Gauge Portion Research Articles

Strain and temperature contributions to structural evolution in a Re-containing 10% Cr-3% Co-3% W steel during creep

ABSTRACT A Re-containing 10% Cr-3% Co-3% W steel with low N and high B contents subjected to normalising at 1050°C and tempering at 770°C was crept at 650°C under the applied stresses ranging from 200 to 100 MPa with a step of 20 MPa. During creep both the temperature and the strain affect the tempered martensite lath structure that causes the thermal and strain-induced recoveries, respectively, and degradation of structure. Both these recoveries lead to premature failure. The microstructure in the gauge and grip portions of the crept samples were investigated separately to estimate the contributions of temperature and strain during creep to the evolution of tempered martensite lath structure of the Re-containing 10% Cr-3% Co-3% W steel. The W depletion from the solid solution was accelerated by creep strain during short-term creep. The lath width and particle sizes were stable during long-term ageing at temperature of 650°C in the grip portions of the crept samples. The width of martensitic laths was 350 nm after 13,600 h of long-term ageing. The creep strain facilitates the recovery of the structure causing the subgrain formation with a mean size of subgrains of 1.1 μm and particle coarsening in the gauge and neck sections. The precipitation of W-rich Laves phase particles was observed in both grip and gauge portions. The constant coarsening rate of M23C6 carbides was significantly lower during long-term ageing than that during creep, whereas the constant coarsening rates of Laves phase particles were comparable in both grip and gauge sections of the crept samples.

Microstructural changes around grain boundary after long-term creep in low-alloy steels

Microstructural changes after long-term creep were investigated in low-alloy Cr–Mo steels, focusing on the precipitation-free zone (PFZ) around the grain boundary. The grip and gauge portions of creep-ruptured samples obtained through the NIMS Creep Data Sheet Project were observed by SEM. The PFZ was clearly observed around the grain boundary after long-term creep in 1Cr-0.5Mo (JIS STBA22) and 1.25Cr-0.5Mo–Si (JIS STBA23) steels. Large precipitates were formed on the grain boundaries near where the PFZ was observed. The width of the PFZ increased with increasing creep time at 550 °C–650 °C. The increase in width was remarkable at higher temperatures. The clear relationship between the PFZ width and the Larson-Miller parameter (LMP) has already been reported for 2.25Cr–1Mo steel. This relationship provides a method for predicting the metal temperatures of serviced components in power plants. A good relationship between the PFZ width and the LMP was confirmed for 1Cr-0.5Mo and 1.25Cr-0.5Mo–Si steels. However, the relationship was not consistent with that for 2.25Cr–1Mo steel. The increase in the PFZ width was faster in 1.25Cr-0.5Mo–Si steel than in 1Cr-0.5Mo steel.

Correlation between Tensile Deformation Behavior and Microstructural Morphology of Nuclear Grade Austenitic Stainless Steel Weld Joints using Infrared Thermography Technique

Tensile deformation behavior of nuclear grade Austenitic Stainless Steel (SS) and its weld joints fabricated by Gas Tungsten Arc Welding (GTAW) and Activated flux Gas Tungsten Arc Welding (AGTAW) processes were studied and correlated with relevant microstructural morphologies using Infrared Thermography (IRT) technique. The microstructure of base metal showed a complete austenite phase. GTAW Fusion Zone (FZ) exhibited both primary ferrite and primary austenite mode of solidification. Meantime, AGTAW FZ exhibited only primary austenite mode of solidification A strain rate of 4.4x10-4 s-1 was used during the tensile test of the base metal and weld samples. The failure locations of the base metal, GTAW and AGTAW samples were noticed at the center of the gauge portion, the base metal side away from Fusion Line (FL) and Heat Affected Zone (HAZ) respectively. Temperature variations of the base metal and weld zones were recorded in the form of thermograms using the IR camera at the different stages of the tensile deformation. During deformation study, peak temperatures of 39.2 oC, 38.8 oC and 34 oC were observed at the base metal, GTAW and AGTAW samples respectively. The lesser peak temperature of the AGTAW sample compared to the base metal and GTAW samples indicated that the AGTAW sample undergone lesser deformation. Moreover, tensile deformation behaviors of the base metal and weld samples were correlated with their microstructural morphologies using corresponding temperature curves.

