This aim of this thesis is to examine the possibility of using Magnetic Barkhausen Noise (MBN) technique in characterizing the microstructures of quenched and tempered low alloy steels as well as annealed low carbon steels. To determine the average grain size by MBN, SAE 1010 steel consisting of dominantly ferrite was used. The specimens were slowly cooled in the furnace after austenitizing at different time and temperature variations. By metallographic examination the average ferrite grain size of specimens was determined. The magnetic parameters were measured by a commercial MBN system. With increasing ferrite grain size, the magnetic Barkhausen jumps caused by the microstructure were decreased due to the reduction in grain boundary density per unit volume. A clear relationship has been observed between average grain size and the magnetic Barkhausen noise signals. SAE 4140, 5140 and 1040 steels were used to characterize the microstructures of quenched and tempered specimens. After austenitizing and quenching identically, the specimens were tempered at various temperatures between 200oC and 600oC. Formation of the desired microstructures was ensured by metallographic examinations and hardness measurements. The results show that as tempering temperature increases the Barkhausen activity increases due to the enhancement of domain wall displacement with softening of the martensite. It has been shown that MBN is a powerful tool for evaluating the microstructures of martensitic and annealed steels.

The main purpose of this thesis is to develop a model that can predict the temperature profile inside a frozen food sample during microwave tempering and infrared assisted microwave tempering processes. Another goal of the study is to compare the tempering time of frozen foods by using microwave, infrared assisted microwave, and ohmic heating methods. Frozen potato puree was used as the food sample for all studies. Three different microwave power levels (30%, 40%, and 50%) were used for microwave tempering studies. Three different microwave power levels (30%, 40%, and 50%) and three different infrared power levels (10%, 20%, and 30%) were combined for infrared assisted microwave tempering. As a control, tempering was done by keeping the sample at 4&deg / C. The increase in microwave power level and infrared power level reduced tempering time in infrared assisted microwave tempering. For the ohmic tempering studies, three different frequencies (10 kHz, 20 kHz, and 30 kHz) and three different salt contents (0.50%, 0.75%, and 1.00%) were used. The increase in frequency of ohmic heating and salt content also decreased tempering times. Microwave tempering and infrared assisted microwave tempering of frozen foods were simulated by using finite difference method. For this purpose, the change in heat capacity and the dielectric properties of frozen potato puree with respect to time were measured. The temperature distribution inside the sample was modeled, and the predicted results were compared with experimental results. The predicted temperatures showed good agreement with the experimental data (r2 &gt / 0.985). It was possible to decrease tempering times by about 75%, 90%, and 95% using ohmic, microwave, and infrared assisted microwave tempering methods, respectively as compared to control.

