Creep Exposure Research Articles

Effect of Long-Term Aging and Creep Exposure on the Microstructure of TiAl-Based Alloy for Industrial Applications

ABSTRACTThe effect of long-term aging and creep exposure on the microstructure of a cast TiAl-based alloy with nominal chemical composition Ti-46Al-2W-0.5Si (at.%) was studied. The aging experiments were performed at temperatures between 973 and 1073 K for various times ranging from 10 to 14000 h in air. Constant load tensile creep tests were performed at applied stresses ranging from 150 to 400 MPa and at temperatures between 973 and 1123 K up to 25677 h. During aging and creep testing the α2(Ti3Al)-phase in the lamellar and feathery regions transforms to the γ(TiAl)-phase and fine needle-like B2 precipitates. Microstructural instabilities lead to a softening of the alloy. The effect of this softening on long-term creep resistance is negligible at temperatures of 973 and 1023 K.

Effect of Nitrogen Content on Microstructural Aspects and Creep Behavior in Extremely Low Carbon 9Cr Heat-resistant Steel

Effects of nitrogen content on microstructure and creep strength in extremely low carbon 9Cr ferritic steel were investigated, focusing on the dislocation structure and the distribution of precipitates. The creep strengthof the high nitrogen steels was lower than that of the base steel, even though the initial lath width and prior austenite grain size in the high nitrogen steels are finer and larger, respectively. MX nitrides as well as large Cr 2 N particles were distributed along lath, block, packet and prior austenite grain boundaries in the high nitrogen steels after tempering. The large particles occupy the MX nitride precipitation sites. The coarsening of the MX nitride during creep exposure in the high nitrogen steels was faster than that of the base steel. The presence of large Cr 2 N particles after tempering and the higher coarsening rate of the MX nitride in the high nitrogen steels cause an increase in mean inter-particle spacing on boundaries, leading to a decrease in creep strength. The Z phase formation was observed after creep exposure in the gauge portion of all the steels. The Z phase formation in the high nitrogen steels occurs at relatively short term in contrast to the base steel. The short term precipitation of the Z phase in high nitrogen steels can contribute to a decrease in creep strength since the Z phase forms at the expense of the MX nitride which is a main strengthening obstacle.

Kinetics of transformation of delta ferrite during creep in a type 316(N) stainless steel weld metal

A quantitative study on the transformation behaviour of δ ferrite in a type 316(N) stainless steel weld metal during creep in the temperature range 823–923 K was carried out. The fraction of δ ferrite transformed was determined from magne-gage measurements on the samples after creep fracture at various stress levels, i.e. after different durations of creep exposure. The transformation was found to follow the Avrami kinetics, X=1−exp(− bt n ) where X is the fraction of δ ferrite transformed, t is the time to rupture and b and n are constants. n was found to be independent of temperature. An apparent activation energy for transformation, Q, was determined from the temperature dependence of the constant b expressed as b 0 exp(− Q/ RT) as about 152 kJ mol −1 which is similar to that for Mo diffusion and for boundary diffusion in 316 SS. X-ray diffraction analysis of the precipitates extracted from the creep tested samples indicated the presence of Mo-rich phases η and χ. It is proposed that the transformation is controlled initially by diffusion of Cr through δ/γ boundaries to form carbides, followed by matrix diffusion of Mo to form the intermetallic phases.

Application of Stress Relaxation Test Methodology for Predicting Creep Life of a Large Steam Turbine Rotor Steel (1CrMoV)

AbstractThe technique proposed here adopts short-term stress relaxation testing (SRT) to determine the metallurgical life of 1CrMoV large rotor steel. SRT has been performed at two temperature levels of 550°C and 600°C, on samples taken from the core region of a large rotor. At 550°C, five initial prestrains ranging from 0.2–1.0% strain were applied, while at 600°C, the specimens were relaxed from 0.6% and 0.8% prestrains. All test durations were kept between 4 to 5 h. To validate the results, a few conventional creep tests have been conducted at same temperature levels and at different stresses. The initial data, stress versus time, were converted into stress versus inelastic strain rate and the creep life was predicted with the help of the Monkman-Grant relationship (ˊεs × tr) = C), where the value of the constant C is taken from experimental creep data. The comparison of creep life, predicted through SRT, with the actual creep data revealed that they were agreeing extremely well at higher test temperature, i.e. 600°C. The computed SRT data have delivered higher activation energy (600 kJ mol−1) than conventional creep testing (344 kJ mol−1). Detailed SEM and TEM studies revealed that degradation and/or development of microstructures had taken place and was commensurable to long-term creep exposure. Therefore, the methodology has the capacity to assess the inherent creep strength of any material.

