Sanicro 28 and Alloy 625 are corrosion-resistant nickel alloys with a fully austenitic structure and a very low carbon content, which means they are both well suited for cold working. Since the millennium shift deformation twinning has been a live research issue as it enhances strength and ductility simultaneously. As nickel has been pointed out as a high stacking fault energy element and deformation twinning should be promoted by a low stacking fault energy level they have been considered as opposite poles. Nonetheless, it is known since long that deformation twins can emerge in high stacking fault face centred cubic elements at low temperatures. In this thesis, we have investigated deformation twinning behaviour in corrosion-resistant nickel alloys. The objective is trying to distinguish between deformation twinning in TWIP steel and corrosion resistant nickel alloys regarding for instance size and bundles. Interrupted uniaxial tensile tests have been performed at several cold working temperatures for the alloys: Sanicro 28 (31% nickel) and Alloy 625 (61% nickel). The microstructure has been characterized in homogeneous deformation volume, by scanning electron microscopy electron backscattering diffraction and electron channelling contrast imaging, transmission electron microscopy and X-ray diffraction. In one investigation fracture behaviour has also been studied with secondary electrons. Ab initio calculations, crystal plasticity modelling and DAMASK simulations have been performed to support emphasizing active deformation mechanisms. It has been revealed that deformation twinning can occur in high Ni alloys. With increasing deformation twinning levels, the diffuse necking decreases. Ab initio calculations indicates that the initiation of deformation twins cannot be determined solely by the stacking fault energy. Distinct features were discovered at low strains that could be rejected from being neither deformation twins nor stacking faults. Level of texture increases with increasing strain and decreasing temperature and the texture modes are changed with decreasing temperature. / Sanicro 28 och Alloy 625 är två legeringar med ett imponerande korrosionsmotstånd, ett lågt kolinnehåll och en helaustenitisk struktur. Det gör dem väl lämpade för kallbearbetning. Sedan millenieskiftet har aktivten varit mycket hög inom forskningsområdet: deformationstvillingar. TWIP (twinning induced plasticity)-effekten har den så eftertraktade egenskapen att både styrkan och duktiliteten förbättras på samma gång. Eftersom nickel har en hög staplingsfelsenergi och TWIP-effekten har uppmätts/beräknats till att aktiveras vid ett snävt och lågt värde, för densamma, har ett ökande nickelinnehåll och TWIP-effekten setts som direkta motpoler. Trots det, har man länge vetat om att deformationstvillingar också framträder, om än, vid låga temperaturer, i legeringar med kubiskt ytcentrerat gitter och hög staplingsfelsenergi. I den här avhandlingen har vi undersökt hur deformationstvillingar utvecklas, om de ens kan bildas i korrosionsbeständiga legeringar med ett högt nickelinnehåll. Målet är att se om det finns några större skillnader i tvillingbeteendet i TWIP-stål i jämförelse med korrosionsbeständiga legeringar med ett högt nickelinnehåll. Några egenskaper vi har tänkt att undersöka är: tjocklek på tvillingarna och om tvillingarna bildas i grupper. Vi hoppas på så sätt kunna svara på den övergripande forskningsfrågan: är det möjligt att designa ett rostfritt TWIP-stål, baserat på det vanligaste legeringssystemet för rostfria austenitiska stål, nämligen: järn-krom-nickel? Enaxliga dragprov har genomförts vid flera kallbearbetningstemperatuer; de har utförts både till brott och till förutbestämda töjningsnivåer. Legeringarna som har testats är: Sanicro 28 (31% nickel) och Alloy 625 (61% nickel). Mikrostrukturen har framför allt karakteriserats i material uttaget från volym där deformationen har varit homogen. De analysmetoder som har använts är: svepelektronmikroskopi, mer specifikt: ”electron backscatter diffraction” och ”electron channelling contrast imaging”. Transmissionselektronmikroskop och röntgendiffraktion har också använts. I en undersökning har också brottbeteende studerats med hjälp av "secondary