IN-519 cast chromiumnickel-niobium. heat-resisting steel. Engineering properties - PDF

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IN-519 cast chromiumnickel-niobium heat-resisting steel Engineering properties databooks Inco, the leading producer and marketer of nickel, conducts research and development programmes on nickel alloys,
IN-519 cast chromiumnickel-niobium heat-resisting steel Engineering properties databooks Inco, the leading producer and marketer of nickel, conducts research and development programmes on nickel alloys, products and processes, establishing engineering and performance data. This knowledge is collated in a library of INCO databooks, which are freely available. Conversion factors for stress and impact energy units Sl metric units have been adopted as the standard throughout this publication. To assist readers who may be more familiar with other units to which they have been accustomed, factors are given below for conversion of the more important of these to Sl metric units and vice versa. Stress units: 1 tonf/in 2 = N/mm 2 1 kgf/mm 2 = N/mm lbf/in 2 = N/mm 2 1 N/mm 2 = tonf/in 2 or kgf/mm 2 or lbf/in 2 or 0.1 hbar Note that the newton per square millimetre (N/mm 2 ), meganewton per square metre (MN/m 2 ) and megapascal (MPa) Sl units of stress are arithmetically identical. Impact energy units 1 ft Ibf = J 1 kgf m = J 1 J = ft Ibf or kgf m 1 J (Charpy V impact) = kgf m/cm 2 The information and data in this publication are as complete and accurate as possible at the time of publication. The characteristics of a material can vary according to the precise method of production and treatment and therefore suppliers should always be consulted concerning the specific properties of their products. INCO is a trademark. Copyright 1976 Inco Europe Limited (formerly International Nickel Limited) IN-519 cast chromium-nickel-niobium heat-resisting steel Engineering properties Alloy IN-519 was developed by Inco primarily for centrifugally-cast catalyst tubes used for steam-hydrocarbon reforming furnaces. The work leading to the development of the new steel consisted essentially of optimising the composition of the established Alloy Castings Institute HK-40 steel (25Cr-20Ni) which is commonly used for this application, in order to provide a material with improved high-temperature stress-rupture strength and retention of good ductility after long-term service at elevated temperatures. The significantly improved stress-rupture strength, in spite of a lower carbon content, is the result of a 1½ per cent addition of niobium, while some improvement in ductility in stress-rupture tests, and in short-time tensile tests after service exposure, has been obtained by lowering the carbon content and modifying the base composition to ensure good structural stability. The improvements are achieved whilst maintaining other essential pre-requisites of a reformer tube alloy, i.e. good weldability and resistance to oxidation, carburization and thermal fatigue. Laboratory tests on trial commercial centri-cast tubes were made around 1965 and since that time the alloy has become firmly established as an attractive alternative to HK-40 as reformer tube material. It is currently produced by several major suppliers of heat-resisting castings throughout the world and is included as Material Number in a revised edition of the German Standard, Stahl Eisen Werkstoffblatt 595 that is to be issued. Other applications include various statically-cast components for heat treatment plant wherever the superior high-temperature strength and ductility of Alloy IN-519 in comparison with HK-40 steel is considered advantageous. Chemical composition Table 1 gives the nominal and recommended range of chemical composition of Alloy IN-519. Within the prescribed limits the balance achieved between the major alloying elements provides optimum stressrupture strength and avoidance of embrittlement after long-term exposure at elevated temperatures. Microstructure As-cast IN-519 contains a proportion of inter-dendritic eutectic niobium carbide and chromium carbide as shown in Figure 1. During exposure at elevated temperatures precipitation of very fine secondary NbC occurs and these particles remain unchanged in size and distribution over long periods of exposure because of the high stability of this type of carbide (Figure 2). The secondary carbide strengthens the matrix and is primarily responsible for the improvement in stress-rupture strength compared with that of HK-40 steel. The mechanism of strength improvement becomes apparent when the morphology and distribution of carbides in Alloy IN-519 is compared with the size and frequency of occurrence of carbides in HK-40 steel. Eutectic chromium carbides in as-cast HK-40 are coarser and although a copious precipitation of fine secondary chromium carbides occurs on ageing at high temperatures, these agglomerate to give fewer and coarser particles as ageing continues. Hence strengthening of the matrix is less than that caused by the persistance of fine carbide particles as in Alloy IN-519. Kihara et al (l) have attributed the superior creep and stress-rupture properties of alloys of the IN-519 type to the effect of fine niobium carbides in restraining grain boundary sliding and in reducing the agglomeration of voids which are nucleated at the boundaries between the matrix and coarser chromium carbides that are also present. Figure 1. Microstructure of as-cast Alloy IN-519. (Etchant-Murakami's reagent). 