Effect of microstructure on stress corrosion cracking of alloy 600 and alloy 690 in 40% NaOH

Stress corrosion cracking (SCC) behaviors of Alloy 600, Alloy 690 and the Ni-10Cr-10Fe alloy have been studied using a C-ring in 40% NaOH solution at 315°C. The current density of Alloy 690 in polarization curves was higher at 200 mV above corrosion potential than that of Alloy 600. SCC resistance increased with Cr content for the chromium carbide free alloys, probably due to facilitation of SCC crack tip blunting with an increase in Cr content. Both thermally treated Alloy 600 and sensitized Alloy 600 have a comparable amount of intergranular carbide. But the former is more resistant to SCC than the latter, which might be attributed to the presence of the slight Cr depletion around the grain boundary in the former one. Sensitized Alloy 600 showed higher SCC resistance than the solution annealed one due to intergranular carbide in sensitized Alloy 600. This implies that the beneficial effect of intergranular carbide overrides the harmful effects of Cr depletion for sensitized Alloy 600. SCC resistance of Alloy 600 increased with grain size. This article based on a presentation made in the symposium “The 4th International Conference on Fracture and Strength of Solid”, held at POSTECH, Pohang, Korea, August 16–18 under the auspices of Far East and Ocean Fracture Society (FEOFS),et al.     

Effect of Ti content on creep properties of Ni-base single crystal superalloys

The effect of Ti content on the creep properties and microstructures of experimental Ni-base single crystal superalloys has been investigated. The experimental alloys were designed to provide better high temperature properties than the commercial single crystal alloy CMSX-4. The creep properties of the experimental alloys, Alloy 2 and Alloy 3, were superior to those of CMSX-4. Alloy 3 showed a longer creep life than Alloy 2 at 900 °C and 950 °C, while it has similar creep life with Alloy 2 at 982 °C. Transmission electron microscopy micrographs of the experimental alloys after the creep test showed distinct deformation features as a function of temperature and Ti content. The dissociation of dislocations into partial dislocations with stacking faults in Alloy 3 was found to improve resistance to creep deformation at 950 °C. The effect of Ti on the creep deformation mechanism was not evident at 982 °C, which resulted in similar creep properties in both experimental alloys. The transition of the γ′ cutting mechanism from dislocations coupled with stacking faults to anti-phase boundary coupled pairs occurred both in Alloy 2 and Alloy 3. However, the transition temperature was higher in Alloy 3 than in Alloy 2 because of the difference in Ti contents.     

Field test of superheater corrosion in a CFB waste boiler: Part II – Scale formation characteristics

This study concerns the scales formed on the steels T22 , Alloy 310, Alloy 28 and the nickel‐based Alloy 65 in a superheater test coil at 460–540°C in a CFB waste boiler. The methods used for the characterisation of the scales included SEM, EDX, Auger spectroscopy and XRD. The deposits on the tubes consisted mainly of alkali chlorides and calcium sulphate. The scales formed consisted of Fe₂O₃ and Fe₃O₄ on the T22 steel, NiFe₂O₃ and Cr₂O₃ on Alloy 310 and Alloy 28, and Cr₂O₃ and NiO on Alloy 65. Rapid corrosion on the steel T22 was associated with the growth of an open columnar iron oxide below a thick porous chlorine‐containing scale. Pitting corrosion on Alloy 310 occurred and it may be associated with selective corrosion, first following the grain boundaries then uniformly attacking the metal. The only protective oxide was observed on Alloy 28 that formed an inner chromium oxide separating the chlorides from the metal. Dense thin chromium oxides were observed in the scale on Alloy 28, but no major cracks were found perpendicular to the tube. Alloy 65 suffered from grain boundary attack and was locally attacked under thick porous chromium oxide with nickel chlorides in the advancing front. Molybdenum was enriched at the interface to the metal on both Alloy 28 and Alloy 65.     

Field test of superheater corrosion in a CFB waste boiler: Part I – Metal loss characteristics

The corrosion was investigated on a superheater test coil in a CFB waste boiler. The alloys ranged from ferritic steel T22 to nickel‐based Alloy 65 and the metal temperatures were between 460 and 540°C. The thickness of the deposit was alloy and temperature dependent. The low‐alloyed steels developed thick deposits at all temperatures while the deposit thickness increased with the temperature on the high‐alloyed steels and the nickel‐based alloy. The corrosion attack was alloy dependent and related to the deposit crest. The nickel‐based Alloy 65 was preferentially attacked directly under the crest of the deposit while the other alloys were preferentially attacked at the edge. The corrosion rate increased with temperature for X20, Alloy 304L, Alloy 310 and Alloy 825; decreased on Alloy 65; and was bell shaped on T22 and Alloy 28. Alloy 310 suffered from severe pitting corrosion in a line following the edge of the deposit crest. The best overall corrosion resistant alloy was Alloy 28.     

