WELD REPAIR OF GRADE 91 STEEL J. D. PARKER, J. A. SIEFERT

WELD REPAIR OF GRADE 91 STEEL J. D. PARKER, J. A. SIEFERT Electric Power Research Institute, 1300 West W. T. Harris Blvd., Charlotte, NC, 28262, USA jparker@epri.com and jsiefert@epri.com Abstract Creep strength enhanced ferritic steels such as Grade 91 are the preferred material for much of the high temperature boiler tubing and piping components used in modern power generating plants. Validation of temperbead welding techniques is particularly important for Grade 91 steel as accurate post weld heat treatment (PWHT) has proven to be difficult in field applications. Even a well controlled PWHT can degrade the base material strength and poorly performed heat treatment can lead to subsequent creep properties being below the minimum expected by codes. Successful demonstration of temperbead techniques for Grade 91 steel would alleviate these concerns and create options for a well-engineered repair. The present paper outlines a major project which will establish information critical to repair of Grade 91 components, and describes details of metallographic characterization of the welds produced. Keywords: Grade 91, CSEF, creep, weld, repair 1. INTRODUCTION Although Grade 91 type steels are used globally, it is noteworthy that different regions will adopt codes and practices which vary in detail. Thus, for example,for a number of reasons there can be differences between the European and United States philosophy on repair. Fossil fired units in the US built to applicable ASME standards are expected to last for many decades; components are typically not constructed with a defined design life. Historically then, this design philosophy has often results in fabrication of many local repairs and there have been many applications of temperbead welding techniques in boiler components made from Grades 11, 12, and 22. The European philosophy is different in that components are constructed to a specific deisgn life. At key stages during operation, the component condition must be formally re-evaluated. Appropriate redmedial action may involved local repair or component replacement. In more metallurgical complex steels such as Grade 91 decisions regarding optimal remediation should consider local effects and the condition of the system. Components made from these materials will benefit from an increased range of repair options and this, in part, drives the need for novel repair techniques. Regardless of the specific reasons for making the repair, there is a worldwide need for novel repair techniques for the new class of creep strength enhanced ferritic (CSEF) steels. Indeed, Grade 91 has been used in high energy applications for more than two decades and there is an increasing need for repair or replacement [1]. To optimize the processes associated with remediation, it is critical to establish key information on removal of damage and on approaches for repair welding. Selected variables and weld processes are under investigation in a current EPRI project. This work will include weldment testing linking performance to details of how the repair was carried out. This aspect is considered important since in contrast to traditional low alloy steels, which are relatively user friendly as far as repairs are concerned; Grade 91 steels introduce additional complications. Because the properties and performance of the base metal will be critically dependent on the original composition and the full heat treatment history, there have been concerns expressed that the reparability of Grade 91 steel will be limited by the condition of the base metal. It is further recognized that in the majority of instances, defects in Grade 91 components will need to be repaired using fusion welding techniques. Because there are a very large number of variables to be

evaluated the project will be performed in two phases. Phase 1 provides a Ranking of Repair performance, the following are the primary factors consided: Weld procedure considerations (including consumabls) and heat treatment; Post repair evaluation of microstructure and damage; Specimen geometry and testing conditions including development of test matrix; Analysis to identify best option repairs best option to be based on factors such as speed of welding, initial quality, creep life, etc. The Phase 2 part of the project, which is not the subject of this paper, is focused on the application of several promising weld procedures (as identified in Phase 1) on the repair of Grade 91 components using several methods and extents of excavation to a variety of base material conditions. 