M.Sc. / Please refer to full text to view abstract

Hot-work tool steels make up an important group of steels that are able to perform with good strength and toughness properties at elevated temperatures and stresses. They are able to gain this behavior through their alloy composition and heat treatment, which relies on the precipitation of alloy carbides to counter the loss in strength as the tempered material becomes more ductile. As demand grows for materials that are suitable for even harsher applications and that show improved mechanical qualities, the steel industry must continuously investigate the development of new steel grades. Within this context, the present work focuses on examining the mechanical properties and microstructure of two hot-work tool steels, of which one is a representative steel grade (Steel A) and the second a higher-alloyed variant (Steel B), at different tempering conditions. To complement the experimental work, precipitation simulations are used to monitor the progression of secondary carbide precipitation and to examine the predicted microstructural changes through varying the alloy composition. The study finds that Steel B does not actually have improved properties with respect to Steel A and suggests that the precipitation behavior of both steels is virtually identical. Despite this, the simulation work reveals that this behavior can change dramatically to favor more positive hardness contributions by increasing the alloy content of V. In short, with the project being part of an ongoing investigation, there remain several areas of analysis that need to be completed before offering a complete picture that can ultimately play a part in the development of a new hot-work tool steel grade.

No description available.

Among the commercial aluminum alloys, aluminum 319 (Al-7wt%Si-4wt%Cu) type alloys are popularly used in automobile engine parts. These alloys have good casting characteristics and excellent mechanical properties resulting from a suitable heat treatment. To get a high strength in the 319 type alloys, grain refining, reducing the porosity, solid solution hardening, and precipitation hardening are preferred. All experimental variables such as solidification condition, composition, and heat treatment are influence on the precipitation behavior; however, precipitation hardening is the most significant because excess alloying elements from supersaturated solid solution form fine particles which act as obstacles to dislocation movement. The challenges of the 319 type alloys arise due to small size of precipitate and complex aging response caused by multi components. It is important to determine the chemical composition, crystal structure, and orientation relationship as well as precipitate morphology in order to understand the precipitation behavior and strengthening mechanism. In this study, the mechanical properties and microstructure were investigated using transmission electron microscopy and three dimensional atom probe tomography. The Mn and Mg effects on the microstructure and mechanical properties are discussed with crystallographic study on the iron intermetallic phases. The microstructural evolution and nucleation study on the precipitates in the low-Si 319 type aluminum alloys are also presented with sample preparation and analysis condition of TEM and 3DAP tomography.

