Super Duplex Stainless Steels (SDSS) are widely used in offshore oil and gas industrial components. They are dual phase materials consisting of ferrite and austenite in similar ratios with high contents of chromium and presence of molybdenum. This combination of microstructure and chemical composition results in enhanced mechanical strength and corrosion resistance. However, this material has poor machinability, exhibiting the following characteristics: (i) tendency to strain-harden; (ii) extreme adhesive behaviour; and (iii) high cutting temperatures. These circumstances not only result in high tool wear rates, but also lead to poor surface integrity due to the work hardening effect, high roughness and tensile residual stress. To minimize these detrimental effects, PVD coating technologies have been widely applied to cutting tools due to their tribological properties exhibited during cutting, which reduce friction and diminish heat. In this work, three different PVD coatings were tested during the turning of super duplex stainless steel of grade UNS S32750. In addition to the tool performance, surface integrity was assessed by surface texture analysis, residual stresses and hardness profile. The electrochemical behaviour of the machined surface was evaluated by potentiodynamic anodic polarization measurements. Stress cracking corrosion (SCC) tests were also performed. Results indicate a relationship between the tool performance and surface electrochemical behaviour, where the tool with best cutting performance, AlTiN, also presented the best electrochemical behaviour. Stress cracking corrosion was found to be associated with residual stresses on the workpiece, among the three tested PVD coated tools the AlCrN/TiSiN showed lowest tensile residual stresses and lowest SCC susceptibility. The surface generated by AlTiN coated tool presented the highest levels of tensile residual stresses, resulting in a higher SCC susceptibility. / Thesis / Master of Applied Science (MASc)

Experiments were conducted to determine the suitability of a chromized steel for use in automotive exhaust applications. Due to government regulations leading to higher temperatures and a more corrosive environment within the automotive exhaust system, Cr-lean alloys such as Type 409 no longer suffice. The high cost of increasing alloying elements to reduce the corrosion susceptibility of exhaust components has led to exhaust manufactures moving toward a sacrificially protected aluminized stainless steel (Type 409Al). Yet, costs remain high due to the stainless steel substrate. Arcanum Alloys have designed a process in which an IF steel coil is chromized using a Cr-rich slurry, creating a thin but corrosion resistant layer. This chromized layer drastically increases the corrosion resistance, without affecting the formability of the interstitial-free (IF) steel substrate and remaining cost-effective. The localized corrosion resistance of the chromized IF steel (XHOM) was measured against current generation ferritic stainless steels in a simulated interior (exhaust gas condensate) and exterior (NaCl (aq)) automotive exhaust environment. Electrochemical polarization measurements along with atmospheric corrosion tests were conducted to characterize and compare the localized corrosion susceptibility of XHOM and benchmark ferritic stainless steels. The specific tests include the following: I. Potentiodynamic polarization curves in NaCl (aq), measuring the corrosion potential (Ecorr), critical current density (icrit) and breakdown potential (Eb). II. Double loop electrochemical potentiokinetic reactivation (DLEPR) testing measuring the ratio of the activation critical current density (ia) and the reactivation critical current density (ir). III. Salt-fog testing (ASTM B117) (external) and exhaust gas condensate exposure testing (internal), measuring the mass loss, pitting density, maximum pit depth and corrosion rate. Although Type 409 and Type 439 exhibited evidence of sensitization in the mill annealed condition, all materials exhibited a resistance to further sensitization during heat treatment, indicating sensitization will not occur during service. The electrochemical polarization curves in the NaCl (aq) resulted in XHOM yielding the highest breakdown potential, yet XHOM also exhibited the highest corrosion rate during the salt fog (ASTM B117) exposure. The latter is due to exposure of the XHOM cut edge where only the plan surface was exposed during the electrochemical polarization measurements. A galvanic couple exists between the chromized coating (cathode) and steel substrate (anode) leading to rapid corrosion of the substrate when exposed. When the cut edge of XHOM is masked, the corrosion rate drops drastically, performing comparably to the highly ferritic stainless steels. During the salt fog (ASTM B117) exposure, pitting of XHOM and Type 409 was caused by cut edge corrosion leading to corrosion product migrating down the panel surfaces and initiating under deposit pitting. A singular pit was observed on the XHOM surface, which led to delamination of the coating surrounding the pit, caused by the galvanic couple at the coating substrate/interface once the substrate was penetrated. The overall corrosion resistance ranking of the materials in the external environment incorporating corrosion rate and pit depth is as follows: Type 436 ≈ XHOM Masked Edges ≈ Type 439 > Type 409Al > Type 409 > XHOM Edges Exposed. Strain was also found to have an effect on the localized corrosion susceptibility of XHOM in NaCl (aq), unlike Type 409, which exhibited no change. The influence of the drain hole manufacturing method (punching and drilling) on the corrosion susceptibility of XHOM and Type 409 was also measured. The punching method caused a smearing effect of the chromized coating, which served to partially cover and protect the cut edge. The main corrosion mechanism that occurred within the external environment is cut edge corrosion, which led to under deposit pitting. Heat treatment of samples prior to testing in the internal exhaust environment led to an intermetallic phase change within the aluminized coating on Type 409Al, drastically reducing the corrosion resistance of the material. An as-received aluminized Type 409 (Type 409Al-A) sample was tested in exhaust condensate exposure conditions to measure the difference in corrosion rate. XHOM with the cut edges exposed exhibited a corrosion rate comparable to Type 409 and heat treated aluminized Type 409, which is promising as XHOM already has an advantage in cost and formability. The overall corrosion resistance ranking of the materials exposed in the internal exhaust environment incorporating corrosion rate and pit depth is as follows: Type 409Al-A > Type 436 > Type 439 > Type 409 ≈ XHOM Edges Exposed ≈Type 409Al-H (heat-treated). / Thesis / Master of Applied Science (MASc)

