Optimisation of long-term creep strength of martensitic steels

Besides the reduction of greenhouse gases the increase of thermal efficiency is one of the major goals in modern material development for process and power plants. Increasing the steam inlet temperatures and pressures is at present the favoured method to increase the thermal efficiency. For the realization of 700°C power plants, new creep resistant ferritic-martensitic 9–12 wt. % Cr steels are required to be applied in the 650°C temperature range. An important task for the optimization of long term creep properties is the characterization of the changes in the microstructure during creep exposure. A sufficient long term creep strength is based on a small initial size and slow coarsening of the M₂₃C₆ precipitates as well as the dynamic precipitation of small V(C, N) particles along with the absence of Z-phase. The paper describes the R&D activities of the MPA University of Stuttgart in the frame work of national and international research projects aimed at the development and long term characterisation of optimised martensitic steels with higher long term creep strength.     

Creep deformation properties of creep strength enhanced ferritic steels

Creep deformation properties of creep strength enhanced ferritic steels were investigated. Good linear relationships between creep strain vs. time and creep rate vs. time were observed within a transient stage in a double logarithmic plot. It was appropriately expressed by a power law rather than exponential law, logarithmic law and Blackburn’s equation. With decrease in stress, a magnitude of creep strain at the onset of accelerating creep stage decreased from about 2% in the short-term to less than 1% in the long-term. Life fraction of the time to specific strain of 1% creep strain and 1% total strain, to time to rupture tended to increase with decrease in stress. A time to 1% total strain, that is a parameter for design of high temperature components, was observed in the transient creep stage in the short-term regime, however, it shifted to the accelerating creep stage in the long-term regime. Difference in stress dependence of the minimum creep rate was observed in the high- and low-stress regimes with a boundary condition of 50% of 0.2% offset yield stress. Stress dependence of the minimum creep rate in the high stress regime was equivalent to a strain rate dependence of flow stress observed in tensile test, and a magnitude of stress exponent, n, in the high stress regime decreased with increase in temperature from 20 at 550°C to 10 at 700°C. On the other hand, n value in the low stress regime was about 5, and creep deformation in the low stress regime was considered to be controlled by dislocation climb.     

Microstructure characteristics of 12Cr ferritic/martensitic steels with various yttrium

12Cr ferritic/martensitic steels with 0, 0.1 wt%, 0.2 wt% and 0.3 wt% theoretical yttrium(Y) additions were fabricated by vacuum inducting melting and casting method. Solubilities of Y in the 12Cr steels are0.027, 0.078 and 0.17 for 12Cr-0.1 Y, 12Cr-0.2 Y and 12Cr-0.3 Y, respectively. Phase transformations and microstructure characteristics under different heat-treatment schedules were investigated. The starting temperature of ferrite-to-austenite transformation A~(c1) are maintained about 850℃, but the finishing temperature of ferrite-to-austenite transformation A~(c3) are about 950, 970, 980 and 1000℃ for 12Cr-0 Y,12Cr-0.1 Y, 12Cr-0.2 Y and 12Cr-0.3 Y, respectively, which indicates that A~(c3) increases gradually with the addition of Y. Martensite accompanied with a few δ-ferrite is the dominant structure in all the steels. The amount of δ-ferrite shows a strong dependence with the Y content and austenitizing temperature. Area fraction of δ-ferrite increases with the content of Y, which is the ferrite favouring element. The minimum amount of δ-ferrite are achieved at 950℃ for 12Cr-0 Y, 12Cr-0.1 Y, 12Cr-0.2 Y and 1000℃ for 12Cr-0.3 Y.Besides, more carbides precipitate along the martensite laths and grain boundaries in the Y-bearing steel due to the redistribution of carbon between austenite and ferrite resulting from the ferrite favouring element of Y.     

