Macrostructure and Micro chemistry Analysis on Stress Corrosion Cracking (SCC) of Alloy 690

Transcription

1 Macrostructure and Micro chemistry Analysis on Stress Corrosion Cracking (SCC) of Alloy 690 Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Lemi Gemechu Geda, B.S. Graduate Program in Mechanical Engineering The Ohio State University 2013 Thesis Committee: Jinsuo Zhang, PhD, Advisor Prasad S Mokashi, PhD

2 Copyright by Lemi Gemechu Ged 2013

3 Abstract Stress Corrosion Cracking (SCC) is a failure that results from the combination of tensile stress, corrosive environment, and material susceptibility. SCC is a serious metallurgical problem with an impact on current and future designs of structural components and nuclear reactors. The initiation of SCC is difficult to detect and control because of highly localized nature of chemical and mechanical conditions. SCC mechanism is dependent on the microstructure and grain boundaries for a given alloy composition. Alloy 690 is used in a pressurized water reactor (PWR) and steam engines due to its extreme resistance to common corrosion in high temperature and aggressive environment. Although alloy 690 has a great tendency to withstand corrosion and aggressive environment SCC has been associated with significant replacement of power and downtime in its application. Alloy 690 has been studied to understand, predict, and avoid costly and dangerous failures that could occur due to SCC. The main goal of the present study is to study the relationship between the structural nature, chemical properties and mechanical properties on and near grain boundaries in alloy 690 to identify their role in the resistance and susceptibility to SCC. The microstructure and microchemistry of alloy 690 at the SCC surface and crack fronts has been studied to improve the fundamental understanding of primary water stress corrosion cracking (PWSCC). ii

4 The alloy 690 was chosen as the material of interest for this study because of its relevance in current application in nuclear industries and application on different structural component at extreme environmental conditions. An integrated approach to study the structural, mechanical and chemical information at nano-scale in selected grain boundaries was developed. Microstructural and micro chemical examinations were conducted using field emission scanning electron microscope (FESEM). High resolution SEM observation were obtained by backscatter electron (BSE) imaging and in-lens FESEM which provide topographical information with virtually unlimited depth of field and less distorted images with special resolution. Also line profiling and mapping were performed using energy dispersive X-ray spectroscopy (EDS) to define the local features and chemical composition. iii

5 Dedication Dedicated to the Students at The Ohio State University and my families who help me throughout my school time iv

6 Acknowledgments The author gratefully acknowledges P.L. Andersen and General Electric Global Research Center for providing alloy 690 test samples. The author also is most grateful to J. Zhang and P. Mokashi for helpful suggestions and detailed discussions and to The Ohio State University Nano tech west lab staff A. Price and D. Ditmer for careful and reliable experimental support. v

7 Vista Jun Hibret Fire High, (Ethiopia) July Bahir Dar University, (Ethiopia) December B.S. Mechanical Engineering, The Ohio State University Field Of Study Major Field: Mechanical Engineering vi

8 Contents Abstract... ii Dedication... iv Acknowledgments... v Vista... vi List of Figures... ix List of Table...xiii Abbreviation... xiv Introduction... 1 Chapter Experimental Procedure... 5 Experimental Test Results (GE)... 8 Specimen c574, Alloy 690, 20% Forge... 8 Specimen c505, Alloy 690, 21% Forge Chapter Discussion Factors Affecting SCC growth rate of alloy Tensile Stress Temperature: Material Heat treatment Chapter SEM Analysis Structure of alloy Post SCC fatigue cracks and SCC cracks Pre-cracking and SCC testing Chapter EDS Analysis vii

9 Test regions SCC and post fatigue test reign SCC and pre fatigue test reign Oxide layer and SCC growth Summary and Conclusion Reference viii

10 List of Figures Figure 1. Schematic of specimen orientation in relation to the longitudinal (L) direction, width (T) and short transverse (S), Rolling is done in the L direction, with reduction occurring in the S direction. Forging reduces in the S direction [1]... 5 Figure 2. Crack length vs. time response of specimen c574 of Alloy 690, forged 20% reduction in thickness at 25 C, tested in the S-L orientation [1] Figure 3. Crack length vs. time response of specimen c505 of Alloy 690, WQ 199 forged 21% reduction in thickness at 25 C, tested in the S-L orientation [1] Figure 4. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness Figure 5. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness Figure 6. Combination of stress, corrosive environment and material susceptibility result SCC [30] Figure 7. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness SCC testing region Figure 8. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness SCC testing region Figure 9. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness SCC testing region ix

11 Figure 10. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness SCC testing region Figure 11. Fracture morphologies of alloy 690 of 20% forge on the left and 21% forge on the right hand side with resolution increasing from top to bottom Figure 12. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness boundary between post testing and SCC testing region Figure 13. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness boundary between post testing and SCC testing region Figure 14. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness boundary between post testing and SCC testing region Figure 15. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness boundary between post testing and SCC testing region Figure 16. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness boundary between Precracking & SCC testing Figure 17. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness boundary between Precracking & SCC testing x

12 Figure 18. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness boundary between Precracking & SCC testing Figure 19. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness boundary between Precracking & SCC testing Figure 20. FESEM machine Figure 21. EDS layer for 20% & 21% Forge between SCC & Post-fatigue test region respectively Figure 22. EDS layer for 20% & 21% Forge SCC & Machined part respectively Figure 23. EDS layer for 20% Forge post fatigue test Figure 24. EDS layer for 21% Forge post fatigue test Figure 25. Map spectrum & Composition for 20% Post-fatigue test Figure 26. Map spectrum & Composition for 21% Post-fatigue test Figure 27. EDS layer for 20% Forge SCC region Figure 28. EDS layer for 21% Forge SCC region Figure 29. Map spectrum & Composition for 20% SCC region Figure 30. Map spectrum & Composition for 21% SCC region Figure 31. EDS layer for 20% Forge SCC region Figure 32. EDS layer for 21% Forge SCC region Figure 33. EDS layer for 20% Forge Machined Region Figure 34. EDS layer for 21% Forge Machined Region Figure 35. Map spectrum & Composition for 20% Machined part xi

