Low-cycle fatigue behaviour of notched AISI 304 stainless steel specimens

Low-cyclic fatigue tests were conducted on semi-circle notched and V-notched specimens made of AISI 304 stainless steel. Extensive scanning electron microscopic examination of the fracture surface was also carried out to correlate the microscopic fracture surface features with the macroscopic fatigue loading parameter for this steel. The elastic-plastic fatigue test results indicated a noticeable cyclic hardening phenomenon and also a great influence of the maximum cyclic stress, the mean stress and the notch geometry on both the fatigue life and the fatigue behaviour process. Using careful sensitivity and regression analysis correlations between the macroscopic fatigue parameters on the one hand and the macroscopic and the microscopic fracture surface features on the other, these correlations are presented and clearly documented and discussed for the two notch geometries investigated.     

Microstructural investigation on strengthening mechanisms of AISI 304L austenitic stainless steel during cryogenic deformation

Abstract

In the present paper, microscopy techniques and mechanical tests were used to investigate in detail the strengthening mechanisms of an AISI 304L austenitic stainless steel during cryogenic deformation. The strain hardening rate–strain response of the alloy indicated three distinct regimes of hardening, being very similar to that previously reported for other low stacking fault energy alloys. The hardening rate initially decreased up to a strain of ～6% (stage I). Then, a second stage of increasing hardening rate began (stage II). At strains larger than ～25%, stage III with decreasing hardening rate followed. It was suggested that the formation of ?-martensite and α′-martensite together is responsible for the appearance of stage II. The high strain hardening values of the alloy in stage II were related to the increased fraction of α′-martensite and dislocation pile-ups behind the Lomer–Cottrell locks. The appearance of stage III was attributed to the difficulty of α′-martensite nucleation and ease of dislocation cross-slip at higher strains.     

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.     

Influence of pad span on fretting fatigue behaviour of AISI 304 stainless steel

The present work deals with the influence of pad span on fretting fatigue behaviour of AISI 304 stainless steel. Relative slip is one of the three primary variables influencing fretting fatigue behaviour. The relative slip can be modified by changing the pad span and/or cyclic stress. In the present study, the effect of relative slip was studied at different cyclic stress levels and by using fretting pads with three different pad span values (15, 20 and 30 mm). The relative slip increased with an increase in pad span and cyclic stress. Samples tested with fretting pads having longer pad span (30 mm) exhibited longer lives. Though the specimens tested with pads having longer pad span experienced higher frictional stress and tangential force coefficient compared with those tested with pads having smaller pad span (15 or 20 mm), the relative slip values were larger in the former. Due to larger relative slip values it was assumed that small cracks initiated by fretting fatigue would have been worn away due to wear damage. Due to this the specimens tested with pads having longer pad span exhibited enhanced fretting fatigue lives. More deformation-induced martensite formed in the samples tested with pads having longer pad span owing to longer lives.     

Yttrium implantation effect on 304L stainless steel high temperature oxidation at 1000°C

Scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDXS) and in situ X-ray diffraction techniques were carried out to observe the oxide scale evolutions of yttrium implanted and unimplanted commercial 304L stainless steels during and after their high temperature oxidation at 1000°C for 100 h. Our results clearly demonstrate that yttrium implantation promotes a faster oxide scale growth and the formation of a more uniform chromia layer due to a higher chromium selective oxidation compared to unimplanted 304L stainless steel. Moreover, the presence of yttrium also leads to the formation of an enriched silicon layer at the metal-oxide interface limiting the growth of iron-based oxides which were not detected (even during cooling) in the case of yttrium implanted samples. These results allow to understand the low weight gain of yttrium implanted 304L stainless steel observed by thermogravimetry and underline the beneficial effect of yttrium implantation on the 304L oxidation resistance at high temperature.     

Growth behaviour of small surface fatigue cracks in AISI 304 stainless steel

A series of axial tensile fatigue tests (R = 0.1) was carried out to investigate the initiation and the growth behaviours of very small surface fatigue cracks under two different surface conditions (viz. smooth and pitted surfaces) of AISI 304 stainless steel at room temperature. This paper deals with both of the two approaches regarding the analysis of fatigue: the approach based on the concept of fracture mechanics and low cycle fatigue. In particular, both the initiation and growth of cracks and the coalescence of small cracks by fatigue in the specimen have been investigated by the methods of surface replicas and photomicrographs. Quantitative information such as the initiation period, growth and coalescence behaviours of small cracks, and crack growth properties were systematically obtained. The results show that the accurate determination of these parameters is critical for the application of fracture mechanics to fatigue life assessment.     