Ｗ含有高Ｃｒフェライト系耐熱鋼の６５０℃における粒界クリープ強化

Creep strengthening mechanism of tungsten containing 9 to 12% chromium steels for high temperature application was experimentally extracted and discussed from the view point of grain boundary shielding by Fe2W type Laves phase and M23C6 type carbide during the creep exposure at 650°C. Increase of tungsten content from 1.8mass% for ASME Gr. 92 to 2.5mass% for model alloys resulted in the abrupt creep rupture strength deterioration for about 1000 hours creep time. Increase of chromium from 9mass% to 11mass% or more also affected the creep rupture curve. Excess tungsten and chromium almost precipitated grain boundary, and a grain boundary shielding ratio by the precipitates explained the creep rupture behavior, bending of the curves, in tungsten or chromium increased model alloys according to the analyze by thin foil observation through the Transmission Electron Microscope (TEM) with Energy Dispersive X-ray Spectroscopy. Grain boundary shielding possibly delayed the recovery of dislocation substructure by rigid grain structure. Grain boundary shieldng ratio f0/f expressed the creep curves of model alloys based on the dislocation density degradation at 650°C. Species of precipitates did not seem to affect the dynamic recrystalization of a grain boundary.

Effect of Carbide Morphology on the Susceptibility to Type IV Cracking of a 1.25Cr-0.5Mo Steel

Type IV cracking is considered to be the likely failure mode of ferritic steel welds when operated for long duration. The carbide morphology of the service-exposed 1.25Cr-0.5Mo steel weldment, which is composed of a forged flange and pipe fabricated from plates, has been examined before and after a creep test. Higher susceptibility to Type IV cracking was observed at the Intercritical HAZ (ICZ) on the flange side despite higher creep resistance of the parent material compared with a pipe. The change in carbide morphology during the creep exposure was the most pronounced at the flange ICZ. The coarse bainitic carbide originally existing depleted the intragranular carbides and significant variation in carbide density inside the ICZ was generated. In contrast, carbides at the pipe ICZ were more uniformly distributed. It was interpreted that higher susceptibility of the flange ICZ was accelerated by the heterogeneous distribution of carbide density and the resultant variation of creep strength and would enhance grain boundary sliding associated with creep strain accumulation. It was proved that the susceptibility to Type IV cracking was highly dependent upon the characteristics of a parent material by experiments using simulated ICZ specimens. Significant difference in the effect of heat treatment to simulate the microstructure at the ICZ upon the creep strength was observed between a flange and pipe. The simulated ICZ specimen generated by a pipe parent showed no apparent change due to the heat treatment compared with a parent material. On the contrary, it reduced the time to rupture and changed the fracture mode from transgranular to intergranular for a flange material. The feature of grain boundary cracking was similar to that of actual weldment which took place preferentially at the inclined grain boundaries to the tensile direction, that was to be considered the evidence of grain boundary sliding.

低炭素・中窒素型１６Ｃｒ‐８Ｎｉ‐２Ｍｏ溶接金属の長時間クリープ破断材の析出挙動

In the past the microstructural observation was mostly applied to understand the materials behavior qualitatively in the R & D of new materials and life prediction for the fast breeder reactor components. However, the correlation between the changes in properties and microstructures must be clarified quantitatively to ensure the structural integrity. Particularly we are interested in the method to correlate the long-term properties with microstructural changes at high temperatures. The current research is to quantify the changes in microstructure of the weld metal for the welded structure of the reactor vessel.In this research we have conducted creep testing of a 16Cr-8Ni-2Mo weld metals at 823, 873 and 923K up to 76, 320h. Based on the change in area fraction of the precipitates, the evaluation of creep-rupture properties with time and temperature was attempted. Correlation between ferrite content and the area fraction of the precipitates was not recognized. It was possible to correlate linearly the amount of the precipitates to log (creep exposure time) with the Larson-Miller parameter.