2. EXPERIMENTAL PROCEDURE It is apparent that welding needs to be carried out carefully using qualified staff and appropriate procedures, further background is provided in reference [2]. The following section provides information and guidelines regarding these issues and the prescribed procedures utilized during the fabrication of 20 unique weldments. 2.1 Base Material In order to evaluation the effect of filler metal strength on the weldment performance, two conditions of base material were also selected. A heat of Grade 91 material, removed from service and with varying material hardness, is classified as A material. The same heat of Grade 91 material, having been renormalized and tempered to the specifications outlined in [2] is classified as B material. It is presumed that the A material will have lower strength and be representative of ex-service material and that the B material represents ideally controlled Grade 91 material with optimum strength and elevated temperature behavior. Tab. 1 Chemical Composition of Grade 91 Base Material for A and B Conditions C Si Mn P S Cr Mo Ni V 0.11 0.31 0.45 0.011 0.009 8.22 0.94 0.19 0.21 Al Cu N Nb Sn As Sb Ni+Mn N/Al Ratio 0.006 0.16 0.039 0.070 0.010 0.005 0.001 0.64 6.5 2.2 Welding Process and Post Weld Heat Treatment The selection of preheat temperature should be linked to the selected weld process. In addition, and because temperbead welding procedures were being explored, the control of preheat and interpass is particularly important to ensure near 100% transformation to martensite during welding. This is critical because if the deposited weld metal and/or heat affect zone in the parent material is to be tempered, it must fully transformation to martensite first. In addition, and because the shielded metal arc welding (SMAW) process was implemented, the use of a depressed preheat and interpass must be accompanied by strict control of welding consumables to avoid hydrogen induced cracking. In these studies, each welding procedure required a minimum preheat of 150 C (300 F), a maximum interpass of 343 C (600 F) and careful control of the welding electrodes to meet these requirements. No issues involving hydrogen induced cracking were reported or documented for the 20 weldments. A total of five unique welding procedures were utilized. Following the completion of welding, two post weld heat treatment scenarios were utilized for specific procedures. The first specified a traditional PWHT temperature in the allowable range for Grade 91 of 746 C (1375 F) for 2 hours. The second scenario simulated a low PWHT where the PWHT temperature was reduced to 675 C (1250 F) for 2 hours. No additional limits on the rate of heat-up or cool-down were specified for PWHT; no issues were reported with distortion caused by residual stresses relaxation.

Tab. 2 Summary of Key Aspects of the Variables Considered in the Phase 1 Evaluation Weld Base Metal Weld Metal, AWS Design. Trade Name 1,2 Weld Procedure 3 PWHT 1A Service-Exposed Grade 91 ( A ) [A] + Typ. PWHT 746 C/2h 2A, 2B Thermanit [A] + Low PWHT 676 C/2h E9015-B91 Chromo 9V Mod. 3A, 3B [B] None 4A, 4B [E] None 5A, 5B Service-Exposed Grade 91 ( A ) E8015-B8 9Cr-1Mo [C] None 6A, 6B 7A, 7B Renormalized and Tempered Grade 91 ( B ) E9015-G (-B23) E9015-G (-B23) Thermanit P23 Thermanit P23 [B] [A] + Typ. PWHT None 746 C/2h 8A, 8B E9018-B3 2.25Cr-1Mo [B] None 9A, 9B EPRI P87 (ASME 2733/2734 4 ) EPRI P87 [D] None 10A, 10B ENiCrFe-2 INCO-WELD A [B] None 1 Thermanit is a registered trademark of Bohler Thyssen Schweisstechnik Deutschland 2 INCO-WELD is a registered trademark of the Special Metals Corporation family of companies. 3 The bracketed letter for each welding procedure was detailed above in section 2.2 Normal Procedure [A] welding procedure performed with no specific guidance. Welders utilized proper sized electrode for given layer (3.2 mm, 1/8 inch in root and 4.0 mm, 5/32 inch for remaining fill). Temperbead [B] three layer technique, where the electrode size was increased in between temperbead layers: 2.5mm (3/32 inch) 3.2mm (1/8 inch) 4.0mm (5/32 inch) and the fill was performed using 4.0mm (5/32 ) electrode. Temperbead [C] two layer technique, where the electrode size was increased in between temperbead layers: 2.5mm (3/32 inch) 4.0mm (5/32 ) and the fill was performed using 1/8 (3.2mm) electrode or 4.0mm (5/32 ) electrode. Temperbead [D] two layer technique, where the electrode size was increased in between temperbead layers: 2.5mm (3/32 inch) 3.2mm (1/8 inch) and the fill was performed using 3.2mm (1/8 inch) electrode or 4.0mm (5/32 ) electrode. Temperbead [E] three layer technique, where the electrode size was increased in between temperbead layers: 2.5mm (3/32 inch) 3.2mm (1/8 inch) 4.0mm (5/32 ) and the fill was performed using 4.0mm (5/32 ) electrode. This procedure was unique in that it was purposely performed to be a poor practice technique where the welders were specified to not use a proper overlap, not use proper spacing and not use proper sequencing within a layer (i.e. between adjacent weld beads) and between each temperbead layer (i.e. between 2.5mm and 3.2mm or 3.2mm and 4.0mm layers). 