Tensile behaviour of 316LN stainless steel at elevated temperatures

Abstract

The tensile deformation behaviour of 316LN stainless steel was investigated from ambient temperature up to 1000°C. The hardness and microstructure of area near tensile fracture were characterised. The results show that the engineering stress increases smoothly with engineering strain when the tensile temperature is at 400°C or below, while the plastic deformation stage displays a serrated/jerky flow at 600°C. At tensile temperatures of 800°C or above, the plastic deformation stage is dramatically prolonged. The deformation mechanisms of 316LN stainless steel are proposed to be sliding and twinning at 400°C or below, tangle dislocations due to cross-slipping at 600°C, dynamic recovery at 700°C, and dynamic recrystallisation at 800°C or above. The finding provides useful guidelines for the processing and service of 316LN stainless steel components at high temperatures.     

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.     

Tensile behaviour of duplex stainless steel at low temperatures

Abstract

The tensile behaviour of the ferrite and austenite phases of Fe–22Cr–5Ni (wt-%) duplex stainless steel containing a maximum of 17·2% austenite was investigated in the temperature range 65–298 K. The results indicate that mechanical twinning occurred in the testing temperature range, and that austenite impeded the growth of twinning. Mechanical twinning in ferrite was well decorated with a ‘dislocation shell’, and the density of dislocations at the coherent twin boundary and within a twin was much higher than in the matrix above the ductile–brittle transition temperature (DBTT). This supported the occurrence of slip localisation next to coherent twin boundaries. Dislocations in the material with no austenite tested below the DBTT were characterised by coplanar slip dislocation on the { 110} plane, and both coplanar slip on { 110} and cross-slip dislocations were observed above the DBTT. Dislocation in ferrite was negligibly affected by the presence of austenitic particles. Strain induced martensite transformation occurred in austenitic particles at or below 220 K, and the characteristics of the transformation were essentially similar to those in type 304 stainless steel. The DBTT of the material was lowered from ～140 to 110 K in the presence of austenite, independent of the volume fraction of austenite. This suggests that the decrease in the DBTT of the material was mainly due to austenite scavenging carbon and other interstitial elements from the ferritic matrix. The fracture of the material at low temperatures was primarily controlled by the fracture of twin boundaries in ferrite.     

Impression creep deformation behaviour of 316LN stainless steel

The impression creep deformation behaviour of 316LN SS was investigated from microstructure, substructure, microhardness and profilometry studies of the creep deformed region. Impression creep tests were conducted on 316LN SS in the temperature range of 923–973?K, at different punching stresses in the range of 472–760?MPa. The impression creep deformation was characterised by a hemispherically shaped plastic zone which developed around the indentation. The study revealed the distinct regions under the punch undergoing deformation to different extents. The deformation was found to occur predominently on (111) planes. The dislocations in the highly deformed region were well dispersed in the matrix. The size of the plastic zone was estimated to be ～1·5 times the diameter of the indenter based on the microhardness and profilometry studies. The critical spacing to be maintained between the adjacent indentations was estimated to be >5 times the diameter of indenter.     

Viscoplastic behaviour of stainless steel AISI 316L under cyclic loading conditions

Summary Experimental results on stainless steel AISI 316L under cyclic loading conditions, at room temperature, showing dependence of the consolidation stress on strain rate are obtained and used for the calibration of a viscoplastic numerical model based on total strain and overstress. An explicit dependence for the evolution of the nonlinear viscosity function on cycle number and strain rate on the one hand, and of the equilibrium stress-strain diagram origin shift at loading reversal on cycle number, have been obtained and the calibrated model is found to yield results which are in very good agreement with the experimental data.     

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.     

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.     

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     

Effects of sensitisation-induced martensitic transformation on the tensile behaviour of 304 austenitic stainless steel

The effects of sensitisation-induced martensitic transformation on the tensile behaviour of 304 austenitic stainless steel have been investigated. Yield strength is reduced by sensitisation, but ultimate tensile strength is nearly unaffected. Strain-hardening behaviour is changed by sensitisation, too. Although sensitisation may induce martensite formation near grain boundary, twin boundary, and austenite/martensite interface, the sensitisation-induced martensite does not exert a great influence on tensile behaviour in the 304 steel. In the unsensitised condition, martensitic transformation in the steel bulk induced by prior deformation and liquid-nitrogen immersion also does not change strain-hardening behaviour in the present steel.     