Effects of Full Annealing Heat Treatment on Long-term Creep Strength of 2.25Cr-1Mo Steel Welded Joint

Long-term creep strength of Cr-Mo steel with fully annealed ferrite and pearlite microstructure is higher than those of the steels with martensite, tempered martensite and bainite microstructures. In this study, influence of post weld heat treatment at the temperature of austenite single phase region(full annealing PWHT) on long-term creep strength at 550°C has been investigated on 2.25Cr-1Mo steel, in comparison with that of normal post weld heat treatment at the temperature below A C1 (standard PWHT). Creep rupture strength of the welded joint subjected to standard PWHT is lower than those of base metal and weld metal in the short-term. However, such differences in creep rupture strength disappear after long-term creep exposure for about 50,000h at 550°C. On the other hand, creep strength of the welded joint subjected to full annealing PWHT is obviously higher than those of the welded joint subjected to standard PWHT, base metal and weld metal in the long-term condition. It may be also expected to reduce a probability of mechanical damage such as Type IV cracking by the full annealing PWHT, since inhomogeneous microstructures in weld metal and heat affected zone have been essentially extinguished. Full annealing at the temperature of austenite single phase region has been proposed as a new post weld heat treatment condition available to obtain high creep strength and to improve a reliability of welded joint for low alloy Cr-Mo steels.

623 炭化物形態変化法による CrMoV タービンロータ鋼のクリープ損傷評価

CrMoV turbine rotor steel is strengthened by dispersed fine carbides, however carbides precipitated and coarsened at grain boundary during creep exposure. The carbide structure also changes from Fe_3C to M_<23>C_6 type carbide as coarsening. The carbide structure was electrochemically isolated by solving the matrix and analyzed with X-ray diffraction analysis. The X-ray peak intensity ratio of M_<23>C_6 to Fe_3C is tentatively defined as J-parameter. In order to evaluate the efficacy of J-parameter for creep damage evaluation, creep interruption test was conducted. The J-parameter increased with increasing creep and also correlated well to mean interparticle spacing. The change in J-parameter was affected by the chemical composition of the steel.

The second phases in Ti40 burn resistant alloy after high temperature exposure for a long time

The Ti40 (Ti–25V–15Cr–0.2Si) alloy is a burn resistant β titanium alloy. Second phase precipitation after high temperature exposure for a long time has been studied. The second phases, i.e. Ti 5Si 3 and α, precipitate from the β matrix after the Ti40 alloy has been exposed at high temperature for a long time. The Ti 5Si 3 phase distributes discontinuously along the grain boundary if the exposure temperature is below 540°C. Exposed at 700°C for 100 h, the coarse Ti 5Si 3 phase rapidly grows, its tensile properties obviously reducing after thermal exposure when conventionally forged. Coarse Ti 5Si 3 and α phases form after exposure at 540°C for 100 h, which leads to serious decrease of tensile properties after thermal exposure for isothermally forged alloys. Ti 5Si 3 precipitates distribute discontinuously along grain boundary in the conventionally forged alloys after creep exposure. There are also many coarse rod-like α phases in isothermally forged alloys after creep exposure.

Microstructural processes in creep of an AZ 91 magnesium-based composite and its matrix alloy

Constant stress tensile creep tests were conducted on an AZ 91–20 vol.% Al 2O 3 short fiber composite and on an unreinforced AZ 91 matrix alloy. The creep resistance of the reinforced material is shown to be considerably improved compared with the matrix alloy. The creep strengthening arises mainly from the effective load transfer between plastic flow in the matrix and the fibers. Microstructural investigations by TEM revealed good fiber–matrix interface bonding during creep exposure. The microstructures of the AZ 91 alloy and its composite were similar with regard to two types of β-phase precipitates; the enhanced precipitation of the Mg 17 (Al, Zn) 12 phase on the fibers is promoted by heterogeneous nucleation due to the Al enrichment of the matrix near to the alumina fibers.

