During the past ten years, considerable interest has developed in the use of the HP-45 alloy for high-temperature furnace components, and this alloy has replaced the conventionally used HK-40 alloy in some critical applications. The wide use of the HP-45 alloy, however, has been retarded by the scarcity of long-time creep-rupture data. This paper presents the results of different research programs conducted at Battelle over the past several years and compares the properties of the HP-45 and HK-40 alloys. Included are comparisons of the short-time tensile and long-time creep-rupture properties, the effects of long-time exposure on the tensile and impact properties, the elastic properties, and the surface and microstructural stability of the two alloys. Creep-rupture comparisons are based upon the results of tests of up to 10 000-h duration at 871, 982, and 1093°C (1600, 1800, and 2000°F) on three commercially produced heats of each alloy.

The short-time tensile properties of the A CI Type HK-40 cast heat-resistant alloy and AISI Type 310 wrought stainless steel were investigated from room temperature to 2000 F. The creep-rupture properties of the HK-40 alloy were studied in the range of 1400 to 2000 F for times long enough to permit extrapolation to 100,000 hr. In addition, the creep-rupture properties of Type 310 were investigated at 1800 to 2000 F, and observations were made of the microstructural changes that occurred in the two materials during creep exposure. The Type 310 material tended to have a higher yield strength and ultimate tensile strength at moderate temperatures than the HK-40; however, from 1200 to 2000 F, the HK-40 was the stronger. The Type 310 was more ductile at all temperatures. The HK-40 displayed about twice the rupture strength of the Type 310 at each test temperature. On the basis of comparable minimum creep rates, the HK-40 showed five to six times the strength of the Type 310 at the same temperature. During exposure at the lower temperatures, chromium carbides precipitated in finely dispersed form in the matrix of the HK-40; isolated islands of sigma phase also tended to form. At high temperatures, the primary eutectic carbides in the HK-40 alloy tended to spheroidize; and both materials absorbed nitrogen from the atmosphere, needles of chromium nitride forming in the matrix.

Two applications of vacuum technology to metallurgy are discussed. One concerns the consumable-electrode vacuum-arc remelting of AISI 316 stainless steel. An air-melted and a vacuum-arc remelted heat are compared with respect to composition, processing and welding, room- and high-temperature mechanical properties, magnetic permeability, inclusion count, and behavior in the Huey corrosion test. The vacuum heat was lower in carbon, oxygen, and hydrogen than the air-melted heat. The low oxygen and hydrogen were attributed to the melting process, while the low carbon was not. Both heats were similar in processing and welding behavior as well as in mechanical and physical properties. In Huey corrosion tests of sensitized material, the air-melted steel was attacked much faster than the vacuum-melted material. However, this difference was ascribed to the difference in carbon content between the two steels. Thus, in this study, no differences attributable to the consumable-electrode vacuum-arc remelting process were observed. The other application was the vacuum annealing of AISI 301 strip. This material was compared with air-annealed and hydrogen-annealed stock. The air-annealed and vacuum-annealed steels had similar mechanical properties. However, the hydrogen-annealed steel had half of the ductility, 80 per cent of the formability and 75 per cent of the strength of the other materials. The increase in hydrogen content of the steel, when annealed in hydrogen, is considered responsible for these differences.