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

Abstract

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).

Historical Context and Perspective on Allowable Stresses and Design Parameters in ASME Section III, Division 5, Subsection HB, Subpart B

“…These values for the aging factors are supported by test and service exposed data, as shown in FIGURE 5 below from a report by Trester et al [15]. More recent data from service-exposed Alloy 800H material were reported by Swindeman et al [16]. These were the data that caused the NRC contractor to raise questions about the conservatism of the aging factors of Alloy 800H in the 2017 edition of HBB.…”

“…As reported by Swindeman at al. [16], the service temperature for section C, with the lowest measured strengths, was in the reported range of 1425 to 1490°F. Since that temperature range is above the maximum allowable temperature in the 2017 edition, these data are not applicable when determining the aging factors for comparison with HBB.…”

“…These were the data that caused the NRC contractor to raise questions about the conservatism of the aging factors of Alloy 800H in the 2017 edition of HBB. The service-exposed materials were sampled from three different components, and they were labeled as sections A, B, and C. The photos of these components can be found in [16].…”

Status of Alloy 800 H in Considerations for the Gen IV Nuclear Energy Systems

“…For Alloy 800 H, the currently provided reduction factor covers temperatures greater than 730 C. Typically, aging studies included the development of temperature-timeprecipitation (TTP) diagrams, hardness studies, toughness studies (Charpy-V impact energy), and tensile properties. The TTP diagram has been developed for Alloy 800 H [41] for times to beyond 10,000 h. Abundant literature also exists on the effect of aging on hardness, Charpy-V impact energy, and tensile properties of Alloy 800 H [42].…”

Next Generation Nuclear Plant Steam Generator and Intermediate Heat Exchanger Materials Research and Development Plan

“…The longest aging time in the original dataset that supported the current strength reduction factors was ~ 20,000 h. Recently, Swindeman et al 117 have compared the yield and tensile strengths of service-exposed Alloy 800H (73,500 h at 752°C to 774°C) with laboratory results from Alloy 800H, thermally aged for 18,600 h at 760°C (see Figure 51). Only very small drops in strengths, within the typical data scatter, are observed.…”