Creep Exposure Research Articles

Microstructure and properties degradation of P/T 91, 92 steels weldments in creep conditions

The studies were performed on dissimilar ferritic/austenitic weldments between 9Cr tempered martensitic steels of the grades either P/T 91 or 92 and unstabilised AISI316H austenitic steel. The welded joints were fabricated using the fusion welding by tungsten inert gas (TIG) method with Ni-based filler metal. Microstructural analyses were performed using light and electron microscopy. Microstructural gradient in heat-affected zone (HAZ) of 9Cr steels remained preserved during creep exposure. All weldments fractured by the type IV failure within their intercritical HAZ (ICHAZ) regions. The most remarkable microstructural change during creep was the precipitation of intermetallic Laves phase. Experimentally determined phases of the samples after creep exposure are in good agreement with equilibrium thermodynamic calculations.

Creep deformation and rupture behaviour of 9Cr–1W–0.2V–0.06Ta Reduced Activation Ferritic–Martensitic steel

This paper presents the creep deformation and rupture behaviour of indigenously produced 9Cr–1W–0.2V–0.06Ta Reduced Activation Ferritic–Martensitic (RAFM) steel for fusion reactor application. Creep studies were carried out at 773, 823 and 873 K over a stress range of 100–300 MPa. The creep deformation of the steel was found to proceed with relatively shorter primary regime followed by an extended tertiary regime with virtually no secondary regime. The variation of minimum creep rate of the material with applied stress followed a power law relation, έm = Aσn, with stress exponent value ‘n’ decreasing with increase in temperature. The product of minimum creep rate and creep rupture life was found to obey the modified Monkman–Grant relation. The time to onset of tertiary stage of deformation was directly proportional to rupture life. TEM studies revealed relatively large changes in martensitic sub-structure and coarsening of precipitates in the steel on creep exposure as compared to thermal exposure. Microstructural degradation was considered as the prime cause of extended tertiary stage of creep deformation, which was also reflected in the damage tolerance factor λ with a value more than 2.5. In view of the microstructural instability of the material on creep exposure, the variation of minimum creep rate with stress and temperature did not obey Dorn's equation modified by invoking Lagneborg and Bergman's concepts of back stress.

The Microstructure Changes in IN713LC during the Creep Exposure

Nickel-based creep resisting alloys (strengthened by γ´) are the basic materials for high-temperature constructional parts in aircraft engines and energy units. These parts are exposed to combined effects of mechanical stresses, high temperature and dioxide-corrosion conditions. The microstructure changes of cast polycrystalline Ni-based superalloy IN713LC after creep exposure were studied. Three specimens with three different diameters were used for creep tests. The degradation stage (damage parameter π) was determined for all parts of specimens. Individual parts of specimens were metallographic observed and analyzed by image analysis after rupture. The results were compared with model of stress distribution in the specimen with potential damage in the centre of the specimen.

The influence of boron on the microstructure of a 9wt% Cr ferritic steel

A comprehensive investigation of the microstructural evolution during heat treatment and creep exposure of a boron-alloyed 9wt% Cr steel, with special emphasis on the role of boron was performed. Samples from different stages of the normalizing and tempering heat treatment cycle, as well as creep tested material were investigated by energy filtered and high resolution transmission electron microscopy combined with electron energy loss- and X-ray-spectroscopy. The substitution of carbon by boron in chromium carbides was found only after 50 hours long-term tempering at 780°C and creep exposure at 650°C for 8,971 hours. No enrichment in boron on the particle matrix interface was detected. Nevertheless, formation of boron-rich carbides B0.38C0.62 and BC0.38Si0.04 was already observed after 4 hours of tempering.