MACHINABILITY ENHANCEMENT OF STAINLESS STEELS THROUGH CONTROL OF BUILT-UP EDGE FORMATION / Demand for parts made from stainless steel is rapidly increasing, especially in the oil and gas industries. Stainless steel provides a number of key advantages, such as high tensile strength, toughness, and excellent corrosion resistance. However, stainless steel cutting faces some serious difficulties. At low cutting speeds, workpiece material and the chips formed during machining tend to adhere to the cutting tool surface, forming a built-up edge (BUE). The BUE is an extremely deformed piece of material which intermittently sticks to the tool at the tool-chip interface throughout the cutting test, affecting tool life and surface integrity. Unstable BUE can cause tool failure and deterioration of the workpiece. However, stable BUE formation can protect the cutting tool from further wear, improving the productivity of stainless steel machining. This thesis presents an in-depth study of machining performance using different coated tools and various coolant conditions to examine the nature of the different tool wear mechanisms present during the turning of stainless steels. Then, different textures are generated on the tool rake face to control the stability of BUE and reduce friction during the machining process. Results show that the BUE can significantly improve the frictional conditions and workpiece surface integrity at low cutting speeds. Finally, square textures on tool rake face were found to control the stability of BUE and minimize the friction at the tool-chip interface. This reduces the average coefficient of friction by 20-24% and flank wear by 41-78% and increases surface finish by 54-68% in comparison to an untextured tool. / Thesis / Doctor of Philosophy (PhD) / Three main objectives are presented in this thesis. The first is a detailed investigation of the performance of stainless steel machining obtained by the use of different coated cutting tools and various cooling conditions. The goal of this research is to assess the reduction of tool service life, productivity, and part quality. The thesis also examines the causes of workpiece material adhesion to the cutting tool during the cutting test and to better explain its effects on tool wear and workpiece surface finish. This phenomenon is known as the "built-up edge" (BUE). Finally, different textures are applied on the cutting tool via a laser to stabilize the BUE formation on the cutting tool, thereby improving the quality of the part.

No / Alpha phase sintering, sinter hardening, and mechanical properties of prealloyed Fe-1·5Mo base powder with and without additions of elemental Si, ferrosilicon, and carbon under various process conditions have been investigated. Liquid paraffin, as a new lubricating agent, was found to be useful in reducing segregation, interparticle and die wall frictions, as well as reducing ejection forces and die and tool wear. It was found that addition of Si to the base powder enhanced the sintering process by stabilisation of the alpha-phase and formation of two kinds of liquid phase at ~1045 and ~1180°C, corresponding to the solidus and liquidus temperatures, respectively. This addition increased the tensile strength of the as sintered Fe-1·5Mo from 174 to 445MPa owing to massive solid solution strengthening effect of Si. An optimum sinter hardenable alloy, of composition Fe-1·5Mo + 3Si + 1·2C, provided a high sintered density of 7·55g cm-3, tensile and bend strengths of7 64 and 1405MPa, respectively, with 2·5% elongation, after sintering at 1250°C for 1h under hydrogen or vacuum using moderate cooling rates of le20K min-1. Faster cooling rates caused brittleness and very low UTS for the high carbon steel. Full heat treatment improved the UTS by ~200MPa which was useful only for the high carbon steel with high cooling rates ge30K min-1. Depending on the cooling rate, the as sintered microstructures consisted of mainly fine or coarse pearlite, bainite, martensite, and some retained austenite with hardness in the range 250-720HV10. Some proeutectoid grain boundary cementites were also present in the as sintered high carbon steel. This work, therefore, has shown that high densities with acceptable microstructures and good mechanical properties are achievable with single stage compaction and single sintering operations by using the optimum process conditions and alloying composition without the need for a post-sintering heat treatment.

