Original Research Article

Influence of the Welding Heat Cycle on the HAZ Properties of T23 Joints Satanphop Amsupan1*, Marcello Consonni2, Bovornchok Poopat3, Kumpanat Sirivedin1 1The

Sirindhorn International Thai-German Graduate School of Engineering (TGGS), KMUTNB, 1518 Piboonsongkram Rd., Bangsue, Bangkok, Thailand, 10800. 2TWI Granta Park, Great Abington, Cambridge, CB21 6AL, United Kingdom. 3King Mongkut’s University of Technology Thonburi (KMUTT) 126 Pracha Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140. *Corresponding author contact no.: +66850434252 E-mail: [email protected] Date received: 7th June 2014 Date revised: 16th January 2015 Date accepted: 4th February 2015

ABSTRACT The creep-strength enhanced ferritic (CSEF) grade 23 steel, commonly known as T23 or P23 and addressed by the ASME Code Case 2199-6, is a 2Cr-1.6W-V-Cb grade strengthened by the presence of vanadium nitride precipitates, specifically designed to allow welding without Post weld heat treatment (PWHT). T23 and the similar CSEF grade known as ‘T/P24’, have found application in the most advanced designs for fossil fuel fired boilers, specifically waterwalls, as well as in heat recovery steam generators (HRSGs). However, concerns have recently been raised with regards to the susceptibility of grades 23 and 24 cracking occurring during fabrication, stress corrosion cracking and sulphide stress corrosion cracking, which have been identified in as-welded joints exhibiting relatively high heat affected zone (HAZ) hardness, hence bringing into question the typical practice regarding preheat and the omission of PWHT. Following a review of recently published literature, this paper presents the findings of an experimental program aimed at evaluating the effect of preheat on the as-welded HAZ hardness of thin welds in T23 and dissimilar metal welds with T91, with a view to suggesting conditions for exemption. The potential use of the temper bead (TB) technique was also evaluated. The experimental program included hardness testing and metallographic evaluations. Keywords: T23; T91; PWHT; HAZ; hardness. 1.0 INTRODUCTION The creep-strength enhanced ferritic (CSEF) grade 23 steel, commonly known as T23 or P23 and addressed by the ASME Code Case 2199 (currently issued as revision 6), was originally developed as a high-strength alternative to the traditional chromium-molybdenum (CrMo) steels, grades 11, 12 and 22. The alloy design philosophy was based on controlled carbon content to improve weldability and eliminating the need for preheating or post weld heat treatment (PWHT), with the addition of Boron (B) to ensure good hardenability and give the alloy a fully bainitic microstructure, the replacement of molybdenum (Mo) with tungsten (W) for improved solid solution strengthening, and the addition of vanadium (V) and niobium (Nb) for precipitation strengthening. The resulting alloy had a nominal chemistry, wt%, of Fe-0.06C-2.25Cr-1.6W0.1Mo-0.25V-0.05Nb-B (nominally 2Cr-1.6W-V-Nb). The tensile strength and creep-rupture strength of T23 are superior to traditional 1%CrMo and 2.25%CrMo materials (T11/T12/T22), and the resulting ASME allowable stress values are close to that of CSEF steel grade 91 which requires PWHT. The significant increase in allowable stress makes T23 an attractive alternative for designs made with T11 or T22 such as superheater tubing, waterwall panels, and heat recovery steam generators (HRSG) high-pressure tubing, since the wall thickness can be reduced or the operating temperature increased. In some instances T23 is chosen as the stub-tube