Stress corrosion cracking of alloy 600 in an aqueous solution containing lead oxide

The stress corrosion cracking (SCC) behavior of Alloy 600 was studied in aqueous solutions containing lead. Electrochemical polarization and current transient experiments were performed at 315 °C in a 40% sodium hydroxide solution to assess the effect of lead on a passive film formed on Alloy 600. The influences of the alloy microstructure and the addition of an inhibitor to the environments on lead-induced SCC were investigated using C-ring and slow strain-rate tensile (SSRT) tests in demineralized high-purity water and caustic solutions containing PbO at 315 °C. The surface films on Alloy 600 were examined using Auger electron spectroscopy (AES) and energy dispersive x-ray spectroscopy (EDXS). The PbO markedly accelerated SCC of Alloy 600 in the caustic solution and the high-purity water at 315 °C. The addition of nickel boride (NiB) or cerium boride (CeB₆) to the test solutions decreased the susceptibility of Alloy 600 to SCC. Thermally treated Alloy 600 (Alloy 600 TT) tended to crack in a transgranular mode, while the solution-annealed Alloy 600 (Alloy 600 SA) tended to crack in an intergranular mode in water containing PbO.     

The effect of prior deformation on stress corrosion cracking growth rates of Alloy 600 materials in a simulated pressurized water reactor primary water

The effect of prior deformation on stress corrosion cracking (SCC) growth rates of Alloy 600 materials in a simulated pressurized water reactor primary water environment is studied. The prior deformation was introduced by welding procedure or by cold working. Values of Vickers hardness in the Alloy 600 weld heat-affected zone (HAZ) and in the cold worked (CW) Alloy 600 materials are higher than that in the base metal. The significantly hardened area in the HAZ is within a distance of about 2-3 mm away from the fusion line. Electron backscatter diffraction (EPSD) results show significant amounts of plastic strain in the Alloy 600 HAZ and in the cold worked Alloy 600 materials. Stress corrosion cracking growth rate tests were performed in a simulated pressurized water reactor primary water environment. Extensive intergranular stress corrosion cracking (IGSCC) was found in the Alloy 600 HAZ, 8% and 20% CW Alloy 600 specimens. The crack growth rate in the Alloy 600 HAZ is close to that in the 8% CW base metal, which is significantly lower than that in the 20% CW base metal, but much higher than that in the as-received base metal. Mixed intergranular and transgranular SCC was found in the 40% CW Alloy 600 specimen. The crack growth rate in the 40% CW Alloy 600 was lower than that in the 20% CW Alloy 600. The effect of hardening on crack growth rate can be related to the crack tip mechanics, the sub-microstructure (or subdivision of grain) after cross-rolling, and their interactions with the oxidation kinetics.     

Nicrofer 5923 hMo,ein neuer hochkorrosionsbeständiger Werkstoff für die Chemische Industrie,die Umwelttechnik und verwandte Anwendungen

Alloy 59, a new highly corrosion resistant material for the chemical process industry, environmental pollutioncontrol and related applications

1 Vortrag anläßlich der ACHEMA '91, Frankfurt/Main, 09.–15. Juni 1991.

A new nickel-chromium-molybdenum alloy, Alloy 59 has been designed to withstand severest corrosive conditions as encountered in chemical process industry and today's environmental pollution control systems. This alloy, which is composed of about 59% nickel, 23% chromium and 16% molybdenum is compared to the common NiCrMo-alloys Alloy C-276, Allo C-4, Alloy 22 and Alloy 625. Conditions of corrosion testing have been varied between oxidizing and reducing mineral acids combined with differing halide contaminations. Furthermore, corrosion data in hot concentrated sulfuric acid and in technical relevant solutions of flue gas desulfurization plants are given. The general resistance to corrosion in sulfuric acid and in hydrochloric acid is highlighted in isocorrosion diagrams. Alloy 59 has an excellent resistance to uniform and localized corrosion in all these environments. The new Alloy 59 clearly outperforms the other NiCrMo-alloys C-276, C-4 and 22 as well under oxidizing conditions as in the strongly reducing hydrochloric acid environment. The time-temperature-sensitization-diagram demonstrates the alloy's excellent thermal stability and reveals an improvement compared to Alloy 22 or the well-known Alloy C-276. Alloy 59, therefore, can be used in the as-welded condition without any additional solution annealing treatment. The excellent corrosion resistance is not impaired if the alloy is processed or fabricated to equipment even in larger sections. Weldability with a matching filler metal is without problems as demonstrated i.a. in the varestraint test.     