electrons". Ab initioberäkningar, modellering av kristallplasticitet och materialbeteende med hjälp av DAMASK har också utförts för att kunna se vilka deformationsmekanismer som är aktiva. Vi upptäckte att deformationstvillingar faktiskt kan bildas i korrosionsbeständiga legeringar med ett högt nickelinnehåll. Den diffusa midjebildningen minskar på samma gång som andelelen deformationstvillingar ökar. Ab initioberäkningarnas resultat indikerar också på att deformationstvillingarnas inträde inte enbart kan bestämmas med staplingsfelsenergin. Tydliga mikrostrukturmönster upptäcktes med hjälp av transmissionsmiroskop och vid låga töjningsnivåer. De mikrostrukturmönstren kunde avfärdas från att vara både deformationstvillingar och staplingsfel. Texturnivån ökar med ökande töjningsnivå och sjunkande temperatur. Typen av textur förändras också med sjunkande temperatur. / <p>Funding agency: Tube division, Alleima AB</p>

Orientadores: Paulo Roberto Mei, Anselmo Eduardo Diniz / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-12T19:31:21Z (GMT). No. of bitstreams: 1 Gravalos_MarcioTadeu_M.pdf: 4613359 bytes, checksum: 86f814729e45d2e34702f6735566c398 (MD5) Previous issue date: 2008 / Resumo: Foram investigados os efeitos das condições de usinagem em operação de torneamento na integridade superficial do aço inoxidável superaustenítico ASTM A744 Gr.CN3MN. A operação de acabamento foi o foco principal, porém também foram realizados estudos com a operação prévia, de desbaste. As principais variáveis no acabamento foram: ângulo de saída da ferramenta, velocidade de corte, profundidade de corte e avanço. No desbaste variou-se somente o ângulo de saída e a velocidade de corte. Os ensaios foram realizados em um torno horizontal CNC, utilizando fluido de corte. Os parâmetros analisados foram: microestrutura por microscopia ótica, rugosidade, encruamento por ensaio de microdureza e tensão residual por difração de raio-X. Para se estudar a resistência à corrosão por pites, superfícies usinadas foram imersas em solução de cloreto de ferro e examinadas no estereoscópio, medindo-se, por área, a densidade de pites e a perda de massa. Os resultados mostraram a presença de tensão residual de tração e de uma camada encruada em todas as superfícies usinadas. A redução da rugosidade elevou a resistência à corrosão por pite. A melhor combinação das condições de corte encontrada para obter os menores valores de rugosidade e de tensão residual de tração, no acabamento, para profundidade de corte de 0,2 ou 0,4 mm, foi a maior velocidade de corte (120 m/min), o menor avanço (0,1 mm/rot) e o maior ângulo de saída (10º), no desbaste; para a profundidade de corte de 2,5 mm e avanço de 0,25 mm/rot., foi a menor velocidade de corte (60 m/min) e o maior ângulo de saída (10º). / Abstract: An investigation was made of the effects of the machining conditions employed in the turning operation on the surface integrity of ASTM A744 Grade CN3MN superaustenitic stainless steel. The main focus was the finishing operation, but studies were also carried out with a prior roughening operation. The main input variables in finishing were: tool rake angle, cutting speed, cutting depth and feed rate. The only input variables in the roughening operation were rake angle and cutting speed. The tests were conducted on a CNC horizontal lathe, using cutting fluid flood. The following parameters were analyzed: microstructure and roughness - examined by optical microscopy; work hardening - determined by microhardness measurements; and residual stress - analyzed by X-ray diffraction. To study the pitting corrosion resistance, machined surfaces were immersed in a ferric chloride solution and examined under a stereoscope, measuring the pits density and mass loss per area. The results revealed the presence of residual tensile stress and a hardened layer on all the machined surfaces. Reducing the roughness caused the pitting corrosion resistance to increase. To obtain the lowest values of roughness and residual tensile stress in finishing at cutting depths of 0.2 or 0.4 mm, the best combination of cutting conditions was the highest cutting speed (120 m/min), the lowest feed rate (0.1 mm/v), and the highest rake angle (10º). In roughening at a cutting depth of 2.5 mm and a feed rate of 0.25 mm/rot., the best combination was the lowest cutting speed (60 m/min) and the highest rake angle (10º). / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

