Metastable Austenitic Stainless Steel AISI Research Articles

Fatigue life evaluation of metastable austenitic stainless steel AISI 347 based on nondestructive testing methods for different environmental conditions

Due to aging nuclear power plants there is an increasing need for methods to evaluate the integrity of components and structures of nuclear engineering. During power plant operation, nuclear materials are exposed to the effects of temperature, corrosion and cyclic loading. These lead to microstructural changes resulting in material degradation, which can be detected by suitable nondestructive testing (NDT) methods. Fatigue testing setups for ambient temperature, distilled water and 300 °C were designed to represent relevant environmental conditions and instrumented with in-situ measurements of temperature, electric, micromagnetic and electrochemical NDT parameters. Constant amplitude and strain increase tests with specimens of metastable austenitic stainless steel AISI 347 (X6CrNiNb18-10, 1.4550) typically used for pipe components were conducted in total strain control. Data obtained were employed in short-time evaluation procedure StrainLife to generate fatigue life data. To establish an understanding between microstructure evolution and NDT data, scanning electron microscopic methods such as electron backscatter diffraction and energy dispersive X-ray spectroscopy were used. Electrochemical data yields fatigue life results as good as conventionally used parameters like stress and strain. These findings could enable usage of presented parameters for mobile or even online testing systems, since accessibility depending on application case can be much improved.

Nano-twinning and uncommon α´-martensite formation as a result of very high cycle fatigue of metastable austenitic stainless steel at 573 K

Microstructural changes were investigated regarding the existence of the true endurance limit of a cyclic loaded metastable austenitic stainless steel AISI 347 in the very high cycle regime at 573 K. The fatigue tests were performed in stress control with a stress load ration R = -1 using a servo-hydraulic testing system with a load frequency of 980 Hz. Local cyclic plastic deformation and slip bands formation were observed even in the specimens that achieved the endurance limit at 5 × 108 cycles. Moreover, in these specimens further microstructural effects like nano-twins and α´-martensite formation were detected by transmission electron microscopy. While nano-twin formation takes place in the bulk material, the α´-martensite nucleates at the specimen's surface. Therefore, the α´-martensitic transformation is not the main mechanism responsible for the true endurance limit observed in the investigated AISI 347 at 573 K.

Cyclic deformation behavior of metastable austenitic stainless steel AISI 347 in the VHCF regime at ambient temperature and 300 °C

Very high cycle fatigue (VHCF) tests on metastable austenitic stainless steels using an ultrasonic fatigue testing (USFT) system are challenging due to the transient material behavior and associated pronounced self-heating effects. Because the conventional measurement of stress-strain hysteresis is not possible in USFT, the cyclic deformation behavior can’t be described in a conventional manner. Hence, an energy-based approach is proposed for the characterization of cyclic deformation behavior of austenitic stainless steels in the VHCF regime at ambient temperature (AT) and elevated temperature. Therefore, in-situ dissipated energy and temperature measurements were performed. At AT both values underwent a change during cyclic loading, while at 300 °C only the dissipated energy changed. The investigated metastable austenitic stainless steel AISI 347 (X6CrNiNb1810, 1.4550) showed cyclic softening in the high cycle fatigue (HCF) regime (Nf < 107) at both testing temperatures, which led to specimen failure. In the VHCF regime, cyclic hardening was observed, which resulted in reaching the limiting number of load cycles Nl = 2 × 109 without specimen failure. This change in the cyclic deformation behavior by a small reduction of the stress amplitude led to a horizontal course of the Woehler curve in the VHCF regime and resulted in the true endurance limit for the investigated material. At AT, ferromagnetic α′-martensite was detected in all fatigued specimens using magnetic measurements. At 300 °C only specimens which reached the limiting number of cycles without failure exhibited a small amount of α′-martensite.

CRYOGENIC MILLING OF METASTABLE AUSTENITIC STAINLESS STEEL AISI 347

The metastable austenitic stainless steel AISI 347 offers the possibility to induce a phase transformation from γ-austenite to ε- and α’-martensite when machining. This knowledge is well understood during cryogenic turning and was already applied to improve the surface morphology of metastable austenitic steel. However, the potential of this in-process hardening method is so far limited to rotationally symmetrical components. The aim of this study is to investigate deformation induced phase transformation during cryogenic milling, aiming at an improved surface morphology and at the resulting beneficial surface properties of the workpiece for parts with complex geometries.

