Very briefly: 304 SS in wrought condition will have only austenite phase. In the solution annealed condition you will see predominantly equiaxed grains and annealing twins in some grains. Autogeneous GTAW will cause localized melting of the weld pool and during solidification delta ferrite forms at high temperature at the inter-dendritic regions and it is retained at room temperature (about 2 - 10%) due to faster cooling. Faster the cooling rate (lower the heat input), higher will be the volume fraction of delta ferrite. The delta ferrite arm spacing decreases with increasing the cooling rate. You can measure the volume fraction of delta ferrite using ferritescope because it is ferromagnetic and austenite is not.
If you are not a skilled welder and cannot keep the arc length constant, then the Ar shielding may not be sufficient. This will ingest nitrogen from the atmosphere into the weld pool and try to stabilize austenite phase. This results in wide variations in the delta ferrite content of the weld. You can evaluate welder's skill by measuring the ferrite number (FN) along the austenitic stainless steel weld bead. If the FN is scattered along the length of the weld, then the skill of the welder is low!
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