Microstructure and impact behaviour of ASTM A105/AISI 304L friction weldments

Abstract

Instrumented impact testing has been used to investigate the influence of the microstructure of the heat affected zone (HAZ) on the impact fracture behaviour of ASTM A105/AISI 304L friction weldments. The friction layer in the HAZ has been found to consist of two different parts. In the A105 side, a mechanically mixed layer made of bainite containing 304L 'protrusions' is present. The hardness of the bainite was found to increase as the friction pressure was decreased. In the 304L side, the friction layer was made of a thick shear band, formed by thermoplastic instability during welding. The impact fracture toughness was found to depend on both the crack nucleation and propagation stages, whose characteristics were related to the dynamic fracture toughness of the friction layers.     

Stellite 21 coatings on AISI 410 martensitic stainless steel by gas tungsten arc welding

Abstract

Degradation of AISI 410 martensitic stainless steel, a typical alloy for many applications such as steam turbine blade, could impair its efficiency and lifetime. To overcome this problem, critical surfaces could be modified by weld cladding via gas tungsten arc welding technique. In the present research, a comparative study of Stellite 21 weld overlays deposited in three different thicknesses, i.e. dilutions, at various preheat and post-weld heat treatment temperatures on the surface of AISI 410 martensitic stainless steel, has been made. The surface of coatings has been examined to reveal their microstructures, phase characterisation and mechanical properties using XRD, microhardness tester and metallographic techniques. The results showed that the deposition of Stellite 21 coating on AISI 410 martensitic stainless steel improved its corrosion resistance. Moreover, the volumetric dilution had a considerable effect on the hardness, microstructure and electrochemical corrosion behaviour of Stellite 21 weld overlays.     

Development of porous 316L stainless steel with controllable microcellular features using selective laser melting

Abstract

The regularly shaped parallel pore gas armed (GASAR) stainless steel porous material with a homogeneous size distribution of unusually micrometer scaled pores (2 μm in average) was successfully prepared using selective laser melting process, by adding 0·10 wt-% gas generating materials in the form of H₃BO₃ and KBF₄. The adjustment of pore morphology, pore direction, and porosity was realised by changing material combinations (such as the content of additive materials) and processing conditions (such as the scan speed of laser beam).     

Microstructure evolution in AISI 304 stainless steel during near rapid directional solidification

Abstract

The microstructure evolution of near rapidly directionally solidified AISI 304 stainless steel was investigated in the present paper. It is found that the microstructure consists of δ ferrite dendrites with developed sidebranches and interdendritic austenite (γ) under the temperature gradient (G) of 20 K mm^–1 and growth rate (V) of 1·0 mm s^–1. Coupled growth microstructures of thin lamellar ferrite and austenite begin to form at a higher growth rate of 2·0 mm s^–1. The formation mechanism of the coupled microstructures is analysed based on the nucleation and constitutional undercooling criterion that the δ ferrite phase and austenite phase form alternately before the steady state growth of each phase is reached due to larger undercooling. With further increase of the growth rate up to 3·0 mm s^–1, the morphology of the δ ferrite transforms from lathy to cellular.     

Corrosion fatigue characterisation of sintered multiphase stainless steels

Abstract

Multiphase stainless steels are produced for their attractive properties of mechanical strength and corrosion resistance relative to their austenite–ferrite structure. The manufacture of these steel by powder metallurgy technology presents some advantages in terms of low cost and formability of complex shapes. Mechanical and corrosion resistances are not at the level of the wrought steels due to their porous nature. In this work the fatigue and corrosion fatigue behaviour of some sintered steels obtained by sintering from 316L and 434L base powders has been studied for characterisation and comparison. The sintered steels were fatigue tested in two different environments: air and NaCl aqueous solution. The tests performed indicate that the chemical and microstructral composition has no great influence on fatigue behaviour in comparison with the manufacturing technology (sintering). This is most evident in the more aggressive environment, like seawater, in which these steels could be advantageously used. The analysis of fracture surfaces using SEM microscopy shows a peculiar crack propagation characterised by cleavage, stress intensification due to porosity, and features of localised ductility on sintering necks and base powder particles.     