Orientadores: Paulo Roberto Mei, Marcelo Martins / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-10T20:59:53Z (GMT). No. of bitstreams: 1 Ritoni_Marcio_M.pdf: 22898482 bytes, checksum: cebeda09f860c091710fa40d369a641d (MD5) Previous issue date: 2007 / Resumo: O aço inoxidável superaustenítico ASTM A 744 Gr. CN3MN é aplicado na fabricação de equipamentos que trabalham em ambientes sob corrosão severa com solicitação mecânica. Neste trabalho investigou-se a influência dos tratamentos térmicos na microestrutura e propriedades desse tipo de material. Foram realizados tratamentos térmicos de solubilização na faixa de temperaturas entre 1100 e 1250 °C, alívio de tensões entre 500 e 800 °C e tratamentos de envelhecimento a 900 °C, variando-se o tempo em 1,5, 12, 24 e 48 horas. Realizaram-se também ensaios mecânicos (dureza e impacto). As análises microestruturais foram feitas por meio de microscopia ótica, eletrônica de varredura, eletrônica de transmissão e difração de raios-X. Concluiu-se que, para maximizar a resistência ao impacto, a solubilização deve ser feita a 1200 °C, que produz a menor fração volumétrica de precipitados. À medida que a temperatura de alívio de tensões aumenta de 500 para 800 °C, a energia absorvida no impacto reduz-se. Além do aumento na fração volumétrica de precipitados, nas temperaturas acima de 550 °C, observa-se precipitados nos contornos de grãos. Quanto maior a exposição do material à temperatura de 900 °C, menor é a energia absorvida no impacto. Com 1,5 horas a 900 °C o material apresentou redução na resistência ao impacto de 127,7 para 25,0 Joules. A dureza do material apresentou resultado inversamente proporcional à energia absorvida no impacto para todas as temperaturas de tratamento térmico. A amostra bruta de fundição apresentou fase sigma (s) na matriz austenítica. As amostras solubilizadas a 1200 e 1240 °C apresentaram fase sigma (s) e carboneto M6C. O tratamento térmico a 900 °C por 48 horas causou a precipitação de algumas fases na matriz austenítica sendo as mais prováveis: sigma (s), chi (c) e carboneto M23C6 / Abstract: ASTM A 744 Grade CN3MN super-austenitic stainless steel is employed in the manufacture of equipment working in severely corrosive environments under mechanical loads. In this work, an investigation was made of the influence of heat treatments on the microstructure and properties of this type of material. Solution heat treatments were carried out at temperatures ranging from 1100 to 1250 ºC, stress relief between 500 and 800 ºC and aging treatments at 900 ºC, for periods of time varying from 1,5, 12, 24 and 48 hours. Mechanical tests (hardness and impact) were also conducted. The microstructural analyses were carried out by X-ray diffraction, optical microscopy, and by scanning and transmission electron microscopy. It was concluded that, to maximize the material¿s impact strength, the solution heat treatment should be done at 1200 ºC, at which temperature the volumetric fraction of precipitates is lower than at other solution heat treatment temperatures. As the stress relief temperature rose from 500 to 800 ºC, the energy absorbed during impact diminishes. In addition to the increase in the volumetric fraction of precipitates at temperatures above 550 ºC, precipitates were visible at the grain boundaries. The longer the material was exposed to a temperature of 900 ºC the lower the energy absorbed during impact. After 1,5 hours at 900 ºC the material¿s impact strength dropped from 127,7 to 25,0 Joules. The steel¿s hardness was also inversely proportional to the energy absorbed during impact at all the heat treatment temperatures. The as-cast sample presented sigma (s) phase in the austenitic matrix. The samples solution heat treated at 1200 and 1240 ºC presented sigma (s) and M6C carbide phases. The heat treatment at 900 ºC for 48 hours caused precipitation of some phases in the austenitic matrix, the most probable of which were sigma (s), chi (c) and M23C6 carbide / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica