Cyclic plastic deformation and damage in 304LN stainless steel

304LN stainless steel samples are tested in room temperature under low cycle fatigue (LCF), ratcheting, pre-ratcheting followed by LCF and tensile, and pre-LCF followed by tensile conditions to understand the deformation behavior and damage evolution during cyclic plastic deformation. It is found that pre-ratcheting have massive detrimental effect on its subsequent LCF life. Systematic alterations in tensile properties are noticed with cyclic plastic damage evolution in this work. Orderly alteration of tensile yield stress with cyclic hardening/softening of the material is also observed.     

Hot deformation behavior of a Nb-containing 316LN stainless steel

A Nb-containing 316LN stainless steel was compressed in the temperature range 900–1200 °C and strain rate range 0.01–10 s^?1. The mechanical behavior has been characterized using stress–strain curve analysis, kinetic analysis, processing maps, etc. The microstructural evolution was observed and the mechanism of flow instability was discussed. It was found that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate. On the contrary, the efficiency of power dissipation increased with them; Flow instability was manifested as cracking and flow localization; The hot deformation equation and the relationships between deformation condition and dynamic recrystallization grain size and fraction were obtained; For Nb-containing 316LN stainless steel, the favorite nucleation sites for dynamic recrystallization are in sequence of triple point, grain boundary, twin boundary and intragranular deformation band; The suggested processing window is given.     

Monotonic vis-à-vis cyclic fracture behaviour of AISI 304LN stainless steel

This investigation is aimed to examine the monotonic and cyclic fracture behaviour of AISI 304LN stainless steel and its weldments, in order to assess their integrity under seismic loading conditions. The monotonic fracture resistance of the steel has been determined using standard J-integral technique; whereas the cyclic fracture resistance has been evaluated using periodic unloading to different extents fixed by pre-determined R-ratio. Comparison of the fracture toughness values of the steel estimated under monotonic and cyclic loading indicates that the latter could be as low as one-fifth of the former. The observed degradation in cyclic fracture resistance has been attributed to crack tip re-sharpening during cyclic loading.     

Cyclic fracture behaviour of 304LN stainless steel under load and displacement control modes

Attempts have been made to understand cyclic fracture behaviour of AISI 304LN stainless steel used for nuclear piping materials under load vis‐à‐vis displacement controlled fracture tests; the former closely simulate the seismic loading conditions. The load controlled tests indicate that a material fails in a limited number of cycles even when the load amplitudes are sufficiently below the maximum load in a monotonic J–R test. The displacement controlled tests, on the other hand, show that the energy absorbing ability of a material gets severely reduced under cyclic loading conditions. The obtained results on standard laboratory specimens have been compared with similar available results on components in order to provide guidelines for maximum load bearing capability of engineering components under cyclic loading.     

Cyclic hardening and fatigue behavior of stainless steel 304L

This article discusses cyclic hardening and fatigue behaviors of stainless steel 304L, the behavior of which is greatly influenced by prior loading. Effects of loading sequence, mean strain and mean stress, and pre-straining (PS) were investigated using constant amplitude as well as step and random loading tests. Contrary to common expectations, fatigue lives in strain-controlled mean strain tests were significantly affected by the mean strain, in spite of mean stress relaxation. PS induced considerable hardening and led to different results on fatigue life, depending on the test control mode. Secondary hardening was observed in some tests, characterized by a continuous increase in the stress response. Possible mechanisms for this behavior are also discussed. To correlate fatigue life data of a material such as stainless steel with strong deformation history effect, it is shown that a damage parameter with both stress and strain is required. The Fatemi–Socie (FS) parameter as such a parameter is shown to correlate the data under different control modes and loading conditions.     

Dynamic behavior of SUS304 stainless steel at elevated temperatures

An elevated temperature tensile impact experimental technique has been developed, using the rotating disk indirect bar-bar tensile impact apparatus with elevated temperature furnaces. Temperatures up to 800°C in the specimen have been obtained by means of rapid contact heating.Tensile impact experiments have been performed to investigate the mechanical behavior of SUS304 stainless steel in the temperature range 25–537°C. In contrast, the quasi-static tensions were conducted on MTS810 at three temperatures 25, 400 and 537°C, respectively. The experimental results show that the elevated temperature tensile impact experimental technique and method adopted are feasible practically. SUS304 stainless steel is a sort of temperature and strain-rate dependent metal, i.e., the strain rate has the effect of strengthening on yield stress and ultimate stress, but embrittling on unstable strain. The unstable strain decreases with increasing temperature at a constant strain rate, exhibiting an elevated temperature embrittlement phenomenon. The microstructure analysis reveals that the elevated temperature embrittlement phenomenon is due to the 'sensitization' of SUS304. In the case of impact loading, the adiabatic temperature rise is also capable of leading to sensitization. The differences of specimens' fractograph between tensile impact and quasi-static tension probably involve different deformation and fracture mechanisms. It is found that the volume fraction of transformation is strain, temperature and strain-rate dependent.     