400X Figure 2. Microstructure of Alloy IN-519 after 2000 hours at 800. (Etchant-Murakami's reagent). 400X. 3 Table 1. Alloy IN-519. Chemical composition, weight per cent. Carbon Silicon Manganese Chromium Nickel Niobium Sulphur Phosphorus Iron Nominal Balance Recommended range max* 1.0 max max max Balance * 0.8 per cent max Si is preferred for retention of highest tensile ductility and toughness after long-term service exposure at elevated temperatures. The proposed new edition of the German Standard SEW 595 stipulates carbon contents of per cent and a range of per cent carbon has been extensively and, as far as is known, satisfactorily adopted by German producers. The progressive replacement of secondary chromium carbide by a fine, stable precipitate of niobium carbide with increasing additions of niobium to the 24Cr-24Ni type of steel is accompanied by a reduction in the amount of secondary carbide formed. Therefore, the improvement in stressrupture properties passes through a maximum at a niobium/carbon ratio of about 5. This fact is recognized in the composition design of Alloy IN-519. The structural stability of IN-519 has been studied in relation to the formation of sigma-phase after long-time exposure at temperatures in the range 600. Of some 53 heats examined by Inco, 33 had compositions within the recommended range given in Table 1 while 20 others were made with silicon, manganese, chromium, nickel or niobium contents somewhat outside that range in order to assess the effects of variations in these elements more completely. The results showed that IN-519 has low susceptibility to formation of this embrittling phase. For example, after 5,000 hours exposure at 800 a critical temperature for sigma-phase formation 30 of the 33 alloys having compositions within the recommended range were completely free from sigma-phase, as judged by optical microscopic examination, while the three remaining alloys had only the smallest detectable trace. The most notable effect of variations in composition outside the recommended range was that attributable to silicon. Increasing the silicon content from 1.0 to 1.8 per cent resulted in the formation of sigma-phase in amounts varying from a trace to 25 vol /o when the nickel/chromium ratio was equal to or less than unity. However, at nickel/chromium ratios above unity the effect of silicon in promoting sigma formation was reduced, to the extent that no sigma-phase was formed in alloys containing 1.0 to 1.3 per cent silicon. To ensure low susceptibility to sigmaphase formation it is desirable, therefore, to conform to the upper limit of 1.0 per cent silicon given in Table 1 and to maintain the nickel/chromium ratio at unity or above. The preferred maximum of 0.8 per cent silicon mentioned in Table 1 for the retention of highest ductility after long-term exposure at elevated temperatures is also to be preferred for avoiding sigma-phase formation. Mechanical properties Tensile and impact properties Representative tensile test data from roomtemperature and short-time elevated temperature tests on as-cast samples of Alloy IN-519 from five centricast commercial tubes are shown in Figure 3. Comparison of these results with published data for as-cast HK-40 steel indicates that the ductility of IN-519 is generally superior at temperatures up to, while the 0-2 per cent proof stresses and tensile strengths of both alloys are similar. Figure 3 also shows the effect of prolonged ageing at 800 on the tensile properties of IN-519. These data indicate that only slight lowering of room-temperature ductility occurs after long-term service exposure, the elongation being typically about 12 per cent in contrast to the behaviour of HK-40 steel which usually has elongation values below 5 per cent after such treatment. The roomtemperature tensile stress values of IN-519 remain unaffected by prior ageing at 800 and no adverse effects of this treatment are shown in the tensile properties at elevated temperatures. Room-temperature Charpy V-notch impact values of Alloy IN-519 in the as-cast condition and after prolonged exposure at temperatures in the range 800 are shown in Figure 4 (Page 6). Some loss of toughness occurs after high-temperature ageing, but a useful level of impact strength is still retained and the results are in accord with microstructural observations that little or no sigma-phase is formed. Stress-rupture properties The results of 339 stress-rupture tests (46 of them on welded specimens) on Alloy IN-519 at temperatures in the range are presented as a Larson-Miller plot in Figure 5 (Page 7) showing a single (mean) master curve* from which rupture-stresses may