Quantification of precipitates in advanced creep resistant 9-12% Cr steels

In this paper a procedure is introduced for the quantification of precipitates appearing in 9-12°Cr-steels. Results gained from conventional transmission electron microscopic (TEM) investigations are compared with results from energy filtering transmission electron microscopic (EFTEM) investigations. The study was performed on a creep rupture specimen of the cast material G-X12CrMoWVNbN10-1-1 exposed at 600°C and ruptured after 33000 h. The size distribution of M₂₃C₆ carbides, MX phase and Laves phase were measured for both the un-stressed head and the stressed shank (gauge length) part of the investigated specimen. In particular, problems and sources of uncertainty concerning the quantitative determination of particle parameters in this type of steel are discussed. It is shown that quantification of the MX-particles by means of TEM bright field images is hardly possible. The size distributions of M₂₃C₆ carbides and MX phase overlap significantly which makes a separation based only on their size nearly impossible. Only Laves phase occupies a different size range. The difference between the size distributions of head and shank is pronounced only for M₂₃C₆ carbides. The measured particle distributions obey more closely a log-normal distribution rather than a normal distribution.     

Microstructural stability during creep of Mo- or W-bearing 12Cr steels

The evolution and stability of particulate phases during creep of molybdenum- or tungsten-bearing 12Cr steels have been investigated in considerable depth. The important finding is that the performance of Laves-phase precipitation in the molybdenum-bearing alloy is significantly different from that in the tungsten-bearing alloy. It is generally believed that such differences in kinetics will influence creep behavior. Data on Laves-phase precipitation kinetics as a function of time and temperature were quantified using the Wert-Zener equation in conjunction with the proprietary Thermo-Calc software, to determine equilibrium solute concentrations in these complex steels. The progressive depletion of Mo and W from the matrix as the particles of Laves phase evolve has been quantitatively modeled using experimental data obtained on both steels over a range of times and temperatures. The Isothermal coarsening rates of M₂₃C₆ and MX carbide particles were measured and found to occur at a constant volume fraction, in accordance with Ostwald ripening kinetics, with no significant differences in rates found between the two steels. The coarsening rates of M₂₃C₆ particles, found on subgrain boundaries, were consistent with a third-power dependence on particle radius, with an activation energy similar to that of volume diffusion. The smaller MX particles, which lay on subgrain-interior dislocation lines, were better explained by dislocation pipe diffusion, with a fifth-power dependence on particle radius and an activation energy approximately half that of volume diffusion.     

Creep rupture strength of tungsten-alloyed 9-12 % Cr steels for piping in power plants

The creep rupture strengths of the three tungsten-alloyed 9-12% Cr-steels E 911, NF 616 and HCM 12A are discussed. It is shown that Larson-Miller assessments may lead to an overestimation of the long-term creep rupture strength because of microstructural changes. The presented assessments indicate that all three alloys will have similar creep rupture strengths in the range of 110-120 MPa for 10⁵ h and 600°C. To obtain reliable estimates for creep rupture strength, however, further long-term tests are needed. In any case, the tungsten-alloyed steels will enable the main steam temperature to be raised by about 20°C over that possible with P 91.     

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.     

The effect of Nb,V, N and Al on the creep rupture strength of 9–12 % Cr steel

The creep resistance of advanced chromium steels can be significantly increased due to precipitation of very small particles of vanadium nitride VN. The solubility and precipitation of VN, Nb(C,N) and AIN in austenite and ferrite was analysed using relevant solubility products. The calculated values of nitrogen in solid solution were used for assessment of creep rupture strength of chromium steel (mean considered chemical composition, mass contents in %: 0.18 C; 10.5 Cr; 1.0 Mo; 0.2 V; 0.07 Nb; 0.05 N; 0.01 Al). Increasing N mass contents from 0.03 to 0.07 % leads to increasing creep rupture strength in 100 000 h at 600°C of about 60 %. Lowering AI mass contents from 0.045 to 0.005 % produces higher creep rupture strength of about 30 %.     

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.     

