Creep deformation and rupture behaviour of 9Cr–1.8W–0.5Mo–VNb steel have been investigated at 873K, 923K and 973K over a stress range of 80–220MPa. The absence of clear primary creep regime and ...prolonged secondary stage of creep deformation have been noticed under lower stress level at 973K. The variation of minimum creep rate with applied stress obeyed Norton's power law of creep. The apparent stress exponents of 15.2, 12.3 and 5.8, and apparent activation energy of 619kJ/mole have been estimated for creep deformation of the steel. The apparent stress exponents and activation energy have been rationalised on the basis of threshold stress. The threshold stress values of 137.5MPa, 83.3MPa and 29.7MPa were obtained at 873K, 923K and 973K respectively. The threshold stress compensated true stress exponent of 4 and true activation energy of 244kJ/mole, and threshold stress normalised by Orowan stress confirms that the lattice diffusion assisted localised climb of dislocation is the rate controlling of creep deformation in the steel. The steel obeyed Monkman and modified Monkman–Grant relationships. Damage tolerance factor of 6 in the steel demonstrates that the microstructural degradation such as coarsening of precipitates and subgrain structure is the dominant creep damaging mechanism in the steel.
Creep rupture behavior of 9Cr–1.8W–0.5Mo–VNb (ASME grade 92) ferritic steel weld joint fabricated by activated TIG (A-TIG) welding process have been investigated at 923K over a stress range of ...80–150MPa. The weld joint was comprise of fusion zone, heat affected zone (HAZ) and base metal. The HAZ consisted of coarse prior-austenite grain (CGHAZ), fine prior-austenite grain (FGHAZ) and intercritical (ICHAZ) regions in an order away from the fusion zone to base metal. A hardness trough was observed at the outer edge of HAZ of the weld joint. TEM investigation revealed the presence of coarse M23C6 precipitates and recovery of martensite lath structure into subgrain in the ICHAZ of the weld joint, leading to the hardness trough. The weld joint exhibited lower creep rupture lives than the base metal at relatively lower stresses. Creep rupture failure location of the weld joint was found to shift with applied stress. At high stresses fracture occurred in the base metal, whereas failure location shifted to FGHAZ at lower stresses with significant decrease in rupture ductility. SEM investigation of the creep ruptured specimens revealed precipitation of Laves phase across the joint, more extensively in the FGHAZ. On creep exposure, the hardness trough was found to shift from the ICHAZ to FGHAZ. Extensive creep cavitation was observed in the FGHAZ and was accompanied with the Laves phase, leading to the premature type IV failure of the steel weld joint at the FGHAZ.
The effect of ‘conventional normalizing and tempering’ (CNT) and ‘double austenitization based normalizing and tempering’ (DNT) heat treatments on the microstructure, tensile, creep and impact ...toughness properties of 9Cr-1Mo steel has been studied. The tempered martensite microstructure obtained through DNT treatment exhibited smaller sizes of prior-austenite grains/martensite packets (28 μm/11 μm) compared to the CNT treatment (44 μm/14 μm). The tempered martensite morphology was largely retained after long-term thermal aging at 550 °C/5000 h, while the M23C6 and M2(C,N) type of precipitates were found to act as nucleation sites for precipitation of brittle Fe2Mo Laves phase. The grain refinement by DNT was found to be beneficial for minimizing the ductile-to-brittle transition characteristics (25 °C lower ductile-to-brittle transition temperature and 70 J higher upper shelf energy) over the CNT. Thermal embrittlement occurred in both heated treated steels, but the transition temperature of aged DNT steel remained below room temperature. Fractured Charpy specimens revealed ductile failure by void coalescence for high temperature tests, and a quasi-cleavage fracture at low temperatures with few isolated occurrence of intergranular crack in thermal embrittled steel. The DNT treated steel resulted in similar or better tensile and creep properties, when compared to the CNT treatment. The homogeneous fine grained tempered martensite microstructure obtained by DNT treatment resulted in improved embrittlement resistance and mechanical properties over the conventional treatment.