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material for making transitions between grade 91[1]. However, concerns have recently been raised with regards to the susceptibility of T23/24 components to cracking occurring due to fabrication cracking, stress corrosion cracking and sulphide stress corrosion cracking. These damage mechanisms have been reported to occur in as-welded joints exhibiting relatively high HAZ hardness. Grade 91 steel, also referred to as P91 in the pipe form, EN designation X10CrMoVNb 9-1, is now widely used in fossil fuel power stations, where it has found application in new build and particularly for headers in existing power plant to replace lower alloy steel counterparts. This paper presents the results of a research program carried out in two parts. The first part consisted of a literature review focused on weldability, current welding practices and experience with in-service damage of T23. This was followed by an experimental program aimed at evaluating the effect of preheat on the as-welded HAZ hardness of thin welds in T23, as well as T23/T91 dissimilar metal welds, with a view to suggest conditions for exemption. In addition, the use of the TB technique and its influence on the HAZ properties was evaluated. Qualitative variations in cooling rates were obtained by welding with and without preheat. The experimental program included hardness testing and metallographic evaluations. 2.0 LITERATURE REVIEW 2.1 General General guidelines on welding of grades 23 and 91, including process selection and application, heating cycles (preheat and PWHT) and consumable selection, have been published by various authors [11, 13, 14, 15] and are summarised in section 2.2. These show how standard welding practice is relatively straight-forward, industry-wide issues and failures continue to occur and, often linked to improper application of such guidelines, lack of monitoring and control, which underline the need for standardised requirements. As reported by Coleman and Newell [12, 14] members of the American Welding Society’s D10 Committee on Piping and Tubing, decided to remove P(T)91 materials from their existing guideline publication on welding CrMo piping and tubing (AWS D10.8, 1996) and prepare a new, specific document for these materials (AWS D10.21, ‘Guidelines for Welding Advanced Cr-Mo Steel Piping and Tubing’) and the other advanced chrome-molybdenum grades such as P(T)92, P911, P92, P122, T23, etc. Some justification for this is the very high creep-strength of these newer steels, which is reflected in the high temperature (typically 740-760 °C) required for PWHT. The need for greater attention to weld consumable selection, preheating, and rigorous PWHT schedules was offered as some of the reasons that the creep strength enhanced ferritic (CSEF) alloys must be treated differently. A draft version of this document is undergoing approval. In addition, topics such as the design and implementation of dissimilar metal welds (see 2.2.4) and the application of welding without PWHT (see 2.3) continue to be a subject of discussion. In particular, the latter has been subject to intense scrutiny, following recent service failures of grade 23 components (see 2.4). 2.2 Current welding practice 2.2.1

Welding consumables

Grade 91 Grade 91 is normally welded with matching consumable, identified by the ‘–B9’ suffix in AWS specifications. Little additions of Ni (0.36%) have been shown to ensure optimum toughness by suppressing retained delta ferrite in rapidly cooled joints [5]. The combined Mn and Ni (Mn+Ni) content helps suppressing retained ferrite, although also it also depresses Ms-Mf and Ac1, with potential issues during PWHT if the latter is exceeded. As a consequence, clauses of ASME Section I (PW5.4 and PW-39) as well as Code Case 2192-8 for grade 91 castings, now impose a 1.2% or 1.0% limit on the Mn+Ni content of filler metals. According to information available to

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the authors, the Mn+Ni limits are being considered by AWS and might be introduced in the specifications for grade 91-matching consumables in the near future. Grade 23 Matching consumables are available for grade 23, with variations such as a low carbon level around 0.05% and deliberate Ni addition, aimed at optimizing as-welded toughness. Typical aswelded weld metal hardness value range between 290 and 350HV, and have been shown to fall below 250HV after PWHT at 715°C for two hours.5 2.2.2

Welding Processes

In the construction of a power plant, welding of tubes and pipes takes place at various phases and in different conditions. For workshop operations, mechanised or semi-automatic processes such as Submerged Arc Welding (SAW) and Flux Core Arc Welding (FCAW) are preferred, with Tungsten Inert Gas Welding (TIG) being limited to root runs and thin-wall components. With regards to site assembly, more manageable processes such as Manual Metal Arc Welding (MMA) are required [1]. According to the authors’ experience16 the most common welding processes for Grade 23 and 91 are TIG, MMA and FCAW and SAW, which are applied as follows:

   

 

Original fabrication: TIG only: mostly for 'thin' wall small bore tubes e.g. 101.6mm diameter, 6.35mm thickness (manual or automatic) TIG and MMA: for root runs and fill+capping runs in 'thick section' pipe joints, respectively TIG and FCAW: for root runs and fill+capping runs in 'thick section' pipe joints, respectively TIG+SAW or TIG+MMA+SAW for 'thick' components (38.mm and greater) welded in workshops Weld repairs: MMA or TIG depending on component, excavation size and original Welding Procedure Specification (WPS), following excavations of the welding defects MMA and FCAW, following excavation of the welding defects, where MMA is used to deposit buttering runs to temper the heat affected zone (HAZ) and FCAW is used for filling and capping runs

