Austenitic Stainless Steel Specimens Research Articles

Hydrogen-Dislocation Interaction in Austenitic Stainless Steel Studied with Mechanical Loss Spectroscopy

Mechanical loss of pre-strained and hydrogen-charged specimens of 316L austenitic stainless steel has been measured by the free-decay method using an inverted torsion pendulum at frequencies around 1 Hz. It is found that in addition to known hydrogen Snoek-like relaxation process observed at about 240 K a new peak of mechanical loss appears around 270 K. The peak has a relaxation origin with enthalpy close to 0.78 eV and pre-exponential factor of relaxation time is found to be about 10-12.5s. Effects of the pre-strain and parameters of electrochemical hydrogen charging on the hydrogen peak in deformed austenitic stainless steels are measured. The obtained results are discussed in terms of the Snoek-Köster mechanism and specifics of hydrogen atomic distribution in a multi-component substitutional alloy of austenitic stainless steel. Based on the analysis the binding energy of hydrogen to dislocation in 316L steel is evaluated to be about 0.16 eV.

Research on Stress Corrosion Behavior of AISI304 Stainless Steel Enhanced by Ultrasonic Peening Treatment

Surface-strengthening treatment was performed on the welded joints and base material specimens of AISI304 austenitic stainless steel using ultrasonic peening treatment. Stress-corrosion test in a 42% boiling magnesium chloride solution were conducted with treated and untreated specimens. Four-point loaded specimens were employed in the stress-corrosion test. Residual stress in the surface layer of the specimens was analyzed with an x-ray stress analyzer before the stress corrosion test. The microstructure of the specimens was observed by a scanning electron microscope after the stress corrosion test. The state of stress in the surface layer of the specimens affected by the ultrasonic peening treatment was analyzed. The stress corrosion cracking behavior was studied by observing the corrosion fractographs. The results showed that the stress corrosion resistance of the welded joint and base material specimens was remarkably improved because of the compressive residual stress employed by the ultrasonic peening treatment. Fracture modes were of the same kinds in the treated and untreated specimens.

Surface modification of austenitic stainless steel with plasma nitriding for biomedical applications

In the present study, plasma nitriding of AISI type 303 austenitic stainless steel (SS) specimens was performed using a microwave system. The nitrided layers were characterized by performing scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and a Vickers microhardness test. The antibacterial activities of the nitrided layers were evaluated. XRD and TEM showed that a single γ N phase was formed by plasma nitriding at the plasma power of 700 W and 450 °C. The analytical results demonstrated that the hardness of type 303 specimens could be enhanced by plasma nitriding because of the formation of the γ N phase. A bacterial test also demonstrated that the nitrided layer exhibited excellent antibacterial properties.

Strain Rate Effect on Material Behavior of TRIP‐Steel/Zirconia Honeycomb Structures

AbstractA pure TRIP‐steel alloy and a novel zirconia reinforced TRIP‐steel matrix composite were implemented in a 2D square‐celled honeycomb structure fabricated by a paste extrusion method, respectively. In terms of a series of compression tests in out‐of‐plane loading direction the buckling and the pronounced strain hardening behavior of the honeycomb structures are described with regard to different material compositions and varied nominal strain rates. Both the compressive flow behavior and the microstructure evolution in the crushed zones are controlled by the rate of formation of strain‐induced martensite and the ceramic particle/steel matrix interactions. The insertion of magnesia partially‐stabilized zirconia (Mg‐PSZ) particles in the austenitic steel matrix cause an increased yield strength and higher compression stresses up to certain deformations degrees. The limited ductility of the composite materials is a consequence of the rearrangement and fracture of zirconia particles initiating cracks and shear bands during deformation. Consistently, the visible strain rate effects on the mechanical responses of the honeycomb structures are similar to AISI 304L austenitic stainless steel specimens in the form of compact rods. However, at high local strain rates generated in drop weight impact tests a micro‐inertia factor support the failure behavior of the cellular structures.

A Feasibility Study on Increasing Surface Hardness of Austenitic Stainless Steels by Pack Co-Deposition of N and Cr

This study is an attempt to codeposit N and Cr into the surface of austenitic stainless steels by pack cementation process to simultaneously increase their surface hardness and corrosion resistance. The pack powders were prepared using Cr2N powder as a source of both N and Cr, NH4Cl as activator and Al2O3 as inert filler. Specimens of the AISI204 austenitic stainless steel were treated in the 2 wt% NH4Cl activated 15Cr2N-85Al2O3 (wt%) pack at 1100 °C for different times. It was demonstrated that a top Cr2N layer with a Cr enriched zone underneath can be formed on the steel surface via the vapour phase generated in the activated powder pack. The effect of adding Cr powder into the pack powders on the surface layer formation and on the hardness profile at the cross-section of the specimen surface was also investigated. Hardness values of more than 1800 HV were obtained at the outermost surface of the treated specimen.

