MACHINABILITY ENHANCEMENT OF STAINLESS STEELS THROUGH CONTROL OF BUILT-UP EDGE FORMATION / Demand for parts made from stainless steel is rapidly increasing, especially in the oil and gas industries. Stainless steel provides a number of key advantages, such as high tensile strength, toughness, and excellent corrosion resistance. However, stainless steel cutting faces some serious difficulties. At low cutting speeds, workpiece material and the chips formed during machining tend to adhere to the cutting tool surface, forming a built-up edge (BUE). The BUE is an extremely deformed piece of material which intermittently sticks to the tool at the tool-chip interface throughout the cutting test, affecting tool life and surface integrity. Unstable BUE can cause tool failure and deterioration of the workpiece. However, stable BUE formation can protect the cutting tool from further wear, improving the productivity of stainless steel machining. This thesis presents an in-depth study of machining performance using different coated tools and various coolant conditions to examine the nature of the different tool wear mechanisms present during the turning of stainless steels. Then, different textures are generated on the tool rake face to control the stability of BUE and reduce friction during the machining process. Results show that the BUE can significantly improve the frictional conditions and workpiece surface integrity at low cutting speeds. Finally, square textures on tool rake face were found to control the stability of BUE and minimize the friction at the tool-chip interface. This reduces the average coefficient of friction by 20-24% and flank wear by 41-78% and increases surface finish by 54-68% in comparison to an untextured tool. / Thesis / Doctor of Philosophy (PhD) / Three main objectives are presented in this thesis. The first is a detailed investigation of the performance of stainless steel machining obtained by the use of different coated cutting tools and various cooling conditions. The goal of this research is to assess the reduction of tool service life, productivity, and part quality. The thesis also examines the causes of workpiece material adhesion to the cutting tool during the cutting test and to better explain its effects on tool wear and workpiece surface finish. This phenomenon is known as the "built-up edge" (BUE). Finally, different textures are applied on the cutting tool via a laser to stabilize the BUE formation on the cutting tool, thereby improving the quality of the part.

Tato práce se zabývá problematikou nárůstku. Jde o řešení konkrétního průmyslového problému. Pro daný odlitek ze slitiny hliníku a řezný nástroj ze slinutého karbidu je studován výskyt nárůstku pro různé řezné podmínky. Nejprve se práce zabýva rozborem tématiky nárůstku, mechanismy jeho vzniku a parametry, které mají na jeho výskyt a růst největší vliv. Dále je věnována pozornost studiu materiálu odlitku, jeho nové koncepce a původního řešení. Během inovačního procesu došlo k obměně použitého materiálu. Tato změna se na procesu řezání projevila vznikem významného nárůstku, který vede až k úplné destrukci nástroje. Byly provedeny zkoušky tvrdosti a metalografické výbrusy pro detailní popis a srovnání vlastností obou slitin hliníku. Výsledky zkoušek prokazují rozdíly v metalografické struktuře (velikosti dendritů, množství precipitátů) a tvrdosti daných materiálů. Experimentální řešení problému spočívalo v testech vrtání do odlitků ze zkoušených slitin hliníku. Pro jednotlivé vrtací zkoušky byla měněna hodnota posuvu a řezné rychlosti. Za pomoci dynamometru byla měřena posuvová síla, dále byl použit wattmetr k určení příkonu stoje a synchroní funkce CN, jejichž výstupem byla aktuální poloha vřetene a řezný moment. Na zákldě naměřených hodnot byla vypočtena měrná řezná síla pro konkrétní testované řezné podmínky.Mimoto po každém pokusu byly pořízeny fotografie nástroje s daným nárůstkem. Z výsledků vyplývá, že měrná řezná síla má tendenci výrazně klesat se zvyšující se hodnotou posuvu, oproti tomu zvyšující se řezná rychlost nemá tak výrazný účinek na její snižování. Co se týče naměřené tloušťky nárůstku, tak se zvyšující se řeznou rychlostí dochází k jejímu poklesu. V případě růstu posuvu, má nárůstek taktéž tendenci klesat, ale menší měrou oproti předešlému případu. V závěru práce je uvedeno rozmezí řezných rychlostí a posuvů vymezující optimalizovanou zónu pro vrtání dané součásti. Následuje nabídka dalších možných experimentů vážících se k tématu studie.