Morphologies and characteristics of deformation induced martensite during tensile deformation of 304 LN stainless steel

The austenite γ (fcc) matrix of 304 LN stainless steel transforms readily to martensites (hcp) and ′ (bcc) on deformation. The formation and nucleation mechanism of deformation induced martensite (DIM) during tensile deformation of 304 LN stainless steel has been studied at various strain rates in room temperature. It is investigated that the enhancement of strain rates during tensile deformation promotes the early formation of DIM, while suppressing its saturation value at fracture. Extensive transmission electron microscopy (TEM) studies showed more than one nucleation site for martensite transformation and the transformation mechanisms were observed to be γ (fcc) →  (hcp), γ (fcc) → ′ (bcc) and γ (fcc) →  (hcp) → ′ (bcc).     

Fatigue deformation behavior of nitrogen-ion-implanted surface layers of type 304 stainless steel

Flexural fatigue experiments were conducted on austenitic type 304 stainless steel specimens whose surface layers had been ion implanted with nitrogen. For comparison, identical tests were carried out on unimplanted specimens. The damage in the surface layer due to ion implantation and the dislocation slip modes after fatigue deformation were investigated by transmission electron microscopy. Persistent slip bands parallel to the {111} crystal planes of individual grains were found in the surface layers of both the fatigued unimplanted and the implanted specimens. Slip was profuse in the unimplanted condition, and sparse and highly localized in the nitrogen-implanted condition.     

Bending ratcheting tests of Z2CND18.12 stainless steel

The ratcheting effect greatly challenges the design of components of the nuclear power plant. Not only the mean stress and the stress amplitude, but the temperature also affects the ratcheting strain. In this study, the tensile properties of Z2CND18.12 stainless steel are studied. The relationship between the Young’s modulus and the temperature was studied experimentally. A series of ratcheting experiments under bending loading for Z2CND18.12 stainless steel were carried out on a dynamic mechanical analyzer. The characteristic of progressive inelastic deformation of the material in the different temperatures and different loadings were investigated. The difference of the domestic and imported stainless steel on the tensile properties and bending ratcheting properties was studied. The experiment results have shown that the imported Z2CND18.12 is better than the domestic one on the tensile properties. The Young’s modulus is linear with the temperature change, and gradually decreased with the increase of temperature. The temperature and the bending loading affect the ratcheting greatly. In addition, the imported one is better than the domestic one on the resistance to the ratcheting deformation at various temperatures. The ratcheting boundary of the imported one is defined based on the ratcheting experiments.     

Impression creep deformation behaviour of 316LN stainless steel

The impression creep deformation behaviour of 316LN SS was investigated from microstructure, substructure, microhardness and profilometry studies of the creep deformed region. Impression creep tests were conducted on 316LN SS in the temperature range of 923–973?K, at different punching stresses in the range of 472–760?MPa. The impression creep deformation was characterised by a hemispherically shaped plastic zone which developed around the indentation. The study revealed the distinct regions under the punch undergoing deformation to different extents. The deformation was found to occur predominently on (111) planes. The dislocations in the highly deformed region were well dispersed in the matrix. The size of the plastic zone was estimated to be ～1·5 times the diameter of the indenter based on the microhardness and profilometry studies. The critical spacing to be maintained between the adjacent indentations was estimated to be >5 times the diameter of indenter.     

Necking propagated deformation behavior of layer-structured steel prepared by co-warm rolled surface nanocrystallized 304 stainless steel

Surface nanocrystallized 304ss sheets prepared by surface mechanical attrition technique were co-warm rolled at 500 °C to obtain layer-structured steel with alternate nanocrystalline layer and coarse grained layer. Tensile test results revealed that a novel tensile deformation behavior characterized by sliding and necking propagation were presented compared to the base material. In the process of tensile deformation, a sliding band occurred firstly when the stress reached the highest value, and then a necking propagated slowly at a constant speed to both ends of the sample gauge. After the first necking extended to the gauge ends, the second necking occurred before the sample cracked. The mechanical properties of the layer-structured steel exhibited both high strength and good ductility.     

Creep behavior of nuclear grade 316LN austenitic stainless steel at 873 K and 923 K

Mechanics of Time-Dependent Materials - Creep deformation and rupture behavior of nitrogen-alloyed (0.14 wt.%) nuclear grade 316LN austenitic stainless steel were investigated for the varying...     

