Creep Exposure Research Articles

Effect of post weld heat treatments on fracture frontier and type IV cracking nature of the crept P91 welded sample

The research work explores the microstructure evolution and creeps rupture behavior of the shielded metal arc welded joint of reduced activation ferritic/martensitic P91 steel plate in the as-welded and different states of post weld heat treatment (PWHT). Two types of heat treatments were employed including (i) post weld heat treatment (PWHT) at 760 °C for 2 h and finally air cooled and (ii) re-austenitizing at 1040 °C for 60 min and air-cooled and tempered at 760 °C for 2 h, followed by air cooling (PWNT). The P91 steel plates were received in the cast and forged condition having fully tempered lath martensite with lath blocks, packet boundaries and prior austenite grain boundaries (PAGBs). In this research, microstructural evolution at fracture frontier of crept P91 weld sample, creep rupture life and effect of creep exposure time on microstructure evolution in the fine-grained heat affected zone (FGHAZ) were studied. The results show a dominating effect of PWNT treatment on fracture behavior and creep rupture life of the P91 welded joints. PWNT treatment results in negligible hardness variation across the transverse section of the weld joint. The creep rupture life of PWNT specimen was 594.895% higher as compared to that of PWHT specimen and 142.7% higher than that of the as-welded specimen for the similar condition of creep exposure (620 °C/150 MPa). In PWHT condition, the most common type IV cracking was observed. However, it was eliminated in PWNT specimens and fracture occurred form the base zone. The fracture frontier of crept PWNT specimen showed a higher amount of creep cavities that can be attributed to the formation of Laves phase.

An improvement in creep strength of thermo-mechanical treated modified 9Cr-1Mo steel weld joint

Modified 9Cr-1Mo steel weld joints generally experience the type IV premature failure in the intercritical region (ICR) of HAZ under long term creep exposure at high temperature. Possibility of improving the resistance of this joint to type IV cracking through thermo-mechanical treatment (TMT) of the steel has been explored. Weld joints have been fabricated from the TMT and conventional normalized and tempered (NT) steels using electron beam (EB) welding process. Creep tests have been carried out on NT and TMT steels joint at 923 K (650°C) and 110–100 MPa applied stress. Creep rupture life of the TMT weld joint was significantly higher than the NT steel weld joint. Significant variations of microstructural constituents such as M23C6 precipitate; lath structure and hardness across the joint have been examined in both the joints. The coarser M23C6 precipitate and lath, and subgrain formation in the ICR resulted in the soft zone formation and was predominant in the ICR of NT steel joint. The enhanced MX precipitation through TMT processing and reduction in coarsening of M23C6 precipitate under thermal cycle resulted in improved creep rupture strength of TMT steel weld joint.

Microstructure Evolution During Creep of Cold Worked Austenitic Stainless Steel

The 14Cr–15Ni austenitic stainless steel (SS) with additions of Ti, Si, and P has been developed for their superior creep strength and better resistance to void swelling during service as nuclear fuel clad and wrapper material. Cold working induces defects such as dislocations that interact with point defects generated by neutron irradiation and facilitates recombination to make the material more resistant to void swelling. In present investigation, creep properties of the SS in mill annealed condition (CW0) and 40 % cold worked (CW4) condition were studied. D9I stainless steel was solution treated at 1333 K for 30 minutes followed by cold rolling. Uniaxial creep tests were performed at 973 K for various stress levels ranging from 175-225 MPa. CW4 samples exhibited better creep resistance as compared to CW0 samples. During creep exposure, cold worked material exhibited phenomena of recovery and recrystallization wherein new strain free grains were observed with lesser dislocation network. In contrast CW0 samples showed no signs of recovery and recrystallization after creep exposure. Partial recrystallization on creep exposure led to higher drop in hardness in cold worked sample as compared to that in mill annealed sample. Accelerated precipitation of carbides at the grain boundaries was observed during creep exposure and this phenomenon was more pronounced in cold worked sample.