Model approaches for closed-loop property control for flow forming

The implementation of control systems in metal forming processes improves product quality and productivity. By controlling workpiece properties during the process, beneficial effects caused by forming can be exploited and integrated in the product design. The overall goal of this investigation is to produce tailored tubular parts with a defined locally graded microstructure by means of reverse flow forming. For this purpose, the proposed system aims to control both the desired geometry of the workpiece and additionally the formation of strain-induced α′-martensite content in the metastable austenitic stainless steel AISI 304 L. The paper introduces an overall control scheme, a geometry model for describing the process and changes in the dimensions of the workpiece, as well as a material model for the process-induced formation of martensite, providing equations based on empirical data. Moreover, measurement systems providing a closed feedback loop are presented, including a novel softsensor for in-situ measurements of the martensite content.

Surface layer hardening of metastable austenitic steel – Comparison of shot peening and cryogenic turning

In this paper, the effect of shot peening and cryogenic turning on the surface morphology of the metastable austenitic stainless steel AISI 347 was investigated. In the shot peening process, the coverage and the Almen intensity, which is related to the kinetic energy of the beads, were varied. During cryogenic turning, the feed rate and the cutting edge radius were varied. The manufactured workpieces were characterized by X-ray diffraction regarding the phase fractions, the residual stresses and the full width at half maximum. The microhardness in the hardened surface layer was measured to compare the hardening effect of the processes. Furthermore, the surface topography was also characterized. The novelty of the research is the direct comparison of the two methods with identical workpieces (same batch) and identical analytics. It was found that shot peening generally leads to a more pronounced surface layer hardening, while cryogenic turning allows the hardening to be realized in a shorter process chain and also leads to a better surface topography. For both hardening processes it was demonstrated how the surface morphology can be modified by adjusting the process parameter.

Characterization of the subsurface properties of metastable austenitic stainless steel AISI 347 manufactured in a two-step turning process

At high mechanical loads and below martensite deformation temperature, metastable austenitic stainless steels undergo a deformation-induced phase transformation from γ-austenite to ε- and/or α’-martensite. During cryogenic turning, this can be exploited in order to realize subsurface hardening, thus avoiding a separate hardening process. An increase of the cutting edge radius, the chamfer angle and the feed rate leads to higher passive forces, consequently a more pronounced phase transformation and ultimately to a higher microhardness. However, increasing these input variables also results in a significant increase in surface roughness.In order to eliminate this conflict of objectives between subsurface properties and surface topography, a two-step turning process is proposed. In the first process step a pronounced phase transformation and high plastic deformation of retained austenite by means of heavily chamfered tools, very high feed rates and precooling are realized in the workpiece subsurface. In the second process step, the pronounced roughness peaks are removed, while maintaining the desired subsurface properties achieved in the first step and even increasing the phase fraction of deformation-induced α’-martensite at and near the surface.In the presented study, the surface and subsurface of workpieces manufactured applying this approach were examined. The residual stresses and the phase fraction of γ-austenite, deformation-induced ε-martensite and α’-martensite after the first and second process step were measured by means of x-ray diffraction. Furthermore, the microhardness was measured in order to quantify the mechanical properties of the hardened subsurface. With the two-step turning process it was possible to generate 87 vol.-% of α’-martensite, while the workpieces manufactured in a one-step cryogenic turning process had a maximum phase fraction of 9 vol.-% α’-martensite. Consequently, the microhardness in the subsurface zone of the workpieces was higher, while also ensuring a good surface topography.

Delayed cracking behavior of a meta-stable austenitic stainless steel under bending condition

The delayed cracking behavior of a meta-stable austenitic stainless steel AISI 301 under bending condition has been investigated at different temperatures, hydrogen contents and external holding forces. The results reveal that the investigated material with an initial hydrogen content of 1 ppm has good bendability at the temperature range between −20 °C and room temperature, which is not susceptible to delayed cracking in atmospheric condition. When the material is pre-charged with 50 ppm hydrogen, the material still shows good bendability. However, it is susceptible to delayed cracking under an external holding force during interrupted bending test. By the measurement of martensitic transformation, the simulation of the stress/strain distributions in the bending specimens and the characterization of fracture surfaces, the effects of hydrogen, stress state and external holding force on delayed cracking behavior have been assessed.

