STRUCTURE STABILITY OF CrMo STEELS AT HIGH TEMPERATURE

Transcription

1 STRUCTURE STABILITY OF CrMo STEELS AT HIGH TEMPERATURE Josef Čmakal, Jiří Kudrman ŠKODA-ÚJP, Praha, a.s., Nad Kamínkou 1345, Praha Zbraslav, ČR, Abstract Changes in the microstructure of CrMo steel and degradation of its mechanical properties during heat treatment were investigated in detail. The changes occurring in the microstructure during long-run exposure at high temperatures were described based on quantitative structure analysis data. At the same time, an extensive study of the changes in the mechanical properties was carried out. The relations between the microstructure and mechanical properties are described, and the feasibility of using non-destructive testing when assessing the steel s mechanical properties in the degraded state after long-term operation at high temperatures is demonstrated. 1. INTRODUCTION The changes in the microstructure caused by long-term exposure to high temperatures were investigated. Exploitation at high temperatures was either simulated in laboratory conditions (steels and 15313), or samples were taken from time to time from steam turbine casing which had been in operation for a sufficiently long time (cast steel ). Creep resistance of CrMo or CrMoV structural steels is achieved through the presence of fine carbides in the usually bainitic or ferritic-bainitic structure. The optimum microstructure and mechanical properties are reached by normalization annealing and high-temperature drawing. The steels are designed for use at temperatures up to 550 C, most frequently as the material for steam lines, piping systems and reactors in the chemical industry and similar applications. The long-term effect of temperature on such steels first brings about changes in the phase composition and chemical composition of the separated carbides. Dissolution of M 3 C carbides and precipitation of carbides of the M 7 C 3 and M 23 C 6 types, particularly chromium-alloyed ones, have been observed. Precipitation of fine vanadium carbides has also been found in vanadiumalloyed steels [1]. This phenomenon takes a very long time and manifests itself, in the bulk of the ferritic-bainitic structures, by a partial dissolution of the carbides in the bainitic grains and their precipitation in the grains of ferrite. Only when those changes are complete, the carbides become coarser through a mechanism consistent with the assumptions of the secondary phase particles growth model [2, 3]. The changes in the particle size can be then described by the equation n n r r0 = K. t The exponent n is governed by the process-controlling mechanism (n = 3 for bulk diffusion, n = 4 for diffusion along planar defects, etc.), r and r 0 stand for the final and starting mean particle diameter, respectively, K is the growth constant (function of the diffusion factor of the element of the particle coarsening-controlling mechanism), and t is the time of annealing.

2 2. EXPERIMENTAL Steels and were examined in the state established by the recommended starting heat treatment. Long-run annealing at temperatures from 625 C to 710 C was applied. Changes in the microstructure, hardness, and strength were investigated during the annealing. Samples of steel were taken periodically from risers on the inlet pipes of the high-power steam turbines. In this case, changes in the notch toughness with the time of operation were also monitored in addition to the changes in microstructure. The temperature of exploitation cannot be defined precisely for such samples, and it can only be concluded that, owing to the same power and the same design of the turbines, the conditions were identical for all samples. The microstructure was evaluated quantitatively by Saltykov s momentum method of quantitative stereological analysis [4]. The chemical composition of the steels is given in Table 1 below. Table 1. Results of chemical analysis of the steels studied (wt.%) Steel C Si Mn P S N Cu Cr Mo V ,16 0,63 0,32 0,016 0,010 0,57 0,42 0, ,12 0,28 0,58 0,011 0,005 0,006 0,03 2,10 0, *) 0,17 0,38 0,46 0,011 0,012 1,50 1,00 0,28 *) Mean chemical composition obtained as the average concentrations of the elements in the individual turbine casings. 3. RESULTS AND DISCUSSION The basic structure parameters defining the status of the steel were determined by stereological analysis for all the conditions examined, i.e. for all temperatures and all times of annealing applied. None of the dependence of the size of the particles precipitated inside the grains or at their boundaries was linear in the logarithmic plot (Figs. 1 and 2). Hence, the particle coarsening model does not apply to any of the steels within the annealing temperature region used; in other words, the observed changes in the dispersion of the carbides precipitated cannot be described in terms of simple atomic mechanisms. If the growth equation is solved for the system of tangents to the experimentally determined dependences, the exponent n will attain values n >> 4 for short annealing times. The exponent decreased with increasing annealing time for both steels, to approach the expected values (n = 3 for particles inside the grains and n = 4 for particles at the grain boundaries) at the longest times of annealing. The calculated exponents for the longest time of annealing warrant the assumption that in this case the particle coarsening model [2, 3] is applicable. This conclusion is borne out by the calculated values of n given in Table 2 below. Table 2. Calculated values of the exponent n in the growth equation Temperature Steel Steel Calculation of n valid n Calculation of n valid n [ C] for annealing times grain boundary for annealing times grain boundary to ,11 3, to ,40 3, to ,91 4, to ,05 3, to ,95 4, to ,30 4,22 In relation to the changes in the dispersion of the precipitates, the dependence between the mechanical properties and structure can be determined by calculating the precipitation strengthening increment in the yield point. This can be calculated based on the relation [5]:

