Term Creep Strength Research Articles

On the Relationship between the Chromium Concentration, the Z‐Phase Formation and the Creep Strength of Ferritic‐Martensitic Steels

In this study, the long term creep strength behavior of commercial heat resistant martensitic/ferritic steels with Cr levels ranging from 1 to 15 wt% is analyzed by linking their computed equilibrium compositions to their creep properties. At lower Cr levels, the calculated strength due to precipitation hardening agrees well with the experimental results. At high chromium levels and longer exposure times, an accelerated strength loss due to the formation of Z‐phase precipitates has been reported. The accelerated strength loss is computationally analyzed and a correlation between accelerated strength loss and Z‐phase formation is confirmed. A study is made to explore the option of adjusting the chemical composition of existing high‐chromium steels to reduce the driving force for Z‐phase formation. However, no proper composition ranges are found which combine a high Cr concentration with a significantly lower driving force for Z‐phase formation.

Quantitative Evaluation of Secondary Phases in a Weld Joint Made of COST F and FB2 Creep Resistant Steels

Conventional (CCT) and accelerated (ACT) creep tests of a weld joint made of COST F and COST FB2 steels were carried out over a temperature range from 550 °C to 650 °C. Fracturing of the crept specimens was located in the heat affected zone (HAZ) of the F steel. Two specimens were selected after CCT and ACT for quantitative evaluation of the precipitates and compared to the weld joint in as-received conditions. Scanning and transmission electron micrographs were used to measure the precipitate size. Both methods were compared and the accuracy of the results was discussed. It was concluded that ACT can simulate the precipitation of chromium carbides and structure recovery during long term creep exposures. However, precipitation of Laves phase during CCT was not recorded after ACT. Therefore, it is difficult to use ACT in this experiment for estimating the long term creep strength.

On the prediction of long term creep strength of creep resistant steels

Abstract When the conventional power law creep equation is applied to rationalise the creep data of creep resistant steels, its parameters depend strongly on stress and temperature and hence cannot be used to predict long term creep properties. Here, it is shown that this problem can be resolved if it is modified to satisfy two boundary conditions, i. e. when σ (stress) = 0, ε ˙ min ${\dot \varepsilon _{{\rm{min}}}}$ (minimum creep rate) = 0, and when σ = σTS (tensile stress at creep temperature T), ε ˙ min ${\dot \varepsilon _{{\rm{min}}}}$ = ∞. This can be achieved by substituting the reference stress σ0 in the conventional equation by the term (σTS – σ). The new power law creep equation describing the stress and temperature dependence of minimum creep rate can then be applied to predict long term creep strength from data of short term measurements. This is demonstrated using the creep and tensile strength data measured for 11Cr-2W-0.4Mo-1Cu-Nb-V steel (tube).

Review of Z phase precipitation in 9–12 wt-%Cr steels

For high temperature applications, 9–12 wt-%Cr steels in fossil fired power plants rely upon precipitate strengthening from (V,Nb)N MX nitrides for long term creep strength. During prolonged exposure at service temperature, another nitride precipitates: Cr(V,Nb)N Z phase. The Z phases lowly replace MX, eventually causing a breakdown in creep strength. The present paper reviews the Z phase and its behaviour in 9–12 wt-%Cr steels including thermodynamic modelling, crystal structure, nucleation process and precipitation rate as a function of chemical composition. The influence of Z phase precipitation upon long term creep strength is assessed from several different 9–12wt-%Cr steel grades and alloy design philosophies.

Damage characterization of an ASTM A 213 grade 91 tube after 116.000 h of service in a reforming plant

ASTM A213 T91 steel is used in power plants and petrochemical industry, for long-term service components. The improved mechanical properties of grade 91 are strictly related to its specific microstructure: a tempered martensite matrix with fine precipitates embedded in. Despite low alloy heat resistant ferritic steels, that have a well known operational experience, T91 service performances are still faintly consolidated, because this material has serviced only in a limited number of plants, since the eighties. Most of the available data were obtained by laboratory tests on relatively short term creep strength and corrosion properties.The investigations reported in this paper represent an important opportunity to describe and better evaluate the damage evolution of the grade T91 steel after more than 100000 h of exposure in severe conditions (580 °C, 18–26 bar, combustion environment).Our results suggest that the steel suffered by different damage forms, which appear on definite portions of the tube cross section. The main degradation forms observed, in fact, into the tube bulk are both the martensite recovery and the microstructural evolution. This latter promoted mostly Laves phase precipitation and coarsening. On the other hand, both the outer and the inner wall side, suffered mainly by severe oxidation/carburization. Especially on the outer surface, the massive carbide precipitation has caused an evident loss of ductility so that the mechanical properties of the tube appear appreciably reduced.