Contributions from research on irradiated ferritic/martensitic steels to materials science and engineering

Ferritic and martensitic steels are finding increased application for structural components in several reactor systems. Low-alloy steels have long been used for pressure vessels in light water fission reactors. Martensitic stainless steels are finding increasing usage in liquid metal fast breeder reactors and are being considered for fusion reactor applications when such systems become commercially viable. Recent efforts have evaluated the applicability of oxide dispersion-strengthened ferritic steels. Experiments on the effect of irradiation on these steels provide several examples where contributions are being made to materials science and engineering. Examples are given demonstrating improvements in basic understanding, small specimen test procedure development, and alloy development. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Evolution of dislocation structure in martensitic steels: the subgrain size as a sensor for creep strain and residual creep life

The results on the evolution of the dislocation structure in martensitic CrMoV-steels published by two research groups are shown to be consistent: The steady state dislocation spacings vary in inverse proportion to shear modulus normalized stress, the subgrains grow with strain at a rate which is determined by the initial subgrain size w₀, the steady state subgrain size w_∞ and the strain rate, independent of the composition of the material. At constant stress and temperature the strain ? and the subgrain size w are uniquely related by ? = ?_wln[log(w₀ / w_∞)/log(w / w_∞)] with ?_w = 0.12. Thus w can be used as a sensor for strain and, if the relation between strain and time is known, for the residual creep life.     

The effect of temperature and microstructure on the creep damage found in low alloy ferritic steels

Constant strain rate tests at 10^-5 s^-1 have been carried out in the temperature range 723 to 973 K on two 1 1/2 pct Cr · 1/2 pct V ferritic steels, the first steel with a 20 pct bainite, 80 pct ferrite microstructure and the second with a fully ferritic structure. Measurements of the quantitative strain, ε_gb, due to grain boundary sliding (gbs), were made and in both steels the γ values (where γ = ε_gb/ε_T) increased with increasing temperature. In both structures, sliding was found to occur on all boundaries. A qualitative study of cavitation damage and final fracture mechanisms was also made. It is suggested that in the mixed structure, cavities are nucleated by gbs at carbides whereas in the fully ferritic structure, cavity nucleation is by the interaction of intragranular slip with a grain boundary. Optical observations showed that the large scale cavitation behavior was superficially very similar in both steels, but scanning electron microscope observations showed remarkable differences in the fine scale cavitation damage. The implications of these results are discussed in terms of the relationship between matrix deformation, grain boundary deformation and creep fracture. Formerly of the Department of Metallurgy, University of Manchester.     

Weldable and corrosion-resistant ferritic stainless steels

High-chromium ferritic stainless steels have been little used as materials of construction because they lose their corrosion resistance and ductility when welded. Good as-welded properties depend on controlling interstitial carbon and nitrogen. Three methods of achieving such control are described: 1) reducing the interstitials to critical low levels; 2) including weld ductilizing additives, which “soften” the matrix; 3) using additives to complex the interstitials. The third method is particularly useful, since it eases the requirements for raw-material purity and melting techniques to control carbon and nitrogen. This paper is based on a presentation made at a symposium on “New Developments in Ferritic and Duplex Stainless Steels,” held at the Fall Meeting in Cleveland, Ohio, on October 19, 1972, under the sponorship of the Corrosion Resistant Metals Committee of TMS-IMD and the Corrosion and Oxidation Activity of the ASM.     

Embrittlement of ferritic stainless steels

The mechanical properties and microstructures of commercial 11 to 29 pct Cr ferritic steels were examined as functions of aging times to 1000 h at 371, 482, and 593°C. Of the properties evaluated, changes in impact transition temperatures were the best measure of embrittlement. Embrittlement at 482°C occurs most rapidly in the 29 pct Cr alloy and somewhat more slowly in the stabilized 26 pct Cr alloy. The stabilized 18 pct Cr alloy embrittles much more slowly while little, if any, embrittlement was detected in a stabilizedll pct Cr alloy. Embrittlement at 482°C was characterized by a rapid change in properties followed by a plateau region and then further property changes. The early property change is attributed to precipitation of interstitial compounds and the later change to classic 475°C embrittlement. The onset of 475°C embrittlement in the two highest Cr alloys was accompanied by clustering of Cr atoms along {100} planes indicative of spinodal decomposition. Concurrent with clustering there was also a change from turbulent slip to a more planar slip along {110} planes. Some embrittlement was observed after longer exposures at 371°C which was attributed to a combination of 475°C embrittlement and the precipitation of interstitial compounds. Two of the alloys also embrittled at 593°C, accompanied by optically observable precipitates. The precipitate in the stabilized 18 pct Cr alloy was identified as Laves (Fe₂Ti) phase. One of the precipitates in the 29 pct Cr alloy was identified as sigma phase. Formerly with Allegheny Ludlum Steel Corporation.     

