unifuse overlay cladding for surface …ommi.co.uk/pdf/articles/98.pdf · unifuse® overlay...

Overlay Cladding in Power Boilers OMMI (Vol.3, Issue 3) December 2004 www.ommi.co.uk UNIFUSE® OVERLAY CLADDING FOR SURFACE PROTECTION AGAINST CORROSION AND EROSION/CORROSION IN POWER BOILERS AND WASTE HEAT RECOVERY SYSTEMS George Y. Lai, Consultant to Welding Services Inc., 2110 Pebble Beach Drive, Suite 100, Carmel, Indiana 46032, USA; Neil Blogg, Welding Services BV, Hammerain House, Suite 12, Hookstone Avenue, Harrogate, North Yorkshire HG2 8ER, UK ABSTRACT Heat exchanger tubes in power boilers and waste heat recovery systems are typically made of ferritic steels. Ferritic steels provide strength and structural integrity for the system as well as excellent resistance to high temperature, pressure water or steam. However, the fireside of the ferritic steel tube components in these systems can suffer significantly higher metal wastage rates on the tube wall due to high temperature corrosion and erosion/corrosion attack from the hot flue gas stream. Without adequate surface protection, the plant can experience frequent shutdowns and excessive maintenance costs. A bi-metallic approach utilizing Unifuse overlay cladding technology employing the automatic gas-metal-arc welding (GMAW) process was developed to deal with fireside corrosion and erosion/corrosion problems for power boilers and many waste heat recovery systems. The technology has been successfully applied to protect boilers and various waste heat recovery systems for more than a decade. A wide range of overlay alloys have been successfully applied to provide protection against high-temperature corrosion and erosion/corrosion attack in waste-to-energy, coal-fired and black-liquor recovery boilers, and waste heat recovery systems such as basic oxygen furnace (BOF) hoods and electric arc furnace (EAF) hoods. Application techniques, selection of overlay alloys, the properties of the weld overlay products, major corrosion modes, and the success of the overlays in different systems are highlighted. INTRODUCTION The heat transfer tubes in boilers and many waste heat recovery systems are typically made of ferritic steels, such as carbon and Cr-Mo steels. Ferritic steels not only provide strength and structural integrity for these tubes but also excellent resistance to high temperature, pressure water and steam used in the heat transfer. However, the ferritic steels, in many cases, do not provide adequate resistance to high temperature corrosion attack from the combustion products in boilers or from the hot, exhaust flue gas stream in waste heat recovery systems. Weld overlay cladding with a high-temperature corrosion-resistant alloy, using automatic overlay welding machines, has been widely accepted to be a long-term, cost-effective protection method for Unifuse is a registered trademark of Welding Services Inc.

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Overlay Cladding in Power Boilers OMMI (Vol.3, Issue 3) December 2004 2

