Investigation of Failure in Automotive Exhausts
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TECHNICAL ARTICLE—PEER-REVIEWED
Investigation of Failure in Automotive Exhausts M. Abdoli • H. Rahimi • A. Godarzizadeh
Submitted: 25 September 2010 / in revised form: 28 May 2011 / Published online: 19 August 2011 Ó ASM International 2011
Abstract Initial investigation of some automotive exhausts that failed during operation in this study revealed that most of the fractures occurred at the position where the diameter of the exhausts changed. Light optical microscopy (OM) and scanning electron microscopy (SEM) examinations have shown that the aluminized coating in the failure area had become cracked during the manufacturing process. This situation led to substrate oxidation and cracking at the elevated temperatures and under cyclic load conditions. It is found that the failure is ultimately due to fatigue caused by the combination of mechanical vibrations and thermal cycles. In addition, oxidation of the crack tip may have accelerated the fatigue crack growth process. A design change mitigated the tendency for failure. Keywords Exhaust failure Automotive exhaust Fatigue Aluminized coating
Introduction Different parts of automotive exhaust systems are subjected to various types of corrosion under operating
conditions. The manifold pipes, catalytic converter, muffler, and connecting pipes are subjected to high-temperature oxidation and de-icing salt contamination, whereas the rear muffler which operates at lower temperatures is subjected to wet corrosion by condensates [1]. Aluminized coatings make mild steel relatively resistant to corrosion and high-temperature oxidation [2]; thus, most of the exhaust parts are made of aluminized mild steel sheet [3, 4]. By means of aluminized plain carbon steel instead of high-alloyed steels for application at temperatures lower than 870 °C, the overall cost of these parts can be decreased [5]. Hot dipping is one of the most economic techniques for aluminizing steel surfaces. In this process, a thin, hard, and brittle aluminum-rich interlayer forms at the aluminum– steel interface. This interlayer primarily consists of intermetallic compounds, such as FeAl, Fe2Al5, and FeAl3 [4]. Exhaust-system failures can cause carbon monoxide release, loss of engine power, and fire. Hence, analyzing the exhaust failures is important, and this article provides an evaluation of several systematic failures of the automotive exhausts.
Exhaust Material and Operating Conditions M. Abdoli (&) Department of Engineering, Tarbiat Modares University, Tehran, Iran e-mail: [email protected] H. Rahimi Research & Development Section, Faragaman Misagh Co., Karaj, Iran A. Godarzizadeh Research & Development Section, Sazehgostar Saipa Co., Tehran, Iran
The exhausts used in this investigation were made of aluminized mild steel sheets. The mild steel sheets (with 3 mm thickness) were coated with an aluminized layer with 25-lm thickness and chemical composition of Al– 10%Si. Most of failures occurred after 20000–30000 km of operation. About two-thirds of the failures occurred at the position where the
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