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Observations on some steels used in high performance gears are presented in this thesis. The object was to understand how microstructure and residual stress influenced mechanical properties, particularly fatigue strength. The investigations were carried out using fatigue testing, metallographic techniques, shot peening, hardness testing and Xray diffraction to determine residual stress and the amount of retained austenite. The work is divided into two main parts. In the first part, the opportunity was taken of investigating a batch of case-carburised gears manufactured from 17CrNiMo6 steel that contained an abnormally high level of retained austenite. . The benefits or otherwise of retained austenite is a matter of some controversy in the literature and this was an opportunity of testing the effect of high retained austenite in gears. In the second part, the properties of a recently formulated through hardening steel was investigated. The steel is inexpensive and capable of being fully hardened by air-cooling. It is believed that use of the material could reduce the cost of gear manufacture by eliminating the carburisation process, oil quenching and the subsequent need to correct distortions that arise from quenching. Gears were manufactured using l7CrNiMo6 steel. After carburization, one batch was found to have an abnormally high level of retained austenite (40 to 60%). Normally, gears with this level of retained austenite would be rejected. Nevertheless, gears from this batch were found to have surface and bending fatigue strengths that were not very different from those with much lower retained austenite contents. Despite the fact that the material is relatively soft, it was found that a very thin surface layer had hardened during back-to-back gear testing, probably by strain-induced transformation of the retained austenite. It was deduced that this thin layer of hardened material is sufficient to maintain a good level of surface contact fatigue strength. The second part of the work includes a basic study of the through-hardening steel. The study includes heat treatment and hardness, estimation of fracture toughness and the development of residual stress during heat treatment. Specialised standing contact fatigue (SCF) tests were also done using this material. The SCF test consists of cyclic loading of a hard ball in contact with the surface of the specimen, which is meant to simulate asperity contact in surface contact fatigue. Radial cracks or ring cracks or both are nucleated just outside the indentation circle in these tests, depending on the load and the development of plasticity. The stresses near the indentation were modelled using Finite Element analysis and were found to be consistent with the experimental results. Finally, the effect of residual compressive stress induced by shot peeing was studied using the SCF test. It was found that shot peening suppressed the formation of radial cracks.

The effect of austenitising and tempering practice on the microstructure and mechanical properties of two martensitic stainless steels was examined with the aim of supplying heat treatment guidelines to the consumer or fabricator that, if followed, would result in a martensitic structure with minimal retained austenite, evenly dispersed carbides and a hardness of between 610 HV and 740 HV (hardness on the Vickers scale) after quenching and tempering. The steels examined during the course of this examination conform in composition to medium-carbon AISI type 420 martensitic stainless steel, except for the addition of 0.13% vanadium and 0.62% molybdenum to one of the alloys. The effect of various austenitising and tempering heat treatments was examined. Steel samples were austenitised at temperatures between 1000°C and 1200°C, followed by quenching in oil. The as-quenched microstructures were found to range from almost fully martensitic structures to martensite with up to 35% retained austenite after quenching, with varying amounts of carbide precipitates. The influence of tempering, double tempering, and sub-zero treatment was investigated. Optical and scanning electron microscopy was used to characterise the as-quenched microstructures, and X-ray diffraction analysis was employed to identify the carbide present in the as-quenched structures and to quantify the retained austenite contents. Hardness tests were performed to determine the effect of heat treatment on mechanical properties. As-quenched hardness values ranged from 700 HV to 270 HV, depending on the amount of retained austenite. Thermodynamic predictions (using the CALPHAD™ model) were used to explain these microstructures based on the solubility of the carbide particles in the matrix at various austenitising temperatures. The carbide particles were found to be mainly in the form of M7C3 at elevated temperatures, transforming to M23C6 on cooling. / Dissertation (MSc)--University of Pretoria, 2010. / Materials Science and Metallurgical Engineering / unrestricted

