Effects of Long-Term Service on the Microstructure and Tensile Properties of Alloy 800H

Three sections of piping removed from a styrene furnace were metallugically examined. The piping was fabricated from alloy 800H and was service-exposed at temperatures in the range of 621 to 774°C (1150 to 1425°F) for times in the range of 73,500 to 90,000 hours. These samples were investigated by metallurgical studies and mechanical testing to determine the effect of the prolonged high-temperature service on the integrity of the components. A few specimens from the samples were re-annealed to determine if the properties could be restored to their original values. A few more specimens were re-annealed and aged for 1000 hours to determine if significant changes would occur during short-time exposure to high temperature. With one exception, the service-exposed samples exhibited microstructures and properties that were comparable to mill annealed and short-time exposed material. Modest increases in strength and reduction in ductility accompanied the exposure. The exception was material exposed to the highest temperature for the longest time. Here, a significant decrease in the ultimate strength and ductility was observed in a test at 704°C (1300°F).