Metal dusting behaviour of welded Ni‐base alloys with different surface finish

Welded Ni‐base alloys and Alloy 800 were exposed under metal dusting conditions at 600°C and 650°C for up to 6000 hours. Alloy 800 was attacked very strongly already in the first days and Alloy 600 also rather soon and widespread, on both materials attack started mainly in the heat affected zone. Several surface states of Alloy 600: brushed, ground, sand‐blasted and pickled were tried only grinding caused a modest delay and decrease of metal dusting attack. Generally the attack was less widespread but deeper at 650°C than at 600°C, also for Alloy 601 and 602. The latter alloys show minor, mainly local attack, but especially the welds are affected. TIG welding led to better resistance than hand‐welding.     

Der Einfluß von Chlorid auf das Korrosionsverhalten von Nicrofer 3033 (1.4591) in mittelkonzentrierter heißer Schwefelsäure

The influence of chloride on the corrosion behaviour of Alloy 33 in medium concentrated hot sulfuric acid In the concentration-temperature field of 15 to 80 mass-% p. a. grade sulfuric acid and temperatures of 50 to 90 °C in which Alloy 33 is either corrosion resistant without and with small additions of Fe³⁺, the influence of chloride concentration on the degradation of its corrosion resistance has been determined. The evaluation of the critical chloride content has been carried out by gravimetrical means in tests of 7 days duration. For comparison materials used in sulfuric acid applications as Alloy 20 (2.4660) and Alloy 31 (1.4562) have been examined in form of random tests in the above mentioned concentration-temperature field. Based on polarization tests the influence of chloride ions in sulfuric acid solutions could be evaluated for the conditions of presence and absence of the oxidant Fe³⁺. The chloride concentration, which can be tolerated, diminishes with increasing acid concentration and temperature. If the oxidant Fe³⁺ is present in the sulfuric acid solution, higher chloride concentrations can be tolerated. Alloy 20 (2.4660) will in comparision to Alloy 33 (1.4591) be activated at significantly lower chloride concentrations. Compared to Alloy 33 (1.4591) the range of application of Alloy 31 (1.4562) is mainly restricted in Fe³⁺ containing sulfuric acid.     

Experimental study of hardfacing materials used as alternatives to alloys containing cobalt

Abstract

In the course of experimental research on hardfacing materials to be used as a substitute for Co-based alloys MMA, TIG and plasma transferred arc (PTA) welding processes were used for hardfacing 1, 2 and 3-layer specimens of Fe 52 and AISI 316L steels. The following filler metals were used:

• Alloy 1 (Co-Cr-W-C) Alloy 4 (Fe-Cr-Mo-V)

• Alloy 2 (Ni-Cr-B-Si) Alloy 5 (Ni-Cr-C-Mo)

• Alloy 3 (Ni-Cr-C-W) Alloy 6 (Fe-Cr-Mo-Mn)

Testpieces were taken from the hardfaced specimens for the following tests: high temperature hardness, impact strength, sliding wear, abrasive wear; dilatometric analysis; corrosion tests; chemical analysis; and metallographic tests.

The results show alloy 1 - chosen as the reference alloy - to have the best hardfacing properties and overall performance, and the existence of valid alternative alloys such as No. 5, 3 and 4, which nevertheless showed that they required greater care in the execution of the hardfacing.     

Double loop electrochemical potentiokinetic reactivation test of Nickel-base Alloy 600 surface-melted by a CO2 laser beam

