## 3 Results
### Polychromatic Imaging  
The CONRAD beamline provides a cold neutron spectrum, which has a maximum intensity peak at around 0.25 nm [18]. This resulting high neutron flux allows us to observe the martensite-austenite phase transformation with very short exposure times and provides a much better signal-to-noise ratio, i.e., a much better image contrast compared to monochromatic imaging.  
The same welding parameters as for the abovementioned measurements with monochromatic neutrons were used for the polychromatic imaging experiment. The exposure time could be as low as 0.5 s, allowing a four times higher image acquisition rate. Figure 5 shows the transmitted intensity for an equal ROI as used for monochromatic imaging. A sheathed type K thermocouple, inserted in a bore hole close to the welded surface, allowed the measurement of the temperature.  
Figure 4: Plot of the transmitted intensity at \(\lambda\) = 0.44 nm taken as mean value from the ROI (yellow rectangle) as a function of time. The insert shows the monochromatic image taken at t \(\approx\) 130 s. The very low neutron intensity results in a poor image quality, showing essentially no visual image contrast in the sample.  
The intensity dip of approx. 4% relatively at t \(\approx\) 25 s can be attributed again to the increasing sample temperature due to the traveling welding torch and the associated change of the Debye-Waller factor. The following intensity increase starting at t \(>\) 30 s is due to the change in transmission because of the evolving austenitic phase. The austenitic phase re-transforms to martensite between t \(\approx\) 102 s and t \(\approx\) 120 s, resulting in a drop in the transmitted neutron intensity by around 7%.  
Again, the welding nozzle can be seen twice due to the normalization of all images by dividing all images by the first image. The starting position is to the right (white nozzle), whereas the final position is to the left (black nozzle). The thermocouple was inserted in the upper right hole and had also moved slightly during welding, showing its initial position (white wire) in all images. The bright and dark streaks at the bottom and the top of the sample were also caused by the image normalization. The thermal expansion bended the sample upwards, and dividing all images by the image of the cold sample at t = 0 results in such streaks at the sample edges.  
The bright area in the upper part of the sample is the austenitic phase in the heat-affected zone (HAZ), which is still present, and which is in the process of re-transformation from austenite to martensite. The peak temperature reading, when the torch is passing by the inserted thermocouple, is larger than 1200 \({}^{\circ}\)C. Measurements with type K thermocouples above such temperatures are not possible. The temperature reading dropped down to around 600 \({}^{\circ}\)C by the end of the welding. Another 2.5 min later the M\({}_{\rm s}\) temperature is reached, and it takes another 7.5 min to reach M\({}_{\rm f}\).  
The increasing vertical dimension of the HAZ from the right to the left is due to the heat accumulation during welding. Additionally, after stopping the torch movement, the electric arc burns for another two seconds, introducing even more energy at this position. The heat protection of both sides of the metal plate restricts the heat dissipation mainly in the two dimensions perpendicular to the neutron flight direction, shaping the temperature profile in the sample.  
Figure 5: Plot of the transmitted intensity as a function of time taken as mean value from the ROI (yellow rectangle). The insert shows the normalized polychromatic image taken just before switching off the welding torch at the end position.