Os aços inoxidáveis endurecidos por precipitação vêm sendo largamente empregados na indústria aeronáutica, por combinar resistência mecânica, tenacidade à fratura e resistência à corrosão. E deste modo, são materiais que possibilitam a substituição dos aços carbonos utilizados atualmente, que necessitam de tratamento superficial adicional, como o cádmio, para melhorar a resistência à corrosão. A utilização desses revestimentos traz desvantagens como o custo, a fabricação, a susceptibilidade à fragilização por hidrogênio além dos aspectos ambientais. Neste contexto, o aço endurecido por precipitação UNS S46500, designado como Custom 465® foi avaliado considerando o efeito da temperatura de envelhecimento sobre as propriedades mecânicas e a resistência à corrosão por pite. Amostras tratadas nas condições solubilizada e envelhecida a 510ºC (H950) e 538ºC (H1000) foram submetidas ao ensaio de tração, caracterização microestrutural e ensaios de polarização potenciodinâmica (PP) para determinar a resistência à corrosão por pite. Os exames microestruturais foram realizados com auxílio de microscopia óptica (MO), microscopia eletrônica de varredura (MEV), espectroscopia de energia dispersiva (EDS) e difração de raios X. Também foram realizadas análises utilizando o software Thermo-Calc. A resistência à corrosão por pite foi avaliada em solução 0,6M NaCl com adições crescentes de Na2SO4. Os resultados obtidos nos ensaios de PP nas duas condições de tratamento térmico foram comparados entre si e com resultados disponíveis na literatura (CALDERÓN-HERNANDEZ, 2012) para o aço inoxidável UNS S30403 (304L). Os exames e análises da microestrutura revelaram que o aço Custom 465® envelhecido apresenta uma matriz martensítica, precipitados de fase chi, austenita e precipitados Ni3Ti. O tratamento H950 apresentou maior resistência mecânica e menor alongamento do que o tratamento H1000. Tal comportamento foi devido à produção de maior porcentagem de fase chi e menor porcentagem de austenita nesse tratamento de envelhecimento. Os diferentes tratamentos térmicos, condição solubilizada, H950 e H1000 apresentaram praticamente a mesma resistência à corrosão por pite. Por outro lado, o aço Custom 465® apresentou ótima resposta à inibição da nucleação de pite com adições crescentes de sulfato em meio de 0,6M NaCl, sendo que a condição H1000 se sobressai sobre a H950 nessa questão. Além disso, através da adição de sulfato foi 7 possível obter maior resistência a corrosão por pite do aço Custom 465® comparativamente ao aço 304L. Tal comportamento foi discutido em termos da afinidade química entre níquel, cloreto e sulfato, levando a maior resistência à corrosão por pite quando o aço contém maior teor de níquel (que é o caso do aço Custom 465®). Este trabalho indicou que o critério na escolha do tratamento de envelhecimento do aço Custom 465® deve ser o das propriedades mecânicas almejadas, uma vez que a resistência à corrosão por pite mostrou-se praticamente independente do tratamento térmico. / The precipitation hardened stainless steels have been widely used in the aircraft industry to combine mechanical strength, fracture toughness and corrosion resistance. And therefore, are materials that enable replacement of the carbon steels used today, which require additional surface treatment, such as cadmium plating, to improve the corrosion resistance. The use of such coatings brings disadvantages such as cost, manufacturing, susceptibility to hydrogen embrittlement beyond environmental aspects. In this context, the precipitation hardened steel UNS S46500, known as Custom 465® were evaluated for the effect of aging temperature on the mechanical properties and the resistance to pitting corrosion .Treated samples in solubilized and aged condition at 510°C (H950) and 538ºC (H1000) were subjected to tensile strength test, microstructural characterization and potentiodynamic polarization (PP) tests to determine the pitting corrosion resistance. The microstructural studies were performed with the aid of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction. Analyzes were also performed using the Thermo -Calc software. The resistance to pitting corrosion was evaluated at 0.6M NaCl solution with increasing additions of Na2SO4. The results obtained from tests of PP in both heat treatment conditions were compared with each other and with results available in the literature (CALDERÓN HERNANDEZ, 2012) to stainless steel UNS S30403 (304L). The analysis of the microstructure revealed that the steel aged Custom 465 ® presents a martensitic matrix, chi precipitates, Ni3Ti precipitates and austenite phase. The H950 age treatment had higher mechanical strength and lower elongation than the treatment H1000. Such behavior was due to the production of higher percentage of chi and a lower percentage of austenite phase in aging treatment. The different heat treatments, solubilized condition, H950 and H1000 showed almost the same pitting corrosion resistance. On the other hand, the Custom 465® showed good response to pitting inhibition with increasing nucleating additions of sulfate in 0.6M NaCl, with enhanced result for H1000 condition. Furthermore, by adding sulfate was possible to obtain greater pitting corrosion resistance of Custom 465® compared to the 304L steel. This behavior has been discussed in terms of the chemical affinity between nickel, chloride and sulphate, leading to higher pitting corrosion resistance when the steel contains a higher nickel 9 content (which is the case of steel Custom 465®). This study indicates that the criterion in selecting the aging heat treatment of Custom 465® steel must be the desired mechanical properties, since the pitting corrosion resistance was found to be substantially independent of heat treatment.

Abstract not available

<p>Commercial alloys are continuously developed to improve their performance. Therefore it is important to develop new optimisation software, which could be used in development of new materials or in materials selection. In this study the mechanical properties which are important in materials selection in mechanical design are investigated. Two types of materials are analysed, aluminium alloys and stainless steels but focus will be on the aluminium alloys.</p><p>Thermodynamic analysis has been used to evaluate the effect of the microstructure. Solid solution hardening has been successfully modelled for both aluminium alloys and stainless steels and follows the theories by Labusch and Nabarro. The precipitation hardening is most dominant for the hardenable aluminium alloys, but the non-hardenable alloys also increase their strength from precipitation hardening. The non-hardenable alloys are divided into tempers, which differ in the amount of strain hardening. This has also been modelled successfully.</p><p>Combining these fundamental results with multiple regression, models for mechanical properties have been created. Separate models are developed for wrought aluminium alloys and stainless steels. For the aluminium alloys this includes the solid solution hardening and the precipitation hardening. For the stainless steels the thickness, nitrogen content and ferrite content are included together with the solid solution hardening.</p>

19 Dec 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 1983" Fei Tang. 12/19/2004. Report is also available in paper and microfiche from NTIS.