Initiation of recrystallized grain structure under high-temperature low-cycle fatigue in 304 stainless steel

Initiation of recrystallized grains was observed under high-temperature low-cycle fatigue conditions in austenitic 18Cr-8Ni stainless steel. The fatigue tests were carried out using unbalanced fast-tension and slow-compression triangular strain wave-shape at 973 K in air. Recrystallized grains were initiated and grew along grain boundaries with the number of fatigue cycles. Accumulated grain-boundary sliding is considered to be the driving force for the recrystallization. Effects of the recrystallized grain structure on high-temperature fatigue failure were then examined under the conditions in which intergranular fatigue failure occurred. The recrystallized grain structure had no detrimental effects on the intergranular failure. The fatigue life was somewhat increased by the initiation of recrystallized grain structure.     

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.     

Corrosion fatigue cracking of AISI 316L stainless steel screen mesh

This paper describes the failure of stainless steel screen mesh grade AISI 316L after being in service for only 8 months. The characterization methods included visual examination, optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and metallography. The results showed that the screen mesh failed by pitting corrosion and subsequent corrosion fatigue cracking. Pitting was initiated by the attack of chlorides from PVC powder and service environment as well as the action of excessive wear. Subsequent corrosion fatigue cracking arose from the presence of chlorides along with the residual and cyclic stress concentration at the pits. Failure prevention can be achieved by annealing after wire drawing, periodic surface cleaning, and proper material selection.     

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.     

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.     

Fabrication of nanostructure in inner-surface of AISI 304 stainless steel pipe with surface plastic deformation

In the present investigation, a pipe inner-surface grinding (PISG) technique was developed to fabricate nanostructure in the inner-surface of an austenitic 304 stainless steel pipe. PISG was performed by high speed shearing with hard sphere tips, leading to gradient distribution of strain, strain rate and strain gradient along depth. Nano-austenite with an average boundary spacing of 20?nm was generated, followed by deformation microstructure characterized by shear bands, multi- and uni-directional twins and planar dislocation arrays. Deformation induced grain refinement of austenitic 304 stainless steel with low stacking fault energy (SFE) covering 4–5 order’s magnitude of length scales toward nanometer regime was unified.     

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     

On the microstructural evolution pattern toward nano-scale of an AISI304 stainless steel during high strain rate surface deformation

In the present investigation, an austenitic AISI 304 stainless steel was subjected to high strain rate surface deformation by Pipe Inner-Surface Grinding(PISG) technique. The depth-dependent deformation parameters(strain, strain rate and strain gradient) were evaluated and the microstructures were systematically characterized. Microstructural evolution from millimeter-to nano-scale was explored, with special attention paid to the localized deformation. Microstructural evolution begins with the formation of planar dislocation arrays and the twin-matrix lamellae, which is followed by the localized deformation characterized by the initiation and the development of shear bands. A twinning-dominated process that was supplemented with dislocation slip-dominated one governed the microstructural evolution inside shear bands. The twin-matrix lamellae transform into extended/lamellar structure and finally the nanosized grains. Austenitic grains were substantially refined and martensitic transformation was effectively suppressed, of which the underlying mechanisms were analyzed.     

Acoustic emission during tensile deformation of annealed and cold-worked AISI type 304 austenitic stainless steel

The influence of prior cold work on the acoustic emission (AE) generated during subsequent plastic deformation of AlSl type 304 stainless steel has been studied. AE parameters such as root mean square voltage, ringdown counts, etc., have been used to analyse the data. AE generated during tensile deformation is affected by prior cold work. The increase in acoustic activity at low strain levels in a less cold-worked (10%) specimen compared to a solutionannealed specimen was attributed to the easy formation of -martensite assisted by prior cold work. The decreased acoustic activity for higher cold-worked (20%, 40% and 50%) specimens at low strain levels was attributed to reduced glide distance for moving dislocations and reduced amount of -martensite formation. The AE activity was found to be maximum during higher strain values in the solution-annealed specimen compared to the cold-worked specimens. This was attributed to relatively larger amount of -martensite formation in the solution-annealed specimen. Eddy current testing, X-ray diffraction, remanent magnetization measurement and magnetic etching techniques have been used to corroborate the AE results. Among these, remanent magnetization results were found to have good correlation with the AE results.     