Microstructure and creep behaviour of P92 steel after HPT

The influence of the application of severe plastic deformation on microstructure changes and creep deformation behaviour was investigated in P92 creep-resistant pipe steel. P92 steel in its as-received state, with a microstructure typical of tempered martensite, was subjected to high-pressure torsion at room temperature. An investigation of the microstructure using an electron backscatter diffraction technique revealed that severe plastic deformation led to the subsequent formation of an ultrafine-grained microstructure, containing {110} fibre texture and randomly misoriented boundaries. The microstructure characteristics and hardness measurements exhibited saturated values at an equivalent strain of higher than about 20. The creep behaviour of the ultrafine-grained specimen produced in the saturated region exhibited a minimum creep rate that was about two orders of magnitude faster, and a larger creep ductility, in comparison with the as-received state. Creep exposure at 873 K with a time to fracture of about 3 h led to a slight grain coarsening and to the formation of Laves phase.

Long-term creep behaviour of cast TiAl-Ta alloy

The long-term creep behaviour of the cast Ti-44.4Al-8.1Ta (at.%) alloy with an initial γ-TiAl(L10)+α2-Ti3Al(D019) microstructure was investigated at 700 °C. The alloy shows a remarkable creep resistance. The minimum creep rate is measured to be as low as 1.47 × 10−10 s−1 and total deformation is only 2.5% after an interrupted 30000 h creep test at 200 MPa. The creep exposure affects the stability of the initial microstructure. The discontinuous precipitation leads to the formation of γ + τ(Ti3Al2Ta) type of microstructure along the lamellar colony and grain boundaries. The decomposition of the α2 phase results in the formation of τ and γ + α2 + L12 segments within the lamellae and cellular γ + α2 + L12 network within the coarse particles. The intensive formation and coalescence of the cavities along the grain boundaries leads to predominantly intergranual type of creep fracture at the applied stresses of 250 and 300 MPa.

Assessment of Creep Deformation, Damage, and Rupture Life of 304HCu Austenitic Stainless Steel Under Multiaxial State of Stress

The role of the multiaxial state of stress on creep deformation and rupture behavior of 304HCu austenitic stainless steel was assessed by performing creep rupture tests on both smooth and notched specimens of the steel. The multiaxial state of stress was introduced by incorporating circumferential U-notches of different root radii ranging from 0.25 to 5.00 mm on the smooth specimens of the steel. Creep tests were carried out at 973 K over the stress range of 140 to 220 MPa. In the presence of notch, the creep rupture strength of the steel was found to increase with the associated decrease in rupture ductility. Over the investigated stress range and notch sharpness, the strengthening was found to increase drastically with notch sharpness and tended toward saturation. The fractographic studies revealed the mixed mode of failure consisting of transgranular dimples and intergranular creep cavitation for shallow notches, whereas the failure was predominantly intergranular for relatively sharper notches. Detailed finite element analysis of stress distribution across the notch throat plane on creep exposure was carried out to assess the creep failure of the material in the presence of notch. The reduction in von-Mises stress across the notch throat plane, which was greater for sharper notches, increased the creep rupture strength of the material. The variation in fracture behavior of the material in the presence of notch was elucidated based on the von-Mises, maximum principal, and hydrostatic stresses. Electron backscatter diffraction analysis of creep strain distribution across the notch revealed localized creep straining at the notch root for sharper notches. A master curve for predicting creep rupture life under the multiaxial state of stress was generated considering the representative stress having contributions from both the von-Mises and principal stress components of the stress field in the notch throat plane. Rupture ductility was also predicted based on the multiaxial state of stress.

Structure and preferred orientation of primary phases of Ni-Al-Co-based alloy alloyed by Rhenium after annealing and creep exposure

