Nickel-Based Furnace Alloy Extends Maximum-Use ... - Rolled Alloys

Z~rcon~um- -Strength, gram-growth contrc Source: Ref 1.Fig 1 Oxrdation rates for selected heat-resistant alloys at temperatures between2000 and 2150'E Samples were cycled weekly to room temperature over a testperiod of 7,500 to 3,400 hours. Source: Ref 2,3.oxide layer on the alloy surface.Typical chromium contentof wrought heat-resistantalloys ranges from 15 to 25%.Alloying with additionalsmall quantities of rare-earthelements (microalloying) hasenhanced the stability of thischromium oxide scale[4].Cerium, yttrium and lanthanumare the more commonlyused elements. Such alloyingadditions are becoming morecommon and are present inrecent alloy developments.Aluminum and silicon additionsimprove scaling resistanceby forming continuous aluminaand silica sublayers below thechromium-oxide scale. Thesesublayers provide additionallevels of protection between theatmosphere and the alloy.Scaling resistance of RA602 CA, even at extreme temperatures,is attributed to itshigh chromium content (25%)enhanced by additions of2.2% aluminum and 0.1%yttrium. The aluminum additionallows for the formationof a continuous homogenousself-repairing A1203 subscale,and the addition of yttriumenhances the adhesion of thechromium and aluminumoxide layers.Another measure of oxidationresistance is the amountof metal affected by internaloxidation and actual metalloss. Oxidation metrics areillustrated in Fig. 2. Theappearance of RA 602 CAand Alloy 601 after 3150hours at a temperature of2100°F (1150°C) are shownin Fig. 3. Internal oxidation isprevalent in the alloy 601sample, while only a thinoxide scale formed on the RA602 CA surface. Freedomfrom internal attack is importantin applications usingsheet material, such as radianttubes. Absence of internaloxidation means a greaterpercentage of the wall thicknessis sound metal. As aresult, the alloy retains agreater level of mechanicalintegrity. A comparison ofoxide penetration in selectedalloys is provided in Table 3.Creep-rupture strengthMetals behave differently athigh temperatures than theydo near room temperature.Loading a metal bar to justbelow its yield strength atroom temperature will notlead to failure or deformationregardless of the amount oftime the stress is applied. Attemperatures above 1000°F(540°C) and higher, mechanicalstrength is no longer independentof time. A metalcomponent stressed to justbelow its yield point at redheat (1500°F, or 81S°C, forexample) will creep (stretch)slowly over time. Dependingon the alloy, the stress leveland the temperature involved,the component could last forhours, weeks or years untilthe metal finally fractures orruptures. Because of creep, analloy component can deformeven under the stressesMetal loss (milslside)[(?)IMaximum penetration (milslside)[C]Total metal affected (milslside)[(y+c)]Fig2 High-temperature oxidation metricsFig3 Surfaces of RA GO2 CA (left) and Alloy GO1 (right) after 3,150 hours exposureat2100"E