High-Temperature Fatigue Testing of Automotive Valve Steels

The elevated temperature bending fatigue strength of seven automotive exhaust valve alloys has been determined. Inconel 751 shows the highest fatigue strength of 45 ksi at 10⁸ cycles 760 C (1400 F). Coatings in air atmosphere at elevated temperature generally reduced the fatigue strength. In corrosive atmospheres consisting of a simulated lead oxide environment or a carbon monoxide/carbon dioxide atmosphere, no protection was afforded by coatings of aluminum or aluminum chromium. Within the varying surface roughness tested only the as-forged surface representing a high surface roughness and decarburization showed a drop of 24 percent in fatigue strength. Compressively stressed Stellite 6 faced valves showed ten times improvement in thermal fatigue life.