The merits of two certain types of accelerated laboratory testing methods for evaluating the indicated life of the coating on hot-dip zinc-coated sheet steel were compared. The two methods studied were the simulated atmospheric corrosion, using a moist gaseous mixture of sulphur dioxide, carbon dioxide, and air, and the spray, using normal solutions of sodium chloride and ammonium chloride separately. A consistent relationship between the " life" and weight of the coating was shown by the resu lts. The time requ ired for the breaking down of the coating was considerably less for the first method than for the second. The coating was corroded, in the first mcthod, in a progressive manner over the entire surface and similar to that reported for galvanized materials corroded in the atmosphere under service conditions, and in the sccond method, in a local and capricious manner. No attempt was made to interpret the experimental results in terms of service lifc in various types of atmospheres prevailing in different climates. Any satisfactory attempt at such an evaluation will have to await the results of long-time field tests on zinc-coated products carried out under several typical atmospherical conditions obtaining at different locations. The presence or absence of about two-tenths of 1 per cent copper in the steel base produced no apparent effect on the results. Tests were also made on specimens which had been annealed for the purpose of converting the zinc coating into an iron-zinc alloy.
A study was made of the creep characteristics at 1,600°F. (870°C.) of 15 alloys covering a range from 1 to 75 percent nickel and from 3 to 55 percent chromium. The results were compared with those of a previous investigation at 1,000°F. (538°C.) on similar alloys.In the investigation at 1,000°F. (538°C.) of the nickel-chromium-iron system, it was found that the alloys containing little or no iron, 50 to 80 percent nickel and 20 to 50 percent chromium exhibited the greatest resistance to creep. At 1,600°F. (870°C.), the strongest alloys are those containing approximately equal parts of nickel and chromium, and not more than 30 to 40 percent iron.As part of a metallographic study the attempt was made to distinguish between the effect on structure of elevated temperature alone and of elevated temperature and stress combined. A comparison was made between the structure in specimens used in the Greep test and that in the unstressed specimens of the same alloys annealed at 1,600°F. (870°C.) for periods ranging from 100 to 1,000 hours or quenched in iced brine from that temperature. In nearly all cases the quenched specimens were similar in structure to the annealed materials which indicates that these alloys were not readily heat treated.Carbide precipitation and agglomeration of the carbide at the grain boundaries were most pronounced in both the binary iron-chromium alloys and the ternary iron-chromium-nickel alloys of higher chromium content. Prolonged heating, of both stressed and unstressed specimens, did not produce any pronounced changes in the structure of the binary iron-chromium alloys or the ternary alloys except those containing 50 percent or more of nickel.
This report relates to flow (elongation) in 0.25 per cent carbon steel subjected to a fixed total load in tension at approximately constant temperature within the range 70 to 1,100°F. (20 to 595°C ). The character of flow is described, and the factors governing selection of maximum allowable stresses are discussed for service in which very long life and freedom from appreciable deformation must be considered.Comparisons are also given between maximum allowable stresses and the stress-strain relations determined in the customary short-time tension tests at various temperatures.
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