be derived by extrapolation to longer for shorter times than those covered by the available test data. These data represent 18 heats of IN-519 from eight commercial suppliers and the tests were made in five test-houses for times extending up to 42,312 hours. Therefore, the results may be considered as representative of the alloy for the normal variations in production and testing likely to be encountered in practice. Figure 5 (Page 7) also shows 95 per cent confidence limits for the prediction of probable variations in test data, while the inset chart of that diagram gives the distribution of rupture-lives for the tests made. Stress rupture vs. rupture-life test results are plotted in Figure 6 (Page 8) for temperatures of 700, 800, and, together with 95 per cent confidence limits corresponding to those shown in Figure 5 (note: the latter also covers tests at other temperatures). The mean and minimum stresses giving various rupture-lives for Alloy IN-519 at several temperatures have been derived from the Larson-Miller plot shown in Figure 5 and are compared in Table 2 (Page 6) with the corresponding stresses for HK-40 steel, similarly derived from test data extending up to 21,700 hours on material from 12 commercial heats. This comparison indicates the superior rupture-strength of IN-519. It should be noted by comparison of Figure 6 (Page 8) and Table 2 (Page 6) that the derivation of long-time rupturestresses for IN-519 involved some extrapolation of the available data to longer rupture times. This extrapolation becomes more problematic the higher the test temperature, since there are fewer data points available giving values greater than 25 for the parameter P (i.e., equivalent to 10 5 hours at 920) in the Larson-Miller plot which forms the basis of the extrapolation. Rather more extrapolation of the stress/ rupture-life test data was incurred in deriving some of the long-time rupture stresses in Table 2 for HK-40 steel. Stress-rupture ductility is an important parameter affecting the performance of stressed components in high-temperature service and Figure 7 (Page 9) compares values for IN-519 and HK-40. Insufficient data are available to provide a clear indication of the relative merits of the two materials at rupture lives in excess of 10,000 hours, but the general trend of the results suggests that IN-519 retains higher ductility, while at lower rupture-lives its superiority in this respect is more clearly evident. Creep properties The time taken to produce a total plastic strain of 1 per cent is plotted as a function * See appendix for the method used to derive the mean curve in the Larson-Miller plot. 4 of stress at various temperatures in Figure 8 (Page 9). These data do not provide a measure of the steady state minimum creep rate since they take into account the initial plastic strain on loading the test piece and also the primary creep extension in addition to steady state creep extension. Limited data for minimum creep rate are given in Table 3 (Page 7). Weldability Alloy IN-519 can be welded by a variety of commercial processes, both manual and automatic. In commercial practice manual metal arc, TIG and MIG welding have all been used successfully; bare filler wires and coated electrodes are commercially available. To ensure good weldability the carbon, niobium and phosphorus contents of the filler material are carefully controlled. The desirable levels of carbon and niobium to avoid weld-metal cracking are inter-related as shown in Figure 9 (Page 10). Niobium is preferably kept in the range 1.4 to 1.8 per cent, as in the parent alloy, while carbon is held to the slightly lower maximum level of 0.33 per cent compared with the 0.35 per cent upper limit of the range recommended for the parent material. In fact, it is preferable if the carbon content of the weld metal does not exceed 0.3 per cent. Similarly, the control of phosphorus in the filler material is slightly more stringent than in the parent alloy and the content should be less than 0.02 per cent. Even with correct levels of carbon and niobium hot-cracking of the weld metal can occur if the phosphorus content is too high. Additionally, filler metal silicon contents below 0.8 per cent are preferable for optimum weldability. Heat-affected-zone cracking of IN-519 parent metal has not been observed in castings having compositions within the recommended range. However, some HAZ cracking has been observed in cast tubes of 24Cr-24Ni-Nb steel with carbon contents of 0.42 and 0.46 per cent. In comparison with the welding of HK-40 steel the welding of IN-519 may require greater control of heat input since the niobium-containing alloy is slightly more susceptible to hot-cracking. However, providing care is taken in this respect, coupled with the aforementioned compositional control, completely sound welds are obtainable. Control of the interpass temperature to less than 200 has generally given crackfree weld-joints. In contrast, one instance of HAZ cracking has been encountered when the interpass temperature was allowed to rise to 400. In commercial