The motivation to use lightweight materials in the construction of the automotive structure is the resultant increased fuel efficiency. However, these materials possess certain drawbacks that make it challenging to adopt them into current automobile manufacturing processes. In this dissertation the microstructural response observed in a magnesium alloy, AZ31, and an advanced high strength steel alloy, QP1180, to uniaxial deformation is analyzed and the results are presented. In AZ31 the required slip modes are not activated at room temperature leading to its low ductility at room temperature. The resulting activity of these twins in response to uniaxial tension is analyzed and its correlations with the microstructure features is reported. Additionally, a neighborhood viscoplastic self-consistent model is developed that will allow more accurate simulation of twin response to outside deformation. Furthermore, activity of slip modes that are usually observed at high temperatures (>200°C) are also observed at lower temperatures (<125°C) and they are compared to the relative twin activity at these temperatures. It is observed that larger grains, with high schmid factors, longer grain boundaries and have misorientation with its neighboring grain greater than 27° are more favorable for twin formation and transmission in the AZ31 microstructure in response to uniaxial tension. The nature of retained austenite (RA) transformation into martensite that gives QP1180 its enhanced ductility, is not clearly understood primarily because of challenges present in characterization of these metastable RA. Further, a 2 dimensional characterization method does not provide the complete information of the RA grain. These challenges are overcome by characterization of a 3 dimensional volume element using serial sectioning and EBSD followed by reconstruction using DREAM3D. The influence of 3d morphology and orientation direction on RA transformation is studied using as-is and uniaxially deformed samples. A novel shear affinity factor is introduced as a metric to describe the ease of RA transformation under uniaxial tension. The 3d nature of the information collected allows a new classification of disk shape in addition to globular and lamellar shapes for RA. It is found that RA that are low volume laths and have low shear affinity factor transform later compared to disk shaped RA’s. Through these guidelines the preparation of a microstructure that is conducive to RA transformation under uniaxial tension is possible.

O processo de tratamento térmico à baixa temperatura é um dos métodos mais promissores para melhorar o desempenho dos materiais. O tratamento criogênico promove a transformação de austenita retida do aço em martensita, o que é atribuída para melhorar a dureza e resistência ao desgaste. Neste trabalho foram analisados os efeitos dos diferentes ciclos de tratamentos térmicos, comparando-se à tempera convencional (têmpera 1050°C + revenido simples e duplo a 200/530°C) respeito à adição do tratamento criogênico, (têmpera 1050°C + criogenia a -125°C + revenido simples e duplo a 200/530°C) com e sem tempo de espera de 24 horas, com a finalidade de avaliar a estabilização térmica da austenita retida no aço em relação a quantidade presente na microestrutura e consequentemente na influencia nas propriedades mecânicas do aço ferramenta para trabalho a frio AISI D2. As análises foram conduzidas através de testes de dureza, impacto, microscopia óptica, microscopia eletrônica de varredura e difração de raios-X. Os resultados encontrados foram uma variação pouco significativa na dureza entre 57 e 58 HRC. Foi evidenciada a baixa tenacidade ao impacto do aço AISI D2, independente das rotas dos ciclos de tratamento térmico, resultado da alta percentagem de carbonetos dispostos na microestrutura. A resistência ao impacto no aço em estudo após o tratamento criogênico, esses resultados foram relacionados à microestrutura do material. / The heat treatment process at low temperature is one of the most promising methods to improve the performance of materials. The cryogenic treatment promotes the transformation of retained austenite from the steel into martensite, which is attributed to improved hardness and wear resistance. In this work the effects of the different cycles of thermal treatments were analyzed, comparing to conventional tempering (tempering 1050°C + single and double annealing at 200/530°C) with respect to the addition of the cryogenic treatment (tempera 1050°C + cryogenics to - 125°C + single and double tempering at 200/530°C) with and without waiting time of 24 hours, in order to evaluate the thermal stabilization of the austenite retained in the steel in relation to the amount present in the microstructure and consequently in the influence on the mechanical properties of cold working tool steel AISI D2. The analyzes were conducted through tests of hardness, impact, optical microscopy, scanning electron microscopy and X-ray diffraction. The results found were a minor variation in hardness between 57 and 58 HRC. It was evidenced the low impact toughness of the AISI D2 steel, independent of the thermal treatment cycle routes, as a result of the high percentage of carbides disposed in the microstructure. The impact resistance in the steel studied after the cryogenic treatment, these results was related to the microstructure of the material.