Formation of the Fe2Hf Laves Phase through Eutectoid Type Reaction of δ→γ+Fe2Hf in Ferritic Heat Resistant Steels

ABSTRACTPeriodically arrayed rows of fine Fe2Hf Laves phase particles were found to form in 9Cr ferritic steel. Microstructural observation demonstrates that the particles were formed on cooling through the interphase precipitation on the phase transformation from the δ ferrite to the γ austenite along the eutectoid transformation route of δ → γ+Fe2Hf and subsequently a phase transformation from the austenite to the α ferrite took place. This eutectoid route is expected to be effectively used for improving the long term creep strength of ferritic steels with Laves phase.

Characterization of precipitates in X12CrMoWVNbN10-1-1 steel during heat treatment

The characterization of precipitates in X12CrMoWVNbN10-1-1 steel during the heat treatment was carried out for revealing the evolution of the precipitates. In addition to other microstructural parameters (such as dislocation and subgrains), the precipitate also plays an important role for microstructural stability which is a prerequisite for long term creep strength. In this paper, the precipitates during the heat treatment for this steel were characterized using physicochemical phase analyses and transmission electron microscopy. It was found that the Fe-rich M3C carbides and Nb-rich MX particles were detected in the samples cooled in furnace from austenitization at 1080°C for 16h. However, after water cooling, only Nb-rich MX particles existed. During tempering at 570°C for 18h, the formation of Cr-rich M7C3 was detected but was replaced partially by Cr-rich M23C6. Additional Cr-rich M2N nitride was also found. After two successive tempering (570°C+690°C) for 24h, Cr-rich M7C3 was completely replaced. The microchemical analyses of the extracted residues during heat treatment were also discussed. The results gave rise to an indication that the precipitation of precipitates nearly completed in first tempering and the transformation from Cr-rich M7C3 to Cr-rich M23C6 mainly occurred in the second tempering.

Exploring the applicability of the LICON methodology for the creep assessment of a dissimilar metal weld

The LICON methodology is an approach for predicting the lifetime of materials under creep loading conditions. The LICON method predicts long-time uniaxial creep strength using the results from several short duration creep crack incubation (CCI) tests in conjunction with the outcome of a mechanical analysis for the adopted multiaxial specimen geometry. The applicability of the methodology for long term creep strength prediction of martensitic 9%Cr and bainitic 1%Cr steels has already been demonstrated. This study has examined the applicability of the procedure for predicting long term uniaxial creep strengths for a dissimilar metal weld (DMW). It has required new developments to the original formulation. Application of the developed formulation for predicting uniaxial creep rupture behaviour of the investigated DMW shows an acceptable agreement with experimental observations which was not previously achievable.

Design of 10%Cr martensitic steels for improved creep resistance in power plant applications

The drive to increase the efficiency of fossil fired power generation to reduce CO2 emissions and to conserve energy resources has led Tata Steel to design new 10%Cr martensitic compositions for use at temperatures of 620°C and above with creep properties superior to steel 92 (9%Cr, 0·5%Mo, and 2%W), which is the best currently available steel of this type. In the new alloys, the chromium content was set at 10% to ensure the required oxidation resistance. The long term creep performance of steels with this level of chromium has been limited by the precipitation of Z phase nitrogen rich particles. These form at the expense of vanadium rich MN precipitates that are vital for long term creep strength. Compositions have been designed with the aim of suppressing or delaying Z phase formation. The principles underlying the new steel chemistries are discussed, and a detailed explanation is given of the role of heat treatment in optimising creep rupture strength at temperatures in excess of 600°C. The new steel compositions are based on published work supplemented by thermodynamic calculations to optimise the heat treatment cycle, to produce a fully martensitic microstructure and to maximise the stable nanoprecipitates that are essential to maximise creep rupture life.

High Temperature Materials for Nuclear Fast Fission and Fusion Reactors and Advanced Fossil Power Plants

