The mechanical stability of austenite and cryogenic toughness of ferritic Fe-Mn-AI Alloys
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FERR1TIC steels have advantages o f l o w e r cost and higher strength for cryogenic application when compared to austenitic steels. But ferritic steels undergo d u c t i l e brittle transition at l o w e r temperatures, and for structural application, the ductile-brittle transition temperature (DBTT) must be suppressed to b e l o w the intended service temperature. In 5 to 12 Ni steels, t~-SI successful metallurgical processes have been developed to l o w e r the DBTT to b e l o w liquid nitrogen temperature, and these steels were extensively used for liquefied natural gas storage tank application. These metallurgical processes include repeated heating to the 3' and upper (a + 3/) region followed by final tempering at the l o w e r (c~ + 3') region. Thermal cycling in the 3' and (c~ + 3') region was known to be an effective way to reduce the grain size, and final tempering at the l o w e r (c~ + 3') region was essential to suppress the DBTT to b e l o w - 1 9 6 °C. Small volume fractions o f fine austenite were precipitated in interlath o r p r i o r austenite grain boundaries during this final tempering. The beneficial effects o f these fine austenite particles on cryogenic toughness are well recognized by many researchers, but the mechanism o f toughening is not well understood yet and still remains a controversial research topic. Application o f identical thermal processing technique to Fe-Mn steel was not very successful mainly bec a u s e [6,7,8] (1) nickel increases the stacking fault energy o f iron to assist the cross slip of screw dislocation while SANG WOO LEE, formerly Graduate Student, Seoul National University, is Research Scientist,Research Institute of Industrial Science and Technology, Pohang, Kyungsangbuk-Do 790-600, Korea. HU-CHUL LEE, Professor, is with the Department of Metallurgical Engineering and Center for Advanced Materials Research, Seoul National University, Kwanak-Ku, Seoul 151-742, Korea. Manuscript submitted June 5 , 1992. METALLURGICAL TRANSACTIONS A
manganese reduces the stacking fault energy to enhance the cleavage fracture, and (2) when Mn contents exceed 8 pet, hexagonal e-martensite precipitates to interrupt the grain refining by thermal cycling. Despite these difficulties, substitution o f nickel by manganese has a substantial advantage o f lowering production costs, which provides sufficient incentives to develop nickel-free ferritic cryogenic steel. Niikura and Morris 191 achieved a good combination o f strength and toughness at - 196 °C in F e - 5 M n steel. In this steel, the manganese content was limited to prevent the intrusion o f e-martensite during thermal treatment. It is also expected that, if the precipitation o f e-martensite could be suppressed by the addition o f a third alloying element, higher manganese steels should be processed by a similar method. In our previous article,~1°1 we reported that the addition o f aluminum could effectively prevent the precipitation o f e-martensite in 12 to 13 pct Mn steel, and Fe-13Mn3A1 steel was successfully processe
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