Browsing by Author "Knutsen, Robert D"

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    A microtexture based analysis of surface roughening in ductile metals during tensile deformation
    (1998) Wittridge, Nicola Janette; Knutsen, Robert D
    This thesis examines the cause and mechanism for the occurrence of parallel surface ridges during the deformation of two specific ductile metal alloys, namely AISI 430 ferritic stainless steel and an aluminium alloy designated AA3002. The investigation considers, in particular, the development of parallel ridges during uniaxial tension, and their effect on the overall surface roughening of the sheet material. A detailed account of the microstructure and texture of the individual sample sheet materials is presented and proposals for the mechanisms of surface roughening are based on plasticity analysis of the actual material data. Both the microstructural characterisation and the texture determination was carried out using mainly electron microscopy techniques. Electron backscattered diffraction techniques were used to measure the microtexture, and analysis of this data allowed the calculation of the plastic flow behaviour of discrete volumes of the sample material. The yield behaviour was implemented in a finite element model to simulate the material behaviour under uniaxial tensile conditions. Analysis of microtexture results has indicated that elongated texture clusters are visible in the aluminium sample material which exhibits severe surface roughening during elongation in the rolling direction. It is proposed that initially surface roughening is the result of a variation in plastic flow of the surface grains due to the local texture clustering. With continued straining, the condition described by the MK analysis for strain localisation is able to arise and this leads to through-thickness strain localisation and necking, and so results in the formation of a ribbed profile. Ridging in stainless steel on the other hand can be attributed to an asymmetric distribution of texture components or plastic flow properties about the mid-plane of the material. An asymmetric arrangement yield properties initiates the development of differential transverse strains about the mid-plane of the material. The variation in transverse strain in turn results in a series of localised bending events which, on a macroscopic level, produces longitudinal corrugations and an overall ridged surface morphology.
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    Analysis of microstructure evolution during Steckel mill rolling of AISI304 stainless steel
    (The Iron and Steel Institute of Japan, 2008) Knutsen, Robert D; Parker, Sa-Aadat
    The microstructural evolution in AISI304 austenitic stainless steel during Steckel mill rolling has been investigated. Particular emphasis is placed on the microstructural behaviour of the strip ends relative to the strip bulk. Good correlation between the development of hard ends that arise in the final strip and strip temperature and mean flow stress has been found by analysing mill log data. Measurement of the recrystallization kinetics under conditions that simulate Steckel mill rolling have shown that deformation temperatures below 950°C can lead to incomplete recrystallisation during the Steckel mill inter-pass. Predictions of the time to 50% recrystallisation (t0.5) are used to quantify the recrystallisation kinetics of the strip ends.
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    Analysis of phase transformations in hydrogenated titanium metals by non-isothermal dilatometry
    (2011) Abbas, Naseeba; Knutsen, Robert D
    Hydrogen was used as a temporary alloying element in CP Ti and Ti-6AI-4V. The microstructural evolution and phase transformations were monitored, before, during and after hydrogenation with in-situ dilatometric testing. Wrought CP Ti and Ti-6AI-4V specimens were pre-annealed and experienced four consecutive thermal cycles (Cycles 1-4) i.e. hydrogenation, post-hydrogenation, dehydrogenation and post-dehydrogenation, during dilatometric testing. The specimen in each thermal cycle was heated to 1000°C, heating rate 1°C/min (with an isothermal hold at 1000°C for three hours for hydrogenation and dehydrogenation cycles) and then cooled to room temperature at cooling rate of 1°C/min.