TIG TIG is generally use for the root run and for small diameter thin wall tubing. As described above (section 2.2.2), TIG welding wire for T23 generally matches base material composition or may carry small addition of Ni [6.] Welding with this process generally results in greater toughness than flux and slag system owing to the reduced oxygen content. In addition, the risk of hydrogeninduced cracking (HIC) is lower compared to other arc-processes, as TIG is inherently lowhydrogen [6]. It is generally recommended that the rod diameter be restricted to 3.2mm maximum for manual TIG. Insufficient heat is available to implement interbead tempering with the puddle size associated with the larger diameter rods. MMA MMA process is widely used for new fabrications and upgrades/repairs. The electrode is generally required for low hydrogen basic flux systems to weld with CrMo steels (equivalent to EXX15/16G, and often with a specified limit of 0.15% moisture in the flux covering). These are designed to satisfy demanding all-positional operability for fixed pipework welding and excellent metallurgical integrity required for critical application [6]. FCAW The FCAW process is considered more advantageous than the MMA process when productivity is the primary concern. The use of rutile-based flux systems is often seen, selected especially for welding fixed pipework [5]. 235

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The integrity and soundness of joints made by FCAW is influenced by the type of the shielding gas used. A high CO2 content is typically recommended. However, higher CO 2 levels will normally increase the weld metal oxygen level which has been shown to be detrimental to impact toughness [6]. SAW The SAW process is generally selected for large diameter and thick section components that can be welded in the flat position and rotated [5]. Semi-automatic or automatic welding of grade 91 can be problematic due to the stiffness of the ‘-B9’ welding wire, when this is fed through the sharp turns of standard semi-automatic welding heads [6]. 2.2.3

Preheat and PWHT

Grade 91 Preheat is essential for grade 91 and it is usually specified as minimum 200 °C, whereas a typical interval of 730-760 °C is specified for PWHT. It may be permissible to use lower preheats if the hydrogen content is reduced to less than 5mls/100gm; for instance when depositing a TIG root pass, fabricators will go as low as 121 °C (250 °F) for root and hot pass layers in thin walled components or when TIG is used. This could be confirmed during welding procedure development Grade 23 Grade 23 was developed specifically to be welded without preheat and PWHT and this was suggested by early research. However, recent reports suggest that preheat is necessary to reduce sensitivity to cold crack [9, 10]. Wang et al [9] reported minimum specified preheat temperatures of 50 °C and 100°C when matching T23 electrodes and 1Cr-0.5Mo electrodes were used, respectively. Further publications recommend a minimum preheat temperature of 150°C for SMAW and SAW, 100 °C for GMAW [5]. For thicker components, a minimum preheat temperature of 170 °C has been recommended, although no indication of a thin/thick threshold thickness has been provided [6]. The heat treatment of creep resistant steel plays an important role in achieving acceptable hardness levels (typically below 350 HV), therefore reducing the risk of cracking due to environmental factor [2]. For T23, the likelihood of obtaining high hardness value (excess 350HV) appears to be governed by the carbon (C) content in the alloy and by the cooling rates. The possibility of variations of the latter having a significant effect on HAZ hardness of T23 weld, was investigated by EPRI [2]. The results obtained showed that only ‘thin’ T23 tubes, i.e. with wall thickness equal to or lower than 12.5 mm, could be welded without PWHT, as although the peak the HAZ hardness exceeded 350 HV, the impact energy in the weld metal was above the threshold of 27 J at room temperature. Further published work suggests that for TIG welded joints, the thin/thick threshold that would allow welding without PWHT could be as low as 10mm [1]. It should be noted that when in-service damage mechanisms are associated with welding residual stresses (eg. Stress corrosion cracking (SCC)), a criterion for PWHT exemption solely based on thickness would not be appropriate, particularly when components are highly restrained (eg. waterwall tubing). In such cases, PWHT may be required regardless of the component’s thickness. This has been confirmed by published work showed that after two years in service operation, a number of cracks were found on T23 waterwall tubes installed in the field without PWHT. On the other hand, tubes manufactured in fabrication shops, where PWHT was performed, were not damaged, highlighting the need to improve field operations to allow reliable performance of PWHT [10]. Cumino et al [1] reported that increase in wall thickness of the pipe, the impact properties of the weld metal decrease, while hardness increases in the weld metal and in the heat affected zone. Therefore, for the larger wall thicknesses PWHT is necessary to reduce residual stresses and peak HAZ hardness. PWHT has also been found to be beneficial to the impact toughness of joints

