Crack growth behaviour of P92 steel under creep and creep–fatigue conditions

Abstract

High temperature creep and creep–fatigue crack growth tests were carried out on standard compact specimens machined from ASME P92 steel pipe. The effects of various loading conditions on crack growth behaviours were investigated. Crack initiation time was found to decrease with the increasing initial stress intensity factor under creep condition and further to decrease by the introduction of fatigue condition. For creep test, the crack growth rate can be well characterised by the facture mechanics parameter C*. For creep–fatigue test, the crack growth behaviour is dominated by the cycle dependent fatigue process when the hold time is shorter, but it becomes dominated by the time dependent creep process when the hold time becomes longer.     

Understanding low cycle fatigue and creep–fatigue interaction behavior of 316 L(N) stainless steel weld joint

The aim of this work is to study the time dependent effects on the low cycle fatigue (LCF) behavior of 316 L(N) stainless steel weld joint. Influence of strain rate, temperature, strain range, hold time and hold duration on fatigue life is evaluated. Occurrence of dynamic strain aging, creep damage, overall distribution of damage across the weld joint and the role of microstructure on the failure mode and failure location of the weld joint is discussed as a function of test parameters.     

Low cycle fatigue and fracture behaviour of cast Cu–Pb alloys

Abstract

The influence of alloy composition on cyclic stress response, cyclic strain resistance, and cyclic fracture behaviour is reported. Three Cu–Pb alloys containing 10, 25, or 40 wt-%Pb were investigated. Alloy composition, i.e. lead content, was observed to have an influence on cyclic stress response, cyclic ductility, and low cycle fatigue life. The cyclic stress response characteristics, fatigue life, and failure process are discussed in the light of the specific roles played by concurrent and mutually interactive influences of plastic strain amplitude, concomitant response stress, and intrinsic microstructural effects.

MST/1918     

Zone-wise investigation of creep behaviour 9Cr–1Mo steel weld joints

Distinct regions such as weld metal, heat-affected zone (HAZ) and base metal of P9 steel weld joints fabricated by various welding processes were investigated using impression creep testing. Smaller prior austenitic grain size, lower density of precipitates and dislocations resulted in faster recovery and higher creep rate of HAZ in comparison to the weld and base metal. Compared to base metal, shielded metal arc weld (SMAW) and activated tungsten inert gas (A-TIG) weld of the P9 steel weld joints exhibited better resistance to creep and displayed higher activation energy due to their coarser prior austenite grain size. A-TIG HAZ exhibited superior creep properties compared to the SMAW and TIG HAZ due to the presence of higher number density of precipitates.     

Effect of temperature on high-cycle fatigue and very high cycle fatigue behaviours of a low-strength Cr–Ni–Mo–V steel welded joint

Axially push–pull fatigue tests of a low-strength Cr–Ni–Mo–V steel welded joint were conducted up to very high cycle fatigue regime at room temperature and 370 °C. The S–N curve at room temperature shows a duplex shape, while the S–N curve at 370 °C is continuously decreasing with lower fatigue strength. The welds at 370 °C undergoes dynamic strain ageing and has an enhanced load–defects interaction, leading to equal distribution of failures among different parts of the welds. The Z parameter model, with micro-defect location incorporated, having sound physical representation, is life-controlling of the welds at high temperature.     

Low cycle fatigue behavior of Cr–Mo–V low alloy steel used for railway brake discs

The cyclic stress–strain response and the low cycle fatigue (LCF) behavior of Cr–Mo–V low alloy steel which was used for forged railway brake discs was studied. Tensile strength and LCF properties were examined over a range from room temperature (RT) to 600 °C using specimens cut from circumferential direction of a forged disk. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The cyclic strain–stress relationships and the strain–life relationships were obtained through the test results, and related LCF parameters of the steel were calculated. The studied steel exhibits cyclic softening behavior and behaves Masing type, especially at higher strain amplitudes. At higher than 600 °C, carbide particles aggregated and a decarburized layer developed near the specimen surface. Micro voids distribute within the depth of 50 μm from the specimen surface could coalesce with fatigue cracks. Multiple crack initiation sites were observed on the fracture surface. The oxide film that generated at 600 °C covered the fatigue striations and accelerated the crack propagation. Final fracture area with bigger and deeper dimples showed better ductility at higher temperature. The investigated LCF behavior can provide reference for brake disc life assessment and fracture mechanisms analysis.     