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    Assessment of the corrosion behaviour of alloys 825 and 625 in stagnant seawater-effect of temperature and welding
    (2017) Chicuba, Pedro Claudio Francisco; Knutsen, Robert D
    Alloy 825 has been extensively used as a cladding alloy in the gas and oil industry for process piping up to and including the manifolds, separators, wellheads, risers and valves. The outstanding corrosion resistance of alloy 825 against general and localised corrosion attack is attributed to its high Ni, Cr and Mo content. However, corrosion failures of alloy 825 equipment have been observed in offshore environments. Alloy 825 has good weldability and for applications that require exceptional resistance to corrosion, Inconel filler metal 625 is used as ''overmatching composition''. Nevertheless, there is always a threat of galvanic corrosion when two dissimilar alloys are electrically connected. In this study, the corrosion behaviour of alloy 825, alloy 625 weld and alloy 825 weldment have been investigated. Potentiodynamic polarization curves for the alloys were recorded in synthetic seawater across a range of temperatures (30 to 60°C). Mixed potential theory was applied to determine corrosion potentials, rates of corrosion and predict the galvanic effect of coupling alloy 825 to alloy 625 filler metal via welding. Three standard methods were considered to determine the critical pitting temperature (CPT) for alloy 825. Lastly, long-term immersion tests in seawater were conducted to determine the relationship between the laboratory accelerated tests results and the performance of the alloys under real service conditions. The results from the experimental tests revealed that alloy 825 and alloy 625 weld exhibit outstanding corrosion resistance to uniform corrosion, despite the effect of temperature on the corrosion rate of both alloys. The galvanic effect of coupling alloy 825 to alloy 625 via welding is insignificant. The corrosion morphology of alloy 825 and its weldment is temperature dependent. At temperatures below 45 °C, grain boundary attack was observed in alloy 825 samples, while pitting corrosion was observed at temperatures higher than 50 °C. Alloy 625 weld exhibited only one mode of corrosion attack, namely the selective dissolution of interdendritic phase throughout the test temperature range. There was no agreement between the CPT results for alloy 825 and its weldment obtained using the three standard methods. No correlation was found between CPT determined by laboratory tests and the temperature above which alloy 825 would suffer pitting corrosion in long term seawater exposure tests.
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    Continuum Damage Mechanics (CDM) modelling of dislocation creep in 9-12% Cr creep resistant steels
    (2016) Stracey, Muhammad Ghalib; Knutsen, Robert D
    The generation of electricity to meet an ever-growing demand has become a defining characteristic of the modern world for both developed and developing nations alike. This, coupled with the intensifying concern with pollution and its effects on the environment has put immense pressure on how quickly and efficiently power is produced. Being the most prevalent source of electricity generation, coal fired power plants have been subject to increasing scrutiny and study in an effort to improve the efficiency at which they operate. Hence, coal fired power plants are being run at increased temperatures and pressures such as those observed in Super-critical and Ultra-super-critical plants. This has by extension put excessive demand on materials used in these plants specifically within the boiler and superheater pipe sections where the most extreme thermodynamic conditions are experienced. The most commonly used materials for these applications are in the family of ferritic/martensitic 9-12% Cr steels chosen for their superior material properties especially during long-term exposure as coal fired power plants typically operate for over 20 years before being decommissioned. One of the lesser understood aspects of 9-12%Cr steels is with regard to their long-term material properties specifically that of creep degradation and deformation. This has been partially due to the reliance of creep life predictions in the past being based on accelerated creep testing and empirically based modelling. With the relatively recent revelations of empirically based modelling shown to be inaccurate when extrapolated to the long-term, a need has been identified amongst researchers to develop more accurate models based on physical relationships and material microstructure. Moreover, the insight obtained from modern experimental techniques and technologies as well as ever-expanding computing capabilities provide an opportunity to produce microstructurally based models with a high degree of complexity. Thus motivated, the focus of this dissertation was to develop a physically based dislocation creep model using the Continuum Damage Mechanics (CDM) approach. A dislocation CDM model was developed and implemented in the current work for uniaxial creep loading using the numerical modelling software Matlabᵀᴹ. The CDM approach was built upon fundamental dislocation theory as well as other microstructural considerations pertaining to dislocation creep including subgrain coarsening, M₂₃C₆ precipitate coarsening and stress redistribution. The CDM model was found to require calibration in order to be applied to specific 9- 12% Cr steels which was implemented using a parameter optimisation routine. The results obtained were compared with experimentally obtained, long-term creep-time and microstructural data for the 11% Cr steel CB8 and the 9% Cr steel P92. The CDM creep-time predictions were found to vary in accuracy depending upon the experimental data against which the model was calibrated. Upon further investigation, it was hypothesised that the discrepancy observed was due to the formation of the Modified Z-phase in some of the long term creep data but not in others which was based primarily on the differing creep exposure times of the various samples. The CDM creep-time predictions for P92 were found to be accurate when compared with experimental results regardless of creep exposure times. The apparent difference in the approximation of the creep deformation for the two steels was concluded as being due to the formation of the Modified Z-phase in CB8 but not in P92 as Modified Zphase formation is intrinsically linked with the Cr content of the steel.