providing life extension for boilers and waste heat recovery systems. Technical discussion on the application and the use of automatic weld overlay cladding technology for providing corrosion and erosion/corrosion protection for boilers have been published elsewhere.(1-7) The present paper reviews briefly the automatic overlay welding technology and highlights the applications of weld overlay protection for power boilers; including waste-to-energy boilers, coal-fired boilers and black liquor recovery boilers, and for some waste heat recovery systems in steelmaking, such as basic oxygen furnace (BOF) hoods and electric arc furnace (EAF) hoods. AUTOMATIC WELD OVERLAY CLADDING In a power boiler, the waterwalls made of tubes and membranes (in few cases, just tubes with no membranes – a tangent tube design) are constructed around the furnace enclosure. High-pressure water flows inside these tubes to extract heat from the high temperature combustion gases on the fireside (outer diameter) of the tubes. The waterwalls, made of carbon or Cr-Mo steels, are subject to high heat flux as well as high temperature corrosion and particulate erosion/corrosion attack. BOF and EAF hoods in steelmaking are also of a tube-membrane construction similar to the boiler’s waterwall. Three application approaches have been successfully used to provide high temperature corrosion and erosion/corrosion protection for boilers and BOF/EAF hoods employing automatic weld overlay cladding technology. These three application approaches are (a) field application performed inside the boiler or hood to apply a high-temperature corrosion-resistant weld overlay on the waterwall area where tube wall thinning has taken place, (b) shop weld overlay application on panels, which are then installed in the boiler or the hood to replace the worn panels, and (c) use of spiral overlay tubes produced by a patented process involving GMAW/GTAW process for the construction of waterwall panels or hood panels. These three application approaches are briefly described below. Field Application of Weld Overlay Cladding Field application of a high-temperature corrosion and erosion-corrosion resistant alloy overlay has accounted for the majority of overlay applications for furnace waterwalls. The automatic gas-metal-arc welding (GMAW) overlay machine deposits weld beads in a vertical down mode starting typically from the membrane and then moving to the tube section following a preprogrammed weld bead sequence to achieve a uniform coverage of the waterwall (i.e., membranes and tubes). Each weld bead is overlapped by subsequent weld bead to ensure a full coverage with no missing spots. A schematic of this process is illustrated in Figure 1. The thickness of the overlay applied to the waterwall is typically 1.78 mm (0.070”) minimum. During overlay welding, a surface layer of the substrate steel is melted and mixed with the molten weld metal of a corrosion-resistant alloy to produce a weld overlay cladding. As a result, the concentrations of the important alloying elements in the weld overlay, such as Cr, will be slightly lower than those of the weld wire due to this dilution effect. In order to maximize corrosion and erosion/corrosion protection, the dilution is typically kept at less than 10% for Unifuse overlays. The weld overlays, performed under WSI’s QA/QC programs, meet ASME Codes as well as similar codes and standards in Europe.


It is a common practice to use many welding machines, for example, 10 or more, at the same time in the boiler to complete the project during a maintenance shutdown. A modern machine can deliver a welding “speed” of approximately 0.14 to 0.19 m2 (1.5 to 2.0 ft2) per hour. Thus, with 10 machines, a total area of about 16.7 to 22.3 m2 (180 to 240 ft2) of the waterwall can be overlaid over a 12-h shift. Figure 2 shows a welder performing waterwall overlay cladding of a coal-fired boiler, and Figure 3 shows the overlay welding in progress in a BOF hood. Shop Fabrication of Weld Overlaid Panels When the thickness of the badly corroded waterwall is reduced to below the ASME Code allowable, a weld metal build-up using a matching filler metal to that of the tube material can be performed to increase the wall thickness to meet the ASME Code before applying a high-temperature corrosion-resistant overlay. Alternatively, the badly corroded waterwall areas can be replaced with waterwall panels overlaid with a high-temperature corrosion-resistant overlay in shop. The overlay procedures in shop are essentially the same as those applied in the field. The weld overlays, performed under WSI’s QA/QC programs, meet ASME Codes as well as similar codes and standards in Europe. Figure 4 shows a welder performing panel overlay in shop. Unifuse Overlay Tubing The panel can also be constructed from individual ferritic steel tubes, which are weld overlaid in a spiral mode to produce individual tubes with a high-temperature corrosion-resistant overlay 360° around the tube. A patented process, developed by Welding Services Inc., involves gas-metal-arc welding (GMAW) for depositing a uniform layer of a corrosion-resistant overlay, which is then followed by gas-tungsten-arc welding (GTAW) to smooth the overlay surface and temper the heat-affected-zone (HAZ) formed in the substrate steel during GMAW process. As a result, the overlay tube exhibits excellent ductility and toughness in the as-overlaid condition. Figure 5 shows Unifuse overlay tubes. In addition to making panels, the majority of the overlay tubes are being used as screen tubes, superheater/reheater tubes, and generating bank tubes in the upper furnace of the boilers. PROPERTIES AND CHARACTERISTICS OF OVERLAY CLADDING One of the important requirements for application of the overlay cladding, in order to achieve the maximum high-temperature corrosion protection, is to maintain the lowest possible dilution for the overlay chemistry. Unifuse overlays typically exhibit less than 10% dilution, with some overlays achieving 5% or less. In addition, the major alloying elements, such as Cr, in the overlay are maintained at a relatively constant level through the thickness of the overlay in order to maintain the protection throughout the life of the overlay. This is illustrated in Figure 6. The Fe concentration profile is shown in Figure 7. Keeping the iron content low in alloy 625 overlay is desirable in certain applications, such as WTE boilers. Excessive dilution in overlay welding that results in 15 or 20% iron content in alloy 625 overlay is known to significantly reduce the overlay’s resistance to high temperature chloride attack in WTE combustion environments.