welding practice the TIG process has been predominantly employed with IN-519 centri-cast tubes, employing Figure 3. Representative tensile properties of Alloy IN-519 as a function of test temperature: as-cast, and after prolonged exposure at 800. filler material from wire or cast sticks of essentially matching composition. Root runs have been made autogenously or by TIG deposition of added filler metal. Additionally, both the MIG and manual metal arc processes have been used for filling runs. Weld preparations have been dictated by the particular experience of individual companies. In general, however, a single U preparation has been used for centricast tubes, with a 2-4 mm root face, a land between 0 and 4 mm, and a root radius of about 3 mm. The bevel angle is sometimes dictated by the process and is typically 15 for TIG welding, or 25 for manual metal 5 arc welding to allow better access. For autogenous TIG welding of the root a closed butt configuration is used. Laboratory trials by Inco have also demonstrated the feasibility of autogenous plasma-arc welding for the forming of a root bead. Such root runs have been made by melting through a 3.2 mm root face having a 1.6 mm root gap, using 200 amps at a welding speed of 0.47 m/min, with argon per cent hydrogen as the plasma gas. Figure 4. Effect of exposure at 800 on the room-temperature impact resistance of Alloy IN-519. Weld-joint properties Stress-rupture data for TIG welds made in IN-519 centricast tubes of commercial manufacture, using cast stick filler material matching the composition of the parent metal, are shown in Figure 6. These data show the weld joints to have stress rupture strengths similar to those of the parent tubes. Similar data for TIG welds have also been reported elsewhere (2). In stress-rupture tests at and on coated-electrode weld joints, rupture-strength joint efficiencies of per cent have been obtained. The stress-rupture properties of weldjoints in Alloy IN-519 may be contrasted with comparable joints in HK-40 steel for which TIG welds give a lower weldefficiency of about 80 per cent, while HK-40 weldments prepared with conventional basic-coated electrodes usually have a joint efficiency of only about 60 per cent. However, it should be noted that recent tests on welds made with rutile-coated electrodes of appropriate composition have indicated that manual metal arc welds can be made in HK-40 steel giving joint efficiencies of per cent. The generally superior stress-rupture Table 2. Comparison of derived-rupture-stresses* for various rupture lives of Alloy IN-519 and HK-40 steel. Test () Duration (hours) Mean derived-rupture-stress Minimum derived-rupture-stress (77% of mean) IN-519 HK-40 IN-519 HK-40 N/mm 2 Ibf/in 2 N/mm 2 Ibf/in 2 N/mm 2 Ibf/in 2 N/mm 2 Ibf/in 2 Ratio of derived -rupture-stress IN-519/ HK ,000 3,000 10,000 30, , ,000 3,000 10,000 30, , ,000 3,000 10,000 30, , (6.8) (981) (5.2) (755) * Rupture stresses were derived from Larson-Miller plots of available test data. Values in parentheses were derived by extrapolation beyond the highest value of the Larson-Miller parameter plotted in Figure 5. Minimum derived-rupture-stresses, taken as the lower 95 per cent confidence limit, were 23 per cent lower than the mean derived-rupture-stresses for both IN-519 and HK-40. Thus the mean- and the minimum-stress ratios of the two materials are the same. 6 Figure 5. Alloy IN-519. Larson-Miller stress-rupture relationship. Table 3. Alloy IN-519. Minimum creep rate/stress data. Minimum creep rate per cent/hour Stress for given minimum creep rate N/mm 2 Ibf/in characteristics of IN-519 weld joints compared with those of HK-40 have prompted the suggestion that IN-519 filler metal can be used for the welding of HK-40 to provide welds having stress-rupture strengths matching those of the parent metal. In fact, trial welds in HK-40, made by Inco using IN-519 consumables, have given stress-rupture lives at given stress levels similar to those of unwelded HK-40, with failure occurring in the parent metal. In common with experience in welding other high-carbon austenitic heat-resisting alloys, the room-temperature ductilities of weld deposits in IN-519 are less than those of the parent metal, whilst tensile strengths of the deposited alloy are higher. However, TIG weld deposits tend to have higher ductilities than manual metal arc deposits. For example, elongations of 8 15 per cent and 4 8 per cent have been obtained in TIG and MMA deposits, respectively, on a 50 mm gauge length, in tensile tests transverse to the joints, whereas elongations for the parent metal are typically per cent. Figure 6. Alloy IN-519. Stress-rupture properties. Time-to-rupture as a function of stress and temperature. (The upper and lower edges of the bands demarcate 95 per cent confidence limits.) Thermal fatigue resistance The ability of a heat-resisting alloy to withstand cyclic thermal stresses without ultimate cracking ensuing is an important factor in determining the service life of components used in
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