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on heavy wall thickness pipe (17.5 and 31.75 mm), with ‘low’ toughness observed in the as-welded condition [1, 11]. With regards to the optimum PWHT parameters: holding temperatures of approximately 740 °C (for two hours) and 710 °C (for 1.5 hours) have been shown to be adequate to restore satisfactory toughness levels, even at relatively low temperature (-20 °C) [1]. Following failures of grade 23 membrane walls described in more details in section 2.4 below, Xcel Energy increased the minimum preheat from 150 to 200 °C and made PWHT mandatory at 700-730 °C for a minimum of 30 minutes, with internal gas purging to minimize root weld pass oxidation during welding [8]. PWHT temperatures recommended by other authors, i.e. 715 °C (nominal 0 [5] or 730±10 °C [6], are in agreement with the ranges mentioned above. Minimizing time between completion of welding and PWHT is also a primary concern for grade 23 as well as all other in Creep Strength Enhanced Ferritic Steels (CSEFs), due to as-welded microstructure exhibiting high hardness and little ductility, hence susceptibility to SCC. In addition, maintaining the material above the dew point is recommended and components should be moved as little as possible and handled carefully [6]. 2.2.4

Dissimilar metal welds

Welding between grade 23 and grade 91 tubes represents is a common occurrence in high temperature components. Up-hill diffusion of interstitial elements, especially of carbon represents a very important mechanism of microstructural degradation of dissimilar metal welds that include grade 23. The creep strength of dissimilar welds is strongly dependent on the microstructural evolution in individual weld zones during long-term creep exposure and it was demonstrated that undissolved MX particles in the partly decarburized zone of the P23 steel significantly delay recrystallisation of the bainitic matrix [7]. Grade 91 matching electrode was shown to give a relatively short time to fracture, due to nitrogen and oxygen produced by welding with an extended arc. On the other hand, the use of 2.5Cr-1Mo-0.25V-1.5W resulted in longer rupture time, hence the use of the lower-alloyed consumable was recommended [7]. The impact toughness of welds deposited with 2.5Cr-1Mo-0.25V-1.5W filler metal was found to be significantly higher. Therefore, the general advice for dissimilar metal joints is to use filler metals matching the grade with the lowest strength and the smallest alloying element content. Lower carbon has also been found to be beneficial [7]. 2.3 Temper bead welding TB welding is a method of tempering the HAZ microstructure, hence potentially avoiding PWHT, by controlling the heat input, thickness and bead placement of subsequent weld layers. The TB welding method is well established in industry and it is mainly applied for weld repairs, when PWHT is impractical or not cost-effective. The main standard reference for the qualification and application of this method are the National Board of Inspectors Code (NBIC), for in-service applications, and the ASME B&PV, which cover the application of such method during fabrication, as well as in-service (the latter limited to nuclear components, see ASME B&PV Section XI). These standards restrict the TB method to specific welding processes and material grades up grade 22 (2.25Cr-1Mo), however, they provide useful guidelines for the application and qualification of the method on any material grades. With regards to the qualification of TB welding, the abovementioned codes call for QW-290 of ASME B&PV section IX, which addresses essential variables, acceptable welding processes, qualification test pieces and acceptance criteria. The latter may be based on hardness or on impact testing, depending on the requirements of the applicable construction code. When hardness requirements apply, TB welding is a method of reducing the hardness of HAZs without the need for PWHT. However, the technique requires very precise placing of weld runs and substantial skill on the part of the welder to ensure a correct and consistent bead overlap and travel speed and that the TB layer does not overlap onto the base metal HAZ. A lengthy training period for the welder is likely to be required before the welder can successfully pass the qualification test and apply the technique in production. 237