Microstructural changes in creep exposed P91 steel weld joint

Cast plates made of 9Cr – 1MoV creep resistant steel were joined together using the GTAW and SMAW methods. Creep testing was carried out at a temperature range from 525°C to 625°C and a stress range from 50 to 240MPa. The microstructure of crept specimens was compared with weld joint at conditions after PWHT and the base material before welding. Substructures were investigated in the base material, the heat affected zones and the weld metal. Different kinetics of precipitation reactions in individual zones caused changes in creep strength and unexpected transition from Type IV cracking to Type I cracking at long creep exposure at 625°C. After creep tests at temperatures above 550°C for about 30,000 hours, coarse particles of Laves phase were observed. In addition, small plates of Z-phase were detected after creep tests at 625°C. Precipitation of Laves phase occurred in the weld metal predominately and caused depletion of solid solution about molybdenum. If the lowest stress were applied at 625°C, the growth of the Laves phase was the dominant process, which caused a significant softening and rupture in the weld metal. This effect prevailed over accumulation of cavitation failure in the heat affected zones that caused fracture of the other specimens.

Comparison of creep rupture behaviour of type 316L(N) austenitic stainless steel joints welded by TIG and activated TIG welding processes

► In the present study, Creep rupture behaviour of 316L(N) base metal, and its weld joints were investigated at 923 K. ► Both the weld joints possessed lower creep rupture strength than the base metal. ► A-TIG joint had higher creep rupture strength than the MP-TIG joint. ► Minimum steady state creep rates of the base metal, A-TIG and MP-TIG weld joints were comparable. ► Reduced cavitation in A-TIG joint has been attributed to lower δ-ferrite content and lesser extent of strength heterogeneity. ► The columnar grains along the applied stress direction in weld metal leading to improvement in creep strength of A-TIG joint. Creep rupture behaviour of type 316L(N) austenitic stainless steel base metal and its weld joints fabricated both by single-pass activated TIG (A-TIG) and multi-pass conventional TIG (MP-TIG) welding processes were studied at 923 K over a stress range of 160–280 MPa. Both the weld joints possessed lower creep rupture lives than the base metal. The A-TIG weld joint displayed higher rupture lives than the MP-TIG weld joint. Failure in the weld joints occurred in the weld metal. Progressive localization of creep deformation in the weld metal of both the joints led to the premature failure. Accumulation of creep deformation at higher rate was observed in the weld metal of the MP-TIG joint than in the A-TIG joint. Finer microstructural features and higher amount of δ-ferrite was observed in the weld metal of MP-TIG joint than in the A-TIG weld joint. Orientation of the columnar grains and δ-ferrite was nearly transverse to the welding direction in the A-TIG joint, whereas it was towards short transverse to the welding direction in the MP-TIG weld joint. TEM investigation of creep exposed weld metal showed the extensive formation of M 23 C 6 carbides, σ-phase and Laves phase along the boundaries in MP-TIG joint, which were less prevalent in the A-TIG joint. With creep exposure, the δ-ferrite transformed to σ and Laves phases, and creep cavitation was found to be associated with the intermetallic phases. Creep cavitation was more pronounced in the MP-TIG weld joint than in the A-TIG weld joint.

Tensile Creep Damage and Rupture of Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub>(sf)/AZ91 Composite

Constant stress tensile creep tests were conducted on an AZ 91–25 vol.% Al2O3-SiO2 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 SEM revealed good fiber–matrix interface bonding during creep exposure. Short fibers have a great function in load bearing and load transfer, and greatly hinder the dislocation movement, thus enhancing the creep resistance of the composite. Damage and multiple rupture of aluminum silicate short fiber, quality of the interface combination between aluminum silicate short fiber reinforcement and the matrix, are two important factors of the creep deformation microstructure process control of Al2O3-SiO2(sf)/AZ91 composite. The creep mechanism of the composite is dislocation and grain boundary sliding control.

