Austenitic High-Performance Materials for Non-Rotating Components in Stationary Gas Turbines

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1 TH AMRICAN SOCITY OF MCHANICAL NGINRS St., New York, N.Y GT-307 ry The Society shall not be responsible for statements or opinions advanced in papers or in dis suasion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASM Journal. Papers are available 0 from ASM for fifteen months after the meeting. Printed in USA. Copyright 1988 by ASM Austenitic High-Performance Materials for Non-Rotating Components in Stationary Gas Turbines K. DRFAHL Metallgesellschaft AG Reuterweg Frankfurt, West Germany DR. FRANZ HOFMANN Vereinigte Deutsche Metallwerke AG (VDM) Nickel-Technology Division P. 0. Box Werdohl, West Germany SUMMARY Five essential requirements for gas turbine materials exposed to high temperatures are discussed: - High resistance to oxidation and high temperature corrosion - High creep strength especially under condition of therrnal stress - High fatigue strength under conditions of vibrational and thermal stress - Good workability since complicated deformation and machining processes are often necessary - Good weldability It is not always possible to make all these requirements totally compatible. This is because certain requirements have to be emphasized for reasons of application. In most cases compromises will have to be made in regard to workability. 1. INTRODUCTION Today's high technical standard in gas turbine engineering is attributable both to improvements in design and, above all, to the materials used. Gas turbines today have to operate at more elevated temperatures and burn more heavily contaminated fuels. In addition, high availability and long life are expected. The characteristic properties of a gas turbine material depend on the different requirements to be met for the various gas turbine components. The non-rotating components of a gas turbine exposed to the greatest loads are the combustion chamber and the hot gas and off-gas systems. Figure 1 shows typical materials for stationary gas turbines, mostly used in the form of flat products, semifinished forged products and filler metals. This figure shows that these alloys are mainly based on the metals nickel and chromium. However, a number of further alemerits have a substantial influence on the properties of the various alloys. These semi-finished products should have the following main properties (Figure 2). 2. PROPRTIS 2.1 Corrosion behaviour While in the past the combustion chambers and hot gas ducts of large stationary gas turbines were manufactured of heat-resistant stainless steel alloys with high strength at elevated temperature such as Cronifer 1613 Nb and Cronifer 2520 (AISI 310), and later of Nicrofer 3220 H (alloy 800 H), nickel-based alloys were used for small gas turbines (e.g. pump drives) on account of experience gained from driving turbines. The main reason was the considerably higher resistance to oxidation behaviour since gas turbines may be subjected to frequent load changes and thus temperature variations. This oxidation behaviour is shown in Figure 3. In addition, Figure 3 shows clearly that there may still be pronounced differences in resistance to cyclic oxidation within the group of the nickel-based alloys. This is particularly obvious at very high temperatures like 1100 C. The reason for this is that these alloys contain alloying elements to increase their strength at elevated temperatures, which may form volatile oxides at very high temperatures. A typical example for this is alloy 625. One of the materials with the highest resistance to cyclic oxidation is the alloy Nicrofer 6023 (alloy 601). This is due particulary to a high chromium content of about 23 % wt. and an aluminium content of about 1,5 % wt. apart from a nickel content of 60 % wt. Aluminium especially reduces the diffusion of cations of the oxidic pro- Presented at the Gas Turbine and Aeroengine Congress Amsterdam, The Netherlands June 6-9, 1988

