Microstructural and electrochemical studies in thermally aged type 316LN stainless steels

Abstract

The effects of nitrogen (680 and 1600 ppm) on the microstructure and electrochemical behaviour of thermally aged type 316LN stainless steels is discussed. Electrochemical potentiokinetic reactivation tests indicated a decrease in chromium depleted regions with increasing nitrogen addition in austenitic stainless steels. Secondary precipitates developed in these alloys during aging at 873 K for 500 h were extracted by an electrochemical method. The precipitates were analysed by X-ray diffraction method. Further TEM investigation on 1600 ppm nitrogen steel was also carried out to help understand the precipitation behaviour. The presence of nitrogen resulted in precipitation of mostly Cr₂ N and χ (chi) phases in the alloy that contained 1600 ppm of nitrogen, in contrast to M₂₃C₆ precipitates in the alloy that contained 680 ppm of nitrogen. The influence of the microstructural evolution and its effect on chromium depletion observed in the present investigation is discussed.     

Microstructural creep damage in welded joints of 316L stainless steel

The present work has been undertaken to study creep damage in welded joints. The complex dual phase microstructure of 316L welds are simulated by manually filling a mould with longitudinally deposited weld beads. Most of the moulded specimens were then aged for 2000 hours at 600°C. High resolution scanning electron microscopy was extensively used to examine the microstructure of the welded material before and after ageing. Columnar grains of austenite constitute a matrix in which thin dendrites of δ-ferrite can be found. The ageing generates the precipitation of carbides, resulting in less transformation in the material. Smooth and notched creep specimens were cut from the mould and tested at 600°C under different stress levels. The creep life of the simulated welded material is shown to be lower than that of the base material. Microstructural observations reveal that creep cavities are preferentially located along the austenite grain boundaries. This analysis of intergranular damage on test specimens is conducted to obtain a predictive damage law which could be used to calculate the lifetime of welded joints.     

Low cycle fatigue resistance of SA533 pressure vessel steels

Abstract

Low cycle fatigue (LCF) tests were conducted on SA533 steels with different levels of sulphur content at room temperature and 300 °C. The fatigue limit shows little or no dependence on sulphur content, but significantly depends on testing temperature. At 300 °C, the fatigue limit is at 0.2% strain amplitude, slightly higher than the 0.1% strain amplitude obtained at room temperature. The fatigue limit of SA533 steel subjected to LCF tests at 300 °C was improved by the combined effects of dynamic strain aging (DSA) and grain size reduction. DSA and grain size reduction increase the steel strength and, accordingly, improve the LCF limit at 300 °C. But, concurrently, the carbide/nitride precipitates in SA533 steel lead to a decrease in steel strength. Grain size reduction and precipitation compete with each other. Grain size reduction is dominant and the net effect is reflected in the increase in hardness value. Fatigue life could be predicted either by means of the strain-life equation or the SWT (Smith, Watson and Topper) parameter for specimens under low cycle fatigue conditions. The strain-life equation for SA533 steel in air is independent of the sulphur content, but significantly dependent on the testing temperature.     

Viscoplastic behaviour of stainless steels AISI 316L and 316H

Summary The theory of viscoplasticity based on total strain and overstress is used in order to simulate the sensitivity to the rate of loading of two commonly used stainless steels, namely AISI 316L and 316H. The consitutive model has been implemented within a transient finite element computer code using a stress update algorithm based on the elastic predictor-return mapping concept. Both monotonic and cyclic loading conditions are considered in one or more space dimensions. Experimental results showing strain-rate dependence at room temperature are reported for both types of steel and used for calibrating the viscoplastic numerical model. An explicit dependence of the nonlinear viscosity function on the strain rate has been obtained and the calibrated model is found to yield results which are in excellent agreement with the experimental data. Finally the calibrated viscoplastic model is applied to predict the response of two representative structures subjected to impulsive loading. The results indicate a significant effect of the rate of loading on the internal stress distribution. With 21 Figures     

Plastic deformation and creep damage evaluations of type 316 austenitic stainless steels by EBSD

The inspection method of plastic and/or creep deformations has been required as the quantitative damage estimation procedure for structural components especially used in electric power plants. In this study, the method using electron backscatter diffraction (EBSD) was applied to the deformation and damage evaluation of austenitic stainless steels strained by tension or compression at room temperature and also tested in creep at high temperature. It was found that the value of Grain Average Misorientation (GAM) which showed the average misorientation for the whole observed area including over several dozen grains, was a very useful parameter for quantifying the microstructural change as either the plastic or creep strain increased. The unique linear correlation was obtained between GAM and plastic strain in tension and compression. For creep damage evaluation, the difference of grain average misorientation from the value of the unstrained specimen (ΔGAM) showed an excellent correlation with the inelastic strain below strain at which the tertiary creep began.     