Low-temperature carburised AISI 316L austenitic stainless steel: Wear and corrosion behaviour

Low temperature carburising (LTC) is a thermochemical treatment designed so as to achieve a good combination of wear and corrosion resistance in stainless and duplex steels. In this work, the influence of LTC on both corrosion and dry sliding behaviour of AISI 316L was investigated. LTC significantly enhanced surface hardness, due to the formation of the carbon-supersaturated S-phase. Consequently, the wear behaviour (evaluated against different countermaterials) improved, due to increased resistance to plastic deformation, as well as to decreased tendency towards adhesion. In order to evaluate the corrosion behaviour, electrochemical measurements were performed both in conventional environments and in reference conditions for the food industry. The results showed a significantly improved corrosion resistance in chloride environments, where the formation of a C-rich surface layer ennobles the treated steel, even though pitting corrosion was observed at very high anodic potentials. Conversely, the treated steel showed comparable (in acetic acid) or worse (in a sanitising solution) behaviour than the untreated one. In sulphuric acid the treated steel did not passivate, but it corroded at a limiting current density much lower that the critical current density for AISI 316L passivation.     

An investigation on boriding kinetics of AISI 316 stainless steel

Boronizing was performed by using a solid medium of Ekabor powders at 1073, 1148 and 1223 K for 2, 4 and 8 h. After boronizing, the major dominant phase was found to be Fe₂B and the minors were CrB and Ni₂B. Boride coating resulted in smooth and dense feature confirmed by optical and SEM. The thickness of boride layer varied from 7 to 87 μm depending on the process time and temperature. Boride layer has a hardness of over 1700 HVN, while the substrate's hardness was about 180 HVN. The growth kinetics of boride layer was found to obey a parabolic rate demonstrating a solid diffusion limited process. The kinetic rates for different process times were plotted by using Arrhenius equation. From this measurement, the activation energy of boride growth for this study was determined as 199 kJ/mol. In addition, the possibility of predicting the iso-thickness of boride layer variation was studied and an empirical relationship between process parameters and boride layer thickness was established. EDS studies showed that Cr concentrated in the coating layer and Ni and Fe concentrated in the substrate.     

Acoustic emission behaviour during stage II fatigue crack growth in an AISI type 316 austenitic stainless steel

Acoustic emission (AE) behaviour during fatigue crack growth (FCG) in a ductile AISI type 316 austenitic stainless steel is reported. The two substages in the stage II Paris regime of FCG could be distinguished by a change in the rate of acoustic activity with increase in crack growth rate. The transition point in the cumulative ringdown count plot coincides with that in the da/dn plot. The AE activity increases with increase in ΔK during stage IIa and decreases during stage IIb. The major source of AE during stage IIa is found to be the plastic deformation within the cyclic plastic zone (CPZ) as compared to the phenomena such as monotonic plastic zone (MPZ) expansion, ductile crack growth, crack closure, etc. The increase in AE activity with increase in ΔK during stage IIa is attributed to the increase in the size of the CPZ which is generated and developed only under plane strain conditions. The decrease in AE activity during stage IIb is attributed to the decrease in the size of the CPZ under plane stress condition. The high acoustic activity during the substage IIa is attributed to irreversible cyclic plasticity with extensive multiplication and rearrangement of dislocations taking place within the CPZ. The AE activity is found to strongly depend on the optimum combination of the volume of the CPZ, average plastic strain range and the number of cycles before each crack extension. Based on this, an empirical relationship between the cumulative RDC and ΔK has been proposed and is found to agree well with experimentally observed values.     

Thermal ageing effects in AISI type 316 stainless steel

The effects of long-term thermal ageing on the microstructure of AlSl type 316 stainless steel are described. The microstructure of the aged steel is related to the reported embrittlement of the steel. The precipitation found is complex and the embrittlement appears to result, in part, from specimen preparation techniques interacting with the microstructure. The dominant creep deformation mechanism under service conditions appears to be diffusion creep.     

Microstructural characterization of AISI 431 martensitic stainless steel laser-deposited coatings

High cooling rates during laser cladding of stainless steels may alter the microstructure and phase constitution of the claddings and consequently change their functional properties. In this research, solidification structures and solid state phase transformation products in single and multi layer AISI 431 martensitic stainless steel coatings deposited by laser cladding at different processing speeds are investigated by optical microscopy, Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), orientation imaging microscopy (OIM), ternary phase diagram, Schaeffler and TTT diagrams. The results of this study show how partitionless solidification and higher solidification rates alter the microstructure and phase constitution of martensitic stainless steel laser deposited coatings. In addition, it is shown that while different cladding speeds have no effect on austenite–martensite orientation relationship in the coatings, increasing the cladding speed has resulted in a reduction of hardness in deposited coatings which is in contrast to the common idea about obtaining higher hardness values at higher cladding speeds.