The main purpose of this thesis is to improve the fundamental knowledge of non-isothermal tempering of martensite phase and its effects on the reduction in hardness (softening) with respect the base metal occurring at the heat affected zone (HAZ) of resistance spot welded dual-phase (DP) steels. This thesis also aims at understanding the influence of HAZ-softening on the joint performance of various DP steel grades. The tempering of martensite occurring at the sub-critical HAZ (SC-HAZ) of resistance spot welded DP600, DP780 and DP980 steels has been systematically evaluated by microhardness testing through Vickers indentation and the degree of tempering has been correlated to the HAZ-softening. From the joint performance analysis of similar and dissimilar steel grade combinations assessed through standardized testing methods, three important issues have been targeted: a) the joint strength (maximum load to failure), b) the location of failure (failure mode), and c) the physical characteristic of the weld that determines certain type of failure (weld nugget size). In addition, a partial tensile test has been conducted in order to evaluate the initiation of failure in dissimilar steel grade combinations. It has been shown that HAZ-softening lowered the weld size at which transition from interfacial to pullout failure mode takes place along with increased load-bearing capacity and higher energy absorption. Thus, it is concluded from mechanical testing that HAZ-softening benefits the lap-shear tensile joint performance of resistance spot welded DP steels by facilitating pullout failures through failure initiation at the SC-HAZ (tempered region). Instrumented nanoindentation testing was employed to further investigate HAZ-softening along the SC-HAZ by evaluating individual phases of ferrite matrix and tempered martensite islands. Although the ferrite matrix presented a slight reduction in hardness at nanoscale, higher reduction in hardness (softening) resulted for tempered martensite; thus confirming that tempered martensite is the major contributor to softening at micro-scale. A comparison between nanohardness and microhardness testing made at different distances from the line of lower critical temperature of transformation (Ac1) allowed revealing the actual extension of the SC-HAZ. In this regard, good correlation was obtained between nanohardness results along the SC-HAZ and the microstructural changes analyzed by electron microscopy (i.e., the tempering of martensite occurring at various distances far from Ac1 was correlated to low temperature tempering of dual phase steels). An in-depth analysis of the tempering of martensite phase at high temperature in DP steel subjected non-isothermal conditions i.e., rapid heating, extremely short time at peak temperature and rapid cooling (resistance spot welding), has been carried out mainly through analytical transmission electron microscopy (TEM). In addition, an isothermal tempering condition (i.e., slow heating and long time at peak temperature) in DP steel has been evaluated for complementing the analysis. Both non-isothermal and isothermal conditions have been correlated to the softening behaviour. TEM analysis of the base metal in the DP steel indicated that the morphology of the martensite phase is dependent on its carbon content, and its tempering characteristics are similar to that of equal carbon containing martensitic steel. The isothermally tempered structure is characterized by coarsening and spheroidization of cementite (θ) and complete recovery of the martensite laths; whereas precipitation of fine quasi-spherical intralath θ-carbides, coarser plate-like interlath θ-carbides, decomposition of retained austenite into elongated θ-carbides, and partial recovery of the lath structure were observed after non-isothermal tempering of DP steel. This difference in tempering behaviour is attributed to synergistic effect of delay in cementite precipitation due to higher heating rate, and insufficient time for diffusion of carbon that delays the third stage of tempering process (cementite coarsening and recrystalization) during non-isothermal. The finer size and the plate-like morphology of the precipitated carbides along with the partial recovery of the lath structure observed after non-isothermal tempering strongly influenced the softening behaviour of DP steel. The chemical analysis of θ-carbides through extraction replicas for three different DP steels revealed that the chemistry of the carbides is inherited from the parent DP steel during non-isothermal tempering at high temperature confirming that non-isothermal tempering DP steel is predominantly controlled by carbon diffusion.

The requirement for lighter, safer and fuel efficient cars has created a major stir in the steel research society to develop advanced automotive steels. Since there is a trade off between strength and ductility, most of the conventional high strength steels do not address the strength-formability combination. With the realization of the TRIP phenomenon first in austenitic stainless steels, a new generation of advanced steels called TRIP steels were realised with an inexpensive and easier to process C-Mn-Si chemistry. TRIP or TRransformation Induced Plasticity is a phenomenon where the timely strain induced transformation of Retained Austenite (RA) to Martensite locally strengthens the steel at the point of plastic instability, causing failure by necking to be postponed and shifted elsewhere along the steel. This phenomenon repeated over and over again allows increased levels of strength and ductility, prior to fracture. / In current TRIP grades, the retained austenite particles present have to posses certain characteristics such as, optimum carbon concentration, optimum grain size and morphology etc. in order to account toward mechanical properties. Such limiting characteristics in turn minimize the processing window and make TRIP processing expensive and difficult to control. In this work, it is suggested that Tempered Martensite Assisted Steels (TMAS) obtained from TRIP steels via subcritical annealing of cold rolled TRIP steels may potentially replace TRIP steels. Relationship between the retained austenite volume fractions and mechanical properties was developed for TRIP steels. The effect of variation of retained austenite on tempered martensite volume fraction in TMAS, which in turn affect the mechanical properties was also investigated in depth. Results indicate that tempered martensite particles in TMAS do not have any limiting factors as in the case of RA in TRIP steels, in order to contribute toward enhancement of mechanical properties. Results also indicate that TMAS offers better strength levels compared to TRIP steels for same the level of formability. / Retained austenite volume fractions in TRIP steels were measured through XRD. Cold rolling of the samples was done in a laboratory scale rolling machine. The microstructures were analysed using conventional and color etching techniques. A new color etching technique for viewing all the four major phases in TRIP steel was developed in this work. The mechanical properties of both TRIP and TMAS were assessed by shear punch testing. And finally, the relationship between tempered martensite volume fraction and TMAS properties was developed and was compared to TRIP properties.

