Eutectoid Transformations in 4.12 Mass Pct Cr 0.88 Mass Pct C Steel
- PDF / 1,740,146 Bytes
- 11 Pages / 593.972 x 792 pts Page_size
- 33 Downloads / 191 Views
INTRODUCTION
THE temperature ranges for the well-known eutectoid transformations in steel, pearlite, and bainite were well defined by the classical work by Davenport and Bain[1] in 1930, where each degree of transformation was represented with a single curve. For hypoeutectoid plain carbon steels, its shape does not reveal that it is composed of two C curves, one for each transformation product, pearlite and bainite, because parts of them overlap. The bainitic transformation has a horizontal asymptote at the bainite start temperature, Bs, which should be depressed to lower temperatures as the carbon content is increased. One may thus expect the temperature range of pearlite to extend to lower temperatures. However, in 1939, Jolivet[2] called attention to the occurrence of more spiky microstructures in the temperature range for the transition between pearlite and bainite. Their shapes sometimes reminded him of dendrites formed on solidification, and he described them as arborescent. He stated that they appear ‘‘only in littleused eutectoid or hypereutectoid steels other than the plain carbon steels.’’ This phenomenon was later studied in steels with 1 mass pct C and 3 mass pct Cr by Klier[3] and by Lyman and Troiano.[4] The latter authors also studied steels with other chromium contents and found a very similar behavior of a steel with 4 mass pct Cr. Several studies were published in more recent years and the field was reviewed recently by Goldenstein et al.[5] in a publication on ‘‘Non classical decomposition products morphologies in Fe-Cr-C and Fe-Cr-Mo-C steels.’’ Several variations of the eutectoid microstructure in plain carbon steels with 1.65 or 1.67 mass pct C were recently discussed.[6] It was found that the microstructure changed, when lowering the temperature, from pearlite P. KOLMSKOG, Postdoctoral Student, and A. BORGENSTAM, Associate Professor, are with the Department of Materials Science and Engineering, KTH (Royal Institute of Technology), SE-10044 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted January 10, 2011. Article published online August 19, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
to inverse bainite to fanlike bainite and finally to bainite. This change seemed possible to explain first as an effect of the increase of the supersaturation of cementite in austenite at decreasing temperature and, second, at lower temperatures, as an effect of the even more rapid increase of the supersaturation of ferrite. It was observed how the increased supersaturation of cementite in austenite favors the tendency to form Widmansta¨tten plates of cementite, which strongly affects the eutectoid microstructure favoring inverse bainite, whereas plates of ferrite may play the same role at lower temperatures for the formation of bainite. With these observations in mind,[6] it would be interesting to examine how much these microstructures resemble those found in steels alloyed with carbide forming elements such as Cr. If there are evident similarities, then the same explanation may apply. Due t
Data Loading...
