Monitoring fatigue degradation in austenitic stainless steels

During cyclic loading of austenitic stainless steel, microstructural changes occurred, which affected both mechanical and physical properties. For certain steels, a strain‐induced martensitic phase transformation was observed. The investigations showed that for the given material and loading conditions the volume fraction of martensite depended on the cycle number, temperature and initial material state. It was found that the martensite content continuously increased with the cycle number. Therefore, the volume fraction of martensite was used for indication of the fatigue usage. The temperature dependence of the martensite formation was described by a Boltzmann function. The martensite content decreased with increasing temperature. Two different heats of the austenitic stainless steel X6CrNiTi18‐10 (AISI 321, DIN 1.4541) were investigated. The martensite formation rate was much higher for the cold‐worked material than for the solution‐annealed one. All applied techniques, neutron diffraction and advanced magnetic methods allowed the detection of martensite in the differently fatigued specimens.     

Microstructural effects on the short crack behaviour of a stainless steel weld metal during low-cycle fatigue

An experimental study into microstructural effects on short fatigue crack behaviour of 19 stainless steel weld metal smooth specimens during low-cycle fatigue is performed by a so-called ‘effective short fatigue crack criterion’. This material has a mixed microstructure in which it is difficult to distinguish the grains and measure the grain diameter. The columnar grain structure is made up of matrix-rich δ ferrite bands, and the distance between the neighbouring rich δ ferrite bands is an appropriate measurement for characterizing this structure. Particularly, the effective short fatigue cracks (ESFCs) always initiate from the bands of δ ferrite in the matrix in the weakest zone on one of the specimen surface zones which is orientated in accordance with the inner or outer surface of welded pipe from which the specimens were machined. These cracks exhibit characteristics of the microstructural short crack (MSC) and the physically small crack (PSC) stages. The average length of the ESFCs at the transition between MSC and PSC behaviour is ≈40 μm, while the corresponding fatigue life fraction is ≈0.3 at this transition. Different from previous test observations, the growth rate of the dominant effective short fatigue crack in the MSC stage still shows a decrease with fatigue cycling under the present low-cycle fatigue loading levels. A statistical evolution analysis of the growth rates reveals that the short fatigue crack growth is a damage process that gradually evolves from a non-ordered (chaotic) to a perfectly independent stochastic process, and then to an ordered (history-dependent) stochastic state. Correspondingly, the microstructural effects gradually evolve from a weak effect to a strong one in the MSC stage, which maximizes at the transition point. In the PSC stage, the effects gradually evolve from a strong to weak state. This improves our understanding that the short crack behaviour in the PSC stage is mainly related to the loading levels rather than microstructural effects.     

A quantitative understanding on deformation twins generation in single-phase austenitic stainless steels

Single-phase austenitic stainless steels (316L) have attracted widespread attention from scientists because of their duplex microstructure. In this paper, to have a quantitative understanding on the microstructure deformation of 316L, a physical model based on dislocation theory and strain gradient theory is established to find out the critical conditions when deformation twins generate. The twinning stress and the stress caused by strain gradients are two factors affecting the deformation twinning process. Numerical simulation results reveal that the twinning stress decreases with the increase of twin spacing and the decreases of volume fraction of twins and the orientation of external shear stress; the stress caused by strain gradients increases with the decrease of matrix grain size.     

Notch fatigue behaviour of low-temperature gaseous carburised 316L austenitic stainless steel

ABSTRACT

The influence of low-temperature gaseous carburisation on notch fatigue behaviour of 316L steel under cyclic axial loading was investigated. After carburisation, the carburised case was well distributed at the surface region and was not influenced by the notch geometry. Low-temperature carburisation considerably enhanced the notch fatigue performance, which led to 32% and 44% increase in the endurance limits for the specimens with stress concentration factors K _t?=?1.91 and 3.91, respectively. The notch sensitivity of 316L steel reduced after carburisation. Irrespective of the applied stress amplitude, the fatigue crack nucleation sites were always at the notch root surface for the untreated specimens. For the carburised specimens, fatigue cracks nucleation changed from surface at high-level stress to subsurface at low-level stress.     

