The mechanical behaviour of porous austenitic stainless steel fibre structures

The mechanical properties of metal fibre porous structures were studied in the light of their potential application as surface coatings of implants. Stainless steel AISI 316 L fibres with diameters of 50 and 100μm were compacted and sintered. The variation of the modulus of elasticity with density, as obtained in tension, corresponds closely with theoretical models. The ultimate failure of the tensile specimens proceeds through the fibres, and not through the sinter bonds, except at lower densities. Differences in yield strength between 50 and 100 μm fibre tensile specimens are explained on the basis of the onset of plastic deformation of the individual fibres. Upon compression the modulus of elasticity is nearly 10 times smaller than in tension. This result is due to the different deformation patterns of the fibres in compression and tension.     

Cold-worked state and annealing behaviour of austenitic stainless steel

An investigation was made into the structural changes accompanying cold working and annealing treatments in seven austenitic stainless steels. The materials studied included five laboratory alloys and two commercial grades of austenitic stainless steels (types 304 and 316). X-ray line profile analysis showed that the stacking-fault energies of the seven steels ranged from 8 MJ m^–2 to 68 MJ m^–2. Transmission electron microscopy (TEM) was used extensively to characterize the cold-worked and annealed states, Measurements of the resistivity change were performed to characterize the recovery and recrystallization behaviours. The cold-worked structure was found to be related to the stacking-fault energy. Dislocations tended to be arranged in planar arrays and to be confined in the original slip planes in alloys of low stacking-fault energy. Dislocation arrangement was less uniform and more random for steel of high stacking-fault energy. In none of the cases studied was the stacking-fault energy high enough to allow the cross-slip necessary to generate the dislocation cell structure often seen in other metals. Isochronal annealing of the steels reveals a distinguishable stage of resistivity recovery prior to recrystallization, which was attributed to the annihilation of vacancies and removal of carbon from the solid solution. A second stage of resistivity drop (above 500° C) resulted from recrystallization. The temperature for the start of recrystallization was found to be related to stacking-fault energy.     

Mechanical properties of friction stir butt welds of high nitrogen-containing austenitic stainless steel

In this study, the friction stir butt welding of 2-mm-thick high nitrogen-containing stainless steel (HNS; Ni-free austenitic stainless steel containing 1 mass% nitrogen) plates was performed using a load-controlled friction stir welding (FSW) machine with a Si₃N₄-based tool at various welding speeds, i.e., 50 mm/min, 100 mm/min, 200 mm/min and 300 mm/min, and a constant tool rotating speed of 400 rpm. To determine the optimum welding conditions to create reliable HNS FSW joints, the effect of the heat input on the mechanical properties of the HNS FSW joints was studied. The mechanical properties were evaluated by the Vickers hardness test and the tensile strength test. Full-penetrated and defect-free butt welded joints were successfully produced, under all the applied welding conditions. The stir zones consisted of very fine grained structures and showed an increase in the Vickers hardness. These joints also showed a higher tensile strength and yield strength than the base metal. In particular, the FSW welds obtained at a welding speed of 100 mm/min, which showed the best mechanical properties, had a relatively higher Vickers hardness, which indicates a good relationship between the welding parameter (heat input) and the hardness profile due to the microstructure refinements. It was estimated that these welding conditions were optimal, and under these conditions both grain growth and α-phase formation were prevented.     

The effect of pre-strain on tensile behaviour of 316L austenitic stainless steel

ABSTRACT

The effect of pre-strain on tensile behaviour of 316L austenitic stainless steel was investigated, focusing on strain rate sensitivity, temperature sensitivity and strain hardening. Experimental data showed that strain rate sensitivity, temperature sensitivity and strain hardening were weakening with pre-strain. Meanwhile, the variation of microstructure with pre-strain was observed by optical microscopy, scanning electronic microscopy and X-ray diffraction. Then, the mechanical properties of pre-strained material were correlated with the increase in dislocation density and mechanical twinning with pre-strain. Finally, an improved Arrhenius-PS model considering the effect of pre-strain was developed.

