alloy 602CA (UNS06025)

CASE HISTORIES ON SUCCESSFUL APPLICATIONS OF ALLOY 602CA, UNS N06025 IN HIGH TEMPERATURE ENVIRONMENTS Jason Wilson Rolled Alloys 125 West Sterns Road, Temperance, MI 48182-9546 D. C. Agarwal ThyssenKrupp VDM USA, Inc. 11210 Steeplecrest Drive #120, Houston, TX 77065-4939, USA dcagarwal@pdq.net ABSTRACT Carbon steel, a workhorse of many industries, loses its usefulness above 538°C (1000°F) both due to strength degradation and corrosion. Alloy steels with chromium and molybdenum additions have expanded the useful temperature range of high temperature applications. However, with the increasing severity of high temperature environments encountered in modern day industries, there has been a singular lack of alloys, which can provide a combination of properties such as good mechanical strength and high temperature corrosion resistance to various modes of degradation (oxidation, carburization, metal dusting, etc.) up to 1200°C. This paper describes the development of one such nickel base alloy – alloy 602CA (UNS N06025) , which has provided a unique combination of properties by optimization of various alloying elements. This alloy since its introduction to the market in the early 1990’s , has found numerous applications in the heat treat industry, annealing furnaces, furnace rolls, furnace belts, heat treat baskets, hydrogen reformer by-pass ducts, chemical vapor deposition retorts, serpentine grids, direct reduction of iron-ore technology to produce sponge-iron, calciners to produce very high purity alumina, calciners for chrome-iron ore for producing ferro-chrome, calciners to reclaim spent nickel catalysts, catalytic converters and glow plugs in the automotive industry, refineries, petrochemical industries, nuclear waste vitrification processes and many others. A brief description of some of these applications is presented in this paper. Keywords: Alloy 602CA, UNS N06025, applications, high temperature, corrosion resistance, oxidation, carburization, metal dusting, high temperature strength, creep, stress – rupture INTRODUCTION Most high temperature nickel base alloys have sufficient amounts of chromium with addition of either aluminum or silicon to form protective oxide scales for resisting high temperature corrosion. Optimization of the various alloying elements led to a new alloy for service temperatures up to 1200°C in various industries. This alloy known as Nicrofer® 6025HT (alloy 602CA – UNS N06025) employs the beneficial effects of high chromium, high aluminum, high carbon and micro alloying with titanium, zirconium and yttrium. Developed in the early 1990’s, the alloy has found numerous applications in various industries as mentioned above. Nicrofer® is a registered trade mark of ThyssenKrupp VDM GmbH Zachary Schulz - Invoice INV-308045-MQBJZ4, downloaded on 2/17/2010 12:01:50 PM - Single-user license only, copying and networking prohibited. The typical chemical composition of the alloy in weight % is given below: Ni Cr Fe Al C Ti Zr Y Bal 25 9.5 2.2 0.18 0.15 0.06 0.08 This alloy is covered in ASTM, ASME via code case # 2359-1 to 1800°F for SC VIII, Div 1 use and SC 1 use for steam service up to 1650°F and other international specifications. AWS coverage has been approved for inclusion in AWS A5.11 (EniCrFe-12) for coated electrodes and A5.14 (ERNiCrFe-12) for bare filler wire. The major beneficial features of this alloy are: • Excellent oxidation resistance up to 1200°C, superior to other wrought nickel base alloys currently available in the market • Good high temperature strength (stress rupture and stress to produce 1% creep at temperatures up to 1200°C), superior to many other Ni-base alloys