► Martensitic lath structure rearranges into cells/subgrains causing cyclic softening. ► Cyclic softening process is highly sensitive to temperature and application of hold. ► Lower CSR of weld joint ...compared to base metal due to soft zone in the HAZ. ► Failure location of WJ depends upon temperature and direction of hold application. ► Enhanced cracking due to strain localization and creep cavity formation in HAZ. ► Compression dwell sensitivity due to the deleterious effect of the surface oxides.
The weld joint consisting of a heterogeneous microstructure exhibits a lower fatigue life than that of the base metal. This is due to the presence of soft zone in the heat affected zone (HAZ). At high temperatures and under hold application, strain localization occurs in the soft intercritical HAZ (ICHAZ). Sub-surface creep cavity formation in the soft region and their linkage causes enhanced crack propagation and that translates into lower fatigue life of the weld joint at high temperatures. Occurrence of compression dwell sensitivity in the material is attributed to the presence of surface oxides.
•SR correlated with deformation under CFI in RAFM steels.•Stress relaxation directly related to plastic strain accumulated, inversely to CFI life.•Optimum combination of W and Ta best for CFI ...life.•RAFM steels demonstrated compressive dwell sensitivity.•SR tends toward constant value at long hold.
The aim of this work is to understand the effect of varying tungsten and tantalum contents on creep–fatigue interaction (CFI) behavior of reduced activation ferritic–martensitic (RAFM) steels. Increase in W improved CFI life. Effect of changing Ta and W upon the resultant CFI life seems to be interrelated and an optimum combination of both W and Ta works out to be the best for CFI life. Stress relaxation obtained during application of hold can be a useful parameter to relate deformation and damage in the RAFM steels.
In the present paper, influence of hold time on the high temperature low cycle fatigue (LCF) behavior of modified 9Cr–1Mo ferritic steel in the normalized and tempered condition is addressed. Total ...axial strain controlled LCF tests at 873
K with hold times at peak strain up to 10
min in tension and compression were carried out employing total strain amplitude of ±0.6%. The alloy in general, showed a gradual and continuous softening regime. Fatigue life was found to decrease with increase in the duration of hold time in both tension and compression. Compression hold was found to be more damaging than the tension hold. The fatigue failure in compression hold tests were marked by extensive crack branching and formation of secondary cracks. Oxidation-assisted crack initiation and propagation contributed to life reduction at high temperatures in hold time tests. Additionally, the substructural changes of the starting microstructure were also found to be responsible for the reduction of the fatigue life. The combined effect of cyclic deformation and elevated temperature on the evolving microstructure have been evaluated. The application of hold during cycling at elevated temperature accelerates the conversion of initial heavily dislocated lath structure to equiaxed cells with low dislocation density and coarse carbides.
The service-exposed (∼60
000 h/873 K) Alloy 625 ammonia cracker tubes showed higher strength and lower ductility compared to the virgin material in the solution annealed state. Precipitation of ...intermetallic γ″ and Ni
2(Cr,Mo) phases and the inter and intragranular carbides were found to be responsible for higher strength of the service-exposed alloy. Subjecting the service-exposed alloy to thermal aging treatments subsequently at 923 K and 1123 K (above the service temperature of the exposed alloy) led to the dissolution of the intermetallic phases that in turn increased the ductility of the alloy. Post-service aging of the alloy at 923 K for short durations resulted in the dissolution of the Ni
2(Cr,Mo)-phase. The dissolution of the Ni
2(Cr,Mo)-phase exhibited significant influence upon yield strength (YS) but negligible effect on ductility. Prolonged aging of the alloy for 500 h at 923 K resulted in the precipitation of intermetallic δ-phase. Post-service aging of the alloy at 1123 K promoted the dissolution of both Ni
2(Cr,Mo) and γ″ formed during service. Longer duration aging at the same temperature led to the precipitation of the δ-phase with an associated increase in strength and loss in ductility. Re-solution annealing of the service-exposed alloy at 1423 K caused the dissolution of the strengthening phases. When the re-solution annealed alloy was subjected to prolonged exposure at 923 K, the yield stress was found to increase rapidly with aging time with attendent loss in ductility due to the precipitation of γ″.