The present paper investigates phase composition and fine structure of Ni-Al-Co-based alloy alloyed by Rhenium (Re) (~3 at.%), as well as studies preferred orientation of the given alloy primary phases by means of scanning electron microscopy and X-ray diffraction analysis. The original state was given as the alloy which underwent a complete cycle of heat treatment: stepwise homogenization, cooling, annealing, and cooling at 100°C/min back to room temperature. The alloy was further subject to annealing at 1000°C for 105 h and 968 h. It was also exposed to creep experiments at the same temperature for 105 h (loading 320 MPa), and for 968 h (220 MPa). γ- and γ′-phases were established to be the primary phases in both states of face-centered cubic crystal lattice. γ′-phase (phase with ordered atom arrangement having L12 superstructure) was the primary phase in all states of the alloy. Investigations showed that Re being able not to dissolve in the primary phases served as a phase-forming element. Phase composition and phase morphology were studied. The influence of high-temperature annealing and creep on superalloy structure and morphology of γ′-phase were defined. The influence of creep on the structure was proven to be different from the annealing effect. First, creep generated larger dissipation of orientations than annealing. Second, anisotropy of γ′-phase cuboids could be observed at meso-level. Cuboids partially joined together. Third, spheroidized cuboids could be often met after creep.

Structure and preferred orientation of primary phases of Ni-Al-Co-based alloy alloyed by Rhenium after annealing and creep exposure

Structure and preferred orientation of primary phases of Ni-Al-Co-based alloy alloyed by Rhenium after annealing and creep exposure

A comparative study on creep behavior of AISI 409 ferritic stainless steel in as-received and as-welded condition (A-TIG and M-TIG)

Activating flux tungsten inert gas (A-TIG) welding process is an evolving welding process to increase the productivity of the conventional or multi-pass tungsten inert gas (M-TIG) welding process. The current work presents the characterization of the microstructure evolution during the creep exposure of Base metal, A-TIG and M-TIG weldments of AISI 409 ferritic stainless steel (FSS). The creep tests were performed at temperature of 600 °C and a stress of 50 MPa. Energy dispersive X-ray spectroscope (EDS) assisted field-emission scanning electron microscopy (FESEM), optical microscopy and microhardness testing were used to evaluate the samples after creep exposure. Creep tests were performed on round creep specimens. The M-TIG and A-TIG creep samples were get ruptured from the heat affected zone during creep test. The rupture time was found nearly equal for A-TIG and M-TIG weldments and maximum for base metal sample.

Influence of type and morphology of carbides on stress-rupture behavior of a cast cobalt-base superalloy

The relationships between the type and morphology of carbides and stress-rupture property have been studied in a cast cobalt-base superalloy. Different types and morphologies of carbides are achieved by varying the heat treatment conditions. After heat treatment at 1240 °C/4 h, the primary M7C3 decomposes into M23C6, and the formed M23C6 and primary MC partially dissolve into the matrix. After heat treatment at 1280 °C/4 h, the primary M7C3 melts, resulting in the formation of a highly developed lamellar structure of M23C6, and the primary MC mostly dissolves into the matrix. The heat treatment at 1100 °C/300 h gives rise to the carbide transformation of M7C3 → M23C6 and a profusion of secondary M23C6 carbide around M23C6 and MC. The samples in stress-rupture test at 980 °C for 83 MPa manifest the following rupture life: 1280 °C sample > 1240 °C sample > as-cast sample > 1100 °C sample. The longer rupture life of the samples of 1240 °C and 1280 °C is derived from the stable microstructure, dispersed MC carbide morphology and the supersaturated matrix. The shortest rupture life of the 1100 °C sample is mainly attributed to the overaging. Additionally, the carbide transformations of M7C3 → M23C6, MC → M23C6, M23C6 → M6C take place in the as-cast sample during creep exposure.

Microstructure-based assessment of creep rupture behaviour of cast-forged P91 steel

The work presented in this study was performed with the intent to characterize the microstructure evolution for short term creep exposure of cast-forged P91 steel. The short-term creep test was performed at temperature range of 620–650°C and stresses ranging from 120 to 200MPa. To characterize the sample after creep exposure, field emission scanning electron microscopy (FESEM) with energy dispersive X-ray spectroscopy (EDS), optical microscope and micro-hardness testing were utilized. Creep tests were performed on round creep specimens. For low temperature service condition, longer creep life was obtained. The fracture surface of creep ruptured specimen were characterized by using the FESEM. The transgranular fracture mode was noticed in all the tests condition. The creep rupture life was found to be decreased with increase in applied stress. The maximum rupture life was measured about to be 3329.28h for the sample exposed at 620°C for 120MPa. A negligible microstructural change was measured in gripping area compared to the gauge area (necking area) of crept sample. The laves phase formation was also noticed along the grain boundaries for creep exposure life of 3329.28h.

