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Automotive exhaust decouplers (flexible couplings) are a critical component that must remain leak free in order to meet extended warranty and pollution control requirements. The key component of a decoupler is the bellows, which allows easier fit-up during exhaust system assembly and absorbs vibration during service. This bellows is most often constructed of austenitic stainless steel. Though known for improved corrosion resistance, there are many possible corrosion related failure mechanisms for stainless steels. Nineteen decouplers, taken from standard endurance track tests and from normal highway use, have been destructively examined in order to document which failure mechanisms may cause premature failure and compare the performance of various component materials of construction.

Flexible couplings are becoming increasingly important in the design and performance of automotive exhaust systems. Higher temperatures, tighter emission requirements along with extended warranties and governmental demands for increased gas mileage are rendering typically selected coupling alloys marginally acceptable or, more often, unacceptable for a growing number of engine platforms. Requirements for longer life are demanding improvements in fatigue and corrosion properties over those of currently employed alloys. This paper describes the optimization of the fatigue properties of alloy 625 for use in automotive flexible couplings. The salt corrosion resistance of typically used 321SS is then compared to that of INCONEL* alloy 625LCF* and two other candidate flexible coupling alloys, INCOLOY* alloys 800 and 825. INCONEL alloy 625LCF is shown to meet the performance requirements of this critical exhaust system component.

Operating requirements for automotive flexible couplings are rapidly becoming increasingly severe. Higher temperatures, tighter emission requirements along with extended warranties and governmental demands for increased gas mileage are rendering present stainless steel coupling alloys marginally acceptable or, more often, unacceptable for a growing number of engine platforms. Requirements for longer life are demanding improvements in fatigue and corrosion resistant properties over those of currently employed alloys. This paper characterizes the strength, fatigue and corrosion resistance of INCONEL® alloy 625LCF® and relates these properties to the ever increasing requirements of automotive flexible couplings. Tight control of property variation is demonstrated for the fatigue properties by presenting data on a range of heats with differing gauges. The effect of aging during service on room temperature tensile properties is examined.

Automotive exhaust system flexible couplings must resist fatigue and a variety of corrosion mechanisms resulting from exposure to road salts. Increasing operating temperatures and extended warranties are creating ever higher demands on flexible coupling materials. In addition to corrosion and fatigue resistance, materials must have good metallurgical stability. This translates to retention of initial mechanical properties after extended operation at high temperature. Comparative time-temperature hot salt attack profiles were determined for several austenitic stainless steels and nickel base alloys over the temperature range of 430-870°C (800-1600°F) for times of 0 to 1,333 hours. Temperature and alloy composition regimes where stress corrosion cracking (SCC) can occur are also outlined. In addition, the effect on mechanical properties of 300 to 2,000 hour exposures at the above temperatures is determined.