Effect of cold deformation on corrosion fatigue behavior of nickel-free high nitrogen austenitic stainless steel for coronary stent application

Due to the excellent mechanical properties, good corrosion resistance, high biocompatibility and nickel-free character, the high nitrogen nickel-free austenitic stainless steel (HNASS) becomes an ideally alternative material for coronary stents. Stent implantation works in harsh blood environment after a balloon dilatation, i.e., the material is used in a corrosive environment with a permanent deformation. The present study attempts to investigate effects of pre-straining on high-cycle fatigue behavior and corrosion fatigue behavior of HNASS in Hank’s solution and the relevant mechanism for coronary stents application. It is found that higher pre-straining on HNASS results in higher strength and maintains almost same corrosion resistance. Fatigue limit of 0% HNASS is 550 MPa, while corrosion fatigue limit is 475 MPa. And improvement in fatigue limit of 20% and 35% pre-strained HNASS is in comparison with the 0% HNASS, while corrosion would undermine the fatigue behavior of HNASS. In a suitable range, the pre-straining had a beneficial effect on corrosion fatigue strength of HNASS, such as nearly 300 MPa improved with 20% cold deformation. This result provides a good reference for predicting the life of HNASS stent and as well its design.     

Some factors exerting an influence on the coaxing effect of austenitic stainless steels

The present paper describes some factors exerting an influence on the coaxing effect of austenitic stainless steels. Particularly, the influence of prestrain was investigated in detail. The materials used were austenitic stainless steels, type 304 and 316. Type 304N2 was also used to examine the properties of the stabilized austenitic phase in type 304. Two types of rotating bending fatigue tests, i.e. the conventional constant amplitude tests and stress‐incremental tests, were performed using the specimens subjected to the several tensile‐prestrain levels. Under the constant amplitude tests, the fatigue strengths of type 304 and 316 increased with increasing prestrain. Under the stress‐incremental tests, type 304 showed a remarkable coaxing effect, where the fatigue failure stress significantly increased regardless of the prestrain level. The coaxing effect in the unprestrained specimens was larger than those of the prestrained ones. Type 304N2 showed lower coaxing effect than type 304. In addition, the strain‐induced martensitic transformation did not occur because of the higher stability of austenitic phase in type 304N2. In type 316, the coaxing effect was dependent on the prestrain level, i.e. below 15% prestrain the coaxing effect became smaller with increasing prestrain, whereas above 25% prestrain the coaxing effect reappeared. Based on the tests results, it was considered that the coaxing effect in austenitic stainless steel was due to the mechanisms such as work hardening, strain ageing and strain‐induced martensitic transformation. The contribution of these mechanisms to the coaxing effect was different among type 304, 304N2 and 316.     

A new life extension method for high cycle fatigue using micro-martensitic transformation in an austenitic stainless steel

Most of the conventional strengthening methods for metals and alloys such as work hardening, precipitation hardening, cause a decrease in ductility and are not very effective for cyclic loading. In this study, a new strengthening method, which is effective for high cycle fatigue, has been developed. The intersections of dislocations in a stainless steel are freezed by very fine martensite particles, which are supposed to suppress dislocation motion at low stress amplitudes. Fatigue life in a high cycle regime increased >60 times, and no decrease in ductility was observed in tensile tests, as compared to a work-hardened stainless steel.     

The effects of saline aqueous corrosion on fatigue crack growth rates in 316 grade stainless steels

A study of fatigue crack propagation rates of 316 grade stainless steels in air and in an aqueous saline environment was carried out in an attempt to assess the fatigue properties encountered when such materials are used as surgical implants. The effects of variables such as temperature, pH, oxygenation level, bulk electrode potential, mean stress, frequency and stress waveform on the Paris crack growth law parameters were determined. Corrosion fatigue effects were observed in the aqueous saline environment, and a mechanism to describe this effect is proposed.     