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As highlighted by Siefert and DuPont [10] in a review of the service experience with grades 23 and 24, weld repair procedures developed on a low-alloy grade are not necessarily applicable to others, due to the significant difference in microstructural constituents, alloy content and material behaviour. The authors went on to observe how procedure for TB weld repair have been developed by the Central Electricity Generating Board (CEGB) in the UK, suggesting that such techniques may be applicable to grades 23 and 24, although they have yet to be proven. 2.4 T23 service failures A number of service failures of grade 23 components have been recently reported, prompting the Power industry, the main used of the grade, to question some of the fabrication practices recommended when the alloy was first introduced. Failures have been attributed to a number of damage mechanisms, such as SCC, HIC (also referred to as ‘cold’ cracking) and reheat cracking. These were due to the service conditions, as well as to the joints’ microstructural constituents and hardness levels, which are directly related to the welding variables described in section 2.2. A summary of the conclusions provided in these publications, and the consequences on fabrication practice of T23 components, is provided below. In a technical report prepared to support code case 2199 [2], the use of lower carbon content T23 is recommended to minimize the likelihood for the development of very hard microstructures when using this material in the as-welded condition. However, control of the chemical analysis of the parent material must be coupled with careful selection of the welding conditions. Wang et al [9] analysed the performance of T23 water wall tubes for ultra-supercritical (USC) power boilers. It was found that such tubes may have water leak problems during operation, all of which were associated to a combination of elevated service stresses, residual stresses and poor toughness. The authors concluded that grade 23 is sensitive to cold cracking, hence preheat is necessary. Susceptibility to reheat cracking was also observed. In addition, PWHT was considered mandatory to reduce residual stresses and improve toughness. As a consequence, a maintenance programme was devised to replace all joints that had operated in the as-welded condition, with new T23 tubes to be welded by TIG with PWHT. Widespread cracking of membrane wall T23 tubes was also reported for Unit 3 of the Comanche Power Plant (750 MW supercritical boiler), operated by Xcel Energy [8]. The majority of the tube cracking was attributed to SCC, with fewer failures associated to HIC. Elevated weld stresses and high as-welded hardness, i.e. susceptible material, were identified as the major contributors to the failures. As-welded joints usually exhibited hardness values in excess of 325 HV for WM and 350 HV for HAZ. Mitigation measures included increasing the minimum preheat temperature and making PWHT mandatory, as described in section 2.2.3. Laboratory test showed how these reduced hardness levels by 70 to 100 HV points, hence reducing drastically the susceptibility to SCC. Siefert and DuPont [10] observed how issues with T23 tubing are significantly less frequent with ‘loose’ applications, such as in reheaters and/or superheaters, compared to applications in which welds are highly restrained, eg waterwall panels, roof tubing, high-crown seals and erection lugs. Additional problems in welding this material were documented in stub-to-header welds and girth welds in thick-section components. This confirms that grade 23 is not intrinsically problematic, provided the welding conditions are carefully selected taking into accounts the requirement for procedure qualification and most importantly the service conditions. In the authors’ experience, careful specification of welding procedure is not necessarily followed by accurate application in the field, particularly when changes to the welding procedures, such as higher preheat temperatures, are introduced. Therefore, any preventive or mitigating measure should be accompanied by training of personnel operating on welding-related tasks, such as welders, welding inspectors, site supervisors etc, who should be made aware of the risks associated with deviations from the required procedures.