Deformation twinning in AISI 316L austenitic stainless steel: role of strain and strain path

Abstract

AISI 316L austenitic stainless steel was deformed at different strain and strain paths. The twin boundaries in the deformed microstructure had two possible origins: decay of original annealing twins and generation of deformation twins. Assuming that rotations of grains, specifically grains on both sides of a twin boundary, are responsible for the twin decay, a simple model was proposed to bring out the domain of relative twin generation. A biaxial strain path, in general, was associated with strong twin generation – an association or dependency linked to the texture estimated values of Taylor factor. Formation of strain induced martensite was also observed to be strain and strain path dependent and was more in biaxial strain path.     

Effect of vapour phase hydrogen peroxide,as a decontaminant for civil aviation applications,on microstructure,tensile properties and corrosion resistance of 2024 and 7075 age hardenable aluminium alloys and 304 austenitic stainless steel

Abstract

A response to the chemical or biological contamination of aircraft requires the use of a suitable decontaminant. Among possible chemical decontaminants, vapour phase hydrogen peroxide appears to be a likely candidate in terms of a combination of efficacy, low environmental impact and potential for materials compatibility. The present paper examines the effect of hydrogen peroxide, both in the vapour phase and as a liquid concentrate on two common structural materials used in aviation, namely 2024 and 7075 age hardenable aluminium alloys and on 304 austenitic stainless steel, the latter as employed in galley and lavatory surfaces. The present paper characterises both the effects of hydrogen peroxide on the microstructure of the materials and the impact that decontamination has on the tensile properties and corrosion resistance of these materials. Microstructural effects are both relatively small in magnitude and confined to a region immediately beside the exposed surface. No systematic effect is found on either the tensile properties or the post-exposure corrosion resistance of the three alloys examined. These observations are encouraging in terms of the use of vapour phase hydrogen peroxide for decontamination applications.     

Assessment and modelling of isothermal forging of intermetallic compounds. Part 3 – Ni3Al

Abstract

An alloy based on Ni₃(Al, Cr), IC­264, has been isothermally forged in the temperature range 950–1100°C and strain rate range 3×10^-4 to 3.5×10^-2 s^-1. Stress–strain curves show flow softening for most forging conditions, suggesting dynamic recrystallisation, as does microstructural examination. Deformation modelling has been carried out using a simple constitutive relationship.     

Prediction of embrittlement during aging of nuclear grade AISI 304 stainless steel TIG welds

Abstract

Simulated weldments of AISI grade 304 stainless steel having a ferrite content of 4–6% with three levels of nitrogen ( 0·03, 0·08, and 0·11 wt-%) were prepared using a modified elemental implant technique. From these weldments, subsize Charpy impact specimens were prepared and subjected to aging treatment at different temperatures, 623–748 K, and for different times, 1000–5000 h. Impact toughness curves for these aged samples were generated by testing at various temperatures from 77 K to 300 K. From the impact curves the upper shelf energy (USE) and lower shelf energy (LSE) were determined. It was observed that both USE and LSE decreased with aging time at all temperatures. Nitrogen seems to offer a beneficial effect as far as impact toughness is concerned, as both USE and LSE values increased with increasing nitrogen content. The worst aging conditions were identified as 748 K, 2000 h at the lowest nitrogen level of 0·03 wt-%. An empirical relation connecting the aging temperature, aging time, and nitrogen content to the LSE was developed, which can be used to predict the time for embrittlement at a given nitrogen level and aging temperature.     

The thermal conductivity of AISI 304L stainless steel

A compilation and critical analysis of the thermal conductivity () of AISI 304 stainless steel (SS) between 100 and 1707 K has been given in the literature. The author represented his recommended values of by an inflection in the A versus temperature relationship between 300 and 500 K. Because a physical mechanism had not been identified that would produce such a temperature dependence in of 304 SS, interest was generated in the possible existence of an as yet undiscovered phenomenon that might cause such an inflection. Consequently, experimental verification of the inflection was sought. The present paper presents recent measurements of , the electrical resistivity, and the absolute Seebeck coefficient of 304L SS from 300 to 1000 K and of the thermal diffusivity () from 297 to 423 K. The values computed from the a measurements were within ± 1.6% of the directly measured An inflection was not observed in the temperature dependence of between 300 and 500 K. After careful evaluation and because a physical mechanism still has not been identified which would produce such an inflection, the authors conclude that the inflection in the vs T relationship reported in the literature was caused by the data analysis technique.     

