Microstructural and surface texture analysis due to machining in Super Austenitic Stainless Steel

Inferior surface quality is a significant problem faced by machinist. The purpose of this study is to present a surface texture analysis undertaken as part of machinability assessment of Super Austenitic Stainless Steel alloy-AL6XN. The surface texture analysis includes measuring the surface roughness and investigating the microstructural behaviour of the machined surfaces. Eight milling trials were conducted using combination of cutting parameters under wet machining. An optical profilometer (non-contact), was used to evaluate the surface texture at three positions. The surface texture was represented using the parameter, average surface roughness. Scanning Electron Microscope was utilised to inspect the machined surface microstructure and co relate with the surface roughness results. Results showed that maximum roughness values recorded at the three positions in the longitudinal direction (perpendicular to the machining grooves) were 1.21 μm (trial 1), 1.63 μm (trial 6) and 1.68 μm (trial 7) respectively whereas the roughness values were greatly reduced in the lateral direction. Also, results showed that the feed rate parameter significantly influences the roughness values compared to the other cutting parameters. The microstructure of the machined surfaces was distorted by the existence of cracks, deformed edges and bands and wear deposition due to machining process.

Impacts of wear and geometry response of the cutting tool on machinability of super austenitic stainless steel

This paper presents a study of tool wear and geometry response whenmachinability tests were applied under milling operations onthe Super Austenitic Stainless Steel alloy AL-6XN. Eight milling trials were executed under two cutting speeds, two feed rates, andtwo depths of cuts. Cutting edge profile measurements were performed to reveal response of cutting edge geometry to the cuttingparameters and wear. A scanning electron microscope (SEM) was used to inspect the cutting edges. Results showed the presenceof various types of wear such as adhesion wear and abrasion wear on the tool rake and flank faces. Adhesion wear represents theformation of the built-up edge, crater wear, and chipping, whereas abrasion wear represents flank wear.Thecommonly formed wearwas crater wear. Therefore, the optimum tool life among the executed cutting trails was identified according to minimum lengthand depth of the crater wear.The profile measurements showed the formation of new geometries for the worn cutting edges due toadhesion and abrasion wear and the cutting parameters.The formation of the built-up edge was observed on the rake face of thecutting tool. The microstructure of the built-up edge was investigated using SEM. The built-up edge was found to have the austeniteshear lamellar structure which is identical to the formed shear lamellae of the produced chip.

FABRICATION, CHARACTERIZATION, AND IRRADIATION OF AN AUSTENITIC OXIDE DISPERSION STRENGTHENED STEEL SUITED FOR NEXT GENERATION NUCLEAR APPLICATIONS

As nuclear energy systems become more advanced, the materials encompassing them need to perform at higher temperatures for longer periods of time. In this Master’s thesis we experiment with an oxide dispersion strengthened (ODS) austenitic steel that has been recently developed. ODS materials have a small concentration of nano oxide particles dispersed in their matrix, and typically have higher strength and better extreme temperature creep resistance characteristics than ordinary steels. However, no ODS materials have ever been installed in a commercial power reactor to date. Being a newer research material, there are many unanswered phenomena that need to be addressed regarding the performance under irradiation. Furthermore, due to the ODS material traditionally needing to follow a powder metallurgy fabrication route, there are many processing parameters that need to be optimized before achieving a nuclear grade material specification. In this Master’s thesis we explore the development of a novel ODS processing technology conducted in Beijing, China, to produce solutionized bulk ODS samples with ~97% theoretical density. This is done using relatively low temperatures and ultra high pressure (UHP) equipment, to compact the mechanically alloyed (MA) steel powder into bulk samples without any thermal phase change influence or oxide precipitation. By having solutionized bulk ODS samples, transmission electron microscopy (TEM) observation of nano oxide precipitation within the steel material can be studied by applying post heat treatments. These types of samples will be very useful to the science and engineering community, to answer questions regarding material powder compacting, oxide synthesis, and performance. Subsequent analysis performed at Queen’s University included X-ray diffraction (XRD) and inductively coupled plasma optical emission spectrometry (ICP-OES). Additional TEM in-situ 1MeV Kr2+ irradiation experiments coupled with energy dispersive X-ray (EDX) techniques, were also performed on large (200nm+) non-stoichiometric oxides embedded within the austenite steel grains, in an attempt to quantify the elemental compositional changes during high temperature (520oC) heavy ion irradiation.