This paper is part of a thematic issue on Nuclear Materials.     

The electrochemical behaviour of direct strip casting stainless steel in acid solutions

The electrochemical behaviour of direct strip casting (DSC) 304 stainless steel in 0.1 M H2SO4 and 0.01M HCl solutions was investigated. The DSC 304 stainless steel strips were produced by using either copper-alloy roller or 304 stainless steel roller. The difference in thermal conductivity of different roller materials resulted in a change in the surface microstructure of the DSC strips. Potentiodynamic polarization curves of the DSC 304 stainless steels produced were measured in 0.1M H2SO4 and 0.01M HCl solutions. The results showed that both alloys could passivate in the above solutions. In the HCl solution, the passive potential range of DSC 304 stainless steel with a higher ferrite content prepared by the copper roller was wider than that with a lower ferrite content. Furthermore, the addition of silicon could cause an expansion while the addition of titanium could lead to a shrinkage in the passive range in 0.01M HCl solution. Potential decay tests in 0.1M H2SO4 solutions showed that the reactivation time decreased as the ferrite content was increased. © 1998 Chapman & Hall     

Nanoindentation creep deformation behaviour of high nitrogen nickel-free austenitic stainless steel

Nanoindentation tests of the high nitrogen nickel-free austenitic stainless steel (HNS) were performed with peak load in a wide range of 100–600?mN to investigate the nanoindentation creep deformation behaviours. The results of the nanoindentation creep tests have demonstrated that the load plateaus, creep strain rate and creep stress of the cold-rolled HNS are larger and its creep stress exponent is smaller than the solution-treated HNS. The analysis reveals that the obvious creep deformation behaviour in the cold-rolled HNS arises from the rapidly relaxed dislocation structures in the initial transition regime, while the small creep deformation behaviour of the solution-treatedHNS is mainly attributed to that the stable dislocation structures for the intensive interactions between dislocations.     

Hall-Petch behaviour of 316L austenitic stainless steel at elevated temperatures

Abstract

The plastic deformation behaviour of two different batches (having differences in chemical composition) of 316L austenitic stainless steel has been explored in the 200-800°C temperature range as a function of grain size. The plastic behaviour is correlated with microstructural observations of annealed and deformed samples. The microstructural parameters measured in this study are grain size, grain size and shape distribution, grain aspect ratio, and the distribution of dihedral angles. Hardness measurements were also performed to assess the hardness profile across the grains. The applicability of Hall-Petch relationship was tested in the 200-800°C temperature range. It is observed that the Hall-Petch relationship is applicable in the coarse grain regime (d≥6 μm) and Kocks composite relationship (σ versus d^-1) in the fine grain regime (d≤6 μm) of batch 1 samples in the 200-600^°C temperature range. At 800°C, the Hall-Petch data is widely scattered and the scatter increases with increasing strain. The variation of Hall-Petch parameters and Kocks parameters with strain and temperature are analysed on the basis of changes in the microstructural parameters. The operating deformation mechanisms in different temperature and strain ranges are discussed on the basis of variation of microstructural parameters with strain and temperature.     

Diffusion bonding behaviour of austenitic stainless steel containing titanium and alumina

A study has been conducted to identify the function of titanium in 1 Cr18Ni9Ti steel and the effects of fabrication temperature, pressure, time and other variables on the strengths of diffusion-bonded alumina-1Cr18Ni9Ti. At temperatures of 750 to 1200°C, 1Cr18Ni9Ti steel was successfully bonded to alumina, with a maximum tensile strength of 19MPa. By EPMA titanium segregated to the interface of the joint, in contrast to the failure of bonding of 1Cr18Ni9 steel and alumina under the same conditions. Titanium can reduce alumina with the reactants of TiO and compounds of titanium and aluminium thermochemical calculation. Thus it is indicated that titanium is an important go-between element for bonding metal to alumina.     