Effect of temperature and strain rate on tensile flow and work hardening behaviour of a Ti modified austenitic stainless steel

Abstract

The tensile flow stress data for a 15Cr - 15Ni - 2.2Mo - Ti modified austenitic stainless steel in the temperature range 300 - 1023 K and in the strain rate range 6.3 × 10^-5- 1.3 × 10^-2 s^-1 was analysed in terms of the Ludwigson and Voce equations. It was found that the Ludwigson equation described the flow behaviour adequately up to the test temperature of 923 K, whereas the Voce equation could be employed over the full temperature range. The peaks/ plateaus observed in the variation of these parameters as a function of temperature and strain rate in the intermediate temperature range have been identified as one of the manifestations of dynamic strain aging (DSA). Also the variation of these parameters with temperature and strain rate could clearly bring out the different domains of DSA observed in this alloy. The work hardening analysis of the flow stress data revealed that, in the DSA regime, the onset of stage III hardening is athermal.     

The influences of sulphur and phosphorus additions on the creep cavitation characteristics in type 304 stainless steels

The effects of impurities on creep cavitation characteristics in type 304 stainless steels with and without additions of sulphur and/or phosphorus have been studied using four experimental heats. Over a limited range of stress at 1000 K, the change in size distribution of creep cavities and carbide precipitates, and the level of impurity segregation with the amount of addition have been investigated. It is found that phosphorus accelerates the nucleation of creep cavities, but retards the growth of them. However, sulphur had little effect on both nucleation and growth of the cavities. It has also been found that ageing prior to testing inhibits creep cavitation. The effects of impurities on cavitation are analysed and discussed from the viewpoint of the change in the segregation of impurities and the precipitation of carbides due to impurity additions.     

Fracture failure analysis of AISI 304L stainless steel shaft

Fracture failure analysis of an agitator shaft in a large vessel is investigated in the present work. This analysis methodology focused on fracture surface examination and finite element method (FEM) simulation using Abaqus software for stress analysis. The results show that the steel shaft failed due to inadequate fillet radius size and more importantly marking defects originated during machining on the shaft. In addition, after visual investigation of the fracture surface, it is concluded that fracture occurred due to torsional–bending fatigue during operation.     

Experimental study of hydrogen embrittlement on AISI 304 stainless steels and Rayleigh wave characterization

Many failures due to hydrogen embrittlement or hydrogen damage are widely reported in oil and refinery industry. Despite many ultrasonic testing methods have been developed to assess hydrogen embrittlement, they are applied well to serious hydrogen attack instead of earlier degradation. This paper aims to characterize nascent hydrogen embrittlement of AISI 304 austenitic stainless steels under cathodic hydrogenation using Rayleigh wave. After cathodic hydrogen charging of AISI 304 stainless steel, XRD and metallographic examination show that martensite transformation occurs within the subsurface region of the specimens. Microhardness testing indicates that hydrogen leads to hardening of the material. It is found that Rayleigh wave are better to inspect local degradation than bulk waves. Rayleigh wave velocity of 5 MHz and 10 MHz decreases significantly with cathodic charging time, while longitudinal wave velocity changes not. Acoustic velocity change is due to elastic modulus reduction resulting from hydrogen-induced phase transformation in the subsurface region.     

Effects of hot deformation and accelerated cooling on microstructural evolution of low carbon microalloyed steels

Abstract

Using a Gleeble 1500 hot simulator, the effects of hot deformation parameters and accelerated cooling conditions on the microstructural characteristics of low carbon microalloyed steels were investigated by means of compression tests. It was found that the grain refinement effect of single pass reduction in the recrystallisation or unrecrystallisation temperature ranges is weaker than that of two pass reduction in the recrystallisation and unrecrystallisation temperature ranges. However, four pass deformation in the recrystallisation and unrecrystallisation temperature ranges could result in rather fine grained microstructures and, when coupled with moderately high cooling rate, partially acicular ferrite microstructure could be obtained. With the increase of cooling rate, the microstructure becomes finer and the content of acicular ferrite increases. Under similar deformation and cooling conditions, the specimens with relatively high carbon content have more refined microstructures.     

