Delta ferrite
A high-temperature form of ferrite that solidifies directly from the melt. Present in small amounts in austenitic weld seams, it effectively prevents hot cracking.
Delta ferrite is the most important lever against hot cracking when welding austenitic steel because it binds sulfur and phosphorus. Too little and the weld cracks during cooling, too much and you risk the brittle sigma phase at high temperatures or lose toughness in the cold. You check it with a ferrite meter directly on the weld, the safe range is usually between ferrite number 3 and 10 – but for high-temperature or cryogenic components you have to bring the value down. You control it via the filler metal: more nickel lowers ferrite, more chromium raises it. And keep the heat input in check, otherwise you promote exactly the sigma phase you want to avoid.