Effect of nickel and cobalt addition on the precipitation-strength of 15Cr ferritic steels

The effect of nickel and cobalt addition on the microstructure of 15 mass% chromium (15Cr) steels and their mechanical properties, such as creep strength and impact toughness, was investigated. The microstructure of 15Cr ferritic steel changes from a ferrite single phase to dual phases of ferrite and martensite by the addition of nickel, which is an austenite-stabilizing element. The creep strength and precipitation behavior of intermetallic compounds (Laves phase, χ-phase and μ-phase) during creep exposure at elevated temperature were strongly influenced by the addition of nickel and cobalt. An increase in cobalt content led to an increase in creep strength by the formation of fine particles and a homogeneous distribution of intermetallic compounds precipitated in the ferritic matrix. The creep rupture life of 15Cr steel at 923 K and 140 MPa was approximately 100 times longer than that of conventional 9Cr ferritic steels with a tempered martensitic microstructure strengthened by carbides and carbonitrides.

Microstructural changes in creep exposed P91 steel weld joint

Cast plates made of 9Cr – 1MoV creep resistant steel were joined together using the GTAW and SMAW methods. Creep testing was carried out at a temperature range from 525°C to 625°C and a stress range from 50 to 240MPa. The microstructure of crept specimens was compared with weld joint at conditions after PWHT and the base material before welding. Substructures were investigated in the base material, the heat affected zones and the weld metal. Different kinetics of precipitation reactions in individual zones caused changes in creep strength and unexpected transition from Type IV cracking to Type I cracking at long creep exposure at 625°C. After creep tests at temperatures above 550°C for about 30,000 hours, coarse particles of Laves phase were observed. In addition, small plates of Z-phase were detected after creep tests at 625°C. Precipitation of Laves phase occurred in the weld metal predominately and caused depletion of solid solution about molybdenum. If the lowest stress were applied at 625°C, the growth of the Laves phase was the dominant process, which caused a significant softening and rupture in the weld metal. This effect prevailed over accumulation of cavitation failure in the heat affected zones that caused fracture of the other specimens.

Development and microstructure of advanced creep resistant ferritic steels

During the past two decades, a new generation of creep resistant ferritic steels has been developed for boiler and turbine component applications in advanced power generation plants operating with inlet steam temperature in the range 600 to 650°C. It is generally accepted that development of new steels with higher creep resistance must be based on detail knowledge of strengthening and degradation processes and structural stability. The aim of the present paper is to point out the most significant strengthening and degradation processes during creep exposure of ferritic steels with respect to chemical composition and structural changes occurring during creep at elevated temperatures. The attention will be devoted to low alloy steels with Cr content up to three mass % and modified chromium steels containing nine to 12 mass % Cr.

Microtomography and Creep Modeling of a Short Fiber Reinforced Aluminum Piston Alloy

AbstractInterconnectivity between eutectic silicon and short fibers in an AlSi12CuNiMg/Al2O3/15s composite increases with long‐term creep exposure time due to diffusion. It is also observed that the stationary creep rate decreases significantly compared to the initial stationary creep rate. Interconnectivity between rigid phases is analyzed and three‐dimensional statistical functions are applied to deduce the representative volume elements of the composite in different conditions. Unit cell geometrical models are generated to simulate the stationary creep behavior using the finite element method.

Quantitative Characterization of Microstructure in Copper Processed by Equal-Channel Angular Pressing

Experiments were conducted on extremely coarse-grained pure copper to evaluate the effect of equal-channel angular pressing (ECAP) on microstructure evolution in the as-pressed state and after creep exposure using various stereological methods. The microstructure formed by severe plastic deformation is an unusual structure which can be hardly characterized only by the mean grain size especially after low number of ECAP passes. The purpose of this paper is a detailed examination of (sub)boundaries and grain boundaries in the microstructures of the pressed material. The inhomogeneity of deformed microstructures is also evaluated. The detailed description of ECAP microstructures should contribute to the better understanding of mechanical properties of the pressed materials.

