| OCR Text |
Show the ability to transfer heat through metal and can also be related to thermal efficiency. This states that the rate of heat transfer is inversely proportional to the thickness and proportional to the surface and temperature difference. The Q /t is the heat energy per unit of time, K = thermal conductivity coefficient, T 2 - T 1 = temperature differences between the outside and inside surfaces, similar to applications of muffles or radiant tubes, A = su rface area, L = mass (wall thickness). As the thickness increases, the rate of heat transfer will decrease. To illustrate the effect of mass on thermal gradients, generally it's more of a problem in conveyor systems, an example is round bars used in a heat treat basket (Figure 1). Making the bar "stronger" by increasing the cross section does not necessarily make it better because it can increase the thermal gradients between the core and outside surfaces. In this particular illustration, replacing a 3/8" diameter with a 1/2" diameter round bar increased the cross section by 78%. The larger cross section increased the thermal gradients that occurred whenever the basket was quenched. I twas unable to absorb the increased and repeated stresses and developed a maze of intergranular fatigue cracks. The bar then fails, not by load, but by thermal gradients. A better way to make the bar stronger is to change the alloy content. Oxidation resistance is another property important in performance levels in thermal equipment. Basically RA alloys have oxidation resistance to 20000F. The relative oxidation resistance of various alloys can vary up or down, depending upon the load and the rate and frequency of heating and coo Ii ng. Whi Ie we do think, along with several other testers, a 20 hour 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 envi ronment contai ns sulphur. I t is considered to be the selective reduction of nickel and i ron under conditions that do not reduce the more stable chromium oxide. The selective reduction forms voids leaving a greenish appearing chromium oxide. Upon initjal inspection it looks like rotted wood that is yellow-green in appearance. It appears that the propensity to form is rela ted to the nickel content; the more the nickel the more likely to occur. Because of the nar;ow parameters required by the environment to produce green rot it will only occasionally be observed, but should be recognized when it 280 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 12000F. 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 their toughness and ductility. This usually happens with high chromium, low nickel grades such as 309 or 310 (Figure 4). The most common problem is the formation of a very brittle phase usually in the grain boundaries identified as sigma. Differentiating this from carbide precipitations, which occur in the same temperature range, there is minimal chromium depletion. Sigma forms in the 1100° to 16000F temperature range peaking at 13000F. It is a time - temperature formation. 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 16000F. FABRICATION AND DESIGN Selection of an alloy for a specific application may involve two considerations : what composition should be used, and should it be wrought or cast form. Although the latter might be considered a design function, it is included here because, (1) the basic principles of design discussed under that subject are applicable to structures in general, whether cast, or w rought, and, (2) the procedure followed in selecting the composition would apply to either cast or wrought form. The Advantages of Cast Are: 1. Initial cost - Since the casting is essentially a finished product as- cast, its cost per pound may be less than a fabricated item. 2. Strength - Simi lar compositions are inherently stronger at elevated temperature in cast form than in wrought; this is attributable to the very coarse grained as- cast structure and the fact that most equivalent cast compositions are modified with hic-her carbon content to improve castabllity. |