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    Critical analysis of simulated thermomechanical processing of aluminium can body stock
    (2015) Hyde, Chase Kennedy; Knutsen, Robert D; George, Sarah L
    Hot Plane Strain Compression (PSC) testing is a thermomechanical testing method used to simulate the deformation condition of industrial rolling. Thermomechanical processing (TMP) factors such as the amount of strain, strain rate and temperature all influence the microstructural evolution. The geometry of the PSC test sample and anvil are important factors in order to achieve the plane strain condition and acceptable strain distribution within deformed sample. Geometrical factors such as the breadth ratio (BR) relates the the samples breadth (b) to anvils face width (w) and this ratio has a significant effect on the breadth spread of the sample. The height ratio (HR) relates w to the samples height (h) and this ratio has a significant effect on the strain distribution. Two different geometric PSC testing configurations were investigated for this study, the one configuration had less favourable geometric ratios with a BR of 3 and a HR of 1 and the other configuration had more favourable ratios, with the BR of 4.62 and the HR of 1.3. This investigation is to evaluate the feasibility of a newly installed TMP machinery, the Gleeble 3800, to simulate the hot finishing rolling conditions by the use of hot PSC tests for the production of the can body stock (CBS) aluminium alloy AA3104. Single hot PSC tests were carried out at temperatures of 300, 350 and 400 ⁰C at strain rates of 10, 30 and 100 sec-1 and multi-pass hot PSC tests were carried out to simulate the different rolling passes experienced on the hot finishing rolling mill of the production of the aluminium alloy AA3104. The strain rate, temperature control, flow stress and microstructural flow were investigate to establish whether PSC testing is feasible on the Gleeble 3800.
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    Development of high performance and efficient coating repair systems for offshore tropical marine environment
    (2018) Agostinho, Francisco José; Knutsen, Robert D
    Rehabilitation coatings of offshore equipment rarely perform as well as the original coating, despite the high cost involved. The performance gap is probably due to high relative humidity, salt contamination and limitations on the use of abrasive blast cleaning. Thus, this research aims to deepen the understanding of surface preparation parameters that affect organic coating performance. Carbon steel samples were subjected to a variety of surface alterations consisting of salt contamination, mechanical (wire brushing) and chemical (rust converter and remover) surface preparations followed by coating application and performance testing. The samples were first pre-corroded in a corrosion chamber to mimic degradation from service then surface preparations were performed after which a coating was applied. Coated new samples (RN) and fully corroded samples (SN) were the reference sets, while other samples were prepared to a variety of surface conditions. Visual inspection and electrochemical impedance spectroscopy (EIS) were performed prior to exposure and periodically during accelerated cycling corrosion testing for a period of 30 days. The visual condition of the samples was used to rank the performance of the prepared samples. These results were used as benchmark to decide the optimum EIS method, either phase angle at high frequency or total impedance at low frequency, for early evaluation of the organic coating performance under the conditions studied. Furthermore, adhesion pull-off testing was performed to rank the effectiveness of the coating over various prepared coating. The reference new samples (RN) proved to be the best surface condition and the corroded samples without preparation (SN) had the worst performance for all tests performed. In addition, it was established that salt contamination had a stronger impact on the coating performance than the amount of corrosion product remaining on the surface. Moreover, it was determined that the best preparation approach after precorrosion of the plates was to apply rust converter to the surface before coating. Adhesion measurement was of secondary concern on the studied coated surfaces as cohesive failure occurred on the pre-treatment layers rather than coating adhesion failure between the coating and the treated surface.