It is also important to ensure that overlay welding application does not result in embrittlement or degradation in the mechanical properties of the overlay product. Table 1 shows the tensile properties of Unifuse 625 weld overlay metal in the as-overlaid condition. The weld overlay was found to exhibit excellent ductility. Furthermore, the mechanical properties of the weld overlay/substrate steel combined product were found to be excellent, as illustrated in Table 2. The tensile properties of the Unifuse 625/SA210 A1 overlay tube samples in the as-overlaid condition were compared with SA210 A1 steel tube samples (Table 2). The test results indicate that the weld overlay not only does not cause mechanical property degradation, but, in fact, provides strengthening to the ferritic steel tube. Similar results were obtained in other weld overlay alloy products. This strengthening of the boiler tube, provided by the weld overlay, helps increase its resistance to overheating and to falling clinkers. The excellent ductility of the Unifuse overlay tube is also illustrated in Figure 8, which shows that the overlay tube in the as-overlaid condition successfully passed the 1T (one tube wall thickness) flattening test. Other issues related to the characteristics and properties of the weld overlay products have been discussed in earlier publications.(1-5) WASTE-TO-ENERGY (WTE) BOILERS Municipal waste typically contains plastic materials, textile, leathers, batteries, food waste, and other miscellaneous materials. These constituents are sources of chlorine, sulphur, sodium, potassium, zinc, lead, and other heavy metals that form corrosive vapors of various chlorides and sulphates during combustion. These chloride and sulphate vapors, along with fly ash, condense and deposit on the cooler surfaces, such as waterwalls, which surround the combustion zone, and heat exchanger surfaces in the convection path, such as screen tubes, superheater tubes and generating banks. These metallic components are subjected to accelerated chloride corrosion because many chlorides exhibit high vapor pressures and low melting points with some as low as 200°C. Wastage rates of 1.3-2.0 mm/yr (50-80 mpy) or higher have been observed for ferritic steel waterwalls, and of 2.5 mm/yr (100 mpy) and higher have been observed for ferritic steel superheater tubes. Flame impingement, higher gas temperatures and flow velocities, and higher concentrations of chlorine and heavy metals in the feed, along with other factors, will further contribute to increased wastage rates. In 1984, WSI pioneered a field overlay technology by using automatic GMAW overlay machines applying a corrosion-resistant alloy to the waterwall on site in a waste-to-energy boiler in Lawrence, Massachusetts. The alloy selected for the overlay was alloy 625 (Ni-22Cr-9Mo-3.5Nb). The overlay had proved to be so successful in its performance in the Lawrence boiler that approximately 126,000 kg (280,000 lbs) of alloy 625 weld overlay metal had been applied by WSI from 1985 to 1990 for about 30 boilers.(1) Today, automatic GMAW applied overlay of alloy 625 has become a “standard” protection cladding for waterwalls, against corrosion or erosion/corrosion in most waste-to-energy boilers in the US. The alloy 625 overlay has proved to be successful in performance in both mass burning (MB) and refuse-derived fuel (RDF) units. In Europe, this weld overlay technology has