Ni-base Alloy 600 has been widely used as a steam generator (S/G) tubing material in nuclear power plants because of its good mechanical and corrosion properties at high temperatures. However, degradations of S/G tubes due to intergranular attack (IGA) and intergranular stress corrosion cracking (IGSCC) during normal operation have been frequently reported. In particular, Alloy 600 can be very susceptible to IGA/IGSCC in some sulfur-bearing environments by sensitization. In this paper, the beneficial effects of laser surface melting (LSM) on intergranular corrosion of the sensitized Alloy 600 is presented from the results of the double loop electrochemical potentiokinetic reactivation (DL-EPR) test. The DL-EPR test was performed in de-aerated 0.01 M H₂SO₄+20 ppm KSCN at a scan rate of 0.5 m V/sec at room temperature. The degree of sensitization (DOS) of the sensitized Alloy 600 measured from the DL-EPR test was considerably reduced by LSM. The sensitized Alloy 600 after LSM also exhibited a relatively low DOS, compared with that of the sensitized but not laser treated Alloy 600. From the microscopic observation, it was found that the microstructural changes brought about by the LSM process, especially changes in the precipitation behavior of grain boundary Cr-rich carbides, caused the improvement of resistance to intergranular corrosion of the laser treated Alloy 600. The resistance to IGSCC of the laser treated Alloy 600 in sulfur-bearing environments was also discussed from the results of measured DOS and microstructural examination. This article based on a presentation made in the symposium “The 4th International Conference on Fracture and Strength of Solid”, held at POSTECH, Pohang, Korea, August 16–18 under the auspices of Far East and Ocean Fracture Society (FEOFS),et al.     

Mitigation of hot cracking of alloy 52M overlay on cast stainless steel CF8A

Abstract

Effects of ER308L buffer layer and welding parameter slope down time on the hot cracking susceptibility of alloy 52M overlay on CF8A base metal were studied. The results indicated that Si segregation was a critical factor affecting Alloy 52M hot cracking. Applying ER308L buffer layers between CF8A and Alloy 52M can reduce the dilution of Alloy 52M weld beads and minimise the contribution of Si from CF8A into Alloy 52M, thereby alleviating Si segregation and the hot cracking susceptibility of Alloy 52M. When the Si content in the grain boundary region was lower than 0·81 wt-%, the hot cracking in the weld bead could be mitigated completely. In many cases, crater cracks occurred in the end crater of the weld bead. Increasing the number of ER308L buffer layers and extending the slope down time could reduce crater crack susceptibility. However, the Si content in the grain boundary region should be controlled to be lower than 0·63 wt-% to prevent crater cracking.     

Development of low thermal expansion superalloy

Alloy 903 and Alloy 909 are well-known Fe-Co-Ni-Al-Ti-Nb alloys with controlled low thermal expan-sion, but they have some properties that can be improved. To improve stress-accelerated grain boundary oxidation embrittlement of Alloy 903 and instability of theγ phase of alloy 909, two new alloys with good stress-rupture ductility, high creep-rupture strength, high tensile strength at high temperature, and good controlled thermal expansion were developed. These property improvements were accomplished by the combination of optimizing the Fe-Co-Ni ratio of the matrix and stabilizing theγ phase with the addition of aluminum.     

Qualifizierung metallischer Werkstoffe für die Eindampfung von Abwässern aus der Rauchgasentschwefelung

Qualification of metallic materials for evaporation of waste water from flue gas desulfurization plants The ecologically-minded processing of waste water from the wet scrubbing of flue gases of coal-fired power plants to produce environmentally acceptable products is carried out in a two-step evaporater operating in closed loop mode. The evaporating process leads to high concentration of chlorides in the two evaporation steps: up to about 100 g/l in the 1st step and up to about 350 g/l in the 2nd step. Therefore in case of metallic design of the evaporation equipment materials of construction with exceptional resistance to chloride induced pitting are required. The corrosion resistance of the high-alloyed stainless steel Alloy 31 (X1NiCrMoCu32-28-7 – UNS N 08031) and of the NiCrMo-alloys Alloy C-276 (NiMo16Cr15W – UNS N 10276) and Alloy 59 (NiCr23Mo16Al – UNS N 06059) including their weldments were to be tested for this application both in the laboratory and in field tests. In addition the behaviour of Alloy 59 heat exchanger tubes had to be determined in field tests under heat-transfer service conditions. The critical pitting corrosion temperatures of the 3 materials after having been GTAW welded under uniform conditions with FM 59 (ERNiCrMo–12) filler were determined in potentiostatic tests in model solutions imitating concentrated waste water products as they may occur in practice, using 5 K temperature intervals. As to be expected the critical corrosion resistance limits of the materials lie at 85 °C at chloride concentrations of 100 g/l Cl⁻ for the Alloy 31 and of 300 g/l Cl⁻ for both the Alloy 59 and the Alloy C–276 respectively. Field tests in waste water evaporation units of flue gas desulfurization plants of coal-fired power stations are carried out as immersion tests with the welded materials and as heat-exchange experiments using longitudinally welded tubes of Alloy 59 (2.4605). The immersion tests over a period of 32 months show the Alloy 31 (1.4562) to be a corrosion resistant construction material for tubes and containers in the first evaporation step, whereas the Alloy 59 (2.4605) and the Alloy C–276 (2.4819) have to be used for the second evaporation step, where the chloride contents are much higher. The Alloy 59 is to value as the most resistant material according to its lower tendency to crevice corrosion. The heat-exchange experiments over a test period of 9 months cause to expect the Alloy 59 to be a suitable construction material for heat-exchanger tubes in both evaporation steps in comparison to graphite which is more succeptible to mechanical destroying.     