Low temperature nitriding, nitrocarburising and carburising of AISI 316L austenitic stainless steel

AbstractAISI 316L grade ASTM F138 austenitic stainless steel specimens were low temperature plasma nitrided (LTPN), nitrocarburised (LTPNC) and carburised (LTPC) using different gas mixtures. Different expanded austenite layers formed after each thermochemical treatment. LTPN and LTPCN led to formation of nitrogen supersaturated expanded austenite (γN). After LTPN, a second carbon expanded austenite (γC) layer was formed beneath the nitrogen expanded austenite layer (γN). LTPC led to formation of a carbon supersaturated expanded austenite (γC). Scanning electron microscopy, XRD and microhardness were used to characterise the expanded austenite layers formed on the surface of the specimens. Different mechanisms of formation and growth of the layers are pointed out. XRD results show that the lattice parameter of nitrogen expanded austenite (γN) is higher than that calculated for carbon expanded austenite γC. As a consequence, the lattice expansion Δa/a for the nitrogen rich (γN) phase is higher than the one...

Model of viscoplasticity with parallel deformation mechanisms. Part 2. Creep and ratcheting

A model of viscoplastic deformation for strengthening materials is considered within which metal deformation is governed by two independent (parallel) mechanisms. Each of these mechanisms corresponds to anisotropic creep theory with linear strengthening into which an isotropic strengthening parameter is introduced common for both mechanisms. The concept of surface flow is not used. In the second part of the article, unilateral strain accumulation (ratcheting) is considered in structures at elevated temperature and with cyclic loading. The possibilities of the model are demonstrated with description of experiments on austenitic stainless steel specimens with different programs for a change in load.

ICONE19-43979 The Effect of Mechanical Loading on Residual Stress Induced by Laser Peening

The stress corrosion cracking(SCC) of austenitic stainless steel and nickel-based alloy is a primary concern for stable operation in Boiling Water Reactors(BWRs) and Pressurized Water Reactors (PWRs). Laser peening is one of the promising technologies for prevention of SCC initiation to induce compressive residual stress near the surface region. However, the compressive residual stress by laser peening is likely affected due to applied stress. In this study, the effect of mechanical loading on the compressive residual stress by laser peening was investigated. The specimens of low carbon austenitic stainless steel (Type316L), nickel-based alloy (Alloy600) and their weldments were prepared, and several levels of cyclic loading were applied to their specimens. The surface residual stress at surface was measured by X-ray diffraction method before and after loading. The relaxation behaviour of the compressive surface residual stress by laser peening was evaluated at the base metal and heat affected zone (HAZ) of the weldment. It was confirmed that compressive surface residual stress remained after stress loading which corresponds to 0.2% proof stress, and that the relaxation of the surface residual stress remarkably observed after initial loading during cyclic loading test. These relaxation behaviour were similar in Type316L, Alloy600 and their HAZ.

Atmospheric corrosion resistance of duplex stainless steels: results of a field exposure program

Stainless steels are often used instead of painted carbon steels or weathering steels in architecture or construction for aesthetic reasons and/or when maintenance cost reduction is considered. With duplex stainless steels the investment cost can also be significantly reduced using their better yield and tensile stresses. Then the choice of material depends on the environment corrosivity. Usual accelerated corrosion tests as salt spray type may help to rank stainless steels but not to select a cost effective material for an outdoor location. Considering the lack of available data, Industeel launched five years ago an extensive field exposure program with the aim to collect representative data. Six locations were chosen in North America and Europe to cover a wide range of humidity, temperature, pH, chloride ions and pollutant contents. In this way more than 200 coupons with and without weld were exposed in marine, industrial, urban and rural atmospheres or combinations of them. This paper shows some of the first results obtained on duplex (lean duplex included), austenitic, martensitic and ferrito-martensitic stainless steel specimens after several years of exposure. Correlation between environmental parameters, alloy chemical composition, microstructure and localized corrosion resistance are discussed.

