.
The best way to realize a transition between T/P 91 and an austenitic steel is by making a transition joint at the shop. First Ni-base weld metal is deposited on one side of the T/P 91 piece followed by an normalizing and tempering (NT) treatment similar to T/P 91 base material. This transition piece can then be welded on site to T/P 91 and to the austenitic steel. The T/P 91 side is a regular similar weld with matching filler and local PWHT, whereas the austenitic side is welded to the Ni-base weld deposit by using a Ni-base filler (Thermanit Nicro 82) without PWHT.
In case of thin wall tubes, a transition between T 91 and an austenitic steel is usually done by direct welding with a Ni-base filler followed by a PWHT at about 760 °C (1.400 °F). In the case of thick wall transitions, a buttering of the P 91 pipe with Ni-base filler should first be made, followed by an ordinary PWHT. Then welding to the austenitic pipe can be performed by using a Ni-base consumable. In order to reduce welding stresses, a multiple bead technique with low heat input should be used (2).
7. Miscellaneous Precautions
�� Interruption of the welding should be avoided. If this is unavoidable, at least 1/3 of the wall thickness should be deposited or the weldment has to be kept at the Interpass temperature. In any case the weld area has to be preheated before welding is resumed.
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�� To minimize crater cracking the Thyssen welding consumables Thyssen Chromo 9 V and Thermanit MTS 3 are produced with low residual elements. The stick electrodes fulfill the "H4"-criteria. Furthermore strict adherence to the preheat and Interpass temperature is required.
�� After welding it is essential to cool down to a temperature below 100 °C (212 °F) before PWHT. In case of storage after welding and before PWHT, the maximum time should be one week and during this time the components must be kept dry. Bending stresses or loading on weldments that have not been PWH'd must be avoided.
�� Depending on the wall thickness it is recommended for SAW weldments prior to cooling below the minimum preheat temperature to heat the weldments to 260 °C (500 °F) and hold for about 4 hours followed by slow cooling under insulation (Dehydrogenation Heat Treatment).
�� Root grooves shall be welded using a backing gas (Argon). The backing gas coverage shall be maintained until completion of the root and hot pass as a minimum.
�� Welding shall be performed in the flat position whenever possible to avoid the thicker beads typically achieved in the vertical and overhead positions.
7. References
1. N.N.: ASTM Specification A335/A335 M-91 „Standard Specification for Seamless Ferritic Alloy Steel Pipe for High Temperature Service“, 1992, Annual Book of ASTM Standards, Volume 01.01. [book]
2. Haarmann, K., Vaillant, J. C., Bendick, W., Arbab, A.: The T91/P91 Book, 1999 – Vallourec & Mannesmann Tubes. [book]
3. N.N.: EUR 14575-Ertüchtigung eines modifizierten 9 Cr-1Mo-Stahles für den praktischen Einsatz bis 600 °C. Kommission der Europäischen Gemeinschaften, Serie: Technische Forschung Stahl, 1993. ISSN 1018-5593. [report]
4. Ambs, E., Tolksdorf, E., Leich, K. E., Schlieben, R., Schwarzwalder, H.: Werkstoff P91 (X10CrMoVNb9 1), VGB Kraftwerkstechnik 73 (1993), Heft 7, S. 634 - 641, Essen. [report]
5. Dittrich, S., Heuser, H.: Schweißzusatzwerkstoffe für den 9 %igen Chromstahl P91. 3. Aachener Schweißtechnik Kolloquium 16./17. März 1993, Aachen. [conference paper]
6. Wortel van, I. C., Etienne, C. F., Arav, F.: Application of modified 9 % chromium steels in power generation components, ESC Information Day; 05.11.93 VdEH Düsseldorf. [conference paper]
7. Bendick, W., Haarmann, K., Wellnitz, G., Zschau, M.: Eigenschaften der 9- bis 12 %igen Chromstähle und ihr Verhalten unter Zeitstandbeanspruchung, VGB Kraftwerkstechnik 73 (1993), Heft 1. [report]
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8. Kalwa, G., Schnabel, E.: Umwandlungsverhalten und Wärmebehandlung der martensitischen Stähle mit 9 - bis 12 %igem Chrom , VGB-Konferenz „Werkstoffe und Schweißtechnik im Kraftwerk 1989“, 1. + 2. März 1989 in Essen. [conference paper]
9. Maile, K., Theofel, H., Bendick, W., Zschau, M.: Zeitstandverhalten von P 91-Schweißverbindungen, Stahl und Eisen 117 (1997), Nr. 8, S 101 –105. [magazine article]