Aging-induced anisotropy of mechanical properties in steel products: Implications for the measurement of engineering properties

Strain aging in low-carbon steels is a well-known strengthening phenomenon, the typical results of which are an increase in yield stress and/or an increase in the extent of discontinuous yielding. Aging effects are generally characterized through the use of results from mechanical tests in which the strain path prior to aging (prestrain) and the strain path after aging are in the same direction. However, these tests do not completely characterize the properties of aged materials, since the effects of aging are reduced when materials are tested in directions different than the direction of prestrain. The result is anisotropy of properties which can affect the performance of industrial products. In this paper, the effect is demonstrated in two examples of industrial products made from low carbon steels, the aging of which during processing results in performance changes that are not predicted through standard tensile testing of as-fabricated products. The first example compares the effect of aging on yield strength and dent resistance of stamped hood panels on an electro-galvanized, Al killed, bake hardenable sheet steel for auto body panel applications. The second example shows the effect of aging on the anisotropy of tensile data from two American Petroleum Institute (API) grade X100 pipe steels in the as-received condition. ta show that the performance gains realized from strain-aging in the tensile tests on base material are not apparent in the tensile data from the stamped panels after aging, but the dent resistance clearly demonstrated the beneficial effect of aging. The high degree of anisotropy in the yield strength and yielding behaviors between the circumferential and longitudinal tensile data in the two pipe steels demonstrates the effect of strain path on a materials response to aging, which may occur during downstream processing or in field service. The manifestation of material properties that are dependent upon the relationship between the pre-aging strain direction and the post-aging strain direction underscores the importance of correct evaluation of mechanical performance in the design of structural components in materials which undergo aging.