Influence of Ratcheting Strain on Tensile Properties of A356 Alloy

Ratcheting, a special kind of fatigue damage, is known to occur under the influence of asymmetric stress-controlled cyclic loading. Engineering structures undergo premature failure once ratcheting damage happens to these. The present study aims to describe the effect of the imposition of previous ratcheting damage on the tensile properties of aluminium A356 alloy. A356 aluminium alloy is being potentially used in automobiles, aerospace components, etc. In such applications, chances of ratcheting deformation cannot be ruled out. Thus, in this investigation, a few ratcheting tests were carried out at ambient temperature on as-received A356 alloy with varying mean stress (σm) and stress amplitude (σa) up to 2000 cycles. The magnitudes of σa and σm were chosen in such a way that the maximum stress (σmax) during cyclic loading must lie above the yield strength of the alloy and below the ultimate tensile strength. All the ratcheting tests were done at a cyclic load frequency of 0.05 Hz using a servo-hydraulic universal testing machine (BISS). The stress rates were varied accordingly in order to avoid any buckling of the specimens. The strain accumulated during the cyclic loading was captured using a 12.5 mm dynamic extensometer fitted at the gauge portion of the specimen. It was aimed to achieve at least 250 stress-strain data points per cycles during each test. Post-ratcheting tensile tests were carried out on the ratcheted specimens at a nominal strain rate of 6.66 × 10-4/s using a screw-driven universal testing machine (Instron). Study of fracture surfaces generated after tensile tests were done using scanning electron microscope (SEM). The results of ratcheting tests indicated that the material was cyclically hardenable in nature obtained through the reduction in stress-strain hysteresis loop areas, with increasing number of cycles. Ratcheting strain increased with increased in the magnitude of σa or σm. Post-ratcheting tensile tests showed that in general, the yield strength (YS) and ultimate tensile strength (UTS) increased with increasing amount of ratcheting strain. Interestingly, post-ratcheting tensile elongation also increased in all the ratcheted specimens as compared to that of the as-received specimen. The fracture surfaces showed presence several shrinkage pores which caused favorable sites for rise of stress concentration resulting initiation and growth of cracks. The elongation depends on the nature of the cracks generated during tensile loading and the extent of quasi-cleavage facets.

ДОСЛІДЖЕННЯ ПРОЦЕСУ ДВОСТОРОННЬОГО ШЛІФУВАННЯ ЦИЛІНДРИЧНИХ ДЕТАЛЕЙ З ТОРЦЯМИ РІЗНИХ ДІАМЕТРІВ ОРІЄНТОВАНИМИ ШЛІФУВАЛЬНИМИ КРУГАМИ

Urgency of the research. To ensure high efficiency of processing the critical end surfaces of parts of different diameters, use double-sided face grinding machines, grinding wheels of which have calibration areas or these areas are absent.Target setting. In a further development of engineering relevant is the task of developing and implementing new methods of polishing end surfaces of parts that provide improved performance processing while maintaining high accuracy and quality.Actual scientific researches and issues analysis. There are known methods of grinding parts ends with the same diameter, as shown modular 3D-model of the workpiece during grinding bilateral ends of parts, which are used to calculate the accuracy of formation and removal allowance.Uninvestigated parts of general matters defining. Investigation of the grinding process of parts with end faces of different diameters by oriented grinding wheels with calibrating sections.The research objective. The article is to study the process of grinding parts from bilateral ends of different diameters and calculation of cutting forces that arise during processing of the ends of different diameters.The statement of basic materials. The presented method bilateral grinding of parts with end faces of different diameters by oriented grinding wheels with calibrating sections of different lengths, provides an increase in processing accuracy. Proposed avoid overlapping input audio items in a processing zone and exit from the gauge portion other, which will improve the processing accuracy.Conclusions. The proposed method bilateral grinding of parts with various diameters of the ends of the grinding wheel with the metering portions of different lengths, which increases the precision in processing.

Effect of Loading Variations on Damage of SA333 C-Mn Steel

The mechanical properties of SA333 C-Mn Steel after fracture under diverse loading conditions have been investigated for further evaluation of damage. Miniature tensile specimens are machined by wire-EDM from the gauge portion of the failed specimens which were subjected to in phase and out of phase multiaxial loading and pure axial loading. From the experiment, it is observed that yield stress and Young’s modulus reduce significantly from the initial values for SA-333 C-Mn Steel. There is considerable variation in the tensile stress strain curve of the material with different types of loading. Finite Element simulations have been performed to estimate the mechanical behavior of the damaged material due to this loading variation. Plasticity model using Chaboche kinematic hardening model had been used to model the degraded mechanical properties of this material.