13 Figure 36. Map spectrum & Composition for 21% Machined part Figure 37. Magnified EDS layer for SCC 20% forge Figure 38. Magnified EDS layer for SCC 21% forge Figure 39. Zoomed in EDS layer for SCC 20% forge Figure 40. Zoomed in EDS layer for SCC 21% forge xii

14 List of Table Table 1. Chemical Composition of Alloy 690 & Table 2 Composition and Properties of Alloy 690 CRDM, Heat NX3297HK12 [1].. 10 Table 3 Composition and Properties of Alloy 690 CRDM, Heat WQ 199 [1] Table 4 Examples of commonly occurring combinations of materials and corrosive environments that exhibit susceptibility to SCC Table 5. Elemental Composition of 20% forge post SCC region Table 6. Elemental Composition of 21% forge post SCC region Table 7. Elemental Composition of 20% forge post SCC region Table 8. Elemental Composition of 21% forge post SCC region Table 9. Elemental Composition of 20% forge machined region Table 10. Elemental Composition of 21% forge machined region Table 11. Elemental Composition of magnified 20% forge machined region Table 12. Elemental Composition of magnified 21% forge machined region xiii

15 Abbreviation AFM ANL BSE BWR CRDM CT CW DC EDS EPRI FESEM GE PWR PWSCC SCC SEM TEM Atomic Force Microscopy Argonne National Laboratory backscatter electron boiling-water reactor control rod drive mechanism compact tension cold worked direct current energy-dispersive x-ray spectroscopy Electric Power Research Institute Field Emission scanning electron microscopy General Electric pressurized water reactor primary water stress corrosion cracking stress corrosion cracking scanning electron microscopy transmission electron microscop xiv

16 Introduction Due to favorable superior tensile strength, low creep ductility and excellent resistance to oxidation at elevated temperatures, Ni base alloys are used in nuclear reactors, electric resistance heaters, gas turbines, aerospace, petrochemical, and heat treating industries [9, 12]. Many nickel based alloys used in the application of structural component in steam engines and water reactors have experienced SCC. The development of newer materials with a change in composition and mechanical processes has improved the resistance of SCC in steam engines and water reactors. Although the resistance of material from SCC has been greatly improved, laboratory and field of experiences have showed that avoiding SCC completely is impossible. Although the roles of cold work on SCC have not been well understood [23], it has been shown that increasing strength is the main effect of cold work on the materials, which would indirectly correlate to susceptibility of SCC. It has been known that high strength materials are typically more susceptible to SCC than comparably softer materials [16]. SCC of alloy 690 has been observed in laboratory test in different conditions, specimen showing that high SCC specimens had high degree of cold work [2]. Although high degree of SCC is observed in higher cold 1

17 work specimens, it is also believed that several other factors contribute to SCC growth [2, 8]. Alloy 690 is a nickel-based alloy with a face centered cubic (FCC) matrix. The microstructure of alloy 690 is very similar to that of the predecessor alloy 600. Alloy 690 forms almost exclusive precipitation of chromium rich M23C6 type carbides, while alloy 600 typically forms M7C3 carbides. The difference is in the effect of the near doubled chromium content in alloy 690 compared to its predecessor alloy 600, which alters the stability for the two carbide types [16]. Studies have been carried out on different commercial super alloys to evaluate their resistance to oxidation at elevated temperature. These studies suggest that oxidation resistance is directly related to chromium content and influenced by other reactive elements [9, 12]. Having a very high content of Cr is well known for high affinity with oxygen and high stability of its oxide Cr2O3 at high temperature [9]. Alloy 600 was also used as a material for the steam generator of pressurized water reactor (PWR). Eventually alloy 690 was introduced as a replacement material for alloy 600, which has nickel content with roughly twice the chromium content as showed on Table 1. This has proven to produce better resistance to SCC, which leads to a stable chromium oxide at high temperature [16]. 2

18 Table 1. Chemical Composition of Alloy 690 & 600 %C %Cr %Fe %Si %Mn %P %S %Cu %Ni Alloy 600 and its weld metals alloy 182 and alloy 82 were originally used in pressurized water reactors (PWRs) because of their resistance to general corrosion in a number of extreme environments. Over the last thirty years, stress corrosion cracking in PWR has been observed in numerous alloy 600 components and associated welds [12]. The occurrence of primary water stress corrosion cracking (PWSCC) is becoming more significant in alloy 600 and its weld materials. Replacement of alloy 600 has been generally utilized by using alloy 690 and its weld materials alloy 52 and alloy 152, which has shown a significant resistance to PWSCC in most laboratory experiment [11]. The main requirement for using alloy 690 as a structural component in steam engine is the ability to withstand corrosion in extreme environment. Using alloy 690 will reduce the susceptibility of corrosion in an environment where an oxide layer can be formed on a variety of different components. The material composition and property of alloy 690 need to withstand this environment for an extended period of time to ensure safe and reliable operation. In order to make sure that the operation is safe and reliable, it is critical to study and understand the growth mechanism of oxide layers [7]. In previous testing programs, the effect of cold work, electrochemical potential, temperature, and stress intensity were 3

19 examined. The results obtained were in close agreement with the data gathered at the GE test lab. Very few quantitative SCC growth rate measurements were seen on alloy 690. Alloy 690 has been studied in order to understand, predict, and avoid costly and dangerous failures that could occur due to SCC. This thesis studies the microstructure and microchemistry at the SCC surface and crack fronts to understand the primary water SCC (PWSCC) mechanisms of Alloy 690. The formation of the micro structural surface of alloy 690 was examined using different imaging methods to gain a better understanding of how this formation will affect the mechanism and reliability of alloy