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    Development of the small punch test platform to evaluate the embrittlement of power plant materials
    (2017) Tshamano, Lavhelesani Oliet; Sonderegger, Bernhard; Knutsen, Robert D; Becker, Thorsten
    Life assessment of critical components and piping is performed in the electrical power plants in order to prevent structural/component failure and prolong safe operation of the equipment. In the event that these components fail, the consequences can be very costly since this may result in power supply disruptions, component replacements, environmental damages and the loss of human life. Regulations, standards and codes are designed to ensure the safe operation of the power plants. However, on their own, they are not adequate to account for aging power plants that have been in service for more than half of their originally designed lifespans, since failures have been experienced due to in-service aging mechanisms (i.e. temper embrittlement, creep, etc.) and poor engineering and maintenance practises. Mechanical, metallurgical and non-destructive techniques are used to evaluate the condition of the in-service materials in order to aid in these life assessments. The structural integrity assessments utilise material toughness properties as determined through fracture toughness testing, which requires a significant quantity of material, and is therefore cumbersome and expensive. Consequently, several other material property testing techniques are used to aid in structural integrity assessments, such as impact energy, tensile and hardness testing. Through empirical correlations, these test results are used to estimate fracture toughness properties and, consequently, the error bands are expected to be as high as 50%. Due to its small size, the small punch test (SPT) technique can be regarded as a quasi-non-destructive test, and is therefore a preferred method for determining the fracture toughness in aid of structural assessment. The SPT technique involves a compression load from the punch to a sample (ϕ8mm x 0.5mm thick) clamped between clamping and receiving dies. This study aims to develop a test rig that will be used to perform the SPT in order to quantify the level of embrittlement on the ex-service, low-pressure steam turbine material (NiCrMoV steel). The data results acquired from the SPT technique are the reaction load of the punch and the deformed displacement of the sample performed at a constant displacement rate according to CWA 15627:2007. Two SPT rigs were designed, manufactured and commissioned. These two were commissioned using FEM and tensile test results for validations. The steel was subjected to three different conditions: as received (AR), de-embrittled (DE) and hardened (HD). The three types of steel illustrated that the SPT can quantify embrittlement levels through the correlation of tensile, Charpy impact energy and fracture toughness testing.
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    The effect of alloy chemistry and strain rate on the Md30 temperature of metastable austenitic stainless steels
    (1994) Papo, Jones Malesela; Knutsen, Robert D
    The work covered in this thesis provides a comprehensive discussion of the transformation behaviour of Type 304 metastable stainless steels with small' variations in alloy composition. The study focuses mainly on the austenite stability with respect to alloy composition, rate of deformation and temperature. To achieve these objectives, uniaxial tensile tests at 0.3 true strain were performed at low and high strain rates (10-3s-1 and 3 x 10-2s-1 respectively), in the temperature range of -60 to 55°C under isothermal testing conditions.