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3.0 METHODOLOGY 3.1 Introduction The lessons learned from the recent failures of T23 components presented in 2.4 seem to suggest that, for highly stressed application, the application of preheating and stress relief cannot be avoided. However, there is a risk that such recommendations are applied to T23 components regardless of the specific service conditions, hence leading to unnecessary extra operations and costs. In addition, as described in section 2.3, the application of specialised welding techniques may allow welding without preheat and/or PWHT, which is particularly desirable when carrying out site repairs following failures or during planned plant outages. This experimental program was aimed at evaluating the effect of preheat (and its omission) on the as-welded HAZ hardness of thin welds in T23, as well as T23/T91 dissimilar metal welds, with a view to suggest conditions for exemption. In addition, the use of the temper bead technique and its influence on the HAZ properties was evaluated. Details on tube grades and dimensions are given in section 3.2. Welds were produced with and without preheat, the complete heating cycle and other welding parameters are detailed in section 3.3. Destructive testing included hardness testing and metallographic evaluations (see 3.4). 3.2 Parent material Lengths of commercial Grade 23 pipe material were supplied by Tenaris Dalmine (TD). The tube dimensions (OD x WT) were 42.7 x 7.10mm (identified as ‘tube A’), 50.8 x 9.14mm (tube B) and 88.9 x 15.0mm (tube C). The composition and mechanical properties met the requirements for ASTM A213-T/P23. Grade 91 tubing was supplied by the Electrical Generating Authority of Thailand (EGAT). The tube dimensions were 38.10 x 3.00mm (tube D) and 42.7 x 7.10mm (tube E). The composition and mechanical properties met the requirements for ASTM A335 Grade 91. 3.3 Welding activities TIG, MMA, and multi-process TIG+MMA T23 welds were prepared. TIG was applied to 7.10 and 9.14mm thick tubes, with no backing gas (test pieces A1, A2, B1, and B2). Multi-process TIG+MMA procedure were applied to thicker tubes (approximately 15mm) with no backing gas (test pieces C1 and C2). T23-matching filler wire was used, namely Metrode 2CrWV (AWS A5.28M: ER90S-G) for TIG and Chroment 23L (AWS A5.5M: E9015-G H4) for MMA. For each experimental setting, test pieces were prepared with a minimum preheat temperature of 100°C and without preheat, and left as-welded. Two further T23 15mm-thick test pieces were prepared using the TB technique (C3 without preheat and C4 with minimum 100 °C preheat). The TIG process was applied for the root pass and second pass. The MMA process was used to deposit a first layer of weld beads against the base metal on either side (dotted beads in Figure 1), followed by a layer of tempering beads (cross-lined beads in Figure 1). Capping runs were deposited on the sides of the joint, then in the middle

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Figure 1. Approximate location of hardness indentations on temper bead welds C3 and C4. Also shown, approximate bead placement. In addition, T91 3mm-thick test pieces (D1: without preheat and PWHT, D2: with minimum 100 °C preheat and without PWHT, D3: without preheat and with PWHT, and D4: with preheat and PWHT), were welded by TIG, with commercially pure Argon backing. The T91 filler metal was KOBE TG-S90B9 (ER90S-B9), supplied by EGAT. Test pieces were prepared with and without preheat, combined with PWHT (750 °C for 15 minutes) and no PWHT. Dissimilar metal welds (E1 and E2) were also produced following the same procedures and using the same consumables as test piece A. See Table 1, experimental programme.

Sample ID, material, thickness A1, T23, 7.10mm A2, T23, 7.10mm B1, T23, 9.14mm B2, T23, 9.14mm C1, T23, 15mm C2, T23, 15mm C3, T23, 15mm C4, T23, 15mm D1, T91, 3mm D2, T91, 3mm D3, T91, 3mm D4, T91, 3mm E1, T23/T91, 7.10mm E2, T23/T91, 7.10mm

Table 1 Test welds. Preheat, min Welding Process °C parameters No TIG 100 No TIG 100 No 100 (TIG) 170 (MMA) TIG+MMA No See appendix A 100 (TIG) 170 (MMA) No 100 TIG No 100 No TIG 100

A-W or PWHT A-W A-W

A-W

A-W PWHT A-W

All test welds were single-Vee butt-welds, with a total length of 300mm, the weld was placed in the middle. The weld circumference was split in four quarters and, in the absence of rotating equipment, the pipe was rotated in between quarters, so that welding could be carried out in the flat position (PA). As-run welding parameters, as well as preheat and PWHT details, are shown in Appendix A for each test piece. 3.4 Testing activities and data collection Full-thickness transverse cross-section specimens were obtained from each weld sample at 90° and 270° with respect to the start position (identified as 0°), with the exception of test pieces C3 and C4, which were sectioned in 150° and 270°. All samples were subject to metallographic examination, to verify weld soundness and, for the TB welds, to assessing bead placement. The testing procedure and specimen preparation were based on BS EN 1321:1997; 10% Nital etchant was applied for 15 seconds holding time. The weld samples were then assessed by Vickers hardness (HV10) testing. All hardness specimens, with the exception of TB specimens C3 and C4, were prepared and tested according to BS EN ISO 9015-1:2011 and BS EN ISO 6507-1:2005 (see Figure 2 for indent location). Specimens C3 and C4 were prepared and tested according to the requirements of ASME section IX QW-462.12, which provides specific requirements for hardness testing when qualifying temper bead welding procedures. A sketch with the position of the hardness indentations is shown in Figure 1. Parent metal and weld metal hardness values were obtained for information and comparison purposes, hence they are not be analysed in this paper.

