Search for: All records

Abstract The individual effects of strain rate and temperature on the strain hardening rate of a quenched and partitioned steel have been examined. During quasistatic tests, resistive heating was used to simulate the deformation-induced heating that occurs during high-strain-rate deformation, while the deformation-induced martensitic transformation was tracked by a combination of x-ray and electron backscatter diffraction. Unique work hardening rates under various thermal–mechanical conditions are discussed, based on the balance between the concurrent dislocation slip and transformation-induced plasticity deformation mechanisms. The diffraction and strain hardening data suggest that the imposed strain rate and temperature exhibited dissonant influences on the martensitic phase transformation. Increasing the strain rate appeared to enhance the martensitic transformation, while increasing the temperature suppressed the martensitic transformation. 

Forming operations are known to be complex, involving many strain states, strain rates, temperatures, strain paths, and friction conditions. Material properties, such as strength and ductility, are large drivers in determining if a material can be formed into a specific part, and for selecting the equipment required for the forming operation. Predicting yielding behavior in situations such as these has been done using yield surfaces to describe material yielding in specific stress states. These models typically use initial mechanical properties, and will require correction if the material has experienced previous straining. Here, we performed interrupted uniaxial tensile testing of a 304 stainless steel to observe the effects of unloading and subsequent reloading on yielding and tensile properties. An increase in yield point developed, in which a higher yield was observed prior to returning to the bulk work hardening behavior, and the magnitude of the yield point varied with unloading conditions and strain imposed. The appearance of a yield point is attributed to strain aging or dislocation trapping at obstacles within the matrix. These results suggest that both strain aging and dislocation trapping mechanisms may be active in the matrix, which may present challenges when forming austenitic stainless and new advanced high strength steels that likely show a similar behavior. These results provide a potential area for refinement in the calculation of yielding criteria that are currently used to predict forming behavior. 

Quenching and partitioning (Q&P) processing of third-generation advanced high strength steels generates multiphase microstructures containing metastable retained austenite. Deformation-induced martensitic transformation of retained austenite improves strength and ductility by increasing instantaneous strain hardening rates. This paper explores the influence of martensitic transformation and strain hardening on tensile performance. Tensile tests were performed on steels with nominally similar compositions and microstructures (11.3 to 12.6 vol. pct retained austenite and 16.7 to 23.4 vol. pct ferrite) at 980 and 1180 MPa ultimate tensile strength levels. For each steel, tensile performance was generally consistent along different orientations in the sheet relative to the rolling direction, but a greater amount of austenite transformation occurred during uniform elongation along the rolling direction. Neither the amount of retained austenite prior to straining nor the total amount of retained austenite transformed during straining could be directly correlated to tensile performance. It is proposed that stability of retained austenite, rather than austenite volume fraction, greatly influences strain hardening rate, and thus controls strength and ductility. If true, this suggests that tailoring austenite stability is critical for optimizing the forming response and crash performance of quenched and partitioned grades.