Introduction of a new term: Ferrite Number Density (FND) to measure the ferrite number of welds on thin 300 series stainless steel sheets

From the analysis of the measured FN done in this research, the following can be concluded: 1. The measured Ferrite Number can decrease with an increase in cooling rate of the welding process, contradicting to what is mentioned in the literature. However, the Ferrite Number Density will increase with an increase in the cooling rate in an FA type of solidification, for low and moderate cooling rates, irrespective of the size of the weld bead. 2. FND is a better indicator of the ferrite content of the weld bead, especially when welding thin sheets. FND follows all the expected trends when compared to the literature, which the FN values don't. 3. The cut-off weld area under which the FND is superior to the FN value was found to be 13 mm2 when the aspect ratio is greater than 2.5. For welds with cross-sectional area larger than this value, FN can be directly used as an indicator of ferrite content, or alternatively, the measured FN must be divided by 13 mm2 instead of the actual cross-sectional area to obtain the FND so long as the minimum aspect ratio criteria of the weld is not violated. 4. For the TIG welding process producing fully penetrated weldments, the minimum cross-sectional area of welds done on 2.4 mm thick sheets was found to be 13.547 mm2, which is more than the FGAB1.3-Fe probe magnetic flux field. Consequently to simplify, the cut-off values for sheets having a thickness greater than 2.4 mm, the FN can be used as a direct indicator of the ferrite content of the weld. For sheets thinner than 0.9 mm with an aspect ratio greater than 2.5, the FND must be used as an indicator of the ferrite content in the weld. For sheet thicknesses in between 0.9 and 2.4, FN or FND may be used depending on the cross-sectional area of the weld and its aspect ratio.