Chemical, energy and processing plants are often subject to heat, aggressive media and high pressure. These circumstances demand steel with high corrosion and acid resistance that are resilient even at high temperatures. It explains the increasingly use of materials that preserve high mechanical characteristics even in those conditions, such as austenitic stainless and austenitic-ferritic (duplex) steels. In these alloys the combination and distribution of austenite, ferrite and martensite is essential for their performance: if the ferrite content is too low, then the welded material is susceptible to hot-cracking; if the ferrite content is too high, the toughness, ductility as well as the corrosion resistance of the steel are reduced.
In particular when welding duplex steel, the ferrite content in the welding area can easily assume unfavorable ferrite values either due to unsuitable welding filler materials or through poor heat input or heat removal. Only an onsite measurement can provide the assurance that the processing did not change the optimum Fe content.
This measurement can be performed by the magnetic induction method: a magnetic field generated by a coil begins to interact with the magnetic portions of the specimen. The changes in the magnetic field induce a voltage proportional to the ferrite content in a second coil. This voltage is then evaluated. All magnetic portions in the otherwise non-magnetic structure are measured, i.e., in addition to delta ferrite and other ferritic portions also strain-induced martensite.
Areas of application are onsite measurements, e.g. of austenitic platings as well as weld seams in stainless steel pipes, containers, boilers or other products made of austenitic or duplex steel.
Metalprove S.r.l. uses portable electronic feritscopes made by HELMUT FISCHER, which can measure low, medium or high content of Ferrite through complete sets of magnetic induction probes in accordance with Basler and DIN 32514-1 standards.