The Electric Power Research Institute is leading a multi-national research project to evaluate the integrity of alternative options for repair of Grade 91 steel components. This research is necessary to overcome problems with conventional repair methods. The challenges with traditional repair procedures are a direct consequence of difficulties in controlling post-weld heat treatment (PWHT) in the field within the currently mandated temperature range. Research achievements have identified significant benefits from performing cold weld repair (i.e., welding with no PWHT) and weld repair followed by PWHT at a temperature significantly below the current minimum.

The EPRI studies have focused on two key component types, namely: repair options for thin-section materials (i.e., tubing) and repair options for thick-section components (i.e., balance-of-plant applications such as headers, piping, stub to header, etc.).

This research has led to the world’s first approved weld repair method for Grade 91 steel that does not mandate PWHT (Welding Method 6 for tube-to-tube weld repairs, which has been incorporated into the National Board Inspection Code [NBIC] Part 3 Repairs and Alterations). Furthermore, the investigations have ignited discussion within the American Society of Mechanical Engineers Boiler and Pressure Vessel (ASME B&PV) Code regard¬ing reduction of the current minimum allowable PWHT of 1,350°F (730°C) to 1,250°F (675°C) for new construction of Grade 91 steel weldments.

Grade 91 steel

Power plant applications of creep strength enhanced ferritic (CSEF) steels have increased greatly during the last two decades. One of the most widely utilized CSEF steels is a 9 percent chromium steel widely known in industry as T91, P91 or Grade 91.

Since the early 1990s, Grade 91 steel has been used in fossil power steam boilers, heat-recovery steam generators (HRSGs) and piping systems. The alloy has enhanced properties, including high creep-rupture strength and fracture toughness, compared to traditional low-alloy steels. Higher strength makes Grade 91 attractive because of the potential for higher operating temperatures – up to 1,100°F (593°C) – and lower wall thickness, with excellent toughness significantly reducing the risk of fast fracture. Grade 91 is, therefore, suitable for use as a retrofit material in conventional subcritical power plants, for plants that will operate in cyclic mode, and as a building material for advanced supercritical plants and HRSGs.

Service experience has shown a significant need for on-site weld repairs to Grade 91 steel components. Rather than simply rely on the use of procedures qualified for new construction for these repairs, research was conducted to establish the technology to support a well-engineered weld repair. These new approaches offer rapid, nonconventional welding procedures for minimum cost replacement and repair.

Critical issues related to weld repair of Grade 91 steel

A number of issues have driven the need to investigate alternative options for weld repair of Grade 91 steel:

• Many current repairs are performed using rules for new con¬struction. Yet no relevant research indicates that these repair methods are technically appropriate for in-service components.

• Field PWHT maintained within the allowable range of 1,350°F to 1,450°F (730 to 790°C) has proven to be very challenging. Much evidence shows that poor heat treatment practice leads to significant over-temperature exposure. Research also indicates that any excursion above the allowable maximum dramatically reduces the long-term creep strength of Grade 91 steel and its weldments.

• Many in-service failures of Grade 91 steel components have been documented. Explanations include poor design, poor operation, bad construction practices, metal¬lurgical risk factors, and other causes. Regardless of the root cause for each documented failure, the number of documented failures highlights the need for validation of weld repair options that can be used for the range of Grade 91 steel components in service.

• The technical details for “best practice” regarding welding of low-alloy bainitic steels should not be applied directly to welding of tempered martensitic steel. Thus, repair techniques established for “conventional” low-alloy steels are not directly relevant for repairs on Grade 91 steels.

Thin-section weld repair: Method 6

For weld repair of thin-section Grade 91 steel, EPRI research investigated the use of a nickel base (Ni-base) filler metal (EPRI P87) using a cold weld repair approach. This research culminated in the acceptance by the National Inspection Code Part 3 of Welding Method 6.