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

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

Dynamic stress analysis of the specimen gauge portion with a circular profile for the ultrasonic fatigue test

An hourglass-shaped specimen has been utilized for the ultrasonic fatigue test (UFT) at a resonance frequency around 20kHz. Although the gauge portion of the actual test specimens was made with a circular arc shape, the stress along the gauge portion has been measured by the conventional stress equation assuming a gauge portion with the form of a hyperbolic cosine. In this study, stress distribution results by finite element analysis (FEA) in forced resonant vibration mode showed that the shape of a circular arc caused considerably higher stresses at the center of the gauge portion than did the hyperbolic cosine. The error of the stress equation occurring due to the difference in the gauge portion profile was compensated for by using the forced vibration factor (C1) and the gauge shape factor (C2). Thus, stress concentration effects in specimens with various gauge lengths were quantified to determine the actual fatigue strength.

The Cause of Premature Tensile Fracture of Gas-Nitrocarburized Steel

The objective of the present study was to investigate the mechanism of premature tensile fracture of the gas-nitrocarburized steel. For this purpose, tempered martensitic steel specimens were nitrocarburized at 853 K (580 °C) for maximum 3.5 hours, and tensile-strained at room temperature. The prolongation of nitrocarburizing time caused the high strength, the short elongation, and the heavy serrations at the yield point elongation range of the tensile flow curve. The serrations were generated by vertical cracking at the specimen surface, which started at the bottom of the gauge portion, and were propagated to the top of the gauge portion with increasing tensile strain up to the yield point elongation. The vertical cracks were triggered at the porous compound layer when the local deformation occurred by the propagation of the Luders band from the bottom to the top of the gauge portion. The cracks readily passed through the diffusion layer with an assistance of e-carbonitride particles, which formed at the boundaries of tempered martensite during nitrocarburizing, resulting in brittle fracture at the diffusion layer and finally in premature failure of the gas-nitrocarburized specimens.

Characterization of High Temperature (873 K) Low Cycle Fatigue Damage in Modified 9Cr–1Mo Steel Using Magnetic Barkhausen Emission Technique

Modified 9Cr–1Mo (P91) ferritic steel is the steam generator structural material for prototype fast breeder reactor. During the start-ups and shutdowns, the material is subjected to conditions leading to low cycle fatigue damage. Detection of initiation of the fatigue damage is required to prevent the catastrophic failure in components of the steam generator. The present study reports the non-destructive characterization of substructural and microstructural changes occurring during low cycle fatigue (LCF) of modified 9Cr–1Mo base metal and weld joint at 873 K, using MBE measurements. MBE studies have been carried out on the samples at various levels of fatigue life by interrupting the fatigue test. MBE RMS voltage has been measured at different locations in the gauge portion of the samples to map the damage profile. The systematic changes in the MBE revealed various stages of progressive LCF damage starting from initial softening to final failure in the ferritic steel.

A study on the effect of variation of aluminide coating thickness on the tensile deformation behavior of a near-α titanium alloy using infrared thermography

Coatings are applied to titanium alloys to prolong the life of the components by improving the high temperature oxidation and corrosion resistance. The coating thickness needs to be optimized depending upon the service conditions. The coating thickness can potentially affect the mechanical properties of the substrate alloy. In the present study, the effect of variation in the coating thickness on the deformation behavior of a near-α titanium alloy coated with Al3Ti aluminide has been studied. Infrared thermography technique has been used to monitor the local heat evolution along the gauge portion of uncoated and coated specimens during tensile deformation. The technique has been used to measure the dissipated heat and to monitor the changes in the stored energy during tensile deformation. The presence of coating led to only marginal variations in strength although it reduced the alloy ductility significantly. The coated specimens failed by propagation of cracks from the coating to the substrate material. It is observed that several cracks penetrate into the substrate alloy and the unstable propagation of these cracks led to failure of the specimen. This study also shows that the localized heat evolution during the crack propagation process can be effectively used as a parameter for studying the in-homogeneous deformation and fracture process typical in coated materials.

