Use of Fe-C Information as Reference for Alloying Effects on B S
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BAINITE is one of the microstructures that can form from austenite on cooling. It has long been of considerable practical importance because it may prevent the complete transformation of austenite to martensite. On the other hand, a completely bainitic microstructure may have unique properties and has found practical use, especially in recent times. Alloying elements have been used for controlling the formation of bainite in various applications. Due to various aspects, it has thus been of considerable interest to be able to control the formation of bainite by alloying elements and information on their effects on the start temperature for bainite formation has been of central interest. The start temperature is denoted as BS and the variation of the BS temperature with the carbon content at fixed alloy content can be represented by a BS line. Several empirical equations for predicting the BS temperature from the steel composition have been derived from large amounts of experimental information on alloyed steels. The result is usually presented as a linear equation with a coefficient for each element, often
obtained by regression analysis. An early example is the equation by Steven and Haynes[1] who determined the BS temperatures of a group of 65 steels and evaluated the coefficients by linear regression analysis. BS ð CÞ ¼ 830-270C-90Mn-37Ni-70Cr-83Mo
½1
The carbon and alloy contents are given in mass pct. In a recent review, Kang et al.[2] listed nine, more recent equations,[3–11] BS ð CÞ ¼ 656-57.7C-75Si-35Mn-15.3Ni-34Cr-41.2Mo ½2 BS ð CÞ ¼ 718 -425C-42.5Mn
½3
BS ð CÞ ¼ 844-597C-63Mn-16Ni-78Cr
½4
BS ð CÞ ¼ 720582:63C þ 126:6C2 91:68Mn 66:34Ni 31:66Cr 42:37Mo þ 9:16Co-36:02Cu-41:15Ru
LINDSAY LEACH, PETER KOLMSKOG, LARS HO¨GLUND, MATS HILLERT, and ANNIKA BORGENSTAM are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, Brinellva¨gen 23, 10044 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted February 8, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
½5 BS ð CÞ ¼ 732202C þ 216Si-85Mn-37Ni-47Cr-39Mo ½6
BS ð CÞ ¼ 711361:9C þ 261:9C2 28:3Mn þ 43:7Si ½7 BS ð CÞ ¼ 630-45Mn-40V-35Si-30Cr-25Mo-20Ni-15W ½8 BS ð CÞ ¼ 745110C-59Mn-39Ni-68Cr-106Mo-17MnNi þ 6Cr2 þ 29Mo2 BS ð CÞ ¼ 839270½1 expð1:33CÞ-86Mn-23Si -67Cr-33Ni-75Mo
½9
½10
It is striking that the numerical values of the coefficients vary considerably between the equations. For Mn it varies from 35 to 90 and for Ni from 15.3 to 66.34. This may partly be caused by the use of different groups of steels by different authors. It may seem that the resulting equation in such a case can give usable predictions for new steels of similar compositions. The equations also differ appreciably when applied to binary Fe-C alloys as illustrated in Figure 1. To indicate that they apply to binary Fe-C alloys with no alloying elements they will all be denoted as Bos lines. It is evident that at least a large majority of the authors have not paid any attention to the consequences for Fe-C
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