20 Chapter 1 Experimental Procedure Rectangular pieces were cut at GE Research, and cold work was introduced by forging at the room temperature ( effectively done by reducing the control rod drive mechanism (CRDM) wall thickness ). The procedure was optimized to achieve as uniform a degree of cold work as possible. Alloy 690 in plate form was forged by 20% and 21% in one direction, and specimens were machined in the S-L orientation [Figure 1]. Figure 1. Schematic of specimen orientation in relation to the longitudinal (L) direction, width (T) and short transverse (S), Rolling is done in the L direction, with reduction occurring in the S direction. Forging reduces in the S direction [1] 0.5 inch thick compact tension (0.5T CT) specimens were machined with 5% side grooves on each side. Because two specimens were tested in tandem, the 5

21 specimens were first individually fatigue pre-cracked in air under identical conditions and to an identical crack depth before being assembled into the autoclave. The compact tension specimens were instrumented with platinum current and potential probe leads for DC potential drop crack length measurements of crack length. In this technique, current flow through the sample is reversed about once per second primarily to reduce measurement errors associated with thermocouple effects. The compact tension specimens were electrically insulated from the loading pins using zirconia sleeves, and within the autoclave a zirconia washer also isolated the upper pull rod from the internal load frame. The lower pull rod was electrically isolated from the autoclave using an Omniseal pressure seal and from the loading actuator using an insulating washer [11]. Deaerated, de-mineralized water was used to mix B (as H3BO3) and Li (as LiOH) to maintain a low-impurity environment throughout the test. H2 was continuously bubbled at the appropriate pressure through a narrow, tall glass column. A low pressure pump provided positive pressure to the high pressure pump, and drew water from and recirculated excess water back into the glass column. Inlet water to the autoclave was heat exchanged with the outlet water to recover heat, and was also heated by a preheater before entering the autoclave. Tests were performed in 4 liter autoclave made of stainless steel at 340 o C and 17.2 MPa (2500 psi) or 360 o C and 20.6 MPa (3000 psi) [11]. Corrosion potentials of the compact tension specimen were measured using a Cu/Cu2O zirconia membrane. 6

22 For characterizing the tested specimens from GE microstructural and micro chemical examinations were conducted using Field Emission Scanning electron microscope (FESEM). High resolution SEM observation were obtained by backscatter electron (BSE) imaging and in-lens FESEM which provides topographical information with virtually unlimited depth of field and less distorted images with special resolution. The surface was carefully cleaned and polished to avoid any damage and minimize dirt or finger print on the surface of the test sample. Comparisons were made in the SEM between in-lens and BSE secondary electron imaging for better understanding and defining local features. Line profiling and mapping were performed using energy dispersive X-ray spectroscopy (EDS). High level of cold rolling or forging in various materials produced significant hardening, however each showed important difference in their damage microstructures. High resolution in-lens imaging under low kv conditions in the SEM proved to be the most effective for sampling a large number of boundaries and identify oxide interfaces and cracked precipitates. 7

23 Experimental Test Results (GE) Successful observation of SCC growth at constant stress intensity factor in materials of low to moderate susceptibility is often dependent on a very smooth and complete transition from the trans-granular fatigue pre-cracking conditions to conditions appropriate to SCC growth. The purpose of GE s experiments was to use such highly sensitive techniques to try to achieve stable, sustained growth in specimens of cold worked Alloy 690. Specimen c574, Alloy 690, 20% Forge Specimen c574 was fabricated from the alloy 690 (heat NX3297HK12, Table 1) that was reduced in thickness by 20% by forging at 25 o C. The forging deformation was one dimensional and the specimen has (S-L) orientation (Figure 1). The specimen was assembled into the autoclave, and the autoclave was purged with N2 and filled with H2 deaerated (26 cc/kg H2) water containing 600 ppm B and 1 ppm Li and heated to 360 o C. After an equilibrium the specimen was loaded to Kmax = 34 MPa m, R = 0.6 and Hz (figure 2). The crack commenced growing at 2.8 x 10-7 mm/s in a very linear, well behaved fashion [1]. After 195 hours, a 9,000 s hold time at Kmax was introduced, and the growth rate decreased to 6.9 x 10-8 mm/s. At 770 hours, a shift to constant K conditions was made, which produced a decrease in growth rate to 1 x 10-8 mm/s. Constant K 8

24 value start increasing slowly, using dk/da control at 908 hours which also increase the proportionality of crack length. At 1726 hours, a return to a 9,000 s hold time was begun, no sudden increase in growth rate was observed. As K value reached 34 MPa m, the growth rate had increased to 4.8 x 10-8 mm/s, then at 1952 hours a 3500 s hold time was initiated, which caused the growth of crack rate decrease to 1.3 x 10 7 mm/s. On return to a 9500 s hold time at 2458 hours, the growth rate decreased to 7.3 x 10-8 mm/s. At 2733 Hours all cycling was stopped and constant K condition was maintained, which produce a decrease in growth rate to 2.6 x 10-8 mm/s. At 3476 hours the specimen was reloaded to Kmax, R = 0.5 and Hz which lead the crack to increase to 1.6 x 10-6 mm/s. A change at 3591 hours to new frequency of Hz decreases the crack to 3.8 x 10-7 mm/s. At 3798 hours, a 9000s hold time and K value increased from 34 MPa m to 35 MPa m was introduced which decrease the crack growth to 6.0 x 10-8 mm/s. at 4170 hours. The crack was again transitioned back to constant K condition, where it slowly increases to the growth rate of 2.2 x 10-8 mm/s. At 6000 hours the specimen was removed and broken open. The fracture surface was even across the specimen width. Scanning electron microscopic (SEM) examination of the fracture surface showed clear evidence primary and secondary cracking along the entire crack front. 9

