Feb/March_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 2 1 2 0 set by diffraction effects, which intro- duce small peaks into the spectra. Fur- thermore, the depth from which in- formation can be obtained from a material is an important factor to con- sider for elemental analysis. The pho- ton source of µ-XRF will have a higher depth of penetration than the electron source of SEM-EDS. Hence, the informa- tion depth is larger in µ-XRF for a given matrix material. This makes it possible to examine thicker layers such as coat- ings and multiple layered structures us- ing µ-XRF without destructive sample preparation. Because the penetration depth is larger, the interaction volume is much larger in µ-XRF than SEM-EDS and although it results in reduced spa- tial resolution, this provides a number of key advantages for large area com- positional mapping of materials. First, because the interaction volume is of- ten closely matched with the pixel size in µ-XRF, the whole area is analyzed, not just a central point of each area as in SEM-EDS. Secondly, the increased depth of penetration and interaction volume means the specimen prepara- tion requirements are much less strin- gent in µ-XRF than in SEM-EDS to the point where is it possible to analyze samples in µ-XRF with minimal prepa- ration/cleaning. Finally, because µ-XRF uses x-rays the technique does not suf- fer from charging effects. Samples may be analyzed with or without vacuum or stigmation effects, which can be signif- icant in large steel samples. All of these factors make µ-XRF a versatile tool for elemental quantification and mapping. At the Electric Power Research In- stitute’s (EPRI) laboratories in Charlotte, North Carolina, a benchtop Bruker M4 TornadoPlus µ-XRF unit with light ele- ment detection capabilities is routinely employed to study elemental distribu- tion in power plant components over large-area material cross sections. This unit uses a Rh x-ray source and two light element large-area silicon drift detec- tors to map the distribution of elements in a material. Apart from compositional analysis, results from such studies are also used to identify areas of interest for further in-depth electron microsco- py-based investigations. This technique has been used in several unique investi- gations, a few of which are presented in the following case studies. CASE STUDY 1: WELD PROCEDURE DEVELOPMENT FOR CLADDING A CARBON STEEL IN WET FLUE GAS DESULFURIZATION SYSTEM There are multiple examples of applications across different industries where a layer of alloy is deposited on a substrate material using a welding pro- cess, either to provide a protective layer of coating in corrosive environments or to enhance the wear/erosion resistance of a component. Common examples in the power generation industry include nickel alloy-based weld overlays on dis- similarmetal welds (DMWs) and pipes to prevent stress corrosion cracking in nu- clear power plants, cladding of carbon- steel pipes using stainless steel or nickel alloy-weld coating material to mitigate corrosion in wet flue gas de- sulfurization systems, or the deposition of cobalt-based alloys for wear resis- tance in valve mating surfaces or stems. In such overlays, there is always a con- cern of metallurgical issues arising from mixing of the substrate and over- lay material that occurs during weld- ing resulting in dilution. Inadequate awareness about the dilution zone char- acteristics could lead to costly failures during service. Well-documented in- dustry-wide failures were observed in valve components of fossil-fired pow- er plants due to the detrimental micro- structure formed in the dilution layer of cobalt-based hardfacing overlay welds made on mainstay power generation steels like 2¼Cr-1Mo (e.g., Grade 22) or 9Cr-1Mo-VNbN (e.g., Grade 91) [1] . A common practice for determin- ing the amount of dilution is to use bulk (area) SEM-EDS analysis to find the approximate amounts of major al- loying elements in the deposited weld as a means to determine the extent of the dilution zone. This would typically involve a sampling strategy where the composition is measured in a specified number of positions across multiple im- aging fields (the field of view in an SEM is typically in the few mm range). In the presented case study, the weld spec- imen is a corner (fillet) weld between the vertical wall and the horizontal base plate (both SA-516 Grade 70 material) of the absorber vessel of a wet flue-gas de- sulfurization unit. The corner weld was made using C-276 nickel alloy filler met- al and two layers of weld overlay were also deposited on the plates near the weld region as shown in Fig. 1. A single field µ-XRF scan across the entire weld joint was generated and the distribu- tion maps of major alloying elements (i.e., Fe, Cr, and Ni) are shown in Fig. 2. Fig. 2 — M4 Tornado µ -XRF elemental distribution maps of Fe, Ni, and Cr of the weld cross section shown in Fig. 1. Fig. 1 — SEM backscattered electron image montage showing the cross section of a corner joint weld of two SA-516 Grade 70 base plates along with two layers of weld overlays made using C-276 weld.