Microstructure and tensile properties of dissimilar weld joint (DWJ) between Alloy 800 and ‘IN82 buttered’ Grade 91 (Gr. 91), fabricated using IN182 electrode and shielded metal arc welding (SMAW) ...technique, were investigated. Tensile properties of DWJ were evaluated at temperatures 298 K, 773 K and 823 K and at strain rates of 3 × 10
–3
, 3 × 10
–4
and 3 × 10
–5
s
−1
. Dendritic microstructure is observed in both IN82 buttered zone and IN182 weld. In the buttered zone, Type-II boundary (parallel to the fusion line) is noticed near the fusion boundary between buttered zone and Gr. 91. Further, in the heat affected zone of Gr. 91, adjacent to this fusion boundary, maximum hardness in the range of 240–350 VHN is observed up to a distance of ~ 1.25 mm from the fusion boundary, with a peak in hardness of ~ 345 ± 5 VHN at ~ 0.5 mm distance from fusion boundary. Under tensile loading, DWJ exhibits characteristic work hardening and post-necking elongation similar to that of Alloy 800 K at 298 K. At 773 K and 823 K, DWJ retains work hardening and exhibits extended post-necking elongation similar to that in Gr. 91. At high temperatures, yield strength of DWJ is intermediate to that of Gr. 91 and Alloy 800, while ultimate tensile strength of DWJ remains lower than that of the base metals. Similarly, uniform elongation decreases in the order of Alloy 800, DWJ and Gr. 91 at all the test conditions. It must be emphasized that, at high temperatures, DWJ exhibited tensile fractures in Gr. 91 base metal, irrespective of the strain rate. In contrast, at room temperature, tensile failures of DWJ have been predominantly observed in Alloy 800 base metal, except at the lowest strain rate wherein fracture occurred in buttered zone with small fraction of interface between buttered zone and Gr. 91 on the fractured surface.
Effect of multiaxial state of stress on creep rupture behaviour of 2.25Cr–1Mo steel has been investigated. Multiaxial state of stress during creep test has been introduced by incorporating two ...different circumferential U-notches in the uniaxial creep specimen. Creep tests on both plain and notched specimens were carried out at 873K over a stress range of 90–210MPa. The creep rupture life of the material was found to increase in presence of the notch with the consequent decrease in creep rupture ductility. The increase in rupture life was found to be more for relatively sharper notch. Finite element analysis coupled with continuum damage mechanics was carried out to understand the creep damage evolution during uniaxial and multiaxial creep exposures. The predicted uniaxial creep deformation and rupture life of the material were in good agreement with the experimental results. The creep rupture life of the notched specimen has been predicted and was found to be controlled predominantly by von-Mises stress. The creep damage in this steel under multiaxial state of stress was predominantly in the form of microstructural degradation with some evidences of intergranular creep cavitation especially at lower stresses.
► Tensile serrated flow behaviour of Hastelloy X was examined. ► A, A+B, B and C types of serrated flow behaviour have been observed. ► Molybdenum solute has been found to be responsible for serrated ...flow behaviour. ► M6C, M23C6 and σ-phase were observed at higher temperatures.
Serrated flow behaviour of Hastelloy X has been examined over a wide range of temperature (300–1023K) and strain rate (3×10−3s−1 to 3×10−5s−1). The alloy exhibited different types of tensile serrated flow in the intermediate temperature range of 473–923K. Normal portevin-Le Chatelier effect (PLE) exhibiting type A and B serrations were observed at temperatures less than 823K and inverse PLE exhibiting type C serrations was noticed at temperatures above 823K. The average activation energy value of 106kJmol−1 for the A and B types of serrated flow has been evaluated. The evaluated activation energy value revealed that the migration of molybdenum in the nickel matrix has been found to be responsible for the occurrence of serrated flow in the alloy.