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     

Influence of laser surface alloying with chromium and nickel on corrosion resistance of type 304L stainless steel

Abstract

The influence of laser surface alloying (LSA) with Cr and Cr + Ni on the corrosion behaviour of type 304L stainless steel (SS) was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) in chloride (0·5M NaCl) and acidic (1 N H₂SO₄) media. Surface alloying was carried out by laser cladding type 304L SS substrate with premixed powders of AISI type 316L SS and the desired alloying elements. The results indicated that Cr surface alloyed specimen exhibited a duplex (γ + α) microstructure with Cr content of ～24 wt-%, whereas Cr + Ni surface alloyed specimen was associated with austenitic microstructure with Cr and Ni contents of ～22 wt-% each. The potentiodynamic polarisation results in chloride solution indicated that LSA with Cr + Ni considerably enhanced the pitting corrosion resistance compared with LSA with Cr alone. In acidic media, such beneficial effects were not observed. Electrochemical impedance spectroscopy results showed an increase in semicircle arc for both chloride and acidic media for both Cr and Cr + Ni clad samples indicating improvement in the oxide film stability compared with untreated specimen. The polarisation resistance was higher and capacitance values of the laser clad specimen were lower than those in the untreated specimen. The microstructural changes and compositional variations produced by LSA are correlated to the corrosion behaviour.     

Characterization of failure modes for different welding processes of AISI/SAE 304 stainless steels

Weld joints manufactured with a welding electrode type 308L and by three different arc welding processes shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and flux cored arc welding (FCAW) in a AISI/SAE 304 were studied in order to compare the failure mechanisms associated with their mechanical and microstructural properties. Chemical compositions were analyzed by optical emission spectroscopy and the ferrite numbers (FN) of the welds were also identified. Relevant microstructural characteristics of the different processes were analyzed by microscopy techniques. Finally, fatigue tests were performed to study the variations in the mechanical properties of each process and to analyze their most probable failure modes by means of a fractographic study, in which the characteristic morphologies of each one (nucleation, propagation, final fracture) were identified by means of optical stereoscopy and scanning electron microscopy (SEM). Three different fracture modes were found at the welding joints that showed correlations with microstructural changes produced during the welding process. The first failure mode displayed that the nucleation of the crack was at the weld root. The second failure mode was generated at the heat affected zone (HAZ), where the crack nucleated due to a variation in the grain size produced by the process and then further propagated through the edge of the weld. The third failure mode appeared due to the presence of exogenous inclusions generated by the welding process, which acted as stress concentrators in the weld and produce the initiation and further propagation of the crack. Lastly, some welding processes presented a combination of the previous failure modes and consequently multiple sites of crack nucleation.     

Effect of Hf additions on phase transformation,microstructural stability,and hardness of nanocrystalline 304L stainless steels synthesized by mechanical alloying

304L stainless steels with Hf additions were nanostructured by mechanical alloying (MA) and annealed at temperatures up to 1100 °C. The results showed that face-centered cubic (fcc) phase in 304L transformed to body-centered cubic (bcc) phase during MA. The in-situ studies revealed that bcc-to-fcc phase transformation completed after 105 min annealing at 900 °C for 304L, whereas Hf addition increased the required time and temperature for the complete transformation. The grain size of 304L stainless steel was ~10 nm after MA and remained ~167 and ~293 nm after annealing at 900 and 1100 °C, respectively, with Hf addition in comparison to 960 nm average grain size of base 304L stainless steel after annealing at 900 °C. The hardness of 304L increased from ~200 HV to 408 HV after MA and remained 329 HV after annealing at 1100 °C with Hf addition as opposed to 195 HV hardness of 304L.     

Fabrication of nanostructure in inner-surface of AISI 304 stainless steel pipe with surface plastic deformation

In the present investigation, a pipe inner-surface grinding (PISG) technique was developed to fabricate nanostructure in the inner-surface of an austenitic 304 stainless steel pipe. PISG was performed by high speed shearing with hard sphere tips, leading to gradient distribution of strain, strain rate and strain gradient along depth. Nano-austenite with an average boundary spacing of 20?nm was generated, followed by deformation microstructure characterized by shear bands, multi- and uni-directional twins and planar dislocation arrays. Deformation induced grain refinement of austenitic 304 stainless steel with low stacking fault energy (SFE) covering 4–5 order’s magnitude of length scales toward nanometer regime was unified.