25 Crack length, mm Test 6043h Table 2 Composition and Properties of Alloy 690 CRDM, Heat NX3297HK12 [1] %Ni %Cr %Fe %Mn %Si %C %Cu %S <0.001 Corrected Data 13.5 Overview - c , ANL NX3297HK12, As-Rec'd, 20% Forge, S-L Outlet conductivity x c tct of 690 AR, 20% Forge, S-L 33 MPa m, 360C, 600B/1Li, 26 cc/kg H Conductivity, ms/cm or Potential, V she 10.5 At 325C, ph = At 300C, ph = CT potential Pt potential Test Time, hours Figure 2. Crack length vs. time response of specimen c574 of Alloy 690, forged 20% reduction in thickness at 25 C, tested in the S-L orientation [1] Specimen c505, Alloy 690, 21% Forge Specimen c505 was fabricated from the alloy 690 (heat WQ199, Table 2) that was reduced in thickness by 21% by forging at 25 o C. The forging deformation was one dimensional and the specimen has (S-L) orientation (Figure 1). The specimen was assembled into the autoclave, and the autoclave was purged with N2 and filled 10

26 with H2 deaerated (26 cc/kg H2) water containing 600 ppm B and 1 ppm Li and heated to 360 o C. After the equilibrium, the specimen was loaded to Kmax = 41 MPa m, R = 0.7 and Hz (Figure 3). The crack commenced growing at 1.4 x 10-7 mm/s in a very linear, well behaved fashion. After 1068 hours, a 9,000 s hold time at Kmax was introduced, the growth rate decreased to 2.1 x 10-8 mm/s. At 1687 hours, a shift to constant K conditions was made, which produced a decrease in growth rate to 2.0 x 10-8 mm/s. A constant value of K start decrease slowly using dk/da control to 1.9 x 10-8 mm/s and increase back to 2.8 x 10-8 mm/s which also increases the proportionality of crack length. At 3614 hours (Figure 3), the specimen was loaded to Kmax, R = 0.6 and 0.01 Hz a very sharp increase in growth rate was observed. As K value reached 39 MPa m, the crack growth rate had increased to 5 x 10-6 mm/s. A change at 3618 hours to new frequency value of Hz leads the crack to decrease to 1 x 10-8 mm/s. At 3643 hours the specimen was reloaded to Kmax, = 39 MPa m and a 9000 hours hold which lead the crack to decrease ~0 mm/s. After 8200 hours the specimen was removed and broken open. The fracture surface was even across the specimen width. Scanning electron microscopic (SEM) examination of the fracture surface showed clear evidence of primary and secondary cracking along the entire crack front. 11

27 Crack length, mm Test 8226h Table 3 Composition and Properties of Alloy 690 CRDM, Heat WQ 199 [1] %Ni %Cr %Fe %Mn %Si %C %Cu %S %Ti %Al %P %Co <0.01 < Corrected Data 12.5 Overview - c , WQ199, As-Rec'd, 21% Forge, S-L Outlet conductivity x c tct of 690 AR, 21% Forge, S-L 39 MPa m, 360C, 600B/1Li, 26 cc/kg H Test Time, hours At 325C, ph = At 300C, ph = 7.40 CT potential Pt potential Conductivity, ms/cm or Potential, V she Figure 3. Crack length vs. time response of specimen c505 of Alloy 690, WQ 199 forged 21% reduction in thickness at 25 C, tested in the S-L orientation [1] 12

28 Chapter 2 Discussion Factors Affecting SCC growth rate of alloy 690 Tensile Stress: The magnitude of the tensile stresses, particularly residual stress from fabrication, has had a major impact on the time for detectable SCC to develop. Only the most highly strained regions of steam generator tubing like larger radius tubes, roll transition regions, expanded regions, and dented areas have exhibited SCC. Consequently, several stress evaluation techniques have been evolved such as local stress relief of first and second row U bends by resistance or induction heating, and shot peening was used to induce compressive stresses on the internal surface [7, 15]. While peening helps to limit initiation of new cracks, it cannot prevent the growth of existing cracks whose depth is greater than that of the induced compressive layer. Peening has been most effective when most tubes have either no cracks or only very small ones. [15] There is a considerable spread in crack growth data, and trends can be hard to identify by comparing data for specific heats or tests, however, it seems clear that the highest crack growth rate appear when aligning the crack plane with the plane of deformation, and the growth direction with direction of rolling [15]. Despite the S-L orientation being the most interesting, many experiments have been performed 13

29 in the T-L orientation because of the limited thickness of the materials. The same orientation CGR dependence has been demonstrated on Alloy 600 [15]. The significant influence of cold work direction on crack growth rates (CGRs) suggests that strain hardening and increased free energy cannot fully explain the increased susceptibility. The role of microstructural inhomogeneities caused by banding may be a partial cause of the directional dependence. Cold worked materials with microstructural banding are associated with elevated CGRs, especially when aligning the cold work and crack growth directions with the bands [15]. Temperature: The first experience of SCC was located at the location where the temperature is typically high. SCC of alloy 600 steam generator tubing becomes a major problem starting from the year The first confirmed primary cracking of alloy 600 tubes of steam generators occurred when leakage at U-bends was experienced. Cracking occurred both in the tight U-bends, mainly on the inner two rows at the apex and at the tangent as well in the tube sheet at the transition expansion [15]. Analyses of temperature effect on average crack growth rate in PWR on alloy 690 were performed in GE lab and other institutes [22, 19]. For meaningful activation energy to be determined, it was necessary to maintain all fundamental parameters constant [16]. The increased kinetics of mass and other factors with increased temperature can significantly affect experimental conditions, such as water purity via release of chromic acid from materials containing Cr and more rapid decomposition of demineralizer resins. Other important phenomena were 14

