(A) study on the low temperature deformation behavior of cryogenic Fe-25Mn-5Al-5Ni-0.3C steel

Abstract

The low temperature deformation behaviors of cryogenic Fe-25Mn-5Al-5Ni-0.3C steel (named as "CAM-2") have been investigated. A particularly interesting observation in this CAM-2 alloy was the increase in elongation with decreasing test temperature. The total elongation at RT was a 25%, compared to a 50% at 77K. The inverse ductility behavior was ideal for cryogenic material requirements. In order to explain the deformation characteristics of this alloy, the new plastic stress-strain equation for low temperature tensile deformation was proposed: $ σ = K ε^Nexp[ (L/2) ε^2 + Mε]$ The extent of true uniform elongation at lower temperatures can be calculated from this model. When no strain-induced phase is formed, the flow equation reduced to the Hollomon``s flow equation. The calculated values of the true uniform elongations were in good agreement with the measured values of this steel at room and subzero temperatures. It is proposed from these results that the application of models to metallic materials which have either a constant or varying strain hardening exponents with strains are possible. It was also found that the yield and tensile strengths of this steel increased with decreasing temperature: the YS increased by 47% from RT to 77K. The Charpy impact energy decreased from 215.3J at RT to 130J at 77K. However, the Charpy impact energy was considerably higher than that of 9%Ni steel. The controlled rolled alloy showed cyclic hardening at RT and LNT. Toal strain-controlled fatigue tests showed that the fatigue resistance at 77K was superior to that at room temperatuer. The reason exhibiting the longer fatigue life at LNT than that at RT for this alloy was caused by the significant increase in the fatigue ductility coefficient at LNT. The fatigue ductility coefficient at RT was 22.5%, compared to 54.1% at LNT.