In tests to date, no reports have been made of a marked reduction in performance or inability to qualify repair welding procedures using Ni-base filler metals for T91 steel. Indeed, this type of repair has been shown to comply with the criteria in ASME B&PV Code Section IX.

With regard to qualification of the welding procedure, the test data showed that for the Gas Tungsten Arc Welding (GTAW) process, the selection of filler mate¬rial was not an important consideration. However, for welds made using the Shielded Metal Arc Welding (SMAW) process, the use of filler metal EPRI P87 could introduce difficulties in passing side bend tests. These prob¬lems were a direct consequence of the weldability challenges using this electrode. It is currently recommended that for qualification of welds made using the SMAW process, ENiCrFe-2 or ENiCrFe-3 be used as the filler.

With regard to high-temperature creep performance, the use of an Ni-base filler does not appear to reduce the creep performance of the weldment. Differences in the location of creep damage have been recorded.

Several limitations have been linked to the use of Welding Method 6, and are highlighted in the National Board Inspection Code Part 3 Repairs and Alterations language. The limitations include tubing 125 mm (5”) or less in diameter and 12.7 mm (0.50”) or less in wall thickness. Repairs can also be performed only within a location internal to the boiler or HRSG setting.

Successful adoption of Welding Method 6 has prompted the need for additional research to provide additional flexibility in the code’s language. The direction of this research is being guided by discussions with end-users who are implementing these innovative repair procedures into their corporate welding pro¬grams. Two initiatives that are the subject of ongoing research include relaxation of the maximum interpass temperature from 400°F (200°C) to 550°F (290°C), and addition of a ferritic filler metal into the allowable filler metal repair options.

Thick-section weld repair: Three weld procedures

The investigation into well-engineered repairs for thick-section components was undertaken in several phases. In the first phase, researchers ranked the repair performance of 10 unique weld repair procedures applied to Grade 91 parent steel in two different conditions.

In the second and third phases, the three best option weld repair methods were applied to an ex-service Grade 91 header. The options included:

• E9015-B9 Filler Metal + Controlled Fill + Low PWHT (675°C, 1,250°F/2 hours)

• E8015-B8 Filler Metal + Controlled Fill (No PWHT)

• Ni-base (EPRI P87) Filler Metal + Controlled Fill (No PWHT)

The research sought to: (1) examine if the condition of parent material modified performance, (2) evaluate options for the removal of damage and (3) establish the important considerations regarding the geometry of the excavation (such as the bevel angle). The investigation resulted in the fabrication of 16 differ¬ent welds, more than 150,000 hours of creep testing, detailed metallographic examination of each weld after manufacture, and evaluation of each specimen after testing.

The results of this research were summarized in an EPRI report entitled Best Practice Guideline for Well-Engineered Weld Repair of Grade 91 Steel. This report contains 20 sections with specific guidance on weld fabrication. In addition, nine appendices provide practical support¬ing technical evidence that validates the recommendations in the document. Prior to publication, the report underwent a critical review and assessment by end-users, industry experts and participants in codes and standards.

One factor highlighted in the Best Practice Guidelines is the importance of the excavation geometry. While strength is one consideration for any weld repair, less emphasis has traditionally been placed on damage tolerance. Examinations of the extent of excavation have shown that repair welds in Grade 91 steel made with an optimized excavation are more damage-tolerant and less susceptible to rapid (potentially cata¬strophic) failure than traditional geometries. Thus, a well-engineered weld repair has the ability to not only exhibit an acceptable life but, more importantly, to provide a safer solution as compared to the traditional fabrication methods. This effect is shown in an actual test, made in a partial repair weld using EPRI P87 filler metal and a controlled fill technique (see Figure 1).

Because of the need for data, continued interaction and guidance based on relevant testing and evaluation, a new series of investiga¬tions has already been initiated. The goal of this follow-on work in Phase 4 is to apply the developed welding techniques to a wider range of materials and component scenarios. Provided the National Board Inspection Code Part 3 Repairs and Alterations approves the proposed welding supplement by the July 2016 meeting, the process for the acceptance of alternative welding methods for thick-section weld repair is expected to take six years.