Stochastic modelling of low-cycle fatigue damage in 316L stainless steel under variable multiaxial loading

In the present study, a stochastic model is developed for the low-cycle fatigue life prediction and reliability assessment of 316L stainless steel under variable multiaxial loading. In the proposed model, fatigue phenomenon is considered as a Markov process, and damage vector and reliability are defined on every plane. Any low-cycle fatigue damage evaluating method can be included in the proposed model. The model enables calculation of statistical reliability and crack initiation direction under variable multiaxial loading, which are generally not available. In the present study, a critical plane method proposed by Kandil et al . ( Metals Soc., London 280, 203–210, 1982) maximum tensile strain range, and von Mises equivalent strain range are used to calculate fatigue damage. When the critical plane method is chosen, the effect of multiple critical planes is also included in the proposed model. Maximum tensile strain and von Mises strain methods are used for the demonstration of the generality of the proposed model. The material properties and the stochastic model parameters are obtained from uniaxial tests only. The stochastic model made of the parameters obtained from the uniaxial tests is applied to the life prediction and reliability assessment of 316L stainless steel under variable multiaxial loading. The predicted results show good aggreement with experimental results.     

Thermomechanical fatigue evaluation and life prediction of 316L(N) stainless steel

An attempt has been made to understand the thermomechanical fatigue (TMF) behaviour of a nitrogen-alloyed type 316L austenitic stainless steel under different temperature domains. Smooth, hollow specimens were subjected to in-phase (IP) and out-of-phase (OP) thermal–mechanical cycling in air under a mechanical strain control mode, at a strain rate of 6.4 × 10^?5 s^?1 and a strain amplitude of ±0.4%. For the sake of comparison, total strain controlled low cycle fatigue (LCF) tests were also performed at the peak temperatures of TMF cycling on similar specimens employing the same strain rate and strain amplitude. Life was found to depend on the thermal/mechanical phasing and temperature. Creep was found to contribute to life reduction in IP tests when the peak temperature of cycling was above 600 °C. A few TMF tests were performed in vacuum in order to assess environmental influence on life. Thermomechanical fatigue cycling led to the development of significant amounts of mean stresses and the stress response was generally higher compared to that of LCF tests at the peak cyclic temperatures. Also, the isothermal tests at the peak temperature of TMF cycling resulted in lower lives compared to those obtained under TMF. An attempt was made to predict the TMF life using the isothermal database and satisfactory predictions were achieved using the Ostergren’s frequency modified damage function (FMDF) approach.     

Dynamic strain aging effect on the fatigue resistance of type 316L stainless steel

Mechanism of dynamic strain aging (DSA) and its effect on the high-temperature low-cycle fatigue resistance in type 316L stainless steel were investigated by carrying out low-cycle fatigue tests in a wide temperature range from 20 to 650 °C with strain rates of 3.2×10⁻⁵–1×10⁻²/s. The regime of DSA was evaluated using the anomalous features of material behavior associated with DSA. The activation energies for each type of serration were about 0.57–0.74 times those for lattice diffusion indicating that a mechanism other than lattice diffusion is involved. It is reasonably concluded that the pipe diffusion of solute atoms along the dislocation core is responsible for DSA. Dynamic strain aging reduced the fatigue resistance by ways of multiple crack initiation, which comes from the DSA-induced inhomogeneity of deformation, and rapid crack propagation due to the DSA-induced hardening.     

Sintered duplex stainless steels from premixes of 316L and 434L powders

Duplex austenite–ferrite stainless steels were prepared from the premixes of 316L and 434L stainless steel atomized powders. Pronounced densification was observed after 1350°C sintering in hydrogen. 316L-60w/o 434L steel composition exhibited maximum transverse rupture strength, while 40 and 60w/o 434L containing compositions showed total immunity in 1N H₂SO₄ even after a exposure time of 360 h. Anodic polarization curves also suggest high-corrosion resistance of those two compositions. Magnetic coercivity decreased with increase in sintering temperature while magnetic saturation follows the reverse trend. Wear resistance of the duplex stainless steels under sliding condition was in between the straight steels.     

