New approach to predict the long-term creep behaviour and evolution of precipitate back-stress of 9–12% chromium steels

Modern advanced 9–12 % Cr steels are complex alloys with excellent creep strength even at high temperatures up to 620°C. The mechanical properties of these steels are significantly influenced by the presence and stability of various precipitate populations. Numerous secondary phases grow, coarsen and, sometimes, dissolve again during heat treatment and service, which leads to a varying obstacle effect of these precipitates on dislocation movement. In this work, the experimentally observed creep rupture strength of an modified 9–12% Cr steel developed in the European COST Group is compared to the calculated maximal obstacle effect (Orowan stress) caused by the precipitates present in these steels for different heat treatment conditions. It is shown that the differences in creep rupture strength caused by different heat treatments disappear after long time service. This observation is discussed on the basis of the calculated evolution of the precipitate microstructure. The concept of boosting long-term creep rupture strength by maximizing the initial creep strength with optimum quality heat treatment parameters for precipitation strengthening is critically assessed.     

STT and DTT Diagrams of Austenitic Cr–Mn–Ni As‐Cast Steels and Crucial Thermodynamic Aspects of γ → α′ Transformation

The mechanical behavior and microstructure evolution during deformation of novel austenitic Cr–Mn–Ni as‐cast steels with varied Ni content were investigated at various temperatures using static tensile tests, optical microscopy, and the magnetic scale for the detection of ferromagnetic phase fraction. To summarize all knowledge about the deformation‐induced processes, the STT and DTT diagrams were developed for Cr–Mn–Ni steels. The diagrams illustrate the different deformation mechanisms depending on temperature and tension load, and quantify the elongation of the deformation mechanisms. The deformation‐induced ε‐ and α' martensite formation and twinning – the TRIP and TWIP effects – occur in the Cr–Mn–Ni steels depending on the chemical composition and temperature. The differences of deformation‐induced processes depend on thermodynamics and are confirmed by thermodynamic calculations. The nucleation threshold of γ → α′ transformation was determined for the investigated Cr–Mn–Ni steels.     

Estimation of liquidus temperatures for steels using thermodynamic approach

Abstract

Alternative schemes for the prediction of liquidus temperatures have been tested against a large compilation of experimental measurements for steels ranging from low carbon to stainless steels. The best agreement overall was obtained with thermodynamic solutions using the IDS (model of interdendritic solidification) database, although certain empirical equations from the literature were adequate for low alloyed steels. Version L of the commercial ThermoCalc thermodynamics package was outperformed for some steel types by the empirical equations, prompting a reassessment of the Fe–Cr–Ni, Fe–Cr–Si, and Fe–Ni–Si systems used in the IDS database. Additionally, regression equations were determined from several thousand predictions computed by the IDS thermodynamic approach, and these performed almost as well as the proper thermodynamic approach at a fraction of the central processing unit time, also outperforming the earlier approaches.     

Thermodynamic Assessment to Chromium Oxidation in the Production of Stainless Steel

A solution model called ε approach is applied to predict the thermodynamic solution behaviour of Cr in steel melts. The calculated results are supported by the available experimental results. Further predictions on chromium scorification are achieved in view of melting and refining processes for Cr‐containing steels. Chromium reduction from slags after meltdown during EAF melting and AOD or VOD refining treatment is important for the production of Cr‐containing steels. It is thermodynamically predicted that chromium is inevitably oxidized during decarburization process which coincides with the industrial and experimental results. Minimum oxidation of chromium can be reached by lowering the partial pressure of CO gas at high temperatures.     

The Creep Rupture Properties of 9% Chromium Steels and the Influence of Oxidation on Strength

Abstract

Creep rupture data for the 9% chromium steels Fe9CrlMoVNb (P91), Fe9CrlMolWVNb (E911) and Fe9Cr Mo2WVNb (P92) have been evaluated using the secondary creep rate as well as the stress rupture life and compared with literature data for Fe9CrlMo (P9) and 12CrlMoV. Extrapolation procedures have been carried out in order to predict the long-terms stress rupture strengths of the 9% Cr Steels. The factors affecting the reliability of the extrapolations are discussed. The 600°C/100 000 h stress rupture strength of P92 was slightly higher than that of E911 based on data of up to 30 000h duration. The effect of oxidation on rupture life was assessed; for components of wall thickness below about 6 mm, the loss of load-bearing cross-section due to oxidation should be taken into account for service life prediction.     

