Creep Strain Modeling of 9 to 12 Pct Cr Steels Based on Microstructure Evolution

Creep deformation is simulated for 9 pct Cr steels by using the Norton equation with the addition of back stresses from dislocations and precipitates. The composite model is used to represent the heterogeneous dislocation structure found in 9 to 12 pct Cr steels. Dislocation evolution is modeled by taking capturing and annihilation of free dislocations into account. Recovery of immobile dislocations is derived from the ability of dislocation climb. In spite of the fact that the initial dislocation density is high and is reduced during creep, primary creep is successfully modeled for a P92 steel. Subgrain growth is evaluated using a model by Sandström (1977). The long time subgrain size corresponds well to a frequently used empirical relation, with subgrain size inversely proportional to the applied stress.     

Microstructural and Finite Element Analysis of Creep Failure in Dissimilar Weldment Between 9Cr and 2.25Cr Heat-Resistant Steels

Microstructural analysis and the creep failure mechanism of dissimilar weldment between ASTM A213 T92 (9Cr1.5W0.5MoVNbTi) and T22 (2.25Cr1Mo) heat-resistant steels are reported. The low-Cr part that has high carbon activity shows a depletion of C during postweld heat treatment. In particular, the soft carbon-depleted zone (CDZ) with the lowest hardness is surrounded by strong weld metal (WM) and the T22 heat-affected zone (HAZ). Load-displacement curves obtained by nanoindentation experiments are used to extract true stress–strain curves of the WM, the CDZ, and the T22 HAZ by using finite element methods (FEMs). Because of the mechanical properties of each region, the soft CDZ confined between harder regions is exposed to multiaxial stress. Therefore, creep voids actively form and coalesce in this CDZ and lead to macroscopic brittle fracture.     

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.     

Microstructural Evolution and Mechanical Properties of Short-Term Thermally Exposed 9/12Cr Heat-Resistant Steels

The microstructural evolution during short-term (up to 3000?hours) thermal exposure of three 9/12Cr heat-resistant steels was studied, as well as the mechanical properties after exposure. The tempered martensitic lath structure, as well as the precipitation of carbide and MX type carbonitrides in the steel matrix, was stable after 3000?hours of exposure at 873?K (600?°C). A microstructure observation showed that during the short-term thermal exposure process, the change of mechanical properties was caused mainly by the formation and growth of Laves-phase precipitates in the steels. On thermal exposure, with an increase of cobalt and tungsten contents, cobalt could promote the segregation of tungsten along the martensite lath to form Laves phase, and a large size and high density of Laves-phase precipitates along the grain boundaries could lead to the brittle intergranular fracture of the steels.     

Large-Strain Softening of Aluminum in Shear at Elevated Temperature: Influence of Dislocation Climb

This communication complements an earlier publication in this journal by the authors describing the basis for large-strain softening in aluminum under pure shear at elevated temperatures. Earlier work by the authors and the materials community only considered changes in the dislocation glide stress with the evolving texture as an explanation for the softening. New analysis finds that changes in the dislocation climb stress with texture development can explain the softening trends.     

The influence of microstructure on creep crack growth in 0.5 Pct Cr, 0.5 Pct Mo, 0.25 Pct V steel

The effect has been determined of changes in bainite content and prior austenite grain size on the creep fracture behavior of a low alloy ferritic steel. Increasing the bainite content is found to increase both crack growth and crack opening displacement rates whereas a simultaneous decrease in both the prior austenite grain size and bainite content is found to lower these rates. The results, coupled with metallo graphic evidence, are interpreted in terms of creep crack growth being controlled by the accumulation of displacement at a crack tip. Formerly with the Dept. of Metallurgy, Univ. of Manchester     

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 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.     

Effect of Compositional Changes of Laves Phase Precipitate on Grain Boundary Embrittlement in Long-Term Annealed 9 Pct Cr Ferritic Steel

In 9 to 12 pct chromium steels, the high-temperature mechanical properties are known to be strongly dependent on the formation and coarsening of Laves phase precipitates at boundaries. During high-temperature deformation, the Laves phase precipitate coarsening to over a critical size has been considered to trigger cavity formation at the precipitate-matrix interfaces. This coarsening, accompanied by the diffusion of W, Mo, and Cr, should change the mechanical properties and chemical composition of both Laves phase precipitates and the matrix. In this study, we aimed to clarify the effects of compositional changes of Laves phase precipitates on cavity formation during coarsening. The values of the Fe/Cr and W/Mo ratios in Laves phase precipitates were shown to induce different levels of strain energy in the vicinity of the Laves phase precipitate, consequently promoting the formation of cavities. Therefore, the compositional change of Laves phase precipitates was found to play a critical role in the grain boundary embrittlement of high Cr steel at elevated temperature.