Milling tests were performed in stainless steel to investigate the formation of built-up edge (BUE). Three variants of tests were conducted which were divided as high-, medium, and low temperature milling tests. These tests were run in the austenitic stainless steel SS2343. The medium temperature milling tests were run in duplex SS2343 and precipitation hardened CORRAX. BUE was found for all tests. With the exception of the high temperature milling tests, BUE was formed locally on the main cutting edge. When milling at higher temperature the BUE covered the entire edge. All inserts used, each with different technologies had the same amount of BUE formation. The tool wear was similar for the CVD-coated inserts used, while the PVD-coated insert suffered less coating detachment along the edge.   Cross sections showed that in addition to BUE, smearing had occurred on the rake face of the inserts, showing that multiple layers of material adhered to the tool surface. Cross sections also showed that the tool wear was similar for the different milling methods, even though it was known that tool failure eventually would be caused by different wear types.   Microhardness tests of the BUE gave results where the austenitic- and duplex stainless steel had in average almost doubled their hardness. Work hardening for CORRAX was lower with an average hardness increase below 9 percent. As CORRAX gave the same amount of BUE formation as the other steels it could be concluded that work hardening did not affect the amount of BUE formation.   For one sample milled at high temperature a new phase had formed on the insert surface. Analysis in EPMA and EDS indicated that it contained chromium, manganese and oxygen. This indicated that the stainless steel had oxidized when milled at higher temperature. When comparing the composition, it was similar for both BUE and the workpiece material SS2343. The only exception was that titanium could be found in the bulk of the BUE, having higher amount when milled at higher temperature. This indicated that the BUE had interacted with the coating, as this was the only source of titanium. / Fräsningstest utfördes I rostfritt stål för att undersöka bildande av lösegg. Tre testvarianter gjordes, uppdelade i hög-, medium-, och låg frästemperatur. Dessa prov kördes i austenitiskt rosfritt stål, SS2343. Fräsning vid mediumtemperatur gjordes även i duplexa SS2377 och utskiljningshärdade CORRAX. Lösegg bildades vid alla tester. Med undantag för högtemperaturstesterna bildades lösegg lokalt på huvudskäreggen. Vid hög temperastur täckte i stället löseggen hela eggen. Alla skär, med olika skärteknologier hade samma mängd bildad lösegg. Den initiala verktygsförlitningen var samma för alla skär, med undantag av det PVD-beläggda skäret, som hade mindre lossnande av beläggning på skäreggen. Tvärsnitt visade att förutom lösegg hade även smearing inträffat på skärens arbetssida, vilket visade att vidhäftande inträffade i flera lager på skäret.Tvärsnitten visade även att skären hade slitits likartat för de olika fräsmtoderna, trots att det var känt att verktygsfel skulle orsakas av olika förslitningstyper. Mikrohårdhetstester av löseggarna gav resultat där det austenitiska-, och duplexa rostfria stålet hade fördubblat sin hårdhet i jämförelse med arbetsmaterialet. Deformationshärdning av CORRAX var lägre i genomsnitt, där dess hårdhet hade ökat med midre än 9 procent. Då CORRAX gav samma mängd löseggsbildning som de andra stålen kunde slutsatsen dras att arbetshärdning inte påverkade BUE-bildningen. För ett prov fräsat vid hög temperatur hade en ny fas bildats på skärytan. Analys i EPMA och EDS indikerade att den innehöll krom, mangan och syre. Detta indikerade att det rostfria stålet oxiderades vid fräsning vid högre temperatur. Jämförelse av kompositionen, visade att den var likartad för lösegg och arbetsmaterialet SS2343. Det enda undantaget var att titan kunde hittas i löseggens bulk, vid högre temperatur. Detta indikerade att löseggen hade interagerat med beläggningen, då det var den enda titankällan