Fatigue Behavior of Metastable Austenitic Stainless Steels in LCF, HCF and VHCF Regimes at Ambient and Elevated Temperatures

Corrosion resistance has been the main scope of the development in high-alloyed low carbon austenitic stainless steels. However, the chemical composition influences not only the passivity but also significantly affects their metastability and, consequently, the transformation as well as the cyclic deformation behavior. In technical applications, the austenitic stainless steels undergo fatigue in low cycle fatigue (LCF), high cycle fatigue (HCF), and very high cycle fatigue (VHCF) regime at room and elevated temperatures. In this context, the paper focuses on fatigue and transformation behavior at ambient temperature and 300 °C of two batches of metastable austenitic stainless steel AISI 347 in the whole fatigue regime from LCF to VHCF. Fatigue tests were performed on two types of testing machines: (i) servohydraulic and (ii) ultrasonic with frequencies: at (i) 0.01 Hz (LCF), 5 and 20 Hz (HCF) and 980 Hz (VHCF); and at (ii) with 20 kHz (VHCF). The results show the significant influence of chemical composition and temperature of deformation induced ´-martensite formation and cyclic deformation behavior. Furthermore, a “true” fatigue limit of investigated metastable austenitic stainless steel AISI 347 was identified including the VHCF regime at ambient temperature and elevated temperatures.

The evolution of mechanical properties of AISI 301 as a result of phase reversion heat treatment, experiment and modeling

Laser heat treatments of metastable austenitic stainless steel AISI 301 are presented aiming to elucidate the relation between heat treatment, transformation and mechanical properties after heat treatment. It is assumed that the observed phase reversion of martensite to austenite is due to a diffusional transformation mechanism governed by nucleation and growth leading to submicron grains. Based on this assumption it is demonstrated that the reverse transformation can be successfully predicted by the proposed model. Subsequently the effect of the heat treatment on the hardness is reviewed. It is shown that the proposed hardness-model, in combination with the proposed isothermal transformation model, is in agreement with the observed behavior. Amongst others it is successfully predicted that the isothermal transformation precedes the recrystallization of the retained austenite and that the post-heat treatment grain size has a large effect on the behavior through the Hall-Petch effect.

Microstructural characterization of cyclic deformation behavior of metastable austenitic stainless steel AISI 347 with different surface morphology

Abstract

Influence of surface condition due to laser beam cutting on the fatigue behavior of metastable austenitic stainless steel AISI 304

Austenitic steels are among the stainless steels the most common used type of material to produce sheet metal parts which can be manufactured by laser beam cutting in a useful manner. However, the use of this technique to manufacture structural thick parts is restricted due to the creation of macroscopic defects and the lack of reliable fatigue strength data. In this context, the fatigue behavior of sheet-like samples made of metastable austenitic stainless steel AISI 304 cut by laser beam is discussed in this paper. The analyses showed that laser beam cutting creates three kinds of macroscopic defects, which are a pronounced relief-like structure along the cut surface, burr in the underside of the cut edge and pores in the interface between the recast layer and base material or inside the recast layer. As a consequence, the fatigue strength of parts cut by laser beam is 40% lower in comparison to specimens in a macroscopically quasi defect-free state. An evaluation of the fatigue results based on comprehensive fractographic analyses allows to explain the reasons for distinct differences between the aforementioned influence factors. The most significant reduction of fatigue life is attributed to the notch effect of the burr, followed by the notch effect created by pores, while the influence of the surface relief is of minor significance.

Influence of loading frequency and role of surface micro-defects on fatigue behavior of metastable austenitic stainless steel AISI 304

