Case study 6: Manufacture of stainless steel automotive exhaust systems

This chapter presents a case study that explores exhaust system components. It illustrates the application of several of the basic principles aimed at the development of an improved product. Exhaust system components are manufactured from ferritic stainless steel. This class of stainless steel derives its name from its body centered cubic or ferritic crystal structure and relies on chromium additions of at least 10.5% for corrosion resistance. For many years, a problem associated with ferritic stainless steel was poor weldability. During welding, chromium carbides form along grain boundaries, removing chromium from the matrix, leading to rapid inter-granular corrosion during service. This phenomenon is usually referred to as sensitization. Many exhaust system components, such as muffler and catalytic converter shells, have complex shapes necessitating sheet with good formability and deep drawability. The rolling process must be designed to produce sheet with the appropriate strain hardening exponent, n, and crystallographic texture. Tubes for exhaust pipes are often ultrasonically welded, whereas tube to muffler casings are metal inert gas (MIG) welded and other seams may be tungsten inert gas (TIG) welded or seam welded.