Microstructure and tensile properties of P92 steel in the normalized and tempered, and thermal aged at 923 K for 5000 h and 10,000 h conditions have been investigated. Laves phase precipitate was ...observed in the thermal-aged steels. The size of Laves phase precipitate increased with increase in thermal exposure. This was also confirmed from the observation that the area fraction of Laves phase precipitate was higher in the 5000 h aged condition which decreased with further increase in thermal exposure. On the other hand, the size and area fraction of M
precipitate were found increased in the 5000 h aged steel, further continued to enhanced precipitation of fine M
in the 10,000 h aged steel. This resulted in significant increase in area fraction and comparable size with the steel aged for 5000 h. Hardness of the steel was decreased with increase in the duration of ageing. Thermal-aged steels exhibited lower yield stress, ultimate tensile strength and relatively higher ductility in comparison with steel in the normalized and tempered condition. The increase in lath width and recovery of dislocation structure under thermal-aged condition resulted in lower tensile strength and hardness. An extensive Laves phase formation and coarsening by loss of tungsten in the matrix led to decrease in the tensile strength predominantly in the 5000 h aged steel. The tensile strength of 10,000 h aged steel was comparable with that of 5000 h aged steel due to enhanced precipitation of fine M
in the steel due to enhanced mobility of carbon in the absence of tungsten in the matrix.
The titanium modified 14Cr–15Ni austenitic stainless steel is used as clad and wrapper material for fast breeder nuclear reactor. Thermo-mechanical treatments consisting of solution annealing at two ...different temperatures of 1273 and 1373K followed by cold-work and thermal ageing have been imparted to the steel to tailor its microstructure for enhancing strength. Tensile tests have been carried out on the thermo-mechanically treated steel at nominal strain rate of 1.6×10−4s−1 over a temperature range of 298–1073K. The yield stress and the ultimate tensile strength of the steel increased with increase in solution treatment temperature and this has been attributed to the fine and higher density of Ti(C,N) precipitate. Tensile flow behaviour of the steel has been analysed using Ludwigson and Voce constitutive equations. The steel heat treated at higher solution temperature exhibited earlier onset of cross slip during tensile deformation. The rate of recovery at higher test temperatures was also influenced by variations in solution heat treatment temperature. In addition, dynamic recrystallization during tensile deformation at higher temperatures was profound for steel solution heat-treated at lower temperature. The differences in flow behaviour and softening mechanisms during tensile testing of the steel after different heat treated conditions have been attributed to the nature of Ti(C,N) precipitation.
Tensile strength and flow behaviour of a Reduced Activation Ferritic–Martensitic (RAFM) steel (9Cr–1W–0.06Ta–0.22V–0.08C) have been investigated over a temperature range of 300–873K at different ...strain rates. Tensile strength of the steel decreased with temperature and increased with strain rate except at intermediate temperatures. Negative strain rate sensitivity of flow stress of the steel at intermediate temperatures revealed the occurrence of dynamic strain ageing in the steel, even though no serrated flow was observed. The tensile flow behaviour of the material was well represented by the Voce strain hardening equation for all the test conditions. Temperature and strain rate dependence of the various parameters of Voce equation were interpreted with the possible deformation mechanisms. The equivalence between the saturation stress at a given strain rate in tensile test and steady state deformation rate at a given stress in creep test was found to be satisfied by the RAFM steel.
Tensile deformation and fracture behaviour of AISI 431 martensitic stainless steel, over the temperature range of 300-823 K, has been examined. Yield and ultimate tensile strength values decreased ...gradually from room temperature to intermediate temperatures, followed by a rapid decrease at high temperatures. At the intermediate temperatures (523-673 K), the steel exhibited jerky/serrated flow and anomalous variations in terms of a plateau in flow stress/strength and work hardening parameters and minima in ductility. These manifestations were identified to be the signatures of dynamic strain ageing, operating at these temperatures. At high temperatures, a rapid decrease in flow stress/strength values and increase in ductility with increasing temperature indicated the dominance of dynamic recovery. The fracture remained transgranular ductile at all temperatures. Work hardening relations that best described the flow behaviour of AISI 431 steel were identified. Variations of the work hardening parameters with temperature were consistent with the variations exhibited by strength and ductility values.
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