Development of materials plays a crucial role in the economic feasibility of fast nuclear fission and fusion power plant. In order to meet this objective, one of the methods is to extend the fuel burnup and decreasing doubling time. The burnup is largely limited by the void swelling and creep resistances of the fuel cladding and wrapping materials. India's 500 MWe Prototype Fast Breeder Reactor (PFBR) is in advanced stage of construction. The major structural materials chosen for PFBR with MOX fuel are alloy D9 as fuel clad and wrapper material, 316LN austenitic stainless steel for reactor components and piping and modified 9Cr-1Mo steel for steam generator. In order to improve the burnup further, titanium, phosphorous and silicon contents in alloy D9 have been optimized for better swelling and creep resistances to develop modified version of alloy D9 as IFAC-1. Creep resistance of inherently void swelling resistance 9Cr-ferritic steel has been improved with the dispersion of nano-size yttria to develop oxide dispersion strengthened (ODS) steel clad tube with long- term creep strength, similar to D9, for increasing the fuel burnup. Development of modified 9Cr-1Mo steel clad tube and 9Cr-1Mo steel wrapper for future metallic fuel reactors being developed for reducing the doubling time are in progress. Extensive studies on resistance of this new generation core materials to void swelling are also under progress along with material development. Improved versions of 316LN stainless steel with nitrogen content of about 0.14 wt.% having higher creep strength to increase the life of fast reactor and modified 9Cr-1Mo steel with reduced nitrogen content and controlled addition of boron to improve type IV cracking resistance for steam generator are other developments. India's participation in ITER programme necessitates the development of India-specific RAFM steel for Test Blanket Module (TBM). A comprehensive research programme is being carried out to develop India-specific 9Cr-W-Ta RAFM steel with the optimization of tungsten and tantalum contents for better combination of strength and toughness. Based of the extensive mechanical tests including impact, tensile, creep and fatigue on four heats of RAFM steels having tungsten in the range 1 – 2 wt. % and tantalum in the range 0.06 -.014 wt., the RAFM steel having 1.4 wt. % tungsten with 0.06 wt. % tantalum is found to possess better combination of strength and toughness. This steel is considered as India-specific RAFM steel and TBM is being manufactured by this RAFM steel. To limit the emission of green house gases, a research and development programme has been initiated to develop advanced ultra super critical fossil fuel fired thermal power plants working at temperature of around 973 K and pressure of 300 bar. High temperature creep strength super 304H austenitic steel and Inconel 617 superalloy tubes are indigenously developed for this purpose.

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.

Microstructural stability and long term creep strength of grade 91 steel

<title/>Long term creep strength of ASTM grade 91 steels was investigated in conjunction with microstructural stability during creep exposure. In the short term, no difference in creep rupture strength was observed among four heats of grade 91 steels; however, the large heat to heat variation of creep rupture strength was observed in the long term at 600°C. Good correspondence between long term creep rupture strength and nickel content was observed, and the long term creep strength of grade 91 steel decreased with an increase in nickel content, although the nickel concentration was 0·28 mass-% in maximum. Only a small number of fine MX carbonitride particles with a large number of coarse Z phase were observed on the creep ruptured specimen of high nickel low strength heat. Although the influence of nickel on the precipitation sequence during creep exposure is not yet clearly understood, the nickel content should be reduced in order to suppress a large drop in long term creep strength of grade 91 steel.

Dependence of Precipitation Behavior and Creep Strength on Cr Content in High Cr Ferritic Heat Resistant Steels

It is known that high temperature tensile strength increases with increasing Cr content in Cr containing heat resistant steels. Recently, however, it was found that long-term creep strength decreased with increasing Cr content in the heat resistant steels containing 8.5-12%Cr. In this study, precipitation behavior of M23C6 carbide and the Z phase after creep tests was investigated using two kinds of high Cr ferritic steels (9Cr and 10.5Cr). As a result, 10.5Cr steel exhibited larger average particle size of M23C6 than 9Cr steel irrespective of creep stress levels, but the amount of M23C6 carbide was almost the same in both steels. On the other hand, the amount of the Z phase became large in 10.5Cr steel compared with 9Cr steel. These experimental results indicate that high level of Cr content accelerates precipitation and coalescence rate of both M23C6 carbide and the Z phase, resulting in degradation of long term creep strength in 10.5 Cr steel compared to 9Cr steel.

An assessment of creep strength reduction factors for 316L(N) SS welds

Nitrogen-alloyed type 316L stainless steel is the major structural material for the high temperature structural components of prototype fast breeder reactor. For the welding electrode, carbon in the normal range of 0.045–0.055 wt% and nitrogen in the range of 0.06–0.1 wt% are used to provide weld joints with adequate long term creep strength. Characterization of the creep properties of the base metal, weld metal and weld joint has been carried out at 873 and 923 K at stress levels of 100–325 MPa with rupture lives in the range of 100–33,000 h. Weld strength reduction factors (WSRFs) based on the weld metal, and weld joint have been evaluated, and compared with the codes. WSRFs for the weld joint were higher than the RCC-MR values. Base metal showed the highest rupture life at all the test conditions whereas the weld metal generally showed the lowest rupture life. All the weld joint specimens failed in the weld metal.