Ferritic/martensitic steels for advanced nuclear reactors

Design concepts for the next generation of nuclear power reactors include water-cooled, gas-cooled, and liquid-metal-cooled reactors. Reactor conditions for several designs offer challenges for engineers and designers concerning which structural and cladding materials to use. Depending on operating conditions, some designs favor elevated-temperature ferritic/martensitic steels for in-core and out-of core applications. Such steels have been investigated in previous work on international fast reactor and fusion reactor research programs. Steels from these fission and fusion programs will provide reference materials for future fission applications. In addition, new elevated-temperature steels have been developed in recent years for conventional power systems that also need to be considered.     

The effect of fine precipitate particles on the creep behaviour of ferritic model steels -I. Experiments

In view of efforts to develop ferritic creep resistant steels for applications above 600°C the effect of fine precipitate particles on the creep behaviour of ferritic model steels was studied as a function of stress, temperature and particle distribution. The chosen model steels contained 20% Cr (by mass), up to 0.9% Nb and up to 0.1 % C to produce NbC volume fractions up to 0.8% with particle sizes of about 0.1 μm (order of magnitude). The alloys and structures are briefly described (NbC solubility, precipitation and ageing behaviour, recrystallization and grain growth, oxidation resistance) as well as the mechanical short-term behaviour. The creep behaviour was studied between 600°C and 800°C (with emphasis on 700°C) at strain rates between 10^?11 and 10^?6 s^?1 with times to rupture up to 20000 h. The creep resistance of the model steels at 700°C (for a strain rate of 10^?8s^?1) increases with increasing NbC content from about 5 MN/m² for the alloy without NbC to about 50 MN/m² for the alloys with 0.6% or 0.8% NbC. The analysis of the obtained results is the subject of the second part of this report.     

A creep technique for monitoring MnS precipitation in Si steels

A newly developed creep method is described for investigating the kinetics of manganese sulfide precipitation in two Si steels at hot working temperatures. The method was also applied to a Ti steel, in which the precipitation kinetics were previously determined using a stress relaxation technique. Prior to loading, the specimens are solution-treated for half an hour and then immediately cooled to the test temperature. A constant stress is applied to the sample by means of a computerized MTS machine, and the strain is recorded continuously during testing. The resulting creep rate is sensitive to the occurrence of precipitation; thus, the slope of the true strain-log (time) curve decreases immediately after the initiation and increases on the completion of precipitation. The precipitation-time-temperature (PTT) diagrams determined in this way on the three tested steels are of classical C shape. Because higher dislocation densities and internal stress levels are maintained, the present technique is more effective for monitoring the precipitation events occurring in both austenitic and ferritic phases than the previously developed stress relaxation method. formerly with the Department of Metallurgical Engineering, McGill University.     

Lattice parameter of ferritic and martensitic Fe−Ni alloys

The lattice parameter of ferritic (0 to 5 at. pct Ni) and martensitic (5 to 30 at. pct Ni) Fe?Ni alloys has been determined by the use of standard X-ray techniques. The lattice parameter first increases with increasing nickel content, reaching a maximum at about 13 at. pct Ni, and then decreases between 13 and 30 at. pct Ni. No simple atomic size concept can explain this behavior. This variation of lattice parameter is, however, similar to the variation of Bohr magmost important factor (s) determining the lattice parameter of Fe?Ni alloys.     

The effect of fine precipitate particles on the creep behaviour of ferritic model steels – II. Analysis of creep results

In view of efforts to develop ferritic creep resistant steels for applications above 600°C the effect of fine precipitate particles on the creep behaviour of ferritic model steels (20% Cr, up to 0.9 % Nb and 0.1 % C) was studied between 600 and 800°C as a function of stress, temperature and particle distribution. The analysis of the experimental results leads to the conclusion that the observed secondary creep rate can be described completely as dislocation creep by means of a modified power law: the temperature dependence is determined by that of the diffusion coefficient and of the shear modulus, the stress dependence is given by (σ-σ_th)³where σ = applied stress and σ_th = threshold stress, and the effect of the particles is described exclusively by the threshold stress which is of the order of the Orowan stress.     