3 0,5 V V σ = β0 lg 2Gb( V ) D In this equation, Δσ is the yield strength increment due to the precipitation mechanism, β 0 is a constant depending on the dislocation slip mechanism, G is the shearing modulus of elasticity, b is Burgers vector, D V is the mean size of the hardening particle, and V V is the volume fraction of the hardening phase precipitated. The yield strength can be then expressed as σ = σ 0 Any state for which the yield strength is known can be taken as the basic σ 0 - value, to which the precipitation strengthening Δσ is added. The σ 0 -value can be then determined readily by subtracting Δσ from the observed yield strength. The yield strength changes accompanying the changes in the structure will then follow from the differences between the increments Δσ in the states being compared. The yield strength dependences so obtained for steel are shown in Fig. 3, and compared to the observed hardness dependences in Fig. 4. The hardness dependences are seen to be similar to the yield strength dependences calculated from the structure data. Extensive mechanical tests were also performed for the two steels to verify the results obtained. The similarity of the hardness dependences was confirmed, and the observed yield strength data agreed with the calculations. Therefore, a relation between the free particle spacing and the yield strength was constructed (Fig. 5). The dependences are virtually unaffected by the temperature and time of annealing which had led to the given structure state; in fact, they are only determined by the structure state itself. The relation between the annealing temperature and time and the yield strength can also be expressed parametrically, by means of the Hollomon- Jaffe parameter P = T (lg t + C). The corresponding plots of the observed yield strength are shown in Fig. 6. The dependences are valuable for prediction of the properties of the steels in operating conditions. The lifetime assessment of bulky castings is associated with many problems. The solidification and cooling patterns are different for the various parts of the casting, bringing about differences in the structure. Thus, either ferritic-bainitic or completely bainitic structures can be found in different points of the turbine casings made of steel The dispersion of the precipitated carbides and their distribution between the basic ferritic bulk and the grain boundaries also varies in dependence on the structure type. Disperse carbides are precipitated in the grains in the purely bainitic structure, in contrast to the ferritic-bainitic structures where the carbides are coarser and precipitate more frequently at the grain boundaries. Their size varies, apparently in dependence on the cooling rate in the given point of the casting. The changes in the notch toughness were examined in dependence on the time of operation for this steel (Fig. 7). The differences in the structure are also responsible for the dispersion of the observed values, particularly for the starting state and for the states after short times of operation. The various structure types also exhibit different behaviour during operation at high temperatures. For the given steel type, cementite dissolves at the beginning and stable carbides particularly vanadium carbides precipitate. The ferritic grains in the ferritic-bainitic structures are thereby hardened. The notch toughness of such structures in the starting state is slightly DV 2b + σ

4 lower than as exhibited by the purely bainitic structures. The differences diminish on hardening of the ferritic grains. A direct dependence between the notch toughness and microstructure has been observed for structural steels [6]. The dependence involved the state of the hardening particles at the grain boundaries, and it was found that the notch toughness decreases with increasing dispersion, and the temperature of transition between the brittle and ductile fracture increases. The state of the grain boundary can be best described in terms of the mean free particle spacing or by the relative area of the grain boundary occupied by carbides. If the reasonable assumptions are adopted that cleavage fracture during the notch toughness tests is initiated at the grain boundaries and that the number of initiation sites depends on the number and size of the carbide particles at the boundary, then the lowest values of the relative area of the boundary occupied by carbides, A GB, should correspond to the highest notch toughness values. This is also borne out by the observed dependence for the material sampled from the steam turbine casing after a long time of operation (Fig. 8). A similar plot was also obtained for the dependence between the mean free particle spacing and the notch toughness. 4. CONCLUSIONS Examination of microstructure changes during a long-term exposure to high temperatures and investigation of the relation between the steel structure and properties constitute the basis for assessment of the degradation of structural materials during operation involving media at high temperatures. This is why considerable attention is paid to this issue, and the degradation processes are modelled and/or monitored on construction units in operation in order to obtain the necessary data for determining the status of the facility based on non-destructive inspections. This paper presented the results of a research programme of this kind for CrMo steels, which are among the conventional materials of pipe systems and facilities handling hot media in the power and chemical industries. The structure analysis procedures and ways of structure data processing by using quantitative stereological analysis have been demonstrated. The limiting factors restricting the applicability of the hardening phase particle coarsening model have been defined, and the conditions have been identified under which this procedure can be applied to predict the structure changes. Furthermore, the relations between the structure parameters and mechanical properties have been highlighted, and the procedures making use of the relations during the examination of steel degradation due to long-term exposure to high temperatures have been explained.. 5. REFERENCES [1] KUDRMAN, J., HOLUB, J., VOBOŘIL, J. Prakt. Met. 16, 1979, 132 [2] LIFŠIC, I.M., SLJOZOV, V.V. J. Phys. Chem. Solids, 19, 1961, 35 [3] WAGNER, C., Z. Elektochem. 65, 1961, 581 [4] SALTYKOV S.A., Stereometricheskaya metallografiya, Metallurgiya, Moscow 1970 [5] STRNADEL S. A., Řešené příklady a technické úlohy z materiálového inženýrství, Ed. K. Mazanec, Ostrava, 1998 [6] WU S.S., CHEN S.Y., GAN D., Mat. Sci. Eng., A127, 1990, L1

5 ACKNOWLEDGMENT Results obtained within research programmes under the umbrella of the Centre for Residual Lifetime Assessment and Application of Technological Units in the Chemical Industry of Controlled Ageing, co-sponsored by the Ministry of Industry and Trade of the Czech Republic, were used in this work. Fig. 1. Changes in the mean carbide size inside the grains in dependence on the annealing time Fig. 2. Changes in the mean carbide size on the grain boundaries in dependence on the annealing time Fig. 3. Dependence of the yield strength of steel 15313, calculated from the structure parameter values, on the annealing time Fig. 4. Dependence of the hardness of steel on the annealing time

6 Fig. 5. Dependence of the yield strength on the mean particle spacing Fig. 6. Parametrically expressed dependence of the yield strength on temperature and annealing time Fig. 7. Dependence of the notch toughness of steel on the time of operation of the steam turbine casing Fig. 8. Dependence of the notch toughness on the relative area of the grain boundaries occupied by carbides