over 1000°C. • Excellent carburization resistance. • Excellent metal dusting resistance. • Excellent grain growth resistance A brief recap of its physical metallurgy, high temperature degradation resistance, strength (stress-rupture, creep, tensile) and fabricability is presented with major emphasis on applications. Further details and data have already been published and are available in the open literature. Information on the original R&D development goals for this alloy, effects of the various alloying elements, physical metallurgy and micro-structural aspects are well documented in the various references. (1-9) ALLOY 602CA DEVELOPMENT GOALS AND PROPERTIES It is a well known fact that all high temperature materials and alloys have certain limitations and the optimum choice is often a compromise between various factors such as (1) the mechanical constraints and compatibility at maximum temperature of operation; (2) environmental constraints as imposed by the process conditions of high temperature, (3) ease of fabricability and repair, and (4) cost effectiveness and availability. Table 1 gives the typical chemical composition of several common high temperature alloys in commercial use today. However, there is a singular lack of an alloy possessing the combination of required properties (high temperature strength, high temperature corrosion resistance, ease of fabricability, cost effectiveness) for application up to 1200°C. Alloy 602CA appears to fulfill this void as proven by the various applications shown below in this paper. TABLE 1 METALLURGICAL OPTIMIZATION OF ALLOY 602CA NOMINAL CHEMISTRY COMPARISON TO SOME OTHER HIGH TEMPERATURE ALLOYS Alloy Fe Ni Cr Si C Others 309 Bal 13 25 0.5 0.15 - 310 Bal 20 25 0.5 0.08 - 253MA Bal 11 21 1.7 - N ,Ce 330 Bal 35 19 1.25 0.05 - 333 18 45 25 1.0 0.05 C0 3 , W 3 800/800H Bal 31 20 0.4 0.08 Ti, Al 0.4 45TM 23 Bal 27 2.7 0.08 RE 600 9 Bal 16 - 0.07 - 601 14 Bal 23 - 0.06 Al 1.4 602CA 9.5 Bal 25 - 0.18 Y, Zr,Ti, Al 2.2 214 2.5 Bal 16 0.10 0.03 Al 4.5, Y X 18 Bal 22 - 0.10 W, Co, Mo 9 625 3 Bal 22 - 0.03 Cb 3.5, Mo 9 617 1.5 Bal 22 - 0.06 Co 12.5 , Mo 9, Al 1.2 Zachary Schulz - Invoice INV-308045-MQBJZ4, downloaded on 2/17/2010 12:01:50 PM - Single-user license only, copying and networking prohibited. TABLE 1 (continued) METALLURGICAL OPTIMIZATION OF ALLOY 602CA NOMINAL CHEMISTRY COMPARISON TO SOME OTHER HIGH TEMPERATURE ALLOYS Alloy Fe Ni Cr Si C Others 188 1.5 22 22 0.3 0.10 W 14 , La 0.04 230 1.5 Bal 22 0.4 0.10 W 14 , Mo 1.2 120 Bal 37 25 0.6 0.05 Cb 0.7 , N 0.2 C4 2.0 Bal 16 0.04 0.005 Mo 16 , Ti 0.4 160 1.5 Bal 28 2.75 0.05 Co 29 690 9.0 Bal 29 0.20 0.02 ---- Alloy 602CA Metallurgy Alloy 602CA employs the beneficial effects of high chromium, high aluminum, high carbon and microalloying with titanium, zirconium and yttrium in a nickel matrix. The relatively high carbon content of approximately 0.18% to 0.2% in conjunction with 25% chromium ensures the precipitation of bulky homogeneously distributed carbides, typically 5 to 10 microns in size. Transmission and scanning electron microscopy suggest these bulky carbides to be of M23C6 type primary precipitates. Microalloying with titanium and zirconium allows the formation of finely distributed carbides and carbonitrides. Solution annealing even up to 1230°C does not lead to complete dissolution of these stable carbides and thus the alloy resists grain growth and maintains relatively high creep strength due to a combination of solid solution hardening and carbide strengthening (9) . This phenomenon of grain growth resistance is responsible for maintaining good ductility, a high creep strength up to 1200°C and superior low cycle