Cutting tool geometries play important roles in tool performance, such as tool life, surface integrity, and cutting force. The most common commercial tools edge geometries are honed, chamfered and hone-chamfered. This study investigates new ways to develop tool geometry. An uncoated carbide tool is used in the orthogonal cutting of AISI 4140. By observing the tool geometry changes in the machining process with white light interferometry, a new tool wear geometry model has been proposed. A non-destructive tool wear measurement is discussed as well. In addition, this study presents the machining result comparison between the new and conventional geometries as well as the failure analysis from both experimental and FEA perspectives. / Thesis / Master of Applied Science (MASc)

AISI 4140 alloy steel has been a very common material to be investigated in automotive and aerospace industries for several decades. AISI 4140 alloy steel is chromium, molybdenum, and manganese containing low alloy steel. It has high fatigue strength, abrasion and impact resistance, toughness, and torsional strength. The functional performance is largely determined by the surface states after machining. The aim of the present study is to explore the polishing methods and surface analysis after machining AISI 4140 alloy steel in different cutting speeds and cooling conditions. The surface analysis includes surface roughness, hardness and residual stresses. Compared to traditional polishing, an innovative experimental work was conducted on electro-polishing technology for removing surface layer before subsurface residual stress measurement. The results of this work show that the electro-polishing method is a significant approach for the residual stress analysis. High cutting speed and cooling conditions can improve the surface quality to achieve lower surface roughness, higher microhardness and more compressive residual stresses after machining AISI 4140 alloy steel.

O procedimento de soldagem convencional do aÃo ABNT 4140, devido à formaÃÃo de martensita na zona termicamente afetada pelo calor (ZAC), contempla um tratamento tÃrmico pÃs-soldagem para aliviar as tensÃes e melhorar a tenacidade desta zona. Esse tratamento tÃrmico, muitas vezes, se torna impraticÃvel devido a alguns fatores como tamanho da peÃa, reparo no campo e custo. A tÃcnica da dupla-camada tem sido pesquisada no sentido de se evitar este tratamento tÃrmico e garantir boas propriedades mecÃnicas da peÃa soldada. O emprego da tÃcnica da dupla-camada exige uma relaÃÃo adequada entre as energias de soldagem das camadas, para que a segunda refina e revina a ZAC grosseria da primeira, pois, nesta regiÃo a possibilidade de formaÃÃo de martensita à grande devido a sua maior temperabilidade. Neste trabalho, foi utilizado um ensaio preliminar chamado de teste de Higuchi que permitiu a escolha das energias. Em seguida foram realizadas soldagens em simples depÃsito e camada Ãnica em corpos de prova temperados e revenidos com o objetivo de avaliar o grau de refino e revenimento proporcionado pelos passes laterais. ApÃs este ensaio, foram soldados corpos de prova temperados e revenidos utilizando-se a dupla-camada para depois, atravÃs do ensaio de microdureza e metalografia, avaliar o grau de refino e revenimento porporcionados pelo uso da tÃcnica de dupla camada. A soldagem de uma junta semi-V permitiu a avaliaÃÃo da tenacidade da ZAC da primeira camada atravÃs da realizaÃÃo do ensaio Charpy. Os resultados deste ensaio mostraram que as energias absorvidas por corpos de prova soldados, com as relaÃÃes de energias de soldagem (KJ/cm) da primeira e segunda camada de 5/5 e 5/10, sÃo semelhantes as dos corpos de prova com o tratamento tÃrmico pÃs-soldagem e superior a do metal base temperado e revendido, mostrando a viabilidade de tÃcnica. / The welding procedure of the conventional steel AISI 4140 due to the formation of martensite in the heat affected zone by heat (HAZ), comprises a post-welding heat treatment to relieve stresses and improve the toughness of this zone. This thermal treatment often becomes impractical due to factors such as part size, and field repair cost. The technique of double-layer has been studied in order to avoid this thermal treatment and to ensure good mechanical properties of the welded part. The use of the technique of double-layer requires an appropriate relationship between the energy of welding of the layers, so that the second and refines the HAZ revina coarseness of the first, since in this region the possibility of formation of martensite is large because of its higher hardenability . In this work, we used a preliminary test called Higuchi allowing the choice of energy. Then welds were carried out in simple deposit and single layer in specimens quenched and tempered in order to assess the degree of refining and tempering provided by lateral passes. After this test, the specimens were welded and tempered using the double-layer and then through metallography and microhardness testing, evaluate the degree of refining and tempering porporcionados to use the technique of double layer. The welding together of a semi-V allowed the evaluation of the toughness of HAZ of the first layer by conducting the Charpy test. The results of this test showed that the energy absorbed by welded specimens, with the relations of welding energy (KJ / cm) of the first and second layer of 5/5 and 5/10, are similar to those of specimens with post-weld heat treatment and superior to the base metal and tempered resold, demonstrating the feasibility of the technique.

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Research on bainite and martensite structures has indicated that lower bainite needles have a refining effect on the lath martensitic structure. Lower bainte needles partitions prior austenite grains and will consequently have a refining effect on the subsequent formed lath martensite. Smaller austenite grains will result in smaller lath martensitic packets and blocks and will result in enhanced mechanical properties.   In order to create a variation of lower bainte structure in a matrix of martensite, two different heat treating methods were tested. The work was focused towards the formation of lower bainite during isothermal heat treating in molten salt, above and below the MS-temperature. Both un-tempered and tempered samples were analyzed .Two different materials were tested, both were AISI 4140 but with a slightly difference in hardenability. The material provided by Ovako Steel is 326C and 326F the later had a higher hardenability. In order to better distinguish the two structures from each other when studied under a microscope, a variation of etching methods were tested.  It was possible to create a variation of lower bainite structures in a matrix of martensite.  326F shows less amount of lower bainite and provides a higher average surface hardness before tempering.

No description available.