Creep properties of simulated heat-affected zone of HR3C austenitic steel

Tensile creep tests on austenitic stainless HR3C steel pipe in an as-received state and after heat affected zone (HAZ) thermal cycle simulation by means of the GLEEBLE 3800 physical simulator were carried out at 923 and 1023K and an applied stress range between 100 and 350MPa to evaluate the effect of welding on its creep properties and behaviour. It was found that creep testing was conducted in a power-law creep regime. The results show a clear detrimental influence of the HAZ simulation on creep properties of HR3C steel at 923K and for very short creep exposure. However, with increasing time to fracture, the effect of weld simulation becomes less significant. By contrast, no such deterioration of creep properties was detected at 1023K. The evaluated similar values of the stress exponents of the minimum creep rate and the time to fracture are explained using the assumption that both creep deformation and the fracture process are controlled by the same mechanism. The main creep fracture mode was intergranular brittle fracture due to grain boundary creep damage.

The effect of reheat treatment on microstructure and stress rupture property of a directionally solidified nickel-based superalloy after long-term thermal exposure

The recovery of microstructure and creep properties of a directionally solidified superalloy after long-term thermal exposure was investigated. After reheat treatment, successful restoration of γ′ microstructure is achieved including morphology, size and chemical composition. However, carbides degradations within interior grains and at grain boundaries are irreversible through the reheat treatment. Short and long-term creep rupture properties are effectively improved by the reheat treatment, but cannot be fully recovered compared to that of the original material. During prolonged thermal and creep exposure, grain boundary carbides degradation occurred rapidly in comparison with that in the original material, which is regarded as the main reason leading to the reduction of stress rupture lifetime.

Microstructure evolution and corrosion behaviour of an ASTM A213 T91 tube after long term creep exposure

Abstract This study deals with microstructural evolution, mechanical properties and oxidation/carburization behaviour of an ASTM A213 T91 superheater tube after long term service in a power plant; approximately 157.000 h (about 18 years) in subcritical conditions (542 °C and 174 bar). This work provides good experience data to better evaluate the performances of this material, it can be referential for readers who are interested in martensitic heat-resistant steels, because the major previous studies were conducted in laboratories focusing on short creep experiences and oxidation behaviour. Our investigations suggest remarkable changes in the core and the walls of the tube. In the bulk material, the recovery of the tempered martensitic matrix and the coarsening of the M 23 C 6 carbides were observed, but the mechanical properties and the creep strength are still good. However, carburization/oxidation represents a major life time limiting factor by affecting about 20% of the tube thickness. The mechanisms of carburization, oxidation and microstructure evolution are here discussed.

Comparative Analysis of Creep Cavitation Damage in 0.5Cr-0.5Mo-0.3V Steel after Long-Term Creep Exposure by Metallography and Replica Testing

Complex metallographic analysis was performed on the steam pipe elbow made of 0.5Cr-0.5Mo-0.3V steel after long-term (more than 240 000 hours) of operation at elevated temperature that revealed the extensive creep damage on the outer surface of the pipe elbow. Metallographic analysis confirmed pronounced creep damage at the outer surface but, at the same time, the non-uniform nature of the cavitation. The density of cavities continuously decreased from outer to inner pipe surface in the most damaged area and rapidly waned along the circumference as well as the length of the elbow. Parallel evaluation of actual extent of the cavitation damage made by metallography and replica methods in various parts of the pipe elbow confirmed that replica method is capable to describe and quantify the cavitation damage of this steel in the same way as metallography, including evaluation of creep damage according to Nordtest NT TR 302.

