Automobile components are often exposed to aggressive environments as a result of aqueous salts from the road coming into contact with unprotected steel. This situation greatly reduces both the life and the appearance of the affected parts. Ultra-high strength steel parts are suspected to exhibit poor corrosion-fatigue properties and be more susceptible to corrosion in general. In this study, the effect of strength level on the decrease in fatigue life of AISI 4130 steel when exposed to an aqueous salt solution is quantified. The observed mechanical properties including corrosion-fatigue behavior are examined with consideration to different microstructural characteristics resulting from heat treatments to the steel. The hardness and tensile properties of the test material were characterized before fatigue testing. Fatigue tests were completed in both air and salt solution to determine the effect on fatigue life of the latter environment. Following fatigue testing, the fracture surface was examined using a scanning electron microscope (SEM) to determine the failure mode. Six strength levels of AISI 4130 steel were investigated ranging from 837 to 1846 MPa (121 268 ksi). The frequency of loading used for corrosion-fatigue tests was 1 Hz and the stress ratio for each test was constant at R = 0.1. The corrosion-fatigue tests consisted of the specimen being submerged in an aqueous solution of sodium chloride, calcium chloride, and sodium bicarbonate and fatigued until failure. The solution was maintained at room temperature with constant aeration to ensure constant oxygen levels. The parameters of interest were the applied loads and the cycles to failure. There were four primary findings of the study. First, decreases in fatigue life of the material caused by the corrosive environment ranged from 100% in the lowest strength level to 190% in the higher strength levels. This result showed that higher strength in this steel corresponds to increasing detriment to fatigue life when the material is exposed to an aqueous salt environment. Second, evidence was found that the salt solution lowered the fatigue limit for each strength level studied in this material. All specimens that were tested in the corrosive environment failed in less than 150,000 cycles, while some specimens fatigued in the air environment experienced run-outs at over 106 cycles. Third, the decrease in fatigue life was attributed to the presence of martensite in the structure of the steel. It was noted that the higher the martensite content, the larger the decrease in fatigue life when exposed to the corrosive environment. Finally, the fracture surfaces of fatigued specimens revealed that a similar cracking mode was present for each strength level in both environments. Enhanced crack initiation was, therefore, assumed to be the cause of the decrease in fatigue life between the air and aqueous salt environments.

Submitted by Fernando Coelho (coelho-fernet@ig.com.br) on 2018-04-17T15:09:25Z No. of bitstreams: 1 DM17ABR201811h.pdf: 2603442 bytes, checksum: c2422dffe1f0841b0b22aec6f8143ac3 (MD5) / Approved for entry into archive by Minervina Teixeira Lopes null (vina_lopes@bauru.unesp.br) on 2018-04-18T14:23:57Z (GMT) No. of bitstreams: 1 coelho_f_me_bauru.pdf: 2533903 bytes, checksum: 74e09316ffac9baa3e8d0973642fff7b (MD5) / Made available in DSpace on 2018-04-18T14:23:57Z (GMT). No. of bitstreams: 1 coelho_f_me_bauru.pdf: 2533903 bytes, checksum: 74e09316ffac9baa3e8d0973642fff7b (MD5) Previous issue date: 2018-02-28 / As indústrias metalúrgicas de diversos segmentos buscam continuamente aperfeiçoar seus processos e reduzir custos de produção de estruturas metálicas. Variações em um processo térmico padronizado ou na configuração do produto podem causar alterações na microestrutura e nas propriedades mecânicas do, bem como, no comportamento mecânico do material. Assim, o presente estudo avaliou os efeitos das variações do ângulo de chanfro da junta em tubos de aço redondos AISI 4130 sem costura, quando soldados no processo TIG (Tungsten Inert Gas). Os tubos foram cortados com arco de plasma e preparados com ângulos, 30°, 45° e 60°, e 1 milímetro de altura da raiz. A soldagem foi realizada em dois passes, sendo o primeiro passe na raiz pelo método autógeno e o segundo de preenchimento com metal de adição arame-eletrodo AWSER80S-B2. As juntas soldadas foram submetidas a análises metalográficas de macrografia e microscopia óptica, para avaliação metalúrgica das zonas fundida e afetada pelo calor. Também foi realizado ensaio mecânico de tração e microdureza Vickers para verificar o desempenho mecânico e a dureza do material. A variável de interesse do estudo foi evidenciada por meio da estabilização do processo de corte e soldagem com uso de uma máquina rotativa digital desenvolvida no presente estudo. Os resultados obtidos evidenciaram uniformidade entre os cordões produzidos, variações nas geometrias dos cordões, no tamanho de grão e nas fases de transformação da microestrutura, com impacto na dureza da ZF (Zona Fundida), assim como no desempenho mecânico verificado com ensaios de tração. Conclui-se que, variações no ângulo da junta alteram a área de contato do arco com a poça de fusão, assim como, da poça de fusão com a ZAC. Com isso têm-se diferentes eficiências de transferência de energia térmica por condução, com diferentes ciclos térmicos que resultam em fases e transformações metalúrgicas distintas. / The metallurgical industries at several segments are continuously looking for improving their process and decrease metallic structures production costs. Modifications in a standardized thermal process or in the product configuration can leads to microstructure changes and in the mechanical properties,as well, in the mechanical behavior from the material. Thus, the current study made an assessment of the angle variation effect in steel round tubes joint AISI 4130 without seam, when welded by mechanized TIG process (Tungsten Inert Gas). The steel tubes were cross cut with plasma and prepared in three different angles 30°, 45° and 60°, with 1 millimeter height of root. The welding was made in two passes, the first one in the root by autogenous methode and the second one for filling using eletrode wire AWS ER80S-B2. The welded joints were submitted to metallographic test of macrograph and optical micrographics for metalurgical assessment in the melted zone and the heat affected zone. In addiction was made traction tests and micro hardness Vickers for verify the mechanical behavior and hardness from the material. The interest variable in this study, was evidenced by cut and welding process stabilization using a digital rotative machine developed in this study. The obtained results showed uniformity between weld beads produced with variation in the geometry, as well, in the grain size and in the fases transformations from the microstructure, with impact in the melted zone hardness profile, and in the mechanical properties verified by traction test. It´s pissible conclude that joint chanfer angle variations cause changes in the contact area size between the arc and the molten puddle, and between the molten puddle with the base metal. With this, diferent energy transfer efficency by conduction are obtained producing diferent thermal cicle, that can result in diferent metallurgical fases and microstructure transformations.