Toshio Narita

Tie-Line Compositions of the σ and (γ, γ', β) phases in the Ni-Al-Re-Cr System at 1423K

A significant number of diffusion barrier coatings have been proposed; however, after a relatively short oxidation time the barrier layer loses the layer structure, resulting in rapid, catastrophic oxidation behavior. Narita et al. proposed a diffusion barrier coating with a duplex layer structure, containing the σ-phase of Re-Cr-Ni as the diffusion barrier layer. In the coating, the σ-layer was sandwiched between a Ni-based superalloy and Ni-aluminides as the Al-reservoir layer. To elucidate the properties of the Re-Cr-Ni alloy, information regarding the phase diagrams and diffusion barrier is required. It is essential to determine the structural stability of the sandwiched Re-Cr-Ni alloy layer. The phase diagrams of the Ni-Cr-Re, Ni-Al-Re, and Ni-Al-Re-Cr systems at 1423 K were investigated. It was found that a very long heat treatment was required to obtain equilibrium compositions of Ar-arc-melted alloys. Two-phase diagrams for the Ni-Cr-Re system have been reported by Slyusarenko et al. and Huang et al. In the present study, a Ni-Cr-Re diagram at 1423 K was experimentally established. From comparison of these results, the three-phase γ, α, σ region was close that of Huang et al., and the δ-phase was close to that of Slyusarenko et al. In the Ni-Al-Re system, δ-phase has tie-lines with γ-Ni, γ’-Ni3Al, and β-NiAl for Al at less than 50 at%, while β-NiAl with more than 50 at% and Ni2Al3 have tie-lines with Re2Al and Re4Al11, respectively. Importantly, the solubility of Re in γ’-Ni3Al and β-NiAl is very low, at less than 1.3 at% Re. In addition, the solubility of Al into the δ-phase is less than 1.8 at%. There is no ternary intermetallic compound in the Ni-Al-Re system at 1423 K. Figure 1 shows a cross sectional microstructure and concentration profiles for Re, Cr, Ni and Al measured along the lines on the Ni-Al-Re-Cr alloys after heat-treatments at 1423 K for 2500 h. The tie-lined compositions of each phase at 1423 K were experimentally determined as follows. The σ-phase tie-lined with the γ-phase (73.5 Ni, 14.6 Al, 2.9 Re, and 9.0 Cr) consists of 56.7 Re, 25.0 Cr, 18.1 Ni, and 0.2 Al. The σ-phase tie-lined with the γ’-phase (73.6 Ni, 21.0 Al, 1.2 Re, and 4.2 Cr) consists of 58.3 Re, 25.0 Cr, 16.6 Ni, and 0.1 Al. The σ-phase tie-lined with the β-phase (50.2 Ni, 48.7 Al, 0.3 Re, and 0.8 Cr) consists of 61.1 Re, 32.5 Cr, 5.2 Ni, and 1.2 Al. The σ-phase tie-lined with the β-phase (65.1 Ni, 29.5 Al, 0.4 Re, and 5.0 Cr) consists of 45.8 Re, 39.3 Cr, 14.6 Ni, and 0.3 Al. The Al4Re phase tie-lined with the β-phase (38.3 Ni, 56.0 Al, 1.0 Re, and 4.7 Cr) consists of 19.7 Re, 4.9 Cr, 2.7 Ni, and 72.7 Al.

Development Of A Sulfidation - Corrosion Resistant Nickel-Base Superalloy For FCC Power Recovery Turbine Rotors.