fatigue strength. Repair and reconditioning of exposed parts is thus easily achieved. The presence of approximately 2.2% aluminum in this alloy allows the formation of a continuous homogenous self- repairing Al2O3 sub-layer beneath the Cr2O3 layer, which synergistically imparts excellent oxidation as well as carburization and metal dusting resistance; “Reactive elements” like yttrium significantly increase the adhesion and spalling resistance of the oxide layers, thereby further enhancing the high temperature corrosion resistant properties. Also, because of its relatively low aluminum content, this alloy does not embrittle due to gamma prime formation, as is the case with higher aluminum containing nickel alloys. High Temperature Mechanical Properties The mechanical properties of interest in designing high -temperature components are “time independent properties”, short term tensile (typically below 600°C) and “time dependent properties” (typically above 600°C), such as stress rupture and creep strength, and thermal stability i.e. maintenance of reasonable impact toughness after long aging. Table 2 lists some of these properties from recent production heats. Comparison with other high temperature alloys is provided elsewhere (2). Table 3 lists the impact strength after aging at various temperatures up to 8000 hours. It is evident that alloy 602CA possesses adequate toughness properties for most industrial applications. TABLE 2 HIGH TEMPERATURE MECHANICAL PROPERTIES OF ALLOY 602CA Typical Short Term Tensile Properties Room Temp. 600°C 800°C 1000°C 1100°C 1200°C UTS (KSI) 105 89 45 15.5 12 5 0.2% Y.S. (KSI) 51 38 35 13 9 4.5 Zachary Schulz - Invoice INV-308045-MQBJZ4, downloaded on 2/17/2010 12:01:50 PM - Single-user license only, copying and networking prohibited. 100,000 hrs and 10,000 hrs Stress Rupture and 1%Creep Strength (KSI) Temperature Rm /105h Rm/104h Rp1.0/105h Rp1.0/104h 650°C (1202°F) 18.8 24.6 16.7 21.0 700°C (1292°F) 13.0 17.4 10.87 14.8 800°C (1382°F) 3.62 5.1 1.74 2.9 900°C (1652°F) 1.67 2.46 0.81 1.36 950°C (1742°F) 1.23 1.74 0.55 0.94 1000°C (1832°F) 0.93 1.35 0.39 0.62 1050°C (1922°F) 0.68 1.09 0.25 0.43 1100°C (2012°F) 0.43 0.74 0.17 0.31 1150°C (2102°F) 0.33 0.58 - 0.20 1200°C (2192°F) 0.20 0.43 - 0.14 Rm/105h = Stress rupture in 100,000 hrs, Rm/104 h = Stress rupture in 10,000 hrs. Rp. 1.0/105 = 1% creep in 100,000 hrs. Rp 1.0/10 4 = 1 % creep in 10,000 hrs. TABLE 3 IMPACT STRENGTH OF ALLOY 602CA IN J AFTER AGING AT VARIOUS TEMPERATURES UP TO 8000 HOURS Exposure Temperature And Condition 1000 Hrs. 4000 Hrs. 8000 Hrs. Annealed Condition Typical Values 78 to 84 J 500°C Exposure 53 35 30 10% Cold Worked + aged 28 26 22 C.W.+ aged + Annealed 76 77 78 640°C Exposure 54 32 30 10% Cold Worked + aged 33 25 27 C.W.+ aged + Annealed 77 77 85 740°C Exposure 55 30 27 10% Cold Worked + aged 40 29 25 C.W.+ aged + Annealed 79 79 76 850°C Exposure 73 62 58 10%Cold Worked + aged 73 70 68 C.W.+ aged + Annealed 76 84 80 High Temperature Corrosion Resistance: Oxidation: It is well known that elements having greater thermodynamic affinity for oxygen tend for form passive barriers in alloy systems, thus providing the required resistance. Chromium, aluminum and silicon are the three major elements, which account for these passive barriers. The usefulness of protective chromia Cr2O3 is limited to around 950°C due to the formation of volatile chromium oxide (CrO3). The higher thermodynamic stability of the alumina sub-layer, at even very low partial pressures of oxygen, improves the alloy 602CA oxidation resistance in cyclic tests. Rare earth elements further reduce the cracking, fissuring and spalling of the protective oxide