The fatigue threshold, surface condition and fatigue limit of steel wire

To test the hypothesis that fatigue cracks in drawn, pearlitic steel wire propagate from pre-existing surface defects which can be treated as cracks, the fatigue limits of five different wires have been statistically determined. The fatigue thresholds were measured using a new AC potential drop method and the initial defect depths then calculated using the equation a_o=1π(ΔK_th/2σ_e)² and compared with observed values.

For higher strength steel wires (σ_u>1800 MPa) the agreement was very good; for the lower strength steel wire (σ_u=1469 MPa), however, the observed values were smaller than the calculated ones. The reasons for this discrepancy are discussed. A quantitative model is developed from the hypothesis and the influence of residual tensile stresses, decarburization and polishing on the fatigue limit of drawn steel wire is assessed.     

Effect of water-shower cooling during welding on tensile residual stress improvement in multi-layer welding of austenitic stainless steel plates

Abstract

A new welding method that uses a water shower behind the welding torch has been developed in order to reduce tensile residual stress in a welded region. When this method is applied to the welding of austenitic stainless steel, the welding and cooling conditions mainly determine how much the residual stress can be reduced. To optimize these conditions, we first used the robust design technique to determine the effects of the interpass temperature, the heat input quantity and the water-shower area on the residual stress distribution of bead-on-plate. We found that, to decrease the tensile residual stress, the interpass temperature should be high, the heat input low, and the water-shower area large. Effect of the water-shower cooling on multi-layer welding was examined analytically and experimentally. It was found that the residual stresses were tensile without water-shower cooling, but compressive with water-shower cooling under the optimized conditions. It can therefore be concluded that the new welding method is appropriate for reducing tensile residual stress in multi-layer welding of austenitic stainless steel.     

Evolution of surface deformation during fatigue of PH 13-8 Mo stainless steel using atomic force microscopy

The relationship between microstructure and nucleation of fatigue cracks in PH 13‐8 Mo stainless steel was explored with the use of atomic force microscopy (AFM) that allowed an accurate quantitative characterization of the surface features. Fully reversed strain‐ controlled fatigue tests were performed at 0.4 and 0.6% strain amplitudes, and the evolution of the surface deformation was observed at various fractions of life. At 0.4% strain amplitude, fatigue surface damage occurred first in the shape of streaks about 4 nm deep that formed at the interface between martensite laths and at prior austenite grain boundaries, and eventually coalesced to form crack nuclei. The increase in strain amplitude to 0.6% led to the formation of large extrusions, on average between 2 and 5 μm long with heights between 10 and 200 nm, which were the preferred crack nucleation sites.     

Volumetric inspection of moderately thick austenitic stainless steels by multifrequency eddy currents

This paper describes the initial phase of a project to develop eddy-current methods to inspect welds joining sections of austenitic stainless steel pipe having walls up to 13 mm (0.5 in.) thick. The objective of this phase was to demonstrate the feasibility of detecting and characterizing flaws in austenitic stainless steel base metals. These materials and welds present challenging eddy-current problems because of their relatively large thickness and ferromagnetism. Multiparameter analysis shows that a reflection coil probe operated with three discrete driver frequencies and phase detection can locate and size a cracklike defect in a single conductor in the presence of variations in conductor resistivity, permeability, and thickness and in the probe-conductor spacing (liftoff). Experiments were performed with a modular three-frequency instrument. Flat-plate specimens of types 304L and 347 stainless steel machined to 12.7 to 15.9 mm thickness simulated pipe walls; saw-cut slots 10 to 30% of nominal specimen thickness simulated cracklike defects. The same slots were used in duplicate experiments as near-side (directly under the test probe) or far-side (in the face opposite the probe) defects. Flaw detection and characterization capability was demonstrated by a series of experimental measurements fitted to specimen properties by least squares techniques. The quality of the fit determined the expected accuracy of measurement. Comparison of accuracy estimates determined the best choice of operating frequencies. From the 1,2,5 sequence of frequencies between 0.5 and 20 kHz, the optimum set of operating frequencies was selected to be 0.5, 2, and 10 kHz. Estimates of measurement accuracy for combined near- and far-side defect cases were: plate thickness, 0.74 mm; probe liftoff, 0.03 mm; defect location (depth of material above defect), 3.48 mm; and defect size (vertical slot depth), 1.09 mm. A few property values were back-calculated from instrument readings; the errors in these values were somewhat larger than the measurement accuracy estimates because of instrument drift and the absence of calibration circuits.     