Effects of the cryogenic cooling on the fatigue strength of the AISI 304 stainless steel ground components

For environmental considerations, the substitution of the conventionally used oil-based grinding fluids has nowadays become strongly recommended. Although several alternatives have been proposed, cryogenic cooling by liquid nitrogen is the non-polluting coolant that has been given relatively more attention because of its very low temperature. In this investigation, in order to contribute to developing this promising cooling mode, its beneficial effects on the ground surface integrity of the AISI 304 stainless steel and their consequences on the fatigue lifetime are explored. Results of this investigation show that grinding under cryogenic cooling mode generates surfaces with lower roughness, less defects, higher work hardening and less tensile residual stresses than those obtained on surfaces ground under oil-based grinding fluid. These surface enhancements result into substantial improvements in the fatigue behaviour of components ground under this cooling mode. An increasing rate of almost 15% of the endurance limit at 2 × 10⁶ cycles could be realized. SEM analyses of the fatigue fracture surfaces have shown that the fatigue cracks observed on the specimens ground under cryogenic cooling are shorter (i.e., 30-50 μm) than those generated under oil-based cooling mode (i.e., 150-200 μm). The realized improvements in the surface integrity and in the fatigue behaviour are thought to be related to the reduction of the grinding zone temperature observed under cryogenic cooling, as no significant differences between the grinding force components for both cooling modes have been observed.     

An experimental investigation of fatigue crack growth of stainless steel 304L

A series of fatigue crack growth experiments were conducted using round compact tension specimens of AISI 304L stainless steel under Mode I loading. The influences of the R-ratio (the ratio of the minimum load to the maximum applied load in a cycle), notch size, the tensile and compressive overloads, and the loading sequence on crack growth were studied. The results show that the material displays sensitivity to the R-ratio. The application of a tensile overload results in a short period of acceleration in the crack growth rate followed by a significant retardation in the crack growth rate. A compressive overload (underload) produces a short period of acceleration in crack growth and the magnitude of such an acceleration depends on the value of the loading amplitude of the constant-amplitude loading. Results from the two-step high-low loading sequence reveal a period of crack growth retardation at the beginning of the lower amplitude step, an effect similar to that of a single overload. Two existing crack growth models which are based on the stress intensity factor concept are evaluated using the experimental results. A two-parameter crack driving force approach together with a modified Wheeler’s model is found to correlate well the crack growth experiments.     

Using ESPI system to measure high temperature fatigue deformation of ceramics thermally sprayed SUS304 steel

The strains in an Al₂O₃/NiCr coating, which was thermally sprayed on SUS304 steel, were analyzed using an electronic speckle pattern interferometry (ESPI) system during fatigue testing (R = 0, _max = 173 MPa) at high temperature of 873 K. The strain changes with the crack initiation in the coatings and the delamination at the coating/substrate interface are accordingly discussed.Surface cracks originated from the top coating of Al₂O₃ and stopped at the bond coating of NiCr after 2 cycles test at 873 K. Many surface cracks and delamination along the NiCr/substrate interface were confirmed after 1 × 10⁵ cycles test. The strain values of un-sprayed specimens obtained from the ESPI system agreed with those measured by the strain gauge when tensile stresses were applied at room temperature. The deformation by thermal expansion and stress application at high temperatures can also be easily measured using this method. The strain on sprayed specimens was almost the same with that on un-sprayed specimens at 873 K, indicating the deformation in the coatings are always associated with that of the substrate surfaces at high temperature. By comparing and analyzing the strain distribution on the coating surface, the presence of cracks in the coatings and delamination at the coating/substrate interface can be in-situ and nondestructively detected.     

Dynamic behaviour of 304 stainless steel during high Z deformation

The deformation behaviour of 304 stainless steel was investigated over a range of strain rates spanning those from typical laboratory conditions through to those more commonly experienced in industry. This study examines the effect of the Zener-Hollomon (Z) parameter on the shape of the deformation flow curves. At Z values typical of laboratory experiments the flow curves display a behaviour indicative of dynamic recrystallization (DRX). A clear peak can be seen in the curve, but as the Z value increases (increasing strain rate/decreasing temperature) the shape of the curve changes to a ‘flat-top’ behaviour which has traditionally been used to indicate that no DRX is occurring and that dynamic recovery (DRV) is the only softening mechanism operating. Examination of the corresponding work hardening curves (θ = δσ/δ?) and deformation microstructures suggests that some level of dynamic recrystallization has occurred. The results suggest that at the higher strain rates the main mechanism for DRX is only the formation from prior austenite grain boundary bulges and that subsequent layers of DRX grains are very difficult to form. The overall deformation textures were also found to be independent of the strain rate.