Comparison of creep behaviour of 2.25Cr–1Mo/9Cr–1Mo dissimilar weld joint with its base and weld metals

Evaluation of the creep behaviour of 2.25Cr–1Mo and 9Cr–1Mo ferritic steel base metals, 9Cr–1Mo steel weld metal, and 2.25Cr–1Mo/9Cr–1Mo ferritic–ferritic dissimilar weld joints has been carried out at 823 K in the stress range 100–260 MPa. The weld joint was fabricated by shielded metal arc welding using basic coated 9Cr–1Mo electrodes. Investigations of the microstructure and hardness variations across the joint in the as welded, post-weld heat treated (973 K/1 h), and creep tested conditions were performed. The heat affected zone (HAZ) in both the steels consisted of a coarse prior austenitic grain region, a fine prior austenitic grain region, and an intercritical structure. In the post-weld heat treated condition, a white etched soft decarburised zone in 2.25Cr–1Mo steel base metal and a black etched hard carburised zone in 9Cr–1Mo steel weld metal around the weld fusion line developed. Hardness troughs also developed in the intercritical HAZ regions of both the steels. The width of the carburised and decarburised zones and hardness differences of these zones were found to increase with creep exposure. The 9Cr–1Mo steel weld metal showed higher creep strength compared to both the base metals. The 9Cr–1Mo steel base metal exhibited better creep resistance than the 2.25Cr–1Mo steel base metal at lower applied stresses. The dissimilar joint revealed lower creep rupture strength than both the base metals and weld metal. The creep strain was found to concentrate in the decarburised zone of 2.25Cr–1Mo steel and in the intercritical HAZ regions of both the steels. Creep failure in the stress range examined occurred in the intercritical HAZ of 2.25Cr–1Mo steel even though this region showed higher hardness than the decarburised zone. Extensive creep cavitation and cracks were observed in the decarburised zone.

Precipitation and deformation behaviour of cast alloy 718 during creep and thermal exposure

The precipitation, deformation, and fracture behaviour of cast alloy 718 during creep rupture tests was investigated, in comparison with thermal exposure tests. Inhomogeneous deformation bands appeared during monotonic or cyclic deformation of alloy 718. The bands were identified as mechanical twins, which are known to be responsible for crystallographic failure during creep rupture at and below 649°C. However, crystallographic failure was observed at temperatures up to 760°C in the present study. No crystallographic failure was observed at and above 816°C. Precipitation of δ phase was observed on deformation bands following creep rupture tests at and above 704°C. The difference in failure mode below 760°C and above 816°C is assumed to be caused by the precipitation of δ phase on the bands. A few discrete δ particles on the bands during 704°C creep rupture tests were not sufficient to prevent decohesion along the twin/matrix interface, and therefore crystallographic failure still occurred. In contrast with little or no precipitation below 704°C, needlelike or platelike δ phase precipitated at and above 816°C. It is postulated that the precipitation of δ phase restricted successive deformation. Since δ phase precipitates on {111} planes where major deformation occurs, this phase usually grows according to the following orientation relationship: (010)δ∥(111)γ, [100]δ∥[11¯0]γ. Restriction of deformation by the precipitation of δ phase caused the change in failure behaviour at and above 816°C.

Development of Advanced Chromium Steels with Respect to Microstructure and Structural Stability

The really promising way how to improve the creep resistance of chromium steels was found in the early 80 th and was based on increasing nitrogen content in the steel up to 0.07 %. Vanadium nitride precipitating especially on dislocations inside subgrains during creep exposure plays the decisive role in the excellent creep strength of modified 9%Cr-1%Mo steel of the P91 type. The beneficial effect of tungsten on solid solution strengthening as well as on the creep resistance was appreciated especially in Japan and steel P 92 has been developed. Nevertheless, high creep resistance of P 92 steel is not only the result of increased tungsten content but boron addition to steel P 92 has also the significant effect. It must be realized that the increasing of Mo and/or W content in the steel should be limited with respect to Laves phase precipitation (about 1%Mo or 2%W) with high coarsening rate and therefore limited effect on the precipitation strengthening. The precipitation strengthening of these steels is ruled by M 23 C 6 and especially by vanadium nitride. Further increasing of CRS is possible by raising nitrogen content.

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.

Microstructural evolution of DS CM186LC during creep and thermal exposure

Thermal exposure tests were carried out at the same temperature and duration as those of creep tests. Growth of γ′ either in the matrix or in the interdendritic regions was observed during high temperature thermal exposure. The growth of γ′ caused an isolated γ island in the γ/γ′ eutectics within which fine HfC particles precipitated. Some directional coarsening of γ′ was found in the specimens exposed at 1050°C. MC carbide reaction with the y matrix was observed in crept and thermally exposed specimens. Needle-like precipitate, a kind of TCP, was found in the specimen crept at 1050°C for 400 hours.

