M Consonni, D J Abson, TWI Ltd
Presented at 10th International EPRI Conference on Welding and Repair Technology for Power Plants. Marco Island, Florida, USA. 26-29 June 2012.
Post-weld heat treatment (PWHT) is generally required for any grade 91 steel fabrication before entering service, as well as after any repair welding. However, this may sometimes be impractical or not cost-effective. A review of current welding practice for grade 91 showed that, whilst welding is relatively well established, there is a lack of published literature and proven procedures for non standard operations, such as weld repair without PWHT. By applying a microstructural refinement criterion (temper bead), SMAW and GTAW weld repairs of grade 91 steel without PWHT have been investigated. Metallographic examination and hardness testing were carried out to evaluate the degree of HAZ refinement and tempering, respectively. For both welding processes, a good level of refinement was produced, but only limited tempering was achieved. Recommendations are provided for the development of weld repair procedures based on an alternative criterion to that explored in this present project.
Grade 91 steel (also referred to as P91 in the pipe form, EN designation X10CrMoVNb 9-1) is widely used in fossil fuel power stations, where it has found application in new build and, particularly for headers in existing power plants, to replace lower alloy steel counterparts. The 9% chromium content and the incorporation of vanadium and niobium as alloying elements have effected a substantial improvement in creep strength compared to the lesser alloyed Cr-Mo grades (e.g. grade 22), and has allowed designers to provide for an increase in steam temperature from around 540 to 580°C or even 600°C. The higher alloy content has also ensured that the microstructure of the parent steel (and the HAZ of any weld) will be martensitic for all likely cooling rates. Moreover, the hardness of such as formed martensite is typically > 400HV. In the light of the high creep resistance of the steel, the hardness decreases only slowly during tempering or PWHT, as shown in Figure 1.
Figure 1. Trend of hardness as function of the Larson Miller parameter. Reproduced by permission of the American Welding Society. The coarse-grain HAZ hardness will be similar to that of the weld metal.
PWHT is generally required for any grade 91 steel fabrication before entering service, to relieve residual stresses, to reduce weld metal and HAZ hardness, and to improve toughness. For the same reasons, after any repair welding there is an expectation, and currently a requirement, that a PWHT is carried out. However, since PWHT may be difficult to perform and often not cost effective, depending on the location and accessibility of the weld repair, a procedure that allows its omission is desirable, and controlled deposition welding procedures (also referred to as 'temper bead' or 'cold repair' procedures) have been devised for C-Mn steel and for the lower alloy steels in creep service. Although at least one study has demonstrated that some HAZ softening can be affected by careful bead placement, industry standards currently do not allow this approach for grade 91 steel components.
Following a review of the available literature on weld repair of grade 91 without PWHT, summarized below, this project investigated the possibility of carrying out non-PWHT weld repairs employing the controlled deposition approach, with some variation in electrode diameter being explored. Low heat input levels were used, as this gives not only a reasonably small coarse-grain HAZ grain size but it also restricts the width of the thermally-damaged region at or beyond the edge of the fine-grain HAZ, which is generally the creep-weak region in a welded joint subjected to cross-weld stress. The preliminary trials reported here represent the first stage in exploring whether repair welding of grade 91 steel without a subsequent PWHT is possible; if progress is to be made, further extensive welding trials and testing will be required, including investigating HAZ toughness and cross-weld creep strength.
Repair using Ni-based consumables
Mitchell and Brett published a review of cold weld repair procedures that have been successfully applied to repairs made to type IV cracking in 2¼Cr 1Mo¼V steels with Ni-based consumables. The main advantages identified by Mitchell and Brett when repairing with Ni-alloys are:
- Lower risk of hydrogen cracking due to the face-centred cubic (FCC) lattice in weld metal.
- Higher fracture toughness of the weld metal compared with weld metal with matching composition and initially lower residual stress.
However, major drawbacks have been identified as well, such as:
- Addition of a 'dissimilar metal transition' joint in the repaired component.
- Issues with NDE and slow relaxation of the residual stresses developed during repair.
Use of ferritic consumables
The use of ferritic consumables has grown in popularity over recent years. For 2¼Cr1Mo steels, both manual metal arc and flux cored arc welding procedures have been applied. However, for the modified 9Cr1Mo¼V steels manual metal arc (SMAW) welding has generally been selected.