Improvement of the prediction accuracy and efficiency of hot strength of austenitic steels with optimised ANN training schemes

The hot strength of austenitic steels of different carbon contents was modelled using an artificial neural network (ANN) model with optimum training data. As training data employed in a traditional neural network model were randomly selected from experimental data, they were not representative and the prediction accuracy and efficiency were therefore significantly affected. In this work, only representatively experimental data were used for training and during the procedure, one tenth of the training data extracted from experiment were used for testing the training model and terminating the modelling. The effects of the carbon con tent on flow stress, peak strains and peak stresses observed from the experiment for both training and test data were accurately represented with the ANN scheme reported in this work.

New insights into the mechanism of low-temperature active-screen plasma nitriding of austenitic stainless steel

Low-temperature active-screen plasma nitriding is an effective surface engineering technology to improve the wear and corrosion resistance of austenitic stainless steel through the formation of expanded austenite. The material sputtered from the active screen and redeposited on the specimens has been suggested to play an important role in the nitriding mechanism involved. This paper reports a patterned deposition layer, which is in correlation with the grain orientation of polycrystalline specimens. This has provided new insights into the nitriding mechanism. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Dry sliding wear of active screen plasma carburised austenitic stainless steel

Low temperature diffusion treatments with nitrogen and carbon have been widely used to increase the tribological performance of austenitic stainless steels. These processes produce a layer of supersaturated austenite, usually called expanded austenite or S-phase, which exhibits good corrosion and wear resistance. The novel active screen technology is said to provide benefits over the conventional DC plasma technology. The improvements result from the reduction in the electric potential applied to the treated components, and the elimination of such defects and processing instabilities as edge effects, hollow cathode effects and arcing. In this study, AISI 316 coupon samples were plasma carburised in DC and active screen plasma furnaces. The respective layers of carbon expanded austenite were characterised and their tribological performance was studied and compared. Detailed post-test examinations included SEM observations of the wear tracks and of the wear debris, EDX mapping of the wear track, EBSD crystal orientation mapping of the cross sections of the wear tracks, and cross-sectional TEM. Based on the results of wear tests and post-test examinations, the wear mechanisms involved are discussed.

Study of active screen plasma processing conditions for carburising and nitriding austenitic stainless steel

Active screen (AS) is an advanced technology for plasma surface engineering, which offers some advantages over conventional direct current (DC) plasma treatments. Such surface defects and process instabilities as arcing, edge and hollow cathode effects can be minimised or completely eliminated by the AS technique, with consequent improvements in surface quality and material properties. However, the lack of information and thorough understanding of the process mechanisms generate scepticism in industrial practitioners. In this project, AISI 316 specimens were plasma carburised and plasma nitrided at low temperature in AS and DC furnaces, and the treated samples were comparatively analysed. Two diagnostic techniques were used to study the plasma: optical fibre assisted optical emission spectroscopy, and a planar electrostatic probe. Optimum windows of treatment conditions for AS plasma nitriding and AS plasma carburising of austenitic stainless steel were identified and some evidence was obtained on the working principles of AS furnaces. These include the sputtering of material from the cathodic mesh and its deposition on the worktable, the generation of additional active species, and the electrostatic confinement of the plasma within the operative volume of the furnace.

Experimental and finite element analysis of machinability of AL-6XN super austenitic stainless steel

This paper presents a finite element cutting modelbased on physical microstructure to investigate the thermomechanicalbehaviour of AL-6XN Super AusteniticStainless Steel in the primary shear zone. Frozen chip rootsamples were created under dry turning operation to observethe plasticity behaviour occurring in the shear zones to comparewith the model for analysis. Chip samples were generatedunder cutting velocities at 65 and 94 m/min, feed rate at0.2 mm/rev and depth of cut at 1 mm. Temperature on thecutting zone was recorded by infrared thermal camera.Secondary and backscatter electron detectors were used toinvestigate the deformed microstructure and to calculate theplastic strain. Experimental results showed the formation ofmicrocracks (build-up edge triggers) at the chip root stagnationzone of both samples. The austenite phase patterns wereevident against the cutting tool tip in the stagnation zone of thechip root fabricated at 65 m/min. The movement of thesepatterns caused the formation of the slip lines within thegrains. The backscatter diffraction maps showed the formationof special grain boundaries within the slip lines, workhardeninglayer and in the chip region. Strain measurementsin the microstructures of the chip roots fabricated at 94 and65 m/min showed high values of 6.5 and 5.7 (mm/mm) respectively.The finite element model was used to measure thestress, strain, temperature and chip morphology. Numericalresults were compared to the outcomes of the experimentalwork to validate the finite element model. The model validatingprocess showed good agreement between theexperimental and numerical results, and the error values werecalculated. For a 94- and 65-m/min cutting speeds, 7.5 and5.2% were the errors in the strain, 3 and 2.5% were the error inthe temperature and 4.7 and 6.8% were the error in the shearplane angles.