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    Effect of oxygen diffusion hardening on fatigue resistance in Ti-6Al-4V alloys
    (2009) Chihoro, Lester Tinashe; Knutsen, Robert D
    The increased usage of titanium and its alloys in the aerospace industry and for biomedical applications has been attributed to its good strength to weight ratio, exceptional corrosion resistance and high melting point. In this research the titanium alloy that was focused on is the Ti-6Al-4V (by weight) alloy. With these good properties the alloys are also susceptible to poor tribological property behaviour. Surface engineering processes help improve these poor properties and one such process is known as Oxygen Boost Diffusion Hardening (OBDH). The OBDH process is a two-step process. The first step involves oxidation of the samples in air at elevated temperatures and the second step is carried out in order to diffusion treat the pre-oxidised specimens in a vacuum or argon environment. This was done using different oxygen and diffusion step temperatures and times. A comparison of the different fatigue lives after the treatments was made using the untreated specimen as the yardstick. The specimens were heat treated, in an oxidation furnace for the first stage and a controlled atmosphere furnace for the second stage. Thereafter, a rotational bending machine was used for the fatigue testing where fatigue failure data was obtained. For assistance in fully analysing the fatigue data; hardness tests, microstructural analysis and fracture morphology were undertaken and quantified.
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    The effect of solution heat treatment on the tensile and creep properties of MarM-002
    (1995) Ming, Vin Ree; Knutsen, Robert D
    The nickel-base superalloy MarM-002 is a high strength precipitation hardening material used in structural applications in the gas turbine field. The microstructure of MarM-002 consists of fine y' precipitates, a y matrix, carbides and a eutectic y- y' where the y' can be composed of coarse lamellae or blocky y' precipitates. Increasing the volume fraction of fine y' by dissolving the coarse eutectic y' during solution treatment can raise the alloy strength. In practice the solution heat treatment temperature does not usually exceed 1220°C because of the danger of incipient melting. At 1220°c the eutectic y' does not dissolve and persists in the alloy structure. In the current project an alternative solution treatment technique, the varied rate solution heat treatment (VRSHT), was determined specifically for MarM-002, and was used to achieve higher solution treatment temperatures up to 1260°C without incipient melting. The microstructural response of MarM-002 to the conventional 1220°C solution heat treatment, and to solution treatment at temperatures above 1220°C, was studied extensively. Microstructural features such as carbide decomposition and the degree of y' and eutectic y' dissolution were noted. The tantalum and titanium rich carbides which form during casting were seen to decompose during solution treatment, while hafnium rich carbides precipitated in the interdendritic regions during heat treatment. The y' solvus and eutectic y' solvus was measured to be1260°C and 1280°C respectively. A 1050°C/12h +_ 870°C/16h ageing heat treatment was applied to the alloy following solution treatment and the effect on the y' characteristics was also studied.
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    The effect of stabilization heat treatment on AA5182 aluminium alloy
    (2013) Morrison, Graham K; George, Sarah; Knutsen, Robert D
    AA5182 aluminium alloy is used for the manufacturing of can ends for beverage cans. The alloy selection for this part is based on the formability of the material and its resistance to softening over time. Owing to the intricate design of the can end opening tab, it is vital that the material maintains its strength during its shelf life. The mechanical properties of the AA5182 aluminium alloy are dependent on the microstructural evolution of the alloy during processing and forming. Al-Mg alloys, like AA5182, can undergo a low temperature heat treatment, which has the effect of stabilizing the microstructure and minimizing the subsequent recovery processes during and after coil coating. The effects of these heat treatments have been investigated in order to understand the effectiveness of the stabilization heat treatment on the AA5182 alloy. This study investigates various stabilization heat treatment temperature profiles, and then aims to characterize the microstructural evolution of the material during a simulation of the coil coating practice that the material is exposed to as the final step in the rolling mill operation.
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    Effect of thermomechanical processing on the surface roughening of AA6061
    (2001) Aphane, Abraham Kalaudia; Knutsen, Robert D; Lang, Candy
    Surface roughening is an undesirable feature in materials. This work investigates the cause of surface roughening in four plates of AA6061 with different thicknesses (Le. 6 mm, 9 mm and two 12 mm). Production of these plates requires thermomechanical processing of the cast slab via a two stage process, namely the initial rolling stage on a hot roughing mill (HRM) and final rolling on the hot finishing mill (HFM). After rolling. the plates are levelled by a tensile stretching process and it is during this process that surface roughening of the plates can arise. Three of these plates exhibited severe roughening and the other 12 mm thick plate exhibited mild surface roughening during the stretch levelling process. Investigations looked closely at how the cause of surface roughening is related to the thermomechanical processing. The four products studied exhibited different degrees of surface roughening during uniaxial tensile deformation. Detailed investigation of the microstructure of these four products was carried out. It was found that the plates that exhibited severe roughening had large flat and elongated grains at the surface. It is the rotation of these surface grains during tensile deformation that was thought to be inducing the roughness in the material. The microstructure evolution was explained relative to the thermomechanical history of the plates. Plane strain compression (PSC) tests were used to simulate the thermomechanical processing cycles on the HFM. Large strain rates used in the industry were compensated with an adjustment in temperature so that an equivalent Zener-Holloman parameter (Z) could be achieved. The microstructure of the PSC specimen was characterised using the electron back scattered technique. It was found that the grain size increases with a decrease in the value of Z.
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    The effects of nitrogen and nickel on the microstructure and mechanical properties of 16 wt.% chromium stainless steels
    (1991) Hutchison, Ross; Knutsen, Robert D
    An investigation has been carried out on the effects of heat treatment on the microstructures and mechanical properties of a number of experimental 16 wt.% chromium dual-phase ferritic-martensitic stainless steels. A comparison was made between an alloy containing 2.5 wt.% nickel (low interstitial content [C + N = 0.03 wt%]), and three alloys possessing low nickel (0 - 1 wt.%) and high nitrogen contents (0.06 - 0.12 wt.%). Samples of AISI 304, 430 and 431 were included in the investigation for comparison with the experimental alloys. The microstructural response of the alloys to heat treatment was examined using light and scanning electron microscopy techniques. Tensile and Charpy V-notch impact tests were carried out on the alloys in their various heat treated conditions. Fracture surfaces, and deformation markings on the tensile gauge surface, were examined in the scanning electron microscope, while cross-sections of fracture surfaces were examined using light microscopy. Dilatometric traces were obtained for the experimental alloys in order to determine the effects of variations in composition on the inter-critical temperature range. The combination of good toughness and tensile strength that can be achieved in the low interstitial, nickel alloyed steel suggests that it could be a favourable alternative to both AISI 430 and 431 in many engineering applications. Toughness values superior to those of AISI 430 and 431 can be achieved in the high nitrogen stainless steels by tempering at 700°C, although the heat treatment results in a substantial loss in strength, and the low toughness exhibited by these alloys in the solution treated condition suggests that their weldability is no better than that of AISI 430 and 431. It is also shown that the formation of a lamellar ferrite/martensite compos.ite pha5e through intercritical annealing can provide attractive combinations of tensile strength, toughness and ductility in certain of the alloys. However, the ductility of alloys containing a lamellar composite phase is dependent on the o-ferrite content, and the toughness of the composite phase is adversely affected by a high nitrogen content. The yielding characteristics of ferritic-martensitic stainless steels are dependent on the hardness difference between the ferrite and martensite phases, and on the volume fraction of martensite. In addition, the morphology of the martensite phase exerts a strong influence on the ductility of dual phase steels. Microvoid initiation in the experimental alloys in the solution treated condition (1000°C/ lhour/air cool) occurs primarily by fracture within the martensite phase. In the 700°C tempered condition alloys having a high nitrogen content may be susceptible to intergranular fracture.
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    Effects of thermomechanical processing on microstructure and hardness profiles in AA6061 plates
    (2003) Hapazari, Innocent; Knutsen, Robert D
    Effects of rolling variables - strain (?), strain rate (?) and temperature (T) - on grain structure and hardness evolution in AA6061 plates were investigated by simulating the rolling process using plane strain compression (PSG). Particular emphasis was exerted on understanding the impact of the variables on the overall mean grain size and hardness, as well as how they influence the grain size and hardness profiles through the thickness of the rolled plates. Prior to PSG investigations, the grain size and hardness profiles of the supplied material (transfer and finished plates) were determined using a Reichert MeF3A optical microscope and Vickers hardness tester. No significant grain size variations were observed through the thickness of transfer plate. On the contrary, finished plates exhibited grain size gradients, with grain size progressively decreasing from the surface to the centre of the plate thickness.