A few examples of repairs of Grade 91 using matching (or near matching) consumables have been published. Storesund and Samuelson indicate that a repair in a grade 91 steam line after 118,000 hours service was only subjected to 23,000 hours further service until a crack developed in the HAZ at the outside of the pipe, which was post weld heat treated. These repair welds had a consistently higher hardness than the original, service aged weld. This particular case considered repair of fabrication flaws, rather than the repair of service degradation such as type IV cracking.
With regard to repairing service degradation flaws, it is recommended that the entire weld is cut out (including HAZ) and re-welded, as the damage accumulation in the creep weak type IV region is likely to be aggravated by the residual stresses associated with any repair. The use of grade 91 matching consumables for such repairs produces welds with have a better creep resistance compared with the parent material which is service-aged. This may aggravate further type IV cracking in the HAZ created by the weld repair.
Vekeman and Huysmans proposed using less alloyed consumables as an alternative to matching the parent material. The rationale behind this is the attempt to match the creep resistance of the service-aged base material. As residual stress relaxation is a 'creep-like' mechanism, less creep resistant weld metal will allow a somewhat greater degree of relaxation of such stresses. A modified T24 filler metal with Nb alloying was used. The authors established that gas backing would be needed, so standard 2¼Cr filler metal was used for the root pass to eliminate the excessive oxidation of the weld bead. The weld repair was performed successfully. However, care needed to be taken with the deposition sequence to ensure that the maximum hardness measured in the HAZ was less than 400HV (to avoid stress corrosion cracking in the absence of PWHT), and it was observed that restrictions on the service temperature (limited to 540-580°C) may be required to avoid reheat cracking during service. At the time of writing this report, there is no evidence of components repaired with the above procedures being put in service.
Two welding processes were selected for the investigation of weld repair procedures without PWHT: shielded metal arc welding (SMAW) and mechanised (machine) gas tungsten arc welding (GTAW). Both processes are allowed by clause IWA-4620 of Section XI of the ASME B&PV Code for in-service temper bead repairs of base materials classified as P-No 1, 3, 12A, 12B or 12C, which do not include grade 91 (P-No 5B). It should be noted that the GTAW process is only allowed by ASME B&PV Section XI if 'automatic' or 'machine' using cold wire feed.
For both processes multi-layer welded pads were deposited on service-aged P91 pipe material with 15mm thickness to simulate weld repairs. The welding parameters for each layer were selected in order to maximise the refinement of the HAZ associated with subsequent welding layers, by using the two-layer refinement criterion suggested by Alberry: 
R ≤ C ≤ P
Where P is the maximum fusion boundary depth (first bead), C is the sum of maximum fusion boundary depth + average layer height (first bead) and R is the maximum depth of the refining zone (second bead). A schematic representation of this criterion is shown in Figure 2
Figure 2. Two-layer refinement parameters 
In addition, it was expected that the deposition of subsequent welding layers would affect some tempering of the HAZ associated with the first layer. The weld repair procedures were then tested by metallographic examination and hardness testing, to evaluate the degree of refinement and tempering obtained, respectively.
Manufacture of weld repair samples
SMAW. For the SMAW welding tests, two-layer welded pads were produced, using different combinations of electrode sizes and heating cycles between the first and second layer. The welding parameters and electrode sizes were based on the work carried out on cold weld repairs of C-Mn steels, which in turn used Alberry's refinement criterion, and were subsequently adjusted to suit the high Cr consumables being used.
Test weld W01 was welded using Ø3.2 and 4.0mm (1/8 and 5/32in) electrodes for the first and second layer, respectively. Test weld W03 was welded using Ø2.5 and 4.0mm (3/32 and 5/32in) electrodes for the first and second layer, respectively. A 'cascade' sequence was applied, with the second layer deposited on a smaller area, to allow sectioning of single and double-layer configurations. All welds were carried out in the vertical-up welding position; with a minimum preheat temperature of 200°C (approximately 390°F) and a maximum interpass temperature of 250°C (approximately 480°F). The SMAW electrical variables are summarised in Table 1.
Table 1. As-recorded MMA welding parameters, the corresponding welded pads are shown in Figures 3 and 4.
|Test weld||L1 ø,|
Notes to Table 1: L= layer number and electrode size, i=welding current, V= voltage, ROL= run-out length assuming a 50mm end stub for 350mm-long stick electrodes, AE=arc energy.
