A new austenitic Ni-free cryogenic alloy of Fe-30Mn-5Al-0.3C-0.1Nb steel possessing excellent toughness and high strength was developed. The tensile strengthes of the hot controlled-rolled steel were equal to those of the commercial cryogenic 9% Ni steel, but the impact toughness of the new cryogenic alloy was twice higher than that of the 9% Ni steel in the temperature range tested from room temperature to -196℃. A particularly interested observation for the steel was the increase in elongation with decreasing test temperature. The steel showed 30% elongation at room temperature, compared to 57% at -196℃.
In order to investigate the inverse temperature dependence of ductility behavior, the aluminum contents varied from 0 to 5%(wt.%) in Fe-30Mn-Al-0.3C-0.1Nb alloys in the controlled rolled and recrystallized material. It was found that the inverse temperature dependence of ductility behavior from RT to -196℃ occurred when Al content exceeded 2% in the alloy. The increased aluminum content suppressed the formation of epsilon martensite, resulting in the predominent formation of strain induced phases of mechanical twinning and alpha martensite during tensile test at -196℃. It was proposed that the inverse temperature dependence of ductility in this alloy was a result of prevention of local necking by the favorable formation of micro-mechanical twinning during deformation; The multiple necking mechanism was responsible for the inverse temperature dependence of ductility behavior. It was also observed that aluminum acted as an austenite stabilizer in this Fe-30Mn-Al-0.3C-0.1Nb alloy.
Since the formation of mechanical twinning was strain rate dependent in the alloy, the behavior of increasing toughness behavior with decreasing temperature did not occur in the high strain rate Charpy impact tests. However, a slow strain rate fracture toughness tests did also exhibit the tendency of increasing toughness with decreasing test temperature.
The effects of test temperatures (RT, -196℃) and inverse temperature dependence of ductility on low and high cycle fatigue behavior in the Fe-30Mn-5Al-0.3C-0.1Nb were studied. The degree of cyclic hardening increased with decreasing test temperature RT to -196℃. Total strain-controlled fatigue tests showed that the fatigue resistance at -196℃ was superior to that tested at RT in the entire life range tested ($10^2$ to $10^5$ cycles). The absolute values of fatigue strength and ductility exponents increased somewhat with the decreasing temperature from RT to -196℃, which were unfavorable for fatigue resistance. However, a pronounced increase in the fatigue ductility coefficient at -196℃ occurred as the result of the inverse temperature dependence of ductility behavior in the alloy. The superior low cycle fatigue resistance at -196℃ was attributed to the significant increase in the fatigue ductility coefficient. The inverse temperature dependence of ductility behavior was very beneficial in increasing strain controlled fatigue resistance at the cryogenic temperature in the alloy. The high cycle fatigue resistance at liquid nitrogen temperature was significantly higher than that at room temperature for the Fe-30Mn-5Al-0.3C-0.1Nb due to the increased strength with decreased temperature. One of the peculiar fatigue behavior in this work is the increased fatigue resistance in the strain controlled low cycle fatigue as well as in the stress controlled high cycle fatigue with decreasing test temperature, which was due to the unique inverse temperature dependence of ductility behavior of this alloy.
