Microstructural Sources of Toughness in QLT-Treated 5.5 Ni Cryogenic Steel.
CALIFORNIA UNIV BERKELEY DEPT OF MATERIALS SCIENCE AND ENGINEERING
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In commercial practice 5.5Ni steel is toughned for cryogenic service by a three-step heat treatment designated the QLT treatment. To determine why this treatment is necessary and successful, a series of two-step heat treatments were applied to 5.5.Ni steel and the resulting microstructural states were characterized and compared with that resulting from the QLT treatment. It was concluded from this analysis that the QLT treatment lowers the ductile-brittle transition essentially be refining the grain size of the alloy through precipitation of thermally stable austenite in a dense distribution along the boundaries of prior martensite laths, hence destroying the crystallographic alignment of martensite packets and preventing cooperative cleavage. The multi-step heat treatment is necessary because of the low nickel content a single step heat treatment leads to an austenite precipitate which is either too lean in solute to be retained or too coarse in its distribution to be effective. The problem is avoided in the QLT treatment since the L intercritical anneal serves to create regions of high solute content along the prior martensite lath boundaries, and the intercritical temper, T, then precipitates a dense distribution of high solute, stable austenite within these enriched regions.Author
- Properties of Metals and Alloys