Welding was carried out using AWS A5.5 E8015-B8 electrodes, matching the composition of Grade 9. The purpose of these tests was to verify the effect of the selected welding procedures on HAZ microstructure and hardness, it was not considered necessary to use consumables matching Grade 91. In addition, type 'B8' consumables have a lower resistance to creep than type B9. These were considered suitable for evaluating repairs on Grade 91 in the service aged condition, i.e. with a somewhat reduced creep strength compared with the as-fabricated condition. After welding, the test pads were not subjected to PWHT.
Mechanised GTAW. GTAW repair using the temperbead (or 'half bead') approach have been developed by Alberry et al and Gandy et al for ASME SA 508 Class 2 steel for application in the nuclear sector. In particular, Gandy et al developed a three-layer procedure, using the same parameters for all layers.
Bead on plate (BoP) tests, which are not detailed in this paper, were carried out to determine the optimum welding parameters. The BoP trials were assessed according to the refinement criterion (Formula (1) and Figure 2) and one set of welding parameters was selected (table 2). Subsequently, in accordance with the abovementioned three-layer procedure, a weld pad (W15) was obtained using the same parameters for all layers. Metallographic specimens were then obtained from three cross-sections, identified as W15-1, 2 and 3, to compare the effect of the subsequent layers on the base metal via visual examination and hardness testing.
AWS SFA A5.28 ER 80S-B8 with Ø1.2mm (0.045in) was used, matching the composition of Grade 9 rather than Grade 91, see discussion above. The TOPTIGTM variant of the GTAW process was used, as this generally allows a smoother HAZ profile for the first layer, compared with standard GTAW welding, hence increasing the possibility of a consistent refinement and tempering of the HAZ itself, when the second and third layer are deposited.
Table 2 GTAW welding parameters
Notes to Table 2: i=welding current, V= voltage, TS=travel speed, WFS=wire feed speed, AE=arc energy, this corresponds to 'heat input' values according to the ASME terminology.
Figure 3 shows photomacrographs of the sections taken from test weld W01 (Ø3.2/4.0mm). The sections taken from test weld W03 (Ø2.5/4.0mm) are shown in Figure 4. The above mentioned figures indicate that if the combination Ø2.5/4.0mm electrode is used for the first/second layer, the latter penetrates through the thickness of the first layer and a new untempered and unrefined HAZ is formed. On the other hand, if the combination Ø3.2/4.0mm electrode is used, the outer regions of the HAZ produced by the second layer appear to overlap with that produced by the first, hence providing the condition for refinement.
Figure 3. Macrosections taken through test weld W01. Scale ruler shown:
(a) W01-A: first layer appearance (SMAW electrode ø3.2mm);
(b) W01-B: appearance after second layer (SMAW electrode ø4.0mm)
Figure 4. Macrosections taken through the samples relating to test weld W03. Scale ruler shown:
(a) W03-A: first layer appearance, showing fairly even HAZ (SMAW electrode ø2.5mm);
(b) W03-B: appearance after second layer, showing elimination of original HAZ and formation of a new HAZ resulting from the higher heat input (SMAW electrode ø4.0mm).
The irregular profiles shown in Figures 3 and 4 are explained by the imposition of welding parameters and heat input levels that are outside the usual ranges for the consumables being used. It should also be noted that at this stage, the primary purpose of the test welds was to determine the effect of different combinations of electrode size and electrical parameters on the HAZ microstructure, hence limited emphasis was put on obtaining a smooth weld pad profile.
The Ø2.5/4.0mm combination was discarded based on the photomacrographs. Subsequent metallographic examination was limited to test weld W01 (Ø3.2/4.0mm). Figure 5 shows the effect of the bead placement and temper bead technique on the HAZ. Good refinement of the HAZ was obtained.
Detail of the weld and HAZ microstructures resulting from test weld W01 (Figure 3) Magnification indicated by micron marker:
(a) Microstructure at one end of the weld pad (see Figure 3), showing limited refinement;
(b) Detail of Figure 5a, showing a predominantly martensitic microstructure;
(c) Microstructure of the HAZ at the centre of the weld, showing good refinement;
(d) Detail of Figure 5c, showing tempered martensite microstructure.