4 ASME Introduced Code Cases 2733 and 2734 to cover the use of EPRI P87 as an F43 filler material in welding performance qualifications for both the GTAW and SMAW processes [3, 4] 3. EVALUATION OF REPAIR PROCEDURES The evaluation of the weldments detailed in Table 1 included non-destructive testing of completed weldments using radiography, extensive metallographic analysis (for procedures specifying PWHT, both aswelded and PWHT conditions were evaluated), large specimen creep and post-test examination of failed samples for failure location and damage assessment. Following the completion of welding, full section transverse cross-sections were taken for analysis using light microscopy and hardness mapping. An example of a completed weldment and associated hardness map is given in Figure 1. Note that the hardness map in Figure 1 is on the left side of the weld, originating in the base metal and continuing through the HAZ, temperbead layers and into the deposited weld metal. Each weldment was examined for cracking, slag inclusions, lack of fusion, and other atypical features. In addition, hardness mapping was conducted using a 200g or 500g load, a spatial distance of 0.20 mm (for 200g load) or 0.25 mm (for 500g load) and a Vickers indenter. The parameters for hardness mapping were developed specifically for Grade 91 and are detailed in [5,6]. The results for hardness mapping were further examined by plotting the data as a color hardness map (Figure 1) and through examination of the overall hardness distribution (Figure 2). Note that the hardness map shown in Figure 1 consists of 10,000 indents taken using

a Vickers indenter, 500g load and 0.25mm spacing. The hardness map is 25 mm (1.0 inch) X 25 mm (1.0 inch). Fig. 1 Cross-section Macro Image of Representative Weldment Note: Hardness map is visible on the left side of weldment and highlighted in yellow Fig. 2 Hardness Distribution for the Hardness Map Detailed in Figure 1 The evaluation of the welding procedures listed in Table 1 included creep testing of all weldments under a similar condition of 625 C (1157 F) and 80 MPa (11.6 ksi) using a large specimen with a cross section of 25.4 mm (1.0 inch) in the through-weld thickness and 20.3 mm (0.80 inch) in the longitudinal-weld orientation (i.e. direction of welding). For weldments 5A/B, 6A/B, 7A/B and 8A/B, additional tests were performed at 600 C (1112 F) and 80 MPa (11.6 ksi) as the filler materials specified for these conditions were undermatching. The gauge length of the creep specimens was generally 115 mm (4.5 inches). To date, >80,000 hours of creep testing has been conducted with the longest test exceeding 6,000 hours. A failed specimen is given in Figure 3.

Fig. 3 Failed Creep Specimen for Representative Weldment from Phase 1 Testing The testing of large diameter specimens is particularly important in the evaluation of cross-weld behavior as it allows for detailed assessment of the unfailed interface/haz. Such assessment may include observations on damage concentration and creep void counting. Specific results are not detailed in this paper, but detailed post test examination has been performed to determine the location of maximum damage and these results will be documented in a future publication. A comprehensive assessment of repair procedures in Grade 91 steel must establish the creep behavior of the repair. Thus, elevated temperature testing using service representative test conditions, i.e. those which result in Type IV failure and specimen geometries that produce failures where the damage mechanisms are relevant to long term service. In general, cross-weld data obtained on Grade 91 steel using relatively high stress (>100MPa, 14.5ksi) and/or temperature (>625 C, 1157 F) conditions does not necessarily result in representative damage comparable to observations in service-removed weldments. Thus, results from tests under these conditions should not be used as a guide to in-service behavior. 