Institute of Metals Division - High-Temperature Short-Time Creep of Graphite. H E Martens

INTEREST in the use of graphite as a high-temperature engineering structural material has recently increased markedly. However, actual use of this material has been limited, in part because information about its high-temperature mechanical properties has been lacking. The information needed includes not only reliable values for the properties, but also how to control these properties. Malmstrom, Keen, and Greenl have reported on the tensile and creep properties of some graphites above 2700° F (1500°C). They observed measurable creep under tensile loading at 4000 OF (2200 °C) and higher, with elongations of about 7 pct at 4500°F. Their typical elongation-vs-time curve showed the normal three stages of creep found for many metals at elevated temperatures. The specimens used were heated by passing an electric current through them. This method of heating caused a significant difference between the surface and axis temperatures, and may have affected the results. More recent work by kattus, in which the specfinens were also heated by their own resistance, showed very little plastic deformation for either slow or fast tensile testing or for creep testing. Work at this Laboratory, already reported,3 showed that for temperatures above 4500 "F and under conditions of slow tensile loading some commercial graphites exhibited elongations up to 20 pct. The work reported here is the continuation of a study of the tensile creep behavior of some commercially available synthetic graphites at temperatures up to 5300°F, and is preliminary to a more systematic investigation of the variables governing the high-temperature mechanical properties of graphite. MATERIALS TESTED The tests were conducted on six blocks from four lots of synthetic graphite bearing four different commercial-grade designations. These blocks were selected because their tensile stress—strain properties had been determined in previous work.3 Table I pre- sents the available information, supplied by the producers, on the processing of these materials. Also included in this Table are the densities and the tensile strengths at room temperature and 4500°F determined by the authors. EXPERIMENTAL TECHNIQUE The furnace used to heat the specimens for testing is shown in Fig. 1. The heating element was a graphite tube, 3-in. OD by 2 1/2 in. ID by 24 in. length. A spiral was cut into the 6-in. center portion of this tube to provide a high-resistance sec-