A nickel-base superalloy for FCC power recovery turbine rotors was developed, which shows excellent sulfidation-corrosion resistance, high temperature strengths, and hot workability. To investigate the influences of the alloying elements of AISI685 on the sulfidation behavior, nickel-base alloys containing each of chromium, cobalt, molybdenum, titanium, and aluminum (the major components of AISI685), and alloys containing multiple such elements were produced and corroded in sulfidizing gas atmospheres at 600°C. It was found that the sulfidation behavior, especially the penetrating behavior into the alloys, was greatly influenced by aluminum, and that the resistance could be improved by increasing the aluminum content in the alloys. Six types of alloys, in which aluminum had been increased to improve the sulfidation resistance of AISI685 and titanium decreased by the same proportion to maintain the high temperature strengths of AlSI685, were forged to test their sulfidation behavior and high temperature strengths. As a result, the alloy with 1.5 percent titanium and 3.0 percent aluminum showed only half the sulfidation depth of AISI685, for the tensile characteristics at 538°C, and at 850°C through 1100°C, equivalent or better than those of AISI685. This means that the alloy with 1.5 percent titanium and 3.0 percent aluminum has twice more sulfidationxad corrosion resistance than AISI685, in addition to equal or higher high temperature strengths and hot workability. A simulation expander rotor disk of 500 mm in diameter was manufactur ed with the newly developed alloy. The disk was successfully produced without shear cracking or buckling problems. Furthermore, test specimens cut from the disk showed a rupture strength equal to that of AISI685 at 650°C to 750°C. INTRODUCTION Gas expander turbines are incorporated in fluid catalytic cracking units (FCC) of oil refineries as power recovery devices. As such, expander turbines are operated in exhausts at 500°C to 700°C, containing corrosive gases such as hydrogen sulfide (H2S), sulfur dioxide (S02), oxygen (02), and so on. AISI685 is used for the rotors, a nickel (Ni)-base gamma-prime (¥)-precipitation hardening alloy proven to be very strong and resistant to high temperature corrosion, as a material suitable for machine parts operated at high temperatures. However, several instances of inservice cracking have recently been experienced in rotating blades of expander turbines operated at about 530°C. Therefore, the cause was investigated in order to determine countermeasures (Dowson, et al., 1995). The authors examined the component in question and found that sulfide had been generated along the alloy grain boundary to a considerable depth around the blade root section, leading to the assumption that the cracks had propagated from the sulfidation-corrosion tip by corrosion fatigue, or some corrosion assisted degradation, and caused fracture. Sulfidation-corrosion with industrial gas turbines has been generally reported as hot corrosion (Stringer, 1979; Birks and Meier, 1983; Pettit and Giggins, 1987), which is a severe corrosion resulting from by-products related to molten sulfate such as sodium sulfate (Na2S04). However, substances that cause hot corrosion, such as sulfate, sodium, and/or chloride, were not detected in the corroded expander rotor. Therefore, we assumed that the sulfidation-corrosion had been caused by direct reaction between the sulfidizing gases and the alloy. It is reported that Nixad base alloys are subject to catastrophic corrosion through the reaction of a sulfidizing gas and the metal at or over about 700°C (Smeltzer, 1979), even without molten salts such as Na2S04. This is due to the fact that the eutectic point of the Ni and Ni-sulfide (Ni3S2) is 635°C (Kullerud and Yund, 1962), thus a melted material is produced at or over that point. However, Ni-base superalloys, in general, are considered to have excellent corrosion resistance in sulfidizing gas atmospheres that are below the eutectic point. Therefore, few cases have been reported in which severe sulfidation-corrosion of alloy grain boundaries had damaged the apparatus. Since many aspects of the mechanism and behavior of sulfidation-corrosion on the rotating blade of the expander turbine remain unexplained, the authors have been working to: • Explain the mechanism of corrosion