The Quenching and Partitioning (Q&P) heat treatment has been proposed in the early 2000s to produce cold-rolled sheets combining high-strength and formability for the automotive market. Q&P consists, first, of an interrupted quench between the martensite-start temperature (Ms) and the martensite-finish temperature (Mf) from intercritical annealing or full austenitization in order to form controlled fractions of martensite. This is followed by a partitioning step in order to stabilize the untransformed austenite through carbon enrichment. In order to maximize the carbon transfer from martensite to austenite, the use of specific alloying elements and the design of appropriate Q&P parameters are required to eliminate or minimize competing phenomena such as carbide formation and austenite decomposition. The final quenched and partitioned microstructure, using full austenitization, ideally consists of carbon-depleted lath martensite and significant fractions of retained austenite providing an improved combination of strength and ductility. Hence, the transformation of retained austenite upon straining at room temperature (TRIP effect) provides supplementary work-hardening and eventually improves the ductility. In the present work, Quenching and Partitioning (Q & P) heat treatments were carried out in a quench dilatometeron a 0.2 wt% carbon steel. The microstructure evolution of the Q & P steels was characterized usingdilatometry, SEM, EBSD and XRD. The martensitic transformation profile was analyzed in order to estimate thefraction of martensite formed at a given temperature below the martensite start temperature Ms. Q & P wasshown to be an effective way to stabilize retained austenite at room temperature. However, the measuredaustenite fractions after Q & P treatments showed significant differences when compared to the calculated valuesconsidering ideal partitioning conditions. Indeed, the measured austenite fractions were found to be less sensitiveto the quench temperature and were never larger than the ideal predicted maximum fraction. Competitivereactions such as austenite decomposition into bainite and carbide precipitation were found to occur in thepresent work.Furthermore, a broad range of mechanical properties was obtained when varying the quenching temperaturesand partitioning times. The direct contributions between Q & P microstructural constituents -such as retainedaustenite as well as tempered/fresh martensite- and resulting mechanical properties were scrutinized. This wascritically discussed and compared to quenching and austempering (QAT) which is a more conventional processingroute of stabilizing retained austenite at room temperature. Finally, Q & P steels were shown to exhibit aninteresting balance between strength and ductility. The achievement of this interesting combination of mechanicalproperties was reached for much shorter processing times compared to QAT steels. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

The ductility of Q&P 1180 steel was studied with regard to retained austenite transformation under different strain paths. Specimens were tested in uniaxial tension in a standard test frame as well as in situ in the scanning electron microscope (SEM). Then digital image correlation (DIC) was used to compute the effective strain at the level of the individual phases in the microstructure. Stretching experiments were also performed using limiting dome height (LDH) tooling, where specimens were strained in both biaxial and plane strain tension. The experiments were done incrementally, for each strain path, and the retained austenite at each level of strain was measured using electron backscatter diffraction (EBSD). Retained austenite levels in the uniaxial tension case dropped from an initial measured level of about 8% to about 2% during an initial strain increment of 0.02, but then stabilized as the specimen was strained to 0.1. In the plane strain and biaxial tension cases retained austenite also dropped significantly during an initial strain increment of about 0.04, but then continued to decrease as the specimens were strained to failure. Biaxial tension, in particular, was the most effective strain path for transforming retained austenite to martensite, resulting in a final volume fraction of 0.3% at an effective strain of 0.3. Retained austenite in the plane-strain tension case dropped at a faster rate than in the biaxial tension case, but finished at about 1% at a strain of 0.1. The greatest limit strains were seen in the biaxial tension case, which may be partly explained by the more effective conversion of retained austenite than was seen in the uniaxial tension case.