Creep Behavior and Degradation of Subgrain Structures Pinned by Nanoscale Precipitates in Strength-Enhanced 5 to 12 Pct Cr Ferritic Steels

Creep behavior and degradation of subgrain structures and precipitates of Gr. 122 type xCr-2W-0.4Mo-1Cu-VNb (x = 5, 7, 9, 10.5, and 12 pct) steels were evaluated during short-term and long-term static aging and creep with regard to the Cr content of steel. Creep rupture life increased from 5 to 12 pct Cr in the short-term creep region, whereas in the long-term creep region, it increased up to 9 pct Cr and then decreased with the addition of Cr from 9 to 12 pct. Behavior of creep rupture life was attributed to the size of elongated subgrains. In the short-term creep region, subgrain size decreased from 5 to 12 pct Cr, corresponding to the longer creep strength. However, in the long-term creep region after 10⁴ hours, subgrain size increased up to 9 pct Cr and then decreased from 9 to 12 pct, corresponding to the behavior of creep rupture life. M₂₃C₆ and MX precipitates had the highest number fraction among all of the precipitates present in the studied steels. Cr concentration dependence of spacing of M₂₃C₆ and MX precipitates exhibited a V-like shape during short-term as well as long-term aging at 923 K (650  °C), and the minimum spacing of precipitates belonged to 9 pct Cr steel, corresponding to the lowest recovery speed of subgrain structures. In the short-term creep region, subgrain coarsening during creep was controlled by strain and proceeded slower with the addition of Cr, whereas in long-term creep region, subgrain coarsening was controlled by the stability of precipitates rather than due to the creep plastic deformation and took place faster from 9 to 12 pct and 9 to 5 pct Cr. However, M₂₃C₆ precipitates played a more important role than MX precipitates in the control of subgrain coarsening, and there was a closer correlation between spacing of M₂₃C₆ precipitates and subgrain size during static aging and long-term creep region.     

Thermodynamic modeling of the interaction of high-level element and oxygen in iron-based melt

The main reactions of calcium, cerium, lanthanum, silicon, and complex alloys with the active additive Ca–Si, Ca–Al, Ce–Si, La–Si, Ce–La, or Ce–Ca–Al are subjected to thermal analysis. On the basis of binary and ternary fusibility diagrams, the thermodynamic data are refined for the main reactions between complex alloys with rare-earth elements and the oxygen present in the liquid metal. Solubility surfaces are plotted for components of the Fe–Si–Ce–O, Fe–Si–La–O, Fe–Ce–La–O, and Fe–Ca–Al–Ce–O systems. From those diagrams, optimal compositions of complex alloys with rare-earth elements in terms of nonmetallic-inclusion formation are established by plotting the consumption of the active components. For each group of steels, the quantitative elementary composition of the active components used for reduction and modification of the nonmetallic inclusions must be calculated. The chemical and phase composition of the nonmetallic inclusions may be very complex even in the final stages of the reduction of oxides. A method is developed for taking account of the polyvalency of cerium in reduction processes. The thermodynamic data and diagrams obtained help provide a better understanding of the complex heterogeneous processes within multicomponent systems that contain liquid metals. In combination with experimental data for the solubility surfaces of the components and the consumption diagrams, it is possible to track the transitions from the nonequilibrium state of the metallurgical system to the equilibrium state. In other words, the degree of refining of the metal and the equilibrium composition of the nonmetallic inclusions may be determined.     

超超临界火电机组用关键锅炉钢性能分析

对T/P92、T/P122、S30432(Super 304H)、S31042(HR3C)、9Cr3W3Co等先进锅炉钢的性能进行了综合评述,对比分析了这几种钢管及其焊接接头的持久强度、抗腐蚀性能和常规力学性能数据,采用等温线法和Larson-Miller法外推了这些锅炉钢的10万h持久强度,探讨了这几种钢管许用应力的设定及在工程设计和应用的问题。     

Thermodynamic Criteria for the Selection of Alloys Suitable for Semi‐Solid Processing

In this work the suitability of alloys for semi‐solid processing was investigated using numeric simulation. The simulation was based on equilibrium calculations, Scheil‐Gulliver calculations and, when necessary, diffusion simulations. For this purpose a new parameter was introduced in addition to the commonly used selection criteria. With the new parameter, the thixo ranges ΔT^40–60 and ΔT^20–40, the specific demands of the different semi‐solid processes thixocasting and thixoforging can be considered. On the basis of thermodynamic simulation, the conventional aluminium alloys A356, AA6082 and A319, the steels 100Cr6, HS6‐5‐2 and X210CrW12 and a number of experimental aluminium‐lithium based alloys were evaluated according to the selection criteria. The thermodynamic calculations showed a large sensitivity of the course of the solidification with respect to variations in the contents of the alloying elements. This shows the necessity of keeping a tight composition control on alloys for semi‐solid processing. For aluminium alloys in particular silicon has to be monitored closely and for steels carbon and chromium.     

