OCR Text |
Show Oxidation resistance Is another property important in performance levels in thermal equipment. Basically RA alloys have oxidation resistance to 2000°F. The relative oxidation resistance of various alloys can vary up or down, depending upon the load and the rate and frequency of heating and cooling. We do think, along with several other testers, a 20 hours cycle test more closely approaches the conditions normally associated with the heat treating industry. Figure 2 illustrates the cyclic scale resistance of various alloys at 20000F for 500 hours. Two other surface conditions can occur to alloy at elevated temperatures-green rot and metal dusting. While not typical sources of failures, both conditions are possible and could be of interest. Because of its appearance, green rot Is sometimes taken for a SUlphurous deposit, and when observed becomes a mystery when neither feed stock or the environment contains sulphur. It Is considered to be the selective reduction of nickel and iron under conditions that do not reduce the more stable chromium oxide. The selective reduction forms voids leaving a greenish appearing chromium oxide. Upon initial inspection it looks like rotted wood that Is 'yellow-green In appearance. It appears that the propensity to form Is related to the nickel content; the more the nickel, the more likely to occur. Because of the narrow parameters required by the environment to produce green rot it will only occasionally be observed, but should be recognized when it does occur. Metal dusting, sometimes known as metal erosion, will also form due to specific atmosphere or environmental conditions. It will usually occur in stagnant atmospheres that are reducing or carburizing to the metal in a temperature range of 800°F to 12000 F. The corrosion product, when still found in place, is a black dust composed of graphite, metal, metal carbides and metal oxides. This dust mixture is usually magnetic. The tendency increases with the nickel content. Sections of anchor bolts are subject to these conditions. The surface appearance of an anchor bolt is shown In Figure 3. Sigma will cause the properties of some heat resistant alloys to change after a few hundred or thousand hours in service and become brittle losing toughness and ductility. This usually happens with high chromium low nickel grades such as 309 or 310 (Figure 4). It is the formation of a very brittle phase usually In the grain boundaries Identified. Differentiating this from carbide preCipitations, which occur In the same temperature range, there is minimal chromium depletion. Sigma forms In the 1100° to 1600°F temperature range peaking at 13000 F. It is a time/temperature fonnation. The amount of sigma usually found in heat resistant alloys is not seriously detrimental to the alloy at high temperature. However, sigma can completely embrittle an alloy when it reaches room temperature and can be a source for failure during frequent thermal cycling. It can be eliminated by heating above 1600°F. 3 |