High frequency fatigue testing always provokes questions about whether the testing conditions are representative for the real application conditions. Therefore, any influences coming from the testing procedure must be known and understood before relying on the fatigue data based on high frequency testing for component design and validation phases. For this reason and because metastable austenitic steels are well known for their strain rate sensitivity (Müller-Bollenhagen, 2011; Sorich, 2016), the steel AISI 304 and the role of surface micro-defects produced by laser beam cutting were analyzed regarding the influence of load frequency on the cyclic response and fatigue behavior, and the findings of the investigation are thoroughly discussed in this paper. Fatigue tests were performed at load frequencies of 100Hz and 1000Hz using two resonance pulsation test systems, as well as by means of a servo-hydraulic test machine at 1Hz and 50Hz. All fatigue experiments were performed at tensile-tensile loading condition (R=0.1). The cyclic deformation behavior was characterized based on the evaluation of stress-strain hysteresis loops and temperature measurements. The deformation-induced phase transformation from γ-austenite to α′-martensite was globally and locally evaluated by means of magneto-inductive measurements and EBSD analysis, respectively. Furthermore, for the first time it was possible to compare test results generated by a 1000Hz resonance pulsation test system with results from a conventional resonance test stand cycling at around 100Hz. The analyses showed that higher amounts of α′-martensite and lower plastic strain amplitudes are observed when the cyclic experiments are carried out at lower frequency, promoting higher fatigue strengths. Nevertheless, the influence of test frequency for specimens containing surface micro-defects is dominant in the low cycle fatigue (LCF) regime while in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) range the fatigue life determining parameter is the severity of the micro-notches present along the laser cut surface.

Influence of surface morphology on fatigue behavior of metastable austenitic stainless steel AISI 347 at ambient temperature and 300°C

The effect of surface modification by cryogenic turning on fatigue behavior of metastable austenitic stainless steel AISI 347 was investigated in stress-controlled fatigue tests at ambient temperature (AT) and 300 °C in air. Five different surface morphologies were manufactured by the variation of turning parameters – with and without cryogenic CO2 snow cooling and feed velocity as well as by the application of polishing for reference surfaces with a very small surface roughness. For a comprehensive characterization of the surface and near surface morphology, X-ray diffraction investigations were performed. Three phases (γ-austenite, α-martensite and ε-martensite) were detected in the near-surface microstructure after cryogenic turning while after turning without cryogenic cooling the only microstructural constituent was γ-austenite. Moreover, residual stress state, micro hardness and surface roughness play an important role in surface morphology. The experimental data on the cyclic deformation behavior and stress-strain response of all surface morphologies are reported. Reference specimens with purely austenitic surface microstructure show the highest plastic strain amplitude during cyclic loading at both AT and 300°C. At elevated temperature these specimens achieved the shortest fatigue life. Martensitic surface layers induced by cryogenic turning result in the reduction of plastic strain amplitude during cyclic loading and significantly enhance fatigue life at both tested temperatures.

Constitutive modeling of hot horming of austenitic stainless steel 316LN by accounting for recrystallization in the dislocation evolution

Hot compression test data taken from Zhang [1] of metastable austenitic stainless steel AISI 316LN over a range of strain rates and temperatures shows typical dynamic recovery and recrystallization behavior. It is proposed to model this behavior by incorporating not only the hardening and recovery into the Bergstrom dislocation evolution equation, but also the recrystallization. It is shown that the initial mechanical response before recrystallization can be accurately represented by assuming that the mean free path evolves as the microstructure evolves from homogeneously spaced dislocations to cell-pattern. Results show that this novel continuum mechanical model can predict the observed behavior, showing a good match to the experimental data and capturing the transition from recrystallization to (almost) no recrystallization.

Modeling and simulation of temperature-dependent cyclic plastic deformation of austenitic stainless steels at the VHCF limit

The exploration of fatigue mechanisms in the VHCF regime is gaining importance since many components have to withstand a very high number of loading cycles due to high frequency or long product life. In this regime, the period of fatigue crack initiation and thus the localization of plastic deformation play an important role. The material that was investigated in this study is the metastable austenitic stainless steel AISI 304 in the initially purely austenitic condition. The experimental investigations during quasi-isothermal fatigue tests revealed that a moderate increase of temperature from room temperature up to 150°C led to a reduced VHCF strength. At both temperatures the 304 grade still undergoes a pronounced localization of plastic deformation in shear bands accompanied by a deformation-induced martensitic phase transformation from the γ-austenite to the α’-martensite during VHCF loading. In the present study, the experimental study is extended by modeling and simulation of the relevant temperature-dependent VHCF deformation mechanisms in order to provide a more profound understanding of the observed cyclic deformation. For this purpose, two-dimensional (2-D) morphologies of microstructures are modeled in the mesoscopic scale by the use of the boundary element method (BEM), and cyclic plastic deformation is considered by certain mechanisms defined in a simulation model. It describes the localization of plastic deformation in shear bands taking into account the formation, plastic sliding deformation and cyclic slip irreversibility of each shear band. The deformation-induced martensitic phase transformation is represented by introducing martensitic nuclei into the modeled microstructure depending on the plastic deformation in shear bands. The influence of temperature is incorporated into the simulation model by the use of empirical data and a kinetic model, each related to the tensile test. The simulated cyclic deformation and phase transformation is compared to experimental observations and allows for assessing the individual influence of deformation mechanisms on the temperature-dependent fatigue behavior. Finally, a temperature-independent ‘limit curve’ for the accumulated irreversible plastic sliding deformation regarding failure in the VHCF regime is proposed.