30 interpreted in terms of temperature influence on solubility usually by changing the ph level from approximately neutral state. The effect of impurities on crack growth both in high temperature water and low temperature water has been found to be significant. Change in chemical equilibrium that occurs with temperature also strongly affects the crack growth rate [16]. Material: Material susceptibility is also a major factor affecting the occurrence of SCC in service. Most PWSCC has occurred in annealed tubing. Tubing manufacturer has employed in different production processes. Whereas some tubing has not experienced any SCC over extended periods of operation, in other cases SCC occurred after only 1 to 2 years of service, particularly at roll transitions. This variability of SCC susceptibility is even observed between the same manufacturers in the same steam generator. The variation in susceptibility to PWSCC of the alloy depends on material and other factors discussed above [1, 15]. The microstructure of alloy 690 is very similar to that of the predecessor alloy 600. However, alloy 690 forms almost exclusive precipitation of chromium rich M23C6 type carbides, while alloy 600 typically forms M7C3 carbides. This difference is an effect of the near doubled chromium content in alloy 690 compared to alloy 600, which alters the stability for the two carbide types [6]. Apart from the chromium based carbides, TiN inclusions are also present in alloy 690. This inclusion bands may have a local effect on grain size, although overall grain size is mainly determined by carbon content [16]. Under tensile stress, materials susceptibility to SCC is strongly dependent on combined effect of their compositions and environmental conditions. New combined 15

31 effects are constantly being discovered as materials in all types of engineering systems progress through their designed services and functions. For the purpose of understanding Table 3 presents examples of commonly used materials and accompanying corrosive environments that are known to be susceptible. Table 4 Examples of commonly occurring combinations of materials and corrosive environments that exhibit susceptibility to SCC Alloy Carbon steel High strength steels Austenitic stainless steels High-nickel alloys a-brass Aluminum alloys Titanium alloys Magnesium alloys Aqueous Cl - Zirconium alloys Environment Nitrate, caustic, and carbonate solutions Aqueous electrolytes containing H2S High concentrated chloride solutions High-purity steam Ammoniac solutions Aqueous Cl -, Br - and I - solutions Aqueous Cl -, Br - and I - solutions, organic liquids and N2SO4 Aqueous Cl -, organic liquids, and I2 Table 3 is not an example which shows the only materials and environments that can combine to result in component failure through SCC. These examples do, however show how failure from SCC covers a wide range of materials and environments making inhibition difficult and expensive to incorporate in to design. Heat treatment: The corrosion resistance of alloy 690 steam generator tubing used for PWR steam generators has been found to be strongly dependent upon the carbide distribution in its microstructure [18]. Alloy 690 was developed as an improvement for alloy 600 after alloy 600 was identified to be susceptible to even 16

32 pure water stress corrosion cracking [6]. Extensive research testing has shown that alloy 690 is immune to pure water SCC in both mill annealed and thermally treated conditions. However, available experimental tests indicated that alloy 690 s resistance to SCC depends on its heat treatments and its carbide distribution [18]. There is a general agreement that both the chemistry and structure of the grain boundaries play a critical role in the improvement of resistance to SCC of alloy 690. Heat treatment can affect corrosion behavior and induce chromium depletion, segregation of impurities to grain boundaries, and the composition, structure and distribution of carbide precipitates and their mechanical effect on stress concentrations [6, 18]. The susceptibility of alloy 690 to SCC was also expressed in terms of the percentage reduction in area (%RA) calculated by the following expression [18]: %RA = 1 (ɸf / ɸi) (1) where ɸf and ɸi are the final and the initial diameters of the tensile specimen, respectively [18]. 17

33 Figure 4. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness. Rate of SCC growth were measured in simulated PWR primary water on 20% and 21% forged alloy 690 at 360 o C for about 6000 hours to 800 hours in GE laboratory using 0.5T CT specimens. Significant depth of SCC was observed in 21% forge alloy 690, and the fracture morphology is shown in Figure 4, also similar resolution factor of alloy 690 with 20% forge is shown in Figure 5 which has completely different structural appearance. Even if, the two materials have small difference of cold work, it had been showed a huge difference in structural appearance of SCC at 360 o C in PWR primary water, which is because of the different heat treatments of the two specimens. Significant SCC was not observed for both 20% and 21% forge alloy 690 with a small resolution but very small and local SCC was observed in Figure 5 of 20% forge alloy 690 at higher resolution of SEM. Increasing strength is the main effect of cold work on the materials and high 18

34 strength materials are typically more susceptible to SCC than relative softer materials. This argument has been proven in this observation in Figure 4 and Figure 5 SCC test result which indicates susceptibility can often be correlated to macroscopic strength [16] Figure 5. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness. 19

35 Chapter 3 SEM Analysis A major objective of this study was to characterize the formation of microstructural surface of alloy 690 in SCC and investigate the relationship between the structural nature and chemical properties on and near the SCC surface and the grain boundary to identify their roles on the resistance to PWSCC. An integrated approach to investigate the structural, mechanical and chemical information using higher resolution SEM at selected boundaries was developed. Microstructural and micro chemical examinations were conducted using filed emission scanning electron and the structure of the boundaries was identified using electron backscatter diffraction (BSE) and in lens FESEM which provide topographical information with higher resolution and less distorted imaging and also line profiling and image mapping were performed using energy x-ray spectroscopy (EDS). Some of the results of observation are shown in Figures [4 to 22]. Microstructure of 20% cold work and 21% cold work alloy 690 were observed on the SCC surface and SCC tips. The result of SEM imaging shows that the microstructure and crack distribution of the two alloy 690 specimens are different. This difference can be attributed to kinds of reasons including different heat treatments of the two specimens. The initiation and propagation SCC of the two specimens are due to the combination of a material susceptibility, the presence of tensile stress and corrosive environment, shown in Figure 6 [18]. It was also observed that crack density was 20

36 higher at the boundary of pre-cracking and SCC testing region and also postcracking and SCC testing region. Figure 6. Combination of stress, corrosive environment and material susceptibility result SCC [30] The presence of stress is the fundamental component of the SCC process. However, an important distinction is that crack propagation does not occur only as a result of local stress in addition it requires the critical stress exceed by fracture mechanics [18]. Sometimes stress required for producing fracture and crack growth is much less than the material critical stress intensity. When this phenomenon occurs it is known as subcritical crack growth [17, 18]. Stress can be residual or applied and it has to be tensile in order to induce SCC. Therefore, compressive stress can be used as a method of SCC inhibition [6]. The corrosive environments that contribute to SCC growth provide sufficient condition for a chemical reaction. Due to these chemical reactions, general corrosion of the material occurs. When the materials pulled apart due to the presence of tensile stress, it creates material exposure to newly localized corrosion. This process continues over time and crack propagation 21