been increasingly accepted as a reliable, long-term solution to the waterwall wastage problems for waste-to-energy boilers. Alloy 625 overlay has been applied to many boilers in the Netherlands, UK, France, Italy and other countries. Many overlaid waterwalls have been in service for a decade or longer. Figure 9(a) shows an overlaid waterwall after 10 years of service revealing no visible corrosion damage on the alloy 625 overlay. Figure 9(b) is an alloy 625 overlay that required minimal repair after 10 years of service. The repair typically consists of grinding the corroded overlay surface followed by re-application of alloy 625 overlay. Figure 10 shows a waterwall tube sample obtained from the alloy 625 overlaid waterwall of a WTE boiler after 16 years of service. Figure 11 shows the full cross-section of the overlay at the crown bead for the sample shown in Figure 10, indicating little or no corrosion. Prior to the application of the 625 overlay, the waterwall of this boiler was corroded through in a matter of months. Unifuse overlay tubes with alloy 625 overlay have also been successfully used in screen tubes, superheaters and generating bank, replacing problematic tube protection methods using stainless steel tube shields or refractories. An example showing excellent performance of Unifuse 625 overlay superheater tubes in a WTE boiler in Europe is illustrated in Figure 12. No evidence of corrosion attack after 4½ years of service in a superheater producing 405°C (761°F)/42 bars (609 psi) superheated steam. BIOMASS FIRED BOILERS Superheaters made of carbon steels or Cr-Mo steels in boilers that burn wood chips, construction wood waste, and hog fuel (i.e. bark and other wood waste materials) can suffer severe wastage problems due to chloride attack. This is due to a mixture of plastic materials in the wood waste materials or to bark from logs that were previously soaked in sea water, at the pulp mills. An example is given here of a boiler burning hog fuel for generation of electricity and process steam in a pulp mill. The superheater, made of 2.25Cr-1Mo steel (T22), had suffered severe high temperature chloride corrosion, as shown in Figure 13. To solve this superheater corrosion problem, the superheater tube bundle was replaced with Unifuse 625/CS overlay tubes, as shown in Figure 14. COAL-FIRED BOILERS

High-temperature corrosion of waterwalls made of ferritic steel is generally a form of oxidation. However, for the last decade the industry has been reducing the boiler’s harmful emissions of nitrogen oxides (NOx) by installing low NOx combustion systems. This has resulted in an oxygen-poor, reducing environment in the lower furnace, thus suppressing NOx formation. Complete combustion is achieved at upper levels in the furnace. As a result, waterwall corrosion has changed from oxidation to sulphidation, thus resulting in a significant increase in tube wall wastage rates. Wastage rates in many boilers have increased from 0.25 mm/y (10 mpy) or less to up to 2.5 mm/y (100 mpy) or more.


