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The mechanical stability of austenite and cryogenic toughness of ferritic Fe-Mn-AI Alloys

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Abstract

In an attempt to understand the role of retained austenite on the cryogenic toughness of a ferritic Fe-Mn-AI steel, the mechanical stability of austenite during cold rolling at room temperature and tensile deformation at ambient and liquid nitrogen temperature was investigated, and the microstructure of strain-induced transformation products was observed by transmission electron microscopy (TEM). The volume fraction of austenite increased with increasing tempering time and reached 54 pct after 650 °C, 1-hour tempering and 36 pct after 550 °C, 16-hour tempering. Saturation Charpy impact values at liquid nitrogen temperature were increased with decreasing tempering temperature, from 105 J after 650 °C tempering to 220 J after 550 °C tempering. The room-temperature stability of austenite varied significantly according to the(α + γ) region tempering temperature;i.e., in 650 °C tempered specimens, 80 to 90 pct of austenite were transformed to lath martensite, while in 550 °C tempered specimens, austenite remained untransformed after 50 pct cold reductions. After tensile fracture (35 pct tensile strain) at -196 °C, no retained austenite was observed in 650 °C tempered specimens, while 16 pct of austenite and 6 pct of e-martensite were observed in 550 °C tempered specimens. Considering the high volume fractions and high mechanical stability of austenite, the crack blunting model seems highly applicable for improved cryogenic toughness in 550 °C tempered steel. Other possible toughening mechanisms were also discussed.

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Authors and Affiliations

  1. Research Institute of Industrial Science and Technology, Pohang, 790-600, Kyungsangbuk-Do, Korea

    Sang Woo Lee (Research Scientist)

  2. Department of Metallurgical Engineering and Center for Advanced Materials Research, Seoul National University, Kwanak-Ku, 151-742, Seoul, Korea

    Hu-Chul Lee (Professor)

Authors
  1. Sang Woo Lee
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  2. Hu-Chul Lee
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Formerly Graduate Student, Seoul National University.

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Lee, S.W., Lee, HC. The mechanical stability of austenite and cryogenic toughness of ferritic Fe-Mn-AI Alloys. Metall Trans A 24, 1333–1343 (1993). https://doi.org/10.1007/BF02668201

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