Tensile behaviour of 316LN stainless steel at elevated temperatures

Abstract

The tensile deformation behaviour of 316LN stainless steel was investigated from ambient temperature up to 1000°C. The hardness and microstructure of area near tensile fracture were characterised. The results show that the engineering stress increases smoothly with engineering strain when the tensile temperature is at 400°C or below, while the plastic deformation stage displays a serrated/jerky flow at 600°C. At tensile temperatures of 800°C or above, the plastic deformation stage is dramatically prolonged. The deformation mechanisms of 316LN stainless steel are proposed to be sliding and twinning at 400°C or below, tangle dislocations due to cross-slipping at 600°C, dynamic recovery at 700°C, and dynamic recrystallisation at 800°C or above. The finding provides useful guidelines for the processing and service of 316LN stainless steel components at high temperatures.     

Characterization of the elevated temperature static load crack extension behavior of type 304 stainless steel

Creep crack extension rates in Type 304 stainless steel, obtained as a function of temperature over the range 650–800°C and as a function of specimen geometry at 750°C, are empirically correlated with both the net section stress and the apparent stress intensity factor. The results indicate that the stress intensity correlation is strongly dependent on specimen geometry, whereas the net section stress correlation appears to be generally valid. A direct correspondence between crack extension and local (crack tip) displacement is noted when creep crack extension rates at 750°C are compared with COD obtained from actual castings of the crack tip. By introducing the concept of a miniature creep specimen at the crack tip, a physical model for creep crack growth is developed, based on local stress relaxation and strain accumulation, that is consistent with both experimental observation and existing theories of steady state creep.     

Influence of martensite formation and grain size on room temperature low cycle fatigue behaviour of AISI 304LN austenitic stainless steel

Abstract

The effects of strain induced martensite formation and grain size on the room temperature low cycle fatigue behaviour of AISI 304LN austenitic stainless steel were considered. Two grain sizes, namely, 60 and 350 μm, were developed via suitable solution annealing treatments. Microstructural changes before and after low cycle fatigue testing were identified. The martensitic transformation was studied using aferritescope, X-ray diffractometry, and optical microscopy. The mechanical response was correlated with the microstructural changes. Secondary hardening as well as a crossover in the strain–life plots for the two grain sizes resulted from martensite formation. Dislocation configurations depended on the strains imposed.

MST/1902     

The edge-cracking of AISI 304 stainless steel during hot-rolling

The hot-rolled plates of AISI 304 stainless steel, containing edge cracks of different intensities, were examined. The austenitic matrix of the steel contained small amounts of ferrite inhomogeneously distributed across the width and the thickness of the plate. A correlation was found between ferrite content and edge cracking: the higher the ferrite content the longer the edge cracks. Among the chemical elements present in the steel, the most critical effect on ferrite content was exerted by carbon and nitrogen. The longest edge cracks were observed for plates with the lowest content of carbon and nitrogen. A possible contribution of steel chemistry and heating temperature to changes in the steel phase composition and the probability of edge cracking is discussed.     

The thermal conductivity of AISI 304L stainless steel

A compilation and critical analysis of the thermal conductivity () of AISI 304 stainless steel (SS) between 100 and 1707 K has been given in the literature. The author represented his recommended values of by an inflection in the A versus temperature relationship between 300 and 500 K. Because a physical mechanism had not been identified that would produce such a temperature dependence in of 304 SS, interest was generated in the possible existence of an as yet undiscovered phenomenon that might cause such an inflection. Consequently, experimental verification of the inflection was sought. The present paper presents recent measurements of , the electrical resistivity, and the absolute Seebeck coefficient of 304L SS from 300 to 1000 K and of the thermal diffusivity () from 297 to 423 K. The values computed from the a measurements were within ± 1.6% of the directly measured An inflection was not observed in the temperature dependence of between 300 and 500 K. After careful evaluation and because a physical mechanism still has not been identified which would produce such an inflection, the authors conclude that the inflection in the vs T relationship reported in the literature was caused by the data analysis technique.     

Evolution of dynamic recrystallisation in AISI 304 stainless steel

Abstract

The nucleation and development of dynamic recrystallisation (DRX) has been studied via hot torsion testing of AISI 304 stainless steel. The DRX behaviour was investigated with microstructural analysis and slope changes of flow stress curves. The characteristics of serrated grain boundaries observed by SEM, electron backscattered diffraction and TEM indicated that the nucleated DRX grain size was similar to that of the bulged part of the original grain boundary. The DRX of the alloy was nucleated and developed by strain induced grain boundary migration and by the necklace mechanism. Before the steady state in the flow curve at 1000 ° C and 0.5 s^-1, the dynamically recrystallised grains did not remain a constant size and gradually grew to the size of fully DRX grains at steady state (30 μm). The calculation of the grain size was based on X _DRX (volume fraction of dynamically recrystallisation) under the assumption that the nucleated DRX grains grow to the steady state continuously. It was found that the calculated grain size of the alloy was good agreement with that of the observed grain size. It is expected that a fine grained steel can be obtained by controlling hot deformation conditions on the basis of newly developed equations for predicting DRX behaviour.