Abstract
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.
Highlights
As multi-phase, complex microstructures are increasingly used to achieve strength and ductility targets, the deformation response of advanced high strength steels becomes more complicated and may deviate from expected with respect to temperature (Tsuchida et al, 2011; Coryell et al, 2013), strain rate (Zou et al, 2017), and strain state (Zou et al, 2018)
The 304 stainless steel exhibited a difference in monotonic tensile properties along the rolling direction (RD) and transverse direction (TD), as shown in the true stress-true
Rathbun et al indicated that strain aging occurred at ambient temperature in 301 and 302 stainless steels and exhibited similar behaviors during straining, strain aging was only present when deformation induced martensite existed in the microstructure (Rathbun et al, 2000)
Summary
As multi-phase, complex microstructures are increasingly used to achieve strength and ductility targets, the deformation response of advanced high strength steels becomes more complicated and may deviate from expected with respect to temperature (Tsuchida et al, 2011; Coryell et al, 2013), strain rate (Zou et al, 2017), and strain state (Zou et al, 2018). The yielding response of metals has been defined using criteria, such as the Hill (1948), Tresca (1864), or Mises (1913), that allow engineers to predict when a material will yield based on the stress state. These criteria focus largely on material that has not experienced straining in various directions prior to deformation. The criteria for yielding have been adapted for the Bauschinger effect and Unloading Effects in 304 Stainless-Steel
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