Automatic GMAW overlay has been considered by the industry to be the most viable long-term solution to the waterwall wastage problems in coal-fired boilers. Common overlay alloys include Type 309 SS (Fe-24Cr-13Ni), alloy 625 (Ni-22Cr-9Mo-3.5Nb) and alloy 622 (Ni-22Cr-13Mo-3W). Up to 2001, approximately 21,000 m2 (227,000 ft2) of waterwalls in more than 100 boilers have been overlaid by WSI, with roughly equal amounts of 309 overlay and 625/622 overlays. Figure 15 shows the approximate total area of waterwalls overlaid for three overlay alloys as a function of year applied by WSI. With some of the overlays approaching 10 years of service, overall performance of these overlays has been excellent in boilers (both subcritical and supercritical units) of major designs (e.g., CE, B&W, Foster Wheeler, Riley Stoker). BLACK LIQUOR RECOVERY BOILERS The recovery boiler in the pulp and paper mill is used to recover liquor, discharged from digesters and followed by a series of evaporators to increase its concentrations. This black liquor generally contains about 50% or more of organic solids and approximately 6% total sulphur in the form of Na2SO4 and Na2S2O3, along with some NaCl.(8) Combustion takes place under reducing conditions, transforming sodium sulphate to sodium sulphide and also forming sodium carbonate. Organic compounds are burned to generate heat for producing steam for various processing applications within the mill. Inorganic solids, such as sodium sulphide, sodium hydroxide, sodium carbonate, sodium sulphate, thiosulphate, sodium chloride and others, are melted in the furnace bed. This molten smelt is then discharged from the furnace bed through smelt spouts. The furnace is constructed with waterwall tubes in the furnace floor as well as on the sidewall. The furnace floor tubes are covered with a layer of solidified smelt, which protects the tubes quite well from molten smelt attack. However, corrosion of carbon steel floor tubes can still be a problem. As a result, co-extruded composite tubes with type 304L as an outer diameter cladding have been used for floor tubes. In recent years, however, many of these composite floor tubes have suffered severe cracking problems. Singbeil, et al(9) provided an excellent review on the cracking of the 304L cladding, concluding that the mechanism of cracking to be a form of environmentally assisted cracking. Alloy 625, known to be highly resistant to environmentally assisted cracking and to thermal fatigue cracking, is a potentially excellent candidate overlay alloy for floor tube applications. Several mills in the U.S. and Canada have used and tested Unifuse 625 overlay tubes for furnace floors since 1995 with excellent performance. Unifuse 625 overlay tubes have also been used successfully for smelt openings. The alloy also works well as a weld overlay for the water-cooled smelt spouts. For the waterwall above the smelt bed, corrosion is primarily due to sulphidation.(10) Wastage rates of carbon steel tubes have been reported to be 0.2-0.8 mm/yr (8-32 mpy).(11) Field application of type 309 SS overlay using automatic overlay machines has been performed in several boilers with great success. One boiler has had type 309 SS overlaid waterwall in service for up to 16 years, so far, with no reports of materials problems. This successful application is briefly described below.


A recovery boiler in a Midwest mill in the US, producing superheated steam of 8.79 MPa (1275 psi) and 482 C (900 F), had experienced severe corrosion problems for the lower furnace wall above the floor bed since its commission in 1968. The lower furnace of the boiler, from the furnace floor up to the 2.1 meters (7 ft) weld line above the primary air ports, was constructed of studded carbon steel tubes. The waterwall was of tangent tube construction. The furnace floor (a decanting design) was protected by refractories. After about 4½ years of operation, the rear wall just above the studded section and up to the liquor gun elevation had suffered severe tube thinning due to corrosion, and had to be replaced with new panels. A year later, the pin studs started to suffer severe corrosion also. Various protection methods including pad welds with a stainless steel, thermal (flame and plasma) sprayed coatings and other protection methods had been tried without success until 1987 when weld overlay applied by automatic overlay machines was installed. During the trial of plasma sprayed coatings, severe sulphidation attack was found to have taken place on the carbon steel tubes underneath the coating. Spallation of the sprayed coatings was another major problem encountered for thermal sprayed coatings. In 1987, WSI was contracted to apply type 309 overlay on site using automatic GMAW overlay welding machines. The overlay was applied on the rear wall of the boiler from the furnace floor below the smelt line up to about 2.1 meters (7 ft) in height including smelt openings and primary air ports. The overlay proved to be so successful that in 1989 the front wall was also overlaid from the floor bed up to about 2.1 meters (7 ft). And in 1992, the overlay was extended to 9 meters (30 ft) in height for both rear and front walls. In July 2000, all overlays were examined. The overlays, applied in 1987, 1989 and 1992, exhibited excellent condition. No cracking was observed on all the areas inspected. Figure 16 shows an overlaid waterwall panel sample on the rear wall removed from the boiler for metallurgical examination after 13 years of boiler operation, revealing no corrosion attack and no cracking. Also shown in Figure 16 is the location of the overlaid waterwall where the panel sample was obtained. Prior to the application of Unifuse overlay, the bare carbon steel rear wall had to be replaced by new panels every 4-5 years. Room temperature tensile tests were also performed to assess the mechanical integrity of the weld overlay tube. The overlay tensile blanks were obtained from the crown bead location, while the bare steel tensile blanks were obtained from the backside of the steel tube. The tensile test results are summarized in Table 3. No degradation in the mechanical properties of the waterwall was observed after 13 years of service. The overlay on the primary air ports and smelt openings was generally in good condition. Unifuse 309 overlay has provided excellent protection for the lower furnace wall from the floor bed up to the liquor gun elevation, including primary air port openings and smelt openings. The overlay was intact and was in good condition with no sign of mechanical degradation after service for up to 13 years. Superheaters in recovery boilers are also subject to high temperature corrosion. Unifuse 310 overlay tubes, manufactured by the patented GMAW/GTAW process, are a cost-effective, long-term solution to this superheater corrosion problem. The performance of Unifuse 310 overlay tubes in black liquor recovery boilers has been excellent. Recent success includes Unifuse 310 overlay tubes showing no sign of corrosion damage after two years of operation in a superheater platen where the previous material, T11 tubes, had been suffering wastage rates of up to 3.9 mm/yr (154 mpy).