Thesis (M. Eng.(Metallurgical engineering))--University of Pretoria, 2005. / Includes bibliographical references. Available on the Internet via the World Wide Web.

A fragilização por hidrogênio é uma potencial causa de falha em diversos componentes industriais. Este tipo de fragilização ocorre principalmente em aços ao carbono e também em aços inoxidáveis martensíticos, que apresentam alta resistência mecânica. Neste trabalho, avaliou-se a influência das temperaturas de revenimento de 300 ºC, 400 ºC e 500 ºC na fragilização por hidrogênio do aço ABNT 10B22-Modificado, após o processo de carbonitretação, o qual é muito utilizado na fabricação de parafusos e fixadores. Foram confeccionados corpos de prova convencionais segundo a Norma NACE TM 177-90 e também foram usados parafusos M4 X50 fabricados conforme a Norma DIN 13 para realização dos ensaios. As amostras foram submetidas a três níveis de introdução forçada de hidrogênio (10mA/cm2, 20mA/cm2 e 30mA/cm2) por controle galvanostático, para as diferentes temperaturas de revenimento. Para efeito comparativo, para cada temperatura de revenido foram produzidas amostras isentas de hidrogênio, caracterizando a condição inicial do material. Todas as amostras foram submetidas ao ensaio de tração com baixa taxa de deformação. Os corpos de prova convencionais foram também ensaiados em tração com carregamento simultâneo de hidrogênio, enquanto os parafusos foram submetidos ao ensaio de torque de fragilização com carregamento prévio. Foram caracterizados os micromecanismos de fratura em todas as condições das amostras confeccionadas. Os resultados obtidos evidenciaram que para menores temperaturas de revenimento as amostras carregadas com hidrogênio apresentaram uma maior perda das propriedades mecânicas como resistência à tração, tenacidade e alongamento quando comparadas a condição inicial, sendo que a geometria do parafuso foi mais propensa aos efeitos deletérios do hidrogênio que os corpos de prova convencionais. O micromecanismo de fratura na camada carbonitretada foi predominantemente intergranular nas amostras carregadas com hidrogênio e o núcleo apresentou coalescimento de microcavidades na maioria dos casos, com microcavidades rasas nas amostras carregadas com hidrogênio. / Hydrogen embrittlement is a potential cause of failure in various industrial components. This type of embrittlement occurs mainly in carbon steels and in martensitic stainless steels, which have high mechanical strength. In this study, it was evaluated the influence of the tempering temperatures of 300 ºC, 400 ºC and 500 ºC on the hydrogen embrittlement effect, for the ABNT 10B22-modified steel, after the process of carbonitriding, which is widely used in the manufacture of screws and fasteners. The specimens were machined according to the NACE TM 177-90 standard and were also employed as specimens in the tests, screws M4 X50 manufactured according to the DIN 13 standard. The samples were subjected to three levels of hydrogen charge (10mA/cm2, 20mA/cm2 and 30mA/cm2) by galvanostatic control for the different tempering temperatures. For comparison, each tempering temperature sample was produced free of hydrogen, characterizing the initial condition of the material. All samples were subjected to slow strain rate tensile test. The specimens were also analyzed in conventional tensile tests with simultaneous charging of hydrogen, while the screws were subjected to the embrittlement torque testing with precharged specimens. Micromechanisms of fracture were characterized in all conditions for each of the samples. The results showed that lower tempering temperatures caused greater losses of mechanical properties such as tensile strength, elongation and toughness for the hydrogen charged samples when compared to the initial condition, and the geometry of the screw was more prone to the deleterious effects of hydrogen that conventional standardized specimens. The case fracture micromechanism was predominantly intergranular in samples charged with hydrogen and the core showed coalescence of microcavities micromechanism, in most cases, with shallow microvoids in the samples charged with hydrogen.

