Bending Formability of Ferritic Stainless Steels for Application to Tubular Exhaust Manifolds

Abstract

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.

Influence of Stabilizing Elements on Ductile-Brittle Transition Temperature (DBTT) of 18Cr Ferritic Stainless Steels

“…Ferritic stainless steels have high thermal conductivity, a low thermal expansion coefficient and low stress corrosion cracking sensitivity compared to austenitic stainless steels. Ferritic stainless steels containing trace amounts of added Ti or Nb are widely used, for instance, in automotive exhaust system parts such as exhaust manifolds in which an antithermal fatigue property is required [1][2][3][4][5] and in building roofs. Addition of Nb or Ti as stabilizing elements can stabilize interstitial elements (C and/or N) in the ferrite matrix by forming carbides or nitrides, thereby suppressing the formation of Cr-depleted zones.…”