O processo de tratamento térmico à baixa temperatura é um dos métodos mais promissores para melhorar o desempenho dos materiais. O tratamento criogênico promove a transformação de austenita retida do aço em martensita, o que é atribuída para melhorar a dureza e resistência ao desgaste. Neste trabalho foram analisados os efeitos dos diferentes ciclos de tratamentos térmicos, comparando-se à tempera convencional (têmpera 1050°C + revenido simples e duplo a 200/530°C) respeito à adição do tratamento criogênico, (têmpera 1050°C + criogenia a -125°C + revenido simples e duplo a 200/530°C) com e sem tempo de espera de 24 horas, com a finalidade de avaliar a estabilização térmica da austenita retida no aço em relação a quantidade presente na microestrutura e consequentemente na influencia nas propriedades mecânicas do aço ferramenta para trabalho a frio AISI D2. As análises foram conduzidas através de testes de dureza, impacto, microscopia óptica, microscopia eletrônica de varredura e difração de raios-X. Os resultados encontrados foram uma variação pouco significativa na dureza entre 57 e 58 HRC. Foi evidenciada a baixa tenacidade ao impacto do aço AISI D2, independente das rotas dos ciclos de tratamento térmico, resultado da alta percentagem de carbonetos dispostos na microestrutura. A resistência ao impacto no aço em estudo após o tratamento criogênico, esses resultados foram relacionados à microestrutura do material. / The heat treatment process at low temperature is one of the most promising methods to improve the performance of materials. The cryogenic treatment promotes the transformation of retained austenite from the steel into martensite, which is attributed to improved hardness and wear resistance. In this work the effects of the different cycles of thermal treatments were analyzed, comparing to conventional tempering (tempering 1050°C + single and double annealing at 200/530°C) with respect to the addition of the cryogenic treatment (tempera 1050°C + cryogenics to - 125°C + single and double tempering at 200/530°C) with and without waiting time of 24 hours, in order to evaluate the thermal stabilization of the austenite retained in the steel in relation to the amount present in the microstructure and consequently in the influence on the mechanical properties of cold working tool steel AISI D2. The analyzes were conducted through tests of hardness, impact, optical microscopy, scanning electron microscopy and X-ray diffraction. The results found were a minor variation in hardness between 57 and 58 HRC. It was evidenced the low impact toughness of the AISI D2 steel, independent of the thermal treatment cycle routes, as a result of the high percentage of carbides disposed in the microstructure. The impact resistance in the steel studied after the cryogenic treatment, these results was related to the microstructure of the material.