alumina sub-layers. Zachary Schulz - Invoice INV-308045-MQBJZ4, downloaded on 2/17/2010 12:01:50 PM - Single-user license only, copying and networking prohibited. Table 4 presents the laboratory test data on cyclic oxidation testing (24 hrs cycles – 1.5 hr heat up, 16 hrs hold at temperature, furnace cool down for test temperatures up to 1100°C, and cooling in air for temperatures higher than 1100°C) for periods up to 1200 hrs. As is evident, alloy 602CA gave superior performance when compared to many other iron-, nickel- and cobalt- based alloys. Metallographic examination of alloy 602CA showed a continuous alumina sub-layer without any selective internal oxidation by comparison to alloy 601 (Figure 1) The higher thermodynamic stability and more than five orders of magnitude lower dissociation pressure of alumina are the primary reasons for formation of the protective alumina layers. TABLE 4 CYCLIC OXIDATION DATA – 1200 HOURS – 24 HR. CYCLES WEIGHT CHANGE ( mg / m2h ) Alloy 750°C 850°C 1000°C 1100°C 1200°C 602CA +0.4 +3 +12 +7 -310 X +1 +8 +5 -5 - 800H +7 +8 -24 -162 - 625 +1 +6 -100 -1410 - 601 +1 +10 +7 -24 -820 188 +1 +4 +7 -302 - 617 +4 +12 +19 -19 - Metallographic examination of alloy 602CA showed a continuous alumina sub-layer without any selective internal oxidation by comparison to alloy 601. Further tests conducted on alloy 602CA and alloy 601 for 3,150 hours at a lower temperature of 2100°F (1148°C) again showed excessive internal oxidation with alloy 601. In contrast, alloy 602CA had no internal attack but only a thin surface oxide scale. This is especially beneficial in applications that utilize thin sheets such as in radiant tubes. No internal oxidation means the entire wall thickness is sound metal and the alloy retains most of its original properties. Another series of test at 2200° and 2250°F for 3040 hours show the excellent oxidation resistance of alloy 602CA at extreme temperatures. Other high creep strength alloys containing molybdenum (alloy 617) or tungsten (alloy 230) for strengthening showed extensive scaling in these tests. In the case of 617, the material had to be removed from the test due to the destruction of the 0.250 inch thick plate sample after one test cycle. Table 5 shows this data. The higher thermodynamic stability and more than five orders of magnitude lower dissociation pressure of alumina are the primary reasons for formation of the protective alumina layers. Work by other authors (10) has also shown good oxidation resistance of alloy 602CA. TABLE 5 WEIGHT GAIN OF VARIOUS ALLOYS AFTER 3,040 HOUR EXPOSURE IN AIR AT 2200°F (1205°C) & 2250°F ( 1232°C) 2200°F (1205°C) Exposure 2250°F (1232°C) Exposure Alloy Weight Gain (mg/cm2) Weight Gain (mg/cm2) 602 CA 47.6 61.8 600 453.4 332.9 601 114. 138 214 ---- 21.9 230 348.3 ---- 353MA 231.9 323.8 333 155.6 ----- 617 ----- 327.63 ( 617 removed after <200 hours) Another series of cyclic oxidation test at 2100°F (1148°C) for 3,000 hours (cycle time of 160 hours) measured the weight loss as well as total penetration in mils by metallographic examination. These results are shown in Table 6 below. Again alloy 602CA gave excellent performance in these tests. Zachary Schulz - Invoice INV-308045-MQBJZ4, downloaded on 2/17/2010 12:01:50 PM - Single-user license only, copying and networking prohibited. TABLE 6 WEIGHT GAIN AND INTERNAL PENETRATION OF VARIOUS ALLOYS AFTER 3,150 HOUR EXPOSURE IN AIR AT 2100°F (1148°C) Alloy Weight Gain (mg/cm2) Max Internal Penetration (mils) 330 55 ---- 333 34 15.4 446 Stainless >530 79.6 353MA 37 16.8 617 30 6.3 230 23 5.0 602CA 18 1.4 214 9 2.8 HR120 206 46.3 800HT 294 54.4 Carburization/Metal Dusting Besides oxygen