9%~12%Cr高中压转子材料发展历程与工程化关键技术

 高中压转子锻件因其尺寸规格和使用工况的特殊性,已成为超超临界机组技术发展过程中具有里程碑意义的关键部件。目前商业化运行的600~620 ℃超超临界机组中,广泛采用9%~12%Cr马氏体耐热钢制造高中压转子锻件。综述了近60年来9%~12%Cr高中压转子锻件耐热材料的发展历程,针对不同使用温度的高中压转子锻件,重点介绍了锻件的选材情况、化学成分和强化机制,并针对工程化制造过程中冶炼工艺、锻造工艺和热处理工艺的难点和关键点进行了简要叙述,为600 ℃及以上超超临界机组高中压转子锻件的工程化制造提供指导性建议。     

Austenite stabilization in Cr 13% Ni 4–6% steels

The present article deals with the study of the effects of concentration changes taking place in the matrix during partial austenitization of Cr 13% Ni 4–6% steel, while it is tempered, in the range of intercritical temperatures, on the structural stability of forming austenite. In the investigation, the authors made use of some special techniques of the potential polarization method and electron microanalysis. Some geometrical factors, such as the morphology and distribution of austenite areas were also studied. Increased austenite content in the matrix provides for an increased level of plastic properties and for improved technological characteristics. The stability of austenite areas depends on their morphology, distribution and, above all, on their dimensions - their thickness should not exceed 0.15 to 0.20 μm. The increase in nickel content observed in these areas was as much as 100% compared with the nominal nickel content of this steel.     

Effect of structural factors on the creep properties of modified chromium steels

The precipitation strengthening of modified chromium steels is effected predominantly by M₂₃C₆ carbides. In molybdenum-modified 9% chromium steels, creep resistance depends on the dispersion of the M₂₃C₆ carbides and on the molybdenum content in the solid solution. There is no point in increasing molybdenum contents of molybdenum-modified steels above approximately 1 wt.%. In vanadium-modified steels precipitation strengthening is effected both by M₂₃C₆ carbides and by VC_xN_y carbonitrides. If the amount of nitrogen in solid solution is insufficient, this reduces the volume fraction of VC_xN_y in the structure and thus impairs the creep resistance of the steel. It is advisable to restrict the aluminium and titanium contents in the vanadium-modified chromium steels.     

Microstructural changes in a 12% chromium steel during creep

A quantitative metallographic study was performed using transmission electron microscopy (TEM) to describe the microstructural changes in a 12% chromium steel (X 20 CrMoV 12 1) during creep at 650°C. The creep experiments were conducted under constant load conditions corresponding to initial stresses of 175 and 80 N/mm². The heat treatment for this steel consists of austenitizing followed by tempering which results in a high density of free dislocations within small elongated subgrains with carbides on or very near some of the subgrain boundaries. During creep, the mean subgrain size increases for both the high and low stress levels. Carbide particle coarsening is observed for the low stress level. These processes result in a softening of the microstructure during creep deformation.     

Subgrain growth during creep of a tempered martensitic 12% Cr-steel

The results of Nilsvang, Eggeler and llschner on subgrain growth in X 20 CrMoV 12 1 during creep are interpreted in terms of dynamic subgrain growth. It turns out that relatively large strains are needed to establish the steady-state subgrain structure, implying that the technically relevant part of creep occurs in the transient range.     

Molecular dynamics simulation of atomic displacement cascades in Fe-9 at % Cr and Fe-9 at % Cr-0.1 at % C alloys

The atomic displacement cascades in Fe-9 at % Cr and Fe-9 at % Cr-0.1 at % C alloys are studied by molecular dynamics simulation at an initial temperature of 600 K. The average fractions of defects that “survive” in a displacement cascade are calculated for primary knocked-on atom (PKA) energies of 0.1–20 keV, and the number and size of the vacancy and interstitial-atom clusters that form in a displacement cascade are determined. Carbon at the concentration under study is found not to affect the number of survived defects and the cascade efficiency. At PKA energies of 15 and 20 keV, the presence of carbon slightly decreases the fraction of point defects forming clusters.     

The effect of molybdenum on high-temperature properties of 9 pct Cr steels

Heat-resistant 9 Cr steels with 1, 2, and 3 pct Mo were tested for mechanical properties, weldability, and creep-rupture properties. The elevated-temperature and rupture strengths increase with increasing molybdenum content. While the 9 Cr-1 Mo steel is martensitic and is precipitation strengthened with carbides, the 9 Cr-2 Mo and 9 Cr-3 Mo steels receive added benefits from precipitation of Laves phase and solid-solution strengthening. The latter cause little decrease in ductility and impact resistance. The 9 Cr-2 Mo and 9 Cr-3 Mo steels are characterized by a duplex microstructure which aids weldability. Weld cracking tests show no need for preheating the latter steels, although the martensitic 9 Cr-1 Mo steel is known to be susceptible to weld cracking if not preheated. Both duplex-structure steels have good resistance to stress-relief cracking. Anisotropy of mechanical properties, due to the orientation of the duplex structure in the rolling direction, is less than that observed in the fully martensitic 9 Cr-1 Mo steel.     