Corrosive and abrasive wear in ore grinding

The relative significance of corrosive and abrasive wear in ore grinding is discussed. Laboratory marked ball wear tests were carried out with magnetic taconite and quartzite under different conditions, namely dry, wet and in the presence of an organic liquid. The effect of different modes of aeration and of pyrrhotite addition on the ball wear using mild steel, high carbon low alloy steel and austenitic stainless steel balls was evaluated. Results indicate that abrasive wear plays a significant role in ore grinding in the absence of sulfides, and rheological properties of the ore slurry influenced such wear. The effect of oxygen on corrosive wear becomes increasingly felt in the presence of a sulfide mineral such as pyrrhotite. Wear characteristics of the three types of ball materials under different grinding conditions are illustrated.

The effect of crack closure on the grain size dependence of fatigue crack growth threshold

Recent studies (I-7) clearly indicate a strong dependence of fatigue threshold parameter, A K on grain size in several alloy systems. Attempts to explain these observations on the basis of crat~tortuosity (1,8), fracture surface roughness (5,9) and crack closure (6) appear to present a fairly clear picture of the mechanisms that cause a reduction in crack growth rates at threshold. In general, it has been shown that coarse grained microstructures exhibit higher fatigue threshold in low carbon steels (1,5) aluminium alloys (7) and titanium alloys (6). In spite of these observations, there exists (10-1#) considerable uncertainity about the manner in which the AK~L depends on prior austenitic grain size in quenched and tempered steels. Studies in quenched and tempered steels demonstrating both an increase (3,12,14) as well as a decrease (11,12) in AKth with an increase in prior austenitic grain size can be sought to illustrate this point. Occasionally , the absence of any sensitivity of AKth to the variations in prior austenitJc grain size has also been reported (11,13). While a few investigators (5-7) comfortably rationalised the grain size effects on AK~L on the basis of crack closure by a comparison in terms of the closure-free component of the thresho~Ifc~, AK -f such an approach has yet to be extended to high strength steels, An attempt has been made in t~et ,pthrg sent study to explai. n the effect of pri, or austeniti.c grain size on &Kth on the basis of crack closure measurements in a high strength steel.

Effect of mechanical cycling on the stress-strain response of a martensitic Nitinol shape memory alloy

An experimental investigation into the ambient temperature, load-controlled tension�tension fatigue behavior of a martensitic Nitinol shape memory alloy (SMA) was conducted. Fatigue life for several stress levels spanning the critical stress for detwinning was determined and compared with that obtained on an alloy similar in composition but in the austenitic state at room temperature. Results show that the fatigue life of the pseudo-plastic alloy is superior to superelastic shape memory alloy. The stress�strain hysteretic response, monitored throughout the fatigue loading, reveals progressive strain accumulation with the cyclic loading. In addition, the area of hysteresis and recoverable and frictional energies were found to decrease with increasing number of fatigue cycles. Post-mortem characterization of the fatigued specimens through calorimetry and fractography was conducted in order to get further insight into the fatigue micromechanisms. These results are discussed in terms of reversible and irreversible microstructural changes that take place during cyclic loading. Aspects associated with self-heating of martensitic alloy undergoing high frequency stress cycling are discussed.

Direct physical evidence for the back-transformation of stress-induced martensite in the vicinity of cracks in pseudoelastic NiTi shape memory alloys

Crack loading and crack extension in pseudoelastic binary NiTi shape memory alloy (SMA) miniature compact tension (CT) specimens with 50.7 at.% Ni (austenitic, pseudoelastic) was investigated using infrared (IR) thermography during in situ loading and unloading. IR thermographic measurements allow for the observation of heat effects associated with the stress-induced transformation of martensite from B2 to BIT during loading and the reverse transformation during unloading. The results are compared with optical images and discussed in terms of the crack growth mechanisms in pseudoelastic NiTi SMAs. Direct experimental evidence is presented which shows that crack growth occurs into a stress-induced martensitic microstructure, which immediately retransforms to austenite in the wake of the crack.