Microstructure Evolution During Short Term Creep of 9Cr–0.5Mo–1.8W Steel

P92 steel (9Cr–0.5Mo–1.8W) was subjected to a heat treatment of 1050 °C/30 min/air cooling/780 °C/120 min/air cooling followed by 1080 °C/30 min/air cooling/740 °C/60 min/air cooling to obtain tempered martensite microstructure, for better creep strength. Stress rupture tests carried at 600 °C in the range of 250–350 MPa resulted in rupture times in the range of 200–3000 h. Straight line plot of stress rupture curve indicated no major change in deformation mechanism. Coarsening of precipitates and substructure development were the main reasons for microstructure degradation, consequently leading to reduced hardness of the sample. Gauge and grip portions of the same sample were sectioned to comparatively evaluate the effects of stress and aging. Gauge portion of 3000 h sample showed considerable change in the microstructure in terms of boundary migration, while that of grip portion hardly evolved. The ruptured samples exhibited predominantly ductile fracture with elongated cavities at higher rupture times.

초음파 피로시험시 시험편의 유한요소 동적 해석

An accelerated ultrasonic fatigue test (UFT) was used for analyzing very high cycle fatigue (VHCF, Nf > 10 ) behaviors of a specimen with a test resonance of 20 kHz. Using the finite element method (FEM), the dynamic behaviors of the specimen was studied by calculating the stresses along its gauge portion, with displacement. The shape of gauge portion profile was assumed to be a hyperbolic according to the stress equation of the UFT. However, as the specimen used in the test had a circular arc profile, the FEM was used for studying the local stresses for two cases of the gauge profile. The results were compared with those obtain from the stress equation of the UFT. The dynamic behavior of the gauge portion could be understood for further comparison with the actual results.

Effect of Long-Term Creep on Microstructure of a 9% Cr Heat Resistant Steel

The effect of long-term creep at 600°C under 137 MPa on the microstructure of a P92-type steel was investigated. The microstructure after tempering consisted of laths with an average thickness of 400 nm. Dispersion of secondary phases consists of M23C6 carbides with an average size of 85 nm located mainly on lath, block and prior austenite boundaries and MX carbonitrides with average size of 31 nm homogeniously distributed throughout. Creep with duration of 40738 hours led to coarsening of M23C6 carbides up to 182 nm. Precipitation of Laves phase with an average size of 290 nm took place in both grip and gauge portions of ruptured specimen. Vanadium-rich MX particles were replaced by particles of Z-phase with sizes of 97 and 48 nm after long-term creep and aging, respectively. The average misorientation of the lath boundaries was approximately 2° and scarcely varied during creep, while the mean lath thickness increased to 890 nm in gauge section of ruptured specimen and remained essentially unchanged in the grip section. Dislocation density decreased slightly under long-range aging and creep.

Effect of creep deformation on Z phase formation in Gr.91 steel

In order to make clear the effect of creep deformation on Z phase formation in Gr.91 steel, creep interrupted tests were performed for 10 000, 20 000, 30 000, 50 000 and 70 000 h at 600°C under 70 MPa. The time to rupture was 80 736·8 h at 600°C under 70 MPa. Z phase was observed in the vicinity of prior austenite grain boundaries in the grip and gauge portions after creep deformation for 10 000 h and more. Number density of Z phase particles increased with increasing creep time in the grip and gauge portions. The number densities of MX carbonitrides particles in the grip and gauge portions clearly started to decrease after 30 000 h corresponding to increase in number density of Z phase. In the gauge portion, the number density of MX carbonitrides particles was almost the same as that of Z phase particles after creep rupture, meaning that a large number of MX carbonitrides particles were disappeared. The number density of Z phase particles in the gauge portion was 2·5 times of that in the grip portion after creep rupture. This indicates that creep deformation promotes the Z phase formation.

Higher-order gradient effects in micropillar compression

Deformation behavior in single-slip oriented micropillars subjected to compressive loading is analyzed using a three-dimensional finite element method incorporating a higher-order gradient crystal plasticity theory in which the spatial gradients of the geometrically necessary dislocation densities are assumed to give rise to backstress that impedes or promotes dislocation motion, depending on its sign. In experimental studies, it has been frequently observed that the gauge portion of micropillar samples is split into a shear zone and two nearly unstrained dead zones under compressive loading. Here it is shown that effects of a higher-order gradient are essential for the formation of this particular deformation mode.