Alternative weld repair options for Grade 91 steel have already been applied in some cases. Recently, the world’s first documented repair using the guid¬ance in the Best Practice Guideline was applied by the Tennessee Valley Authority (TVA) in its Southaven Combined Cycle Plant. In this through-section weld repair, a 152 mm O.D. (6”) x 22 mm (0.864”) wall Grade 91 manual dump valve was welded to the main steam line using ER80S-B8 and E8015-B8 filler metals without PWHT. At the first outage, this repair weld was inspected using conventional ultra¬sonic non-destructive evaluation, and no defects were documented. It has now operated at ~1,065°F (575°C) for several thousand hours.

Conclusions

EPRI is currently engaged with all stakeholders involved with the electric supply industry (including the codes and standards com¬mittees, insurance regulators and expert working groups) to gain acceptance of the proposed repair welding procedures and tech¬niques for Grade 91 steel. The benefits from well-engineered repairs are driving a series of follow-on studies to provide the necessary technology transfer to these various groups.

Three welding procedures have been successfully developed for the weld repair of Grade 91 steel piping and components. Test results have demonstrated that these procedures achieve cross-weld creep lives above the minimum of the required scatter band for a range of parent material conditions and under representative creep conditions.

The well-controlled welding procedures tested in research to date offer significant advantages over the con¬ventional repair procedures. The reduction in the allowable PWHT temperature to 1,250°F (675°C), or the elimination of PWHT altogether, has an immediate tangible benefit in alleviating many of the problems associated with field PWHT. Lastly, with proper well-engineered approaches, the design and application of weld repairs in Grade 91 steel will not only achieve performance similar to the weld it is replacing, but more importantly can introduce damage tolerance, an attribute that is not inherent to conventional weldments (new or repair) in Grade 91 steel.

Resources

• Best Practice Guideline for Well-Engineered Weld Repair of Grade 91 Steel. EPRI. Palo Alto, CA: 2014. 3002003833.

• Alternative Well-Engineered Weld Repair Options for Grade 91 Steel: An Executive Summary of Results from 2010 to 2014. EPRI. Palo Alto, CA: May 2015. 3002006403.

• TVA Applies an Alternative Well-Engineered Weld Repair Method for Grade 91 Steel. EPRI. Palo Alto, CA: 2014. 3002006394.

• Guidelines and Specifications for High-Reliability Fossil Power Plants, 2nd Edition: Best Practice Guideline for Manufacturing and Construction of Grade 91 Steel Components. EPRI. Palo Alto, CA: 2015. 3002006390.

• A Perspective on the Selection of Preheat, Interpass, and Post-weld Cool Temperatures Using Grade 91 Steel as an Example. EPRI. Palo Alto, CA: 2015. 3002005351.

• A Well-Engineered Approach for Establishing the Minimum Allowable Post Weld Heat Treatment for Power Generation Applications of Grade 91 Steel. EPRI. Palo Alto, CA: 2015. 3002005350.

John Siefert is a senior technical leader supporting EPRI’s Fossil Materials and Repair Program. Siefert previously worked for Babcock & Wilcox and has experience in welding research that includes dissimilar metal welding; development of processes for advanced ferritic, austenitic, and nickel-based alloys; post-weld heat-treatment; waterwall fabrication; and procedure qualifications. You may contact him by emailing editorial@woodwardbizmedia.com.

Jonathan Parker is a technical executive in EPRI’s Boiler Life and Availability Program. He provides expert support for projects associated with understanding the factors affecting damage in critical components. Previous employment includes positions with Swansea University (UK), the Central Electricity Generating Board (UK), Ontario Hydro (Canada), Replication Technology Inc., Failure Analysis Associates, and Structural Integrity Associates Inc. (USA). You may contact him by emailing editorial@woodwardbizmedia.com.