Components found in industrial plants – whether chemical, energy, petrochemical or other – are often subject to heat, aggressive agents and high pressure. These conditions demand steel types that are extremely corrosion and acid resistant even at high temperatures. When austenitic steels are used, it is important to make sure the ferrite content of the weld seams is within strict norms, because only the optimal ferrite content can ensure the best corrosion protection. For this reason some industries have set standards, specifications and regulations for ferrite content.
During the welding of joints on e.g. boilers and pipelines made of austenitic steel, the heat causes modifications in the crystal lattice structure which lead to the formation of ferrite. Weld seams that are poor in ferrite do not have as much yield strength, but too much ferrite reduces their fracture toughness, ductility and corrosion resistance, so it is important that the welding process produces just the right amount.
With duplex steel in particular, the ferrite content in the heat affected zone can easily deviate from the target values, either due to unsuitable filler materials or through incorrect heat input or cooling during the welding. Only onsite spot measurements can provide assurance that the processing did not change the ferrite content at the expense of crucial mechanical or corrosion-resistance properties.