Occasionally in the application of the austenitic chromium-nickel steels to corrosive conditions, failures have occurred by cracking without serious general over-all attack of the metal. As pointed out by Hoyt and Scheil(1), and by Scheil, et al.(2) as well as by Hodge and Miller(3), the stress-corrosion failures that have occurred have been limited in number, and have taken place only when the steels were exposed to certain corrodents. These investigators have stated that while stress-corrosion cracking can be intergranular in nature and originate at the grain boundaries of the austenitic chromium-nickel steels, it can also take place in transgranular fashion. They have shown that the cracking may be either initiated at the grain boundaries and may propagate along grain boundaries for some distance and then suddenly extend across grains, or it may begin in a transgranular fashion and suddenly proceed along grain boundaries until the cracking stops. Their data further show that when austenitic stainless steel is subject to intergranular attack, stress will concentrate and cause cracking in service.
SYNOPSISThe demand in recent years for lightweight high-strength structures, especially in the transportation industry, has created considerable interest in the corrosion-resisting chromium-nickel and chromium-manganese-nickel steels. These steels are austenitic in character, and their strength can be most economically increased by application of cold-work, which, together with composition, determines their ultimate mechanical properties. Data have been published on the cold-rolled chromium-nickel steels, but practically no information is available on the austenitic type of chromium-manganese-nickel steels. The present paper gives data on these latter steels. The effects of various degrees •of cold-work on steels of different compositions are presented in •order to describe those steels that have high strength associated with good ductility, which is important from the standpoint of fabrication of the sections required for lightweight high-strength construction.The paper gives information on the stress-strain properties of the steels in both the longitudinal and transverse directions to rolling and shows the improvement obtained in these properties by application of the low temperature (200° to 300 °C.) stressrelieving heat-treatment. It further shows that the 17 per cent chromium-7 per cent nickel steels, and the 17 per cent chromium-5.50 per cent manganese-4.50 per cent nickel steels have better tension and compression properties longitudinal to the direction of rolling than do the 18 per cent chromium-8 per cent nickel steels, particularly when the steels are cold-rolled to a tensile strength exceeding about 150,000 lbs. per sq.in. All these steels have better compressive properties transverse to the direction of rolling than longitudinal to the direction of rolling, but this difference is less marked in the 17 per cent chromium-7 per cent nickel steels and the 17 per cent chromium-5.50 per cent manganese-4.50 per cent nickel steels than in the 18 per cent chro^-mium-8 per cent nickel steels. An attempt has been made to present the data on the steels so they will be of greatest value to the designer of lightweight high-strength structures.
The austenitic wrought chromium-nickel steels for which data are given in Section B of Part I are those which contain basically the following: 17 per cent chromium, 7 per cent nickel, 18 per cent chromium, 8 per cent nickel, 24 per cent chromium, 12 per cent nickel, and 25 per cent chromium, 20 per cent nickel. The detailed chemical compositions of these steels appear in Table XIX. Information as to their physical properties, and on the forging and heat-treating practices to be used in fabricating these chromium-nickel steels is given in Table XX. The mechanical properties of the steels in various wrought forms, such as bars, plates, wire, strip, sheet, seamless tubes, etc., are shown in Tables XXI to XXXIV. The creep characteristics and short-time tensile properties at elevated temperatures are presented graphically by the curves in Figs. 12 to 26.
Data in Section B cover the following types (the carbon content usually being higher than for similar corrosion-resistant alloys): 1. Straight chromium alloys with 26 to 30 per cent chromium and 7 per cent maximum nickel. 2. Chromium-nickel alloys with 18 to 32 per cent chromium and 8 to 22 per cent nickel; chromium always being in excess of nickel. 3. Nickel-chromium alloys with 10 to 21 per cent chromium and 33 to 68 per cent nickel. The detailed chemical compositions of alloys are given in Table XXXVIII and physical properties in Table XXXIX. The mechanical properties of the alloys at room temperature are shown in Table XL. It should be recognized, however, that room temperature data are of little use in estimating the behavior of the materials at high temperatures. Short-time and long-time elevated temperature properties are represented in Table XLI. Relative heat-resistant characteristics of the different grades appear in Table XLII. In this table the alloys are indicated to be excellent, good, fair, or poor in various types of application, based on the consensus of the alloy casting manufacturers. The data are included merely as a general guide; not as recommendations for specific uses.
Analysis of surface finishes on machined polystyrene foam presents a unique challenge when the cell size of the foam is of the order of or larger than desired surface finishes. Ideally, the surface could be defined as the geometric surface formed by the locus of the severed edges of the cell walls. However, both machining and grinding tend to rip and fracture cell walls and leave asperities formed by agglomerations of fragmented cell walls. Machined geometric surfaces can be defined as the locus of the tips of the asperities, but the surface in between asperities can extend several cell layers below the asperities. The severe nature of this problem is emphasized by stereoscopic examinations of fractured, machined and ground, and cryo-vibratomed polystyrene surfaces in the SEM.Since coating does not seriously distort low-density polystyrene foam, the specimens were gold-palladium coated for examination in a Hitachi S-800 FESEM at 5 kV. Stereo pairs were obtained using tilts of + and − 3 1/2 degrees. The polystyrene foam had a cell size that varied between 2 to 11 μm.
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