2 tective coat, thus reducing its growth rate. The thinner oxide coat lessens the stresses between oxide and metal, which cause the oxide to peel off. As, however, this alloy does not meet all requirements regarding strength, a new material Nicrofer 5520 Co (alloy 617) has been developed recently. By adding the alloying elements molybdenum and cobalt, the creep resistance in particular could be increased considerably. However, at very high temperatures of about 1100 C the resistance to cyclic oxidation is somewhat reduced. This is due to the volatilization of oxides, especially at high gas velocities. 2.2 Creep behaviour The second important requirement for alloys used in combustion chambers of stationary gas turbines is a high creep resistance even at very high temperatures. M echanical stresses are mostly caused by large temperature differences both at different locations on the surface and across the material thickness. Low creep resistance results in deformation or rupture of the combustion chamber casing. It was, above all, the required high creep resistance that influenced the development of materials substantially. In urope the highly heat-resistant heating conductor alloy Nicrofer 7520 (alloy 75) was taken as a basis. Figure 4 shows the creep rupture strength of this alloy in comparison with the other gas turbine materials. The addition of titanium and aluminium (approx. 3,5-4,5 %) as alloying elements increased the creep resistance of this alloy considerably. Decisive for this was the formation of y'-phase Ni 3(Al, Ti) inhibiting the dislocating movement supporting the creep process and thus reducing the creep rate. A result of this development is the alloy Nicrofer 7520 Ti (alloy BOA). However, the influence of the y'-phase on the creep resistance is only effective up to about 800 C as above this temperature a considerable phase coagulation or dissolution occurs. The loss of creep resistance can be counteracted by adding approx. 20 % wt. cobalt to the alloy Nicrofer 5120 CoTi (alloy C 263). This alloy has a creep rupture strength up to about 900 C. Precipitationhardenable materials have, however, not been accepted as yet in the field of stationary gas turbine engineering because of their problematic workability, although recently an American gas turbine manufacturer has produced highly-loaded hot gas ducts of Nicrofer 5120 CoTi (alloy C 263). In the USA the development of materials for combustion chambers started at first from solid solution hardening alloys. Nicrofer 4722 Co (alloy X) has proved satisfactory in many gas turbines over many years and is still being applied today. The development then proceeded to precipitation-hardenable materials. This was attained by the alloying element niobium, which forms the y"-phase Ni 3Nb with nickel. As the phase formation proceeds more slowly than in y'-hardening alloys, these alloys are easier to weld. xamples of such alloys are Nicrofer 5219 Nb (alloy 718) and Nicrofer 6022 hmo (alloy 625). These materials are, however, not used above 800 C because of rapid over-ageing and, thus, decreasing creep resistance. Furthermore the resistance to oxidation is limited at high temperatures. Therefore the development returned to solid solution hardening alloys such as Nicrofer 5520 Co (alloy 617). This material has both a high creep resistance and a high resistance to oxidation up to very high temperatures. 2.3 Fatigue behaviour Apart from the time-dependent creep behaviour, the fatigue behaviour of combustion chamber materials must also be known since greatly varying stresses due to vibration may occur. As such investigations are often made with a view to specific applications, the results are hardly comparable. Figure 5 shows the fatigue behaviour of the materials Nicrofer 3220 H (alloy 800 H) and Nicrofer 5520 Co (alloy 617). It is apparent that the highly-alloyed nickel-based Nicrofer 5520 Co can be loaded with a much higher stress amplitude than the austenitic stainless steel Nicrofer 3220 H (alloy 800 H). Apart form the high resistance to cyclic oxidation (Figure 3) and the high creep rupture strength (Figure 4), this feature was the main reason for using this material today in combustion chambers of large gas turbines. 2.4 Workability Optimum application characteristics of a material are useless if it cannot be worked. Combustion chambers often consist of many complicated formed individual parts, which have to be welded together. A problem with certain materials is, for instance, that on punching air slots, cracks may occur at the slot ends (Figure 6). This is almost always due to heavy carbide precipitation at grain boundaries. Figure 7 shows the microstructure of alloy 333 under both poor and good deformation properties. The coarse carbide precipitations are due to insufficient heat treatment. In this connection the richsen deep drawing test has proven itself as test criterion. According to experience, a drawing depth of around 9 mm represents a sufficient deformability. The sheet with the poor structure showed a drawing depth of about 9,5 mm and the sheet without cracks a depth of 12 mm. Further problems resulting from the sheet metal production may lead to difficulties on deep drawing metal sheets. This may he for example an anisotropic effect, which can be controlled by appropriate deformation measures such as longitudinal or transverse rolling and defined deformation degrees during cold rolling. Another problem in deep drawing metal sheets may be caused by locally deformed surface area of the original sheet metal. This may result in heavy coarse grain formation during intermediate annealing operations impairing both the deformation and the application properties. These local deformation areas are occasioned by the final straightening procedure of the sheets after annealing. Investigations have shown that this problem occurred during stretcher straightening, but not during roller straightening; therefore the latter should be used. Apart from deformation, the manufacture of gas turbine combustion chambers and hot gas ducts always involves machining of the semi-finished products. Thus, holes must be drilled, weld edges planed or ground and slots milled or sawn. Because of the properties required, the VDM high-temperature materials have a complicated alloy structure, making these materials highly subject to workhardening. Therefore these materials are much more difficult to machine than ferritic or simple austenitic steels. 2