Microstructural banding of directed energy deposition-additively manufactured 316L stainless steel

The microstructures of 316L stainless steel created by rapid solidification are investigated by comparing the similar microstructures of individual hatches of directed energy deposition additive manufacturing(DED-AM) and those of single, laser surface-melted tracks formed on a solid plate. High recoil pressure,which is exponentially dependent on the laser beam power density, induces convection of the melt pool,which causes formation of microstructural bands in the as-solidified microstructure. The microstructural bands are associated with changes in the chromium concentration and are a significant component of the inhomogeneous microstructure of DED-AM.     

Non‐destructive evaluation of fatigue damage and fatigue crack initiation in type 316 stainless steel by positron annihilation line‐shape and lifetime analyses

Rotating bending fatigue tests were conducted using type 316 stainless steel. The fatigue tests were periodically terminated, and fatigue damage and fatigue crack initiation were non‐destructively and sequentially evaluated by positron annihilation line‐shape and lifetime analyses. The counter‐jig and anticoincidence methods were used for positron annihilation line‐shape and lifetime analyses, respectively, to enhance the analytical precision. The fatigue crack lengths were monitored by a plastic replication technique, and related to the parameters in both analyses. S‐parameter obtained in the line‐shape analysis increased with increasing fatigue damage, while it was difficult to detect fatigue crack initiation and subsequent small fatigue crack growth. That was because the precision of line‐shape analysis was limited. On the other hand, both fatigue damage and fatigue crack initiation were successfully detected by lifetime analysis. Positron annihilation lifetime also increased with increasing fatigue damage, and lifetime was longer at the notch root with fatigue crack than at the smooth section without crack. It was considered that the precision of lifetime analysis was high enough to detect high dislocation density areas at the fatigue crack tips.     

Relation between tensile properties and microstructure in type 316 stainless steel SA weld metal

Tensile properties of a thick, multipass, submerged-arc (SA) weld-deposited type 316 are investigated by tests at room temperature and at 400 ° C and by microstructural and compositional analyses. The as-deposited metal, which shows a lower yield strength, a comparable ultimate tensile strength and a lower total elongation compared to the (solution-annealed) parent metal, is characterized by systematic variations in tensile properties across its thickness, with the highest strength and the lowest ductility in the weld centre. These variations are related to material variability (mainly changes in dislocation density) within the weld metal due to local dissimilarities in thermal and mechanical histories during welding.     

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.     

Effects of temperature on the low cycle fatigue behaviour of nitrogen alloyed type 316L stainless steel

Strain-controlled low cycle fatigue tests have been conducted in air between 298–873 K to ascertain the influence of temperature on LCF behaviour of nitrogen-alloyed type 316L stainless steel. A strain amplitude of ± 0.60% and a symmetrical triangular waveform at a constant strain rate of 3 × 10⁻³ s⁻¹ were employed for all tests. Crack initiation and propagation modes were evaluated, and the deformation and damage mechanisms which influence the cyclic stress response and fatigue life identified. The cyclic stress response at all temperatures was characterized by an initial hardening to the maximum stress, followed by gradual softening prior to attaining saturation. Temperature dependence of fatigue life showed a maximum in the intermediate temperature range. The drastic reduction in fatigue life at elevated temperatures has been ascribed primarily to the combined influence of dynamic strain ageing effects and oxidation-enhanced crack initiation, while the lower life at room temperature is attributed to detrimental effects associated with deformation-induced martensite.     

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.     

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.     

Microstructural evolution of injection molded gas- and water-atomized 316L stainless steel powder during sintering

The present study investigates the microstructural evolution and densification behavior of water- and gas-atomized 316L stainless steel powder. Dilatometry and quenching studies were conducted to determine the extent of densification and corresponding microstructural changes. Results indicate that water-atomized powder could be sintered to 97% of theoretical density, while gas-atomized powders could be sintered to near-full density. The difference in the densification behavior is examined in terms of the particle morphology, initial green density and the particle chemistry.