     

Role of Boundary Strengthening on Prevention of Type IV Failure in High Cr Ferritic Heat-Resistant Steels

Microstructure evolution of newly developed 9Cr-3W-3Co-V, Nb steel with boron addition (B steel) has been analyzed during HAZ thermal cycle at the peak temperature of around A_c3 (A_c3 HAZ) and post-weld heat treatment (PWHT) to elucidate the prevention mechanism of type IV failure by boron addition. It was found that enhancement of the boundary strengthening by precipitates is the main reason for prevention of type IV failure by boron addition. In B steel HAZ, original austenite is reconstituted through martensitic α to γ reverse transformation during the heating and original martensite is reconstituted through martensitic transformation during cooling of the A_c3 HAZ thermal cycle. This process allows M₂₃C₆ carbides to precipitate at the prior austenite grain (PAG) and block boundaries during PWHT even if the chemical segregation of carbide forming elements exists. The effect of boundary strengthening on the creep property has also been investigated. Microstructure evolution during creep was compared among Gr.92 with different A_c3 HAZ microstructures prepared by three kinds of heat treatments and B steel. The results revealed that both the boundary length and kernel average misorientation value decreased in all samples during creep. However, this process occurred very rapidly in A_c3 HAZ simulated Gr.92, whereas it was significantly retarded in the other samples with sufficient boundary strengthening by precipitates. This result confirms that the precipitates formed at PAG and block boundaries play the most important role to stabilize the microstructure of A_c3 HAZ simulated samples during creep and prolong the creep life.     

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.     

Growth Kinetics of Laves Phase and Its Effect on Creep Rupture Behavior in 9Cr Heat Resistant Steel

The effects of Laves phase formation and growth on creep rupture behaviors of P92 steel at 883 K were studied.The microstructural evolution was characterized using scanning electron microscopy and transmission elec-tron microscopy.Kinetic modeling was carried out using the software DICTRA.The results indicated Fe2 (W,Mo) Laves phase has formed during creep with 200 MPa applied stress at 883 K for 243 h.The experimental results showed a good agreement with thermodynamic calculations.The plastic deformation of laths is the main reason of creep rupture under the applied stress beyond 160 MPa,whereas,creep voids initiated by coarser Laves phase play an effective role in creep rupture under the applied stress lower than 160 MPa.Laves phase particles with the mean size of 243 nm lead to the change of creep rupture feature.Microstructures at the vicinity of fracture surface,the gage portion and the threaded ends of creep rupture specimens were also observed,indicating that creep tensile stress enhances the coarsening of Laves phase.     

The impact of welding on the creep properties of advanced 9–12% Cr steels

The term “long-term creep properties” for heat resistant steels is mainly reflected by the 100.000 hour creep rupture strength at elevated testing temperature. Often, results of high stress, short-term creep tests are extrapolated to this 100.000 hour target value. Results of long-term creep tests are rather rare because of high testing costs and the time consuming testing procedure. Especially, long-running crossweld creep tests have not been performed in a sufficient extent so far, although, the heat-affected zone of crosswelds of ferritic chromium steels is known as a possible weak point. The long-term creep properties of crosswelds is linked to microstructure of the heat-affected zone of 9–12% chromium steels. The formation of heat-affected zone microstructures is studied by dilatometry, in-situ X-ray diffraction using synchrotron radiation, optical microscopy as well as most advanced electron microscopic methods. Results of crossweld creep tests up to duration of 40,000 hours are directly linked to the heat-affected zone microstructure at the location of fracture. The most predominant failure mechanism at lower stress levels and long term duration is Type IV cracking in the fine-grained heat-affected zone region of crosswelds. The failure mechanism is discussed in detail in this paper.     

稀土在低硫16Mn钢中的作用

长条状的MnS夹杂对钢的横向韧塑性极为不利。喷吹Si—Ca或加入RE,通过脱硫和变质硫化物的作用,都可显著地消除其危害。喷吹Si—Ca变质硫化物的程度与钢中Ca/S有关。喷吹Si—Ca后加入少量RE,有利于控制MnS完全变质,此时钢中所需的RE/S可明显低于3。在大幅度脱硫的基础上加入少量RE,是使钢材韧塑性获得稳定大幅度提高的一条有效途径。