Hydrogen Effect against Hydrogen Embrittlement

The well-known term “hydrogen embrittlement” (HE) expresses undesirable effects due to hydrogen such as loss of ductility, decreased fracture toughness, and degradation of fatigue properties of metals. However, this article shows, surprisingly, that hydrogen can have an effect against HE. A dramatic phenomenon was found in which charging a supersaturated level of hydrogen into specimens of austenitic stainless steels of types 304 and 316L drastically improved the fatigue crack growth resistance, rather than accelerating fatigue crack growth rates. Although this mysterious phenomenon has not previously been observed in the history of HE research, its mechanism can be understood as an interaction between hydrogen and dislocations. Hydrogen can play two roles in terms of dislocation mobility: pinning (or dragging) and enhancement of mobility. Competition between these two roles determines whether the resulting phenomenon is damaging or, unexpectedly, desirable. This finding will, not only be the crucial key factor to elucidate the mechanism of HE, but also be a trigger to review all existing theories on HE in which hydrogen is regarded as a dangerous culprit.

On the Notch Effect in Low Temperature Carburized Stainless Steel under Fatigue

The present paper describes the fatigue behaviour of carburized notched AISI 316 austenitic stainless steel specimens. Rotary four point bending fatigue tests have been performed using carburized smooth specimens with two different values of surface rugosity and notched specimen with two different stress concentration factors Kt, of 3.55 and 6.50 and the effects of carburizing on fatigue strength and notch sensitivity were discussed. Results show a general improvement of the fatigue life due to the treatment for all the series with an apparent notch sensitivity lower than one in the case of blunt notches due to secondary effects that were singled out.

High temperature oxidation of plasma and HVOF thermal sprayed CoNiCrAlY coatings in simulated gas turbine and furnace environments

CoNiCrAlY powders were thermal sprayed using plasma (atmospheric plasma spraying) and high velocity oxyfuel procedures onto AISI 310 austenitic stainless steel specimens in order to study the cyclic oxidation behaviour of these coatings in air and in a simulated gas turbine environment (10% oxygen) at 800 and 1000°C. The oxidation behaviour and hardness evolution of both coatings were significantly different, and the observed weight gain for the plasma spray coating was always much greater due to the presence of substantial open interconnected porosity. In both cases, protective alumina (Al2O3) and spinel CoAl2O4 layers were created.

Controlled generation of ferromagnetic martensite from paramagnetic austenite in AISI 316L austenitic stainless steel

The strain-induced austenite (γ) to martensite (α′) transformation in AISI 316L austenitic stainless steel, either in powders or bulk specimens, has been investigated. The phase transformation is accomplished using either ball-milling processes (in powders)—dynamic approach—or by uniaxial compression procedures (in bulk specimens)—quasi-static approach. Remarkably, an increase in the loading rate causes opposite effects in each case: (i) it increases the amount of transformed α′ in ball-milling procedures, but (ii) it decreases the amount of α′ in pressed samples. Both the microstructural changes (e.g., crystallite size refinement, microstrains, or type of stacking faults) in the parent γ phase and the role of the concomitant temperature rise during deformation seem to be responsible for these opposite trends. Furthermore, the results show the correlation between the γ → α′ phase transformation and the development of magnetism and enhanced hardness.

Microstructural analysis and oxidation protection behavior of Cr–Ni surface coatings of 347 stainless steel by rapid mirror scanning of a high power CO2 laser

A surface coating technique that uses a 1 kW CO2 laser beam actuated by two high-speed raster-scanning mirrors was utilized to selectively consolidate predeposited powder coatings consisting of 50%Cr-50% Ni (wt %) over the surface of 347 austenitic stainless steel specimens. The oxidation resistance of the coated samples was evaluated by heating them in air at 750 °C for up to 300 h. The migration behavior of the coating elements was studied. Comprehensive characterization of the samples was performed using scanning electron microscopy, energy dispersive spectrochemical analysis, and optical metallography. Surface protective coatings produced by this technique demonstrated a strong metallurgical bond with homogeneous dilution, low porosity, and uniform thickness. Metallographical analysis showed migration of coating species toward the bulk preferentially by means of grain boundary diffusion as a result of high-temperature exposure. Preliminary results from this study demonstrate that the surface modification by high-speed selective raster-scanning of a high-power laser beam is a versatile and expedient technique that could be used for applications that require large surface area coverage for oxidation protection.

Experimental setup for fully coupled kinematic and thermal measurements at the microstructure scale of an AISI 316L steel

In polycrystalline metallic materials, above a certain threshold, mechanical deformations induce plastic strains at the grain scale. The development of plasticity also triggers a thermal dissipation due to local mechanical irreversibilities. These phenomena conduct to a heterogeneous state of deformation and temperature which has never been simultaneously observed at the micrometric scale. In this paper, an original experimental setup is presented in order to obtain fully coupled measurements. The objective is to get at the same time strain and temperature fields in the same zone at the microstructure scale of an AISI 316L austenitic stainless steel specimen during a tensile test. The fully coupled measurements underscore the relationship between the local plastic and thermal heterogeneities during the first stages of the deformation until a more general plastic state of the specimen. These results may also be used to perform energy balances at the micrometric scale.