Microstructure of creep-exposed single crystal nickel base superalloy CSMX4

Main changes which appear in microstructure of Ni base superalloys during creep exposure is the directional coarsening (rafting) of originally cubic γ' precipitates embedded in γ matrix. The progress in γ' degradation can be assessed with help of the analysis of geometrical parameters of the γ' microstructure. Determination of the morphological changes of γ' in creep-exposed single-crystal Ni-base superalloy CMSX4 using SANS was carried out. A new sample design (conic specimen) was tested in order to get continually changing stress levels in one sample and corresponding variety of micro structural change. Evolution of rafts with stress magnitude is clearly observable from the parameters obtained using SANS (the mean thickness of the γ channel in different crystallographic directions of the single-crystal, the evolution of the γ' precipitate size and distance, the evolution of the specific interface). The use of conic specimen and spatial scan facilitate determination of microstructure evolution in dependence on the exposure parameters.

Precipitation behavior of Laves phase in 10%Cr steel X12CrMoWVNbN10-1-1 during short-term creep exposure

Precipitation behavior of Laves phase in steel X12CrMoWVNbN10-1-1 during short-term (≤400 h) creep exposure was investigated using a transmission electron microscope (TEM) and an ultra high resolution thermal field emission scanning electron microscope (UHR-FESEM). The results demonstrate that Laves phase precipitates are detected in the creep specimens exposed for such a short-term of 400 h at 600 °C and 204 h at 650 °C. Stacking faults are observed in them and the selected area electron diffraction patterns of the phase are characteristic of streaked. There are two mechanisms of the nucleation and growth for Laves phase, one of which is that Laves phases occur alone on martensite lath boundaries and they are coherent with a grain but grow into the adjacent grain which has no rational orientation relationship with them. The other one is that Laves phases are formed in the regions adjacent to M 23C 6 particles, and then they grow at the expense of the Cr-rich M 23C 6 carbides in close vicinity. It is established that the former one is the dominant formation mechanism for Laves phase in steel X12CrMoWVNbN10-1-1. Laves phases prefer to locate on the prior austenite grain boundaries and the martensite lath boundaries. The mean diameter and the number density of the phase show significant change with temperature, but slightly change with stress. Moreover, it is proved that the formation of Laves phase in this steel is a thermodynamically possible process. The nucleation process has not finished after heat-treatment and the number of nuclei for Laves phase formed in the steel decreases with raising the temperature.

Creep behavior of ageing hardened Mg-10Gd-3Y alloy

Tensile creep behaviors of the ageing hardened Mg-10Gd-3Y alloy (referred to GW103) were investigated at temperatures up to 300 °C. The extruded-T5 specimen exhibited high creep resistance, i.e. the low steady-state creep rate and long creep rupture time, while the better creep properties were observed in the cast-T6 one. The low steady-state creep rate of 1.71×10 −9 s −1 is obtained at 200 °C and 80 MPa for the extruded-T5 GW103 alloy. In addition, the microstructure development of GW103-T5 alloy was also examined after creep exposure at different temperatures. On the other hand, the stress exponent and activation energy were studied in the temperature range of 200–300 °C for the extruded-T5 specimens, and the creep mechanism was also discussed.

Tensile properties and high temperature creep behavior of microalloyed Ti–Ti 3Al–Nb alloys by directional solidification