Evaluation of the density of steels

Data on physical properties of steels have been collected from the open literature and put into a database. The influence of composition on the density of steels has been analyzed. An overview over former studies is given. The steels have been investigated by regression analysis in two groups, i) ferritic and low alloy steels, and ii) austenitic steels. For ferritic steels two models are provided. The first model is based on the assumption that all C is bound in cementite and other solutes are insoluble in cementite. The second model employs the result of a thermodynamic analysis where the amount of cementite and the solubilities in ferrite and cementite were determined with computational thermodynamics. The non‐linear effect of Cr and Mn in cementite was computed and regression analysis of the effect of solutes on the density of ferrite was performed. For Ti‐stabilized austenitic steels, the amount of TiC and the solubilities were assessed in a thermodynamic analysis. The effect of solutes on the density of austenite was studied by regression analysis. For estimations of the density of steels containing components that are not covered by the regression analysis, the regression coefficients can be supplemented with literature data or theoretically determined values. The results obtained by the present regression analysis are: Cu and Mo increase the density of ferritic steels, and C, Cr, Mn, S, Si, and V decrease it. TiC, C, Cr, Mn, N, Si, and Ti reduce the density of austenitic steels and Cu, Co, Mo, and Ni increase it.     

Effects of boron on the creep properties,phosphorus segregation behaviour,and stress relief cracking susceptibility of 1.5% Cr–0.5% Mo steels

The effects of 25 ppm boron and of 0.05% phosphorus on the creep behaviour at 550 °C and on the fracture temperature in the constant load fracture test were tested for a 1.5% Cr–0.5% Mo steel. B and P decrease the creep strength, the rupture elongation is increased by B. B and P lower stress relief cracking susceptibility, however, it is increased at low stress. In the B doped material the rate of P grain boundary segregation is accelerated and the level of equilibrium segregation is somewhat higher, the equilibrium segregation is somewhat lower in the bainitic than in the martensitic structure.     

Creep Strength of Dissimilar Welded Joints Using High B-9Cr Steel for Advanced USC Boiler

The commercialization of a 973 K (700 °C) class pulverized coal power system, advanced ultra-supercritical (A-USC) pressure power generation, is the target of an ongoing research project initiated in Japan in 2008. In the A-USC boiler, Ni or Ni-Fe base alloys are used for high-temperature parts at 923 K to 973 K (650 °C to 700 °C), and advanced high-Cr ferritic steels are planned to be used at temperatures lower than 923 K (650 °C). In the dissimilar welds between Ni base alloys and high-Cr ferritic steels, Type IV failure in the heat-affected zone (HAZ) is a concern. Thus, the high B-9Cr steel developed at the National Institute for Materials Science, which has improved creep strength in weldments, is a candidate material for the Japanese A-USC boiler. In the present study, creep tests were conducted on the dissimilar welded joints between Ni base alloys and high B-9Cr steels. Microstructures and creep damage in the dissimilar welded joints were investigated. In the HAZ of the high B-9Cr steels, fine-grained microstructures were not formed and the grain size of the base metal was retained. Consequently, the creep rupture life of the dissimilar welded joints using high B-9Cr steel was 5 to 10 times longer than that of the conventional 9Cr steel welded joints at 923 K (650 °C).     