A new equation for the Cr equivalent in 9 to 12 pct Cr steels

In advanced 9 to 12 pct Cr steels, the Cr equivalent is used as a measure to check the formation of δ-ferrite. In the present analysis, 29 alloys of varying composition were vacuum induction melted, and the amounts of δ-ferrite were measured in as-tempered conditions. Based on this and previous results on 9 to 12 pct Cr steels, a new equation for the Cr equivalent is proposed and correlated with the amount of δ-ferrite formation. Results indicate that the new Cr equivalent equation shows better correlation than previous equations and predicts the amount of δ-ferrite formed reasonably well.     

Effect of impurity content on creep crack growth resistance in 1Cr1Mo0.25V ferritic steels

The effect of impurity content on creep crack growth (CCG) rate and, more generally, on hot ductility of a typical lCrlMo0.25V ferritic steel was evaluated. Four heats intentionally doped with various amounts of impurities were characterized after heat treatments simulating the industrial thermal cycle taking place in a 1000-mm-diameter high-pressure rotor at two positions: near the outside surface and at the center in the cases of air cooling and oil quenching, respectively. Results indicate that the highest crack growth rates occur in the grade with a low P content (40 ppm) and Sn and Sb values (100 to 200 ppm) comparable with those characteristic of commercial steels. A marked reduction in brittleness is achieved only through a substantial reduction in the amount of Sn and Sb, even when medium-to-high P levels (100 ppm) are present. Creep resistance in terms of both time to rupture and minimum growth rate is not influenced by the impurity content, at least within the range of stresses investigated. Auger analyses on crept specimens demonstrate the presence of a selective segregation of impurity elements similar to that found in other ferritic steels: P is the only segregating element at non-cavitated grain boundaries, while cavitated areas contain Sn, Sb, and Cu in addition to P. The embrittlement at high Sn and Sb levels depends on two factors: at low P levels, cracks rapidly propagate under surface diffusivity control; at high P levels, excess P segregates at the grain boundaries, and crack propagation proceeds by an intergranular decohesion mechanism.     

Coherent precipitation and age hardening mechanisms in ferritic and martensitic alloys

In the course of tempering ferritic and martensitic structures obtained after quenching with water, an isomorphous, ordered, metastable and transient phase of Fe₃Si was observed in the form of spherical particles coherent with the matrix. Coarsening of the precipitate is controlled by bulk diffusion as in the model of Lifshitz and Wagner. If the temper is prolonged, the transient phase is reduced, to the benefit of an isomorphous phase in equilibrium with Laves’ Fe₂Ti phase. A study was carried out on this precipitation in an iron-chromium ferritic matrix, an iron-manganese martensitic matrix and a two phased ferrite-martensite matrix of iron-chromium-manganese. The different cases were considered and the mechanical properties of the hardened alloys were examined. Observation by transmission electron microscopy showed that the variations can be explained by an interaction mechanism between the precipitate and the dislocations. It has been seen that at diameters less than a critical value, a shear mechanism exists in the ordered precipitate, followed, when the average diameter is greater, by an Orowan bypass mechanism. The type of fracture observed by scanning electron microscopy has been interpreted in terms of the mechanism.     

Coherent precipitation and age hardening mechanisms in ferritic and martensitic alloys

In the course of tempering ferritic and martensitic structures obtained after quenching with water, an isomorphous, ordered, metastable and transient phase of Fe₃Si was observed in the form of spherical particles coherent with the matrix. Coarsening of the precipitate is controlled by bulk diffusion as in the model of Lifshitz and Wagner. If the temper is prolonged, the transient phase is reduced, to the benefit of an isomorphous phase in equilibrium with Laves’ Fe₂Ti phase. A study was carried out on this precipitation in an iron-chromium ferritic matrix, an iron-manganese martensitic matrix and a two phased ferrite-martensite matrix of iron-chromium-manganese. The different cases were considered and the mechanical properties of the hardened alloys were examined. Observation by transmission electron microscopy showed that the variations can be explained by an interaction mechanism between the precipitate and the dislocations. It has been seen that at diameters less than a critical value, a shear mechanism exists in the ordered precipitate, followed, when the average diameter is greater, by an Orowan bypass mechanism. The type of fracture observed by scanning electron microscopy has been interpreted in terms of the mechanism.