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    Evaluation of corrosion behaviour of hot dip Zn and Zn-Al alloy coatings on steel wire using laboratory and field tests
    (2014) Tevera, Tapiwa; Knutsen, Robert D
    The use of galvanised coatings on steel for structural ropes, bundles and wires has become standard practice as unprotected steel is prone to corrosion degradation. Galvanised coatings increase the service lifespan of steel by providing barrier and cathodic corrosion protection. Zinc (Zn) and zinc-aluminium (Zn-Al) alloys are the most commonly used metallic coatings on steel wire. Zn-Al coatings outperform Zn coatings, most notably in marine environments, as they combine the highly insulating oxide film associated with Al corrosion and the cathodic protection of Zn to the underlying steel.
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    Evaluation of stress corrosion cracking of high-nitrogen Cr-Mn stainless steel
    (1997) Mginqi, Lungile Ngubekhaya; Knutsen, Robert D
    The stress corrosion cracking susceptibility of an experimental high nitrogen Cr-Mn stainless steel, known as Cromanite ™, and conventional AISI 304 stainless steel were investigated in order to compare their stress corrosion performance in solutions where AISI 304 stainless steel is known to be susceptible. Slow strain rate tests (SSRT) were performed on solution treated specimens a t30°C in aerated aqueous sodium chloride (NaCI) solution containing hydrochloric acid (HCI) of varying concentration at open circuit potentials. Static tests in the form of bent-beam tests were performed on both solution treated and aged specimens in 3M NaCI solution containing 0.05 M HCI. Potentiodynamic scans and Tafel plots were used to assess corrosion behaviour and corrosion rate respectively, while the electrochemical potentiokinetic reactivation (EPR) method was used to quantify the degree of sensitisation for the materials. The SSRT revealed poor corrosion behaviour of Cromanite TM in the presence of hydrochloric acid. Whilst AISI 304 could be examined for stress corrosion cracking at HCI concentrations up to 0.5 M HCI, Cromanite ™ exhibited corrosion rates which were too fast to permit assessment of stress corrosion susceptibility at HCI concentrations of 0.15 M or above. SCC started in a salt solution containing 0.05M HCI for AISI 304 while Cromanite TM cracked in both salt solution (3M NaCI) and in 0.05 M HCI +3M NaCI. The bent-beam test performed on solution treated specimens revealed no evidence of cracking for both alloys after 100 days of exposure; however, Cromanite ™ suffered substantial mass loss after this period. While aged Cromanite TM suffered intergranular cracking after only 25 days in the test solution, no cracking was observed for the aged AISI 304 after 75 days.
