Parameters that control the solidification of castings also control the solidification and microstructure of welds. However, various physical processes that occur due to the interaction of the heat source with the metal during welding add a new dimension to the understanding of the weld pool solidification. In certain cases, because of rapid cooling rate effects, it is not unusual to observe non-equilibrium microstructures (Figure 1). Along with the formation of microstructures chemical heterogeneity can occur, which can lead to local mechanical or corrosion failures. For troubleshooting investigation, it is therefore important to understand such heterogeneity and micrographic analysis is not always sufficient.
Figure 1. A detail of microstructures typically occurring in weld samples observed with a scanning electron microscope (SEM)
A common technique usually employed is scanning electron microscopy (SEM) equipped for energy dispersive spectroscopy (EDS) analysis. However, this technique requires extensive sample preparation, long measurement times and can only be performed on small scale. X-ray fluorescence spectrometry enabling small spot analysis and elemental distribution mapping represents a solution for this type of analysis allowing both bulk and small spot analyses.
The present study demonstrates the capability of practical, fast small spot analysis and elemental distribution mapping for the characterization of weld samples.
Instrumentation and software
Measurements were performed using a Zetium XRF spectrometer configured with a 4 kW Rh-anode SST R-mAX X-ray tube and the WD core for WDXRF analysis. To enable the spot analysis and mapping capability, the spectrometer was equipped with an ED core, high-precision translation mechanics for sample positioning and a high-resolution camera for sample imaging. Using the ED core offers the advantage of performing simultaneous analysis for all elements present, whereas using the WD core would imply already knowing which elements are present and repeating each spot analysis for the number of elements needed to be mapped.
All measurements were conducted using the state-of-the-art SuperQ software package.
A weld sample mounted in epoxy (Figure 2) was placed into a special mapping sample holder and imaged using a high- resolution camera. The sample was then loaded into the measurement position using a special turret mechanism and analyzed using SuperQ software.
Figure 2. Weld sample mounted in epoxy for in situ characterization. The sample consists of three distinguished areas: FT (ferritic), ZF (fused zone) and AS (austenitic)
In order to map the elemental distribution of a small area, a conventional calibration for small spot (0.5 mm) was set up using 9 standards, including six NiFeCo setup samples and two standards from the ECRM series (298/1 and 289-1). The sample consists of two compositionally different types of steel weld together (Figure 2). One side presents a ferritic composition (17,5 % Cr - 0,5 % Nb, described as FT), the other side is the austenitic type 304L (17,5 % Cr - 8,0 % Ni, described as AS). The weld area (described as ZF, fused zone) is of intermediate composition between FT, AS and a third type of metal used as a solder (Type 316LSi, Mo enriched) containing Mo, Cr, Ni and Nb. The weld sample was measured against the calibration using a step size of 250 µm and analyzed in 1142 spots.
Compositional images reported in Figure 3 show the variations in elemental distribution between the three different regions of the sample (namely FT, ZF and AS). The elemental distribution mapping can help to identify and localize the depletion of specific elements, which can be ultimately responsible for the occurance of corrosion at a certain stage of the product lifecycle (i.e. variations in Mo content). Moreover, it can also be used to identify microstructures where micrographic analyses do not provide clear evidence.
Figure 3. 2D contour plots for Fe, Nb, Ni, Mo and Mn in a weld sample and high-resolution image indicating sample area mapped
The results clearly demonstrate that the integration of EDXRF (equipped to perform small spot analysis and mapping) in the Zetium spectrometer delivers fast and accurate analysis of a weld sample on a small scale (0.5 mm) for troubleshooting purposes. The possibility to perform small spot analysis and mapping using the ED core enables fast simultaneous analysis of all elements present without compromising the analytical performance of the WDXRF optics. Furthermore it is considerably faster than using a conventional WDXRF, where each element present needs to be measured sequentially. The inclusion of the ED core has other advantages, such as fast sample screening, with or without Omnian, and identification and flagging of unexpected elements in process control.