37 starts on this site and branches out to different area, and leads to the material failure, as shown on figure 12, 14 and figure 15 [18]. Figure 7. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness SCC testing region Structure of alloy 690 The microstructure of a material is an important factor in susceptibility to SCC and quite often decisive in controlling the corrosion of the material [3, 17]. Since our present study has focused on the corrosion behavior of alloy 690 in different heat treated conditions, results will be discussed from the perspective of structure. Cracking can occur in different forms depending on different conditions at grain boundaries and grain interiors [18]. Crack can grow across grain boundaries, known as trans granular SCC (TGSCC), and along grain boundaries, known as inter granular 22

38 SCC (IGSCC). One of the purposes of this research is to verify the main engineering factors that affect SCC such as cold work and temperature. The effect of cold work on SCC microstructure has been studied to figure out how the cold work and other source aggregate to form cavities at the grain boundaries as crack grows near sites of crack initiation and crack tips. It has been reported that there will be a considerable difference in crack growth rate with different cold work. Figure 7 and Figure 8 shows a clear structural difference at 20% cold work and 21% cold work even if the test condition for both case were in S-L orientation.. From the observation in Figure 11 the direction of plain deformation are upward on the same direction for both 20% and 21% cold work but the crack growth was observed along the grain boundary (IGSCC) on 21% forged alloy 690 as shown in Figure 4. On the other hand the crack growth was observed across grain boundary (TGSCC) on 20% alloy 690 in Figure 5 and Figure 13. Significant influence of cold work on crack growth suggest that strain hardening will increase the susceptibility of the material [6]. This explanation of susceptibility of material due to higher percentage of cold work is consistent with the result obtained in Figure 4 and Figure 5 which shows high degree of crack growth on 21% cold work than 20%. Change in the mechanical properties of the material at the surface due to electrochemical interactions with the environment is another reason proposed to explain the mechanism of SCC [18]. The absorption of ions from the local environment can serve to weaken the bonds of the crystal lattice and result the stress required to initiate cracks is reduced [16]. In caustic environments, alloy 690 in different heat treated conditions has displayed unstable condition. 23

39 Figure 8. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness SCC testing region Figure 9. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness SCC testing region 24

40 The OH - ions have either caused a block in formation of protective film or created a premature breakdown of protective film leading to localized attack on alloys 690. As a result the bond weakening on the surface will occur or facilitate the formation of oxide on the surface of alloy 690. Figure 10. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness SCC testing region The oxide formations on alloy 690 in a PWR at 360 o C are similar in nature although qualitatively different [5]. It was observed that 21% cold work alloy 690 has less quantity of oxide at the surface as shown in Figure 9 while 20% cold work alloy 690 exhibit higher quantity of oxide as shown in Figure 10. It was also observed from Figures 4 and 6 the oxide particles are different in size and structure, the inner layer contains very small crystal size and the outer layer contains much bigger crystal size and structure. 25

41 Figure 11 Continued 26

42 Figure 11 Continued 27

43 Figure 11. Fracture morphologies of alloy 690 of 20% forge on the left and 21% forge on the right hand side with resolution increasing from top to bottom Post SCC fatigue cracks and SCC cracks The interfaces between smaller crystals of different orientation are known as grain boundaries and are the most important aspect of microstructure. The stress field that exists due to the disorientation of neighboring grain results as a barrier to the translation of dislocation or crack propagation [18]. The networks formed by grain boundaries drastically affect the mechanical properties of a material. The effects on mechanical property of a material are not homogeneous on alloy 690. In addition to exhibiting varying mechanical properties, grain boundaries can also be based on their chemical composition. 28

44 Figure 12. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness boundary between post testing and SCC testing region SEM examinations were carried out in post fatigue crack surface and SCC testing regions on both 20% cold work and 21% cold work alloy 690 which is displayed on figure [12-15] showing the microstructure of the specimen in detail. Various structural differences was observed between 20% and 21% cold work alloy 690, the crack propagations are more visible and larger on 21% cold work than 20% cold work. This difference in crack propagation might be attributed to their varying mechanical property and heat treatment. A higher material strength created due to elevated cold work on 21% forge alloy 690 increase the susceptibility of material for crack to begin in structure. This condition was facilitated in extreme environmental situation during experimental testing. 29

45 Figure 13. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness boundary between post testing and SCC testing region It was also interesting to note that, even though crack formation was higher on 21% cold work specimen than 20%, the quantity of crack observed on both specimen are quite low. Several possible reasons can be given for lack of extensive crack propagation in these materials. The first and obvious reason is alloy 690 microstructures are more resistant to the corrosion by PWR water and also possible that there may be insufficient cold work to accelerate SCC [13]. It is also important to note that a very big crack formation are observed on post testing and boundary between the two region than SCC region, which is directly relate to applied tensile force to separate the two piece after experimental procedure. 30

46 Figure 14. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness boundary between post testing and SCC testing region Not only structural difference between SCC and post testing region, there was also a varying chemical composition of individual element are observed. EDS examination of elemental mapping on Figure 21 and map spectrum of individual composition in Figure 25 and figure 26 also table 4 and 6 shows the difference between the two regions. The crystalline structure observed on Figure 15 also shows the presence of oxide in SCC region and no oxide formation is observed in post testing region because crack formation takes a place in atmosphere at room temperature. This is because there was electrochemical interaction between alloy 690 and caustic environment during experimental testing in SCC region and there might be absorption of ions from local environment to create a bond of the crystal lattice which result in the formation of oxide. A uniform structural appearance on 31

47 post testing region and random and more scattered structural appearance was observed on SCC region. Figure 15. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness boundary between post testing and SCC testing region Pre-cracking and SCC testing SCC has long been thought to occur when the alloy is susceptible, in a particular environment and in the presence of tensile stress. The known environments and susceptible material that result in stress corrosion cracking has to include a wide range of combinations but the first requirement for SCC to occur is the presence of tensile stress. The maximum stress has to be applied prior to experiment to initiate the crack propagation. 32