4. DISCUSSION The overall performance of a weld repair in Grade 91 is ultimately dependent on a number of factors including the strength of the filler material, condition of parent material, inspectability of proposed filler metals, residual stress state of the repair weld, welding procedure (temperbead versus PWHT), excavation method of damage and others. Although once thought absurd, the application of a temperbead procedure in Grade 91 has been demonstrated for 14 unique weldments. In addition, the use of a temperbead welding procedure may be more applicable to martensitic steels than for their bainitic cousins; note that temperbead welding procedures were originally applied to notoriously difficult to repair CrMoV alloys and to Grades 11, 12, and 22 [7-8]. The successful application of temperbead welding to a given ferritic material may be most dependent on the accumulation of residual stresses. An illustration of how phase transformations can influence the buildup of residual stress is presented in Figure 4 [9-12]. Using bainitic, martensitic, and stable austenitic steels, it has been demonstrated that transformation plasticity during the cooling of a uniaxially constrained sample from the austenite phase field acts to relieve the buildup of thermal stress as the sample cools [9]. By contrast, the non transforming austenitic steel exhibited a continuous increase in residual stress with decreasing temperature, as might be expected from the thermal contraction of a constrained sample. When the steels were transformed to bainite (2.25Cr-1Mo) or martensite (9Cr-1Mo and Grade 91 HAZ), the transformation strain compensated for any thermal contraction strains that arose during cooling. Significant residual stresses were therefore found to build up

only after the bainitic or martensitic transformation was completed and the specimens approached ambient temperature [9-12]. In the case of the Grade 91 HAZ microstructure, the peak temperature simulating the FGHAZ (peak temp. of 950 C) results in lower residual stress than either the subcritical Grade 91 HAZ (peak temp. of 780 C) or the low alloy steel [12]. This observation is profound in that, it may be entirely possible to utilize a temperbead procedure for Grade 91. Fig. 4 Interpretation of experimental data showing how residual stresses develop on cooling for an austenitic steel (no transformation), a bainitic low alloy steel (relatively high temperature of transformation) and a martensitic steel (relatively low temperature of transformation) [9-12]. 5. CONCLUSIONS Repair welding procedures encompassing both traditional welding techniques (including PWHT) and temper bead welding procedures were demonstrated for both service-exposed and renormalized and tempered Grade 91 material. The application of welding techniques to these materials has demonstrated that there are numerous options regarding the repair of Grade 91 material; a fact that was not previously demonstrated across such a large number of filler materials, PWHT procedures and base material conditions. REFERENCES [1] Service Experience with Grade 91 Components. EPRI, Palo Alto, CA: 2009. 1018151. [2] Guidelines and Specifications for High-Reliability Fossil Power Plants: Best Practice Guideline for Manufacturing and Construction of Grade 91 Steel Components. EPRI, Palo Alto, CA: 2011. 1023199. [3] ASME Code Case 2733. Cases of ASME Boiler and Pressure Vessel Code. Approved June 28, 2012. [4] ASME Code Case 2734. Cases of ASME Boiler and Pressure Vessel Code. Approved June 28, 2012. [5] Siefert, J. A., Shingledecker, J. P. and Parker, J. D. Optimization of Vickers Hardness Parameters for Micro and Macro Indentation of Grade 91 Steel. Journal of Testing and Evaluation, accepted for publication. [6] Parker, J. D. Siefert, J. A., and Coleman, K. Optimization of Vickers Hardness Parameters for Micro and Macro Indentation of Grade 91 Steel. Materials Performance and Characterization, in review. [7] Temper bead Welding of P-Nos 4 and 5 Materials. EPRI, Palo Alto, 1998. TR-111757. [8] Mitchell, K. C. and Brett, S. J. Review of Cold Weld Repair Applications. OMMI 2 (1), 2003. [9] Bhadeshia, H. K. D. H. Effect of Materials and Processing: Material Factors. Handbook of Residual Stress and Deformation of Steel, 2002, ASM International, Ohio, USA. [10] Jones, W.K.C. and Alberry, P.J. Ferritic Steels for Fast Reactor Steam Generators. Metals Technology, 1977, 11, pp. 557 566. [11] Jones, W.K.C. and Alberry, P.J. Residual Stress in Welded Constructions. The Welding Institute, 1977, Paper 2. [12] Paddea, S., Francis, J. A., Paradowska, A. M., Bouchard, P. J. and Shibli, I. A. Residual stress distributions in a P91 steel pipe girth weld before and after post weld heat treatment. Materials Science and Engineering A, 2012, 534, pp. 663-672.