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.
극저온(영하 150 ℃ 이하) 구조용 재료(CAM-2)의 저온 변형 거동에 관하여 연구하였다. 극저온 재료는 극한 분위기에서 사용되기 때문에 요구되는 물성도 상당히 까다로우며, 이러한 저온의 변형 특성에 대한 이해는 저온 재료의 성질을 향상시키는데 필수적인 것으로 생각된다.
독특한 가공 경화 특성을 토대로 CAM-2 합금강의 저온 변형 모델을 제시하였으며, -100 ℃ 이하의 온도에서는 다음과 같다.
$ σ = K ε^N exp[(L/2) ε^2 + M ε]$
여기서 K,N,L,M은 온도에 따른 상수이고, 이 식에 instability 조건을 고려하면 true uniform elongation은 다음과 같이 구할 수 있다.
$ ε_u = [(1-M)-[(M-1)62-4LN]^{0.5}]/2L$
이 식을 이용해서 계산한 값과 실험치와는 잘 일치하며, 이 재료는 저온으로 갈수록 elongation이 증가하는 이상 거동을 나타낸다.
본 재료의 다른 저온 기계적 성질을 조사하기 위하여 충격 및 low cycle fatigue 시험을 상온과 저온에서 행하였으며, impact absorption energy는 저온에서 약간 감소 하였으나 fatigue life는 오히려 증가 하였다. 이와같이 저온 (-196 ℃)에o}U} fatigue property가 우수한 것은 상온보다 저온에서 fatigue ductility coefficient가 상당히 크기 때문이다.