48 Figure 16. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness boundary between Pre-cracking & SCC testing SEM examinations were carried out in pre testing and SCC regions on both 20% cold work and 21% cold work of alloy 690. From this observation alloy 690 specimen exhibits very well in a similar fashion throughout pre-testing and SCC region on both 20% cold work and 21% cold work. Comparisons are displayed on figure [16-19] for both 20% cold work and 21% cold work alloy 690. Even though the structural appearance are different on SCC region and pre-testing region, the result found on EDS elemental mapping indicate similar composition of individual element between the two regions on figure

49 Figure 17. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness boundary between Pre-cracking & SCC testing Figure 18. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c505 of Alloy 690, forged to 21% reduction in thickness boundary between Pre-cracking & SCC testing 34

50 Detailed examination of microstructural appearance was done on SCC surface and pre-testing region and a clear difference is observed. Figure 17 and 18 shows a high resolution image of SCC region and machined part where machined part toward the bottom. There was no visible crack formation are observed in a pre-test region but very aggressive horizontal fractured surface are observed at the transition between testing region and pre-testing region. This crack formation is assumed to be created due to applied tensile stress used to initiate air fatigue pre-crack rather than extreme environmental test condition in SCC experiment. It was also observed crack formations are very minimum going away from transition between the two regions toward SCC region, this is also a clear indication that formation of aggressive horizontal fractured surfaces observed at the border are not from test condition, rater it is an effect created during air fatigue pre-crack Figure 19. Scanning electron micrograph of the fracture surface of 0.5TCT specimen c574 of Alloy 690, forged to 20% reduction in thickness boundary between Pre-cracking & SCC testing 35

51 Chapter 4 EDS Analysis Figure 20. FESEM machine Determination of the elemental composition of area of interest on the sample surface was accomplished using dispersive x-ray spectroscopy (EDS). EDS is analyzed using FESEM equipped with an in-lens and backscatter diffraction camera. Electrons are impinged on the area of interest on the surface of the sample, which is 36

52 tilted toward the diffraction camera or in-lens. The electrons interact with the lattice planes on the surface of the sample and collide with the phosphor screen fitted on in-lens and backscattered diffraction camera. The collisions of the electrons with the phosphor screen produce a pattern, which are detected by the charge coupled device (CCD) camera which is mounted on FESEM. These patterns can then be indexed to recognize the crystallographic orientation and elemental composition of the plane that formed the pattern. A major objective of this study was to characterize the formation of oxide at the surface of alloy 690. The formation of oxide at the surface of alloy 690 were examined using FESEM and some of the results of the observation are shown in figure [21-36], which were observed on the surface formed at 21% forged and 20% forged alloy 690. The formations of oxides were observed on the surface and near the opened cracks. The result of SEM imaging shows that the oxide distributions are different, this might be attributed to the test condition on the alloy or the time it takes for the test to be completed. It was also observed that formation of crack was higher on 21% forge alloy than 20% that is attributed to different material property. FESEM technique was also used to analyze surface morphology of alloy 690 at higher resolution. EDS analysis was used to identify an approximate percentage of each element present at the surface. 37

53 Test regions Figure 21. EDS layer for 20% & 21% Forge between SCC & Post-fatigue test region respectively Figure 22. EDS layer for 20% & 21% Forge SCC & Machined part respectively 38

54 The oxidation behavior could be affected by high temperature steam and water existing in the oxide film [19]. There are many factors contributed to oxidation of alloy 690, steam is considered the main contributor to affect oxidation procedure in the form of molecules in PWR [19]. The ionization of water results in hydrogen ions and hydroxyls which would result in electrochemical interaction between water and metal. These molecules and ions will have different effect on the oxidation behavior; the location of each alloying element in the oxide layer depends on whether or not it diffuses faster than the base metal in its oxide. Faster diffusing elements will pass through to the outer layer, while slower diffusing atoms will be oxidized without movement and remain in the inner layer [19]. Some of the oxides formed on alloy 690 are Fe3O4, NiO, and Cr2O3. Figure 21 shows the EDS mapping on the region between the part which experimental procedure takes a place and a region after experiment (SCC region and post fatigue test region) of cold work alloy 690 at 20% and 21%. Also Figure [22] shows the region where fatigue test takes place and pre fatigue test region at 20% forge and 21% forge consecutively. A clear color difference observed in Figure 21 shows, the difference in oxide concentration throughout the surface of alloy 690 but the color difference was not obviously observed on Figure 22 because oxide concentrations are relatively similar on the surface of the SCC and pre- fatigue region. 39

55 Figure 23. EDS layer for 20% Forge post fatigue test Table 5. Elemental Composition of 20% forge post SCC region Element Apparent Concentration k Ratio Wt% Wt% Sigma Standard Label O SiO2 Al Al2O3 Si SiO2 Cl NaCl Ti Ti Cr Cr Fe Fe Ni Ni Total:

56 Figure 24. EDS layer for 21% Forge post fatigue test Table 6. Elemental Composition of 21% forge post SCC region Element Apparent Concentration k Ratio Wt% Wt% Sigma Standard Label C C Vit F CaF2 Al Al2O3 Si SiO2 Ti Ti Cr Cr Fe Fe Ni Ni Total:

57 SCC and post fatigue test reign Color difference observed on Figure 21 and Figure 22 shows the region of different test condition due to their concentration of oxide layer. The oxide presence in fatigue test (SCC) region is much higher than the oxide in post fatigue test region. This is because the test specimen was exposed to extreme environment during SCC test and oxide formation was expected during the process which can be observed on Figure 27 and Figure 28. The composition of individual element shown on Figure 29 and Figure 30 and Table 6 and 7 also support expected result. The amount of oxide in SCC region is more than post fatigue test region because, in a post fatigue region the specimen was broken open and there was no exposure to extreme environment to produce oxide layer. The observation of lower oxide presence in figure 23 and figure 24 was supported using map spectrum and composition on figure 25 and figure 26 and elemental composition on table 4 and 5, which indicates the composition and spectrum of individual element are very higher in post fatigue region. 42