In the present work, the deformation and damage evolution in 316L(N) stainless steel during low cycle fatigue (LCF) and creep–fatigue interaction (CFI) loadings have been compared by evaluating the ...residual tensile properties. Towards this, LCF and CFI experiments were carried out at constant strain amplitude of ±0.6 pct, strain rate of 3 × 10
−3
s
−1
and temperature of 873 K (600 °C). During CFI tests, 30 minutes hold period was introduced at peak tensile strain. Experiments were interrupted up to various levels of fatigue life
viz
. 5, 10, 30, 50, and 60 pct of the total fatigue life (
N
f
) under both LCF and CFI conditions. The specimens subjected to interrupted fatigue loadings were subsequently monotonically strained at the same strain rate and temperature up to fracture. Optical and scanning electron microscopy and profilometry were conducted on the untested and tested samples to elucidate the damage evolution during the fatigue cycling under both LCF and CFI conditions. The yield strength (YS) increased sharply with the progress of fatigue damage and attained saturation within 10 pct of
N
f
under LCF condition. On the contrary, under CFI loading condition, the YS continuously increased up to 50 pct of
N
f
, with a sharp increase of YS up to 5 pct of
N
f
followed by a more gradual increase up to 50 pct of
N
f
. The difference in the evolution of remnant tensile properties was correlated with the synergistic effects of the underlying deformation and damage processes such as cyclic hardening/softening, oxidation, and creep. The evolution of tensile properties with prior fatigue damage has been correlated with the change in surface roughness and other surface features estimated by surface replica technique and fractography.
This study demonstrates that, three parameters which are microstrain, lattice parameter and crystallite size, obtained from X-ray diffraction line profile analysis, can be used in a complementary way ...to study the precipitation/dissolution of various intermetallics and carbides in nickel base superalloy Inconel 625, without extracting the precipitates from the matrix.
P91 is a 9Cr-1Mo ferritic–martensitic steel which is used as steam generator (SG) material for fast reactor applications. Construction of SGs involves several weld joints (WJs) that are weak links ...also in the structure. Application of various types of cyclic loading and creep combinations can lead to different responses of the P91 steel WJ and thereby the resultant fatigue life. The scope of the present work is to identify the deformation and damage under the effects of various combinations of creep–fatigue interaction (CFI) waveforms. Towards this, location-specific damage has been assessed through various particles present in the steel and their cavitation propensity and severity to cause final failure under the effects of various fatigue-creep combinations. First time, it is demonstrated through 2D hardness mapping that in an unsymmetrical CFI loading waveform, short creep hold has more damaging effects in the form of strength loss as against the longer strain or creep holds. Interestingly, whereas under unsymmetrical loading, the initial strength of the weld metal was either uniformly retained (30 min CH or 30 min TH) or uniformly lost (10 min CH, 10 min TH), under symmetrical loading (CC and 5 min TCH), only a part of the weld metal lost the initial strength and the rest of it retained.
Graphical Abstract
Dynamic strain aging (DSA) has a pronounced effect on low cycle fatigue (LCF) behavior of materials. One of the manifestations of occurrence of DSA under LCF is serrated hysteresis loops (HLs). ...Analysis of the evolving serrated HLs under LCF loading has a large potential to provide information on the underlying principles of the occurrence of DSA. However, as manual analysis of serrated HLs is cumbersome, it has not been studied so well. A methodology for automated analysis of the serrated flow behavior under LCF loading is developed and used in the present paper for comprehensive analysis of evolution of DSA behavior in Alloy 617M during low cycle fatigue. For the first time, the magnitude of the evolving serrations both within a cycle and through the entire fatigue life has been represented as 2D stress drop maps to visualize the scale of DSA occurrence at any moment within the entire fatigue event for a given strain amplitude-temperature combination. Quantitative characterization of serrated behavior is reported as a function of fatigue cycle for different total strain amplitude and temperature combinations. Critical parameters for the initiation of DSA determined by stress drop and strain jump methods indicate a progressive nature, typical of fatigue deformation. Pronounced DSA activity at 773 K as compared to that at 573 K and 973 K is also reflected in terms of micro-strain in deformed specimens as observed by electron back-scatter diffraction (EBSD) analysis.