BOF AND EAF HOODS IN STEELMAKING The basic oxygen furnace (BOF) hood is used to collect, direct and cool the furnace effluent gas stream from the steel melting campaign before the flue gas stream is cleaned and discharged to atmosphere. The hood, which is typically made of carbon steel and cooled by water, has to handle a hot effluent gas stream with temperatures up to 1650°C and, thus, is subject to severe erosion/corrosion attack. Due to recent improved practices, the furnace life has significantly improved. As a result, the hood has become a limiting factor in BOF campaign life and requires an innovative protection method to improve the availability and productivity of BOF shops and, thus, melting campaigns. Since 1998, many US steel mills have relied on Unifuse 625 overlay to provide protection for their BOF hoods against erosion/corrosion with great success. Prior to the use of this technology, many of these steel mills had used hardfacing coatings, such as tungsten carbide (WC) coatings, which frequently failed to provide adequate protection. One such example is give here. In a major steel mill in the Pittsburgh area, the BOF hood is made of carbon steel (SA192). Without protection, the hood suffered a wastage rate of about 50 mpy (1.3 mm/y). A side-by-side test was performed at a “J” bend area of the hood to compare the Unifuse 625 overlay with an HVOF (high velocity oxyfuel) tungsten carbide (WC) coating. After 8 months of testing, a panel containing both 625 overlay and HVOF coating was removed for examination. Figure 17 shows the removed test panel. The alloy 625 overlay on the middle three tubes along with the membranes revealed no sign of erosion/corrosion attack, with the original weld bead ripples being clearly visible. On the other hand, the tubes and the membranes on both sides of the overlaid section were coated with an HVOF tungsten carbide coating. The coating was essentially gone. The mill’s two hoods have since then been overlaid with alloy 625. In electric arc furnace (EAF) melting, the scrap used in the charge is often contaminated with both organic and inorganic compounds, such as oil, plastic, paint and others. Plastic is the major source of corrosive constituents (chlorides). Steels are particularly susceptible to high temperature chloride attack. With the high velocity flue gas stream containing particulates, erosion/corrosion plays a major role in the degradation process. Unifuse 625 overlay has also been very successful in mitigating the tube wall wastage problems in a number of major steel mills. Figure 18 shows a section of EAF hood awaiting overlay welding in shop. SUMMARY Unifuse overlays, using the automatic GMAW process, have been successfully applied to provide high-temperature corrosion and erosion/corrosion protection for boilers and waste heat recovery systems, including furnace off-gas systems such as BOF and EAF hoods in steelmaking. Depending on the combustion conditions in boilers and waste heat recovery systems, a wide variety of corrosion and erosion/corrosion resistant alloys, which include stainless steels and nickel-base alloys, have been used successfully. Applications and successful performance of different Unifuse overlays for the protection of power boilers, such as waste-to-


energy boilers, biomass-fired boilers, coal-fired boilers and black liquor recovery boilers are highlighted. Also highlighted are Unifuse overlays for protection of BOF and EAF hoods in steelmaking. REFERENCES

1. P. Hulsizer, Corrosion/91, Paper No. 246, NACE, Houston, TX, 1991. 2. G. Lai, M. Jirinec and P. Hulsizer, 1998 TAPPI Engineering Conference Proceedings, Book