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Os aços inoxidáveis endurecidos por precipitação vêm sendo largamente empregados na indústria aeronáutica, por combinar resistência mecânica, tenacidade à fratura e resistência à corrosão. E deste modo, são materiais que possibilitam a substituição dos aços carbonos utilizados atualmente, que necessitam de tratamento superficial adicional, como o cádmio, para melhorar a resistência à corrosão. A utilização desses revestimentos traz desvantagens como o custo, a fabricação, a susceptibilidade à fragilização por hidrogênio além dos aspectos ambientais. Neste contexto, o aço endurecido por precipitação UNS S46500, designado como Custom 465® foi avaliado considerando o efeito da temperatura de envelhecimento sobre as propriedades mecânicas e a resistência à corrosão por pite. Amostras tratadas nas condições solubilizada e envelhecida a 510ºC (H950) e 538ºC (H1000) foram submetidas ao ensaio de tração, caracterização microestrutural e ensaios de polarização potenciodinâmica (PP) para determinar a resistência à corrosão por pite. Os exames microestruturais foram realizados com auxílio de microscopia óptica (MO), microscopia eletrônica de varredura (MEV), espectroscopia de energia dispersiva (EDS) e difração de raios X. Também foram realizadas análises utilizando o software Thermo-Calc. A resistência à corrosão por pite foi avaliada em solução 0,6M NaCl com adições crescentes de Na2SO4. Os resultados obtidos nos ensaios de PP nas duas condições de tratamento térmico foram comparados entre si e com resultados disponíveis na literatura (CALDERÓN-HERNANDEZ, 2012) para o aço inoxidável UNS S30403 (304L). Os exames e análises da microestrutura revelaram que o aço Custom 465® envelhecido apresenta uma matriz martensítica, precipitados de fase chi, austenita e precipitados Ni3Ti. O tratamento H950 apresentou maior resistência mecânica e menor alongamento do que o tratamento H1000. Tal comportamento foi devido à produção de maior porcentagem de fase chi e menor porcentagem de austenita nesse tratamento de envelhecimento. Os diferentes tratamentos térmicos, condição solubilizada, H950 e H1000 apresentaram praticamente a mesma resistência à corrosão por pite. Por outro lado, o aço Custom 465® apresentou ótima resposta à inibição da nucleação de pite com adições crescentes de sulfato em meio de 0,6M NaCl, sendo que a condição H1000 se sobressai sobre a H950 nessa questão. Além disso, através da adição de sulfato foi 7 possível obter maior resistência a corrosão por pite do aço Custom 465® comparativamente ao aço 304L. Tal comportamento foi discutido em termos da afinidade química entre níquel, cloreto e sulfato, levando a maior resistência à corrosão por pite quando o aço contém maior teor de níquel (que é o caso do aço Custom 465®). Este trabalho indicou que o critério na escolha do tratamento de envelhecimento do aço Custom 465® deve ser o das propriedades mecânicas almejadas, uma vez que a resistência à corrosão por pite mostrou-se praticamente independente do tratamento térmico. / The precipitation hardened stainless steels have been widely used in the aircraft industry to combine mechanical strength, fracture toughness and corrosion resistance. And therefore, are materials that enable replacement of the carbon steels used today, which require additional surface treatment, such as cadmium plating, to improve the corrosion resistance. The use of such coatings brings disadvantages such as cost, manufacturing, susceptibility to hydrogen embrittlement beyond environmental aspects. In this context, the precipitation hardened steel UNS S46500, known as Custom 465® were evaluated for the effect of aging temperature on the mechanical properties and the resistance to pitting corrosion .Treated samples in solubilized and aged condition at 510°C (H950) and 538ºC (H1000) were subjected to tensile strength test, microstructural characterization and potentiodynamic polarization (PP) tests to determine the pitting corrosion resistance. The microstructural studies were performed with the aid of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction. Analyzes were also performed using the Thermo -Calc software. The resistance to pitting corrosion was evaluated at 0.6M NaCl solution with increasing additions of Na2SO4. The results obtained from tests of PP in both heat treatment conditions were compared with each other and with results available in the literature (CALDERÓN HERNANDEZ, 2012) to stainless steel UNS S30403 (304L). The analysis of the microstructure revealed that the steel aged Custom 465 ® presents a martensitic matrix, chi precipitates, Ni3Ti precipitates and austenite phase. The H950 age treatment had higher mechanical strength and lower elongation than the treatment H1000. Such behavior was due to the production of higher percentage of chi and a lower percentage of austenite phase in aging treatment. The different heat treatments, solubilized condition, H950 and H1000 showed almost the same pitting corrosion resistance. On the other hand, the Custom 465® showed good response to pitting inhibition with increasing nucleating additions of sulfate in 0.6M NaCl, with enhanced result for H1000 condition. Furthermore, by adding sulfate was possible to obtain greater pitting corrosion resistance of Custom 465® compared to the 304L steel. This behavior has been discussed in terms of the chemical affinity between nickel, chloride and sulphate, leading to higher pitting corrosion resistance when the steel contains a higher nickel 9 content (which is the case of steel Custom 465®). This study indicates that the criterion in selecting the aging heat treatment of Custom 465® steel must be the desired mechanical properties, since the pitting corrosion resistance was found to be substantially independent of heat treatment.