Bacterial biofilms attached to food contact surfaces are an ongoing concern for the food industry due to the resistance of bacteria within biofilms to detergents and sanitizers. Within food manufacturing facilities, stainless steel is a common food-contact surface in which microbial cell attachment and biofilm formation may occur. Identifying methods to prevent and remove biofilms during standard cleaning and sanitation practices could prove useful, as mature biofilms can release planktonic cells into an aqueous environment, causing continual low-level contamination. Dental studies involving delmopinol hydrochloride, a cationic surfactant, have found a preventative and dissociating affect on biofilms, where food applications have scarcely been researched. This study demonstrates the prevention and removal of Listeria monocytogenes 1/2a and S. enterica Agona biofilms on stainless steel with pre- and post-exposures of delmopinol hydrochloride. Stainless steel blanks (#304, 16 gauge, 2cm x 2cm, finish #4) were submerged in a 0.2% or 0.5% delmopinol solution before or after biofilm formation. Treatment times were 1, 5 or 10 minutes, whereas controls were not exposed to the delmopinol solution. Disinfected stainless steel blanks were spot-inoculated with 20µL of a 10⁹ CFU/mL liquid culture, and pre-exposed blanks were additionally submerged in delmopinol and dried prior to inoculation. Biofilms were exclusively formed on the finished and inoculated side by placing the surface face-down on TSA. After cell attachment and biofilm development for 24 hours at 25°C, blanks were rinsed with phosphate buffer. Post-exposed blanks were submerged in 0.2% or 0.5% delmopinol for 1, 5 or 10 minutes before all blanks were individually vortexed for 90 seconds to dislodge films. Bacterial populations were determined by surface plating onto TSA followed by incubation at 32°C for L. monocytogenes and 37°C for S. Agona for 48 hours. Treatments were in-duplicate and repeated three times for each microorganism. Pre-exposure of 0.2% delmopinol resulted in a significant decrease in L. monocytogenes concentration at 1, 5 and 10 minute exposures (P < 0.05). Pre-exposures with the 0.5% solution had no significant effect on L. monocytogenes biofilm populations (P > 0.05), whereas all post-exposures lead to a significant decline in biofilm concentrations (P < 0.0001). Post-exposures of 10 minutes exhibited a mean log₁₀ reduction of 5.59 and 6.40 log₁₀ for 0.2% and 0.5% delmopinol solutions, respectively. For S. Agona, 0.2% pre-exposure resulted in no significant log10 reduction (P > 0.05), while the 10 minute 0.5% pre-exposure exhibited a minimal reduction in bacterial growth (P < 0.05). Post-exposures of 10 minutes exhibited a mean log10 reduction of 7.65 and 7.75 log10 for 0.2% and 0.5% delmopinol solutions, respectively. For L. monocytogenes and S. Agona, post-exposure to delmopinol hydrochloride caused a notable log10 reduction. The removal effect of delmopinol on biofilms is significantly greater the preventative effect. / Master of Science in Life Sciences

The ability of Listeria monocytogenes, to attach to various food contact surfaces such as stainless steel, polypropylene, and rubber compounds is well documented. The retention of these or other pathogenic bacteria on food contact surfaces increases the risk of transmission to food products. The objective of this study was to compare several methods for quantitative recovery of Listeria monocytogenes from stainless steel surfaces. A cocktail of four serotypes of Listeria monocytogenes (Scott A (4b)), 1/2b, 3b, and 4b) were mixed in equivalent concentrations and inoculated onto type 304 stainless steel coupons in a 2cm x 2cm area. After a one hour exposure, coupons were sampled by one of the following methods: 1) swabbing using a pre-moistened Dacron swab, 2) rinsing with phosphate buffered saline, 3) direct contact onto a Tryptic Soy Agar containing 0.6% yeast extract (TSA+YE) plate surface for 10 seconds, 4) sonication in an ultrasonic water bath (40 kHz), 5) contact with the bristles of a sonicating brush head for 1 min, and 6) indirect contact (2-4 mm) with the bristles of a sonicating brush head for 1 min. Coupon rinses were plated onto TSA containing 0.6% yeast extract and incubated for 24 hours at 35°C. The three sonication methods yielded higher recovery than the other three methods (p < 0.05). Brushing the coupons with the sonicating brush head yielded a recovery level of 58% and indirect exposure to the sonicating brush head permitted a recovery level of 65% from the initial microbial load. The lowest cell recovery (~20%) was observed with the swab and direct agar contact methods. / Master of Science

An investigation was conducted to determine the effect of nitrogen concentration between 0.27 percent and 1.30 percent on the internal friction peaks in 304L stainless steel. The amplitude of the internal friction peak associated with the stress-induced diffusion of interstitial nitrogen increased as a linear function of the nitrogen content. The activation energy of diffusion was found to decrease with an increase in nitrogen content. The presence of another internal friction peak was observed in the spectrum of the sar.iple containing 1.30 percent nitrogen. A metallographic investigation and a change in the magnetic properties of the specimens after testing along with the disappearance of the internal friction peak caused by nitrogen diffusion when the specimen was rerun indicated that the second peak probably resulted form a chromium-nitrogen interaction. / M.S.

