Assessment of the amount of deformation induced martensite: a calibration curve for the Ferritoscope

Ruch, Marta; CNEA - AAENDE; Argentina

Fava, J.O.; Comision Nacional de Energia Atomica; Argentina
Carabedo, F.D.; Comision Nacional de Energia Atomica; Argentina
Spinosa, C.C.; Comision Nacional de Energia Atomica; Argentina
Ruch, M.C.; Comision Nacional de Energia Atomica; Argentina

ID: ECNDT-0046-2018
Download: PDF
Session: Materials Characterization - Magnetic Techniques 1
Room: J2
Date: 2018-06-14
Time: 13:50 - 14:10

Austenitic stainless steels of the 300 series, when submitted to plastic deformation at low temperatures, can undergo the martensitic transformation to α’ (a ferromagnetic phase). The amount of deformation induced martensite (DIM) depends on composition, processing temperature and the degree of applied deformation. Through heat treatments at temperatures above As (temperature at which the α´→ γr transformation starts), the austenite can be recovered.
In the present work, specimens of austenitic stainless steels 304, 316 and 347 were submitted to 63% reduction in thickness by rolling at a temperature of -70ºC, to achieve high DIM content. Partial reversion of DIM to austenite was made by means of one hour long isothermal heat treatments at temperatures between 200 and 900 ºC. Thus, series of biphasic (α’ and γr) specimens with different contents of martensite and non-magnetic austenite were obtained.
On specimens of the three above mentioned stainless steels magnetic saturation measurements were performed; the magnetic permeability was determined by means of an inverse eddy currents technique and electrical conductivity was measured with
Van der Pauw’s technique.
Through correlation of the results, two calibration curves were constructed, which allow to determine the amount of martensite in the specimens. One curve relates the amount of martensite with the permeability determined by the inverse eddy current technique; the other one relates this concentration with the reading from a commercial ferritescope. The latter allows an extension of the measuring range of the commercial equipment to readings greater than 30% δ-ferrite, limit suggested by the manufacturer.