2, p. 417, TAPPI Press, Atlanta, Georgia 3. G. Lai, P. Hulsizer and R. Lee, Presented at 1999 EPRI Fossil Plant Maintenance

Conference, June 21-23, 1999, Atlanta, Georgia 4. G. Lai and P. Hulsizer, Corrosion/2000, Paper No. 258, NACE, Houston, TX, 2000. 5. G. Lai and P. Hulsizer, Presented at TAPPI 2001 Engineering Conference, September 16-20,

2001, San Antonio, TX, and published in the Conference Proceedings, TAPPI Press, Atlanta, Georgia

6. W. T. Bakker, J. L. Blough, S. C. Kung, T. L. Banfield and P. Cunningham, Paper No. 2384, Corrosion 2002, NACE, Houston, Texas

7. G. Lai, Presented at EPRI Conference on Welding and Repair Technology for Power Plants, June 26-28, 2002, Point Clear, Alabama, and published in the Conference Proceedings, and also published in PowerPlant Chemistry 2002, 4(12), p.712.

8. Corrosion in the Recovery Boilers, in Metals Handbook, Ninth Edition, 13, Corrosion, ASM International, Materials Park, Ohio, (1987), p.1198

9. D. Singbeil, et al., Conference Proceedings, TAPPI Engineering & Papermaking Conference, TAPPI, Atlanta, Georgia (1997), p.1001

10. W. Sharp, Conference Proceedings, 7th International Symposium on Corrosion, TAPPI, Atlanta, Georgia, p.23

11. D. Bowers, Pulp and Paper, 61(7), (1987), p.118


Table 1. Tensile Properties of Unifuse 625 Weld Overlay Metal* in As-Overlaid Condition

Test Temp, °F (°C) UTS, ksi (MPa) 0.2% Offset YS, ksi (MPa) %EL

70 (21) 114.1 (787) 66.7 (460) 54 600 (316) 95.8 (661) 55.6 (383) 45 800 (427) 93.9 (647) 54.6 (377) 53 1000 (538) 89.1 (614) 52.2 (360) 48 1200 (649) 87.1 (601) 51.4 (354) 42

-------------- * Overlay metal was applied to carbon steel plate, which was subsequently machined off. Table 2. Tensile Properties of Unifuse 625/SA210A1 Overlay Tube Samples in As-Overlaid Condition in Comparison with SA210 A1 Carbon Steel Tube Samples Test Temp Material UTS, ksi (MPa) 0.2% YS, ksi (MPa) % EL RT SA 210 A1 68.8 (474) 47.7 (329) 42 625/SA 210 A1 91.1 (628) 64.0 (441) 38 600° F SA 210 A1 74.3 (512) 35.1 (242) 34 625/SA 210 A1 87.2 (601) 58.1 (401) 29 800° F SA 210 A1 60.0 (414) 30.5 (210) 37 625/SA 210 A1 81.6 (563) 55.5 (383) 35 1000° F SA 210 A1 37.0 (255) 21.9 (151) 59 625/SA 210 A1 70.1 (483) 51.3 (354) 33 1200° F SA 210 A1 19.4 (134) 14.4 (99) 94 625/SA 210 A1 46.3 (319) 27.5 (190) 57


Table 3. Room temperature tensile properties of Unifuse 309 overlay specimens and carbon steel specimens cut from the 309 overlaid waterwall panel sample removed from the boiler after 13 years of service in a black liquor recovery boiler. Carbon steel specimens were cut from the unoverlaid side (backside) of the tube Test Specimen UTS, psi (MPa) 0.2% YS, psi (MPa) % Elongation Carbon steel specimen 52,100 (359) 32,900 (227) 39.0 Carbon steel specimen 52,800 (364) 30,700 (212) 42.0 Overlaid specimen* 69,900 (482) 38,100 (263) 38.0 Overlaid specimen* 69,700 (481) 36,100 (249) 45.0 -------------------------- * Overlay + carbon steel substrate (specimens were cut from the overlay side of the tube).