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Figure 2. Approximate position of hardness indentation in the cap and root area for test welds A, B, C1, C2, D and E. Two variables considered significant to evaluate the effect of welding parameters and heat cycle on hardness were determined: the heat input (HI) and the time taken, during cooling, for a weld run and its heat affected zone to pass through the temperature range from 800 °C to 500 °C (t 8/5). The heat input was determined according to the following formula provided in the ASME Boiler&Pressure Vessel Code, Section IX, QW-409.1:

HI 

60  V  I 1000  v

[A]

Where HI is heat input in kJ/mm, V is the voltage in Volts, I is the current used, in Amperes, v is the travel speed of the welding torch, in millimetres per minute. The t8/5 interval was determined according to Appendix D of EN 1011-2 and considering a threedimensional heat flow, independent of thickness, as per Figure D.4 in the same standard. 4.0 RESULTS Macroscopic examination of the welded samples (Figure 3), aimed at verifying weld soundness, revealed no evidence of cracking or other detectable flaws. Average HAZ hardness obtained in different welding conditions are summarised in Table 2. With regards to the T23 test welds (Figure 7), the average as-welded HAZ hardness was found to vary only slightly for the thinner samples, 7.10 mm and 9.14 mm thick, with the application or omission of preheat. The values varied from 289 to 296HV10, and from 294 to 284HV10, when welding without and with preheat, respectively. Such variations cannot be considered significant. Figure 7 also shows that the peak HAZ hardness (309HV) was obtained from ‘thick wall’ (15 mm) tubes welded without preheat, as expected. Thicker test welds also corresponded to a larger variation in average HAZ hardness obtained with and without the application of preheat (309 to 293HV10, respectively), therefore suggesting that at such thickness level, the effect of preheat on HAZ hardening becomes significant.

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Table 2. Summary of test results: average hardness (HV10) for different positions across the welded joints, t8/5 (calculated) and heat input. HI Sample ID PM HAZ WM t8/5 (s) (kJ/mm) A1 189 289 283 6.17 1.52 A2 185 296 279 8.72 1.53 B1 199 294 282 9.00 2.13 B2 193 284 285 11.59 1.83 C1 191 309 307 6.81 1.50 C2 185 293 273 8.38 1.45 C3 196 275 308 4.06 0.84 C4 195 264 306 6.42 1.14 E1 91 224 359 335 7.41 1.74 E1 23 203 317 322 9.71 1.70 E2 91 236 357 318 7.41 1.74 E2 23 219 326 322 9.71 1.70 D1 229 299 297 6.35 1.41 D2 231 301 303 7.95 1.39 D3 195 190 190 6.21 1.33 D4 195 204 211 6.96 1.17

(a) test weld A2-270°

(b) test weld B2-270°

(c) test weld C1-270°

(d) test weld C4-270°

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(e) test weld E2-270 Figure 3. Macrographic sections of test welds, images (a) to (e) correspond to test welds A to E. See Table 1 for preheat and PWHT conditions. Figure 4 shows that the overall HAZ results in 15mm-thick welds appeared to be significantly influenced by the application of temper bead, with hardness reductions from 293 to 264 HV for C2 (with preheat) and C4 (TB, with preheat), respectively and from 309 to 275 HV for C1 (no preheat) and C3 (TB, no preheat), respectively. In addition, Figure 4 shows that an overall reduction of more than 40HV10 points if a 15 mm weld obtained without preheat and a standard welding technique (C1) is compared with one where both preheat and temper bead were applied (C4).

Figure 4. Average HV vs weld zones for 15mm thick T23 welds made using a standard welding procedure (C1 and C2) and a temper bead welding procedure (C3 and C4). See Table 1 for preheat and PWHT conditions. With regards to the T91 to T23 7.10mm-thick dissimilar welds (Figure 5), relatively high HAZ hardness values (>350HV10) were obtained on the T91 side, regardless of the application of preheat, which confirm that PWHT is required whenever Grade 91 is used, to obtain hardness values below 300HV, which is commonly considered an acceptable level for this steel grade. This is further confirmed by Figure 6, which shows how PWHT had a significant effect on the overall hardness of T91 welds, even with thin section (3mm) joints.