attack, high- temperature alloys are frequently subjected to attack by carbon. The degradation of metallic systems in carburizing environments can take two forms, namely carburization and metal dusting (some times referred to as catastrophic carburization). Metal dusting is a kind of corrosion, which manifests itself as pits and which proceeds at a rapid metal wastage rate at approximately 500-800°C in media with carbon activities significantly greater than one. Investigations by Grabke et al (4,8, 9) over a period of 10,000 hours showed that in contrast to the materials currently in common use, such as alloy 601 and alloy 600, the new alloy 602CA exhibited no metal dusting and had very good corrosion resistance under the applicable test conditions after 10,000 hours. Due to the very low solubility of carbon in nickel, materials with high nickel content are considered beneficial for imparting carburization resistance. Alloys high in chromium, aluminum and silicon form a protective oxide layer, which prevents the ingress of carbonaceous corrosive species thus providing improved resistance. However, if alternating exposure to carburizing and oxidizing environments is experienced, the precipitated carbides are converted to oxides and the liberated CO widens the grain boundaries thus loosening the oxide layer, whereby causing accelerated deterioration. The higher nickel plus chromium coupled with high aluminum content of alloy 602CA results in lowest weight gain in the temperature range tested as shown in Table 6. The reason for improved carburization behavior is due to the formation of an alumina sub-layer rather than via the nickel content alone as exhibited by the oxidation data in Table 4 at 1200°C for alloy 602CA and alloy 601. In a recent study on metal dusting behavior of nine nickel base alloys and four Fe-Ni-Cr alloys (4) tested in a carburizing H2-CO-H20 gas with carbon activity ac > >1 at 650°C, alloy 602CA was one of the most resistant material. Figure 2 shows the metal wastage rate of three nickel base alloys due to metal dusting. Table 7 gives the tabular data for the various materials tested. One very important point to note is that these results were obtained on unstressed coupons. In the real world the components exposed to metal-dusting type environments are stressed and hence have certain amount of strain. Alloy 602CA, even with 1% strain maintained its passive oxide layers thus preventing any accelerated attack whereas in alloy 690, the passive layer is damaged leading to accelerated metal wastage. TABLE 6 CYCLIC CARBURIZATION BEHAVIOUR IN CH4 / H2 ENVIRONMENT ( Ac = 0.8 ) IN TEMPERATURE RANGE 750°C – 1000°C Weight change (mg/m2h) Alloy 750°C 850°C 1000°C 310 2 130 305 800H 4 143 339 625 4 105 204 Zachary Schulz - Invoice INV-308045-MQBJZ4, downloaded on 2/17/2010 12:01:50 PM - Single-user license only, copying and networking prohibited. TABLE 6 ( continued) CYCLIC CARBURIZATION BEHAVIOUR IN CH4 / H2 ENVIRONMENT ( Ac = 0.8 ) IN TEMPERATURE RANGE 750°C – 1000°C Weight change (mg/m2h) Alloy 750°C 850°C 1000°C 617 2 50 64 X 2 93 204 601 2 69 152 602CA 0 44 58 TABLE 7 TOTAL EXPOSURE TIME & FINAL WASTAGE RATE AFTER EXPOSURE IN METAL DUSTING ENVIRONMENT (CO- H2-H2O GAS AT 650°C) Alloy Surface Total Exposure Time Final Metal Wastage Condition in hours Rate in mg/cm2h 800H ground 95 0.21 HK-40 - 190 0.04 HP-40 - 190 0.038 DS ground 1988 4.3 x 10-3 600H ground 5000 0.003 601 black 6697 7.3 x 10-3 601 polished 1988 4.9 x 10-3 601 ground 10000 5.8 x 10-4 C-4 ground 10000 1.1 x 10-3 214 ground 96651) 1.2 x 10-3 160 ground 96651) 6.3 x 10-4 45TM black 10000 1.0 x 10-5 602CA black 10000 1.1 x 10-5 6172) ground 70001) 3.7 x 10