The long-term aging embrittlement of Fe-12Cr steels below 773 K

When aged for long times at elevated temperatures, Fe-12Cr steels can experience a significant decrease in the fracture toughness, as observed by an increase in the fracture appearance transition temperature (FATT). The mechanism responsible for this decrease in toughness has never been unequivocally explained, but it has been generally attributed to the precipitation of second phases or impurity segregation. The objective of this study was to characterize the microstructural changes in conventional and super clean, electroslag remelted (high-purity) M152 steel aged between 616 K and 783 K up to 50 K hours for correlation to the toughness behavior. Analytical electron microscopy techniques were used to characterize the microstructure and impurity segregation. After aging at 727 K, conventional M152 contained large quantities of alpha prime (Cr-rich, bcc structure) in addition to Cr-rich M₂₃C₆ carbides and complex Cr-Fe-Mo-Ni-Si-rich precipitates. High-purity M152 aged at 727 K had a similar microstructure as the conventional material, except for the absence of the Si-rich precipitates. Aging at 616 K revealed the same phases as in the as-tempered material. Aged conventional M152 possessed segregated Sn on the prior austenitic grain boundaries, with equivalent levels of P segregation in both the conventional and high-purity materials. A “deembrittling” treatment of 873 K for 2 hours restored much of the original toughness, correlated with a decrease in hardness and a reduction in the amount of alpha prime phase. These results reveal that the degradation in toughness of M152 upon aging up to ∼753 K is primarily due to the combination of alpha prime formation and Sn segregation. These results are consistent with temper embrittlement mechanisms, where impurity segregation and second-phase formation synergistically embrittle NiCr steels. Improvements in the long-term aging embrittlement resistance of M152 below 773 K require ultralow levels of embrittling elements, especially Sn, and a reduction in the alloy Cr content to prevent alpha prime formation.     

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.     

Microstructural evolution of the martensitic cast steel GX12CrMoVNbN9-1 during long-term annealing and creep

The microstructure of four specimens of the martensitic cast steel GX12CrMoVNbN9-1 crept to fracture at 873 K for 220-33027 h was quantified by electron microscopy with regard to large M₂₃C₆-particles, small V- and Nb-containing particles, dislocations, pores, and oxidation layers. The laws of time-dependent coarsening of particles and strain-controlled growth of subgrains are consistent with those previously established for X20(22)CrMoV12-1. Using these data the microstructural model of deformation developed for X20(22)CrMoV12-1 can explain the creep behaviour of GX12CrMoVNbN9-1. The minimum in creep rate is related to the coarsening of particles and subgrains. At low stresses (< 140 MPa) the fraction of small slowly coarsening V- and Nb-containing particles leads to improved creep resistance. The material resists against pore formation; fracture occurs by necking.     

Thermal treatment improving creep properties of nitrogen-added Mod.9Cr-1Mo steels

Generation IV reactors are being developed to produce a reliable energy safely and with an economic benefit, because nuclear energy is being seriously considered to meet the increasing demand for a world-wide energy supply without environmental effects. Ferritic/martensitic steels are attracting attention as candidate materials for the Gen-IV reactors due to their high strength and thermal conductivity, low thermal expansion, and good resistance to corrosion. In recent years, new ferritic/martensitic steels have been developed for ultra supercritical fossil power plants through advanced technologies for steel fabrication. The microstructural stability of these materials for the pressure vessel, cladding and core structure of the VHTR and SFR is very important. Nitrogen is a precipitation hardening element, and the thermal stability of nitrides is superior to that of carbides. So the formation of nitrides may improve the thermal stability of the microstructure and eventually increase the creep rupture strength of high Cr steels. The effect of nitrogen on the creep rupture strength and microstructure evolution of nitrogen-added Mod.9Cr-1Mo steels has been studied. Creep testing was carried out at 873 and 923 K under constant load conditions. The optimum controlled Cr₂X precipitates were developed by special heat treatment, and they were not dissolved after a creep deformation. These fine and stable Cr₂X precipitates contributed to the increase of the creep rupture strength. The prior austenite grain size and martensite lath width were decreased by the resultant stable nitrides.