58 Figure 25. Map spectrum & Composition for 20% Post-fatigue test Figure 26. Map spectrum & Composition for 21% Post-fatigue test 43

59 Figure 27. EDS layer for 20% Forge SCC region Table 7. Elemental Composition of 20% forge post SCC region Element Apparent Concentration k Ratio Wt% Wt% Sigma Standard Label O SiO2 Si SiO2 Ti Ti Cr Cr Fe Fe Ni Ni Total:

60 Figure 28. EDS layer for 21% Forge SCC region Table 8. Elemental Composition of 21% forge post SCC region Element Apparent Concentration k Ratio Wt% Wt% Sigma Standard Label C C Vit F CaF2 Al Al2O3 Si SiO2 Ti Ti Cr Cr Fe Fe Ni Ni Total:

61 Figure 29. Map spectrum & Composition for 20% SCC region Figure 30. Map spectrum & Composition for 21% SCC region 46

62 SCC and pre fatigue test reign Figure shows the EDS mappings and elemental spectrum for SCC region and pre fatigue region. From this observation the color similarity in Figure 31 and Figure 32 and closer values shown on the elemental compositions in Figure 35 and Figure 36 and table 8 and 9 indicate both regions have been exposed to similar environment which create oxide layer. Although the crack propagation and presence of different fractured surface on SCC region is high, it does not affect the formation of oxide. It was suggested that the presence of oxide with other environmental condition will facilitate the trend in crack orientation in SCC region but it did not have bigger effect on change in chemical composition. The oxide film formed during high temperature steam and high pressure steam on alloy 690 is composed of hydroxide and oxide layers [5]. The formation of oxide film in high temperature water is almost the same in the presence of applied stress or without external force. The only difference is the crack formation was observed in the presence of stress in SCC region and no visible crack formation was observed in a pre-fatigue test region, suggesting crack formation did not have any influence in formation of oxide. 47

63 Figure 31. EDS layer for 20% Forge SCC region Table 9. Elemental Composition of 20% forge machined region Element Apparent Concentration k Ratio Wt% Wt% Sigma Standard Label Al Al2O3 Si SiO2 Ti Ti Cr Cr Fe Fe Ni Ni Total:

64 Figure 32. EDS layer for 21% Forge SCC region Figure 31 and 32 shows that x-ray diffraction of the oxide film formed on alloy 690, also table 8 and 9 show higher value of Fe in a precracked surface. According to FESEM result in both cases, higher percentage iron oxide is presented in pre-cracked machined surface. The presence of Fe in machined part of alloy 690 indicates that formation of iron oxide due to the presence of deaerated environment and high temperature. There was no visible crack formation observed in a pre-test region but very aggressive horizontal fractured surface are observed at the transition between testing 49

65 region and pre-testing region. This crack formation is assumed to be created due to applied tensile stress used to initiate air fatigue pre-crack. One can conclude that the presence of oxide in pre cracking region has nothing to do with SCC formation. Figure 33. EDS layer for 20% Forge Machined Region 50

66 Figure 34. EDS layer for 21% Forge Machined Region Table 10. Elemental Composition of 21% forge machined region Element Apparent Concentration k Ratio Wt% Wt% Sigma Standard Label C C Vit O SiO2 Al Al2O3 Si SiO2 Ti Ti Cr Cr Fe Fe Ni Ni 51

67 Figure 35. Map spectrum & Composition for 20% Machined part Figure 36. Map spectrum & Composition for 21% Machined part 52

68 Oxide layer and SCC growth Knowledge of the oxidation process of Ni base alloys in PWR has major importance for several practical reasons. The oxidation process is at the origin of the initiation of inter granular stress corrosion in alloy 600. Understanding the mechanisms cause of this cracking should also allow the safety margins offered by alloy 690 that have replaced the former one to be evaluated more accurately [5]. It is generally recognized that growth of corrosion cracking is strongly related to the protective properties of passive films formed on the metal surface [20]. Therefore understanding of the composition and structure of the passive films formed on alloy 690 can provide important information on its corrosion resistance in high temperature aqueous environments. The formation of oxide layer on alloy 690 was observed using EDS at higher resolution. Figure shows the presences of high concentration oxide on both 20% forge and 21% forge specimens. It was observed that Chromium, Nickel and oxygen are the major element in scale on both specimens which shows oxides of chromium and nickel are present. The presence of higher crack growth in 21% cold work than 20% are observed, which might be attributed to its higher strength on 21% cold work. It is well known that higher strength materials are typically more susceptible to SCC than comparable softer materials [16]. Yield strength and hardness of 21% forge alloy are larger than 20% forge alloy. The presence of oxide in 21% forge alloy will facilitate the susceptibility of material 53

69 to SCC because the oxide particle will stuck in to existing crack which is created due to cold work and increase the rate of crack growth as shown on figure 4. Among the features of oxidation in PWR presented above it is clear that the penetration of oxygen into grain boundaries, the kinetic movement of molecules in a base metal, and selective oxidation of Cr will play a significant role in crack initiation. Figure 37. Magnified EDS layer for SCC 20% forge 54

70 Table 11. Elemental Composition of magnified 20% forge machined region Element Apparent Concentration k Ratio Wt% Wt% Sigma Standard Label O SiO2 Si SiO2 Ti Ti Cr Cr Fe Fe Ni Ni Total: 100 Figure 38. Magnified EDS layer for SCC 21% forge 55

71 Table 12. Elemental Composition of magnified 21% forge machined region Element Apparent Concentration k Ratio Wt% Wt% Sigma Standard Label O SiO2 Si SiO2 Cr Cr Fe Fe Ni Ni Total: 100 Figure 39. Zoomed in EDS layer for SCC 20% forge 56

72 Figure 40. Zoomed in EDS layer for SCC 21% forge 57