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Figure 5. Average HV vs weld zones for T23/T91 dissimilar metal welds, 7.10mm thick; ‘91’= T91 side; ‘23’ = T23 side. See Table 1 for preheat and PWHT conditions.

Figure 6. Average HV10 vs weld zones for T91 welds, 3mm thick. See Table 1 for preheat and PWHT conditions. Figure 5 also confirms that, for 7.10 mm thick welds, the effect of preheat is negligible, as observed above, with little variations in hardness values if the T23 and T91 side are considered separately. 5.0 DISCUSSION No major defects were detected that would have warranted the welds unusable for test purpose. A hardness limit of 350HV was originally established as the design maximum hardness for grade 23 [3] and is also indicated as a maximum limit for HAZ and weld metal in Cr-Mo-V alloys according to ISO 15614-1 (the most common ISO standards for the qualification of welding procedures). All hardness measurements obtained for T23 welds were found to be within these limits. If the results obtained from 7.10 and 9.14mm thick test pieces are considered, the hardness levels observed in the current programme were somewhat lower than those reported in for a previous research project by Alexandrov et al [17]. These authors conducted a study aimed at establishing the effect of cooling rates on the microstructural transformation and hardness of T23 welds. This showed that due to the significantly higher t 8/5, hence slower cooling rate, thin wall components (approximately 5mm thick) resulted in significantly lower as-welded hardness levels, with coarse grain HAZ (CG-HAZ) hardness ranging from 200 to 375HV10 and weld metal hardness between 244

Influence of the Welding Heat Cycle on the Haz Properties of T23 Joints

275 and 375 HV10, compared to thicker welds (approximately 8mm), for which hardnesses up to 415 and 405HV10 were observed in CG-HAZ and weld metal. As a consequence, the authors concluded that any variations in variables that affect the weld cooling rate, i.e. joint design, thickness, heat input and preheat, is likely to affect the performance of T23 joints with regards to damage mechanisms for which hardness is critical, such as SCC and HIC, as well as environmentally assisted-cracking in general. Considering the results presented in section 4 above and as shown in Figure 7, variations in thickness did not correspond to clear trends in hardness levels, with similar HAZ average values obtained in parent material, HAZ and weld metal for 7.10 to 15 mm thick welds. This can be explained considering the relatively high heat input (average values around 1.5 kJ/mm), which produced relatively small variations in the calculated t8/5 (Table 2). At the same time, the application or omission of preheat did not appear to have a significant effect on hardness levels for thin wall (100

300

A-W

T I G

113

13

1

1.9 3

2.1 3

2.3 3

No

300

A-W

1 6

T I G

113

13

1

1.1 2

1.8 3

2.0 2

>100

300

A-W

1 2

T I G

115

13

1

1.0 0

1.4 1

1.8 2

No

300

A-W

3 1 0

M M A

100

26

2

1.2 4

1.5 3

1.8 3

No

300

A-W

1 2

T I G

115

13

1

0.7 6

1.1 9

1.7 1

>100

300

A-W

3 1 0

M M A

100

26

2

1.0 6

1.5 3

2.4 3

>100

300

A-W

1 2

T I G

116

12

1

0.8 0

1.1 5

1.4 9

No

300

A-W

3 1 8

M M A

94

25

3

0.4 3

0.8 1

1.7 2

No

300

A-W

1 2

T I G

116

16

1

0.9 0

1.3 6

1.8 6

>100

300

A-W

3 1 8

M M A

95

29

3

0.6 4

1.1 1

2.2 1

>100

300

A-W

248

Heat Input (kJ/mm)

Influence of the Welding Heat Cycle on the Haz Properties of T23 Joints

E1

1 6

T I G

113

13

1

1.6 4

1.7 4

1.8 4

No

300

A-W

E2

1 6

T I G

113

13

1

1.6 2

1.7

1.7 9

>100

300

A-W

D1

1 2

T I G

78

14

1

1.1 7

1.4 1

1.6 2

No

300

A-W

D2

1 2

T I G

78

14

1

1.2 1

1.3 9

1.6 1

>100

300

A-W

D3

1 2

T I G

80

14

1

1.1 2

1.3 3

1.6 8

No

300

PW HT

D4

1 2

T I G

78

14

1

1.0 4

1.1 7

1.3 9

>100

300

PW HT

249