In this study, the bending formability of tubular pipes made of ferritic stainless steels during the rotary bending process was investigated. Three different types of ferritic stainless steel-STS 439, STS 429EM, and STS 441-were selected as the test materials. Finite element (FE) simulations were introduced to predict maximum bending angles, or equivalently the bending formability, for both as-received and annealed tubes. The results from experiments and FE simulations suggest the following main conclusions. First, the pipe materials used in the rotary bending process were subjected to prior work hardening during the tubing process, which resulted in reduced formability. However, proper heat treatment could enhance the bending formability. The optimum annealing conditions were determined from the microstructure analysis and mechanical assessments by uni-axial tensile tests for various heat-treated samples. An annealing temperature/holding time of 900°C/10-60 s resulted in enhanced formability without grain coarsening for the three tested ferritic stainless steels. Second, a FE model predicted maximum bending angles and thinning profiles at the extrados of pipes for both as-received and heat-treated tubes when the boundary conditions and friction coefficients were properly optimized.
Experimental and numerical investigations of the ridging in ferritic stainless steels were presented in this paper. Two kinds of ferritic stainless steels exhibiting different levels of ridging were selected as model materials. The measured roughness of the uniaxially elongated specimens up to 15% in rolling direction (RD) was compared to the prediction using a rate-dependent crystal plasticity FEM (CPFEM). Initial textures of the two materials on 5 equi-spaced sequential RD planes were obtained by EBSD measurement. The initial textures were utilized as input data for the constitutive parameters of the crystal plasticity. Measured respective single planar textures were collected all together so that the 5-layer textures complete 3-dimensional structure and they were mapped onto the FE mesh. Ridging profiles predicted by the CPFEM using both every single layer texture and multilayer texture were compared to the experimental results. Predicted ridging profile of a material exhibiting weak ridging by using 5-layer EBSD mapping was in good agreement with the experimental result. On the other hand, prediction by using only single layer texture was efficient to estimate the ridging in a material exhibiting severe ridging due to the elongated cluster of analogous orientations along RD.
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