The usage of titanium alloys in many industries has increased significantly over the years due to their superior properties. However, they are extremely difficult to machine because of their distinctive characteristics such as their high temperature strength, low thermal conductivity, and high chemical affinity for tool materials. Hence, despite their increased usage, they are still expensive to machine when compared to other metals. The current research aims to address the machinability issues of titanium alloys by developing novel compositions of a new generation of self-adaptive Physical Vapor Deposition (PVD) coatings that function by forming beneficial tribo-films through their interaction with the environment. These tribo-films form during cutting and provide enhanced lubricity, hardness, strength, and thermal barrier characteristics to the cutting tool. It was found that during Ti6Al4V machining, significant BUE and crater wear formation occurs; however, one is dominant over the other depending on the cutting conditions. Therefore, the coatings investigated were designed by taking into consideration the dominant tool wear mechanisms and the complex tribological phenomena that occur in the cutting zone. The current research investigated monolayer TiB2 and CrN self-adaptive PVD coatings for the rough (cutting speed - 45 m/min, feed -0.15 mm/rev, and depth of cut – 2 mm) and finish (cutting speed - 150 m/min, feed -0.1225 mm/rev, and depth of cut – 0.25 mm) turning of Ti6Al4V alloy. Detailed experimental studies were performed to study the effectiveness of the coatings during machining. Micro-mechanical characteristics of the coatings were also studied to understand how coating properties affect the coatings performance in machining and tribo-film formation. The results obtained show that both the TiB2 and CrN coatings significantly improve tool performance during the rough turning of Ti6Al4V alloy compared to the current industrial standard, which is due to certain micro-mechanical coating properties and the beneficial tribo-films formed. A coating of CrN coating was found to increase tool life during finish turning. It was also established that for machining applications where intensive adhesive interaction occurs at the tool-chip interface, coatings with lower hardness values perform significantly better than harder ones. / Thesis / Doctor of Philosophy (PhD) / Titanium alloys are increasingly becoming the material of choice for many industrial applications due to their superior properties. However, they are very difficult to machine since they have high chemical affinity towards tool materials, low thermal conductivity, and high temperature strength. These properties cause rapid failure of the tool. The objective of the current research is to address machinability issues during Ti6Al4V machining and improve tool performance. One effective strategy to minimize tool wear is to apply self-adaptive PVD tool coatings that can form beneficial tribo-films through their interaction with the environment and provide enhanced lubricity, hardness, strength, and thermal barrier characteristics to the cutting tool. In the current research, two self-adaptive PVD coatings were developed that offset the dominant tool wear mechanisms prevalent during the rough and finish turning of Ti6Al4V alloy and reduced the tool wear rate by more than 60% compared to the current industrial standard.

O presente trabalho trata sobre a influência do teor de enxofre, em quatro aços com uma composição química similar (famílias ABNT 1045 e ABNT 1145), na microestrutura, nas propriedades estáticas, dinâmicas e nos processos de usinagem. Para esta análise foi feita uma caracterização microestrutural de cada material para determinar parâmetros tais como a fração de inclusões de sulfeto de manganês (MnS) e a fração volumétrica de perlita. Também foi feita uma caracterização mecânica que consistiu em ensaios estáticos mediante o ensaio de tração e dureza, e um ensaio dinâmico utilizando a barra de Hopkinson, com o objetivo de observar o comportamento das inclusões e do próprio material quando deformado com altas e baixas taxas de deformação. Para a caracterização durante a usinagem destes aços foram feitos ensaios de torneamento para avaliar as forças de corte e de avanço em velocidades de corte de 190, 110, 45 e 15 m/min. A rugosidade dos corpos-de-prova também foi medida. Os resultados obtidos nos ensaios de torneamento e da caracterização microestrutural foram analisados estatisticamente para observar variações do comportamento das forças de usinagem de cada aço sob diferentes condições de velocidade de corte, e tentar correlacionar esse comportamento com a microestrutura do material. Observou-se que o aço 1045-A apresentou forças de usinagem (força de corte e força de avanço) superiores que os demais aços, já o aço que apresentou menores forças de usinagem foi o aço 1145-B. Isto é apenas uma tendência, devido que não houve diferença estatística que avaliasse esse comportamento. Também se observou que a rugosidade é um parâmetro que depende mais da velocidade de corte que da distribuição e/ou morfologia das inclusões. Evidenciou-se a formação de aresta postiça de corte (APC) numa faixa de velocidades (15-50 m/min), o que influenciou na rugosidade para estas