Aging response and its effect on the functional properties of Cu–Al–Ni shape memory alloys

The β-phase aging response of Cu–Al–Ni single crystal shape memory alloys (SMAs) within the temperature range of 473–573 K has been investigated. Alloys in austenitic (Cu–14.1Al–4Ni wt.%, alloy A) and martensitic (Cu–13.4Al–4Ni wt.%, alloy M) conditions at room temperature were considered. Aged samples show presence of β1′ and γ1′ martensites in both the alloys and formation of γ2 precipitates in the alloy A. The differential scanning calorimetry (DSC) thermograms of the aged samples show increase in transformation temperatures as well as transformation hysteresis with aging. Dynamic mechanical analysis (DMA) was conducted on both the alloys to ascertain the role of precipitates and martensitic transition on tan δ, which characterizes the damping behaviour of the material. With aging, a steady decrease in tan δ value was observed in both the alloys, which was attributed to the decrease in the number of interfaces per unit area with increasing aging temperature. Moreover, in alloy A, as the volume fraction of precipitate increases with aging, the movement of martensitic interfaces is restricted causing a decreased tan δ.

Micromechanics Of Fatigue Crack-Growth At Low Stress Intensities In A High-Strength Steel

An attempt to systematically investigate the effects of microstructural parameters in influencing the resistance to fatigue crack growth (FCG) in the near-threshold region under three different temper levels has been made for a high strength low alloy steel to observe in general, widely different trends in the dependence of both the total threshold stress intensity range, DELTA-K(th) and the intrinsic or effective threshold stress intensity range, DELTA-K(eff-th) on the prior austenitic grain size (PAGS). While a low strain hardening microstructure obtained by tempering at high temperatures exhibited strong dependence of DELTA-K(th) on the PAGS by virtue of strong interactions of crack tip slip with the grain boundary, a high strength, high strain hardening microstructure as a result of tempering at low temperature exhibited a weak dependence. The lack of a systematic variation of the near-threshold parameters with respect to grain size in temper embrittled structures appears to be related to the wide variations in the amount of intergranular fracture near threshold. Crack closure, to some extent provides a basis on which the increases in DELTA-K(th) at larger grain sizes can be rationalised. This study, in addition, provides a wide perspective on the relative roles of slip behaviour embrittlement and environment that result in the different trends observed in the grain size dependence of near-threshold fatigue parameters, based on which the inconsistency in the results reported in the literature can be clearly understood. Assessment of fracture modes through extensive fractography revealed that prior austenitic grain boundaries are effective barriers to cyclic crack growth compared to martensitic packet boundaries, especially at low stress intensities. Fracture morphologies comprising of low energy flat transgranular fracture can occur close to threshold depending on the combinations of strain hardening behaviour, yield strength and embrittlement effects. A detailed consideration is given to the discussion of cyclic stress strain behaviour, embrittlement and environmental effects and the implications of these phenomena on the crack growth behaviour near threshold.

An assessment of creep deformation and fracture behavior of 2.25Cr-1Mo similar and dissimilar weld joints

The evaluation of the creep deformation and fracture behavior of a 2.25Cr-1Mo steel base metal, a 2.25Cr-1Mo/2.25Cr-1Mo similar weld joint, and a 2.25Cr-1Mo/Alloy 800 dissimilar weld joint at 823 K over a stress range of 90 to WO MPa has been carried out. The specimens for creep testing were taken from single-V weld pads fabricated by a shielded metal arc-welding process using 2.25Cr-1Mo steel (for similar-joint) and INCONEL 182 (for dissimilar-joint) electrodes. The weld pads were subsequently given a postweld hear treatment (PWHT) of 973 K for I hour. The microstructure and microhardness of the weld joints were evaluated in the as-welded, postweld heat-treated, and creep-tested conditions. The heat-affected zone (HAZ) of similar weld joint consisted of bainite in the coarse-prior-austenitic-grain (CPAG) region near the fusion line, followed by bainite in the fine-prior-austenitic-grain (FPAG) and intercritical regions merging with the unaffected base metal. In addition to the HAZ structures in the 2.25Cr-1Mo steel, the dissimilar weld joint displayed a definite INCONEL/2.25Cr-1Mo weld interface structure present either as a sharp line or as a diffuse region. A hardness trough was observed in the intercritical region of the HAZ in both weld joints, while a maxima in hardness was seen at the weld interface of the dissimilar weld joint. Both weld joints exhibited significantly lower rupture lives compared to the 2.25Cr-1Mo base metal. The dissimilar weld joint exhibited poor rupture life compared to the similar weld joint, at applied stresses lower than 130 MPa. In both weld joints, the strain distribution across the specimen gage length during creep testing varied significantly. During creep testing, localization of deformation occurred in the intercritical HAZ. In the similar weld joint, at all stress levels investigated, and in the dissimilar weld joint, at stresses greater than or equal to 150 MPa, the creep failure occulted in the intercritical HAZ. The fracture occurred by transgranular mode with a large number of dimples. At stresses below 150 MPa, the failure in the dissimilar weld joint occurred in the CPAG HAZ near to the weld interface. The failure occurred by extensive intergranular creep cavity formation.