Contribution of recovery mechanisms of microstructure during long-term creep of Gr.91 steels

Creep rupture life and microstructural degradation have been studied in two heats of Gr.91 steels. The coarsening of subgrains and precipitates, mainly M23C6 and MX, has been evaluated during static aging and creep. Hardness of head (static aging) and gauge (creep) portions of crept samples were measured to know their relation with microstructural degradation during long-term exposure. The correlation between subgrain size and spacing of precipitates and hardness has been equated. As an example, there is a close correlation between hardness value and inverse subgrains size in Gr.91 steels, regardless of aging or creep conditions. The appearance of three recovery mechanisms was found during long-term creep, namely: strain-induced recovery, pure static recovery and strain-assisted static recovery. By equated correlations between subgrain size, precipitates and hardness, the contribution of three recovery mechanisms to subgrain coarsening and hardness drop were calculated for two creep conditions at 700°C in long-term creep region, where breakdown of creep strength has happen. The calculated data accord well with experimental data obtained from aged and crept samples. The contribution of static recovery to the subgrain coarsening and consequent hardness drop during long-term creep increases with increasing creep time. The significant contribution of both static recovery mechanisms can result in the breakdown of creep strength in long-term creep region.

Microstructure and Creep Rupture of P92-grade weld metal

The search for new more creep-resistant materials for supercritical components of modern power plants stimulates also welding research to provide joints adequate to withstand highly demanding operation conditions. Seeking for improvement of existing welding consumables, experimental Ti-doped P92 grade weld metals were developed of exceptionally long life in creep rupture tests. In response to this request SEM and TEM investigations were carried out to identify the features of microstructure, which could be responsible for so different behaviour in the creep rupture tests. In this research the microstructures of the Ti-doped FCAW P92 weld metals were also compared with these of a standard P92 pipe material as well as with the submerged arc weld metal deposit W0 manufactured by flux-cored consumable containing no addition of Ti. Transformation of microstructures of this “reference” P92 pipe material and of the Ti-free P92 weld metal was accomplished in the accelerated creep tests on Gleeble physical simulator. An attempt has been made to correlate the fracture appearance of the creep-ruptured samples with the fine structure in their head/grip and gauge portions. In the microstructure of non-affected by the creep strain head portions of the creep-ruptured samples appeared numerous delta-ferrite grains with no carbide precipitates. Evidences were found that creep voids and cracks formed fast in the initially precipitate-free delta-ferrite grains of the sample W1, while in sample W2 during the creep test a peculiar strain-induced precipitation hardening of the delta-ferrite appeared, however no Ti was identified there in the ultra-fine precipitates.

G030061 単軸クリープ試験片へのスリット導入と変形一様性([G03006]材料力学部門一般セッション(6):高温・環境)

Specific numerical analyses have been required at designing modern high temperature machines and structures. Since analytical results usually depend on a constitutive equation of creep, it is necessary to derive a high-precision equation. Creep properties on deformation can be normally obtained using constant load creep machines, but constant stress creep tests are more suitable to acquire high-precision creep data. In order to keep the true stress constant, an applied load or the lever ratio should be changed during the test duration. Conventional uniaxial creep specimens have ridges attached on both sides of gauge length to measure deformation outside an electric furnace. For ductile materials, however, even if the constant stress creep testing machine are used, these ridges constrain area reduction around them, and unequal-uniform deformation breaks the constant volume condition in the gauge portion during deformation. Authors have proposed introducing slits into the ridge to release the circumferential constraint by the extensometer ridges. In this study, taking into account a realistic analytic condition, the authors carried out elastic creep Finite Element (FE) analyses to investigate an optimum depth of the slit. Subsequently, a series of constant stress creep tests using SUS304 steel are conducted to examine the effect of introducing slits on the performance of uniform deformation. The analytical and the experimental results show that the introduction of these slits into the ridges would improve uniform deformation effectively.

Changes in contributions of matrix and block boundary strengths to macroscopic hardness of high Cr ferritic steel during creep

The instrumented indentation test was applied to the interrupted creep specimens of turbine rotor steel (Fe-10Cr-1Mo-1W-VNbN) to investigate the changes in contributions of matrix and block boundary strengths to macroscopic hardness during creep. The indentation tests were performed under the loads of 1 and 10 mN to determine the matrix strength and the block boundary strength, respectively. The matrix strength of the grip portion decreased gradually with increasing life fraction and, for the gauge portion, the decrease was more pronounced, particularly, at the latter half of the life. On the other hand, the experimental results revealed that the block boundary strength decreased significantly at the early stage of the creep life, but this decrease was no longer pronounced when the creep life fraction exceeded 20%. Additionally, there was a good correlation between the block boundary strength and the width of block boundary carbide and the carbide coarsening was considered to be closely associated with the decrease in block boundary strength.