condições de velocidades. Verificou-se que o comportamento das inclusões em baixas taxas de deformação é de caráter frágil, entanto que em altas taxas seu comportamento é plástico e deforma junto com a matriz. / This work deals with the sulfur influence on the microstructure and on the static, dynamic and machining behavior of four steels with similar chemical composition. (ABNT 1045 and ABNT 1145). Microstructure characterization of the materials was performed in order to obtain the area fraction of the phases of perlite and sulfide inclusions. A mechanical characterization of the materials was also performed, consisting in a set of static (tension and hardness test) and dynamic tests (Split Hopkinson Pressure Bar Test) with the objective of observing the deformation behavior of the sulfide inclusions at low and high strain rates. Various machining tests were carried out at different cutting speeds, namely 190, 110, 45 e 15 m min-1, for obtaining the cutting forces during de machining process. After the machining tests, the roughness of the steels was also measured. Later on, the results of the different experiments were analyzed with statistical tools and then compared to establish a correlation between the cutting forces and microstructure. The higher cutting forces were registered for the 1045-A steel and the lower for the 1145-B steel. However, this was considered merely a trend given that no statistical difference was found to support any conclusion. It was also observed a stronger roughness dependency on the cutting speed than in the distribution and/or morphology of the inclusions. The steels were observed to form a built-up edge (BUE) in a range of cutting velocities of 15-50 m/min. This phenomenon affected the roughness for these cutting velocities. The behavior of the sulfide inclusions was observed to be brittle under low strain rates. On the other hand, under high strain rates, a plastic deformation behavior was observed with inclusions participating in the plastic flow of the metal matrix.

O presente trabalho trata sobre a influência do teor de enxofre, em quatro aços com uma composição química similar (famílias ABNT 1045 e ABNT 1145), na microestrutura, nas propriedades estáticas, dinâmicas e nos processos de usinagem. Para esta análise foi feita uma caracterização microestrutural de cada material para determinar parâmetros tais como a fração de inclusões de sulfeto de manganês (MnS) e a fração volumétrica de perlita. Também foi feita uma caracterização mecânica que consistiu em ensaios estáticos mediante o ensaio de tração e dureza, e um ensaio dinâmico utilizando a barra de Hopkinson, com o objetivo de observar o comportamento das inclusões e do próprio material quando deformado com altas e baixas taxas de deformação. Para a caracterização durante a usinagem destes aços foram feitos ensaios de torneamento para avaliar as forças de corte e de avanço em velocidades de corte de 190, 110, 45 e 15 m/min. A rugosidade dos corpos-de-prova também foi medida. Os resultados obtidos nos ensaios de torneamento e da caracterização microestrutural foram analisados estatisticamente para observar variações do comportamento das forças de usinagem de cada aço sob diferentes condições de velocidade de corte, e tentar correlacionar esse comportamento com a microestrutura do material. Observou-se que o aço 1045-A apresentou forças de usinagem (força de corte e força de avanço) superiores que os demais aços, já o aço que apresentou menores forças de usinagem foi o aço 1145-B. Isto é apenas uma tendência, devido que não houve diferença estatística que avaliasse esse comportamento. Também se observou que a rugosidade é um parâmetro que depende mais da velocidade de corte que da distribuição e/ou morfologia das inclusões. Evidenciou-se a formação de aresta postiça de corte (APC) numa faixa de velocidades (15-50 m/min), o que influenciou na rugosidade para estas condições de velocidades. Verificou-se que o comportamento das inclusões em baixas taxas de deformação é de caráter frágil, entanto que em altas taxas seu comportamento é plástico e deforma junto com a matriz. / This work deals with the sulfur influence on the microstructure and on the static, dynamic and machining behavior of four steels with similar chemical composition. (ABNT 1045 and ABNT 1145). Microstructure characterization of the materials was performed in order to obtain the area fraction of the phases of perlite and sulfide inclusions. A mechanical characterization of the materials was also performed, consisting in a set of static (tension and hardness test) and dynamic tests (Split Hopkinson Pressure Bar Test) with the objective of observing the deformation