Comparison of creep behaviour of 2.25Cr–1Mo/9Cr–1Mo dissimilar weld joint with its base and weld metals

Abstract

Evaluation of the creep behaviour of 2.25Cr–1Mo and 9Cr–1Mo ferritic steel base metals, 9Cr–1Mo steel weld metal, and 2.25Cr–1Mo/9Cr–1Mo ferritic–ferritic dissimilar weld joints has been carried out at 823 K in the stress range 100–260 MPa. The weld joint was fabricated by shielded metal arc welding using basic coated 9Cr–1Mo electrodes. Investigations of the microstructure and hardness variations across the joint in the as welded, post-weld heat treated (973 K/1 h), and creep tested conditions were performed. The heat affected zone (HAZ) in both the steels consisted of a coarse prior austenitic grain region, a fine prior austenitic grain region, and an intercritical structure. In the post-weld heat treated condition, a white etched soft decarburised zone in 2.25Cr–1Mo steel base metal and a black etched hard carburised zone in 9Cr–1Mo steel weld metal around the weld fusion line developed. Hardness troughs also developed in the intercritical HAZ regions of both the steels. The width of the carburised and decarburised zones and hardness differences of these zones were found to increase with creep exposure. The 9Cr–1Mo steel weld metal showed higher creep strength compared to both the base metals. The 9Cr–1Mo steel base metal exhibited better creep resistance than the 2.25Cr–1Mo steel base metal at lower applied stresses. The dissimilar joint revealed lower creep rupture strength than both the base metals and weld metal. The creep strain was found to concentrate in the decarburised zone of 2.25Cr–1Mo steel and in the intercritical HAZ regions of both the steels. Creep failure in the stress range examined occurred in the intercritical HAZ of 2.25Cr–1Mo steel even though this region showed higher hardness than the decarburised zone. Extensive creep cavitation and cracks were observed in the decarburised zone.     

Low cycle fatigue behaviour of 15-15Ti at 20 and 550°C under vacuum

15-15Ti is one of the potential candidate materials for fuel rod cladding of lead-based reactors, and its fatigue properties have been investigated. Low cycle fatigue (LCF) tests of 15-15Ti have been carried out under total strain amplitudes ranging from ±0.3% to ±0.9% at 20 and 550°C in vacuum. The results show the LCF life decreases with the increase of temperature. Crack propagates more tortuously and the fatigue crack propagation (FCP) rate is lower when it is tested at 20°C compared to 550°C. In addition, the effect of twin boundaries (TBs) and stacking fault energy (SFE) on FCP rate is discussed in this paper.     

Fretting–fatigue behavior of steel wires in low cycle fatigue

The effect of strain amplitude on fretting–fatigue behavior of steel wires in low cycle fatigue was investigated using a fretting–fatigue test rig which was capable of applying a constant normal contact load. The fretting regime was identified based on the shape of the hysteresis loop of tangential force versus displacement amplitude. The variations of the normalized tangential force with increasing cycle numbers and fretting–fatigue lives at different strain amplitudes were explored. The morphologies of fretting contact scars after fretting–fatigue tests were observed by scanning electron microscopy and optical microscopy to examine the failure mechanisms of steel wires. The acoustic emission technique was used to characterize the fretting–fatigue damage in the fretting–fatigue test. The results show that the fretting regimes are all located in mixed fretting regimes at different strain amplitudes. The increase in strain amplitude increases the normalized tangential force and decreases the fretting fatigue life. The abrasive wear, adhesive wear and fatigue wear are main wear mechanisms for all fretting–fatigue tests at different strain amplitudes. The accumulative total acoustic emission events during fretting–fatigue until fracture of the tensile steel wire decrease with increasing strain amplitude. An increase of the strain amplitude results in the accelerated crack nucleation and propagation and thereby the decreased life.     

Degradation in tensile and creep properties of 2.25Cr–1Mo steel by long term service in plant

Abstract

Degradation in tensile and creep properties has been investigated for 2·25Cr–1Mo steel, after long term service at 577°C for 1·9 × 10⁵ h. Creep tests were carried out at 550–690°C for up to about 10 000 h for the long term serviced material. The results are compared with those for virgin material tested for up to 100 000 h. The creep rupture time is shorter but creep ductility is larger for the long term serviced material than for the virgin material at high stress and short time conditions. The difference between the two materials becomes decreased with decreasing stress and increasing time. Microstructure evolution during long term service causes a softening and promotes dynamic recovery or recrystallisation during subsequent creep, which accelerates the onset of acceleration creep. This results in a higher minimum creep rate and a shorter rupture time for the long term serviced material than for the virgin material. The deviation from Monkman–Grant relationship is correlated with a decrease in total elongation.     