Hardness measured in the HAZ of specimen W01 ranged between 329-435HV5 (average 385HV5) and 258-420HV5 (average 360 HV5) after deposition of one and two layers, respectively, against 224-229 HV5 for the parent metal.
Figure 6 shows the degree of overlap between the HAZs of the three welding layers in test weld W15. In particular, Figure 6b shows that the HAZ associated with the second layer consistently overlapped that of the first layer, whereas, due to the variation in layer thickness, the HAZ produced by the third layer appears to have extended over the fusion line only in a few positions (Figure 6c).
Macrosections taken through the samples relating to test weld W15. Scale ruler shown:
(a) First layer (W15-1);
(b) Two layers (W15-2) - arrows indicate the limit of the 2nd layer HAZ (darker);
(c) Three layers (W15-3) - arrows indicate the limit of the 3rd layer HAZ (lighter).
Figure 7 shows the degree of HAZ refinement obtained when depositing two and three layers (Figure 7b and c, respectively), compared with the HAZ produced by the deposition of a single layer (Figure 7a).
Detail of the weld and HAZ microstructures resulting from test weld W15 (GTAW). Magnification indicated by micron marker:
(a) HAZ associated with a single layer (W15-1);
(b) HAZ associated with two layers (W15-2).
(c) HAZ following the deposition of the third layer (W15-3).
The HAZ hardness associated with a single layer ranged from 391 to 473HV5, and averaged 430HV5. After the deposition of a second layer, the HAZ hardness ranged from 374 to 453HV5 and the average value slightly decreased to 423HV5. The values measured after the deposition of the third layer varied between 312 and 466HV5 with an average of 391HV5. This indicated that, whilst the third layer did not contribute to the refinement of the HAZ, it had a measurable effect on HAZ tempering.
The review of current welding practice for grade 91 showed that whilst conventional welding and repair procedure are relatively established and can be reliably assessed, there is a lack of published literature and proven procedures for non-standard operations, such as welding without PWHT, which are highly desirable in a variety of fabrication and maintenance scenarios.
In this respect, the experimental results showed that, if a refinement criterion previously applied to C-Mn steels, is applied to investigate the possibility to repair grade 91 components without PWHT, a substantial degree of HAZ refinement can be obtained for both the SMAW and GTAW processes. However, the tempering produced by such approach in the HAZ cannot be considered significant for SMAW, with an average hardness drop of approximately 25HV5. On the other hand, when GTAW was applied, a moderate degree of tempering was obtained, with a hardness reduction of 40HV5 after the deposition of three weld layers. It should however be noted that grade 91, being resistant to creep damage, is intrinsically resistant to tempering.
The wide range of hardness values observed indicated the range of maximum temperatures experienced in the HAZ regions with the lowest hardness values, corresponding to the areas that had been more effectively tempered by the heat cycle induced by the subsequent layers. This confirms that the depth of penetration of the second and following welding layers, as well as their associated HAZ, must be made more uniform, if consistent tempering is to be obtained.
- No procedures for 'cold' weld repairs of P91 components are available in the public domain, which have been successfully employed in-service.
- SMAW and GTAW weld repair procedures for grade 91 without PWHT were investigated by applying a refinement criterion.
- A combination of SMAW electrodes with diameters 3.2mm (1/8in, first layer) and 4.0mm (5/32in, second layer) produced some refinement, but only limited tempering occurred in the HAZ.
- A three-layer GTAW repair procedure provided a higher degree of microstructural refinement, with limited tempering.
If weld repair procedures for grade 91 without PWHT are to be developed, the investigation of a controlled deposition procedure, based on a shallower penetration by the second layer than that explored in this present project, is recommended to achieve maximum tempering rather than maximum refinement.
In order to assess the performance of such repair procedures for in service applications, extensive welding trials and testing are required, including investigating HAZ toughness and cross-weld creep strength.
This work was funded by the Industrial Members of TWI, as part of TWI's internal research programme. The author would like to thank TWI colleagues: Nigel Allison, Ray Banham, Rita Banks, Jackie Brand, Chris Dungey, Harry Froment, Chen Fun Wee, Alan Parker-Murrell, Diane Shaw, Shaun Smart, Ramin Taheri, Mark Tiplady and David Welsh.
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