Survey of current literature

This Section is intended to provide the most current phase diagram data. Guidelines for the inclusion of new information in this section are (1) systems for which no phase diagrams are given inBinary Alloy Phase Diagrams, second edition; (2) complete diagrams that are substantially different from earlier versions published inBinary Alloy Phase Diagrams, second edition, theBulletin of Alloy Phase Diagrams, or single-topic monographs; (3) partial diagrams that alter or clarify earlier versions in the above-mentioned publications; and (4) relevant new literature of interest. Thermodynamic consistency of the new phase diagrams was checked based on phase rules, and the diagrams were modified if necessary. However, the diagrams and texts have not gone through the ordinary reviewing process, and the final evaluations may be carried out by relevant category editors of the Alloy Phase Diagram Program. For convenience, reaction tables and crystal structure data are added when new information is available.     

The Effect of Nb and S Segregation on the Solidification Cracking of Alloy 52M Weld Overlay on CF8 Stainless Steel

The weld overlay of Alloy 52M (a nickel-based filler metal) on a cast 304 (CF8) stainless steel (SS) was made to simulate overlay welding of the safe end of reactor pressure vessels in nuclear power plants. The deteriorated effect of sulfur on solidification cracking of the Alloy 52M overlay was highlighted by using a CF8 substrate with 0.14 wt.% S. Severe solidification cracking was observed when Alloy 52M was directly overlaid on the CF8 substrate. To lower the cracking susceptibility, ER 308L was deposited on the CF8 SS as a buffer layer before the subsequent deposition of Alloy 52M. Under such circumstances, the region near the weld interface between the SS buffer layer and Alloy 52M overlay was susceptible to solidification cracking. The formation of γ-NbC(N), γ-Laves, and γ-(Fe-Ni-S) eutectic-type constituents at the solidification boundaries was responsible for cracking near the weld interface. Nevertheless, depositing two layers of 308L prior to applying Alloy 52M could effectively reduce the cracking susceptibility of the overlay.     

Survey of current literature

This Section is intended to provide the most current phase diagram data. Guidelines for the inclusion of new information in this section are (1) systems for which no phase diagrams are given inBinary Alloy Phase Diagrams, second edition; (2) complete diagrams that are substantially different from earlier versions published inBinary Alloy Phase Diagrams, second edition, theBulletin of Alloy Phase Diagrams, or single-topic monographs; (3) partial diagrams that alter or clarify earlier versions in the above-mentioned publications; and (4) relevant new literature of interest. Thermodynamic consistency of the new phase diagrams was checked based on phase rules, and the diagrams were modified if necessary. However, the diagrams and texts have not gone through the ordinary reviewing process, and the final evaluations may be carried out by relevant category editors of the Alloy Phase Diagram Program. For convenience, reaction tables and crystal structure data are added when new information is available.     

Survey of current literature

This Section is intended to provide the most current phase diagram data. Guidelines for the inclusion of new information in this section are (1) systems for which no phase diagrams are given in Binary Alloy Phase Diagrams, second edition; (2) complete diagrams that are substantially different from earlier versions published in Binary Alloy Phase Diagrams, second edition, the Bulletin of Alloy Phase Diagrams, or single-topic monographs; (3) partial diagrams that alter or clarify earlier versions in the above-mentioned publications; and (4) relevant new literature of interest Thermodynamic consistency of the new phase diagrams was checked based on phase rules, and the diagrams were modified if necessary. However, the diagrams and texts have not gone through the ordinary reviewing process, and the final evaluations may be carried out by relevant category editors of the Alloy Phase Diagram Program. For convenience, reaction tables and crystal structure data are added when new information is available.     

Corrosion behavior for Alloy 690 and Alloy 800 tubes in simulated primary water

Corrosion behavior for Alloy 690 and Alloy 800 in simulated primary water is studied by open-circuit potential (OCP) measurement, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. Rise of pressure and temperature lead to negative shift of corrosion potential for both Alloy 690 and Alloy 800. EIS results show that the effect of pressure on corrosion only exists in low-frequency region, while the effect of temperature presents at the entire frequency range. Alloy 690 shows a better corrosion resistance than Alloy 800 at present investigated conditions.