Direct observation and analysis of the oxide scale formed on Y-treated austenitic stainless steels at high temperature

Abstract

We have studied the oxidation behavior of conventional austenitic stainless steels using same small amounts of Y as is added for deoxygenation and desulphurisaton in steel making.

The direct observation and analysis of the oxide scale formed on 19Cr–10Ni–l .5Si steels with and without small amounts of Y at high temperature have been carried out using several types of equipment. The following results were found: (1) Steel with 0.03Y showed good resistance to oxidation at l,000°C.

(2) Oxide scale was composed mainly of Cr oxide, and Si oxide was also detected at the oxide scale–metal interface and in the internal oxides. The Si oxide formed a network cell structure in the inner oxide scale with deeper internal penetrations. The steel with Y formed a uniform oxide scale in every oxide layer.

(3) Small amounts of Y and Si were detected at the grain boundaries of the inner oxide scale, but no Y was detected in the oxide grains.

The beneficial effect of Y addition was more notable in the Si containing austenitic stainless steels, as the existence of Y or Si prevents the diffusion of cations and anions through the oxide grain boundaries. As consequence, the steel treated with Y showed good resistance to oxidation.     

A note on fatigue damage assessment by x-ray diffraction techniques for a 304L stainless steel specimen

The progress of fatigue damage in type 304L solution treated stainless steel was studied by surface residual stress measurements and measurements of the formation of martensite, as determined from x-ray diffraction techniques. The specimen was subjected to tension-tension (R=0.1) cycling in air at a maximum stress _max=422 MPa, which was approximately 160% off the yield stress. The initial longitudinal residual stress distribution was determined for 13 locations along the gage length and found to vary around –80±17 MPa (compression). After several fatigue cycles, the residual stress changes to approximately +115±17 MPa (tension). At about 7% of the life (N/N _f =0.07), the variation of the residual stress along the gage length reaches a minimum. With additional cycling, the residual stress first drops, reaching a minimum at approximately 10% of the life (100±8 MPa), then increases to a maximum at between 20 and 40% of the life (125±18MPa), followed by a marked relaxation to a minimum of 90±18 MPa at approximately 65% of the life. The surface martensite content of the sample continuously increases from its initial value of around 0.6%, first slowly up to 10% of the total fatigue life, and more rapidly thereafter to final failure. On the basis of additional measurements of the residual stress and martensite content as a function of depth below the surface, surface layer hardening of austenite and martensite is suggested as the principal reason for the residual stress development.     

Assessment of low cycle fatigue improvement of machined AISI 316 stainless steel by brush hammering

The effects of wire brush hammering on low cycle fatigue behaviour of AISI 316 austenitic stainless steel has been investigated on turned samples through an experimental study combining strain controlled fatigue tests, scanning electron microscope examination and X‐ray diffraction analysis. An increase in fatigue life by 266% was reported at an imposed strain amplitude of Δε_t/2 = 0.2%. This improvement is limited to Δε_t/2 ≤ 0.5%. It is found that wire brush hammering produces a surface texture that favours, under cyclic loading, nucleation of randomly dispersed short cracks of the order of 50 µm in length stabilized by a compressive residual stress field. In contrast, turned surface showed much longer unstable cracks of the order of 200 µm in length nucleated in the machining groves and propagated under the effect of a tensile residual stress field. It has also been established that wire brush hammering can be used as intermittent treatment to improve the residual fatigue life of components subjected to cyclic loading. The treatment is very efficient if it is performed at a fraction of service lifetime n_i/N_r lower than 0.5.     