Made available in DSpace on 2014-10-09T12:53:35Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:58:29Z (GMT). No. of bitstreams: 1 12220.pdf: 3040591 bytes, checksum: cf07d18c47b6b9bc7f1a361b3a6fe286 (MD5) / Dissertação (Mestrado) / IPEN/D / Universidade Mackenzie. Departamento de Engenharia de Materiais, São Paulo

Made available in DSpace on 2014-10-09T12:45:57Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:37Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:53:34Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:09:34Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2016-12-08T17:19:39Z (GMT). No. of bitstreams: 1 Capa-Introducao.pdf: 151731 bytes, checksum: 85462ba1c7359a14191412d891696a53 (MD5) Previous issue date: 2011-07-26 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Secondary graphitization in ductile cast iron occurs in quenched and tempered irons for long times. Usually the phenomenon is prevented since decreases mechanical properties of the material. On the other hand, cast irons with high graphite particles number present good resistance to hydrogen embrittlement.This work presents the development of secondary graphitization in gray cast iron. Samples of special compositions of gray iron, with high silicon content and low Cr content (2,56% Si and 0,025% Cr; 2,80% Si and 0,03% Cr), were casted. The heat treatment for graphitization consisted in austenitizing and quenching, in order to obtain a martensite matrix, susceptible to secondary graphite nodule development, and then tempering. For each composition, two austenitizing temperatures were defined, for the higher Si composition, 850° and 900°C and, for the lower Si composition, 900° and 950°C. The tempering temperatures were 600° and 650°C for periods of 2, 5 and 7 hours. The highest secondary graphite nodule number found were 261 nodules per mm2, from the higher Si composition sample, austenitized at 950°C and tempered at 650°C for 5 hours. The results show higher number of graphite nodules and secondary graphite volume for higher austenitizing temperatures. As the tempering time increased, martensite tempers and, then, a ferrite matrix takes place with spheroidal cementite particles. Cementite transforms to ferrite and small graphite particles. In the sequence, the particles grow and coalesce, aggregating higher graphite volume. The reaction controlling factor is the carbon diffusion. The nodules morphology is similar to the morphology of malleable iron graphite particles. The nodule numbers reveals a peak at about 5 hours of tempering for the majority of composition and heat treating combinations. After this time, small nodules reprecipitate over the larger ones. The preferable sites for secondary graphite nodules formation were regions far from primary graphite veins, interdendritic and inside the eutectic cells. The secondary graphite precipitation over the primary graphite was also observed. The secondary graphite volume increases with tempering time. / A grafitização secundária em ferro fundido nodular é obtida após tratamento de têmpera seguido de revenido por longos períodos. Geralmente, o fenômeno é evitado, pois reduz as propriedades mecânicas do material. Por outro lado, ferros fundidos com grande número de partículas de grafita apresentam boa resistência à fragilização por hidrogênio. O presente trabalho trata do estudo da formação de grafita secundária em ferro fundido cinzento Duas composições de ferro fundido cinzento com alto teor de Si e baixo teor de Cr (2,56% de Si e 0,025% de Cr; 2,80% de Si e 0,03% de Cr), foram fundidas. O tratamento térmico para a grafitização consistiu em austenitização e têmpera, para a obtenção de matriz martensítica, mais propensa à formação de nódulos de grafita secundária, e posterior revenido. Para cada material, duas temperaturas de austenitização foram escolhidas, a saber, para o material de menor teor de Si, 850° e 900°C e, para o de maior, 900° e 950°C. As temperaturas de revenido foram definidas como 600° e 650°C por tempos de 2, 5 e 7 horas. O maior número de nódulos de grafita secundária encontrado foi o de 261 nódulos por mm2, da amostra de composição de maior teor de Si, austenitizada a 950°C, revenida a 650°C, por 5 horas. Os resultados mostram maior formação de nódulos de grafita secundária, bem como maior volume grafitizado, para maiores temperaturas de austenitização. Com o passar do tempo do revenido, tem-se martensita revenida e, então, esta dá lugar à matriz ferrítica com carbonetos esferoidizados. A cementita transforma-se em ferrita e pequenas partículas de grafita. Com o tempo, as partículas crescem e coalescem, agregando maior volume grafitizado. O mecanismo controlador dessa reação de grafitização é a difusão de carbono. A morfologia dos nódulos é similar à das partículas de grafita de ferro maleável. O número de nódulos mostra um pico em cerca de 5 horas de revenido para a maioria das combinações de composição e tratamento térmico. Após esse período, os pequenos nódulos reprecipitam sobre os maiores. Os locais preferenciais para a formação de nódulos de grafita secundária são regiões distantes de lamelas de grafita, interdendríticas e no interior de células eutéticas. Foi possível observar a precipitação de grafita secundária também sobre as lamelas de grafita primária. O volume grafitizado é crescente com o tempo de revenido.