Low cycle fatigue resistance of Al–Li alloys

Abstract

Low cycle fatigue (LCF) resistance data from binary Al–Li, ternary Al–Li–Cu, and quaternary Al–Li–Cu–Mg alloys have been compiled and discussed. The LCF resistance is measured in terms of the variation of the number of reversals to failure 2N _fwith the plastic strain amplitude Δ? _p /2 as well as a modified average plastic strain energy per cycle (ΔW _p )_modified , obtained at different applied total strain amplitudes (Δ? _t /2). The data show the effects of microstructural features, namely dominant strengthening precipitates and the degree of recrystallisation as well as crystallographic texture. Lithium content, when in excess of 2·5 wt-%in aluminium decreases the low cycle fatigue resistance the most. The degree of aging, the degree of recrystallisation, and the degree of texture also influence the LCF resistance; among which the effect of the degree of aging is the most pronounced. The effects of lithium content in aluminium solid solution and strengthening precipitates obtainable by the change in the Li/Cu ratio are found to be marginal. Based on a modified total cyclic plastic strain energy till fracture, it is shown that most of the Al–Li alloys exhibit degradation in their LCF resistance in both hypotransition (higher fatigue lives) and hypertransition (lower fatigue lives) regions. Such degradation is attributed to the combined effects of mechanical fatigue, strain localisation through dislocation–precipitate interaction, environmental effects, and finally strain localisation through the high angle grain boundaries. In comparison with the currently used 2XXX and 7XXX series aluminium alloys, Al–Li alloys require substantial improvement before they can be considered for fatigue critical applications.     

Effect of heat treatment on creep and fracture behaviour of 1·25Cr–O·5Mo steel

Abstract

The effect of tempering treatments on the microstructure and creep behaviour of multipass 1·25Cr–0·5Mo steel weldments has been evaluated. While tempering invariably reduced the hardness, significant changes in microstructure were only found after heat treatment at 750°C. In this case ferrite bands developed adjacent to the fusion boundary. Tempering increased creep deformation and reduced failure lifetime for base metal specimens. For crossweld testpieces, the susceptibility to low ductility failures in the heat affected zone was found to be linked to the development of creep cavities and cracks. Thus, brittle failure modes were a function of stress, temperature, microstructure, post weld heat treatment, and to a lesser extent, specimen geometry.

MST/3069     

Anomalous creep behaviour of 316 stainless steel at 550°C

The results offifteen constant-load creep tests at 550°C, with nominal stresses in the range 200 to 360 MPa and with test durations ofup to 14 000 h, are presented. The usual primary, secondary and tertiary creep behaviour was exhibited for nominal stresses greater than about 330 MPa. At lower stresses, ‘renewed’ primary and secondary creep regions were observed. The renewed secondary creep strain rates were found to be about an order of magnitude greater than the initial secondary creep strain rates. The results indicate that the occurrence ofthe renewed primary and secondary creep regions is associated with time-dependent exposure to a temperature of 550°C. The presence or magnitude of the prior stress level does not appear to have any significant effect.

The results are relevant to design procedures because extrapolation of short duration or high stress data to long-term design lifetimes is often required. Unless the possibility ofthe occurrence ofrenewed primary and secondary creep is taken into account, gross errors in strain predictions could occur.     

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C have been studied to analyse defect assessment in a steam generator. Different grades of alloy 800 have been investigated to reproduce the in service conditions. Fatigue crack growth (FCG) tests were conducted on CT20 and tubular specimens, then on welded tubes. Furthermore the influence of hold times on fatigue crack growth behaviour was studied.

The results obtained on material simulating the weld heat affected zone are in agreement with the tests conducted on welded tubes. Fatigue crack growth characteristics of aged and cold-worked aged material seem to be slightly improved in comparison with base material. Finally a hold time of one minute increases strongly the FCG threshold value determined in pure fatigue but has a negligible influence on crack growth rates.     