Dynamical behaviour and microstructural evolution of a nitrogen-alloyed austenitic stainless steel

The aim of this work is to study the mechanical properties of a nitrogen austenitic stainless steel (Uranus B66) and their relation to its microstructural evolution. Quasi-static (10⁻³ s⁻¹) and quasi-dynamic (1 s⁻¹) compression tests have been carried out with a universal servo-hydraulic testing machine. Dynamic (>10³ s⁻¹) compression tests have been performed on a classical split-Hopkinson bar apparatus. These tests, which cover a wide range of plastic strain, show that the material has a high-strain hardening rate, a good ductility and a great strain rate sensitivity. The temperature sensitivity has been determined over a large range, going from 77 K to 673 K. Transmission electron microscopy (TEM) observations have been conducted in order to correlate the microstructure to the mechanical behaviour. Uranus B66 undergoes basically the same structure evolution during both quasi-static and dynamic compression tests. The plastic deformation is governed initially by planar gliding, followed by mechanical twinning when the dislocation density is saturated.     

Mechanical behaviour of nitrogen-alloyed austenitic stainless steel hardened by warm rolling

This investigation deals with the hardening effect, by warm rolling, of the alloy Uranus B66^®, a nitrogen-alloyed austenitic stainless steel produced by Creusot-Loire Industrie. The warm rolling process leads to substructural changes from the appearance of planar slips at low deformation, the micro-twins formation followed by sequences of their bending, breaking and disappearance at intermediate deformation, and finally to the formation of heavily deformed domains at the highest warm rolling reduction. The mechanical behaviour of the warm rolled Uranus B66, under quasi-static tensile and quasi-static and dynamic compression tests, has been analysed. Warm rolling increases the mechanical resistance to a saturation level and decreases the ductility when compared to that of the as-received material. The dynamic flow stress after warm rolling up to 85% increases to such a level that brittle fracture occurs after small plastic deformation. The origin of the strength saturation is related to the terminal microstructure derived from the warm rolling deformation.     

Fatigue behaviour of friction welded medium carbon steel and austenitic stainless steel dissimilar joints

This paper reports the fatigue behaviour of friction welded medium carbon steel–austenitic stainless steel (MCS–ASS) dissimilar joints. Commercial grade medium carbon steel rods of 12 mm diameter and AISI 304 grade austenitic stainless steel rods of 12 mm diameter were used to fabricate the joints. A constant speed, continuous drive friction welding machine was used to fabricate the joints. Fatigue life of the joints was evaluated conducting the experiments using rotary bending fatigue testing machine (R = −1). Applied stress vs. number of cycles to failure (S–N) curve was plotted for unnotched and notched specimens. Basquin constants, fatigue strength, fatigue notch factor and notch sensitivity factor were evaluated for the dissimilar joints. Fatigue strength of the joints is correlated with microstructure, microhardness and tensile properties of the joints.     

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.     

Precipitation behaviour of a sensitized AISI type 316 austenitic stainless steel in hydrogen

The purpose of this study was to characterize the precipitation behaviour of AISI type 316 steel in hydrogen. The different precipitates (M₂₃C₆, M₆C), the intermetallicχ-phase and the martensitic phase (α′,ε) were determined by using transmission electron microscopy (TEM) and X-ray diffraction techniques. All the specimens were sensitized at 650? C for 24 h. Some samples were carburized up to 2 wt% C. Additions of carbon content decrease the time required for sensitization. Short-term (24 h) exposure of this steel to sensitization temperature results in a complex precipitation reaction of various carbides and intermetallic phases. Hydrogen was introduced by severe cathodic charging at room temperature. This study indicates that by conventional X-ray techniques it is possible to detect those precipitates and their behaviour in a hydrogen environment. The zero shift as observed by X-ray diffraction from the carbides (M₂₃C₆, M₆C) and the intermetallicχ-phase, indicates that those phases absorb far less hydrogen than the austenitic matrix. TEM studies reveal that hydrogen inducesα′ martensite at chromium-depleted grain-boundary zones, near the formation of the carbides.     

