No abstract

Apparatus has been designed and built for conducting creep tests under constant-stress or constant-load conditions. An exact mathematical solution describing the shape of the cam lever for constant stress is given. This solution applies for the large strains usually found for ductile metals and alloys, as long as the strain is uniform. The initial specimen length employed in a constant-stress creep test ordinarily is fixed by the dimensions of the cam system used. It is shown by the present analysis that the initial specimen length may be changed if an approximate adjustment is made in the initial setting of a properly designed cam lever. The direction of the load axis remains fixed and therefore the apparatus has the advantage of a more complex level-beam machine. The results of a number of creep tests under constant-stress conditions are reported.

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.

The usual method of preparing specimens for through-thickness tension tests is to weld extensions (prolongations) to the plate by shielded metal-arc, electronbeam, or friction welding and to machine ASTM standard round tension specimens from these weldments. All three methods have disadvantages. Shielded metal-arc welding is time-consuming because small electrodes must be used and the sample must cool between weld passes to minimize the size of the heat-affected zone. The equipment for friction and electron-beam welding is relatively expensive. Consequently, a rapid and low-cost stud-welding method has been developed for attaching prolongations to plate coupons to produce weldments from which tension specimens up to 12.8 mm (0.505 in.) in diameter can be prepared. A portable stud-welding gun, mounted in a drill press to ensure axial alignment, is being used for welding studs to plate to produce weldments suitable for through-thickness testing. Welds have been made successfully on A36, A588, A514, A516, A537, and A633 steel plates, and through-thickness tension specimens machined from these weldments have been tested with a stud- or weld-failure rate of less than 2 percent. A stud fabricated from AISI 8620 steel bar and quenched and tempered to a minimum hardness of 30 Rockwell C appears to be suitable for through-thickness testing of plate products having strength levels up to and including that of A514 steel. Data show that this procedure is suitable for determining through-thickness reduction-of-area values for plate of about 25.4-mm (l-in.) thickness or more. Test results indicated that the strength level of studs does not have a significant effect on the tensile properties of the test plate (provided that the strength of the stud is greater than that of the test plate).

If Types 304 and 316 steels with improved elevated temperature strength existed, equipment manufacturers could design to higher stresses, thereby reducing the weight and consequently the cost of fabricated equipment. The addition of nitrogen appears to be one means of improving the strength of Types 304 and 316 steels with little additional cost. Although appreciable work has been done, most of it over a limited temperature range, the effect of nitrogen on the creep and creep-rupture properties of Types 304 and 316 steels has not been quantitatively established. This paper reviews studies to determine quantitatively the effect of nitrogen on the (a) creep and creep-rupture properties of Types 304 and 316 steels within AISI composition limits over a large temperature range and (b) toughness of Types 304 and 316 steels after long time exposures at elevated temperatures.