Post-forming Room Temperature Brittle Fracture in a High-Strength Low-Alloy Steel Sheet After Various Forming Modes
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JMEPEG https://doi.org/10.1007/s11665-019-04445-w
Post-forming Room Temperature Brittle Fracture in a High-Strength Low-Alloy Steel Sheet After Various Forming Modes Philippe L. Charpentier (Submitted July 7, 2019; in revised form October 27, 2019) The effects of various sheet forming modes on the post-forming room temperature fracture behavior of a 550-MPa yield strength high-strength low-alloy (HSLA) steel are investigated. In-plane biaxial stretching, plane strain (cold rolling), uniaxial tension, cylindrical cup drawing, and in-plane compression (IPC) are examined up to a von Mises effective prestrain eeff = 0.7. Most of the results pertain to sub-size Charpy-type impact specimens prepared from the prestrained material. After sufficiently large prestrains, the fracture behavior is highly anisotropic. Low-energy fracture modes, namely cleavage and intergranular fracture, occur after all modes of prestraining along plane directions that are perpendicular to the principal compressive prestrain. These planes are parallel to the sheet surface after biaxial stretching and cold rolling and correspond to brittle splits extending into the primary, ductile fracture surface. After cup drawing, the brittle planes are oriented perpendicular to the circumferential compressive prestrain. This results in very low energy fractures propagating in the length direction of a fully drawn cup. After in-plane compression, brittle fracture occurs along planes that are perpendicular to the compression direction. The fracture toughness for a crack propagating along such a plane after IPC to eeff = 0.38 was very low (12.7 MPa m), much lower than in the undeformed condition (247 MPam). Changes in grain shape and in crystallographic texture caused by the various prestraining modes, as well as the microstructural damage at nonmetallic inclusions and carbides, are examined to try to understand the fracture behavior. Keywords
anisotropic embrittlement, compressive prestrain, HSLA steels, post-forming properties, prestraining, sheet forming, splitting fractures
1. Introduction 1.1 Background Sheet metal parts, which are used in many industries, are often produced by cold forming. The post-forming mechanical properties are different from those in the as-received material since, in general, there is an increase in strength and a decrease in ductility during cold forming. In addition, some sheet forming operations produce a marked mechanical anisotropy. Several investigations have been conducted in the past on the effect of one or several sheet forming modes (primarily uniaxial tension and biaxial stretching) on the post-forming flow behavior in steel sheets (Ref 1-5). The influence of deep drawing, which, unlike the two previous modes, involves a compressive prestrain in the plane of the sheet, on flow (Ref 68) and on fracture (Ref 6, 9, 10) of steel sheets has also received some attention. This mode of prestraining was found to result
Philippe L. Laboratory.
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Philippe L. Charpentier, Bettis Ato
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