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•Methodology is proposed for comprehensive study of evolution of DSA during LCF.•Visualization of scale of DSA at any moment within entire fatigue event as 2D maps.•Dynamic nature of DSA with strain cycling under LCF is observed.•Optimum plastic strain-temperature-cycle number combination only initiates DSA.•Most conducive temperature for DSA occurrence in Alloy 617 M during LCF is 773 K.
Alloy 625 ammonia cracker tubes were service exposed for 60,000 hours at 873K. These were then subjected to a solution-annealing treatment at 1473 K for 0.5 hours. The effects of temperature and ...strain rate on the tensile properties of the solution-annealed alloy were examined in the temperature range of 300 to 1023 K, employing the strain rates in the range of 3 X 10-5 s-1 to 3 X 10-3 s-1. At intermediate temperatures (523 to 923 K), various manifestations of dynamic strain aging (DSA) such as serrated flow, peaks, and plateaus in the variations of yield strength (YS) and ultimate tensile strength (UTS) and work-hardening rate with temperature were observed. The activation energy for serrated flow (Q) was determined by employing various methodologies for T < 823 K, where a normal Portevien-Le Chatelier effect (PLE) was observed. The value of Q was found to be independent of the method employed. The average Q value of 98 kJ/mol was found to be in agreement with that for Mo migration in a Ni matrix. At elevated temperatures (T # > 823 K), type-C serrations and an inverse PLE was noticed. The decrease in uniform elongation beyond 873 K for 3 X 10-5 s-1 and 3 X 10-3 s-1 and beyond 923 K for 3 X 10-4 s-1 strain rates seen in this alloy has been ascribed to reduction in ductility due to precipitation of carbides and *d phase on the grain boundaries.
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•Texture formation in NC Ni under self-ion irradiation at 250–550 °C was studied using EBSD.•Irradiation at 250 °C strengthens the as-deposited texture by ion beam mixing at ...GBs.•Irradiation-induced recrystallization occurs at 350 °C, downward shifting of 100 °C relative to thermal.•At 350 °C, 450 °C, irradiation enhances the thermally-induced texture, by radiation enhanced diffusion.•Thermally-activated texture reversal occurred at 550 °C and irradiation enhances this texture.
The application of nickel coatings on structural materials in molten salt reactors for corrosion resistance is being researched. For corrosion resistance, the coatings must have specific texture and grain boundary characteristics that withstand irradiation. While most reported results concern textural changes caused by room-temperature irradiation, research in the thermally-assisted regime, which is the operating temperature range of nuclear reactors, is limited. In this study, we investigate the effect of 1.4 MeV Ni+ ion irradiation on the texture, recrystallization, and grain boundary characteristics of nanocrystalline Ni at temperatures ranging from 250 °C to 550 °C. Thermal activation at 350 °C and 450 °C results in partially and fully recrystallized microstructures, respectively. A concurrent decrease of //ED and evolution of //ED, as well as minor orientations that reduces surface energy, are detected up to 450 °C; nevertheless, textural reversal is found at higher temperatures. Irradiation accelerated the recrystallization kinetics, resulting in a fully recrystallized microstructure at temperatures 100 °C lower. Irradiation at temperatures above 350 °C increased the thermally-induced texture; at 250 °C, this trend reversed. Calculated radiation-enhanced diffusion (RED) coefficients at 250 °C revealed that ion mixing drives the growth of by consuming orientations. Conversely, the increasing RED contribution at higher temperatures strengthens the thermally-induced textures.