Mechanical behavior and related microstructural aspects of a nano-lamellar TiAl alloy at elevated temperatures

Advanced intermetallic γ-TiAl based alloys, which solidify via the disordered β phase, such as the TNM+ alloy, are considered as most promising candidates for structural applications at high temperatures in aero and automotive industries, where they are applied increasingly. Particularly creep resistant microstructures required for high-temperature application, i.e. fine fully lamellar microstructures, can be attained via two-step heat-treatments. Thereby, an increasing creep resistance is observed with decreasing lamellar interface spacing. Once lamellar structures reach nano-scaled dimensions, deformation mechanisms are altered dramatically. Hence, this study deals with a detailed characterization of the elevated temperature deformation phenomena prevailing in nano-lamellar TiAl alloys by the use of tensile creep experiments and mechanical spectroscopy. Upon creep exposure, microstructural changes occur in the lamellar structure, which are analyzed by the comparative utilization of X-ray diffraction, scanning and transmission electron microscopy as well as atom probe tomography. Creep activation parameters determined by mechanical characterization suggest the dominance of dislocation climb by a jog-pair formation process. The dislocations involved in deformation are, in nano-lamellar TiAl alloys, situated at the lamellar interfaces. During creep exposure the precipitation of βo phase and ζ-silicide particles is observed emanating from the α2 phase, which is due to the accumulation of Mo and Si at lamellar interfaces.

Mechanical properties of T23 steel welded joints without post-weld heat treatment for fossil fired boilers

Abstract

Creep Damage and Fracture in Advanced Tungsten Modified 9%Cr Ferritic Steel

Advanced tungsten modified 9%Cr ferritic steel (ASTM Grade P92) is a promising material for the next generation of fossil and nuclear power plants. Unfortunately, there are rather few published reports on damage processes in P92 steel during high temperature creep and the effect of damage evolution on the creep strength is not fully understood. In this work, the creep behaviour of P92 steel in as-received condition and after long-term isothermal ageing was investigated at 600 and 650°C using uniaxial tension creep tests. To quantify the effect of each damage process on the loss of creep strength, most of creep tests were followed by microstructural and fractographic investigations. It was found that the large Laves phase particles, which coarsened during creep exposure, served as preferential sites for creep cavity nucleation.

Type IV cracking behaviour of modified 9Cr-1Mo steel weld joints

Creep deformation and rupture behaviour of modified 9Cr–1Mo steel weld joints fabricated by single-pass activated TIG (A-TIG) and shielded metal arc welding (SMAW) processes have been investigated at 923 K over a stress range of 50–110 MPa after post-weld heat treatment. The formation of coarse M23C6 precipitates and recovery of martensite lath structure into subgrain in the ICHAZ of the weld joints resulted in hardness trough. The weld joints exhibited significantly lower creep rupture lives than the base metal at lower applied stresses. Creep rupture location of the weld joints was found to occur in the ICHAZ. The reduced ductile mode of failure was observed in the low stress level in comparison with high stress level. An extensive localised creep deformation, coarsening of M23C6 precipitates in the ICHAZ with creep exposure led to the premature Type IV failure of the joints. The coarsening of M23C6 precipitates was extensive in the mid-section of the ICHAZ than the sub-surface of the joints, and was more predominant in the SMAW joint. While A-TIG weld joint exhibited reduced creep cavitation and coarsening of M23C6 precipitates due to lower deformation constraints by adjacent regions in the ICHAZ. Hence, A-TIG weld joint exhibited higher creep rupture life than the SMAW joint.

Failure transition mechanism in creep rupture of modified casting 9Cr-1.5Mo-1Co welded joint

Creep rupture behavior and microstructure evolution of modified casting 9Cr-1.5Mo-1Co steel welded joint were systematically investigated in this paper. Based on a series of creep rupture tests at 620°C under 110–170MPa, a curve of stress versus rupture time was achieved to evaluate the service life of welded joint. The results indicated that creep rupture is considered to occur in over tempered zone (OTZ) under relatively high stress above 130MPa while failure transferred to weld metal (WM) under lower stress below 130MPa. The ruptured specimens showed less ductility and necking with the decrease of stress. Microstructure characterization revealed OTZ with less grain boundaries than WM and relatively lower hardness than BM was more possible to deform in short-term creep rupture accompanied by the coarsening of M23C6. Besides, formation of Laves phase and deterioration of martensite lath structure were detected during long-term creep exposure. Mo segregation near M23C6 provided the condition for Laves phase to form adjacent to M23C6. The difference of plastic deformation resistance in each zones of welded joint and nucleation and coarsening of Laves phase in WM are considered as the main factors to result in the failure transition.