## 2 Materials and Methods  
The experiments were carried out at the Cold Neutron RADiography 2 (CONRAD 2) instrument located at the research reactor BER II of the Helmholtz-Zentrum Berlin fur Materialien und Energie [10]. The imaging set-up consists of the sample holder system and the welding torch located approx. 10 cm in front of a 200 \(\upmu\)m thick \({}^{6}\)LiF:ZnS scintillator. The scintillator converts the transmitted neutrons into visible light and a mirror reflects the visible light image of the sample out of the neutron beam and towards a high-sensitive CCD camera with magnifying optics. A double-crystal monochromator allows the adjustment of the neutron wavelength in the range between 0.2 nm and 0.6 nm with a resolution of \(\Delta\lambda/\lambda\approx 3\%\). More technical information regarding the CONRAD 2 instrument can be found elsewhere [11]. The material used was LTT martensitic steel. The sample sheets were made by build-up welding using LTT steel wire and subsequent machining to achieve plates of the required dimensions. The sample dimensions in length, width, height are 100 mm, 5 mm, 13 mm, where the width of 5 mm was the penetration length for the neutrons. The microstructure consists mainly of cellular martensite in rows because of the build-up welding process. The primary solidification state of the LTT steel was austenitic. Micrographs showed that some retained austenite can be found as a fine network in the microstructure due to segregation effects of Ni and Cr. The martensite transformation temperature for this LTT steel upon cooling is \(\mathrm{M_{s}}\approx 250\)\({}^{\circ}\)C and the transformation is finalized at around \(\mathrm{M_{f}}\approx 80\)\({}^{\circ}\)C. The chemical composition of the LTT steel is given in Table 1. The composition was measured by spark emission spectroscopy with a relative measurement error of \(\Delta\mathrm{c/c}\approx 10\%\). A type K thermocouple covered by InconelTM sheathing was positioned in the sample side 2 mm below the surface and 20 mm behind the starting point of welding (see Figure 1). The temperature was monitored during the complete welding process. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**C** & **Ni** & **Cr** & **Si** & **Mn** & **Mo** & **Fe** \\ \hline
0.054 & 10.2 & 10.3 & 0.55 & 0.90 & 0.28 & Bal. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the LTT steel in wt.-%. The scintillator in the background is also protected against weld spatter by alumina felt, which is almost transparent for neutrons. Welding direction is from right to left. Gas tungsten arc welding (GTAW) was used to perform remote-controlled bead-on-plate welds with argon as a shielding gas. The arc welding power source used was a Castolin CastoTIG 1611 DC. The welding length for each parameter set was 45 mm, which allowed for four weldments per sample (two on each side). The set-up is shown in Figure 1. The used welding parameters together with the respective imaging parameters are listed in Table 2. The energy input could be controlled by varying both the welding current and the welding velocity. The welding experiments were performed twice, each time with a different neutron wavelength as shown in Table 2. To obtain good contrast between the austenitic and martensitic crystallographic phases, a neutron wavelength of \(\lambda\) = 0.395 nm was used. This wavelength has been defined by performing a wavelength scan using the original set-up. Although the monochromatic imaging allowed the recording of the phase transformation step by step, the needed exposure time of t = 2 s to gain a reasonable signal-to-noise ratio allowed only for a few frames per weldment. To prolong the cooling state and thus gain more images during the phase transformation of austenite to martensite, the cooling rate was reduced by thermal isolation of the samples.