behavior of the sulfide inclusions at low and high strain rates. Various machining tests were carried out at different cutting speeds, namely 190, 110, 45 e 15 m min-1, for obtaining the cutting forces during de machining process. After the machining tests, the roughness of the steels was also measured. Later on, the results of the different experiments were analyzed with statistical tools and then compared to establish a correlation between the cutting forces and microstructure. The higher cutting forces were registered for the 1045-A steel and the lower for the 1145-B steel. However, this was considered merely a trend given that no statistical difference was found to support any conclusion. It was also observed a stronger roughness dependency on the cutting speed than in the distribution and/or morphology of the inclusions. The steels were observed to form a built-up edge (BUE) in a range of cutting velocities of 15-50 m/min. This phenomenon affected the roughness for these cutting velocities. The behavior of the sulfide inclusions was observed to be brittle under low strain rates. On the other hand, under high strain rates, a plastic deformation behavior was observed with inclusions participating in the plastic flow of the metal matrix.

Stainless steel is a metal material widely used in many industries because of its high tensile strength, toughness, and corrosion resistance. Machining stainless steel is challenging due to its high work hardening tendency, low thermal conductivity, and ductility of the material resulting in built-up edge formation. Machining stainless steel at lower cutting speeds must be performed with coolant, which adds to the cost of the process and increases concerns for the environment and the operator's health and safety. Industries such as the aerospace and die-mold industries demand high-speed machining to realize productivity targets. Therefore, a cermet tool material was selected for the present study because of its high temperature resistance, high bending strength, and fracture toughness. The study focused on investigating wear mechanisms and developing a coating on a cermet tool for dry high-speed machining stainless steel to increase tool life. The wear mechanisms of tools were investigated at a fixed cutting interval in relation to the tool's composition and microstructure. Scanning Electron Microscope (SEM) was used to study the microstructure and identify elements on the tool. X-ray diffraction (XRD) was used to identify the phases and concentrations of key elements on the tool. The new advanced in-house coating was developed with Super Fine Cathode (SFC) technology on a Kobelco AIP-20 Physical Vapour Deposition (PVD) coater. The micromechanical properties of the commercial coating and in-house coatings were investigated with the help of nanoindentation and scratch tests. Atomic Force Microscopy (AFM) and SEM were used to investigate the coating microstructure and surface topography. An Alicona variable focus 3D microscope was used to investigate wear volume and wear behaviour. It was discovered that various secondary carbides used by manufacturers to manufacture cermet tools change the microstructure, which affects the machining performance of the cermet tool material. Microchipping at the depth of cut (DOC) causes catastrophic notch wear. It was found that the developed in-house coatings were able to delay the initial wear (microchipping), which improved the tool's life by 318%. This research contributes to meeting the manufacturing industry's challenging demand for dry-high speed machining with reduced manufacturing costs. / Thesis / Doctor of Philosophy (PhD) / Cutting is the process of removing unwanted material from the bulk material to obtain the desired shape. Each metal material has unique mechanical properties that lead to various machining challenges. The cutting process is done with the help of a cutting tool that wears out during the process, and a coating layer is often used to protect the tool. Stainless steel 304 is a widely used material that is difficult to machine. This study includes a systematic approach to understanding the wear mechanisms of tools and existing commercial coatings during the dry machining of stainless steel 304. An in-house coating was developed and deposited on the selected cutting tool to protect it, reduce tool wear and extend its working life. The research results will help reduce machining costs by reducing tool and coolant costs and meet the current industry demand for dry high-speed machining. It will also reduce environmental impact by reducing waste and hazardous chemicals and addressing occupational health and safety concerns.