The microstructures, tensile properties and creep behavior of the Cr- and Mo-microalloyed Ti–Ti 3Al–Nb alloys processed by directional solidification were investigated. The experimental results indicated that the strength, ductility and creep resistance of ternary Ti–Al–Nb alloy were remarkably enhanced by additions of small amounts of Cr and Mo. The stress exponent for creep showed a transition and varied from a lower value of 3.5–5.3 (LSR-region I) to a higher value of 8.3–12.6 (HSR-region II) with increasing stress at 600 and 650 °C whereas a single stress exponent value of 5.3–6.0 was obtained at 700 °C. Conversely, the true activation energy for creep was calculated to be 303–324 kJ/mol and fell into or was quite close to that for the self-diffusion of Ti in α 2-Ti 3Al (288–312 kJ/mol). TEM examinations revealed that ordinary dislocations in the lath dominated the deformed microstructures. However, the initial microstructure of the alloys was unstable during long-term creep exposure and dynamic recrystallization occurred at high stress and moderately high temperature. The creep deformation in region I was dislocation-controlled whereas the abnormally high stress exponent in region II was associated with the effects of recrystallization resulting in a reduction in the overall grain size of the initial structure. The fracture modes and total strain were dependent on the composition, test temperature, and stress level. The accelerating strain rate in the extended tertiary stage was attributed to the microstructural instabilities or the nucleation, coalescence and merge of voids or microcracks.

Prediction of type IV cracking behaviour of 2.25Cr-1Mo steel weld joint based on finite element analysis

Creep tests were carried out on 2.25Cr-1Mo ferritic steel base metal and its fusion welded joint at 823 K over a stress range of 100–240 MPa. The weld joint possessed lower creep rupture strength than the base metal and the reduction was more at lower applied stresses. The failure occurred in the intercritical region of heat-affected zone (HAZ) of the joint, commonly known as Type IV cracking. Type IV cracking in the joint was manifested as pronounced localization of creep deformation in the soft intercritical region of HAZ coupled with preferential creep cavitation. The creep cavitation in intercritical HAZ was found to initiate at the central region of the creep specimen and propagate outwards to the surface. To explain the above observations, the stress and strain distributions across the weld joint during creep exposure were estimated by using finite element analysis. For this purpose creep tests were also carried out on the deposited weld metal and simulated HAZ structures (viz. coarse-grain structure, fine-grain structure, and intercritically annealed structure) of the joint. Creep rupture strength of different constituents of joint were in the increasing order of intercritical HAZ, fine-grain HAZ, base metal, weld metal and coarse-grain HAZ. Localized preferential creep straining in the intercritical HAZ of weld joint as observed experimentally was supported by the finite element analysis. Estimated higher principal stress at the interior regions of intercritical HAZ explained the pronounced creep cavitation at these regions leading to Type IV failure of the joint.

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.

Study of the microstructure of the Grade 91 steel after more than 100,000 h of creep exposure at 600 °C

This paper presents results on the evolution of microstructure (both matrix and precipitates) of an ASME Grade 91 steel that has been creep tested for 113,431 h at 600 °C under a load of 80 MPa. The microstructure was investigated using transmission electron microscopy (TEM) and revealed chromium rich M 23C 6 carbides, MX-type precipitates, Laves phases and modified Z-phases. Only a small amount of modified Z-phase was found. In order to quantify coarsening of precipitates and growth of new phases during creep, the size distributions of the identified precipitates were determined by analysis of TEM images. In addition to this, the size distribution of Laves phases was determined by image analysis of scanning electron micrographs. Substructure modifications and creep damage were investigated on cross sections of the creep specimen using Electron Backscatter Diffraction and Scanning Electron Microscopy.

Evolution of dislocation density, size of subgrains and MX-type precipitates in a P91 steel during creep and during thermal ageing at 600 [formula omitted]C for more than 100,000 h

There are rather few quantitative data on the microstructure of the 9–12%Cr heat resistant steels after long-term creep. This paper presents results of the quantitative measurement of the size of MX precipitates, subgrain size and dislocation density in a P91 steel that had been creep tested for 113,431 h at 600 ° C. The same measurements were conducted in the same P91 steel in the as received conditions. Transmission electron microscopy investigations were conducted using thin foils and revealed a decrease in dislocation density and an increase in subgrain size after creep exposure. MX carbonitrides are very stable during thermal and creep exposure of P91 steel at 600 ° C up to 113,431 h. Electron backscatter diffraction (EBSD) investigations also revealed a significant change in the substructure of the steel after creep exposure.