Interaction between the bauschinger effect and strain aging
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[4]
Equation [3] can be written as Y~X~bQ'/ QB = Q'B + 1
-
~X~ b
[5]
Equation [5] is a more general form of Eq. [9] in Ref. 2 (note that there was a mathematical sign error for the expression of Eq. [5] in the original paper). If boron atoms are the only ones to segregate to the grain boundaries in significant numbers, then Eq. [5] reduces to QB = Q'B/(1 - X ~ b)
[6]
In a recent work on Fe-3 pct Si alloys, 3 Auger studies indicated that for ferrite the grain boundary concentration of B could exceed the matrix concentration by 1.5 • 104 at 1025 ~ If such a large value of the enrichment factor were to apply to austenite, our
Interaction Between the Bauschinger Effect and Strain Aging D. N. W I L L I A M S When a metal is strained plastically, the yield strength in the direction opposite the direction of strain is reduced. The magnitude of the reduction can be quite significant, more than 50 pct after a few percent strain) This phenomenon, which is called the Bauschinger effect, can exert an important influence on the properties of metals. During an investigation of the Bauschinger effect in line-pipe steels, an interesting interaction between the Bauschinger effect and strain aging was observed. These studies were carried out using transverse roundbar specimens obtained from a 0.625-inch-thick plate of controlled-rolled X60-grade steel. The chemical composition and yield strength of this H S L A steel are described in Table I. The steel exhibited discontinuousyield behavior as received, with about 1.6 pct yield strain in both tension and compression. Specimens with a 20.6-mm-long • 7.9 mm diam reduced section ( L / D = 2.6) were installed in a mechanically loaded universal testing machine equipped with D. N. WILLIAMS is Principal Metallurgist, Battelle Columbus Laboratories, 505 King Ave, Columbus, OH 43201. Manuscript submitted March 12, 1980.
previous work 2 would predict that X$b "-~ 0.3 and, therefore, according to Eq. [6], QB - 57 K J / m o l , a value comparable to that predicted by Hoch 4 from the molar enthalpy of liquid and ~, iron. We conclude that the reported value of 40 K J / m o l was a lower limit for the binding energy, QB. In order to improve on this value, one would need autoradiography data which contains sufficient points to analyze the curvature of In D vs 1 / T curves. For example, by combining Eqs. [1] and [6], a plot of Q'n vs D yields a straight line if the model applies. The intercept of the line on the Q~ axis is QB and that on the D axis is 8. The value of QB is the desired binding energy. The value of 8 permits calculation of an average grain boundary concentration via Eq. [1]. This work was supported in part by a grant from the Foote Mineral C o m p a n y to The University of Connecticut Foundation.
1. J. E. Morral and T. B. Cameron: Boron in Steel, pp. 19-32, AIME, NY, 1980. 2. W. F. Jandeska and J. E. Morral: Met. Trans., 1972, vol. 3, p. 2933. 3. R. G. Rowe: Met. Trans. A, 1979, vol. 10A, p. 997. 4. M. Hoch: Tech. Rep. G.E.-Tm64-7-11, General Electric, Atomic Products Divisi
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