Electrochemical behaviour of Mo‐containing austenitic stainless steels in buffer solution

The effect of pH on the electrochemical behavior of austenitic stainless steels in naturally aerated and nitrogen‐deaerated buffer solutions has been studied. Potentiodynamic and ac impedance techniques were used. The results indicate that corrosion current (I_corr) decreases with pH. By examining the impedance data for the seven tested steels under various conditions, it seems that the total resistance (R_T) value is a suitable indication parameter for explaining the role of molybdenum and the other supplementary alloying additions (Ni, N, Cu and Mn) on the passive film behaviors as a function of pH of the forming solutions. As a general trend, the R_T values for all steels tended to be lower in the basic buffer than in the acidic solutions.     

Metadynamic recrystallization of austenitic stainless steel

Interrupted torsion tests were performed in the temperature range of 900–1100°C, strain rate range of 5.0 × 10^–2–5.0 × 10⁰/sec and interpass time range of 0.5–100 seconds to study the characteristics of metadynamic recrystallization (MDRX) for austenitic stainless steel. To compare the MDRX with static recrystallization (SRX), the pass strain was applied above the critical strain (_c) (_c = 2.2 × 10^–3 D^1/2 ₀ Z ^0.089, where Z is Zener-Hollomon parameter, Z = exp((380000 J/mol)/RT) and D ₀ is as-received grain size) to obtain the MDRX during interpass time. It was found that the kinetics of MDRX were dependent of the strain rate and deformation temperature but were nearly independent of the change in pass strain after the peak strain. The time for 50% metadynamic softening, t ₅₀, was determined as follows: t ₅₀ = 1.33 × 10^{–11 –0.41} D ₀ exp((230000 J/mol)/RT) and this calculated value was consistent with the measured value. The Zener-Hollomon parameter was impossible to evaluate the MDRX fraction, because the fractional softening values were different at the same Z values. The new parameter (MDRX parameter) considering deformation temperature, strain rate and interpass time was proposed to evaluate the MDRX fraction. The MDRX-parameter was determined as 3.25 × 10^{–19 0.3} t _i ^0.6 T ¹².     

Creep of austenitic stainless steel welds

Austenitic stainless steels (SS) find extensive application in power, petrochemical and nuclear industries in view of their excellent elevated temperature mechanical properties, corrosion resistance, formability and weldability. However, they are susceptible to hot cracking during fusion welding. To avoid this problem, chemical composition of the welding consumable is generally adjusted to promote primary ferrite mode of solidification and retain about 3 to 10%δ-ferrite in the as-welded condition. The duplex microstructure of the weld metal undergoes transformation to carbides and a variety of intermetallic phases during elevated temperature service and causes deterioration in the mechanical properties. This paper presents a comprehensive review of the current understanding of the solidification microstructures, ageing processes and their influence on the creep behaviour of types 308 and 316 SS weld metals. The effects of varying chemical composition,δ-ferrite content, electrode coating and welding processes on creep strength and ductility are examined. Current trends in the design of welded components for creep application are also discussed.     

Effect of strain rate on quasistatic tensile flow behaviour of solution annealed 304 austenitic stainless steel at room temperature

Room temperature tensile test results of solution annealed 304 stainless steel at strain rates ranging between 5 × 10⁻⁴ and 1 × 10⁻¹ s⁻¹ reveal that with increase in strain rate yield strength increases and tensile strength decreases, both maintaining power–law relationships with strain rate. The decrease in tensile strength with increasing strain rate is attributed to the lesser amount of deformation-induced martensite formation and greater role of thermal softening over work hardening at higher strain rates. Tensile deformation of the steel is found to occur in three stages. The deformation transition strains are found to depend on strain rate in such a manner that Stage-I deformation (planar slip) is favoured at lower strain rate. A continuously decreasing linear function of strain rate sensitivity with true strain has been observed. Reasonably good estimation for the stress exponent relating dislocation velocity and stress has been made. The linear plot of reciprocal of strain rate sensitivity with true strain suggests that after some critical amount of deformation the increased dislocation density in austenite due to the formation of some critical amount of deformation-induced martensite plays important role in carrying out the imposed strain rate.