3 It is therefore most important to avoid work-hardening. A hole should for instance be drilled with continuous advance travel at slow drilling speed and greater shaving thickness. Or, the shaving thickness should be raised accordingly on turning a rod or ring exhibiting surface hardening. Hard metal tools have proved optimal for cutting operations. 2.5 Weldability The last but very important requirement for materials used for gas turbine combustion chambers is their weldability since, as already mentioned, the casings are mostly complicated welded constructions. Figure 8 shows the internal casing of a large gas turbine of Nicrofer 5520 Co. It is essential that the properties are similar to those of the base metal. Therefore, on selecting the base metal, there will always arise the question of the appropriate welding procedure and filler metal. Thin sheets up to about 6 mm thickness are plasmawelded without filler metal or TIG-welded with filler metal, depending on the location of the weld. On applying these welding procedures the weld zone should be inert gas shielded to prevent the molten metal from reacting with air. The latter would lead to slagging of important reactive alloying elements such as Cr, Ti, Al, Nb etc. Moreover, these slags may be included in the filler metal. Both would deteriorate the properties of the weld. Thicker sheets may be TIG, MIG or submerged-arc welded. Frequently, the use of coated welding rods is required for manual welding. In the case of very sensitive alloys tending to crack formation, such as Nicrofer 5120 CoTi, a welding procedure with very low base metal heating, namely the TIG procedure, is mandatory. properties of the weld comparable to those of the base metal. Figure 11 compares the creep rupture strength of Nicrofer 3220H (alloy 800 H) with the creep rupture strength of the deposited filler metal Nicrofer S On using a similar filler metal, the creep rupture strength of the base metal is not reached by a long way; therefore the creep rupture strength of the base metal must not be used as basis for the strength calculation. The material combination shown in Figure 11 has been proven in practice and has become generally accepted. There may be a few cases where the corrosion resistance of the higher alloyed filler metal is not sufficient. This problem is mastered by depositing a similar top pass on the filler metal determining the strength properties. Of course, this is only possible with multi-layer welds, i.e. thicker plates. Apart from the welding procedure and filler metal, the properties of the base metal to be welded should of course also meet the specific requirements. A problem faced time and again with the materials in question is the occurrence of macro and micro-segregations, which can only be avoided by careful fabrication of the semifinished products. Such segregation zones, which as a rule have a lower melting point than the surrounding matrix, are melted in the heat-affected zone and lead to cracks on cooling (Figure 12). When exposed to fatigue loading these may then quickly result in failure of the structural part. xaminations of the filler metal Nicrofer S 6020 have shown that the welding procedure may affect the creep rupture strength properties of the metal deposited. Figure 9 shows that a metal deposited under an inert gas shield has better creep rupture strength properties than a metal deposited under a slag blanket. It is assumed that micro-slag inclusions influence the creep processes. Apart from the selection of the appropriate welding procedure, the selection of the most suitable filler metal is also of great importance, as already mentioned. As a rule, almost all materials listed in Figure 1 are welded using a adequate filler metal. This will produce s with properties coming close to those of the base metal. This is shown in Figure 10, which compares the creep rupture strength of the base metal Nicrofer 5520 Co with the weld made with a similar filler metal. The similar high temperature corrosion behaviour and the comparable expansion coefficient are also facts favouring similar filler metals. Disregarding the expansion coefficient has frequently led to problems caused by fatigue cracks in the filler metal or the heat-affected zone. After all, most of the materials listed in Figure 1 are complicated multi-component systems whose individual elements are matched to avoid the formation of undesired, possibly harmful, phases. On using dissimilar materials, new alloy systems with properties difficult to assess may be formed mainly in the heat-affected zone. For some materials such as Nicrofer 3220 H (alloy 800 H) it is, however, inevitable to use a dissimilar filler metal to obtain creep rupture strength 3

4 VDM - tradenarce Alloy Constituents (% wt.) NICROFR Alloy Ni Cr Fe Mo Co Ti Al C others 3220 H 800 H ,4 0,3 0, ,4 1,5 0,06 Zr < ,4 0,2 0, MoW ,2-0,05 W 3 / Si Co X , ,07 W 0, Co < ,4 1,2 0, Mo < < 0,3 < 0,3 0,05 Ca / Mg 6022 hmo < ,06 Nb 4 / Ca 5120 CoTi C ,0 0,5 0,07 Zr 5219 Nb ,0 0,6 0,06 Nb TiNb X ,5 1,0 0,05 Nb 1 / Zr Fig. 1 Typical chemical composition High resistance against oxidation and high temperature corrosion High creep strength especially due to thermal stresses High fatigue strength due to vibrational and thermal stresses Good workability since often complicated deformation and 500 machining processes are necessery alloy 625 Good weldability 200 alloy C263 Fig. 2 Requirements on the material of 100 stationary gas turbines alloy X z 50 alloy 75 alloy N alloy 601 FeCrAl alloy 800 H 20 0 alloy o/!o X alloy ) 3 C -30 v en c - 40 U O y^ I' o a ^ S Temperature ( C) Fig h-creep rupture strength of different alloys for stationary gas turbines Time (h) Fig. 3 Weight changes of different alloys over time due to cyclic exposure (15 min 1100 C - 5 min cooling) 4

5 m C Z v V?00 1 <a ood Fig. 7 Microstructure of alloy mm sheet cycles to failure Fig. 5 Fatigue behavious of alloy 617 and 800 H at 850 C Fig. 8 Welded internal casing of a large gas turbine of alloy 617 Fig. 6 Punching air slots with cracks at the slot ends 5

6 Nicrofer S 6020 MIG 100 Nicrofer S 6020 MIG 6 z N I base metal ^\ 10 Nicroter S 6020 coated electrode Nicrofer S 6020`4 coated electrode temperature, "C tenmperature, C Fig h-creep rupture strength of MIG-welded and coated electrodewelded joints of S 6020 Fig h- Creep rupture strength of alloy 800 H welded with S z w too 50 Fa i^^ C 800 c goo C 1000 C temperature Fig. 12 Microstructure of alloy 625 H submerged arc welded Fig h-creep rupture strength of and base metal of alloy 617 C7