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The comparison of mechanical properties between a “super-clean” heat of a 12% chromium steel and a heat with an identical chemical analysis, but “industrially” produced, does not yield any definite differences. At most it can be ascertained that the “super-clean” heat displays better ductility and impact strength while keeping a comparable strength to that of the other heats.
The comparisons of structures are more interesting and give better results since the study showed (1) that trace elements, especially antimony, (comparison between castings 7–8 and 91) seem to be responsible for the embrittlement of austenitic intergranular boundaries and (2) that no brittle intergranular fracture has been noticed, which distinguishes this superclean heat from the others. Besides, the following comments may be made on superclean heat: (1) the carbides that are visible under the optical microscope (0.5 to 1 m) are still heterogeneously distributed and (2) few carbides are detected in the platelike crystals of martensite.
The main characteristic of this clean heat is to provide a good impact energy and a low transition temperature. For the more in spite of the very clear and relatively abundant precipitations of carbides and carbonitrides in the grain boundaries, no intergranular rupture as shown up during the scanning electron microscope (SEM) examination of the fracture surfaces. One can deduce from this, that the brittle intergranular ruptures observed in the case of heats 7–8 and similar heats are more as a result of the intergranular segregations of trace elements than that of carbides and carbonitrides derived from contents of carbon, nitrogen, neon, niobium, vanadium, and so forth, since all these steels undergo the same thermal treatments.
But it clearly appears that the relationship between the content and distribution of these various carbides and the mechanical properties measured (creep, impact strength) is of prime interest for future research. This paper does not claim to establish this close relationship. Moreover, it is not absolutely certain that this study would be interesting to the mechanics specialist. A materials specialist might well have an opposed opinion.
martensitic steels, steam turbines, turbine rotors, creep resistance, residual elements, carbides
Research manager, Alsthom, Belfort,