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In recent years, there has been a trend toward the use of increasingly higher operating temperatures in many industrial processes to obtain greater operating efficiencies. This trend has created many design- and material-selection problems. As is now widely recognized, the difference between the behavior of metals at elevated temperatures and at room temperature necessitates an approach to the problem of design for elevated-temperature service different from that for room-temperature service. The occurrence of creep during elevated-temperature service, as well as the need for preventing actual rupture (the end result of creep), requires knowledge of the stresses and temperatures which materials must withstand without producing excessive creep or rupture. However, the designer is often faced with very limited engineering information of this type. For example, little information is available on the creep and creep-rupture properties of stainless steels at temperatures above 1500 F. To provide a basis for the selection of stainless steels for service between 1600 and 2000 F, creep and creep-rupture tests and metallographic and X-ray diffraction studies were conducted on sheet product of AISI Types 302, 309S, 310S, 314, and 316, and Type 310 Cb stainless steels at 1600, 1800, and 2000 F, and on sheet product of AISI Type 446 stainless steel at 1600 and 1800 F.

The present paper summarizes the results of a number of investigations in which the effects of nitrogen on the mechanical properties of an austenitic stainless steel were evaluated. The results of studies on commercially produced plate indicated that increasing the nitrogen content of Type 304L steel to the range 0.10 to 0.13 percent increased the room and elevated temperature strength of this steel such that its strength was equivalent to that of Type 304 steel in both the welded and unwelded condition. The results of room and elevated temperature tension tests on product from four commercial heats of high-nitrogen (0.12 to 0.16 percent) Type 304 steel indicated that the yield and tensile strength of this steel was significantly increased. Creep-rupture tests were too limited to assess the effect of nitrogen on the creep-rupture strength of the steel. Some failures on punch marks of the creep-rupture test specimens were observed suggesting the possibility of some notch sensitivity in this steel. The results of a limited study on laboratory heats of Type 316L containing nitrogen in the range 0.02 to 0.19 percent indicated a significant increase in the elevated temperature yield and tensile strength of this steel. The creep-rupture strength at 1300 F also increased with increasing nitrogen content.

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A study was made to determine whether decreasing the chromium or the chromium and molybdenum contents of AISI Type 316 stainless steels would decrease the tendency for sigma-phase formation without impairing the elevated-temperature strength of the steel. The results of this study showed that in comparison with AISI Type 316, a modified Type 316 steel with about 14 per cent chromium instead of the normal 16 to 18 per cent chromium had about the same coefficient of thermal expansion, similar room- and elevated-temperature tensile properties, similar creep and creep-rupture strengths, and slightly better hot workability. Moreover, the impact strength of modified Type 316 steel after exposure for 6000 hr at 1100, 1300, and 1500 F was markedly superior to that of AISI Type 316 steel. The results of X-ray and metallographic studies indicated that the superior impact strength of the modified Type 316 was the result of lesser amounts of sigma formed in the new steel.