The effects of stainless steel fiber additions on the resistance of castables to CO and steam were investigated. A series of high and intermediate alumina calcium-aluminate bonded castables was prepared containing several commercial stainless steel fibers. Compressive strength and abrasion resistance of the castables were measured after exposure to high pressure carbon monoxide and steam at 500ºC. Strength and abrasion resistance values were comparable to those of samples without stainless steel fibers. The addition of stainless steel fibers to refractory castables was found to not decrease resistance to carbon monoxide only if the castables were not fired in air prior to CO exposure. Firing in air was found to create oxide layers on the fibers which catalyze CO decomposition, ultimately causing disintegration of the castable. / Master of Science

Diffusion couples between nickel and S.A. 212 ferritic steel and between nickel and AISI 304 austenitic stainless steel were studied to determine the effect of nickel on the structure of these steels after diffusion anneals at 1300 ℉. Diffusion times varied from 50 to 4000 hours. The migration of nickel resulted in the formation of a martensitic band between nickel and S.A. 212 ferritic steel and an austenitic band between nickel and AISI 304 austenitic stainless steel. The width of the bands increased exponentially with the time of annealing. The band width increased faster in nickel-S.A. 212 couple than in nickel-AISI 304 couple. Hardness values were obtained within the band of both diffusion couples and varied across the band. Generally, the hardness was greatest in the band. In the nickel-AISI 304 diffusion couple, chromium carbides were observed in the nickel plate after diffusion. / M.S.

This thesis investigates the use of laser shock peening (LSP) to improve mechanical properties, electrochemical behavior, and stress corrosion cracking (SCC) resistance in laser powder bed fusion (LPBF) stainless steel. The thesis begins by introducing metal additive manufacturing and reviews the current technological frontiers of LSP before elucidating the fundamentals behind the imaging, experimental, and theoretical frameworks used in the subsequent chapters. The experimental work is roughly divided into two parts; the first half is dedicated to study of the plasticity response augmentation by LSP in anisotropic stainless steel. The prevalence of back stress hardening occurring in anisotropic metal parts causes reduced fatigue life under random loading. LSP is known to improve fatigue life by inducing compressive residual stress and has been applied with promising results to AM metal parts. It is here demonstrated that LSP may also be used as a tool for mitigating tensile back-stress hardening. This discussion is initially applied to rolled and annealed 304L stainless steel which is shown to exhibit material anisotropy. Back stress is extracted from hysteresis tensile testing for treated and untreated samples. Analysis of plasticity response by orientation imaging microscopy (OIM) and finite element analysis (FEA) describes back stress and residual stress development during tensile testing and LSP treatment. The research indicates LSP's potential to address manufacturing design challenges caused by yield asymmetry due to back stress and is thus next applied to additively manufactured 316L. The microstructure and texture in additively manufactured metal lead to anisotropic hardening behavior. Comparison of LSPed and as-built LPBF samples shows LSPed samples processed along the build direction demonstrate significant back-stress reduction. Electron backscatter diffraction (EBSD) illuminates grain morphologies' role, while crystal plasticity finite element (CPFE) modeling reveals mechanisms underlying back-stress reduction across different build orientations and crystal planes.In the second half of the thesis, LSP’s effect upon LPBF 316L material performance in corrosive environments is investigated. This effort begins with analysis of LSP’s improvement to electrochemical and wetting behavior of as-built LPBF surfaces. The corrosion performance of LPBF stainless steel varies between studies and build parameters, thus motivating the search for postprocessing methods that enable wetted surface applications. The study examines electrochemical properties before and after LSP, measuring pitting potential, electrochemical impedance, contact angle, surface free energy, and surface finish. LSP imparts surface improvement which is attributed to morphology and chemistry alterations as well as compressive residual stress. LPBF stainless steel is also particularly susceptible to SCC due to surface-level tensile residual stress. The final study demonstrates LSP's ability to enhance SCC behavior in LPBF stainless steel by increasing time to crack initiation. Analyses of residual stress, texture, dislocation distribution, hardness, microstructure, and fracture surfaces are conducted to understand the mechanisms underlying SCC improvement. Dynamic crack modeling supports observed outcomes, linking residual stress and failure modes to LSP's effects. This work highlights LSP's potential as a versatile tool for enhancing the performance and reliability of LPBF stainless steel components in demanding engineering applications. Further, it identifies the key relevance of the anisotropy of LPBF material structure to mechanical behavior and also to the effectiveness of LSP surface processing.