Effect of temperature and aluminium additions on the mechanical properties of the 13% chromium ferrite stainless steels

The potential interest of the ferritic stainless steels as component materials for nuclear power reactors led to investigate how aluminium influences the mechanical properties of 13% chromium ferritic stainless steels between room temperature and about 7000C. Nominal 13% chromium and 0.04 to 0.08% carbon ferritic stainless steels containing 0, 0.13, 2.19 and 4.15% aluminium, respectively, were obtained by vacuum remelting of a commercial martensitic-ferritic stainless steel and suitable additions of aluminium. After successive rolling operations and recrystallizations performed in order to obtain final 0.5 mm thick sheets with similar average grain sizes the specimens of the above mentioned steels were tested in a tensile test Instron machine, with a constant strain rate (approximately equal to 1.6 x 10-3 min-1), at room temperature, 140, 265, 415, 565 and 7150C. The results obtained show that strengthening by aluminium is strongly temperature dependent. At 2650C all the steels presentes serrated plastic deformation (Portevin-Le Chatelier effect), which is attributed to interactions of the interstitial and substitutional solute atoms with dislocations in the body centered cubic structure. Flow stress drops were still observed at 4650C, although the tests performed at 565 and 7150C showed work-softening of the materials and total absence of serrations. Stress relaxation tests at room temperature yielded values of the apparent activation volumes, which are scattered between about 100 and 130 b3 (b-Burgers vector), being almost constant with stress, strain and aluminium content. Therefore, although aluminium appreciably strengthens the 13% chromium steel, the behaviour summarized suggests that the mechanism controlling plastic deformation at room temperature is the same for all the tested steels, the values of the apparent activitation volumes being probably determined by the interstitial content. Stress relaxation tests at 20, 75, 140 and 2650C for the 4.15% aluminium steel yielded values of the apparent activation volume and internal shear stress tausub(i) which seems to be directly affected by temperature and aluminium content