Interpretation of creep behaviour of a 9Cr–Mo–Nb–V–N (T91) steel using threshold stress concept

The creep behaviour and the microstructural evolution of a 9Cr–Mo–Nb–V (T91) steel were extensively evaluated by means of short term constant load creep tests and TEM analysis. Statistical analysis of the microstructural data revealed that the precipitated phases M23 C6 (where M is a metal, mainly Cr or Fe) and MX (where M is Nb or V, and X is C and/or N) were subject to coarsening during creep exposure. The coarsening law and its dependence on applied stress were identified, and the model was used to predict the magnitude of the Orowan stress at the time corresponding to the minimum creep rate. The minimum creep rate dependence on applied stress at 873 K was described by incorporating the threshold stress concept in a power law with stress exponent n = 5. In the resulting phenomenological model, the strengthening effect of the dispersed phases was thus expressed by a threshold stress proportional to the Orowan stress.

The Influence of Microstructural Aspects on the Service Behaviour of Advanced Power Plant Steels.

There is a world-wide substantial demand to increase the application temperature and design stresses of advanced creep resistant ferritic-martensitic 9-12 % Cr steels to increase the efficiency of thermal power plants. They show, compared to austenite grades, favourable physical properties-like good thermal conductivity and low coefficient of thermal expansion coupled with better service behaviour. In the frame work of the European research action COST 501 intensive investigations of these types of steels were performed. These alloys show during service conditions pronounced microstructural changes in the aged as well as in the stressed conditions. Therefore an extrapolation of short term creep tests to long times-i.e. 100 kh-using conventional methods is not successful because of changes in the acting creep mechanism governed by microstructural development. Therefore an intensive investigation and modelling of the microstructure of W- and Mo-alloyed advanced 9-12 % Cr steels, mainly G-X12 CrMoVWNbN 10-1-1, was applied to understand the mechanisms. Hardness tests, optical microscopy, SEM, TEM methods and thermodynamic equilibrium phase diagram calculations were performed. Very promising is the use of the EFTEM (Energy filtering TEM) method to quantify the precipitation sequence of the different types of precipitate: M23C6, MX, Laves Phase and Z-Phase as a function of time, temperature and straining. A new approach (Graz Model) was proposed for the improvement of the creep resistant alloys in terms of the α/γ-transformation and the Curie temperature respectively, which influence the microstructural stability. Both were found as important influencing factors. Experimentally it was found, that the alloys showing the highest creep resistance i.e. the Japanese grade NF616/P92 and the alloy B2, one of the COST 501 activity, show the highest degree of microstructural stability after creep exposure. Further investigations and modelling is necessary to develop an optimised alloy composition of advanced 9-12% Cr steels.

Evolution of microstructure in a modified 9Cr–1Mo steel during short term creep

An investigation of the effect of creep exposure on the microstructure of a 9Cr–1Mo alloy for steam tubing was performed. The samples were machined from a tube, austenised at 1323 K for 15 min and air cooled to room temperature, followed by tempering at 1023 K for 1 h. Creep tests were performed at 848, 873, 898 and 923 K for different loading conditions. The conventional power law was used to describe the minimum creep rate dependence on applied stress; the stress exponent was found to increase when temperature decreased. Transmission electron microscopy (TEM) of the crept samples showed that during creep both subgrain and particle size increased; the statistical analysis of the dimensions of the precipitates revealed a bimodal distribution of particles that coarsen during creep exposure at testing temperatures. A linear dependence of subgrain size on the inverse of the modulus compensated stress was used to describe the softening of the dislocation substructure. A similar relationship was found to be also valid for particle carbides.

Creep and fracture behaviour of English Electric Mark III transition welds: Part (b) Ex-service weldments

The microstructure and high temperature behaviour of an ex-service weldment have been assessed. Exposure at elevated temperature resulted in some carbide development in the niobium stabilised buttered layer. However, significant decarburisation in the low alloy steel was not detected. Cross-weld uniaxial creep testing invariably led to low ductility failures in the heat affected zone on the 2.25 Cr1Mo steel side of the weld. These failures were associated with the nucleation, growth and link-up of cavities on prior austenite grain boundaries. Assessment of tests interrupted at selected life fractions indicated that the development of cavitation followed a sensible trend with creep exposure. The cavitation behaviour of the transition weld was in general agreement with data obtained from creep tests undertaken on material heat treated to simulate heat affected zone structures. The level of creep damage in these welds can be assessed by evaluation of local strain accumulation or through monitoring the presence of cavities.