Figure 1. Schematic illustrating weld overlay bead overlapping to cover the tube-membrane waterwall or hood construction in either field or shop applications


Figure 2. Automatic overlay welding in progress in field overlay inside a boiler

Figure 3. Automatic overlay welding in progress in field overlay inside a BOF hood


Figure 4. Automatic overlay welding in progress in shop fabrication of Unifuse overlay panel

Figure 5. Patented GMAW/GTAW Unifuse overlay tubes


Figure 6. Chromium concentration profile across Unifuse 625 overlay at the crown bead location on a ½Cr-½Mo steel (T2) waterwall panel. The overlay surface is on the right-hand side of the scale. Cr content was measured by SEM/EDX. Unit conversion: 0.02 in = 0.5 mm

Figure 7. Iron concentration profile across the 625 overlay at the crown bead location on a ½Cr-½Mo steel (T2) waterwall panel (same sample as Figure 6). The overlay surface is on the right-hand side of the scale. Fe content was maintained at less than 5% (by wt.) for more than 60 mils (1.5 mm) from the overlay surface. It was found to increase only slightly near the fusion boundary. Fe content was measured by SEM/EDX. Unit conversion: 0.02 in = 0.5 mm

0

5

10

15

20

25

0 0.02 0.04 0.06 0.08 0.1

Distance from Fusion Boundary (in)

Cr C

onte

nt (W

t. %

)

05

1015202530

0 0.02 0.04 0.06 0.08 0.1

Distance from Fusion Boundary (in)

Fe C

onte

nt (W

t. %

)


Figure 8. Unifuse 625/SA210 A1 overlay tube in the as-overlaid condition successfully passed 1T (one tube wall thickness) flattening test without cracking

(a) (b) Figure 9. (a) Unifuse 625 overlay showing excellent condition after 10 years of service in a waste-to-energy boiler. (b) In some cases, minor repair involving grinding the corroded area and re-application of 625 overlay may be needed, such as in this case after 10 years of service

Figure 10. Unifuse 625 overlaid waterwall sample obtained from a WTE boiler after 16 years of boiler operation


Figure 11. Optical photomicrograph showing the full cross-section of the overlay at the crown bead for the 625 overlaid waterwall tube sample, as shown in Figure 10, after 16 years of operation in a WTE boiler

Figure 12. Overview (left) and close-up view (right) of Unifuse 625 overlay tubes after 4½ years of service in a superheater bundle of a WTE boiler. Weld bead ripples were still clearly visible


Figure 13. Close-up view of the severely corroded T22 superheater tube in a biomass boiler burning hog fuel (bark and other wood waste materials) in a pulp mill

Figure 14. Installation of Unifuse 625 overlay tubes to combat high temperature corrosion attack in a boiler burning hog fuel, where T22 tubes suffered premature failure as shown in Figure 13


Figure 15. Approximate total overlay areas of waterwalls for Unifuse 309 SS, 625 and 622 overlays performed by Welding Services Inc. as a function of year applied in coal-fired utility boilers. (1 ft2 = 0.093 m2)

Figure 16. A Unifuse 309 overlaid waterwall panel sample (left) was cut from the overlaid waterwall at the location as shown in the photograph (right) after 13 years of service in a black liquor recovery boiler

0100002000030000400005000060000

Y93 Y94 Y95 Y96 Y97 Y98 Y99 Y00 Y01

Year Overlay Performed

Tota

l Ove

rlay

Are

a (ft

2)


Figure 17. Photograph showing a small test panel with HVOF tungsten carbide coating on both sides of the Unifuse 625 overlay (center) installed for testing in a BOF hood and removed for evaluation after 8 months of service in a major US steel mill. The HVOF coating was completely gone, while the overlay exhibited no sign of high temperature corrosion or erosion/corrosion attack.

Figure 18. A section of EAF duct awaiting Unifuse overlay welding with alloy 625 in shop

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