[pt] Aços de alta resistência contendo frações significativas de austenita retida têm alcançado grande interesse comercial principalmente quando associados ao fenômeno TRIP durante o processo de conformação final. Recentemente, um novo conceito de tratamento térmico, denominado Têmpera e Partição, vem sendo estudado como mais uma alternativa no desenvolvimento de aços multifásicos. Neste processo, o controle da fração volumétrica da austenita retida é possível uma vez que durante o tratamento de partição, a supersaturação de carbono na martensita temperada é utilizada para estabilizar a austenita não transformada, evitando assim transformações futuras que poderiam ocorrer em temperaturas mais baixas. A seqüência de processamento térmico envolve o tratamento de têmpera numa faixa de temperatura entre Ms e Mf, seguido de partição numa temperatura igual ou superior à temperatura de têmpera. A partição do carbono da martenista para a austenita é possível caso reações competitivas, como por exemplo, a precipitação de carbetos, sejam suprimidas pela adição de elementos de liga tais como Si e/ou Al. Uma condição básica para o modelo está relacionada à restrição de movimentação da interface martensita/austenita, uma vez que a difusão em temperaturas baixas está limitada aos átomos interticiais. Essa restrição leva a um novo conceito de equilíbrio denominado Equilíbrio Constrito de Carbono, que é caracterizado pela igualdade do potencial químico na interface austenita-martensita apenas para o carbono. Nesse trabalho foram desenvolvidos quatro aços, contendo diferentes percentuais de C e Ni e com a presença dos elementos Si, Mn, Mo e Cr. A adição desses elementos teve finalidade reduzir a temperatura Bs, visando desacoplar o tratamento de têmpera e partição de uma eventual transformação bainítica. Um conjunto de condições para o tratamento de têmpera e partição foi então desenhado, envolvendo diferentes temperaturas de têmpera e diferentes temperaturas e tempos de partição. A avaliação microestrutural foi realizada utilizando recursos de microscopia ótica e microscopia eletrônica de varredura e de transmissão. A técnica de difração de raios-X foi empregada para quantificar a fração de austenita retida e seu enriquecimento em carbono. Foi modelado o processo de partição do carbono utilizando o programa DICTRATM. Os resultados dessas simulações foram analisados em termos dos parâmetros microestruturais, do tempo e da temperatura, e como essa combinação influência a cinética de partição do carbono. Os resultados obtidos para as amostras ensaiadas em tração indicaram uma vasta combinação de resistência e ductilidade, confirmando o potencial do processo na otimização das propriedades mecânicas. / [en] High strength steels containing significant fractions of retained austenite have been developed in recent years and are the subject of growing commercial interest when associated with the TRIP phenomenon during deformation. A new process concept, Quenching and Partitioning, has been recently proposed for production of steel microstructures containing carbon-enriched austenite. The heat treatment sequence involves quenching to a temperature between the martensite-start (Ms) and martensite-finish (Mf) temperatures, followed by a partitioning treatment, above or at the initial quench temperature, designed to enrich the remaining untransformed austenite with the carbon escaping from the supersaturated martensite phase, thereby stabilizing the retained austenite phase during the subsequent quench to room temperature. To enable the austenite enrichment, competing reactions, principally carbide precipitation, must be suppressed by appropriate alloying elements, such as Si and/or Al. The concept assumes a stationary martensite/austenite interface and the absence of shortrange movements of iron and substitutionals elements. The condition under which partitioning occur has been called Constrained Carbon Equilibrium (ECC), due to the restriction in movement of the interface and the assumption that only carbon equilibrates its chemical potencial at the interface. In this work, a group of four alloys was investigated, containing different additions of C and Ni and containing Si, Mn, Mo e Cr. These alloys were designed to preclude bainite formation at the partitioning temperatures of interest. Several heat-treatments, were performed in these alloys, using the Q&P concept, to evaluate its effect on the resulting microstructure and mechanical properties. Each alloy was quenched at selected temperatures and partitioned from 350 to 450°C for times ranging from 10 to 1000s. Microstructural characterization was performed by optical microcoscopy, scanning and transmission electron microscopy, while X-ray diffraction was used to determine both the fraction and the carbon content of the retained austenite. Partitioning kinetics were simulated with DICTRATM. The results were analyzed taking into consideration the scale of the microstructure, as well as the partitioning temperature. Tensile test results indicated that very high levels of strength with moderate toughness can be achieved confirming the potential of the Q&P to produce a superior combination of mechanical properties.

The diploma thesis deals with phase composition measurement of the TRIP steel, using EBSD method. The steel was delivered as thermo-mechanically treated via two different routes. The phase composition of the steel was examined during gradual plastic deformation in the range 0 to10.99%. One route of thermo-mechanical treatment exhibited good agreement with the literature in terms of measured fraction of the retained austenite (15.6%) as well as its decrease during the deformation (to 8.9% at the maximum imposed strain). The samples of the second route did not show any agreement in either of the parameters spoken.