Evaluation of the electrical resistivity of steels

Literature data on the physical properties of steels have been collected and put into a database. The resistivity of steels has been analyzed as a function of composition and microstructure. An overview over former studies is given. The steels have been investigated in two groups, ferritic steels and austenitic steels. A thermodynamic analysis with ThermoCalc has been performed. Regression analysis on the influence of composition on the resistivity was then carried out. The results for ferritic steels are: Si and Al have the highest elemental resistivity, followed by Mn, Cu, Ni, Mo, and Cr. C precipitated in cementite shows a high coefficient in the analysis when the amount of Fe bound in cementite is not considered separately. C in solution with ferrite shows no significant effect. Cr bound in cementite shows a significant effect but Mn, though present in cementite in comparable amounts, has no significant effect on the resistivity. N and C have the highest elemental resistivity in austenite, followed by the substitutional solutes Nb, Si, Ti, Cu, Ni, Mo, and Cr. The carbides NbC and TiC appear with a higher coefficient in the regression model than can be explained by phase‐mixture models providing upper and lower bounds for the resistivity of two‐phase alloys. Cr₂₃C₆ shows no significant effect. The regression results can be used to predict the resistivity of steels with known composition. The model predicts the resistivity of ferritic steels with a maximum deviation between experimental and computed value of 12 nōm and a standard deviation of 5.6 nōm. For austenitic steels, the model prediction shows a maximum deviation of 52 μōcm and a standard deviation of 20 nōm.     

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 %.     

(C+N)复合强化的Fe-Cr-Mn(W,V)钢高温性能的研究

研究了用于核反应推低放射性结构材料Ｆｅ－Ｃｒ－Ｍｎ（Ｗ,Ｖ）奥氏体钢。通过（Ｃ＋Ｎ）复合强化有效地提高Ｆｅ－１２％Ｃｒ０１５％Ｍｎ（Ｗ,Ｖ）钢高温强度和蠕变断裂寿命,并改善高温塑性。在温度６７３Ｋ以下,合金比ＳＵＳ３１６钢和ＪＰＣＡＳ钢强度和塑性优良。合金强度和塑性与形变的相互关系是和合金形变组织变化;密切相关。对６７３Ｋ以上塑性降低的原因进行断口和显微组织分析,控制晶界碳化物粗化是进一步提高高温     

On Physico‐Chemical and Technical Limits in Clean Steel Production

Great progresses in steel cleanliness have been attained during the last decades. In the measures of oxygen the lowest levels are approaching 5 ppm in O_tot i.e. close to the thermodynamic limit of Al deoxidation. As the thermodynamics of the reaction system is firmly anchored to the steel chemistry and thus to the properties of steel and the final product, it is useful to examine the thermodynamic constraints for selected steels in the refining and casting processes. In most steels Al‐O equilibrium determines the limit of deoxidation. Calculations by applying thermodynamic software showed that in selected “common” steels the equilibrium oxygen content varied from below 5 ppm up to 30 ppm or higher depending on the aluminium content, interaction effect of other alloying elements and temperature. For lower oxygen content there are several possibilities. The first one is to use stronger deoxidizing additions like Ca, Zr, Ce etc. However, they can be problematic as substitutes as they produce inclusions which influence steel properties. The second way is to intensify the deoxidizing power of certain elements e.g. Si by bringing the steel in intimate contact with a proper slag with low SiO₂ activity in ladle treatment with violent stirring. In the calculation example it was possible to decrease oxygen content from 30 ppm to the level below 10 ppm in C/Mn/Si steel at very low Al level. The third potential but unused process is vacuum deoxidation especially for medium and high carbon steels. Equilibrium oxygen contents below 1 ppm are thermodynamically easily attainable. But in practice the vacuum treatment should be designed to intensify the carbon‐oxygen reaction on the top surface of the stirred steel liquid. Also the eventual reactions with refractory materials should be suppressed.     

The effect of tungsten on creep

The effect of tungsten on creep behavior and microstructural evolution was investigated for tempered martensitic 9Cr steels with various W concentrations from 0 to 4 wt pct. The creep rupture testing was carried out at 823, 873, and 923 K for up to 54 Ms (15,000 hours). The creep and creep rupture strength increased linearly with W concentration up to about 3 wt pct, where the steels consisted of the single constituent of the tempered martensite. It increased only slightly above 3 wt pct, where the matrix consisted of the tempered martensite and δ-ferrite. The minimum creep rate was described by a power law. The apparent activation energy for the minimum creep rate showed a tendency similar to the W concentration dependence of the creep-rupture strength and was larger than the activation energy for self-diffusion at high W concentrations above 1 wt pct. The martensite lath microstructure with fine carbides along lath boundaries was responsible for a high resistance to creep deformation. With increasing W con- centration, the martensite lath microstructure became stabilized, which decreased the minimum creep rate and increased the apparent activation energy for the minimum creep rate.