Deformation mechanisms induced under high cycle fatigue tests in a metastable austenitic stainless steel

Advanced techniques were used to study the deformation mechanisms induced by fatigue tests in a metastable austenitic stainless steel AISI 301LN. Observations by Atomic Force Microscopy were carried out to study the evolution of a pre-existing martensite platelet at increasing number of cycles. The sub-superficial deformation mechanisms of the austenitic grains were studied considering the cross-section microstructure obtained by Focused Ion Beam and analysed by Scanning Electron Microscopy and Transmission Electron Microscopy. The results revealed no deformation surrounding the pre-existing martensitic platelet during fatigue tests, only the growth on height was observed. Martensite formation was associated with shear bands on austenite, mainly in the {111} plane, and with the activation of the other intersecting austenite {111}〈110〉 slip system. Furthermore, transmission electron microscopy results showed that the nucleation of ε-martensite follows a two stages phase transformation (γfcc→εhcp→α'bcc).

Elastic strain evolution and ε-martensite formation in individual austenite grains during in situ loading of a metastable stainless steel

The (hcp) ε-martensite formation and the elastic strain evolution of individual (fcc) austenite grains in metastable austenitic stainless steel AISI 301 has been investigated during in situ tensile loading up to 5% applied strain. The experiment was conducted using high-energy X-rays and the 3DXRD technique, enabling studies of individual grains embedded in the bulk of the steel. Out of the 47 probed austenite grains, one could be coupled with the formation of ε-martensite, using the reported orientation relationship between the two phases. The formation of ε-martensite occurred in the austenite grain with the highest Schmid factor for the active {111}<12¯1> slip system.

Evolution of Residual Strains in Metastable Austenitic Stainless Steels and the Accompanying Strain Induced Martensitic Transformation

The deformation behavior of metastable austenitic stainless steel AISI 301, suffering different initial cold rolling reduction, has been investigated during uniaxial tensile loading. In situ highenergy x-ray diffraction was employed to characterize the residual strain evolution and the strain induced martensitic transformation. Moreover, the 3DXRD technique was employed to characterize the deformation behavior of individual austenite grains during elastic and early plastic deformation. The cold rolling reduction was found to induce compressive residual strains in the austenite along rolling direction and balancing tensile residual strains in the ά-martensite. The opposite residual strain state was found in the transverse direction. The residual strain states of five individual austenite grains in the bulk of a sample suffering 2% cold rolling reduction was found to be divergent. The difference among the grains, considering both the residual strains and the evolution of these, could not be solely explained by elastic and plastic anisotropy. The strain states of the five austenite grains are also a consequence of the local neighborhood.

LOW‐TEMPERATURE AUTOFRETTAGE: AN IMPROVED TECHNIQUE TO ENHANCE THE FATIGUE RESISTANCE OF THICK‐WALLED TUBES AGAINST PULSATING INTERNAL PRESSURE

Abstract— It is shown that autofrettage at low temperatures is superior to autofrettage at room temperature in enhancing the fatigue resistance of thick‐walled tubes against pulsating internal pressure. The physical reason is based on the well‐known temperature dependence of the mechanical behaviour of metals and alloys which generally exhibit an enhancement of both the yield stress and strain hardening behaviour at lower temperatures. As a consequence, significantly larger compressive residual hoop stresses can be introduced during pressurization at low temperatures than at room temperature. Experimental data obtained on thick‐walled tubes of the metastable austenitic stainless steel AISI 304 L which were subjected to pulsating internal pressure at room temperature after autofrettage at temperatures between‐110°C and room temperature are presented. These data demonstrate convincingly the advantages offered by low‐temperature autofrettage in enhancing both the fatigue life in the finite‐life region and the fatigue endurance limit in comparison with autofrettage at room temperature. In conclusion, some specific materials requirements for optimum low‐temperature autofrettage performance are discussed.