and cracking, • Develop technology for life prediction of actual machines, • Develop surface modification technology for improving a sulfidation-corrosion resistance, and • Develop a sulfidation-corrosion resistant alloy. Some portions of the studies on corrosion behavior of AISI685 (Yakuwa, et al., 1996) and on the development of the surface modification technology for improving sulfidation-corrosion resistance (Nakahama, et al., 1995) have already been reported. This paper describes the effects of alloying elements on sulfidation behavior of AISI685 and the development of a high temperature sulfidation-corrosion resistant Ni-base superalloy. EFFECTS OF ALLOYING ELEMENTS ON CORROSION BEHAVIOR Description of Experiment AISI685 is a Ni-base superalloy composed of nickel (Ni), chromium (Cr), cobalt (Co), molybdenum (Mo), titanium (Ti), and aluminum (Al) as major alloying elements, as shown in Table 1. To clearly illustrate the effects of the alloying elements on corrosion behavior, various alloys with different components were produced by the argon arc melting method. That is, starting from pure Ni, another element was added to the previous mixture, in accordance with the chemical composition of AISI685, e.g., Ni-20Cr, Ni-20Cr-13.5Co, and so on. Alloys containing zero percent up to 4.5 percent Ti and Al, respectively, were also prepared, to study the effects in detail of variations in the contents. The ingots of the alloys were heat-treated at 1024°C for 24 hours and cooled in the furnace for homogenization. Then, each ingot was sliced into approximately 1 mm thick strips, both surfaces of which were polished to 1 j.Lm diameter diamond paste level and degreased with acetone. Corrosion tests were conducted at 600°C for nine hours through 144 hours. Though the partial pressures of sulfur (pS2) and oxygen (p02) in a 600°C atmosphere of actual machines are w-12 to w-10.5 atm and lQ-23.5 atm, respectively, the test gas was adjusted by hydrogen (H2) and hydrogen sulfide (H2S) mixtures, so that the pS2 became either 10-12 or lQ-10.5 atm and the p02 became lower than that of actual machines, in order to give sulfidation priority over oxidation. The corrosion behavior was evaluated by examining the weight gain after the test, observing corrosion morphologies by using a scanning electron microscope (SEM), and identifying corrosion products by x-ray diffract meter (XRD) and electron probe micro analyzer (EPMA). DEVELOPMENT OF A SULFIDATION-CORROSION RESISTANT NICKEL-BASE SUPERALLOY FOR FCC POWER RECOVERY TURBINE ROTORS 51 Table 1. Chemical Composition of AJS/685 (Wt.% ). Ni Cr Co Mo Ti AI c Zr B Fe Si s p Mn 18.0 12.0 3.5 2.75 1.2 0.02 0.02 0.003 ::;;; ::;;; ::;;; ::;;; :ii bal. 2 i .o I I I I I I I 2.0 0.15 0.015 0.015 0.1 15.0 5.0 3.25 1.6 0.10 0.08 0.010 Experimental Results Figure 1 shows the weight gain of the individual alloys corroded at 600°C for 49 hours under pS2 = IQ-10.5 atm. The corrosion amount of Ni was dramatically decreased by the addition of 20 percent Cr. As the weight gain of Ni-20Cr-13.5Co alloy is almost equal to that of Ni-20Cr, Co does not have much influence on the corrosion amount of Ni-20Cr. When four percent Mo was added to Ni-20Cr-13.5Co alloy, the weight gain decreased to approximately two-thirds, indicating that the addition of four percent Mo effectively reduced the corrosion. Then, the effects of adding Ti and AI were studied. Three percent Ti was added to Ni-20Cr13.5Co-4Mo alloy, revealing that the weight gain was almost the same with, or more than, that of Ni-20Cr-13.5Co-4Mo alloy, while the weight gain was almost two-thirds for the alloy containing 1.5 percent AI. Therefore, the addition of three percent Ti to Ni-20Cr13.5Co-4Mo alloy has almost no effect on the improvement of sulfidation-corrosion resistance (weight gain) or no effect, but that the addition of 1.5 percent AI works. Following this experiment, alloys containing different amounts of Ti and AI were tested to examine in detail the effects of Ti and AI contents in the Ni-20Cr13.5Co-4Mo alloy. An alloy of 3Ti-1.5Al has a simulated composition as that of AISI685. The weight gain of the 4.5Ti-1.5Al alloy, which contains more Ti than AISI685, was more than that of the 3Ti-1.5Al. On the other hand, 2.5Ti-2.5Al alloy, with higher AI content and lower Ti content, showed approximately half the corrosion of 3Ti-1.5Al.