Peculiarity of Notch Effect on High Cycle Fatigue Strength of a Stainless Steel SUS316NG at 300°C

Rotating bending fatigue tests were carried out for notched specimens of a SUS316NG austenitic stainless steel at 300°C. Nevertheless any non-propagating crack was not recognized at the root of the notch for specimens endured at 108 stress cycles, the fatigue strength at 108 cycles did not decrease continuously with increasing the stress concentration factor. An area of the root of notch hardened during fatigue test at 300°C by dynamic strain aging. The hardening behavior became remarkably with increasing the stress concentration factor. Effects of the stress concentration factor and hardening behavior on the fatigue strength cancel each other, and then dependency of the fatigue strength on the stress concentration factor becomes insensitive.

1247 SUS304鋼の軸ねじりPPおよびCC試験寿命におよぼす位相角の影響(G03-8 材料力学(8)疲労・強度1,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

The present paper investigates the effect of phase angle on the PP and CC test lives of SUS304 steel. The PP and CC type combined push-pull and cyclic torsion tests have been conducted on thin-walled tubular specimens of SUS304 austenitic stainless steel at 1023 K in air under in-phase and 30 to 90 deg out-of-phase conditions. The experimentally obtained lives are compared with those predicted by the new parameters, that have been derived based on both the Tresca and Mises type effective inelastic strain ranges starting from the inelastic-strain-based parameter previously proposed for 90 deg out-of-phase condition.

SUS304 ステンレス鋼のスパッタエッチングによって形成した円錐状およびリング状突起物層の変形挙動

Austenitic stainless steel (SUS304) specimens were sputter-etched by using argon ions, and the formation process of protrusions on the surface was investigated by changing the sputter etching time. In addition, tensile and bending tests of the specimens were carried out to examine whether the delamination between the protrusion and matrix occurs or not. When the sputter etching time is 0.9 ks, fine conical protrusions with diameter smaller than 2 μm are formed relatively homogeneously on the surface. When the sputter etching time is 1.8 ks, they grow to more than 5 μm. In some cases, the holes and cracks are formed on the wall of protrusions. When the sputter etching time is 3.6 ks, the protrusions grow further and occupy whole surface, and finally they collapse to form ring-shaped protrusions. The tensile test of the specimens with conical protrusions shows that, even at a fracture strain of 0.53, the shape of the cones is almost maintained and the plastic deformation occurs mainly in the region around them without delamination of the cones. Also in the bending test, the similar behavior to the tensile test is observed. The tensile test of the specimen with ring-shaped protrusions also shows that the deformation does not largely occur in the protrusion but occurs in the region outside of them. The reasons for the above characteristics seem to be that the outer shell of the protrusion with higher Cr content is strong and the protrusion grows from interior of specimen keeping high coherency to matrix.

Characterisation of microstructural evolution during thermomechanical processing of 15Cr–15Ni–2·2Mo–Ti modified austenitic stainless steel (alloy D9) using ultrasonic measurements

Ultrasonic attenuation measurements have been used for imaging the variations in the grain size in 15Cr–15Ni–2·2Mo–Ti modified (D9) austenitic stainless steel specimens thermo mechanically processed at 1273 K for different strain amplitudes in the range of 0·1 to 0·5. Ultrasonic attenuation measurements could identify various deformation zones such as macroscopic intense shear zones, moderate deformation zones and dead metal zones in the specimen strained to 0·5 exhibiting dynamic recrystallisation. The initiation of the recrystallisation and skew in the loading during the thermo mechanical processing could also be identified using ultrasonic attenuation measurements.

Preparation of fatigue specimens with controlled surface characteristics

The response surface methodology (RSM) coupled with central composite design (CCD) have been employed to design fatigue specimens of austenitic stainless steels with controlled surface characteristics, for the purpose of investigating the effects of machining-induced residual stresses and roughness on fatigue behaviour. Simple cylindrical specimens having various surface characteristics were first produced by changing the final cutting conditions (spindle speed, feed rate and cutting depth) of the lathe. The fitting of the response surface model for the data was studied by analysis of variance (ANOVA). The response surface model employed in the analysis adequately represented the largest peak to valley height (roughness R y ) and the axial residual stress, although a good fit was not always achieved on the responses of the mean spacing of adjacent local peaks (roughness S) and the microhardness. Fatigue specimens were then prepared for several surface conditions, selected by interpolation of the response surface model, and the predictions were successfully compared with experiments. This approach to prepare fatigue specimens of austenitic stainless steels can form the basis of a systematic study of the effect of surface preparation parameters on fatigue behaviour.