Comparison of different methods for measuring strain induced α′-martensite content in austenitic steels

Several techniques for measuring the strain induced α′- martensite content in EN 1.4318 (AISI 301LN) and EN 1.4301 (AISI 304) austenitic stainless steels were compared in order to determine a correlation curve between Ferritescope measurement results and actual α′-martensite contents. The studied methods involved Satmagan measurement, magnetic balance measurement, X-ray diffraction, density measurement and quantitative optical metallography. Satmagan, magnetic balance and density measurements were found to give equal α′-martensite contents. X-ray diffraction results were affected by the texture but averaging of several diffraction peaks improved the reliability of the results. It was shown that α′-martensite can be detected by means of optical metallography, but quantitative analysis is time consuming and inaccurate. The relationship between the Ferritescope results and actual α′-martensite contents measured with the other techniques was found to be linear. According to the results, the Ferritescope readings can be converted to actual α′-martensite contents by multiplying with a correction factor of 1·7.