Cumulative fatigue damage of stress below the fatigue limit in weldment steel under block loading

To investigate the cumulative fatigue damage below the fatigue limit of multipass weldment martensitic stainless steel, and to clarify the effect of cycle ratios and high‐stress level in the statement, fatigue tests were conducted under constant and combined high‐ and low‐stress amplitude relative to stress above and below the fatigue limit. The outcomes indicate that neither modified Miner's nor Haibach's approach provided accurate evaluation under repeated two‐step amplitude loading. Moreover, effect of cycle ratios has been determined. Additionally, the cumulative fatigue damage saturated model is established and validated. Cumulative fatigue damage contributed by low‐stress below the fatigue limit in high stress of 700 MPa is higher than that with 650 MPa at identical conditions (fatigue limit 575 MPa). Thus, high stress affects fatigue damage behaviour below the fatigue limit. A new predicted approach has been proposed based on Corten‐Dolan law, whose accuracy and applicability have been proven.     

Crack closure and fatigue crack growth near threshold of a metastable austenitic stainless steel

In this paper R-ratio effects on fatigue crack growth near threshold region of a metastable austenitic stainless steel (MASS) in two different conditions, i.e. annealed and cold rolled, is investigated. The authors present two approaches to correlate FCGR data for R = 0.1, 0.3, 0.5, 0.7 and K_max = 23 MPa√m using a two-parameters approach (ΔK, K_max and α in Kujawski’s model) and crack closure model (using Elber’s K_op and in Donald’s ACRn2 approaches). The K_op and ACRn2 were experimentally measured on a single edge tension specimens. The K_op measurements were performed using a modified method and based on ASTM standards. While the two driving force approaches correlate data well in the Paris region, they fail to correlate them in the threshold region. However, this correlation can be improved in the threshold region when a different α value from the Paris region is used. The authors indicated that two different mechanisms operate; one in the Paris region and another in the near threshold. Hence, they proposed to combine the two-parameter and crack closure approaches where ΔK is replaced by ΔK_eff (estimated by a new method proposed in this paper), which is shown to correlate the FCGR data for different stress ratios for annealed steel. The correlation for cold rolled condition shows improvement with the new approach but is not as good as for the annealed one. The author further suggests to modify K_max in the two-parameter approach.     

Cyclic strain rate effect on martensitic transformation and fatigue behaviour of an austenitic stainless steel

In this study, the effect of strain rate on the cyclic behaviour of 304L stainless steel is investigated to unveil the complex interrelationship between martensitic phase transformation, secondary hardening, cyclic deformation and fatigue behaviour of this alloy. A series of uniaxial strain controlled fatigue tests with varying cyclic strain rates were conducted at zero and non‐zero mean strain conditions. Secondary hardening was found to be closely related to the volume fraction of strain‐induced martensite which was affected by adiabatic heating due to increasing cyclic strain rates. Tests with lower secondary hardening rates maintained lower stress amplitudes during cyclic loading which resulted in longer fatigue lives for similar strain amplitudes. Fatigue resistance of 304L stainless steel was found to be more sensitive to changes in strain rate than the presence of mean strain. The mean strain effect was minimal due to the significant mean stress relaxation in this material.     

奥氏体不锈钢Cr15Mn9Cu2Ni1N连铸坯壳层的热塑性

为认识连铸坯壳层中热塑性的变化及其原因,从奥氏体不锈钢Cr15Mn9Cu2Ni1N连铸坯壳层不同深度区域及芯部制取小型试样,在热模拟试验机上进行高温拉伸试验.结果表明:变形温度高于1050℃时,表层试样断面收缩率较低,由表及里逐渐增加,在距壳层表面27 mm左右处达到最高;变形温度低于1050℃时,壳层各深度区域断面收...