Effect of niobium on creep and creep crack growth of cast Ni–Cr austenitic steel

Abstract

An investigation of the effect of Nb on creep properties and creep crack growth rate in a 25Cr–35Ni–0·4C (wt-%) cast steel at 871 and 950°C was carried out. Tensile tests were also carried out at room temperature, 871, and 950°C. The tensile strength and elongation increased with an increase in Nb content at high temperatures. There existed an optimum Nb content for the creep properties and creep crack growth rate. Creep crack growth is controlled by creep deformation.

MST/1222     

Hall–Petch behaviour of 316L austenitic stainless steel at room temperature

Abstract

The room temperature plastic deformation behaviour of two different batches (with differences in chemical composition) of 316L austenitic stainless steel has been studied. By thermomechanical treatments, a wide range of grain sizes varying from 2·7 to 64·0 νm was obtained in this study. The different microstructural parameters, such as grain size, distribution of grain size and shape, dihedral angle distribution, and grain aspect ratio were measured for annealed and deformed specimens of the two batches. The Hall–Petch behaviour of batch 1 showed two distinctly different linear regions, one in the fine grain size range (d≤6νm) and the other in the coarse grain size range (d6νm). The Hall–Petch parameter K _H (?) was significantly higher in the fine grain regime than coarse grain regime at all strains. Hardness measurements were also performed across the grain at different strain levels. The applicability of the Hall–Petch relationship was assessed in batch 1 and batch 2. It was observed that the Hall–Petch relationship was applicable in the coarse grain regime and Kocks composite relationship in the fine grain regime of batch 1. In batch 2 of 316L austenitic stainless steel, a single linear Hall–Petch relationship could describe the deformation behaviour over the entire range of grain size (from 2.9 to 46 νm) studied. The variation of the Hall–Petch and Kocks composite parameters with strain was discussed in terms of changes in the microstructural parameters.     

Magnetic nondestructive evaluation of thermally degraded 2.25Cr–1Mo steel

Aging was performed to understand the microstructural degradation in 2.25Cr–1Mo steel. Microstructural parameters (mean equivalent carbide size, number of carbides per unit area), mechanical properties (UTS, Vickers hardness) and magnetic properties (coercivity, remanence) were measured to investigate the relationship among these parameters. The magnetic coercivity and remanence were observed to decrease rapidly in the initial 1000 h of aging time and then decrease slowly thereafter. Linear correlations between mechanical and magnetic properties were found.     

Effect of cyclic loading on subsequent creep behaviour and its implications in creep–fatigue life assessment

Abstract

Evaluation of creep–fatigue failure is essential in design and fitness evaluation of high-temperature components in power generation plants. Cyclic deformation may alter the creep properties of the material and taking cyclic effects into account may improve the accuracy of creep–fatigue failure life prediction. To evaluate such a possibility, creep tests were conducted on 316FR and modified 9Cr–1Mo steel specimens subjected to prior cyclic loading; their creep deformation and rupture behaviours were compared with those of as-received materials. It was found that creep rupture life and elongation generally decreased following cyclic loading in both materials. In particular, the rupture elongation of 316FR in long-term creep conditions drastically decreases as a result of being cyclically deformed at a large strain range. Use of creep rupture properties after cyclic deformation, instead of those of as-received material, in strain-based and energy-based life estimation approaches brought about a clear improvement of creep–fatigue life prediction.     

Cyclic creep behaviour under tension–tension loading cycles with hold time of modified 9Cr–1Mo steel

Abstract

Cyclic creep behaviour of modified 9Cr–1Mo steel was investigated by a series of cyclic creep (CC) tests at 600°C, which were performed under controlled tension–tension loading cycles with the magnitude of stress ranges in a constant stress ratio (R?=?0·1). Hold time was applied for a 10 min hold at the maximum stress (σ_max) and minimum stress (σ_min). The CC properties were compared with the static creep (SC) using Norton’s power law, Larson–Miller plot, and Monkman–Grant relation, and the microstructure was examined. For the test conditions employed in the present investigation, retardation in the CC behaviour in terms of a lower creep rate and longer rupture time compared to those in the SC was obtained. The retardation was ascribed to the effects associated with anelastic recovery during the 10 min hold time at the minimum load of the cyclic loading. The creep rupture ductility decreased with a general decrease in stress, and there was no difference in the creep ductility between the CC and SC. The steel displayed a transgranular fracture characterised by the presence of dimples resulting from micro-void coalescence. Carbide precipitation was more coarsened with increasing in exposure time in the CC tests.