Alloy design and creep strength of advanced 9%Cr USC boiler steels containing high concentration of boron

The research and development project of National Institute for Materials Science (NIMS) for advanced ferritic heat resistant steels for 650°C ultra super critical (USC) plants, revealed that the addition of >0·01 mass% boron to a 0·08C–9Cr–3W–3Co–V–Nb–,0·003N steel remarkably improves the long term creep strength. Boron enriched in M23C6 carbides near prior austenite grain boundaries suppresses the coarsening of carbides during creep deformation, leading to excellent microstructural stability and creep strength. If creep strength was further improved by the addition of nitrogen, it was found to enhance precipitation of fine MX. Addition of excess nitrogen to the high boron containing steel was found to reduce creep rupture lives and ductility. This results from a decrease in the amount of effective boron, which is dissolved in M23C6 and suppresses its coarsening, resulting from the formation of coarse BN at normalising temperature. The highest creep strength is obtained with steel of the following composition: 0·08C–9Cr–3W–3Co–0·2V–0·05Nb–0·008N–0·014B (mass%), which has an improved creep strength compared to P92. The 105 h extrapolated creep rupture strength at 650°C is ∼100 MPa. This steel also shows good creep ductility even in the long term. In conclusion, high boron bearing 9Cr–3W–3Co–V–Nb steel combined with the addition of 0·008 mass% nitrogen is a promising candidate for thick section components in the 650°C USC plants as it shows superior creep strength without impaired creep ductility.

Behaviour of Z phase in 9–12%Cr steels

AbstractThe literature on the behaviour of modified Z phase Cr(V,Nb)N in creep resistant martensitic 9–12%Cr steels is briefly reviewed. Ten different 9–12%Cr steels were investigated after prolonged exposure at 600–660°C; the modified Z phase was found in all of them. In steels with high Cr content (11–12%), Z phase precipitates much faster than in 9%Cr steels. Precipitation of Z phase is associated with dissolution of MX carbonitrides, and causes a breakdown in long term creep strength in 9–12%Cr steels. High Cr steels show creep instabilities accompanied with Z phase precipitation, whereas low Cr steels show good long term creep stability. A niobium free CrVN variant of the modified Z phase was observed for the first time during the course of this work. The solution temperature of the Cr(V,Nb)N and CrVN modified Z phases was found to be close to 800°C for 11–12%Cr steels, much lower than the 1200–1250°C solution temperature of the unmodified CrNbN Z phase. Above the solution temperature the modified Z ...

高Crフェライト系耐熱鋼における長時間クリープ強度に注目したCr含有量の最適化

高Crフェライト系耐熱鋼における長時間クリープ強度に注目したCr含有量の最適化

Quantification of precipitates in a 10%Cr steel using TEM and EFTEM

Modern (9–12)%Cr steels designated for power plants with higher steam parameters show a pronounced time dependent change in microstructure during purely thermal or creep stress exposure at temperatures around 600°C that determines their properties in service. In addition to other microstructural parameters, the state of the precipitates plays an important role for microstructural stability which is a prerequisite for long term creep strength. In order to support theoretical studies on precipitation growth and coarsening with more reliable experimental data, in this study a method is introduced for the quantification of the state of precipitates in (9–12)%Cr steels which is based on the application of different TEM methods. Therefore up to about 33 000 h aged specimens of the martensitic cast alloy G-X12CrMoWVNbN-10-1-1 were investigated by means of electron microscopy. The application of energy filtering transmission electron microscopy (EFTEM) allowed a reliable quantitative distinction between M23C6, VN, and Laves phases to establish the size distribution of these precipitates in different specimens conditions.

低合金フェライト鋼の長時間クリープ強度に及ぼす初期組織の影響

低合金フェライト鋼の長時間クリープ強度に及ぼす初期組織の影響

Prediction of atomic configurations in alloys

Improvements in the description of configurational thermodynamics have enabled appropriate evaluations of atomic configurations in the ordered and disordered phases. From the cluster variation method (CVM) calculations, the substitution behaviors of the third element in Ti 3Al and TiAl intermetallic compounds are in good agreement with those evaluated by ALCHEMI and X-ray diffraction. The equilibrium concentrations of solute elements and atomic pairs in the ferrite matrix were estimated by thermodynamic calculations with the sublattice model (Thermo-calc.) and CAM, respectively. A good correlation is observed between the long term creep strength of carbon steels at 773 K and 88 MPa and the concentrations of Mn–C and Mo–C atomic pairs, suggesting that the strength is controlled by these atomic pairs. In the case of Cr–Mo steels, however, the strengthening effect of M–C atomic pair is not observed at 823 K and 88 MPa, and the contribution of solute elements such as Cr, Mn and Mo in the ferrite matrix seems to be dominant.