In Neutron-Bragg-Edge Imaging (NBEI) experiments, we studied the phase transition during butt-welding of martensitic Low Temperature Transformation (LTT) Steel [1]. Tungsten inert gas (TIG) welding was used with a moveable torch allowing for automated weldments. The austenitization in the heat affected zone (HAZ) underneath the welding head could be clearly visualized at λ = 0.39 nm, a wavelength smaller than the Bragg edge wavelengths of both austenite and martensite. Also, the re-transformation upon cooling from austenite into martensitic phase was detected. However, we observed an unexpected additional change in transmission at λ = 0.44 nm that is a wavelength larger than the wavelength of the Bragg edges of both the martensitic and austenitic phases. We attribute this change to the Deybe-Waller-Factor that describes the temperature dependence of coherent scattering at a crystal lattice [2]. The observed two-dimensional attenuation map corresponds well with a temperature distribution modelling by software macros in ANSYS [3]. Here, the absolute temperature values could be achieved by calibrating the modelled attenuation with help of a thermocouple placed at the steel sample plate. This allows in return for a direct two-dimensional temperature reading based on the Debye-Waller-relation between neutron attenuation and sample temperature.

Figure 1. Comparison between experiment and simulation of the temperature distribution during welding (GTAW) of LTT steel plates [2].

References

[1] A, Griesche; B, Pfretzschner; UA, Taparli; N, Kardjilov Appl. Sci, 2021, 11, 10866.

[2] R, Jamro; T, Mente; N, Kardjilov; H, Markötter; AM, Al-Falahat; R, Woracek; I, Manke; A, Griesche J. of Phys.: Conf. Series 2023, 2605(1): 012026.