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    The evaluation of the corrosion behaviour of Hercules™ alloy
    (2017) Nkomo, Duduzile Zamavezi; Knutsen, Robert D
    Ni contributes about 60% of austenitic stainless steel manufacturing material price. This means that the price of austenitic stainless steel increases with an increase of Ni. Ni price fluctuation has led to major efforts to reduce its content in austenitic steels. Ni has been replaced with readily available, cheap elements such as Mn and N. Hercules™ is a low Ni austenitic stainless steel alloy that was developed at Mintek in the Advanced Materials Division as part of the new development of low Ni austenitic stainless steels. The typical content of Hercules™ comprises of 2 wt.% Ni, 9 wt.% Mn and 2.5 wt.% N When Hercules™ alloy was tested at Mintek for mechanical properties, it was found that it had higher tensile strength than Type 304 in the hot rolled and annealed condition. Therefore, it was concluded that it can be used for structural applications where high strength is required. The target applications for Hercules™ were reinforcement bars and fasteners. There has been work done in order to improve corrosion resistance of Hercules™ in order for it to be used in corrosive environments or for general purposes. A more corrosion resistant Hercules™ alloy with 0.5 wt.% Mo addition was developed. The current project focused on further characterisation of the corrosion resistance of Hercules™ B (with 0.5 wt.% Mo) and Hercules™ A (without Mo addition) against Type 304 and Type 202. Cyclic polarisation technique was used to test the susceptibility of Hercules™ to pitting, crevice and general corrosion in different solutions. Hercules™ B showed better resistance to pitting in 3.56 wt.% NaCl compared to Hercules™ A and Type 304 but, it showed poor resistance in the presence of an artificial crevice. When the concentration of NaCl was reduced to 1 wt.%, Hercules™ B showed better resistance to crevice corrosion compared to Type 304, while Type 202 consistently showed poor corrosion resistance during each test. Immersion tests in 6 wt.% FeCl3.6H2O were consistent with the results that were obtained from the cyclic polarisation technique. The critical pitting temperature (CPT) test was investigated using 6 wt. % FeCl3.6H2O immersion testing. All test alloys started pitting at 25℃, meaning that they all have a CPT value that is less than 25℃. The results obtained from ten-days immersion and cyclic polarisation test in 5 wt.% H2SO4 were also consistent with each other. All test alloys showed good performance in 5 wt.% H2SO4 by achieving a corrosion rate that is less than 0.1 mm/y. It was therefore, concluded that Hercules™ B has an overall corrosion resistance that is comparable to that of Type 304 in 5 wt.% H2SO4 and 1 wt.% NaCl.
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    Evaluation of the formability properties of nitrogen alloyed metastable austenitic stainless steels
    (1994) Sibanda, Mandla; Knutsen, Robert D
    This study examines the formability of an AISI 301 based metastable austenitic stainless steel, in which nitrogen partially substitutes nickel. In order to understand the formability of the experimental alloys, the tensile behaviour of the alloys is characterised. The tensile properties of metastable austenitic stainless steels are governed by austenite stability which is related to alloy composition and test temperature. At certain alloy compositions, transformation induced plasticity (TRIP) occurs. TRIP depends on the manner in which deformation induced martensite forms in the steels. Incipient necking is resisted if the martensite forms gradually and selectively, preventing propagation of micronecks and microcracks. Tensile tests performed from -5 to 100°C were used to study the effect of TRIP on the ductility of these alloys and optimum tensile properties were obtained at room temperature. In addition, the effect of copper on TRIP and subsequently formability were ascertained using copper alloyed stainless steels. Important stretch formability parameters were obtained from the tensile test which is an intrinsic formability test. TRIP results in improved formability of metastable austenitic stainless steels, and a simulative Engelhardt test was performed to ascertain the effect of TRIP on drawability of the test alloys. It was found that alloys with TRIP characteristics exhibited good drawability and in all cases the test alloys had better limiting drawing ratios than AISI 304 stainless steel. Delayed cracking occurred in alloys with more than 0.2 percent nitrogen content and a low austenite stability, probably as a result of embrittlement of the deformation induced martensite by nitrogen. A study of the cavitation erosion of the test alloys was initiated because it is known that TRIP enhances cavitation erosion resistance in stainless steels. Results indicate that the metastable test alloys demonstrate superior erosion resistance when compared with the stable experimental alloys. Cavitation induced martensite was found in metastable alloys using x-ray diffraction.
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    Evaluation of the sag properties in a dual stabilized ferritic stainless steel
    (2002) Muller, Bruce Walter; Knutsen, Robert D
    The dual stabilised type 441 (DIN 1.4509) ferritic stainless steel is primarily used in the automative industry for catalytic converters. The steel, in addition to oxidation and corrosion resistance, is required to have a certain degree of creep resistance in the operating environment. The customer, by means of a modified creep test known as the sag test, usually specifies this requirement. This requirement is attained in most cases; however, in certain instances the customer places a severe requirement on the material, which exceeds the normal capabilities of type 441 material for sag resistance.